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Radiation chemistry of dimethylsulfoxide Cooper, Terry Kenneth 1972-12-31

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RADIATION CHEMISTRY OF DIMETHYLSULFOXIDE  BY  TERRY KENNETH COOPER B.Sc., U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1968  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Department of CHEMISTRY  We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e required standard  THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1972  In p r e s e n t i n g t h i s t h e s i s  i n p a r t i a l f u l f i l m e n t of the requirements  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e  for  I agree t h a t and s t u d y .  copying of t h i s  thesis  f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . of t h i s t h e s i s  It  i s understood t h a t copying or p u b l i c a t i o n  f o r f i n a n c i a l g a i n s h a l l not be allowed without my  written permission.  Department The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada  ABSTRACT A comprehensive s t u d y o f t h e r a d i a t i o n c h e m i s t r y o f d i m e t h y l s u l f o x i d e (DMSO), and DMSO-H 0 m i x t u r e s , by s t e a d y - s t a t e y - r a d i o l y s i s , 2  p u l s e r a d i o l y s i s and l o w temperature  m a t r i x i s o l a t i o n t e c h n i q u e s has  r e v e a l e d s e v e r a l s i g n i f i c a n t f e a t u r e s about the t r a n s i e n t i o n i c p r o d u c t s o f the r a d i o l y s i s .  Very s h o r t - l i v e d (T^^  ^5 n s e c ) ,  <  w e a k l y bound s o l v a t e d e l e c t r o n s a r e formed i n q u i t e h i g h y i e l d s i n t h e s e l i q u i d s a t room t e m p e r a t u r e .  S o l v a t e d e l e c t r o n s i n the b i n a r y  DMSO-H^O m i x t u r e s e x h i b i t s i n g l e o p t i c a l a b s o r p t i o n bands w i t h maxima w h i c h v a r y smoothly w i t h c o m p o s i t i o n from p u r e w a t e r ( ^ t o p u r e DMSO (X > 1500 nm). max v  = m a x  720 nm)  The e l e c t r o n s t a b i l i t y v a r i e s w i t h J  c o m p o s i t i o n , showing a minimum l i f e t i m e c o i n c i d i n g w i t h DMSO-H^O m i x t u r e s w h i c h e x h i b i t maximum i n t e r - m o l e c u l a r s t r u c t u r e . a t 77°K, e l e c t r o n s g e n e r a t e d t r a p p e d aid s t a b i l i z e d . are longer l i v e d  (T-^^  I n glassy mixtures  r a d i o l y t i c a l l y or photochemically are not  The p o s i t i v e i o n s formed from i r r a d i a t e d DMSO %  ^ y s e c ) than the s o l v a t e d e l e c t o n s and have  a s t r o n g a b s o r p t i o n band c e n t e r e d a t X  ^ 550 nm.  I n DMSO-H-0 z.  in cix  m i x t u r e s t h i s same a b s o r p t i o n band i s formed.  The c a t i o n i c y i e l d i s  d i r e c t l y p r o p o r t i o n a l t o t h e f r a c t i o n o f the t o t a l dose i n i t i a l l y by DMSO w h i c h s u g g e s t s  t h e r e i s no charge t r a n s f e r ,  exchange or p o s i t i v e i o n s c a v e n g i n g both e  g  absorbed  i n these mixtures.  proton  The y i e l d o f  and c a t i o n s was ^ 30% h i g h e r i n the f u l l y d e u t e r a t e d DMSO  than i n the h y d r o g e n a t e d m a t e r i a l i n d i c a t i n g some a l t e r a t i o n i n t h e e l e c t r o n escape p r o b a b i l i t y due t o i s o t o p i c  substitution.  Most o f the above o b s e r v a t i o n s are i n t e r p r e t e d i n terms o f t h e p h y s i c a l p r o p e r t i e s o f DMSO and i t s aqueous m i x t u r e s .  The r a d i a t i o n  - iii -  y i e l d of free ions i s measured to be 1.30 +0.15 which i s i n reasonably good accord with a l i q u i d having a d i e l e c t r i c constant of 48.  The y i e l d of solvated electrons was ascertained by observing the  formation of anthracene anions i n solutions of anthracene and corresponded very c l o s e l y to the y i e l d of p o s i t i v e ions determined from the formation of  i n KBr solutions.  The near i n f r a r e d  absorption  band of solvated electrons i n DMSO indicates a weakly s t a b i l i z e d species which i s consistent with the very poor solvating power for negative ±ms  afforded by DMSO due to the aprotic nature of the  solvent.  On the other hand the lack of charge transfer or proton exchange with H^O shown by the DMSO cation i s consistent with the dipolar  character  of DMSO and i t s strong solvation of p o s i t i v e ions. A v a r i e t y of free r a d i c a l s have been observed by electron spin resonance in Y ~ i r r a d i a t e d p o l y c r y s t a l l i n e DMSO at 77°K amongst which the  •CH.j and a s u l f u r containing r a d i c a l are distinguished.  Y - r a d i o l y s i s l i q u i d DMSO produces H values of 0.20 + 0.01,  3.3 + 0.1,  2 >  CH^,  and ( C H ^ S  Under with G  0.49 + 0.03 and 1.2 + 0.2 respectively.  - iv -  TABLE OF CONTENTS Chapter I.  Page INTRODUCTION  1  A.  I n t e r a c t i o n of High-Energy R a d i a t i o n w i t h M a t t e r  2  1.  Electromagnetic  2  2.  High-energy e l e c t r o n s  3.  Range of e l e c t r o n s  4.  T r a c k e n t i t i e s : s p u r s , b l o b s and  B.  8 13 short tracks  C h e m i c a l Consequences F o l l o w i n g A b s o r p t i o n  14  of  High-Energy R a d i a t i o n  17  1.  Time s c a l e of e v e n t s  18  2.  Phase-dependent phenomena  21  3.  S t u d i e s on the c h e m i c a l  4. C.  radiation  e v e n t s i n condensed  phases  22  Chemical y i e l d s  24  Stabilized Electrons  26  1.  Stabilization  26  2.  Properties  27  2.1  O p t i c a l spectrum  28  2.2  E l e c t r o n s p i n resonance  32  2.3  Conductivity  34  3.  Models  35  4.  Free i o n y i e l d s  43  D.  Binary Mixtures  48  E.  Scope of Study  53  - v -  Chapter II.  P a  EXPERIMENTAL A.  B.  ,  C.  ^Co y  _ R a  §  e  58 diolysis  58  1.  Materials  58  2.  R a d i a t i o n source  59  3.  Dosimetry  59  4.  Sample p r e p a r a t i o n  65  5.  Product a n a l y s i s  71  Pulse Radiolysis  77  1.  O u t l i n e o f the t e c h n i q u e  77  2.  R a d i a t i o n source  79  3.  I r r a d i a t i o n c e l l and o p t i c a l d e t e c t i o n system  79  4.  O s c i l l o s c o p e measurements  82  5.  Cerenkov e m i s s i o n  84  6,  Dosimetry  88  7.  M a t e r i a l s and p u r i f i c a t i o n  91  E l e c t r o n S p i n Resonance  92  1.  M a t e r i a l s and p u r i f i c a t i o n  92  2.  R a d i a t i o n source  92  3.  Sample p r e p a r a t i o n  92  4.  E l e c t r o n s p i n resonance s p e c t r a  93  5.  P h o t o l y s i s apparatus  94  -  VI  -  Chapter III.  Page STUDIES ON LIQUID DMSO  95  A.  ^ C o y-Radiolysis  95  1.  Gaseous p r o d u c t s  95  2.  L i q u i d products  98  3.  Scavenger s t u d i e s  4.  Discussion  B.  IV.  V.  100 H3  Pulse Radiolysis  1  2  2  1.  Absorption  1  2  2  2.  Free i o n y i e l d s  132  3.  Geminate i o n s c a v e n g i n g  148  4.  D i e l e c t r i c constant  154  s p e c t r a i n DMSO and DMSO^  and e l e c t r o n s t a b i l i z a t i o n  PULSE RADIOLYSIS OF DMSO-WATER BINARY MIXTURES  156  1.  Solvated electrons  157  2.  DMSO p o s i t i v e i o n s  169  3.  Transient intermediates  a t 77°K  173  ELECTRON SPIN RESONANCE STUDIES ON DMSO AND DMSOH 0 MIXTURES AT 77°K 2  A.  B.  ..  I  7  7  Introduction  177  1.  Basic p r i n c i p l e s of e s r  177  2.  Amorphous and p o l y c r y s t a l l i n e media  183  R e s u l t s and D i s c u s s i o n 1.  187  P u r e DMSO  I  8  7  1.1  Ultraviolet irradiated  I  8  7  1.2  y-Irradiated  190  - vii Chapter  Page  V (continued) 2.  DMSO-water m a t r i c e s  193  2.1 y - i r r a d i a t e d p o l y c r y s t a l l i n e H^O  193  2.2 y - i r r a d i a t e d DMSO-H^O m i x t u r e s  195  2.3 p h o t o i o n i z a t i o n o f K^Fe(CN)^ i n aqueous glasses  REFERENCES  205  211  - viii LIST OF TABLES  O p t i c a l data f o r e l e c t r o n s s t a b i l i z e d i n l i q u i d media a t room temperature R a d i a t i o n y i e l d s from i r r a d i a t e d DMSO c o n t a i n i n g 0.05  M ^ 0 p l u s the second scavenger  indicated i n  column 1 a t c o n c e n t r a t i o n g i v e n i n column 2  R a t i o o f k_/k_^  o b t a i n e d f o r the p r e c u r s o r o f N„  i n DMSO w i t h v a r i o u s second scavengers t o a 0.05 M s o l u t i o n o f ^ 0 . r e f e r s t o the  (S) added  Column headed w a t e r  published rate constant r a t i o f o r  the h y d r a t e d e l e c t r o n Summary o f d a t a o b t a i n e d from s t u d i e s on p u l s e i r r a d i a t e d DMSO-water m i x t u r e s a t room  temperature.  Summary o f d a t a o b t a i n e d from s t u d i e s on Y ' i r r a d i a t e d DMSO-water m a t r i c e s a t 77°K  - ix -  LIST OF FIGURES Figure 1  Page Atomic absorption Total absorption  c o e f f i c i e n t s f o r water.  Curve  A,  c o e f f i c i e n t (with coherent  s c a t t e r i n g ) ; B, p h o t o e l e c t r i c a b s o r p t i o n c o e f f i c i e n t ; C, Compton c o e f f i c i e n t ( w i t h c o h e r e n t s c a t t e r i n g ) ; D,  Compton c o e f f i c i e n t ( w i t h o u t  coherent s c a t t e r i n g ) ; -24  E, p a i r - p r o d u c t i o n 2  coefficient.  2  1 barn =10  cm  Huygens c o n s t r u c t i o n of e l e c t r o n t r a j e c t o r y  ..  and  r e s u l t i n g Cerenkov waveform 3  short tracks for electrons i n water.  logarithm  The  numbers are the n e g a t i v e  of t i m e (pt = - l o g t ( s e c ) )  Schematic r e p r e s e n t a t i o n  of e l e c t r o n  20 localization  (shaded r e g i o n ) p r o d u c e d by p o l a r i z a t i o n o f  the  medium. The  or  dotted  area represents  the v o i d  c a v i t y i n w h i c h the e l e c t r o n i s c e n t e r e d 6  30  C o r r e l a t i o n c u r v e of the t r a n s i t i o n energy of e l e c t r o n i n DMSO and  i t s c a v i t y radius using  c o n t i n u o u s d i e l e c t r i c model w i t h an  the the  adiabatic  approximation 7  16  T h e o r e t i c a l t i m e s c a l e f o r the i n i t i a l p r o c e s s e s i n r a d i a t i o n chemistry.  5  11  S c h e m a t i c p l o t of p e r c e n t a g e of energy s p l i t between s p u r s , b l o b s and  4  6  39  P l o t of G ( f r e e i o n ) as a f u n c t i o n of the d i e l e c t r i c c o n s t a n t of the medium. references  32,  33,  34, 46,  101  static  Data t a k e n from 46  -  x  -  Figure 8  Page F r i c k e d o s i m e t e r r e s u l t s o b t a i n e d from the r a d i o l y s i s of the s o l u t i o n s i n the i r r a d i a t i o n c e l l used i n t h i s study  9  ...  3  P y r e x i r r a d i a t i o n c e l l s used f o r d e o x y g e n a t i o n o f the l i q u i d samples  10  6  ^6  S c h e m a t i c diagram o f vacuum l i n e used f o r d e g a s s i n g the l i q u i d samples and a d d i n g n i t r o u s o x i d e t o t h e samples  11  68  P l o t showing the r e l a t i o n s h i p o f t h e p a r t i a l p r e s s u r e of n i t r o u s o x i d e t o i t s s o l u b i l i t y i n DMSO a t 23°C for  the two b u b b l e r c e l l s c o n t a i n i n g  a medium  (cell  A) and f i n e ( c e l l B) p o r o s i t y s i n t e r e d d i s k 12  S c h e m a t i c diagram o f a p p a r a t u s used f o r f l u s h i n g t h e v o l a t i l e gaseous p r o d u c t s i n t o the gas chromatograph.  13  70  72  T y p i c a l chromatograph o b t a i n e d f o r 20 m l DMSO sample c o n t a i n i n g 0.05 M n i t r o u s o x i d e and r e c e i v i n g an 4 absorbed dose o f 8 x 10  14  rads  75  T y p i c a l chromatograph o b t a i n e d a f t e r i n j e c t i o n o f 25 u£ o f i r r a d i a t e d DMSO sample.  T o t a l absorbed  dose was 6 Mrad 15  L a y - o u t o f the p u l s e r a d i o l y s i s equipment  76 at the  N a t i o n a l Research C o u n c i l r a d i a t i o n l a b o r a t o r y i n Ottawa, O n t a r i o  78  - xi -  Figure 16  Page S c h e m a t i c diagram of i r r a d i a t i o n c e l l used f o r deoxyg e n a t i o n of l i q u i d samples used i n the p u l s e  radiolysis  study.  tipping  The  s p e c t r o s c o p i c c e l l was  f i l l e d by  the c e l l h o r i z o n t a l l y a f t e r f l u s h i n g w i t h h i g h p u r i t y argon 17  80  H y p o t h e t i c a l o s c i l l o s c o p e t r a c e showing the b u i l d and decay of t r a n s i e n t a b s o r b i n g s p e c i e s .  The  up  time  p r o f i l e o f the e l e c t r o n p u l s e i s shown as the d o t t e d curve 18  83  T y p i c a l o s c i l l o s c o p e t r a c e s showing the r e s p o n s e of the d e t e c t i o n a p p a r a t u s ted  19  t o Cerenkov l i g h t  genera-  u s i n g a 40 n s e c wide e l e c t r o n p u l s e  85  E f f e c t o f Cerenkov e m i s s i o n on the a b s o r p t i o n of t r a n s i e n t s a b s o r b i n g a t 500 nm  the  i n pure DMSO. (a) pure  Cerenkov ( a n a l y z i n g l i g h t o f f ) ; (b) observed  absorption;  (c) a b s o r p t i o n t h a t would have been o b s e r v e d  had  been no Cerenkov e m i s s i o n . The p h o t o d i o d e w i t h a 93 ohm w i d t h was 20  time  d e t e c t o r was  load r e s i s t o r .  there  a Si  The  pulse  40 n s e c  87  T y p i c a l o s c i l l o s c o p e t r a c e s f o r the f o r m a t i o n decay o f ( C N S ^  a t 500 nm  n s e c p u l s e of 35 MeV  o b t a i n e d by u s i n g a 40  e l e c t r o n s on a n i t r o u s o x i d e -3  s a t u r a t e d s o l u t i o n o f 5 x 10 (a) S i p h o t o d i o d e , m u l t i p l i e r , 470 ohm  and  93 ohm  M thiocyanate i n water,  load r e s i s t o r ;  load r e s i s t o r  (b) p h o t o 90  - x i i-  Figure 21  Page R a d i a t i o n y i e l d s as a f u n c t i o n o f absorbed dose: Q,  ^2^6  a n <  ^ 0>  H  2*  ^  e  I n e t  '  i a n e  c  u  r  v  e  w  a  obtained  s  by t a k i n g the s l o p e a t v a r i o u s p o r t i o n s o f t h e c u r v e 96  shown i n F i g u r e 22 22  Accumulated gas y i e l d s as a f u n c t i o n of accumulated dose: 9 , A  O, 23  H  , £\ ,  , CH. a t v a r i o u s doses: Q , C_H,; 97  2  Accumulated d i m e t h y l s u l f i d e y i e l d as a f u n c t i o n o f a c c u m u l a t e d dose. The e x t r a p o l a t e d p o r t i o n o f the 99  c u r v e c o r r e s p o n d s t o G(DMS) = 1.2 + 0 . 2 24  R a d i a t i o n y i e l d s of C H  4  (A),  as a f u n c t i o n o f t h e ^ 0  C ^  (•) and H  concentration.  (O)  2  I n the case 101  o f CH^, a l l doses were < 1.8 x 10^ r a d s 25  R a d i a t i o n y i e l d o f n i t r o g e n as a f u n c t i o n o f ^ 0 102  concentration 26  Accumulated n i t r o g e n y i e l d as a f u n c t i o n o f a c c u m u l a t e d dose  27  103  P l o t o f 1/G(N ) as a f u n c t i o n o f [ S c a v e n g e r ] / [ N 0 ] . 2  2  The N 0 c o n c e n t r a t i o n was 0.05 M and the second 2  s c a v e n g e r c o n c e n t r a t i o n was v a r i e d . t a k e n from T a b l e I I . © , O > Ag 28  +  and D  C C l ^ ; A,  The d a t a was I  2  ; B,  CHC1 ; 3  , acetone.  108  P l o t o f 1/G(N ) as a f u n c t i o n o f [ H ] / [ N 0 ] i n w h i c h +  2  O  >  t i o n s : O, M N 0. © T  2  D correspond to d i f f e r e n t N 0 2  0.05 M N O ; A,  0.04 M N 0 and • , 2  concentra0.07  Corresponds t o 0.5 M methanol added t o t h e  acid solution  109  - xiii -  Figure 29  Page . R a d i a t i o n y i e l d s o f CH^ (A), as a f u n c t i o n o f  (•) , and H  concentration.  (O)  2  The doses were  < 5 x 1 0 rads  I l l  4  30  Radiation y i e l d s of CH as a f u n c t i o n o f H  +  4  (A),  C ^  (•) and H  concentration.  (O)  2  corresponds t o  the y i e l d o f CH, when 0.05 M N.O was added t o t h e 4 2 c o r r e s p o n d i n g a c i d s o l u t i o n o f DMSO.  I n t h e case o f  CH. , a l l doses were < 1.8 x 10^ r a d s 4 31  Transient The  112  s p e c t r a o b s e r v e d i n ( C H ^ ) S 0 and (CD,j) S0. 2  2  c i r c l e s r e f e r t o t h e s o l v a t e d e l e c t r o n band  c o r r e c t e d f o r t h e d e t e c t o r r e s p o n s e time and t h e t r i a n g l e s r e f e r t o t h e DMSO p o s i t i v e i o n , o r o x i d i z i n g species. (CD.j) S0. 2  O and A a r e f o r ( C H ^ ) S 0 ; 2  9 and  are f o r  A l m o s t a l l d a t a p o i n t s a r e t h e mean o f a t  l e a s t two measurements.  The X f o r the A spectrum max  was e s t a b l i s h e d t o be a t 550 nm from a p r e v i o u s set o f experiments 32  123  T y p i c a l o s c i l l o s c o p e t r a c e s showing t h e decay o f t h e s o l v a t e d e l e c t r o n i n DMSO.  Both t r a c e s were  obtained  u s i n g a Ge p h o t o d i o d e w i t h a 50 ohm l o a d r e s i s t a n c e . (a) p u l s e w i d t h 10 n s e c ; (b) p u l s e w i d t h 40 nsec 33  125  T y p i c a l o s c i l l o s c o p e t r a c e s showing t h e decay o f t h e DMSO p o s i t i v e i o n o r o x i d i z i n g s p e c i e s a t 550 nm. The f a s t i n i t i a l decay i n (a) i s due t o t h e s o l v a t e d e l e c t r o n . B o t h t r a c e s were o b t a i n e d  u s i n g a p u l s e w i d t h o f 40  n s e c . (a) S i p h o t o d i o d e w i t h 93 ohm l o a d  resistor;  (b) p h o t o m u l t i p l i e r w i t h 470 ohm l o a d r e s i s t o r  126  - xiv -  Figure 34  • Page F i r s t - o r d e r decay p l o t o f t h e s o l v a t e d e l e c t r o n i n DMSO t a k e n a t 1300 nm.  The p u l s e w i d t h was 10 n s e c ,  g i v i n g an absorbed dose o f 900 r a d s p e r p u l s e . The decay was measured u s i n g a Ge p h o t o d i o d e w i t h a 50 ohm load r e s i s t o r 35  130  F i r s t - o r d e r p l o t o f t h e decay o f the DMSO p o s i t i v e i o n . The decay was measured a t 550 nm u s i n g the p h o t o m u l t i p l i e r w i t h a 470 ohm l o a d r e s i s t o r .  The p u l s e was  40 n s e c , g i v i n g an absorbed dose o f 2200 r a d s p e r pulse 36  ^31  End o f p u l s e a b s o r p t i o n spectrum of the a n t h r a c e n e r a d i c a l a n i o n o b t a i n e d from a DMSO s o l u t i o n 0.02 M i n a n t h r a c e n e a f t e r absorbances due t o t h e e l e c t r o n and o x i d i z i n g s p e c i e s had been s u b t r a c t e d  37  134  T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay o f t h e e l e c t r o n and b u i l d up o f the a n t h r a c e n e r a d i c a l a n i o n a t 750 nm. (a) no a n t h r a c e n e added;  (b) 0.001  M a n t h r a c e n e i n DMSO; (c) 0.005 M a n t h r a c e n e i n DMSO. 38  I  3  5  I  3  6  T y p i c a l o s c i l l o s c o p e t r a c e s showing the b u i l d up and decay o f the a n t h r a c e n e r a d i c a l a n i o n a t 750 nm. (a) 0.01 M a n t h r a c e n e i n DMSO; (b) 0.02 M a n t h r a c e n e i n DMSO  -  X  V  -  Figure 39  Page Graph showing t h e s c a v e n g i n g  of solvated electrons i n  pure DMSO by a n t h r a c e n e and t h e f o r m a t i o n o f anthracene r a d i c a l anions. © due  to A  immediately  , absorbance a t 750 nm  a t t h e end o f the p u l s e ; O »  maximum i n t h e absorbance a t 750 nm due t o A  after  the p u l s e ; A » absorbance due t o t h e p o s i t i v e i o n s a t 550 nm; B , absorbance due t o s o l v a t e d e l e c t r o n s a t 1275  nm ( n o t c o r r e c t e d f o r decay d u r i n g t h e p u l s e n o r  f o r t h e d e t e c t o r response time) 40  138  A b s o r p t i o n s p e c t r a f o r KBr s o l u t i o n s i n DMSO.  Solid  s u r v e r e f e r s t o B r ^ s p e c t r a ; t h e d o t t e d curve r e f e r s t o t h e t r a n s i e n t p r e c u r s o r o f B r • <^, B r 2  2  from 0.1  M K B r s o l u t i o n ; # , B r ~ from 0.01 M K B r s o l u t i o n ; 2  13 , t r a n s i e n t p r e c u r s o r o f B r  41  2  a t 0.01 M K B r .  The  i n s e t i s a p l o t o f Ge a t 375 nm f o r B r  log  ([Br~]/M)  2  against 142  T y p i c a l o s c i l l o s c o p e t r a c e s showing t h e b u i l d up and decay o f B r  2  a t 365 nm i n pure DMSO.  D e t e c t i o n made  u s i n g t h e p h o t o m u l t i p l i e r w i t h a 470 ohm l o a d resistor.  (a) 0.1 M KBr;  (b) 0.01 M KBr;  ( c ) 0.001  M KBr 42  P l o t showing f i r s t - o r d e r b u i l d up o f B r 4 1 ( c ) ) f o r DMSO s o l u t i o n 0.001 M i n KBr  143 2  (Figure 145  - xvi -  Figure 43  Page T y p i c a l o s c i l l o s c o p e traces Br  2  showing the decay o f  and i t s t r a n s i e n t p r e c u r s o r .  The f a s t  initial  decay i n (b) and (c) i s a t t r i b u t e d t o the t r a n s i e n t complex.  Detection  was made u s i n g  the  photomultiplier  w i t h a 470 ohm l o a d r e s i s t o r 44  End o f p u l s e a b s o r p t i o n i o n i n 0.2 M H„S0.. L 4  spectrum o f the DMSO p o s i t i v e  The d o t t e d  l i n e r e f e r s to the  absorbance o f the p o s i t i v e i o n i n pure DMSO. t o t h e l o n g - l i v e d S0^ y  intermediate  X refers  produced i n t h e  acid solution 45  L  F i r s t - o r d e r decay p l o t o f the DMSO p o s i t i v e i o n i n the p r e s e n c e o f 0.2 M H^SO^. nm u s i n g  the p h o t o m u l t i p l i e r  Decay measured  a t 550  w i t h a 470 ohm l o a d ^1  resistance 46  ^  Typical oscilloscope traces  showing the decay o f t h e  DMSO p o s i t i v e i o n i n the p r e s e n c e o f 0.2 M H^SO^ a t 650 nm and 450 nm.  The l o n g e r - l i v e d SO^  transient  i s r e a d i l y o b s e r v e d a t 450 nm 47  End o f p u l s e spectrum o f t r a n s i e n t s produced i n a DMSO s o l u t i o n 0.5 M i n A g  48  152  +  Absorption spectra of solvated m i x t u r e s ; 0, 0.20, 0.28, mole f r a c t i o n DMSO. nm i n t e r v a l s .  1 5  electrons  3  i n DMSO-H^O  0.43, 0.72, 0.93 and 1.0  Data p o i n t s were o b t a i n e d a t 50  The d a t a f o r pure w a t e r have been  m u l t i p l i e d by a f a c t o r o f 0.3 r e l a t i v e t o the o t h e r s .  159  -• x v i i -  Figure 49  Page P l o t of the v a l u e s o f Ge f o r the s o l v a t e d e l e c t r o n max a b s o r p t i o n bands p r e s e n t e d  i n F i g u r e 48 as a f u n c t i o n  of t h e mole f r a c t i o n DMSO f o r t h e DMS0-H 0 m i x t u r e s . 2  The  n o n - l i n e a r a x i s showing t h e change as a f u n c t i o n  of s t a t i c d i e l e c t r i c c o n s t a n t  of the b u l k m i x t u r e i s  shown on t h e top a b s c i s s a . O > a c t u a l o b s e r v e d absorbance peak h e i g h t s . 9 , c o r r e c t e d f o r decay d u r i n g the p u l s e and response time o f the d e t e c t o r 50  162  P l o t o f t h e photon energy o f t h e a b s o r p t i o n band maximum f o r t h e s o l v a t e d e l e c t r o n i n t h e DMSO-H^O mixtures  against the bulk s t a t i c d i e l e c t r i c  of the mixtures  ( a t 25°C).  showing t h e c o r r e s p o n d i n g  constant  The n o n - l i n e a r a x i s mole f r a c t i o n DMSO i s shown  i n t h e upper a b s c i s s a 51  P l o t o f t h e photon energy o f t h e a b s o r p t i o n band maximum f o r t h e s o l v a t e d e l e c t r o n i n t h e DMSO-l^O mixtures  a g a i n s t t h e mole f r a c t i o n o f w a t e r .  l i n e a r a x i s showing t h e c o r r e s p o n d i n g constants  o f the m i x t u r e s  bulk  The non-  dielectric  i s shown i n t h e upper  abscissa 52  (a) A b s o r p t i o n  168 s p e c t r a a t t r i b u t e d t o the DMSO p o s i t i v e  i o n produced i n DMSO-H^O m i x t u r e s .  Curve 6 i s pure  DMSO; 5, 0.93; 4, 0.72; 3, 0.43; 2, 0.28; and 1, 0.20 mole f r a c t i o n DMSO. (b) Peak absorbance ( a t 550 nm) of t h e bands shown i n (a) p l o t t e d a g a i n s t t h e f r a c t i o n of dose absorbed i n i t i a l l y by DMSO  170  - xviii -  Figure 53  Page A b s o r p t i o n spectrum o f t r a n s i e n t s produced by the p u l s e r a d i o l y s i s o f a DMSO-H 0 g l a s s (39 mole % DMSO) 2  at 77°K.  The p u l s e w i d t h was 500 n s e c , t h e dose p e r  p u l s e b e i n g ^ 10 k r a d . The d o t t e d c u r v e i s t h a t o f t h e DMSO p o s i t i v e i o n i n pure DMSO n o r m a l i z e d 54  a t 600 nm..  T h e o r e t i c a l e s r l i n e shapes f o r (a) a x i a l l y and (b) c o m p l e t e l y  asymmetric g t e n s o r s .  175  symmetric  The upper  curves r e f e r to absorption s p e c t r a of the paramagnetic species.  The l o w e r c u r v e s r e f e r t o t h e  e x p e r i m e n t a l l y observed f i r s t 55  derivative spectra....  185  T h e o r e t i c a l f i r s t d e r i v a t i v e e s r spectrum f o r a p a r a m a g n e t i c s p e c i e s w i t h S = 1/2, I = 1/2 and w i t h a x i a l l y asymmetric g and A t e n s o r s .  The c e n t r a l  d o t t e d p o r t i o n i s t h e t h e o r e t i c a l spectrum i n t h e absence o f the h y p e r f i n e i n t e r a c t i o n s 56  186  E l e c t r o n s p i n resonance spectrum o b t a i n e d  after the  u l t r a v i o l e t p h o t o l y s i s o f p o l y c r y s t a l l i n e DMSO a t 77°K. The arrows c o r r e s p o n d t o t h e asymmetric gf a c t o r s o f t h e s u l f u r r a d i c a l CH^SO. The m e t h y l r a d i c a l q u a r t e t i s i n d i c a t e d by t h e s t i c k p l o t 57  E l e c t r o n s p i n resonance s p e c t r a of Y i -  r r a  189  d i a t e d DMSO.  The sample was i r r a d i a t e d i n the dark a t 77°K t o a t o t a l absorbed dose o f 0.72 Mrad.  (a) microwave  power  0.44 mW; (b) microwave power 10 mW. g--,.,,, = 2.0036.. DP r H  191  - xixFigure 58  Page E l e c t r o n s p i n resonance s p e c t r a o f Y i -  a f t e r b l e a c h i n g i r r a d i a t e d sample w i t h  r r a  d i t e d DMSO a  ultraviolet  l i g h t f o r f o r t y m i n u t e s (in s p e c t r o m e t e r c a v i t y ) . Sample y - i r r a d i a t e d a t 77°K i n t h e dark t o a t o t a l a b s o r b e d dose o f 0.72 Mrad. (b) microwave power 0.52 mW; (b) microwave power 10 mW 59  192  E l e c t r o n s p i n resonance spectrum o f Y ~ i r r a d i a t e d p o l y c r y s t a l l i n e i c e a t 77°K. Resonance p a t t e r n c o r r e s p o n d s t o t h a t o f t h e «0H r a d i c a l  60  194  Resonance p a t t e r n showing b e h a v i o u r o f »X w i t h (a) i n c r e a s i n g w a t e r c o m p o s i t i o n (microwave power 0.42 mW) and (b) i n c r e a s i n g microwave power (pure DMSO).  Numbers c o r r e s p o n d i n g t o s p e c t r a on l e f t  r e f e r t o mole f r a c t i o n DMSO. The arrows r e f e r t o 8  61  D  P  P  H  *  1  9  8  E l e c t r o n s p i n resonance s p e c t r a o f p o l y c r y s t a l l i n e Y - i r r a d i a t e d DMSO-water m i x t u r e (0.80 mole f r a c t i o n DMSO) a t 77°K. (a) microwave power 0.42 mW; (b) a f t e r b l e a c h i n g w i t h u l t r a v i o l e t l i g h t f o r 20 m i n u t e s , microwave power 0.42 mW; ( c ) same as ( b ) , microwave power 10 mW  62  E l e c t r o n s p i n resonance s p e c t r a o f Y ~ i  199 r r a  d i a t e d DMSO-  w a t e r g l a s s (0.20 mole f r a c t i o n DMSO) a t 77°K. The " s u l f u r p a t t e r n " and m e t h y l r a d i c a l q u a r t e t a r e r e a d i l y o b s e r v e d , (a) microwave power 0.42 mW; (b) microwave power 10 mW  200  - XX -  Figure 63  Page E l e c t r o n s p i n resonance s p e c t r a of y i r r a d i a t e d p o l y c r y s t a l l i n e DMSO-water m i x t u r e (0.01 mole f r a c t i o n DMSO) a t 77 °K. (a) microwave power 0.42 mW; (b) microwave power 10 mW. The low f i e l d "hump" and d o u b l e t of t h e »0H r a d i c a l s a r e e v i d e n t power  64  a t 10 mW  (see F i g u r e 59)  202  E l e c t r o n s p i n resonance spectrum obtained  after  p h o t o i o n i z a t i o n a t 77°K o f 0.01 M K ^ F e ( C N ) i n 6  0.20 mole f r a c t i o n DMSO-water g l a s s  (compare t o  Figure 62(a)) 65  207  E l e c t r o n s p i n resonance s p e c t r a obtained p h o t o i o n i z a t i o n o f 0.01 M K ^ F e ( C N ) g l a s s a t 77°K.  6  f o r the  i n 8 M NaOH  (a) no DMSO added; (b) 1.0 M DMSO  present i n glass 66  E l e c t r o n s p i n resonance s p e c t r a obtained irradiated  208 f o r y-  (0.24 Mrad) o f 8 M NaOH g l a s s a t 77°K.  (a) no DMSO added; (b) 1.0 M DMSO added; ( c ) a f t e r p h o t o b l e a c h i n g (b) w i t h u l t r a v i o l e t l i g h t f o r 20 m i n u t e s  209  - xxi -  ACKNOWLEDGMENTS The a u t h o r would l i k e t o s i n c e r e l y thank Dr. D.C. Walker f o r h i s encouragement and guidance d u r i n g t h e c o u r s e o f t h i s s t u d y as w e l l as f o r i n t r o d u c i n g him t o R a d i a t i o n C h e m i s t r y . I t i s t h e a u t h o r ' s p l e a s u r e t o acknowledge t h e c o o p e r a t i o n o f t h e N a t i o n a l R e s e a r c h C o u n c i l f o r use o f t h e p u l s e r a d i a t i o n i n Ottawa.  facilities  The a u t h o r i s e s p e c i a l l y g r a t e f u l t o Dr. Hugh A. G i l l i s ,  Dr. Norman V. K l a s s e n and Mr. George G. l e a t h e r f o r t h e i r assistance i n performing the pulse r a d i o l y s i s  invaluable  experiments.  S p e c i a l thanks a r e due t o t h e N a t i o n a l Research C o u n c i l and t h e F.J.  N i c h o l s o n F a m i l y f o r f i n a n c i a l s u p p o r t i n t h e form o f p o s t  graduate  scholarships.  F i n a l l y , t h e a u t h o r would l i k e t o thank h i s w i f e C a r o l , who has shown g r e a t u n d e r s t a n d i n g and f o r b e a r a n c e d u r i n g t h e p r e p a r a t i o n and writing of this  thesis.  - 1 -  CHAPTER I INTRODUCTION R a d i a t i o n c h e m i s t r y encompasses t h e s t u d y o f t h e c h e m i c a l e f f e c t s i n d u c e d i n a system by h i g h - e n e r g y r a d i a t i o n s such as those made a v a i l a b l e by r a d i o a c t i v e s u b s t a n c e s , p a r t i c l e a c c e l e r a t o r s and n u c l e a r reactors.  I n c a r r y i n g o u t such a s t u d y an attempt i s made t o i d e n t i f y  t h e p r o d u c t s formed, d e c i d e how they were formed and what t h e i r p r e c u r s o r s were w i t h t h e hope o f e l u c i d a t i n g and u n d e r s t a n d i n g t h e c h e m i c a l and p h y s i c a l p r o c e s s e s i n v o l v e d .  This d i s s e r t a t i o n i s  concerned w i t h t h e e f f e c t o f i o n i z i n g r a d i a t i o n on d i m e t h y l s u l f o x i d e (DMSO) and b i n a r y m i x t u r e s o f DMSO and w a t e r . i s p l a c e d on  P a r t i c u l a r emphasis  one o f t h e e a r l y p r o c e s s e s i n r a d i a t i o n c h e m i s t r y ,  namely e l e c t r o n s t a b i l i z a t i o n , and how t h i s p r o c e s s i s r e l a t e d t o t h e p h y s i c a l and c h e m i c a l p r o p e r t i e s o f t h e medium. I n o r d e r t o u n r a v e l t h e many c o m p l e x i t i e s i n v o l v e d i n s t u d y i n g r a d i a t i o n - c h e m i c a l phenomena, i t i s n e c e s s a r y t o know how h i g h - e n e r g y r a d i a t i o n i n t e r a c t s w i t h m a t t e r t o produce t h e u l t i m a t e c h e m i c a l e f f e c t .  - 2 A.  INTERACTION OF HIGH-ENERGY RADIATION WITH MATTER  I n the s t u d y o f the r a d i a t i o n c h e m i s t r y of d i m e t h y l s u l f o x i d e , two s o u r c e s of h i g h - e n e r g y r a d i a t i o n were used:  (1) e l e c t r o m a g n e t i c  60 r a d i a t i o n i n the form o f  Co y ~ y s and r a  (2) s h o r t p u l s e s o f h i g h -  energy e l e c t r o n s from a l i n e a r a c c e l e r a t o r . 1.  Electromagnetic Radiation There a r e f o u r p r i n c i p a l p r o c e s s e s by w h i c h e l e c t r o m a g n e t i c  r a d i a t i o n may  be absorbed by matter"'":  ( i i i ) p a i r - p r o d u c t i o n , and  (i) photoelectric,  ( i v ) photonuclear reactions.  ( i i ) Compton, Each o f t h e s e  p r o c e s s e s depends p r i m a r i l y on the energy o f the i n c i d e n t r a d i a t i o n b u t does depend upon the atomic number (Z) o f the a b s o r b i n g medium t o some extent.  Furthermore,  the i n c i d e n t e l e c t r o m a g n e t i c beam may  be  scattered  by the e l e c t r o n s o f the medium w i t h l i t t l e o r no l o s s i n energy and i s termed c o h e r e n t s c a t t e r i n g . f o r high-Z  T h i s s c a t t e r i n g becomes i m p o r t a n t o n l y  m a t e r i a l s and low photon e n e r g i e s (< 0.1 MeV).  whenever  e l e c t r o m a g n e t i c r a d i a t i o n passes through m a t t e r , i t s i n t e n s i t y i s governed by the r e l a t i o n s h i p  I  =  I e  (1.1)  Q  where I i s the i n t e n s i t y o f the r a d i a t i o n t r a n s m i t t e d t h r o u g h a t h i c k n e s s , x, of a b s o r b e r and I  i s the i n t e n s i t y o f the i n c i d e n t o The l i n e a r a b s o r p t i o n c o e f f i c i e n t , y , i s the sum o f a l l 3  radiation.  3.  the p a r t i a l c o e f f i c i e n t s r e p r e s e n t i n g the v a r i o u s p r o c e s s e s o f a b s o r p t i o n mentioned  above.  - 3 -  I n the p h o t o e l e c t r i c e f f e c t , the e n t i r e energy of the p h o t o n , i s t r a n s f e r r e d to a s i n g l e atomic e l e c t r o n . e j e c t e d from the atom w i t h an energy, E , £  E^,  This e l e c t r o n i s then  e q u a l t o the d i f f e r e n c e between  the photon energy and the b i n d i n g e n e r g y , B, o f the e l e c t r o n i n the atom.  E  e  =  E  P h o t o e l e c t r o n s may atom.  -  p  B  (1.2)  be e j e c t e d from any o f the K, L, M,...  s h e l l s of  an  However a f r e e e l e c t r o n cannot absorb a photon t o become a p h o t o -  e l e c t r o n because a t h i r d body, i n t h i s case the n u c l e u s , i s i n o r d e r to c o n s e r v e momentum.  The p r o b a b i l i t y of  necessary  photoelectron  a b s o r p t i o n i n c r e a s e s w i t h the t i g h t n e s s of the b i n d i n g e l e c t r o n so t h a t a t photon e n e r g i e s g r e a t e r t h a n the b i n d i n g e n e r g i e s o f the K- and s h e l l s , e j e c t i o n from the o u t e r s h e l l s i s n e g l i g i b l e .  The  L-  vacancy  c r e a t e d by the l o s s o f an e l e c t r o n from the i n n e r s h e l l w i l l be  filled  by an e l e c t r o n from the o u t e r s h e l l and the e x c e s s energy i s d i s s i p a t e d by the e m i s s i o n o f low - energy Auger e l e c t r o n s o r X - r a y s . a b s o r p t i o n i s predominant a t low photon e n e r g i e s  (< 0.1  Photoelectric  MeV).  In  m a t e r i a l s o f h i g h a t o m i c number the a b s o r p t i o n c r o s s s e c t i o n , x , 3.  i s given approximately  T  a  where k i s a  *  kZ /E 4  by^  3  p  (1.3)  constant.  At e n e r g i e s > 0.1 MeV,  the photon i n t e r a c t s w i t h l o o s e l y bound o r f r e e  e l e c t r o n s r e s u l t i n g i n a r e f l e c t i o n o f the photon w i t h reduced energy.  - 4 -  The e l e c t r o n i s e j e c t e d w i t h e n e r g y , E , e q u a l t o t h e d i f f e r e n c e e  between  t h e i n c i d e n t and s c a t t e r e d photon energy ( t h e b i n d i n g energy i s n e g l e c t e d ) .  E  e  =  E  p  -  E  (1.4) Y  The energy o f t h e s c a t t e r e d photon i s g i v e n by"'"  E E  = Y  \ 1 + (E /m c ) (1 - cose) p o  (1.5)  2 where 0 i s t h e s c a t t e r i n g a n g l e and m c o  e l e c t r o n (0.51 MeV).  i s t h e r e s t mass energy o f t h e  These r e c o i l e l e c t r o n s a r e c a l l e d Compton  e l e c t r o n s and have t h e i r maximum energy when t h e s c a t t e r i n g a n g l e i s 180°.  I n g e n e r a l , however, Compton e l e c t r o n s have a f a i r l y u n i f o r m  d i s t r i b u t i o n i n e n e r g y , t h e average b e i n g about h a l f t h e i n c i d e n t photon energy.  Compton i n t e r a c t i o n s a r e t h e predominant a b s o r p t i o n p r o c e s s f o r  photon e n e r g i e s between 1 and 5 MeV i n h i g h a t o m i c number m a t e r i a l s such as l e a d .  I n low-Z m a t e r i a l s such as w a t e r and DMSO, Compton i n t e r a c t i o n s  a r e dominant 20 MeV  o v e r a much w i d e r r a n g e ; t h a t i s , from about 30 keV t o  (see F i g u r e 1 ) .  The complete a b s o r p t i o n o f a photon i n t h e v i c i n i t y o f an a t o m i c n u c l e u s r e s u l t i n g i n t h e p r o d u c t i o n o f two p a r t i c l e s , an e l e c t r o n and a positron, i s called pair-production.  Because p a r t o f t h e photon  energy i s used t o c r e a t e t h e p o s i t r o n and e l e c t r o n , p a i r - p r o d u c t i o n cannot o c c u r a t photon e n e r g i e s l e s s than the sum o f t h e i r r e s t mass e n e r g i e s , namely 1.02 MeV. between  The r e m a i n i n g photon energy i s d i v i d e d  t h e k i n e t i c e n e r g i e s o f the e l e c t r o n and p o s i t r o n s i n c e energy  - 5 -  t r a n s f e r t o the n u c l e u s negligible.  and i t s subsequent r e c o i l i s c o n s i d e r e d  The p o s i t r o n , e i t h e r b e f o r e or a f t e r l o s i n g i t s k i n e t i c  e n e r g y , i s a n n i h i l a t e d by c o m b i n i n g w i t h an e l e c t r o n w i t h subsequent e m i s s i o n , i n o p p o s i t e d i r e c t i o n s , o f two 0.51  the MeV  Y  - r a  y « s  P a i r p r o d u c t i o n i s o f major i m p o r t a n c e o n l y w i t h h i g h a t o m i c number m a t e r i a l s and photon e n e r g i e s > 10  MeV.  I n the p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n p r o c e s s e s , photons e i t h e r e j e c t o r c r e a t e h i g h - e n e r g y e l e c t r o n s . energies  above about 8 MeV  10 t o 20 MeV  However, a t  f o r h i g h - Z m a t e r i a l s and i n the r e g i o n of  f o r low-Z m a t e r i a l s , the  energy to e j e c t a p r o t o n or n e u t r o n ever p h o t o n u c l e a r  photons may  have s u f f i c i e n t  from the n u c l e u s  o f an atom.  How-  c r o s s s e c t i o n s a r e g e n e r a l l y s m a l l e r than Compton  and p a i r - p r o d u c t i o n c r o s s s e c t i o n s a t the same energy so t h a t photonuclear  the  process  the  g e n e r a l l y makes a n e g l i g i b l e c o n t r i b u t i o n t o the  t o t a l energy a b s o r p t i o n . The  t h r e e main p r o c e s s e s  t h e n i n w h i c h h i g h - e n e r g y photons may  be  a b s o r b e d by m a t t e r are the p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n processes. molecular energy.  F i g u r e 1 shows the v a r i a t i o n of the t o t a l and  partial  a b s o r p t i o n c o e f f i c i e n t s f o r w a t e r as a f u n c t i o n of photon The  atomic a b s o r p t i o n c o e f f i c i e n t ,  y , i s r e l a t e d t o the 3. 3.  l i n e a r a b s o r p t i o n c o e f f i c i e n t by the r e l a t i o n s h i p ' ' '  3  y  3  =  y A/pN 3  O  2 -1 cm atom  (1.6)  where p i s the d e n s i t y and A the a t o m i c w e i g h t o f the s t o p p i n g m a t e r i a l , N  b e i n g Avogadro's number.  S i n c e the p r o b a b i l i t y o f a photon  i  PHOTON E N E R G Y ( M e V ) Figure 1.  Atomic absorption c o e f f i c i e n t s s c a t t e r i n g ) ; B, photoelectric  for water.  Curve A, t o t a l absorption c o e f f i c i e n t  absorption c o e f f i c i e n t ;  C, Compton c o e f f i c i e n t  (with  (with  coherent  coherent  s c a t t e r i n g ) ; D, Compton c o e f f i c i e n t (without coherent s c a t t e r i n g ) ; E , pair-production c o e f f i c i e n t . -24 2' 1 barn = 10 cm . (Adapted from Figure 3.6, page 54, reference 1).  - 7 -  i n t e r a c t i n g w i t h an atom i s independent o f i t s e n v i r o n m e n t , t h e molecular  a b s o r p t i o n c o e f f i c i e n t i s s i m p l y t h e sum o f t h e a t o m i c  a b s o r p t i o n c o e f f i c i e n t s . T h u s the m o l e c u l a r  absorption c o e f f i c i e n t f o r  water i s :  WR O  "  2  W H  2  Wo  +  (1  '  7)  As i n d i c a t e d p r e v i o u s l y , t h e a b s o r p t i o n c o e f f i c i e n t s f o r t h e v a r i o u s processes  mentioned above may be added t o g i v e t h e t o t a l  absorption  coefficient,  y  =  a  T  +  a  a  a  +  K  (1.8)  a  where T , a and K a r e t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t s f o r t h e a a a c  p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n p r o c e s s e s  r e s p e c t i v e l y . The  s m a l l c o n t r i b u t i o n s from coherent s c a t t e r i n g and p h o t o n u c l e a r are  neglected.  S i n c e t h e two Y ~  r a  y  s  reactions  e m i t t e d by ^ C o a r e 1.33 MeV  and 1.17 MeV i n e n e r g y , t h e a b s o r p t i o n i n w a t e r and o t h e r low-Z m a t e r i a l s (such as DMSO) w i l l be p r e d o m i n a n t l y for  t h e primary Y  - r a  y  s  o  that y a  - a . a  by t h e Compton  process  However Compton photons o f  l o w e r energy w i l l have a much more i m p o r t a n t  photoelectric contribution.  The Compton e l e c t r o n i c a b s o r p t i o n c o e f f i c i e n t , o " , i s r e l a t e d e  to  a  t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t i n t h e f o l l o w i n g manner,  (a  a  /o) =  a (Z/A)N cm gm e a o 2  1  (1.9)  where p i s t h e d e n s i t y , A i s the a t o m i c w e i g h t ( m o l e c u l a r w e i g h t ) and Z i s  -  8  -  the atomic number ( o r sum o f atomic numbers i f a compound) of t h e s t o p p i n g medium.  F o r r a d i a t i o n of a g i v e n energy t h e Compton e l e c t r o n  energy c o e f f i c i e n t i s the same f o r a l l materials"'" so t h a t (a /p) a (Z/A). Thus f o r r a d i a t i o n e n e r g i e s f o r which m a t e r i a l s absorb p r e d o m i n a n t l y by t h e Compton p r o c e s s ,  energy  energy a b s o r p t i o n i s  p r o p o r t i o n a l t o the e l e c t r o n d e n s i t y ( t h e number o f e l e c t r o n s p e r gram). Most o f t h e y - r a y energy i s t r a n s f e r r e d t o the k i n e t i c energy o f one or  two h i g h - e n e r g y e l e c t r o n s .  S i n c e these Compton e l e c t r o n s a r e  r e s p o n s i b l e f o r the c h e m i c a l e f f e c t s o b s e r v e d , i t i s n e c e s s a r y t o know how h i g h - e n e r g y e l e c t r o n s s u b s e q u e n t l y i n t e r a c t w i t h t h e c h e m i c a l constituents of matter.  2.  High-Energy E l e c t r o n s High-energy  e l e c t r o n s i n t e r a c t w i t h m a t t e r through i n e l a s t i c and e l a s t i c  c o l l i s i o n s and by the e m i s s i o n o f e l e c t r o m a g n e t i c r a d i a t i o n  called  bremsstrahlung emission. When a h i g h - e n e r g y e l e c t r o n passes c l o s e t o t h e n u c l e u s o f an atom, i t i s d e c e l e r a t e d by t h e e l e c t r i c f i e l d and r a d i a t e s magnetic 2 2  to e Z /m  electro-  r a d i a t i o n ( b r e m s s t r a h l u n g ) w i t h a r a t e , -dE/dx, p r o p o r t i o n a l 2  where e and Z a r e t h e charge on the e l e c t r o n and n u c l e u s  r e s p e c t i v e l y and m i s the e l e c t r o n mass.  As a r e s u l t ,  bremsstrahlung  e m i s s i o n w i l l be g r e a t e s t f o r s t o p p i n g m a t e r i a l s of h i g h atomic number. Thus l e a d and t u n g s t e n a r e g e n e r a l l y used f o r e f f i c i e n t X-ray p r o d u c t i o n from e l e c t r o n beams.  The X - r a d i a t i o n e m i t t e d may then be p a r t i a l l y  - 9 -  absorbed  i n " t h e medium by t h e p r o c e s s e s d e s c r i b e d e a r l i e r .  B r e m s s t r a h l u n g e m i s s i o n i s u n i m p o r t a n t below 100 keV b u t p r e d o m i n a t e s above 100 MeV.  F o r 10 MeV e l e c t r o n s bombarding t u n g s t e n % 50% o f t h e  energy i s d i s s i p a t e d by b r e m s s t r a h l u n g e m i s s i o n . A t l o w e n e r g i e s where b r e m s s t r a h l u n g e m i s s i o n i s u n i m p o r t a n t , e l e c t r o n d e c e l e r a t i o n i s predominantly through coulombic w i t h t h e e l e c t r o n s o f the s t o p p i n g m a t e r i a l .  interaction  The l i n e a r r a t e o f  energy l o s s , c a l l e d l i n e a r energy t r a n s f e r (LET) and e x p r e s s e d as -dE/dx, f o r an e l e c t r o n h a v i n g r e l a t i v i s t i c v e l o c i t y v i s g i v e n by t h e 3 Bethe e x p r e s s i o n .  -dE dx  2irNe Z m v o  2  [In  m v E o 2 2 2I (1-3 ) Z  - (2>JlH5 2  1 + g )ln 2 + 1 - 3 2  2  Z  (1.10)  Here,  N  i s t h e number o f atoms p e r c u b i c c e n t i m e t e r ,  e  i s the e l e c t r o n  Z  i s the a t o m i c number o f t h e s t o p p i n g m a t e r i a l ,  m  o  charge,  i s t h e e l e c t r o n mass,  E  i s t h e k i n e t i c energy o f t h e e l e c t r o n , e r g s ,  v  i s t h e v e l o c i t y of t h e e l e c t r o n ,  g  e q u a l s v / c , where c i s t h e v e l o c i t y o f l i g h t ,  I  i s the average e x c i t a t i o n p o t e n t i a l o f t h e s t o p p i n g medium.  - 10 As t h e s e e l e c t r o n s a r e slowed down by these i n e l a s t i c  collisions,  s u f f i c i e n t energy i s t r a n s f e r r e d t o t h e medium o r s t o p p i n g m a t e r i a l t o cause i o n i z a t i o n s and e x c i t a t i o n s .  From e x p r e s s i o n ( 1 . 1 0 ) , i t can be  seen t h a t t h e LET f o r t h i s p r o c e s s i n c r e a s e s w i t h t h e a t o m i c number of the medium (Z) and d e c r e a s e s w i t h i n c r e a s i n g k i n e t i c energy of t h e electron.  I n w a t e r , the LET i s ^ 0.02 eV/A° f o r e l e c t r o n s h a v i n g  a p p r o x i m a t e l y 1 MeV o f energy b u t i n c r e a s e s t o about 0.22 eV/A° as the k i n e t i c energy f a l l s t o 10 keV."'' The r a t i o o f energy l o s s p e r u n i t p a t h l e n g t h by b r e m s s t r a h l u n g e m i s s i o n t o t h a t by i n e l a s t i c c o l l i s i o n s i s g i v e n by"*"  (- / >brem (-dE/dx) coll dE  EZ  dx  2 1600 m c o  n i  (1.11)  where tie terms have a l r e a d y been d e f i n e d .  F o r Compton e l e c t r o n s o f  up t o 1.3 MeV from ^ C o y r a y s t r a v e r s i n g DMSO, l e s s than 1% o f the -  energy w i l l be e m i t t e d as bremmstrahlung  radiation.  However, i n the  case of 35 MeV e l e c t r o n s from the l i n e a r a c c e l e r a t o r , a p p r o x i m a t e l y 25% of  t h e energy i s l o s t as b r e m s s t r a h l u n g e m i s s i o n . In  a d d i t i o n t o b r e m s s t r a h l u n g r a d i a t i o n , t h e r e i s a n o t h e r form o f  r a d i a t i v e e m i s s i o n w h i c h o c c u r s when h i g h - e n e r g y e l e c t r o n s pass m a t t e r , namely Cerenkov  radiation.  through  T h i s o c c u r s whenever a charged  p a r t i c l e p a s s e s through any medium w i t h a v e l o c i t y g r e a t e r than t h e phas v e l o c i t y of l i g h t i n the m a t e r i a l .  As the e l e c t r o n s t r a v e r s e t h e  d i e l e c t r i c medium, t h e m o l e c u l e s become t e m p o r a r i l y p o l a r i z e d .  When  - l i -  t h e e l e c t r o n p a s s e s , they r e l a x and emit an e l e c t r o m a g n e t i c wave which i s i n the v i s i b l e and u l t r a v i o l e t r e g i o n o f the spectrum. photons  Only  those  e m i t t e d a t an a n g l e 6 w i t h r e s p e c t t o t h e e l e c t r o n ' s  trajectory w i l l  c o n s t r u c t i v e l y i n t e r f e r e as shown i n the s i m p l e Huygens  c o n s t r u c t i o n i n F i g u r e 2.  d i r e c t i o n of propagation  F i g u r e 2.  d i r e c t i o n of propagation  Huygens c o n s t r u c t i o n o f e l e c t r o n t r a j e c t o r y and r e s u l t i n g Cerenkov waveform.  If  the e l e c t r o n t r a v e l s a d i s t a n c e  t o P^ i n the time t h e l i g h t  wave t r a v e l s from P^ t o X, then t h e w a v e l e t s from P^,  and P^ w i l l 4  constructively interfere.  cos6  =  1/Bn  From F i g u r e 2, t h e "Cerenkov r e l a t i o n "  (1.12)  i s e s t a b l i s h e d where B i s the v e l o c i t y o f the p a r t i c l e r e l a t i v e t o the  - 12 -  v e l o c i t y of l i g h t i n vacuo, v / c , and n i s t h e r e f r a c t i v e i n d e x o f t h e medium.  S i n c e cose < 1, Cerenkov e m i s s i o n w i l l o c c u r o n l y i f  f3 > 1/n; t h a t i s , i f the v e l o c i t y of t h e e l e c t r o n i s g r e a t e r than t h e phase v e l o c i t y o f l i g h t i n the medium. theory"* t h e r a t e o f energy  A c c o r d i n g t o t h e Frank-Tamm  l o s s p e r u n i t d i s t a n c e i n t h e form o f 4  Cerenkov r a d i a t i o n i s g i v e n by  c  where to =  2TTV.  J  gn >  I  6  N  S i n c e t h e l i g h t i n t e n s i t y o f f r e q u e n c y co i s E = Nfito,  where N i s t h e number of p h o t o n s ,  i t can be shown t h a t e q u a t i o n  (1.13) 4  may be w r i t t e n as e q u a t i o n (1.14) f o r a l l f r e q u e n c i e s where n > 1.  d N  1  2  f  A  A c c o r d i n g t o e q u a t i o n (1.14) the number o f photons e m i t t e d p e r u n i t p a t h l e n g t h , £.,  of the e l e c t r o n ' s t r a c k per u n i t wavelength  interval follows  2 a 1/X  s p e c t r a l d i s t r i b u t i o n i n the v i s i b l e and u l t r a v i o l e t r e g i o n s . A l t h o u g h energy  to  l o s s by Cerenkov e m i s s i o n i s n e g l i g i b l e compared  t h a t by i n e l a s t i c c o l l i s i o n s and b r e m s s t r a h l u n g  radiation, i t s  i n t e n s i t y exceeds the b r e m s s t r a h l u n g by a v e r y l a r g e f a c t o r i n t h e v i s i b l e region.  F o r t h i s r e a s o n , and the f a c t t h a t t h e e m i s s i o n i s  " i n s t a n t a n e o u s " , Cerenkov e m i s s i o n i s o f t e n used t o m o n i t o r shape o f an e l e c t r o n a c c e l e r a t o r . later.  the p u l s e  T h i s w i l l be d i s c u s s e d more  fully  - 13 -  3.  Range o f E l e c t r o n s U n l i k e electromagnetic r a d i a t i o n , high-energy  f i n i t e range i n an a b s o r b i n g medium.  e l e c t r o n s have a  As e l e c t r o n s t r a v e r s e the medium  they a r e c o n s t a n t l y d e f l e c t e d and slowed  down by e l a s t i c and  inelastic  c o l l i s i o n s w i t h the medium e l e c t r o n s o r by i n t e r a c t i o n s w i t h the atomic n u c l e i (bremsstrahlung).  Because o f these d e f l e c t i o n s t h e i r  p a t h l e n g t h w i l l exceed t h e i r depth o f p e n e t r a t i o n . T h e r e f o r e  total  the p a t h l e n g t h  i s d e f i n e d as the d i s t a n c e t r a v e l l e d by t h e i m p i n g i n g e l e c t r o n a l o n g i t s p a t h b e f o r e i t i s b r o u g h t t o r e s t whereas t h e range or p e n e t r a t i o n i s the d i s t a n c e t r a v e l l e d i n t h e d i r e c t i o n o f o r i g i n a l momentum. the range may  In theory,  be o b t a i n e d by n u m e r i c a l i n t e g r a t i o n o f a s u i t a b l e  stopping-power formula s i m i l a r to equation v e l o c i t y dependence of LET  ( 1 . 1 0 ) . I f one n e g l e c t s the  then the range would be p r o p o r t i o n a l t o t h e  s q u a r e o f the e l e c t r o n energy.  For high-energy  electrons this  a p p r o x i m a t i o n i s r e a s o n a b l e s i n c e the range i s r o u g h l y p r o p o r t i o n a l t o E , m  m b e i n g o n l y s l i g h t l y l e s s than 2 b u t d e c r e a s i n g as E  E m p i r i c a l formulae have been d e v e l o p e d of e l e c t r o n s i n aluminum a b s o r b e r s . 2.5 MeV,  the range i n mg  Range  =  412  cm  -2  t o r e l a t e the range and  i s g i v e n by  =  530E -  106  to  1  1.265-0.0954 InE  e n e r g i e s g r e a t e r than 2.5 MeV,  energy  For e l e c t r o n s of energy 0.01  where E i s the k i n e t i c energy of the e l e c t r o n s (MeV).  Range  decreases.**  the range i s g i v e n by  (1.15)  For e l e c t r o n (1.16).''"  (1.16)  - 14 -  These f o r m u l a e can be a p p l i e d t o o t h e r l i g h t elements s i n c e the range (mg cm  ) v a r i e s o n l y s l i g h t l y w i t h a t o m i c number.  Thus f o r a 1  MeV  e l e c t r o n i n DMSO, the r a n g e , c a l c u l a t e d u s i n g ( 1 . 1 5 ) , c o r r e s p o n d s t o -2 1.41  gm cm  o r t o a t h i c k n e s s o f 1.28  cm.  U s i n g 35 MeV  e l e c t r o n s , an  approximate range, c a l c u l a t e d u s i n g ( 1 . 1 6 ) , i s found t o be 17  cm.  T h i s i s o n l y a rough e s t i m a t e s i n c e e q u a t i o n (1.16) i s good o n l y up t o about 20 4.  MeV.  Track E n t i t i e s ;  S p u r s , B l o b s , and S h o r t T r a c k s  I n r a d i a t i o n c h e m i s t r y the p r i m a r y c h e m i c a l e v e n t s a r i s e from the i o n i z a t i o n s and e x c i t a t i o n s o f t h e m o l e c u l e s produced by the s e c o n d a r y h i g h - e n e r g y e l e c t r o n s o r o t h e r charged p a r t i c l e s .  The  distribution  o f t h e s e a c t i v e s p e c i e s i n the i r r a d i a t e d m a t e r i a l i s n o t homogeneous o r random.  I n s t e a d the a c t i v e s p e c i e s a r e produced o n l y a l o n g the  t r a c k o f the i i c i d e n t p a r t i c l e and i t i s o n l y a f t e r t h e s e s p e c i e s have d i f f u s e d throughout the r e a c t i o n volume t h a t the system can be c o n s i d e r e d homogeneous.  I n a d d i t i o n , the a c t u a l y i e l d s o f a c t i v e  s p e c i e s and t h e i r s p e c i f i c d i s t r i b u t i o n i n space depend upon the o f the p a r t i c u l a r i n c i d e n t p a r t i c l e .  Consequently, d i f f e r e n t  LET  overall  c h e m i c a l changes can a r i s e from d i f f e r e n t i o n i z i n g r a d i a t i o n s s i m p l y because of the v a r i o u s s p a t i a l d i s t r i b u t i o n s of the p r i m a r y s p e c i e s formed. As the p r i m a r y e l e c t r o n s ( o r Compton e l e c t r o n s ) a r e slowed down by i n t e r a c t i o n s w i t h t h e medium, they produce a t r a i l o f e x c i t e d i o n i z e d species along t h e i r t r a c k s .  and  The e l e c t r o n s w h i c h a r e e j e c t e d  - 15 as a consequence o f the i o n i z a t i o n s may  themselves be  sufficiently  e n e r g e t i c to produce f u r t h e r i o n i z a t i o n s and e x c i t a t i o n s .  I f the  energy o f t h e s e secondary e l e c t r o n s i s s m a l l (< 100 e V ) , t h e i r range i n a l i q u i d o r s o l i d w i l l be s h o r t . or  C o n s e q u e n t l y any f u r t h e r  ionizations  e x c i t a t i o n s produced w i l l be l o c a l i z e d i n a s m a l l r e g i o n  (perhaps o r o u g h l y a p p r o x i m a t e d t o a sphere) w i t h a mean d i a m e t e r of ^20 A. These s m a l l c l u s t e r s o f e x c i t e d and/or i o n i z e d s p e c i e s a r e c a l l e d s p u r s . -2 For  a 1 MeV  e l e c t r o n , whose LET i s ^ 10  0  eV/A,  the s p u r s w i l l  be  s e p a r a t e d , on the a v e r a g e , by s e v e r a l thousand Angstrom u n i t s . However t h e r e w i l l be a c o n t i n u o u s d i s t r i b u t i o n o f e j e c t e d e l e c t r o n e n e r g i e s and t h e r e f o r e a d i s t r i b u t i o n i n i n t e r - s p u r d i s t a n c e s .  In  some cases the secondary e l e c t r o n s may have s u f f i c i e n t energy t o form b r a n c h t r a c k s o f t h e i r own. cases the s p u r s may  These e l e c t r o n s are c a l l e d 6 - r a y s .  In other  o v e r l a p so t h a t t h e r e i s e s s e n t i a l l y a c o n t i n u o u s  c y l i n d r i c a l region of a c t i v a t e d species.  Mozumder and Magee^ c o n s i d e r e d  t h i s d i s t r i b u t i o n of s p e c i e s f o r the case of w a t e r i n the f o l l o w i n g Those e l e c t r o n s w h i c h have e n e r g i e s i n e x c e s s of 100 eV but  way.  insufficient  energy t o a l l o w them t o escape t h e c o u l o m b i c a t t r a c t i o n o f t h e i r p o s i t i v e " h o l e " a r e c a l l e d super s p u r s o r " b l o b s " . I f the e l e c t r o n s a r e e n e r g e t i c enough t o escape the " h o l e " b u t n o t e n e r g e t i c enough t o p r e v e n t spur o v e r l a p , they form " s h o r t t r a c k s " . I n the case of w a t e r , these e n e r g i e s are  divided as f o l l o w s :  t r a c k , 'v, 500 eV-5 keV. the  s p u r , ^6-100 eV; b l o b , ^ 100-500 eV; s h o r t Mozumder and Magee^ e s t i m a t e d the p a r t i t i o n of  p r i m a r y energy of the e l e c t r o n i n t o these v a r i o u s t r a c k  entities  and the r e s u l t s o f t h e i r c a l c u l a t i o n s are shown i n F i g u r e 3. ^ C o  yrays  PRIMARY E L E C T R O N F i g u r e 3.  ENERGY (MeV)  Schematic p l o t o f percentage of energy s p l i t between s p u r s , b l o b s and s h o r t t r a c k s f o r e l e c t r o n s i n water.  (Adapted from F i g u r e 5, page 211, r e f e r e n c e 6).  - 17 -  i n water keV.  g i v e r i s e t o e l e c t r o n s h a v i n g a mean energy o f 440  About 64% o f t h i s energy  i s d e p o s i t e d i n t h e form o f i s o l a t e d  s p u r s , about 25% i n the form o f s h o r t t r a c k s , and about 1 1 % as " b l o b s " . From F i g u r e  3,  i t can be seen t h a t as t h e p r i m a r y e l e c t r o n  i n c r e a s e s , t h e f r a c t i o n o f energy d e p o s i t e d i n Knowledge o f t h i s approximate  energy  i s o l a t e d spurs i n c r e a s e s .  distribution i s useful i n describing  spur d i f f u s i o n processes s i n c e r e a c t i o n s i n i s o l a t e d spurs are e x p e c t e d t o be d i f f e r e n t from those  i n " b l o b s " and'short  tracks"where  i o n and r a d i c a l r e c o m b i n a t i o n r e a c t i o n s w i l l be more h i g h l y f a v o u r e d .  B.  CHEMICAL CONSEQUENCES FOLLOWING ABSORPTION OF HIGH-ENERGY RADIATION  The o v e r a l l p r o c e s s o f p r o d u c i n g c h e m i c a l changes i n a medium w i t h i o n i z i n g r a d i a t i o n b e g i n s w i t h t h e bombardment and p r o d u c t i o n of i o n i z e d and e x c i t e d s p e c i e s and t e r m i n a t e s w i t h the r e e s t a b l i s h m e n t of c h e m i c a l e q u i l i b r i u m .  However t h e r e a r e s e v e r a l o r d e r s o f magnitude  i n time between these two s t a g e s .  The  chemical events  observed  b e f o r e o r a f t e r t h e a t t a i n m e n t o f c h e m i c a l e q u i l i b r i u m ma}' n o t n e c e s s a r i l y a r i s e d i r e c t l y from the p r i m a r y p r o c e s s e s b u t r a t h e r the i n t e r m e d i a t e s so produced.  from  For t h i s reason i t i s necessary to  know t h e t y p e s o f p r o c e s s e s w h i c h may o c c u r and t h e i r r e l a t i v e s c a l e i n o r d e r t o c o r r e c t l y i n t e r p r e t the c h e m i c a l e v e n t s  time  observed.  - 18 1.  Time S c a l e o f Events In r a d i a t i o n c h e m i s t r y one u s u a l l y r e c o g n i z e s t h r e e s t a g e s  f o l l o w i n g the a b s o r p t i o n of high-energy  r a d i a t i o n and l a s t i n g up t o  the time o f the p r o d u c t i o n o f the u l t i m a t e c h e m i c a l e f f e c t .  These a r e  r e f e r r e d t o , i n o r d e r o f i n c r e a s i n g t i m e , as the p h y s i c a l s t a g e , t h e p h y s i c o - c h e m i c a l s t a g e and the c h e m i c a l s t a g e . I n t h e p h y s i c a l s t a g e , energy i s t r a n s f e r r e d t o t h e system by t h e p r o c e s s e s mentioned e a r l i e r .  T h i s i n v o l v e s t h e p r i m a r y p r o c e s s and  i t s d u r a t i o n i s o f t h e o r d e r o f 10  t o 10  s e c , t h e upper  b e i n g f i x e d by t h e H e i s e n b e r g U n c e r t a i n t y P r i n c i p l e e l e c t r o n d e p o s i t i n g 20 eV t o 30 eV o f energy.  limit  (AE«At ^ "fl) f o r an  As t h e p r i m a r y ,  and h i g h e r - o r d e r e l e c t r o n s a r e slowed down by i n e l a s t i c  secondary  collisions  w i t h the medium, they l o s e t h e i r e x c e s s energy by m o l e c u l a r e x c i t a t i o n s or i o n i z a t i o n s .  When t h e i r energy has dropped below t h e l o w e s t  e l e c t r o n i c e x c i t a t i o n l e v e l ( u s u a l l y 1 t o 10 e V ) , t h e e l e c t r o n s a r e c a l l e d " s u b e x c i t a t i o n e l e c t r o n s " and they s u b s e q u e n t l y l o s e energy by e x c i t a t i o n o f m o l e c u l a r v i b r a t i o n s and r o t a t i o n s .  their S i n c e the -14  p e r i o d o f a m o l e c u l a r v i b r a t i o n o r r o t a t i o n i s o f t h e o r d e r 10  to  -12 10  s e c , energy  range.  t r a n s f e r by t h e s e p r o c e s s e s o c c u r s over t h i s  time  These i n t e r a c t i o n s a r e t h e germane i n i t i a l events and thus the  p r i m a r y r e a c t i v e s p e c i e s a r e the p o s i t i v e i o n s , n e a r l y t h e r m a l i z e d e l e c t r o n s , v i b r a t i o n a l l y and e l e c t r o n i c a l l y e x c i t e d m o l e c u l e s , a l l o f w h i c h a r e the p r e c u r s o r s o f the observed c h e m i c a l consequences o f the r a d i a t i o n absorption. The n e x t s t a g e i s c a l l e d the p h y s i c o - c h e m i c a l s t a g e and l a s t s up to about 10  ± X  sec.  I t i s d u r i n g t h i s p e r i o d t h a t the u n s t a b l e p r i m a r y  - 19 -  s p e c i e s undergo secondary  r e a c t i o n s , e i t h e r s p o n t a n e o u s l y o r by  c o l l i s i o n s w i t h adjacent ions or molecules.  The p o s i t i v e i o n s may be  i n v o l v e d i n charge n e u t r a l i z a t i o n , p r o t o n t r a n s f e r o r they may decompose i n t o o t h e r fragments whereas the h i g h l y e x c i t e d  molecules  may decompose i n t o r a d i c a l s p e c i e s o r l o s e energy by i n t e r n a l c o n v e r s i o n . The e l e c t r o n reaches t h e r m a l e q u i l i b r i u m w i t h i t s environment, E % kT (0.025 eV) , a t which time i t f a c e s s e v e r a l d i f f e r e n t  with  possibilities.  I t may be ( i ) r e c a p t u r e d by i t s p a r e n t i o n , ( i i ) c a p t u r e d by a s o l v e n t m o l e c u l e t o produce a n e g a t i v e i o n , ( i i i ) c a p t u r e d by a r a d i a t i o n produced  i o n o r p r o d u c t o t h e r than i t s p a r e n t , ( i v ) c a p t u r e d by a  scavenger m o l e c u l e o r i o n i n i t i a l l y p r e s e n t i n t h e system, o r i t may (v) become t r a p p e d o r s t a b i l i z e d among t h e m o l e c u l e s of the medium.  The  f o r m a t i o n and p r o p e r t i e s o f these t e m p o r a r i l y s t a b i l i z e d , o r s o l v a t e d e l e c t r o n s w i l l be d i s c u s s e d l a t e r .  U l t i m a t e l y the system a t t a i n s t h e r m a l e q u i l i b r i u m  and the c h e m i c a l s t a g e b e g i n s . In ed  the c h e m i c a l s t a g e t h e newly formed r e a c t i v e s p e c i e s w h i c h have escap-  geminate  r e c o m b i n a t i o n i n the s p u r s d i f f u s e out i n t o the b u l k o f  the medium and e v e n t u a l l y they become homogeneously d i s t r i b u t e d . p r i m a r y o r secondary  These  s p e c i e s undergo t h e r m a l c h e m i c a l r e a c t i o n s w i t h  each o t h e r , w i t h scavengers o r w i t h t h e medium i t s e l f .  Excited  m o l e c u l e s , formed e i t h e r i n t h e p r i m a r y p r o c e s s o r from i o n recombinat i o n , can undergo v a r i o u s l u m i n e s c e n t p r o c e s s e s .  These e v e n t s c o n t i n u e  u n t i l the system once again a t t a i n s c h e m i c a l e q u i l i b r i u m . The above i s a r a t h e r g e n e r a l d e s c r i p t i o n of the types of c h e m i c a l p r o c e s s e s p o s s i b l e when a system i s bombarded by i o n i z i n g  radiation.  These p r o c e s s e s and t h e i r time s c a l e s a r e i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 4.  The time s c a l e r e f e r s t o the l i q u i d phase and would be  Dielectric Relaxation Spur Diffusion Processes  15  16  14 —>  <—  Electronic Energy Deposit  13  Figure  4.  <  >  12  11  10  Electron Thermalization  Subexcitation Electron <  Radiative Electronic Transitions  Molecules Jump  Molecular Vibrations  8 Ion Recombination  Electron Recapture >  Internal Conversion of Electronic States (non - radiative)  T h e o r e t i c a l time s c a l e f o r the i n i t i a l p r o c e s s e s i n r a d i a t i o n c h e m i s t r y . negative logarithm  o  o f time (pt = - l o g t ( s e c ) ) .  The numbers a r e the  The time s c a l e r e f e r s t o the l i q u i d  state.  - 21 -  d i f f e r e n t i n s e v e r a l r e s p e c t s when t h i s d e s c r i p t i o n i s a p p l i e d to the s o l i d o r gas phase. 2.  Phase-Dependent Phenomena whenever a compound i s exposed  t o a s o u r c e of i o n i z i n g  the p r i m a r y p r o c e s s e s are l a r g e l y independent medium.  However the subsequent  on the phase.  radiation  of the phase o f the  c h e m i c a l p r o c e s s e s w i l l depend  markedly  For i n s t a n c e , t h e r e a r e no t r a c k e f f e c t s o r s p u r - c o n t r o l l e d  r e a c t i o n s of consequence i n the gas phase.  The i o n - p a i r s formed i n the  gas phase have a n e g l i g i b l e p r o b a b i l i t y of u n d e r g o i n g  geminate  r e c o m b i n a t i o n because the mean f r e e p a t h o f the e l e c t r o n i s too l o n g . Consequently  the p o s i t i v e i o n may  experience several c o l l i s i o n s w i t h  n e u t r a l m o l e c u l e s b e f o r e n e u t r a l i z a t i o n o c c u r s and thus has  the  o p p o r t u n i t y t o decompose o r undergo i o n - m o l e c u l e r e a c t i o n s . e l e c t r o n , on the o t h e r hand, may  attach i t s e l f  and the r e s u l t i n g n e g a t i v e i o n may  to a n e u t r a l  disproportionate before  neutralization with a positive ion.  However, i n an  The molecule undergoing  identical  i o n i z a t i o n event i n the l i q u i d o r s o l i d phase, the i o n i z e d and m o l e c u l e s and m o l e c u l a r fragments m o l e c u l e s a r e produced  excited  formed by d e c o m p o s i t i o n of e x c i t e d  a t h i g h l o c a l c o n c e n t r a t i o n s i n the t r a c k s and  s p u r s . Consequently the p r o b a b i l i t y of t h e i r r e a c t i o n w i t h each o t h e r i s i n c r e a s e d r e l a t i v e t o t h e i r r e a c t i o n w i t h the medium o r added scavengers. of  Furthermore,  c o n c o m i t a n t p a r t n e r s formed by the r u p t u r e  a g i v e n m o l e c u l e w i l l be t r a p p e d w i t h i n the same s o l v e n t cage t h e r e b y  i n c r e a s i n g the p r o b a b i l i t y of geminate r e c o m b i n a t i o n .  Because o f  this  d i f f e r e n c e i n s p a t i a l d i s t r i b u t i o n o f the r e a c t i v e i n t e r m e d i a t e s , the r a d i a t i o n c h e m i s t r y of a gaseous system i s o f t e n d i f f e r e n t from i t s  - 22  -  l i q u i d or s o l i d phase counterpart,  Since only studies on the l i q u i d  and s o l i d phase were undertaken i n this work, further discussions w i l l be l i m i t e d to the condensed phase.  3.  Studies on the Chemical Events i n Condensed Phases Radiation chemical studies are not just concerned with the net  e f f e c t but with an understanding  chemical  of the detailed mechanism leading to the  change. The ionized and excited molecules  i n i t i a l l y formed subsequently  give r i s e to a series of ±>nic or r a d i c a l intermediates which then produce the stable chemical products.  By i d e n t i f y i n g these products and studying  the e f f e c t s on t h e i r y i e l d s caused by the addition of various r a d i c a l , i o n i c or electron scavengers, i t i s possible, by inference, to speculate on the i d e n t i t y of the intermediates and thereby propose a reaction scheme.  The l i f e t i m e of the transient species which have escaped -9  intra-spur reactions i s often of the order 10 and assuming the scavengers -1 M  -6 seconds to 10  -1 , scavenger  seconds,  react at d i f f u s i o n - c o n t r o l l e d rates,  -1 sec  the  concentrations of the order 10  10"^  -4 M to 10  M are  required to compete e f f e c t i v e l y with t h e i r alternative decay processes. Those species which decay by intra-spur reactions and hence cannot be scavenged at these concentrations give r i s e to products which are referred to as "molecular products".  By the method of pulse  7-9 radiolysis,  i t i s possible to observe the formation and decay of  many of these transient intermediates.  In this technique short pulses  ( t y p i c a l l y 10 ^ to 10 ^ sec) of high energy electrons are used as the radiation source at i n t e n s i t i e s s u f f i c i e n t l y high to produce "instantaneous" concentrations of transient species which may  be detected  - 23 and i d e n t i f i e d by various fast physical methods, p a r t i c u l a r l y by o p t i c a l absorption spectroscopy.  From the absorption spectra,  i t is  often possible to i d e n t i f y free r a d i c a l s , molecular i o n s , solvated electrons or excited molecules.  Sometimes this i d e n t i f i c a t i o n comes  from comparison to other systems where the species have been w e l l characterized but i n many cases they have to be assigned on the basis of t h e i r chemical behaviour to various scavengers. In the l i q u i d phase these active species w i l l usually react with each other or with the solvent or scavengers i n times of microseconds or l e s s .  As a r e s u l t , they can only be observed, i f at a l l , by using  very fast pulse r a d i o l y s i s techniques.  However, i f the medium i s i n  the s o l i d s t a t e , and at s u f f i c i e n t l y low temperatures  (usually 77°K or  lower), the rate of reaction of these transient intermediates may be slowed down so that they can be observed over periods of minutes or even years.  This approach i s often referred to as the " i s o l a t i o n  technique".  If these species are paramagnetic, such as r a d i c a l s  and trapped electrons, resonance spectroscopy.  they can then be studied by electron spin They may also be observed by o p t i c a l spectro-  scopy i f the sample i s s u f f i c i e n t l y transparent.  The formation and decay  of the various intermediates, i n p a r t i c u l a r the e l e c t r o n , depend to a great extent upon whether or not the medium i s glassy (amorphous) or polycrystalline.  Often studies on the s o l i d state give a valuable  insight into the processes occurring i n the l i q u i d s t a t e , e s p e c i a l l y since not a l l transients can be p o s i t i v e l y i d e n t i f i e d through their o p t i c a l and chemical behaviour alone. The techniques of o p t i c a l spectroscopy, electron spin resonance  - 24  ( e s r ) s p e c t r o s c o p y and the n e x t c h a p t e r .  p u l s e r a d i o l y s i s w i l l be d e s c r i b e d  However, i t must be  i n t e r m e d i a t e s observed u s i n g reacting during  the  have been r e s o l v e d  -  stressed  t h e s e t e c h n i q u e s are  chemical stage only. experimentally  The  p o l a t i o n t o the e v e n t s o c c u r r i n g  transient  those a c t i v e  species  e a r l i e s t events which  from 10  c o n s e q u e n t l y any t o 10  extra-  seconds can  ± X  only  speculative.  4.  Chemical Y i e l d s Any  successful  must i n v o l v e chemistry events  and,  d e s c r i p t i o n of the e f f e c t of r a d i a t i o n on  the y i e l d s of the r a d i o l y t i c p r o d u c t s .  o f a s p e c i f i c k i n d i n d u c e d i n a medium per The  u n i t energy, 100  eV,  matter  In r a d i a t i o n  c h e m i c a l y i e l d s are e x p r e s s e d as G v a l u e s ,  absorbed.  the number of  100  eV of energy  i s an e n t i r e l y a r b i t r a r y magnitude  as s u c h , G v a l u e s have no i n t r i n s i c or s t o i c h i o m e t r i c s i g n i f i c a n c e .  I n g e n e r a l the y i e l d s are w r i t t e n as G(X) ions, r a d i c a l s , excited species energy d e p o s i t i o n .  required  o r m o l e c u l e s used up  phase and  i n ion-pair formation.  the r e g i o n o f 30 eV.  where X r e f e r s to the  atoms,  or produced by  the  O f t e n c o r r e l a t i o n s are drawn between the G v a l u e  f o r i o n i z a t i o n i n the gas  it  in  using pulse r a d i o l y s i s occur l a t e r  t h a n 10 p i c o s e c o n d s a f t e r energy d e p o s i t i o n ;  be  t h a t the  more f u l l y  the W v a l u e , w h i c h i s the mean energ  T y p i c a l W v a l u e s f o r gases are  in  S i n c e i o n i z a t i o n p o t e n t i a l s are t y p i c a l l y ^ 10  eV  f o l l o w s t h a t o n l y 30-40% of the energy i s d i s s i p a t e d i n i o n i z i n g  processes.  The  r e l a t i o n s h i p may  G (ion-pairs)  =  100/W  be e x p r e s s e d as  follows:  (1.17)  - 25' -  I f one assumes t h a t the W v a l u e f o r the condensed phase i s the same as i t s gas phase c o u n t e r p a r t , then G ( i o n - p a i r s ) i n the condensed phases s h o u l d be  3-4.  Scavenging  s t u d i e s at very h i g h c o n c e n t r a t i o n s  have i n d i c a t e d i o n i c y i e l d s not j u s t between 3 and t o i n d i c a t e t h a t W v a l u e s may  4 but even up t o 5,  be somewhat s m a l l e r i n the condensed  phase than i n the gas phase. Another parameter which i s o f t e n used t o c h a r a c t e r i z e r a d i a t i o n e f f e c t s i s the i o n - p a i r y i e l d , M/N,  where M i s the number o f s p e c i e s X  p r o d u c e d and N i s the number of i o n - p a i r s .  T h i s parameter was  widely  used i n the e a r l i e r y e a r s o f r a d i a t i o n c h e m i s t r y when i t was b e l i e v e d t h a t v i r t u a l l y a l l i n d u c e d c h e m i c a l changes a r o s e from i o n i c p r e c u r s o r s . I t i s r e l a t e d to the G v a l u e by e q u a t i o n  G(X,  -  f  •  (1.18).  2f  (1.18)  However, e x p r e s s i o n (1.18) i s not u s e f u l when a p p l i e d t o condensed systems s i n c e N and W cannot be measured d i r e c t l y .  For t h i s  reason,  G v a l u e s a r e used t o e x p r e s s r a d i a t i o n c h e m i c a l y i e l d s s i n c e they  can  be o b t a i n e d d i r e c t l y and do not i m p l y , as does the i o n - p a i r y i e l d , t h a t t h e c h e m i c a l a c t i o n i s c o n t r o l l e d by the number of i o n s formed. A l l t h a t i s r e q u i r e d i s a knowledge of the number o f s p e c i e s produced and absorbed  by the medium.  described  The  l a t t e r i s o b t a i n e d by d o s i m e t r y and w i l l  later.  G e n e r a l l y the G v a l u e s o f s p e c i e s produced by i o n i z i n g range from C t o 5. reactions.  the dose  Y i e l d s g r e a t e r than 5 u s u a l l y s i g n i f y  radiation  chain  However i t must be emphasized t h a t the G v a l u e s measured  be  - 26 -  represent average values.  They are averaged over the range of LET  involved and therefore small y i e l d s can a r i s e , f o r instance, e x c l u s i v e l y from the s p e c i f i c chemistry p e c u l i a r to short tracks or "blobs" and may not be at a l l representative of i s o l a t e d spurs.  C.  STABILIZED ELECTRONS  In many systems s t a b i l i z e d electrons are the p r i n c i p a l chemicallyreducing species produced by the i n t e r a c t i o n of i o n i z i n g r a d i a t i o n with matter.  Although  the existence of s t a b i l i z e d electrons i n solutions of  a l k a l i metals i n ammonia has been known for over f i f t y years, i t was only with the advent of pulse r a d i o l y s i s that they were d i r e c t l y observed  as intermediates i n the r a d i a t i o n chemistry of water. ® L  This  was because the electrons have a high reduction p o t e n t i a l and as such are extremely  reactive and very s h o r t - l i v e d .  In the l a s t decade  s t a b i l i z e d electrons have been i d e n t i f i e d and studied i n many other systems; indeed, no other species studied i n radiation chemistry has l i k e l y commanded as much attention."''"''  1.  Stabilization In gaseous media the thermalized electrons c o l l i d e e l a s t i c a l l y or  i n e l a s t i c a l l y with unreactive molecules captured by reactive molecules or d i s s o c i a t i v e reactions.  u n t i l they are eventually  or ions, undergoing electron attachment  However, i n condensed media electrons may  become confined to a cavity, either by the thermalized electron  - 27  -  p o l a r i z i n g the medium through r e p u l s i v e f o r c e s f o l l o w e d by r e o r i e n t a t i o n of the m o l e c u l e s to produce a b e t t e r t r a p , or by the e l e c t r o n  "falling  into" a pre-existing, suitably oriented void.  these  The  l i f e t i m e of  l o c a l i z e d o r s t a b i l i z e d e l e c t r o n s w i l l depend upon the t h e r m a l m o t i o n of the m o l e c u l e s f o r m i n g the c a v i t y w a l l s .  I n the l i q u i d phase  l i f e t i m e of t h e s e e l e c t r o n s , w h i c h w i l l be  c a l l e d solvated  and  denoted by e , g  i s o f t e n l e s s than 10 ^ sec due  the  electrons  to t h e i r m o b i l i t y  and h i g h r e a c t i v i t y towards the medium, o t h e r r a d i c a l s , p o s i t i v e i o n s , r a d i a t i o n p r o d u c t s or s c a v e n g e r s .  I n the s o l i d phase, however, and  low enough t e m p e r a t u r e s , the c a v i t i e s may e l e c t r o n l i f e t i m e may  be  be extended by s e v e r a l o r d e r s  e l e c t r o n s are c a l l e d t r a p p e d e l e c t r o n s and The  s i m i l a r i t y between the c h e m i c a l and  s o l v a t e d i n l i q u i d s and the two  species  electrons  those t r a p p e d i n the s o l i d phase s u g g e s t t h a t  regarding  By s t u d y i n g  the s t a b i l i z a t i o n p r o c e s s can  s t a b i l i z e d e l e c t r o n , no s a t i s f a c t o r y t h e o r y  2.  Such  by e^ .  p h y s i c a l p r o p e r t i e s of  f o r , d e s p i t e the amount of work b e i n g  the t r a p p i n g s i t e s n o r  the  of magnitude.  are d e s i g n a t e d  are i d e n t i c a l e x c e p t f o r m o b i l i t y .  properties information be o b t a i n e d ,  " f r o z e n i n " and  at  these hopefully  c a r r i e d out on  regarding  the mechanism of s o l v a t i o n has  the  the n a t u r e of y e t been p r o p o s e d .  Properties The  s t a b i l i z e d e l e c t r o n i s r e g a r d e d as the s i m p l e s t and  r e a c t i v e chemical e n t i t y . absorption  most  I t i s h i g h l y c h a r a c t e r i z e d by i t s i n t e n s e  spectrum i n the v i s i b l e and n e a r i n f r a - r e d r e g i o n , i t s  paramagnetism and  i t s high m o b i l i t y .  - 28 2.1  O p t i c a l Spectrum The most p r o m i n e n t f e a t u r e o f s t a b i l i z e d e l e c t r o n s i s t h e i r  i n t e n s e a b s o r p t i o n s p e c t r a i n t h e v i s i b l e and near i n f r a r e d r e g i o n . The  s p e c t r a a r e c h a r a c t e r i z e d by t h e i r broadness  and l a c k o f s t r u c t u r e ,  t h e i r asymmetry on t h e h i g h - e n e r g y s i d e and by t h e i r i n t e n s i t y , w i t h 4 t y p i c a l m o l a r e x t i n c t i o n c o e f f i c i e n t s > 10  -1 M  cm  -1 a t t h e maximum.  A l l c u r r e n t t h e o r i e s r e g a r d t h e s t a b i l i z e d e l e c t r o n as b e i n g  confined  to a  type o f p o t e n t i a l w e l l , o r c a v i t y , t h e depth o f w h i c h depends upon t h e p o l a r i z a t i o n energy a c t i n g back on t h e e l e c t r o n . system, t h e r e w i l l be q u a n t i z e d The  L i k e any l o c a l i z e d  energy l e v e l s a s s o c i a t e d w i t h each c a v i t y .  a b s o r p t i o n spectrum may then be a t t r i b u t e d t o t r a n s i t i o n s between  t h e s e l e v e l s and thus t h e t r a n s i t i o n energy may be t a k e n as r e p r e s e n t a t i v e of t h e w e l l depth o r the s o l v a t i o n energy o f the e l e c t r o n . I n n o n p o l a r media, such as t h e h y d r o c a r b o n s , s t a b i l i z a t i o n can o n l y occur through short-range of t h e s u r r o u n d i n g shallow.  r e p u l s i o n s or induced e l e c t r o n i c p o l a r i z a t i o n  molecules,  so t h a t t h e c a v i t y depth w i l l be r a t h e r  On t h e o t h e r hand, w i t h p o l a r m o l e c u l e s such as w a t e r , a l c o h o l s  and amines, o r i e n t a t i o n (atomic thereby  and d i p o l e ) p o l a r i z a t i o n can a l s o o c c u r  increasing stabilization.  S i n c e the a b s o r p t i o n s p e c t r a o f e i n t  v i t r e o u s , g l a s s y s o l i d s a r e v i r t u a l l y t h e same as those o f t h e i r counterpart,  liquid  t h i s s u g g e s t s t h a t t h e s e c a v i t i e s , w i t h t h e optimum d i p o l e  arrangement, e x i s t e d b e f o r e  the a r r i v a l o f the e l e c t r o n .  This i s  p a r t i c u l a r l y t r u e o f p o l a r media w h i c h owe t h e i r s t r o n g s o l v a t i o n t o o r i e n t a t i o n p o l a r i z a t i o n . I n t h e l i q u i d s t a t e t h i s d i p o l e r e l a x a t i o n may f o l l o w t h e i n i t i a l e l e c t r o n i c p o l a r i z a t i o n by the e l e c t r o n b u t may s e v e r a l orders  take  o f magnitude l o n g e r a t l o w e r t e m p e r a t u r e s . However d i e l e c t r i c  r e l a x a t i o n times a r e measured on a m a c r o s c o p i c s c a l e whereas r o t a t i o n a l  - 29 -  o s c i l l a t i o n s by the p o l a r m o l e c u l e s f o r m i n g t h e c a v i t y w a l l s may be p o s s i b l e under the e l e c t r o n ' s f i e l d a t a l l t e m p e r a t u r e s .  Recently  12  Richards  and Thomas  observed a s h i f t i n the absorption  spectra of  t r a p p e d e l e c t r o n s i n g l a s s y e t h a n o l a t 77°K towards t h e b l u e on a m i c r o s e c o n d time s c a l e .  S i m i l a r observations  i n binary mixtures of  13  a l c o h o l s and w a t e r were made by Kevan.  T h i s b l u e s h i f t was  a t t r i b u t e d t o i n i t i a l t r a p p i n g o f t h e t h e r m a l e l e c t r o n s i n l e s s than optimum t r a p s f o l l o w e d by m i c r o s c o p i c more s t a b l e t r a p s .  r e l a x a t i o n t o produce deeper,  Thus the s t a b i l i z a t i o n o f e l e c t r o n s i s p r o b a b l y  f a c i l i t a t e d by t h e p r e - e x i s t e n c e  i n t h e medium o f s u i t a b l y o r i e n t e d  v o i d s , b u t r e o r g a n i z a t i o n o f the c a v i t i e s o c c u r s a f t e r e l e c t r o n Unfortunately  capture.  the time s c a l e o f c u r r e n t a p p a r a t u s i s l i m i t e d t o t h e 14  20 p i c o s e c o n d range,  by w h i c h time e l e c t r o n s i n l i q u i d media a t  room t e m p e r a t u r e have a l r e a d y been s o l v a t e d .  Extension  of these  s t u d i e s t o l o w t e m p e r a t u r e l i q u i d systems i s a l s o p o s s i b l e . In  p o l y c r y s t a l l i n e media, t h e s p e c t r a a r e s i m i l a r t o t h e l i q u i d  phase b u t t h e y i e l d o f t r a p p e d e l e c t r o n s i s much l o w e r .  Because o f  the l o n g - r a n g e o r d e r i n c r y s t a l s , t h e r e a r e l e s s s u i t a b l y o r i e n t e d voids As  fori n i t i a l l y  t r a p p i n g t h e e l e c t r o n compared t o t h e g l a s s y s t a t e .  a r e s u l t , t h e e l e c t r o n s can o n l y be t r a p p e d a t d e f e c t s i t e s i n the  crystal  lattice.  B e s i d e s the medium p o l a r i z a b i l i t y , t h e c a v i t y o r v o i d r a d i u s i s a l s o a f a c t o r i n g o v e r n i n g t h e s o l v a t i o n energy o f t h e e l e c t r o n . A l l t h e o r i e s c o n c e r n i n g t h e l o c a l i z e d e l e c t r o n p r e d i c t t h a t t h e energy separations  should  diminish with increasing cavity radius.  level  That i s ,  the s e l f - i n d u c e d energy a c t i n g on the e l e c t r o n f a l l s o f f w i t h the  -  30  d i s t a n c e from the c a v i t y c e n t e r .  -  However, i t s h o u l d be n o t e d t h a t t h e  c a v i t y r a d i u s i s n o t t o be confused w i t h t h e e f f e c t i v e i o n i c r a d i u s . The  s t a b i l i z e d e l e c t r o n has a s m a l l mass and low momentum and i s  t h e r e f o r e "smeared o u t " o v e r a l a r g e volume i n accordance w i t h t h e uncertainty principle. e l e c t r o n , represented molecules. moments.  This i s represented  i n F i g u r e 5 where t h e  by t h e shaded p o r t i o n , i s s p r e a d o v e r s e v e r a l  The arrows r e p r e s e n t  the induced  o r permanent d i p o l e  The t i p s o f t h e d i p o l e v e c t o r s r e p r e s e n t  the c a v i t y , o r v o i d ,  i n t h e medium.  Figure 5 .  Schematic r e p r e s e n t a t i o n of e l e c t r o n l o c a l i z a t i o n r e g i o n ) produced by p o l a r i z a t i o n o f t h e medium. area represents  (shaded The d o t t e d  the v o i d o r c a v i t y i n w h i c h t h e e l e c t r o n i s  centered.  As the c a v i t y r a d i u s i s d e c r e a s e d , the s e l f - e n e r g y i n c r e a s e s , and the s o l v a t i o n energy i s i n c r e a s e d .  T h i s has been demonstrated i n s t u d i e s  on systems a t low t e m p e r a t u r e s and h i g h p r e s s u r e s where t h e s h i f t i n t h e e l e c t r o n a b s o r p t i o n band towards the b l u e has been a t t r i b u t e d t o a d e c r e a s e i n t h e c a v i t y r a d i u s caused by these e x t e r n a l e f f e c t s . The  shape and p o s i t i o n of the a b s o r p t i o n band w i l l depend upon t h e  - 31 -  manner i n w h i c h the e n e r g i e s  o f the ground and e x c i t e d s t a t e s v a r y w i t h  the shape o f the c a v i t y due t o the v a r i o u s arrangements o f the m o l e c u l e s forming  the c a v i t y w a l l s as w e l l as any d i s t r i b u t i o n i n c a v i t y s i z e s .  T h i s would suggest a r e a s o n a b l e  e x p l a n a t i o n f o r the e x c e p t i o n a l width  (% 1 eV) o f t h e o p t i c a l s p e c t r a o f s t a b i l i z e d e l e c t r o n s and why they show no s t r u c t u r e .  Each e l e c t r o n w i l l be s t a b i l i z e d i n a p a r t i c u l a r  environment and w i l l e x h i b i t a c h a r a c t e r i s t i c a b s o r p t i o n . spectrum i s then s i m p l y the maximum r e p r e s e n t s The  The o b s e r v e d  the e n v e l o p e o f t h e i n d i v i d u a l s p e c t r a i n w h i c h t h e most p r o b a b l e  c a v i t y or p o t e n t i a l w e l l .  asymmetric t a i l o f the a b s o r p t i o n band on t h e h i g h - e n e r g y s i d e o f  the a b s o r p t i o n maximum may be i n t e r p r e t e d , p e r h a p s , i n terms o f t r a n s i t i o n s o f the s t a b i l i z e d e l e c t r o n s t o h i g h e r e x c i t e d s t a t e s o r an energy continuum ( c o n d u c t i o n  band).  I n t h e d i s c u s s i o n so f a r i t has been assumed t h a t the e x c i t e d s t a t e o f t h e e l e c t r o n i s a bound s t a t e .  I f one t r e a t s t h e c a v i t y as a  s p h e r i c a l l y symmetric p o t e n t i a l w e l l and t h e e l e c t r o n i c wave f u n c t i o n s as h y d r o g e n i c  i n n a t u r e , t h e n t h e s e p a r a t i o n between s u c c e s s i v e  q u a n t i z e d energy l e v e l s w i l l c o n v e r g e , t h e f i r s t t r a n s i t i o n , (2p I s ) , b e i n g t h r e e q u a r t e r s o f t h e w e l l depth.  However t h e f i r s t e x c i t e d s t a t e  may be v e r y c l o s e t o , o r perhaps o v e r l a p , w i t h e i t h e r the c o n d u c t i o n band o r an a u t o - i o n i z a t i o n s t a t e . observations  This i s i n keeping w i t h the  o b t a i n e d when t h e e l e c t r o n s a r e p h o t o b l e a c h e d i n low  temperature g l a s s e s .  By i l l u m i n a t i n g Y i - a d i a t e d n-propanol"'""' and -  r r  a l k a l i n e aqueous glasses"'"^ a t 77°K a l t e r n a t e l y w i t h r e d (X > 640 nm) and b l u e ( X < 500 nm) l i g h t i t was p o s s i b l e t o " p h o t o s h u t t l e " t h e t r a p p e d e l e c t r o n s between s h a l l o w and deep t r a p s w i t h o u t  c a u s i n g any l o s s o f  - 32  electrons.  A l l t h a t changed was  -  the shape o f the a b s o r p t i o n band.  F u r t h e r m o r e , by u s i n g near-monochromatic l i g h t , i t was show t h a t p h o t o c o n d u c t i v i t y c o u l d be i n d u c e d a l k a l i n e glasses  17  and m e t h y l t e t r a h y d r o f u r a n  in y  _ : L r r a  glasses  p o s s i b l e to  d i a t e d aqueous  18  by l i g h t w h i c h  matches the a b s o r p t i o n spectrum.  2.2  E l e c t r o n s p i n resonance I n c o n t r a s t t o the o p t i c a l a b s o r p t i o n spectrum of the  stabilized  e l e c t r o n , e l e c t r o n s p i n resonance ( e s r ) can be used t o g i v e a more s e n s i t i v e i n d i c a t i o n of the e l e c t r o n ' s immediate e n v i r o n m e n t .  The  e s r s i g n a l c o n s i s t s of a s i n g l e , narrow l i n e w i t h a g - f a c t o r n e a r t h a t of the f r e e e l e c t r o n , g = 2.0023. l e s s t h a n 0.1 broader,  I n l i q u i d s the l i n e w i d t h i s t y p i c a l l y  gauss whereas i n s o l i d media the l i n e w i d t h i s much  g e n e r a l l y 5-15  g a u s s , due  to the m a g n e t i c i n t e r a c t i o n  o f the e l e c t r o n s p i n w i t h the n u c l e a r m a g n e t i c moments o f the n u c l e i surrounding  it.  The narrowness of the l i n e i n the l i q u i d s t a t e i s due  t o the e x t e n s i v e time a v e r a g i n g The  o f the n u c l e a r d i p o l a r i n t e r a c t i o n s .  absence o f any r e s o l v a b l e h y p e r f i n e s p l i t t i n g i n d i c a t e s t h a t  the  e l e c t r o n i s not s t r o n g l y l o c a l i z e d on any p a r t i c u l a r m o l e c u l e b u t r a t h e r i n t e r a c t s w e a k l y w i t h the m o l e c u l e s f o r m i n g  the c a v i t y .  the s i g n a l s a r e e a s i l y power s a t u r a t e d a t low t e m p e r a t u r e s . nonpolar o n l y 0.02  Furthermore,  In  h y d r o c a r b o n m a t r i c e s , the e s r s i g n a l becomes s a t u r a t e d a t mw",  whereas i n p o l a r media, s a t u r a t i o n becomes a p p r e c i a b l e llh  if  the microwave power s a t u r a t i o n exceeds 0.2 One  mW.  i n t e r e s t i n g f e a t u r e o f e s r s t u d i e s on s t a b i l i z e d e l e c t r o n s i s  t h a t the l i n e w i d t h s  of t r a p p e d e l e c t r o n s i n p o l a r media a r e  considerably  - 33 -  b r o a d e r than those i n n o n p o l a r  systems.  The  cavity r a d i i for  nonpolar  media are l a r g e r , c o n s e q u e n t l y  there i s a diminished i n t e r a c t i o n of  the e l e c t r o n w i t h the n u c l e i o f the c a v i t y w a l l s as compared t o  those  o f p o l a r media. I n the m a j o r i t y o f s t u d i e s to d a t e , trapped e l e c t r o n s have shown no resolvable hyperfine s t r u c t u r e suggesting strongly localized.  t h a t the e l e c t r o n i s n o t  However a few cases o f o b s e r v a b l e  hyperfine  s t r u c t u r e have been r e p o r t e d , i n p a r t i c u l a r t h a t by B e n n e t t , M i l n e 19 and Thomas  i n w h i c h t r a p p e d e l e c t r o n s a t 77°K were p r e p a r e d  by  the  c o - d e p o s i t i o n of sodium and w a t e r vapour on a r o t a t i n g c r y o s t a t . s l o w l y warming the sample t o 140°K, the s i n g l e e s r l i n e was seven e q u a l l y spaced l i n e s and was  spit  into  a t t r i b u t e d t o the i n t e r a c t i o n o f  e l e c t r o n w i t h s i x e q u i v a l e n t protons electron.  Upon  arranged  the  o c t a h e d r a l l y around the  I t has been s u g g e s t e d t h a t t h i s o b s e r v a t i o n may  be due  to a  phase t r a n s i t i o n of i c e from the amorphous t o the c u b i c s t a t e w h i c h lie l e a d s t o the p r e f e r r e d o r i e n t a t i o n o f the s o l v a t i o n s h e l l . O t h e r examples o f h y p e r f i n e i n t e r a c t i o n were found i n the case o f 20 21 deuterated p y r r o l i d i n e and c r y s t a l l i n e d e u t e r a t e d a c e t o n i t r i l e at 77°K.  I n the f o r m e r case a seven l i n e spectrum was  observed, i n d i c a t i n g  i n t e r a c t i o n o f the e l e c t r o n w i t h t h r e e o r f o u r e q u i v a l e n t n i t r o g e n atoms. I n the l a t t e r c a s e , a f i v e l i n e spectrum was s u g g e s t e d t h a t the e l e c t r o n was w i t h the two n i t r o g e n n u c l e i .  observed and i t was  t r a p p e d between the d i p o l e s and However t h i s o b s e r v a t i o n has 22  a t t r i b u t e d t o a dimer r a d i c a l a n i o n o f a c e t o n i t r i l e .  interacting  s i n c e been  23 '  To d a t e ,  no  o t h e r e x p e r i m e n t s have been r e p o r t e d r e g a r d i n g h y p e r f i n e s t r u c t u r e of stabilized electrons.  - 34 -  A more d e t a i l e d d e s c r i p t i o n o f e s r s p e c t r o s c o p y and i t s a p p l i c a t i o n t o t h i s s t u d y on d i m e t h y l s u l f o x i d e i s g i v e n  2.3  i n C h a p t e r V.  Conductivity A t h i r d important property  conductivity. of i t s c a v i t y .  of s t a b i l i z e d electrons i s t h e i r high  The s t a b i l i z e d e l e c t r o n i s a s s o c i a t e d w i t h s e v e r a l m o l e c u l e s Therefore, i f the c l a s s i c a l p i c t u r e o f i o n i c  m o b i l i t y , i n w h i c h t h e i o n moves c a r r y i n g i t s s o l v a t i o n s h e a t h w i t h i t , h o l d s f o r s t a b i l i z e d e l e c t r o n s , t h e n t h e e l e c t r o n ' s m o b i l i t y s h o u l d be comparable t o o t h e r i o n i c s p e c i e s . greater  However, i t s m o b i l i t y i s s i g n i f i c a n t l y  t h a n most o t h e r n e g a t i v e i o n s , s u g g e s t i n g t h a t , the e l e c t r o n moves  from t r a p - t o - t r a p by quantum m e c h a n i c a l t u n n e l l i n g o r t h r o u g h  voids  formed i n t h e c a v i t y w a l l by the r o t a t i o n o f one o f t h e c a v i t y m o l e c u l e s . I n t h e s o l i d s t a t e a t low t e m p e r a t u r e s such r o t a t i o n i s i n h i b i t e d and the e l e c t r o n s  can o n l y be m o b i l i z e d by the a b s o r p t i o n  c o n d u c t i v i t y ) o r by t h e r m a l r e l e a s e .  of l i g h t  (photo-  Measurements o f e l e c t r o n  c o n d u c t i v i t i e s are h e l p f u l i n determining the extent of e l e c t r o n solvation.  Those e l e c t r o n s w h i c h a r e s t r o n g l y s o l v a t e d , as i n p o l a r  a l c o h o l s and w a t e r , e x h i b i t a much l o w e r m o b i l i t y t h a n t h o s e i n hydrocarbons  where  t h e p o l a r i z a t i o n f o r c e s a r e v e r y weak.  However,  because o f t h e h i g h background conductance i n p o l a r and p a r t i a l l y i o n i c s o l v e n t s , experiments d e a l i n g w i t h r a d i a t i o n - i n d u c e d usually r e s t r i c t e d to materials  o f low d i e l e c t r i c c o n s t a n t ,  s a t u r a t e d h y d r o c a r b o n s and e t h e r s . i n DMSO were n o t u n d e r t a k e n .  conductances a r e  Studies  on e l e c t r o n  such as t h e  conductivity  - 35  3.  -  Models I n o r d e r to account f o r the o b s e r v e d p h y s i c a l  of s t a b i l i z e d e l e c t r o n s have been p r o p o s e d . all  i n v a r i o u s media, s e v e r a l  and  chemical  t h e o r e t i c a l models  Such models are e s s e n t i a l i f one  i s to understand  f e a t u r e s o f the e x p e r i m e n t a l r e s u l t s , i n p a r t i c u l a r the  i n properties  variation  between d i f f e r e n t systems. 24  25  P r o b a b l y the most w i d e l y a c c e p t e d t h e o r y i s J o r t n e r ' s continuum model i n w h i c h the e l e c t r o n by  i n i t i a l l y polarizes i n the  '  i s r e g a r d e d as b e i n g s t a b i l i z e d  the i n d u c e d p o l a r i z a t i o n f i e l d of the medium.  followed,  The  electron  the medium by e l e c t r o n i c p o l a r i z a t i o n w h i c h i s  l i q u i d phase, by o r i e n t a t i o n  ( a t o m i c and  dipole)  p o l a r i z a t i o n i f the m o l e c u l e s of the medium have a permanent I n the  properties  case of low  t e m p e r a t u r e g l a s s e s t h e s e t r a p s are c o n s i d e r e d  b e i n g p r e f o r m e d w i t h the o f the e l e c t r o n .  dipole.  dipoles  I n t h i s way  of s t a b i l i z e d e l e c t r o n s  i n the  Once s t a b i l i z e d , the e l e c t r o n  one  suitably oriented can  explain  l i q u i d and  b e f o r e the  as  arrival  the s i m i l a r s p e c t r a l  glassy  properties  phases.  i s r e g a r d e d as b e i n g l a r g e l y c o n f i n e d  to  a s p h e r i c a l c a v i t y o f r a d i u s R i n w h i c h the e l e c t r o s t a t i c p o t e n t i a l function  i s constant.  O u t s i d e the  c a v i t y the f u n c t i o n  i s continuous.  The  s t a b i l i z a t i o n i s a t t r i b u t e d to l o n g - r a n g e p o l a r i z a t i o n  the  s h o r t - r a n g e a t t r a c t i o n s b e i n g o f f s e t by  short-range  repulsions.  The  s e l f - e n e r g y , E.,  represented  by  E  i  where T i s the  =  T  of the e l e c t r o n may +  be  interactions,  V(r)  i n h e r e n t k i n e t i c energy (< kT)  (1.19)  and  V(r)  i s the  potential  energy a c t i n g on the e l e c t r o n produced by the p o l a r i z a t i o n . o r i e n t a t i o n p o t e n t i a l energy f u n c t i o n , P ( r ) , i s g i v e n as  P(r)  =  -ge /r  The  Landau  follows:  for r > R  2  (1.20)  2 =  -ge  /R  for r < R  where ft = (1/D - 1/D ) . ^ ' op s  D  op  and D  s  a r e the o p t i c a l and s t a t i c d i r  e l e c t r i c c o n s t a n t s and r i s the d i s t a n c e from the c e n t r e of the cavity.  The  ground and f i r s t e x c i t e d s t a t e  electron  (presumed t o be bound) o f  the e l e c t r o n are c o n s i d e r e d as b e i n g s i m i l a r t o one-parameter h y d r o g e n i c type I s and 2p wave f u n c t i o n s , w h i c h take the f o l l o w i n g  , 3, .1/2 *ls i|>2p  =  ( y  =  / 7 r )  form:  -yr 6  (1.21)  ,5,  .1/2 (a /TT)  Q  r cosy  -ar e  where y and a a r e the v a r i a t i o n a l p a r a m e t e r s . The  energy o f the ground s t a t e can be r e p r e s e n t e d by the e x p r e s s i o n  w  is  -7*  * [ls  ^~2~ 8TT  V  2  + P(r)]*  l s  dx  (1.22)  m  2 where V  i s the L a p l a c i a n  operator,  P ( r ) i s the p o t e n t i a l energy  o p e r a t o r , h i s P l a n c k ' s c o n s t a n t and m i s the e l e c t r o n mass. g i v e n v a l u e o f R and 3, W^  g  the v a r i a t i o n  procedure  i s t h e n o b t a i n e d as a f u n c t i o n o f  For a y  Using  - 37 -  the b e s t v a l u e o f y i s o b t a i n e d w h i c h , when s u b s t i t u t e d i n t o  (1.21)  and (1.22) y i e l d s t h e wave f u n c t i o n and energy f o r t h e ground s t a t e o f the s t a b i l i z e d e l e c t r o n . s i m i l a r manner.  The f i r s t e x c i t e d s t a t e i s t r e a t e d i n a  However, a c c o r d i n g t o t h e Frank-Condon p r i n c i p l e , t h e  v a l u e o f R and the form o f t h e p o t e n t i a l s e l e c t e d f o r t h e ground  state  must be t h e same f o r the 2p-type s t a t e s i n c e t h e m o l e c u l e s cannot r e o r i e n t t h e m s e l v e s f a s t enough t o f o l l o w t h e e x c i t a t i o n . The e l e c t r o n i c p o l a r i z a t i o n energy i s r e p r e s e n t e d , a p p r o x i m a t e l y , by t h e e x p r e s s i o n  S  =  x  -e  (1 - 1/D  2  op  )  (1.24)  where r ^ i s t h e mean r a d i u s o f the charge d i s t r i b u t i o n i n t h e i s t a t e , 3 being  5  ^  and  — ^  f o r t h e ground and f i r s t e x c i t e d s t a t e  respectively.  Thus the total energy o f t h e ground and f i r s t e x c i t e d s t a t e o f t h e e l e c t r o n may be r e p r e s e n t e d a s :  E  ls  =  l s  W  +  S  l s (1.25)  E  2p 0  =  W  +  2p  0  The energy f o r t h e (2p  ^  =  E  2 p  -  S_ 2p  I s ) t r a n s i t i o n i s t h e n g i v e n by:  E  l s  (1.26)  w h i c h i s r e g a r d e d t o c o r r e s p o n d t o t h e e x c i t a t i o n energy a t the a b s o r p t i o n maximum.  - 38  -  I t s h o u l d be s t r e s s e d , however, t h a t t h i s model i s b e c a u s e the c a v i t y r a d i u s i s i n t r o d u c e d and  does n o t p r o v i d e  any  g  as an a d j u s t a b l e  interactions.  i s f a i r l y l a r g e f o r most p o l a r media, and D  m e d i a , t h e terms ( 1 / D ^ are f a i r l y constant  parameter  r e a l p h y s i c a l information regarding  range s t r u c t u r a l m o d i f i c a t i o n s and D  semi-empirical  - 1/D )  and  g  (1 - 1/D^)  so t h a t t h e s p e c t r a l and  the  short-  F u r t h e r m o r e , because i s % 1 to 2 for a l l  i n the energy  expressions  thermodynamic p r o p e r t i e s  o f s t a b i l i z e d e l e c t r o n s are o f t e n a t t r i b u t e d t o changes i n the c a v i t y 26 radius.  Assuming a c o n s t a n t  have drawn up a c o n t o u r map steps  v a l u e o f 2 f o r D p» Noda, F u e k i and Q  Kuri  of the o p t i c a l t r a n s i t i o n energy i n 0.1  as a f u n c t i o n of R and D .  From t h i s map  g  i t i s p o s s i b l e t o draw  a c o r r e l a t i o n c u r v e of the t r a n s i t i o n energy and  cavity radius f o r  s o l v e n t knowing i t s s t a t i c d i e l e c t r i c c o n s t a n t .  The  shown i n F i g u r e 6, t a k i n g D  g  =  c u r v e f o r DMSO i s  d a t a can be f i t t e d  of R) and has been  m e d i a , i n p a r t i c u l a r ammonia, w a t e r and success.  any  48.  U s i n g Jortner's model, experimental (using e m p i r i c a l values  eV  done f o r  f o r any  solvent  several polar  the a l c o h o l s , w i t h  reasonable  However, i n s p i t e of the apparent s u c c e s s of t h i s q u a l i t a t i v e  a p p r o a c h , t h e r e e x i s t v e r y s e r i o u s t h e o r e t i c a l arguments a g a i n s t i t s application, especially for strongly solvated electrons.  The  treatment  i s r e a l l y an e l e c t r o n i c a d i a b a t i c a p p r o x i m a t i o n s i n c e i t assumes t h a t the e x t r a e l e c t r o n i s much more l o o s e l y bound and  therefore of  mean v e l o c i t y than the v a l e n c e o r c o r e e l e c t r o n s of the medium.  lower In  p o l a r media the b i n d i n g o r s o l v a t i o n energy i s *v. 1 t o 2 eV so t h a t  the  e l e c t r o n i s not a p p r e c i a b l y more w e a k l y bound than the v a l e n c e e l e c t r o n s . The  e x t r a e l e c t r o n s h o u l d be  considered  on an e q u a l b a s i s w i t h  the  I 0 F i g u r e 6.  I  1.0  I  I  2.0 3.0 CAVITY RADIUS (A°)  I  I  4.0  C o r r e l a t i o n curve of the t r a n s i t i o n energy o f the e l e c t r o n i n DMSO and i t s c a v i t y r a d i u s u s i n g t h e continuous  d i e l e c t r i c model w i t h an a d i a b a t i c  approximation.  - AO -  medium e l e c t r o n s by i n c l u d i n g t h e e l e c t r o n i c p o l a r i z a t i o n p o t e n t i a l i n the eigenvalue equation f o r determining the wavefunction and e n e r g i e s o f t h e e l e c t r o n i c s t a t e s .  T r e a t i n g the t o t a l p o l a r i z a t i o n  energy i n t h i s way thus c o n s t i t u t e s an independent consistent f i e l d  parameters  p a r t i c l e or s e l f -  ( H a r t r e e - F o c k ) type a p p r o x i m a t i o n .  When t h i s  approximation i s applied to the hydrated e l e c t r o n , the c a v i t y radius must be made v a n i s h i n g l y s m a l l (R ^ 0 ) f o r t h e t h e o r e t i c a l o p t i c a l 24 t r a n s i t i o n energy t o agree r e a s o n a b l y w i t h t h e e x p e r i m e n t a l r e s u l t s . However s h o r t - r a n g e o v e r l a p i n t e r a c t i o n s a r e n o t a c c o u n t e d f o r by t h i s continuum model and i s r e a l l y o n l y a p p l i c a b l e p r o v i d e d t h a t t h e c a v i t y r a d i u s i s t a k e n t o be e q u a l o r l a r g e r than a m o l e c u l a r o r a t o m i c radius.  Moreover, t h e s h i f t i n t h e a b s o r p t i o n spectrum w i t h  temperature  and p r e s s u r e cannot be a d e q u a t e l y e x p l a i n e d assuming a n e a r z e r o cavity radius.  For these reasons both the s e m i - e m p i r i c a l a d i a b a t i c  and s e l f - c o n s i s t e n t f i e l d a p p r o x i m a t i o n t o t h e c o n t i n u o u s  dielectric  model a r e t o o l i m i t e d t o p r o v i d e a p r o p e r i n t e r p r e t a t i o n o f t h e p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s i n p o l a r solvents. Whereas t h e c o n t i n u o u s d i e l e c t r i c model a c c o u n t s f o r e l e c t r o n s t a b i l i z a t i o n t h r o u g h l o n g - r a n g e p o l a r i z a t i o n f o r c e s , o t h e r models a t t r i b u t e these f o r c e s t o short-range e l e c t r o n - s o l v e n t i n t e r a c t i o n s . 27 N a t o r i and Watanabe  proposed  a s t r u c t u r a l model f o r t h e h y d r a t e d  e l e c t r o n which i n v o l v e s a t r a p p i n g center c o n s i s t i n g o f four water m o l e c u l e s w i t h one OH bond o f each w a t e r m o l e c u l e t e t r a h e d r a l l y around  the e l e c t r o n .  oriented  The t h e o r e t i c a l t r a n s i t i o n energy was f i t t e d t o  the e x p e r i m e n t a l v a l u e by v a r y i n g the oxygen-oxygen d i s t a n c e and by s t r e t c h i n g t h e bond d i s t a n c e s and a n g l e s .  However, t h e c a l c u l a t e d v a l u e  - 41 -  was much l o v e r than t h e e x p e r i m e n t a l l y o b s e r v e d v a l u e (0.8 eV v e r s u s 1.7 e V ) .  T h i s was u n d o u b t e d l y due t o t h e f a c t t h e dominant  long-  range p o l a r i z a t i o n i n t e r a c t i o n s were n e g l e c t e d . A second model, c a l l e d an o r i e n t e d  d i p o l e model, has been s u g g e s t e d  28 by I g u c h i .  I n t h i s model t h e p o t e n t i a l f i e l d a r i s e s from t h e  molecular dipoles electron.  oriented  i n a s p h e r i c a l manner about t h e e x c e s s  The a p p r o x i m a t i o n s used a r e e s s e n t i a l l y t h e same as f o r t h e  electronic adiabatic  continuum model i n w h i c h t h e e l e c t r o n i c p o l a r i z a t i o n  energy i s added as a c o r r e c t i o n term t o t h e computed energy. t h e r m a l dependence o f t h e a b s o r p t i o n s p e c t r a degree o f o r i e n t a t i o n o f t h e d i p o l e s of the l i q u i d r a t h e r  than a  The  a r e a t t r i b u t e d t o the  as w e l l as by t h e t h e r m a l e x p a n s i o n  s i m p l e change i n c a v i t y  radius.  The s u c c e s s o f any o f t h e s e t r e a t m e n t s f o r e l e c t r o n s o l v a t i o n and trapping  a r e j u d g e d e s s e n t i a l l y i n terms o f t h e i r a b i l i t y t o produce  "agreement" between t h e o r y and e x p e r i m e n t .  However, i n v i e w o f the  crude a p p r o x i m a t i o n s u s e d , such as t h e n e g l e c t o f e i t h e r t h e s h o r t or l o n g - r a n g e i n t e r a c t i o n s , none o f these models can be c o n s i d e r e d as g i v i n g a t r u e p i c t u r e o f e l e c t r o n s t a b i l i z a t i o n . the c a v i t y r a d i u s  Furthermore, e i t h e r  (continuum model) o r a s p e c i f i c d i p o l e  orientation  ( a t o m i c models) a r e v a r i e d u n t i l t h e o r y and experiment a g r e e , these are the only f a c t o r s  implying  involved.  R e c e n t l y a semicontinuum model f o r t h e s t a b i l i z e d e l e c t r o n i n 29 30 31 ammonia, water and methanol was d e v e l o p e d w h i c h i n c l u d e d b o t h l o n g - and s h o r t - r a n g e i n t e r a c t i o n s . electron interacts with  I n t h i s treatment the excess  t h e i n d u c e d and permanent d i p o l e moments o f the  m o l e c u l e s i n the f i r s t s o l v a t i o n s h e l l by a s h o r t - r a n g e c h a r g e - d i p o l e  -  42  -  a t t r a c t i v e p o t e n t i a l , s i m i l a r t o t h a t proposed by I g u c h i , the s o l v e n t  whereas  m o l e c u l e s beyond the f i r s t s o l v a t i o n s h e a t h a r e t r e a t e d  a c o n t i n u o u s d i e l e c t r i c medium w i t h w h i c h the e l e c t r o n l o n g - r a n g e p o l a r i z a t i o n p o t e n t i a l (continuum model).  i n t e r a c t s by a By p e r f o r m i n g  v a r i a t i o n a l c a l c u l a t i o n s s i m i l a r to those described p r e v i o u s l y v a r i o u s c a v i t y r a d i i , the a u t h o r s were a b l e t o c o n s t r u c t c o o r d i n a t e diagrams f o r the ground and e x c i t e d s t a t e s .  for  configurational I n t h i s manner  a minimum i n the t o t a l energy o f the system c o u l d be e s t a b l i s h e d , c o u l d n o t be o b t a i n e d a t a f i n i t e r a d i u s by o n l y i n c l u d i n g p o l a r i z a t i o n i n t e r a c t i o n s , and a unique c a v i t y r a d i u s predicted.  as  which  long-range  could  be  I n the case o f ammonia, a s e l f - c o n s i s t e n t f i e l d t r e a t m e n t  was used f o r s h o r t - r a n g e i n t e r a c t i o n s and a Landau-type p o t e n t i a l f o r long-range i n t e r a c t i o n s  ( a d i a b a t i c a p p r o x i m a t i o n ) whereas b o t h  short-  and l o n g - r a n g e i n t e r a c t i o n s were t r e a t e d s e l f - c o n s i s t e n t l y i n the case o f the e l e c t r o n i n methanol and w a t e r .  U s i n g such a t r e a t m e n t , the  observed o p t i c a l a b s o r p t i o n of both the solvated  e l e c t r o n i n ammonia  and methanol and the h y d r a t e d e l e c t r o n i n w a t e r ( u s i n g D p o l y c r y s t a l l i n e i c e a t 77°K ( u s i n g D explained.  g  = 3) c o u l d be  = 78) and i n  satisfactorily  Moreover, the energy l e v e l s were moved toward t h e continuum  so t h a t the e x c i t e d s t a t e s a r e l e s s s t r o n g l y bound. with  g  the p h o t o b l e a c h i n g and c o n d u c t i v i t y  g l a s s e s mentioned e a r l i e r .  studies  This i s i n keeping  on low t e m p e r a t u r e  Thermodynamic and s t r u c t u r a l d a t a , such as  the h e a t o f s o l u t i o n and charge d i s t r i b u t i o n o f the s t a b i l i z e d e l e c t r o n , were i n good agreement w i t h  the e x p e r i m e n t a l r e s u l t s .  A l t h o u g h the semicontinuum model i s o n l y a p p r o x i m a t e i n n a t u r e , i t does show t h a t b o t h s h o r t - and l o n g - r a n g e i n t e r a c t i o n s must be  -  considered  43  -  when d e s c r i b i n g t h e t r a p p i n g and s t a b i l i z i n g o f e l e c t r o n s  and when a n t i c i p a t i n g t h e i r i n h e r e n t Such c o n s i d e r a t i o n s  c h e m i c a l and p h y s i c a l p r o p e r t i e s .  a r e e s s e n t i a l i f one i s t o u n d e r s t a n d a l l t h e  features of the experimental  r e s u l t s , e s p e c i a l l y the v a r i a t i o n i n  p r o p e r t i e s between d i f f e r e n t s o l v e n t s .  S t u d i e s o f o t h e r systems, i n  p a r t i c u l a r p o l a r a p r o t i c s o l v e n t s such as DMSO, s h o u l d p r o v i d e rigorous  a  t e s t t o t h e s e models and a b e t t e r i n s i g h t i n t o t h e s t r u c t u r a l ,  thermodynamic and o p t i c a l p r o p e r t i e s o f t h e s t a b i l i z e d e l e c t r o n .  4.  Free Ion Y i e l d s In the preceding  s e c t i o n i t was s u g g e s t e d t h a t t h e s o l v a t i o n  energy o f t h e e l e c t r o n depends t o a g r e a t e x t e n t o f t h e medium.  S i m i l a r l y , the r a d i a t i o n y i e l d of s t a b i l i z e d  depends on t h e b u l k d i e l e c t r i c c o n s t a n t The  on t h e p o l a r i z a b i l i t y  of the medium.  p r i m a r y , s e c o n d a r y and h i g h e r - o r d e r  e l e c t r o n s produced by  the i n c i d e n t r a d i a t i o n a r e reduced t o t h e r m a l e n e r g i e s mentioned e a r l i e r . and  electrons  by t h e p r o c e s s e s  Because o f t h e random n a t u r e o f t h e c o l l i s i o n s  s c a t t e r i n g , t h e r e w i l l be a d i s t r i b u t i o n o f the t h e r m a l i z a t i o n  d i s t a n c e s r e s u l t i n g i n a range o f i n i t i a l s e p a r a t i o n d i s t a n c e s  between  the t h e r m a l i z e d e l e c t r o n s and t h e i r p a r e n t p o s i t i v e i o n .  thermalized,  Once  t h e e l e c t r o n s a r e doomed e i t h e r t o geminate r e c o m b i n a t i o n w i t h  their  c o n c o m i t a n t p a r t n e r o r t o d i f f u s i o n outwards i n t o t h e m i l i e u t o undergo reaction there.  Those t h a t escape a r e r e f e r r e d t o as " f r e e i o n s " , those  u n d e r g o i n g geminate c o m b i n a t i o n s as "geminate i o n s " . energy o f a t t r a c t i o n , E  , i s g i v e n as f o l l o w s ,  Since  the coulombic  E  att  ^  =  /  D  (1.27)  r  where q i s the e l e c t r o n c h a r g e , D i s t h e d i e l e c t r i c c o n s t a n t of the medium and r i s t h e s e p a r a t i o n d i s t a n c e , i t f o l l o w s t h a t f o r a g i v e n i n i t i a l s e p a r a t i o n d i s t r i b u t i o n the f r e e i o n y i e l d w i l l i n c r e a s e w i t h dielectric The  constant.  d i s t a n c e y a t w h i c h t h e t h e r m a l k i n e t i c e n e r g y , kT, o f t h e  e l e c t r o n e q u a l s t h a t o f the c o u l o m b i c  a t t r a c t i v e p o t e n t i a l i s g i v e n as  follows:  y  =  q /DkT 2  ( 1 <  The p r o b a b i l i t y o f e s c a p i n g geminate r e c o m b i n a t i o n , by e q u a t i o n  2 ) 8  <j>(esc), i s g i v e n  (1.29)  ' att E  .{.(esc)  =  e  a  c  /  k  T  (1.29)  c  w h i c h , when combined w i t h (1.27) and ( 1 . 2 8 ) , y i e l d s t h e c e l e b r a t e d Onsager r e l a t i o n s h i p  <J>(esc)  =  (1.30).  e~ ^ V  (1.30)  T  I f one were t o make t h e crude a p p r o x i m a t i o n s  that ( i ) the d i s t r i b u t i o n  o f s e p a r a t i o n d i s t a n c e s and ( i i ) t h e i n i t i a l i o n i z a t i o n y i e l d  were t h e  same f o r a l l m e d i a , then t h e y i e l d o f f r e e i o n s s h o u l d v a r y e x p o n e n t i a l l y w i t h t h e d i e l e c t r i c c o n s t a n t o f t h e medium.  The dependence o f the  f r e e i o n y i e l d on the s t a t i c d i e l e c t r i c c o n s t a n t o f t h e l i q u i d s i s shown i n F i g u r e 7. The e m p i r i c a l c u r v e appears t o f o l l o w t h e e x p o n e n t i a l b e h a v i o r p r e d i c t e d a l t h o u g h t h e r e a r e some s i g n i f i c a n t e x c e p t i o n s such as l i q u i d ammonia. A p a r t from the a p p r o x i m a t i o n s s t a t e d e a r l i e r t h e r e a r e o t h e r f a c t o r s w h i c h must be i n c l u d e d when c o n s i d e r i n g t h i s model. of  electrostatic  I f e l e c t r o n s o l v a t i o n i s t o be a t t r i b u t e d t o t h e r e l a x a t i o n  t h e medium around t h e t h e r m a l e l e c t r o n , then t h e s o l v a t i o n time  w i l l have t o be comparable t o t h e m a c r o s c o p i c d i e l e c t r i c r e l a x a t i o n of  t h e medium.  time  However i f t h e s o l v a t i o n time i s i n t h e v i c i n i t y o r  s m a l l e r t h a n t h e d i e l e c t r i c r e l a x a t i o n time, T , o f t h e l i q u i d , then a t i m e - a v e r a g e d d i e l e c t r i c c o n s t a n t must be used. D Op  For nonpolar  = D , and t h e d i e l e c t r i c c o n s t a n t i s t i m e independent. s' r  liquids,  In polar r  media the permanent m o l e c u l a r d i p o l e s r e q u i r e a c e r t a i n time t o r o t a t e and l i n e up w i t h t h e e l e c t r i c f i e l d o f the e l e c t r o n so t h a t D i n e q u a t i o n (1.27) w i l l be l e s s t h a n D . g  T h i s i s s u p p o r t e d by t h e f a c t  t h a t r e c e n t s t u d i e s on t h e p u l s e r a d i o l y s i s o f w a t e r i n d i c a t e t h a t t h e s o l v a t i o n time o f t h e h y d r a t e d e l e c t r o n i s s h o r t e r than i t s b u l k 14 35 d i e l e c t r i c r e l a x a t i o n time. T h e r e f o r e , as s u g g e s t e d by Mozumder, the d i e l e c t r i c c o n s t a n t s h o u l d be t r e a t e d as a time-dependent v a r i a b l e , g i v e n as f o l l o w s , D D(t)  =  22 /D ) (1 - e " op s  1 - (1 - D  where 6 = (D /D )x op s  ( 1 t/<!)  .31)  )  and t i s t h e time a f t e r t h e s t a r t o f t h e p o l a r i z a t i o n  of the medium by t h e e l e c t r o n .  r  U n f o r t u n a t e l y the time r e s o l u t i o n o f p r e s e n t  o p t i c a l d e t e c t i o n equipment i s l i m i t e d t o 20 p i c o s e c o n d s , by w h i c h  FORMAMIDE  -  3.0  —  D  1 T  c  o 20 <u  —  NITRILES  L.  S ^ /  2°  .  1  PROPYLENE CARBONATE  •c-  ON  61  H—  o  1.0  ALCOHOLS  ooro  ' r  F i g u r e 7.  o  . • AMMONIA HYDROCARBONS . i . I  20  40  1  60  .  1  80  fl—  1  __,  100  P l o t o f G ( f r e e i o n ) as a f u n c t i o n o f the s t a t i c d i e l e c t r i c c o n s t a n t o f the medium, Data taken from r e f e r e n c e s 32, 33', 34, 46, 101.  - 47  -  time the e l e c t r o n s are a l r e a d y s o l v a t e d , so t h a t a p p l i c a t i o n of  this  r e l a t i o n s h i p i s not y e t p o s s i b l e . A n o t h e r p a r a m e t e r w h i c h has n o t r e c e i v e d the a t t e n t i o n i t d e s e r v e s i s the i n i t i a l d i s t r i b u t i o n of t h e r m a l i z a t i o n d i s t a n c e s .  The  low  energy  e l e c t r o n s l o s e t h e i r e x c e s s energy m a i n l y by e l e c t r o n i c a l l y e x c i t i n g the m o l e c u l e s they e n c o u n t e r .  As t h e i r k i n e t i c energy drops below the  l o w e s t e x c i t e d l e v e l , f u r t h e r energy d e g r a d a t i o n i n t r a m o l e c u l a r and The  p r o c e e d s by e x c i t i n g  i n t e r m o l e c u l a r v i b r a t i o n s and m o l e c u l a r r o t a t i o n s .  d i s t a n c e t r a v e l l e d by the e l e c t r o n d u r i n g . t h i s second p a r t o f i t s  t h e r m a l i z a t i o n , the s o - c a l l e d s u b e x c i t a t i o n range, makes the c o n t r i b u t i o n t o the t o t a l e l e c t r o n range.  According  greatest  to L a s s e t t r e  and  36 Silverman,  the Born a p p r o x i m a t i o n , upon w h i c h the Bethe s t o p p i n g  formula given i n equation  (1.10) i s b a s e d , i s o n l y good above about  120 eV so t h a t t h e o r e t i c a l t h e r m a l i z a t i o n d i s t a n c e s using t h i s formula.  Unfortunately  below 100  eV.  cannot be  no s u c c e s s f u l t r e a t m e n t has  p r o p o s e d to account f o r t h e r m a l i z a t i o n d i s t a n c e s energies  power  calculated been  f o r electrons having  I t w o u l d appear t h a t e l e c t r o n s have s h o r t e r  t h e r m a l i z a t i o n p a t h s i n media i n w h i c h the m o l e c u l e s have s e v e r a l i n t e r n a l degrees o f freedom. d u r i n g and  However, the i o n i c m o b i l i t y o f the e l e c t r o n  before,  a f t e r the medium b e g i n s t o r e l a x w i l l a l s o c o n t r i b u t e t o  thermalization  the  length.  A l t h o u g h the f r e e i o n y i e l d i s dependent on s e v e r a l v a r i a b l e s , the s t a t i c d i e l e c t r i c constant  i s the o n l y parameter w i t h w h i c h y i e l d s i n  v a r i o u s media have been compared. Figure  From the e m p i r i c a l r e l a t i o n s h i p shown i n  7, i t appears t h a t t h i s parameter does i n d e e d p l a y the dominant  r o l e i n most media; b u t  t h i s r e l a t i o n s h i p needs t o be much more w i d e l y  t e s t e d , p a r t i c u l a r l y i n media such as the a p r o t i c p o l a r  solvents.  - 48 -  D.  The  chemical  BINARY MIXTURES  y i e l d s of s o l v a t e d e l e c t r o n s and t h e i r  optical  a b s o r p t i o n s p e c t r a i n b i n a r y s o l v e n t systems a r e o f i n t e r e s t because they  o f f e r a t e s t o f the t h e o r e t i c a l models d e s c r i b e d p r e v i o u s l y .  If  the continuum o r semi-continuum models a r e a p p l i c a b l e , then the e l e c t r o n s s h o u l d be d e l o c a l i z e d , thereby  sampling the average environment o f the  m i x t u r e and thus t h e o p t i c a l p r o p e r t i e s and y i e l d s s h o u l d be determined by t h e m a c r o s c o p i c p r o p e r t i e s o f the medium. electrons are strongly localized  On the o t h e r hand, i f the  through s h o r t - r a n g e  i n t e r a c t i o n s , then  the y i e l d s and o p t i c a l c h a r a c t e r i s t i c s s h o u l d be governed by t h e s o l v a t i o n due  t o a s m a l l number o f s o l v e n t m o l e c u l e s ,  perhaps 4 t o 6.  Thus an  e x a m i n a t i o n o f e l e c t r o n y i e l d s , a b s o r p t i o n band maxima and the w i d t h - a t h a l f - h e i g h t , AW, o f the a b s o r p t i o n bands as a f u n c t i o n o f b i n a r y solvent composition  should provide  electron solvation.  a b e t t e r i n s i g h t i n t o the n a t u r e o f  Perhaps f o r these  reasons s e v e r a l i n v e s t i g a t i o n s  have been undertaken v e r y r e c e n t l y on b i n a r y m i x t u r e s o f w i d e l y  different  polarity.  37 A r a i and Sauer at v a r i o u s  have s t u d i e d b i n a r y m i x t u r e s o f water and a l c o h o l s  concentrations.  I n a l l the m i x t u r e s examined the e l e c t r o n  a b s o r p t i o n had o n l y one peak and the a b s o r p t i o n maxima and h a l f - w i d t h were i n t e r m e d i a t e  t o those  o f t h e pure components.  I f the a b s o r p t i o n  s p e c t r a were a mere s u p e r p o s i t i o n o f the bands a r i s i n g components, a much b r o a d e r band at intermediate  from t h e i n d i v i d u a l  (enhanced AW) w i t h w e l l - d e f i n e d  c o n c e n t r a t i o n s would be e x p e c t e d .  case o f the e t h a n o l - w a t e r m i x t u r e ,  shoulders  F u r t h e r m o r e , i n the  G(e ) i n c r e a s e d l i n e a r l y w i t h the  - 49 -  d i e l e c t r i c constant of the mixture.  These r e s u l t s were t a k e n as  s u g g e s t i n g t h a t t h e s o l v a t i o n o f the e l e c t r o n depends on the a g g r e g a t e p r o p e r t i e s o f the m i x t u r e , such as the m a c r o s c o p i c d i e l e c t r i c c o n s t a n t , i n w h i c h i t i n t e r a c t s w i t h a l a r g e number o f m o l e c u l e s . 38 s i o n s were a r r i v e d a t by Dorfman e t a l . from t h e i r  Similar  conclu-  investigations  on b i n a r y m i x t u r e s o f w a t e r w i t h ammonia and e t h y l e n e d i a m i n e .  Whereas  the Xmax o f t h e pure components i n t h e w a t e r - a l c o h o l b i n a rJy m i x t u r e s a r e tr r v e r y c l o s e (see T a b l e 1 ) , so t h a t any changes would be s m a l l , t h o s e of w a t e r (720 nm), e t h y l e n e d i a m i n e (1350 nm) and ammonia (1550 nm) a r e s u f f i c i e n t l y w i d e l y s e p a r a t e d so t h a t any b r o a d e n i n g e f f e c t s would be r e a d i l y observed. B i n a r y m i x t u r e s o f p r o t i c p o l a r media w i t h n o n p o l a r h y d r o c a r b o n s o r s l i g h t l y p o l a r e t h e r s have a l s o been s t u d i e d b u t t h e r e s u l t s a r e 39 d i f f e r e n t from t h e m i x t u r e s mentioned above.  Kemp e t a l .  pulse  i r r a d i a t e d b i n a r y m i x t u r e s o f methanol i n t e t r a h y d r o f u r a n (67 mole % methanol)  and c y c l o h e x a n e ( 4 % m e t h a n o l ) .  I n both mixtures the l i f e t i m e  o f t h e s o l v a t e d e l e c t r o n was unchanged w i t h r e s p e c t t o t h a t i n pure methanol.  A l t h o u g h t h e a b s o r p t i o n maximum was s h i f t e d  slightly  (% 80 nm), t h e band was n o t broadened. The maximum a b s o r b a n c e s , e x p r e s s e d as G(e ) e , were 13,900 (pure m e t h a n o l ) , 14,000 ( 3 3 % s insx t e t r a h y d r o f u r a n , 67% methanol) and 3500 ( 9 6 % c y c l o h e x a n e - 4 % i o n s (100 eV) Hi "'"cm . L  of e  g  I f one assumes t h a t the e x t i n c t i o n  methanol) coefficients  a r e n o t d r a s t i c a l l y changed i n these m i x t u r e s from t h a t i n  m e t h a n o l , then the l a t t e r v a l u e s u g g e s t s t h a t a g g r e g a t e s o f methanol m o l e c u l e s must e x i s t i n the m i x t u r e and t h a t these a g g r e g a t e s a r e c a p a b l e o f t r a p p i n g and s o l v a t i n g e l e c t r o n s formed i n the h y d r o c a r b o n component.  T h i s c o n c l u s i o n a r i s e s because  the y i e l d of s o l v a t e d e l e c t r o n s  - 50 -  was a p p r o x i m a t e l y  s i x times g r e a t e r than t h a t e x p e c t e d  mole f r a c t i o n o f m e t h a n o l i n t h e m i x t u r e .  s i m p l y from t h e  Once t r a p p e d i n the m e t h a n o l  c l u s t e r , the e l e c t r o n decays by the same p r o c e s s e s o f e l e c t r o n decay i n p u r e m e t h a n o l , hence i t s l i f e t i m e i s unchanged. observed  f o r sclvated electrons  A s i m i l a r r e s u l t was  i n m i x t u r e s o f 3-methylhexane i n  AO e t h a n o l and methanol.  Analogous i n v e s t i g a t i o n s i n e t h a n o l / n - h e x a n e  m i x t u r e s o v e r a c o n c e n t r a t i o n range o f 2-100 mole % e t h a n o l have a l s o 41 been r e p o r t e d .  No s h i f t i n the p o s i t i o n o f the a b s o r p t i o n s p e c t r u m  maximum o f t h e s o l v a t e d e l e c t r o n from t h a t i n e t h a n o l was  observed.  T h i s , t o g e t h e r w i t h t h e f a c t t h a t the e l e c t r o n had a c o n s t a n t  half-life,  was i n t e r p r e t e d t o mean t h a t t h e e l e c t r o n was t r a p p e d i n b a s i c a l l y t h e same type o f p o t e n t i a l w e l l , r e g a r d l e s s o f t h e n-hexane c o n c e n t r a t i o n . The e l e c t r o n y i e l d s i n t h e e t h a n o l / n - h e x a n e m i x t u r e s were o b s e r v e d t o be g r e a t e r than those f r e e i o n y i e l d s p r e d i c t e d ( u s i n g F i g u r e 7) f o r pure l i q u i d s w h i c h have d i e l e c t r i c c o n s t a n t s e q u a l t o t h e b u l k d i e l e c t r i c c o n s t a n t s o f the m i x t u r e s . 42 c r i t i c i z e d by Freeman  on two p o i n t s .  However t h i s comparison was F i r s t l y , the bulk  dielectric  c o n s t a n t o f t h e e t h a n o l / n - h e x a n e m i x t u r e i s l o w e r than t h e average microscopic d i e l e c t r i c constant.  S e c o n d l y , some o f t h e e l e c t r o n s and  i o n s g e n e r a t e d by t h e i o n i z a t i o n o f the h y d r o c a r b o n by c l u s t e r s o f e t h a n o l m o l e c u l e s  a r e p r o b a b l y scavenged  r e s u l t i n g i n an i n c r e a s e i n t h e  average d i e l e c t r i c c o n s t a n t i n the v i c i n i t y o f t h e geminate i o n p a i r s , t h e r e b y d e c r e a s i n g t h e p r o b a b i l i t y o f geminate 43 R e c e n t l y Dorfman e t a l . ,  recombination.  using fast infrared o p t i c a l  techniques,  were a b l e t o i n v e s t i g a t e t h e o p t i c a l p r o p e r t i e s o f t e t r a h y d r o f u r a n - w a t e r m i x t u r e s o v e r t h e complete range o f c o m p o s i t i o n s .  They o b s e r v e d  that  - 51 -  both  the peak p o s i t i o n and h a l f - w i d t h , a l t h o u g h  the two  pure components, were dominated by  i n t e r m e d i a t e between  the water d e s p i t e the  t h a t the v a l u e of the m a c r o s c o p i c d i e l e c t r i c c o n s t a n t was  dominated by  tetrahydrofuran.  of the  fact  mixtures  A s i m i l a r o b s e r v a t i o n was  observed  for  44 water-1,4 dioxane m i x t u r e s . shift  In the l a t t e r study  t h e r e was  a  gradual  of the e l e c t r o n band w i t h i n c r e a s i n g water c o n t e n t u n t i l at 34 mole  % water the peak a b s o r p t i o n corresponded  to t h a t of the h y d r a t e d e l e c t r o n .  A p o s s i b l e e x p l a n a t i o n i s t h a t these s l i g h t l y p o l a r o r g a n i c  molecules  are a b l e to d i s r u p t the water aggregates somewhat through weak d i p o l e i n t e r a c t i o n s thereby  c a u s i n g a v a r i e t y of t r a p s of d i f f e r e n t p o l a r i t y t o be  formed  a t low water c o n c e n t r a t i o n s . At h i g h e r water c o n c e n t r a t i o n s , t h e r e i s presumably a predominance of s u i t a b l e "pure w a t e r " t r a p s i n which the e l e c t r o n s are more s t a b l e and  t h e r e f o r e the o p t i c a l spectrum i s governed by  When e t h y l e n e d i a m i n e c o n c e n t r a t i o n s , both  was  the water  aggregates.  added t o t e t r a h y d r o f u r a n i n v a r i o u s  the peak p o s i t i o n and h a l f - w i d t h showed an  almost  l i n e a r dependence upon c o m p o s i t i o n analogous to the m i x t u r e s of p r o t i c 43 40 p o l a r media. Magnusson e t a l . i n v e s t i g a t e d m i x t u r e s o f 3-methyl hexane and  d i e t h y l e t h e r and  observed  an almost l i n e a r dependence o f  e l e c t r o n y i e l d on the e t h e r c o n c e n t r a t i o n .  The  spectrum showed no  w i t h c o n c e n t r a t i o n , b e i n g the same as t h a t o f the pure e t h e r . the d e t e c t i o n apparatus was  the shift  However,  too slow to observe the pure h y d r o c a r b o n  spectrum so the:,question of whether o r not ether concentrations i s uncertain.  t h e r e was  a s h i f t at  Because o f the l i n e a r i t y of  w i t h e t h e r c o n c e n t r a t i o n , the authors  suggest  low yield  t h a t the e l e c t r o n  s t a b i l i z a t i o n i s by an e l e c t r o n - e t h e r m o l e c u l e complex w i t h the e l e c t r o n b e i n g bound by a c h a r g e - d i p o l e  interaction.  -  Recently  52  -  a s t u d y on b i n a r y m i x t u r e s  s o l v e n t , formamide (D  s  w h i c h o n l y the h y d r a t e d  = 109  of a h i g h l y p o l a r a p r o t i c  a t 20°C), w i t h w a t e r was  electron  band was  reported  45  in  o b s e r v e d , i t s p o s i t i o n and  shape b e i n g u n a l t e r e d by changes i n c o m p o s i t i o n .  S i n c e no  absorption  band t h a t c o u l d be a t t r i b u t e d t o the s o l v a t e d e l e c t r o n i n formamide o b s e r v e d , i t was  s u g g e s t e d t h a t the h i g h f r e e i o n y i e l d  was  (G(free ion) =  46  3.3)  reported e a r l i e r  was  due  t o r e a c t i v e r a d i c a l a n i o n s produced by  e l e c t r o n r e a c t i o n w i t h the s o l v e n t m o l e c u l e s . of the a b s o r p t i o n band of the h y d r a t e d  The  diminished  intensity  e l e c t r o n i n these mixtures  was  r a t i o n a l i z e d on the b a s i s t h a t w a t e r a g g r e g a t e s were competing w i t h formamide m o l e c u l e s f o r the t h e r m a l i z e d e l e c t r o n . t h a t the h y d r a t e d  the  K i n e t i c d a t a showed  e l e c t r o n formed then decayed by r e a c t i o n w i t h  the  formamide m i l i e u . Thus, from the s t u d i e s t o d a t e on b i n a r y m i x t u r e s , the m a c r o s c o p i c d i e l e c t r i c c o n s t a n t determining  i s not  i t appears t h a t  the major f a c t o r i n  t h e e l e c t r o n y i e l d s and s o l v a t i o n p r o p e r t i e s ; but  rather  I t i s the b i n a r y s o l v e n t s t r u c t u r e t h a t c o n t r o l s t h e s e p r o p e r t i e s . Mixtures  i n w h i c h the pure components do n o t i n t e r a c t s t r o n g l y , such  as t h e p r o t i c p o l a r s o l v e n t s and n o n p o l a r m e d i a , the e l e c t r o n y i e l d  and  o p t i c a l p r o p e r t i e s a r e d e t e r m i n e d by the a b i l i t y of the p o l a r a g g r e g a t e s t o scavenge and may  s t a b i l i z e the e l e c t r o n s . On the o t h e r hand, components w h i c h  i n t e r a c t s t r o n g l y so t h a t the medium becomes a homogeneous  mixture,  the e l e c t r o n y i e l d and o p t i c a l c h a r a c t e r i s t i c s appear t o depend upon the m a c r o s c o p i c p r o p e r t i e s . I t s h o u l d be n o t e d t h a t the b i n a r y m i x t u r e s  mentioned above were 47  f o r s t u d i e s on the l i q u i d s t a t e .  Binary mixtures  of g l a s s y  alcohols  -  and w a t e r - a l c o h o l s  53  -  have been i n v e s t i g a t e d . They showed s i m i l a r  character-  i s t i c s t o t h e i r l i q u i d c o u n t e r p a r t s ; t h a t i s , e l e c t r o n a b s o r p t i o n maxima v a r y c o n t i n u o u s l y w i t h change i n c o m p o s i t i o n ,  s h i f t i n g towards t h e  s h o r t e r w a v e l e n g t h s w i t h i n c r e a s i n g c o n c e n t r a t i o n of the more p o l a r component.  Support f o r the f o r m a t i o n of p o l a r a g g r e g a t e s i n n o n p o l a r  media comes from the o b s e r v a t i o n t h a t y - i r r a d i a t e d g l a s s y m i x t u r e s  of  47  n - p r o p a n o l w i t h 3-methylpentane to  trapped  y i e l d two a b s o r p t i o n bands a t t r i b u t a b l e  e l e c t r o n s , one c h a r a c t e r i s t i c o f each p u r e component.  S i m i l a r l y , y - i r r a d i a t e d glassy mixtures  of methyltetrahydrofuran  and  e t h a n o l show two a b s o r p t i o n p e a k s , one a t t r i b u t a b l e t o m e t h y l t e t r a 49  hydrofuran  and t h e o t h e r t o e t h a n o l .  E.  SCOPE OF STUDY  The d i e l e c t r i c c o n s t a n t has been used e x t e n s i v e l y i n a t t e m p t s a t e x p l a n a t i o n s o f v a r i o u s types o f s o l v e n t e f f e c t s .  I n the f i e l d o f  r a d i a t i o n c h e m i s t r y b o t h t h e y i e l d o f f r e e i o n s and the s t a b i l i z a t i o n o f e l e c t r o n s a r e c o n s i d e r e d as b e i n g dominated by t h e d i e l e c t r i c constant.  However t h i s parameter i s a m a c r o s c o p i c p r o p e r t y o f t h e  medium and flie e l e c t r o s t a t i c s i t u a t i o n i n t h e v i c i n i t y o f any p a r t i c u l a r s o l v e n t m o l e c u l e o r " c l u s t e r " may be q u i t e d i f f e r e n t from t h i s average value.  I n g e n e r a l t h e r e i s a d i r e c t c o r r e s p o n d e n c e between t h e  s o l v a t i o n energy of s t a b i l i z e d e l e c t r o n s , as c h a r a c t e r i z e d by t h e i r o p t i c a l a b s o r p t i o n band maxima, and the s t a t i c d i e l e c t r i c c o n s t a n t the medium.  of  Dorfman showed t h a t f o r a homologous s e r i e s o f a l i p h a t i c  - 54 -  a l c o h o l s , t h e energy c o r r e s p o n d i n g t o t h e a b s o r p t i o n smoothly w i t h an i n c r e a s e  maximum  i n dielectric c o n s t a n t . A  increases  similar correlation  was found by E k s t r o m and W i l l a r d f o r a s e r i e s o f o r g a n i c g l a s s e s a t 47 77°K, r a n g i n g from 3-methylpentane ( D = 2.0) t o g l y c e r o l ( D = 42.5). g  g  However, n e i t h e r w a t e r n o r ammonia obey t h i s e m p i r i c a l r e l a t i o n s h i p (see T a b l e I ) s u g g e s t i n g t h a t o t h e r f a c t o r s may be i n v o l v e d . p r o p e r t y i s t h e " s o l v a t i n g power" o f t h e medium. studied  F o r t h e systems  t o d a t e , those media w i t h h i g h d i e l e c t r i c c o n s t a n t s a r e a l s o  p r o t i c solvents bonding.  and hence r e a d i l y s o l v a t e i o n i c s p e c i e s  by hydrogen  These s o l v e n t s , s u c h as w a t e r and t h e a l c o h o l s , a r e  characterized  by h i g h d i e l e c t r i c c o n s t a n t s , e x c e l l e n t i o n i c s o l v a t i n g  a b i l i t y , h i g h f r e e i o n y i e l d s and a b s o r p t i o n r e g i o n o f the spectrum. and  One such  maxima i n the v i s i b l e  On t h e o t h e r hand, a p r o t i c n o n p o l a r h y d r o c a r b o n s  s l i g h t l y p o l a r ethers are characterized  by l o w s t a t i c  dielectric  c o n s t a n t s , poor i o n i c s o l v a t i n g a b i l i t y , l o w f r e e i o n y i e l d s and absorption studies  maxima i n t h e i n f r a r e d r e g i o n o f t h e spectrum.  As a r e s u l t ,  on t h e s e media a r e n o t i n d i c a t i v e o f whether o r n o t t h e  medium's s o l v a t i n g power i s a d e t e r m i n i n g f a c t o r i n e l e c t r o n  stabilization.  The d i e l e c t r i c c o n s t a n t i n f l u e n c e s t h e s t a b i l i t y o f an i o n , b u t t h e i n f l u e n c e i s o n l y i m p o r t a n t when t h e d i e l e c t r i c c o n s t a n t i s s m a l l . I n media o f d i e l e c t r i c c o n s t a n t l e s s than 10 o r so t h e e f f e c t o f t h e d i e l e c t r i c c o n s t a n t s h o u l d be a t l e a s t comparable t o t h a t o f i t s s p e c i f i c s o l v a t i n g power. greater  However, i n media of d i e l e c t r i c  constants  t h a n 30 o r so t h e e f f e c t o f t h e d i e l e c t r i c c o n s t a n t i s o f 51 52  m i n o r i m p o r t a n c e compared w i t h t h e s p e c i f i c s o l v a t i n g a c t i o n . t h i s r e a s o n DMSO seemed p a r t i c u l a r l y s u i t e d t o such a s t u d y .  '  For  DMSO i s a  - 55 -  p o l a r a p r o t i c solvent which i s widely  used i n o r g a n i c  and i n o r g a n i c  c h e m i s t r y because o f i t s i n a b i l i t y t o s o l v a t e n e g a t i v e i o n s ; i n d e e d bases appear e x c e p t i o n a l l y s t r o n g i n t h i s medium.  However DMSO has a  f a i r l y h i g h s t a t i c d i e l e c t r i c c o n s t a n t ( D = 48 a t 20°C) and l a r g e g  d i p o l e moment (4.3 D ) . ^  T h e r e f o r e , i f the s t a t i c d i e l e c t r i c  of the continuum i s the i m p o r t a n t c r i t e r i u m i n d e t e r m i n i n g  electron  s t a b i l i t y , then one would e x p e c t the s o l v a t i o n energy t o be between t h a t o f the p o l a r a l c o h o l s and w a t e r ; t h a t i s , the absorption On  constant  intermediate electron  band maximum s h o u l d be i n the v i s i b l e r e g i o n o f the  spectrum.  the o t h e r hand, i f the s o l v a t i n g power o f the medium i s the  f a c t o r , as g i v e n by i t s s p e c i f i c i n t e r a c t i o n s , then the  dominant  stabilized  e l e c t r o n i n DMSO s h o u l d resemble the a p r o t i c h y d r o c a r b o n s .  In addition,  c o m p a r i s o n o f the f r e e i o n y i e l d w i t h the e m p i r i c a l c o r r e l a t i o n shown i n Figure  7 s h o u l d s i m i l a r l y i n d i c a t e whether o r n o t  the  static  d i e l e c t r i c has a commanding i n f l u e n c e on the y i e l d i n DMSO. M i x t u r e s o f DMSO and w a t e r were a l s o i n v e s t i g a t e d i n o r d e r t o d e t e r m i n e whether o r not  the f r e e i o n y i e l d and o p t i c a l p r o p e r t i e s o f  the s t a b i l i z e d e l e c t r o n s are dependent upon the b u l k o r l o c a l o f the d i e l e c t r i c medium. a l l proportions,  properties  DMSO and w a t e r are c o m p l e t e l y m i s c i b l e i n  consequently aggregate e f f e c t s which hinder  studies  on o t h e r p r o t i c - a p r o t i c m i x t u r e s s h o u l d be absent i n t h i s system. I n a d d i t i o n , s t u d i e s were made on t r a p p e d e l e c t r o n s and o t h e r species  i n the s o l i d s t a t e a t 77°K on y - i r r a d i a t e d p o l y c r y s t a l l i n e DMSO  as w e l l as p o l y c r y s t a l l i n e and g l a s s y s o l i d s o f DMSO-water mixtures.  Such studies were o f i n t e r e s t i n t h e i r own r i g h t but  c o r r e l a t i o n w i t h the l i q u i d phase work.  also for  TABLE I.  Optical data f o r electrons s t a b i l i z e d i n l i q u i d media at room temperature.  Medium  Amax (nm)  E  (eV)  x  G(e;>  f  3  max  , -1 -1. e (M cm ) max  AW (eV) 1/2  s  0.65  18,500  0.92  78  —  —  1.5  43  1.2  0.68  14,000  1.35  39  1.97  1.1  0.78  17,000  1.29  33  700  1.77  1.0  0.87  15,000  1.55  25  n-propanol  740  1.67  1.0  0.59  13,000  isopropanol  820  1.51  1.0  0.67  14,000  1.22  19  n-butanol  680  1.82  —  —  —  1.47  18  ethylenediamine  1350  0.92  —  —  20,000  0.88  —  ammonia  1550  0.80  0.45  0.77  49,000  0.46  22  tetrahydrofuran  2100+50  0.59  —  —  14,000  d ime thoxye th ane  1900+150  0.65  —  —  diethylether  2050+150  0.60  0.19  —  7,500  diethylamine  1900+80  0.65  —  — —  10,000  water  720  1.72  2.7  glycerol  528  2.35  —  ethylene glycol  580  2.16  methanol  630  ethanol  dioxane  >1100  <1.1  0.04,0.10  21  d  — d  —  C  —  7.4  —  7.2  —  4.3  — —  3.6 2.2  TABLE I  (continued)  Medium  X (nm) max  E, (eV) X max  G(e~) s  f  a  e (M" cm ) max 1  _1  AW. ,.(eV) 1/2  D s  b  n-hexane  >1500  <0.80  0.10  —  >10,000  ~  1.9  methylcyclohexane  >1500  <0.80  —  —  >10,000  --  2.0  Data taken from r e f e r e n c e s l i e , 2 9 , 3 7 , 3 8 , 4 3 , 4 4 , 1 0 2 , 1 0 3  Oscillator b  s t r e n g t h o f the s o l v a t e d e l e c t r o n band.  S t a t i c d i e l e c t r i c c o n s t a n t a t 20°C.  C  -33°C.  ^  U n c e r t a i n t y may be as much as 5 0 % .  - 58 -  CHAPTER I I EXPERIMENTAL A.  1.  6  ° C o Y-RAMOLYSIS  Materials DMSO (Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade) was  s t i r r e d i n a c l o s e d v e s s e l over calcium h y d r i d e  f o r a t l e a s t two days  i n a n i t r o g e n d r y box.  I t was t h e n d i s t i l l e d under reduced  (> 1 t o r r a t 48-50°C).  Only the m i d d l e 50% was c o l l e c t e d and s t o r e d  o v e r a l a y e r o f L i n d e 4A m o l e c u l a r atmosphere o f pure n i t r o g e n .  pressure  s i e v e s and s e a l e d i n an  P r i o r t o u s e , a p o r t i o n o f i t was  r e d i s t i l l e d and t h e samples i m m e d i a t e l y p r e p a r e d .  A n a l y s i s by gas  chromatography showed t h a t t h e w a t e r c o n t e n t was l e s s than 0.001% by volume.  The o n l y o t h e r i m p u r i t y d e t e c t e d was a t r a c e (> 0.005%  by volume) o f d i m e t h y l  sulfide.  T r i p l y d i s t i l l e d w a t e r was p r e p a r e d by f i r s t l y r e f l u x i n g an a c i d i c d i c h r o m a t e s o l u t i o n made w i t h s i n g l y d i s t i l l e d w a t e r .  The  d i s t i l l a t e was y - i r r a d i a t e d w i t h a dose o f ^ 0.5 Mrad t o remove any  trace impurities.  distillation  I t was then p l a c e d under c o n t i n u o u s r e f l u x  from a l k a l i n e permanganate u n t i l  Nitrous oxide, obtained distillation  required.  from Matheson, was p u r i f i e d by " t r a p - t o - t r a p "  i n a h i g h vacuum system t o remove any t r a c e s o f n i t r o g e n  - 59 -  and oxygen.  I t was  vacuum l i n e u n t i l the l i q u i d  s u b s e q u e n t l y s t o r e d i n a f i v e l i t r e f l a s k on  required.  the  The argon and h e l i u m used f o r d e g a s s i n g  samples and as a c a r r i e r gas f o r the gas  chromatographs  were o b t a i n e d from Canada L i q u i d A i r . A l l o t h e r c h e m i c a l s used i n t h i s s t u d y were a n a l y t i c a l grade o r b e t t e r and were n o t p u r i f i e d  further.  A l l g l a s s w a r e used i n the e x p e r i m e n t s was  s c r u p u l o u s l y c l e a n e d by  w a s h i n g i n permanganic a c i d f o l l o w e d by r i n s i n g w i t h d i s t i l l e d  water  and a s o l u t i o n of c o n c e n t r a t e d hydrogen p e r o x i d e and n i t r i c a c i d t o remove any r e s i d u a l MnC^ was  on the g l a s s s u r f a c e .  F i n a l l y the  glassware  t h o r o u g h l y r i n s e d w i t h s i n g l y d i s t i l l e d and t h e n t r i p l y  distilled  w a t e r b e f o r e d r y i n g i n the oven a t 250°C. i r r a d i a t i o n sample c e l l s , stoppers  was  first  I n the case of the  the e x c e s s grease from the s t o p c o c k s removed w i t h hexane and t h e c e l l was  and then  r i n s e d w i t h hexane and w a t e r b e f o r e p u t t i n g them i n the a c i d b a t h . G e n e r a l l y , however, the sample c e l l s were annealed  i n the g l a s s b l o w e r s  1  oven p r i o r t o w a s h i n g i n the a c i d because some o f t h e r a d i a t i o n p r o d u c t s c o u l d not be removed u s i n g the above p r o c e d u r e .  2.  R a d i a t i o n Source The- r a d i a t i o n s o u r c e was  Canada Gammacell  3.  a 4000 c u r i e ^ C o  A t o m i c Energy of  220.  Dosimetry In  o r d e r t o c a l c u l a t e G v a l u e s i t i s n e c e s s a r y t o know the  r a d i a t i o n dose.  The u n i t o f absorbed  absorbed  dose i s c a l l e d the r a d and i s e q u a l  - 60 -  13 to  100 ergs p e r gram o r 6.24  absorbed  -1  x 10  eV gm  O f t e n one r e f e r s t o the  dose r a t e o f the s o u r c e w h i c h i s the absorbed  dose p e r  unit  time. S i n c e the p r i m a r y energy a b s o r p t i o n by the Compton p r o c e s s depends o n l y on the e l e c t r o n d e n s i t y o f the medium, the energy absorbed  by a sample can be d e t e r m i n e d  from a s t a n d a r d s i m p l y by  comparing the e l e c t r o n d e n s i t i e s o f the two samples as g i v e n by relationship R  the  (2.1)  . sample  -  where R i s ftie absorbed atomic weight  R „. x std.  (Z/A)  P (Z/A) , std. s  a  m  . l  (2.1)  e  dose r a t e , Z the a t o m i c number and A the gram  o f the medium c o n s i d e r e d .  The  chemical standard, or  d o s i m e t e r , most commonly u s e d , and the one used i n t h i s s t u d y , i s the F r i c k e d o s i m e t e r w h i c h measures the o x i d a t i o n o f f e r r o u s i o n s t o f e r r i c ions. The F r i c k e s o l u t i o n was  p r e p a r e d by d i s s o l v i n g 0.4  grams o f  F e ( N H ^ ) ( S 0 ) * 6 H 0 , 0.060 grams N a C l and 22 ml c o n c e n t r a t e d 2  4  2  2  (95-98%)  H S 0 ^ i n s u f f i c i e n t t r i p l y d i s t i l l e d w a t e r t o make 1 l i t r e o f s o l u t i o n . 2  The  s o l u t i o n was  s u l f a t e , 0.001  then 0.001  M w i t h r e s p e c t t o f e r r o u s ammonium  M w i t h r e s p e c t t o sodium c h l o r i d e and 0.4  to s u l f u r i c a c i d .  M with respect  A l l the r e a g e n t s used were a n a l y t i c a l grade o r b e t t e r .  S i n c e the r a d i a t i o n f i e l d i n the Gammacell 220 chamber i s not  uniform,  i r r a d i a t i o n s o f the F r i c k e s o l u t i o n s were done i n the same c e l l s p l a c e d i n the same p o s i t i o n i n the c a v i t y as the DMSO samples. When w a t e r i s i r r a d i a t e d , the f o l l o w i n g p r o c e s s  occurs:  and  - 61 -  I n t h e p r e s e n c e o f 0.8 N a c i d , a i r (oxygen) and f e r r o u s i o n s  e~ aq  +  H -  0  +  H0 2  H 0 2  •OH  so t h a t  H  +  Fe  2 +  +  Fe  + 2  Fe  2 +  +  (2)  *-  (3)  2  +  2  >• H-  H C y  +  H  +  +  H  +  +  H  +  *- F e  + 3  *• F e  3 +  Fe  3 +  +  H 0  +  «0H  +  H 0  2  (4)  2  +  H 0 (5) 2  (6)  2  G ( E e ) = 3[G(e ) + G(H•)] + G(-OH) + 2G(H.0 ) aq 11 3 +  o  (2.2)  w h i c h e q u a l s 15.5. T h i s v a l u e has been d e t e r m i n e d on an a b s o l u t e b a s i s 53 by m e a s u r i n g by c a l o r i m e t r y t h e energy absorbed by t h e F r i c k e s o l u t i o n . Any f a c t o r w h i c h a l t e r s t h e m o l e c u l a r o r r a d i c a l y i e l d i n w a t e r 3+ w i l l a l t e r t h e y i e l d o f Fe dosimeter  as seen by e q u a t i o n  (2.2).  i s very s e n s i t i v e to organic i m p u r i t i e s .  The F r i c k e  Oxidation of the  o r g a n i c i m p u r i t y by 'OH r a d i c a l s and subsequent r e a c t i o n o f t h e o r g a n i c r a d i c a l w i t h oxygen v i a r e a c t i o n s (7) and (8) produces an o r g a n i c p e r o x i d e , R0_•.  - 62  *0H  R*  +  +  0  RH  R-  +  2  R0 -  -  H0  (7)  2  (8)  2  The o r g a n i c p e r o x i d e then r e a c t s w i t h the f e r r o u s i o n i n an manner t o H0 ',  thus i n c r e a s i n g  2  analogous  the y i e l d o f the f e r r i c i o n .  3+ As a r e s u l t , G(Fe impurities.  ) > 15.5  i n the p r e s e n c e o f some o x i d i z a b l e o r g a n i c  To s u p p r e s s r e a c t i o n (7) t h e c h l o r i d e i o n was  the d o s i m e t e r s o l u t i o n .  added t o  I n i t s p r e s e n c e , the h y d r o x y l r a d i c a l i s  r e a d i l y reduced and the c h l o r i n e atom then o x i d i z e s the f e r r o u s i o n so 3+ t h a t the o v e r a l l y i e l d o f G(Fe ) i s the same.  The  •OH  +  Cl"  Cl-  +  Fe  *~  OH  Fe  2 +  c o n c e n t r a t i o n of f e r r i c  p h o t o m e t r i c a l l y on a Cary IA  +  3 +  Cl-  +  Cl  (9)  (10)  i o n s produced was measured s p e c t r o -  spectrophotometer using  F r i c k e sample as the b l a n k and r e a d i n g the absorbance  an u n i r r a d i a t e d a t 30A nm.  The  3+ l i n e a r i t y o f t h e absorbance shown i n F i g u r e 8 .  o f Fe  v e r s u s the i r r a d i a t i o n time i s  The i r r a d i a t i o n time i s t h a t o f the a u t o m a t i c  t i m e r on the Gammacell.  The p o s i t i v e i n t e r c e p t i s due t o the f a c t  t h a t t h e m i c r o - s w i t c h w h i c h a c t i v a t e s the a u t o m a t i c t i m e r i s engaged o n l y when the Gammacell drawer reaches i t s f u l l y lowered p o s i t i o n . C o n s e q u e n t l y , the samples were exposed  t o the r a d i a t i o n f i e l d f o r a  s h o r t p e r i o d d u r i n g the r a i s i n g and l o w e r i n g o f the drawer w h i c h was  not  IRRADIATION TIME ( m i n u t e s ) Figure  8.  Fricke dosimeter r e s u l t s used i n t h i s  study.  obtained from the r a d i o l y s i s  of  the s o l u t i o n s  in  the i r r a d i a t i o n  cell  - 64 -  a c c o u n t e d f o r by the  timer.  T h i s s m a l l c o r r e c t i o n f a c t o r was t a k e n  i n t o a c c o u n t i n subsequent dose c a l c u l a t i o n s and i s e q u i v a l e n t 5 sec e x p o s u r e w i t h  the t i m e r  to a  operating.  From the s l o p e o f the g r a p h , the absorbed dose r a t e , R, o f t h e 53 F r i c k e s o l u t i o n was c a l c u l a t e d from the  following  0.965 x 1 0 x (AO.D./At) TT. „ 3-K 304 ^ ' 9  R_ . , Fricke  =  £  X  X  P  relationship  . -1 r a d s mm  ,„ (2.3)  X  3+ where e_„. i s the m o l a r e x t i n c t i o n c o e f f i c i e n t . o f Fe a t 304 nm 304 M ''"cm ) , £ i s the p a t h l e n g t h L  o f the c e l l  (2174  (1 cm) , p i s the d e n s i t y o f 3+  the F r i c k e s o l u t i o n (1.024 + 0.001 15.5.  between 15 and 25°C) and G(Fe  )=  A l l i r r a d i a t i o n s on the d o s i m e t r y s o l u t i o n s and subsequent DMSO  samples were c o n d u c t e d a t 23 + 2°C.  The dose r a t e c o r r e s p o n d i n g t o  F i g u r e 8 was found t o be 5500 r a d s m i n u t e . ±  Since ( Z / A ) f o r the  F r i c k e s o l u t i o n i s 0.553 and ( Z / A ) f o r DMSO i s 0.538, the dose r a t e for  DMSO i s 0.971 t h a t o f the F r i c k e s o l u t i o n f o r DMSO i n the same c e l l  and  i n the same p o s i t i o n i n the Gammacell chamber. A f u r t h e r c o r r e c t i o n must be made f o r the r a d i o a c t i v e  decay o f  The a c t i v i t y , A^, a f t e r a p e r i o d o f decay, t , i s r e l a t e d t o 53 the o r i g i n a l a c t i v i t y A by the e x p r e s s i o n the ^ C o .  q  A^  t  =  A  o  e"  where X i s the decay c o n s t a n t (X = 0.693/T^^2^ of ^ C o ,  5.27 y e a r s .  (2.4)  X t  T  i/2  i  s  t  *  ie  half-life  As a r e s u l t , the dose r a t e , R^, a t day t a f t e r  the d o s i m e t r y was p e r f o r m e d , R , was o b t a i n e d from r e l a t i o n s h i p  (2.5).  - 65 -  =  R  t  R  E  "(0-693  x  t/1925  days)  o 54  A computer program was w r i t t e n  t o g i v e the dose r a t e and the absorbed  dose for a g i v e n i r r a d i a t i o n time f o r a p a r t i c u l a r l i q u i d on any g i v e n day. 4.  Sample P r e p a r a t i o n Two  d i f f e r e n t t y p e s o f p y r e x g l a s s sample c e l l s were u s e d ,  depending upon whether l i q u i d o r gaseous p r o d u c t s were t o be a n a l y z e d . S c h e m a t i c diagrams o f the sample c e l l s a r e shown i n F i g u r e 9 . F o r gas a n a l y s i s , the DMSO was d i s t i l l e d d i r e c t l y i n t o t h e p r e weighed sample c e l l  ( F i g u r e 9 ( a ) ) through the B 7 s o c k e t u s i n g a  modified Perkin triangle.  A f t e r about  20-25  m l o f DMSO had been  c o l l e c t e d , the c e l l was q u i c k l y removed and s t o p p e r e d .  The B 7 cone  and s o c k e t j o i n t were g r e a s e d s p a r i n g l y w i t h h i g h vacuum A p i e z o n N g r e a s e and h e l d t o g e t h e r w i t h two s t a i n l e s s s t e e l s p r i n g s . four-way  The  s t o p c o c k was s i m i l a r l y g r e a s e d w i t h A p i e z o n N and was h e l d i n  p l a c e w i t h an aluminum s t o p c o c k r e t a i n e r .  The s t o p c o c k r e t a i n e r and  s p r i n g s were r e q u i r e d because the c e l l was p r e s s u r i z e d d u r i n g t h e a n a l y s i s procedure.  The c e l l was then reweighed on a beam b a l a n c e t o  t h e n e a r e s t 0 . 0 1 gram and t h e sample w e i g h t determined.  T h i s was  n e c e s s a r y i n o r d e r t o c a l c u l a t e the t o t a l absorbed dose and hence G values.  F o l l o w i n g t h i s w e i g h i n g and g r e a s i n g , the sample was  deoxygenated  by f l u s h i n g w i t h argon f o r 3 0 minutes a f t e r w h i c h t h e  c e l l s t o p c o c k was t u r n e d about 4 5 ° , t h e r e b y s e a l i n g the c e l l under an argon atmosphere.  The s t o p c o c k was o n l y t u r n e d 4 5 ° so t h a t the sample  (a) Irradiation cell for gas products F i g u r e 9.  (b) Irradiation cell for liquid products  P y r e x i r r a d i a t i o n c e l l s used f o r deoxygenation o f the l i q u i d  samples.  - 67 -  would remain above the f r i t t e d glass disk.  I f i t was turned 90°,  the sample tended to flow through the sintered disk and f i l l the opposite sidearm of the c e l l .  Since the dosimetry had been done with  the s o l u t i o n above the disk, the actual absorbed dose would be d i f f e r e n t . A f t e r deoxygenation, the sample was i r r a d i a t e d immediately or else attached to the vacuum l i n e f o r degassing and addition of nitrous oxide p r i o r to i r r a d i a t i o n .  DMSO samples containing s o l i d and l i q u i d  scavengers were prepared by weighing a given quantity of the scavenger i n t o a volumetric f l a s k and immediately made up to the required volume with freshly d i s t i l l e d  DMSO. The sample was then added to the pre-veighed  through the B7 cone, a f t e r which the c e l l was greased, reweighed  cell  and degassed.  For nitrous oxide studies, the deoxygenated DMSO samples were attached to the vacuum l i n e i l l u s t r a t e d i n Figure 10 and degassed. The S13 b a l l j o i n t s of the c e l l were greased and then connected to the two S13 sockets of the vacuum l i n e .  The b a l l and sockets were  held together by metal c l i p s i n order to produce a hard vacuum. of  Each  the connections was i s o l a t e d from the vacuum l i n e by small stopcocks, and S^. A t h i r d stopcock, S^, further separated the c e l l and these  external connections from the main vacuum manifold.  With the four-way  stopcock turned i n i t i a l l y at 45°, the vacuum l i n e up to c e l l bore was pumped to a hard vacuum, t y p i c a l l y 10 ^ t o r r , using a three-stage mercury d i f f u s i o n pump backed by a rotary o i l pump. S  2  Then, with stopcock  closed and S^ p a r t i a l l y closed, the four-way stopcock of the c e l l was  gradually rotated u n t i l the argon inside the c e l l started to bubble out through S^. I t was necessary to turn the stopcock slowly because opening the sample to the vacuum l i n e too quickly resulted i n excessive  to mercury diffusion pump  5 litre n i t r o u s oxide s t o r a g e flask  U y  A  xcell  ure  10.  mercury manometer  Schematic diagram of vacuum line used for degassing the l i q u i d samples and adding nitrous oxide to the samples.  - 69 -  bubbling  and l o s s o f sample by s p l a s h i n g i n t o the c o n n e c t o r s .  the b u b b l i n g  had s u b s i d e d ,  Once  was s l o w l y opened and t r a c e s o f gas  r e m a i n i n g on t h a t s i d e o f the s i n t e r e d d i s k were removed. down t o a good vacuum, s t o p c o c k s S^,  and  were  A f t e r pumping  closed.  N i t r o u s o x i d e , w h i c h had been p r e v i o u s l y degassed and t r a p p e d out i n b u l b T u s i n g  l i q u i d n i t r o g e n , was s l o w l y v a p o u r i z e d i n t o t h e  e v a c u a t e d l i n e between s t o p c o c k s S^, oxide pressure,  and  From the i n i t i a l  nitrous  as r e a d from the mercury manometer, and from t h e  p r e v i o u s l y d e t e r m i n e d volume o f the l i n e between the s t o p c o c k s S^, and  S^, t h e i n i t i a l number o f moles o f n i t r o u s o x i d e c o u l d be o b t a i n e d  from the i d e a l gas law, n = PV/RT. n i t r o u s oxide allowed  Then  was s l o w l y opened and t h e  t o b u b b l e u n t i l e q u i l i b r i u m was e s t a b l i s h e d ,  u s u a l l y about 30 m i n u t e s .  was t h e n opened, the four-way s t o p c o c k  t u r n e d 45° and the f i n a l p r e s s u r e r e a d .  Knowing the volume o f t h e  DMSO sample i n the c e l l , t h e volume o f the u n f i l l e d c e l l and t h e volume between the b o r e and s t o p c o c k s S^ and S^, one c o u l d the amount o f n i t r o u s o x i d e d i s s o l v e d i n the sample. n i t r o u s oxide concentration shown i n F i g u r e  The v a r i a t i o n o f  w i t h the e q u i l i b r i u m p a r t i a l p r e s s u r e i s  11 f o r two s i m i l a r c e l l s .  One c e l l  p o r o s i t y s i n t e r e d d i s k whereas the second c e l l disk.  calculate  (A) had a medium  (B) had a f i n e p o r o s i t y  I t can be seen t h a t n e i t h e r c e l l g i v e s a " z e r o "  a l t h o u g h b o t h have the same s l o p e .  intercept  The " n o n - z e r o " i n t e r c e p t i s  b e l i e v e d t o be an e m p i r i c a l a r t i f a c t o f the system, p r o b a b l y  arising  because o f an " e f f e c t i v e " b a c k - p r e s s u r e due t o the p r e s e n c e o f t h e sintered disk.  From the i n v e r s e o f the s l o p e , the s o l u b i l i t y -4  for  n i t r o u s o x i d e i n DMSO was found t o be 1.10 + 0.05 x 10  a t 23°C.  factor -1 M torr  [N 0] 2  F i g u r e 11.  (M)  P l o t showing the r e l a t i o n s h i p of the p a r t i a l p r e s s u r e of n i t r o u s 23°C f o r the two bubbler c e l l s c o n t a i n i n g  o x i d e to i t s s o l u b i l i t y  a medium ( c e l l A) and f i n e  ( c e l l B) p o r o s i t y  i n DMSO a t sintered  disk.  - 71 -  The sample c e l l u s e d . f o r l i q u i d p r o d u c t a n a l y s i s i s shown i n Figure 9(b).  T h i s c e l l c o n s i s t e d o f a t h r e e - n e c k e d , 50 m l f l a s k w i t h  BIO s o c k e t s .  A s i n t e r e d g l a s s b u b b l e r was a t t a c h e d t o the c e n t r e neck  by means o f a BIO cone.  The 25 m l sample was p i p e t t e d i n t o t h e f l a s k  t h r o u g h a s i d e arm w h i c h was then s e a l e d w i t h a r u b b e r septum.  The  sample was deoxygenated by f l u s h i n g w i t h h e l i u m and then s e a l e d under a s m a l l excess pressure o f helium.  5.  Product Analysis F o l l o w i n g i r r a d i a t i o n t h e sample c e l l f o r gaseous a n a l y s i s was  a t t a c h e d to t h e e x t e r n a l l o o p o f t h e gas chromatograph v i a S13 s o c k e t s . A s c h e m a t i c diagram o f t h e e x p e r i m e n t a l s e t u p i s shown i n F i g u r e 12. A t t a c h e d t o t h e e x t e r n a l l o o p were the sample c e l l , an 18 x 1/8 i n c h Porapak Q "pre-column" and an " o n - l i n e " sample l o o p .  The Porapak Q  " p r e - c o l u m n " was used t o p r e v e n t DMSO v a p o u r s from e n t e r i n g the gas chromatograph system.  A f t e r each e x p e r i m e n t , the "pre-column" was  b a c k - f l u s h e d f o r about 30 m i n u t e s t o remove the c o l l e c t e d v a p o u r . The " o n - l i n e " sample l o o p c o n t a i n e d a four-way s t o p c o c k so t h a t t h e l o o p c o u l d be b y p a s s e d . I n i t i a l l y , t h e four-way s t o p c o c k o f t h e sample c e l l was t u r n e d so t h a t t h e e x t e r i o r l o o p o f the gas chromatograph and i t s a t t a c h m e n t s , i n c l u d i n g t h e o u t s i d e b o r e o f t h e sample c e l l , c o u l d be f l u s h e d o f a i r . Once the a i r had e l u t e d , the s t o p c o c k was r o t a t e d by 90° and t h e gases were f l u s h e d i n t o t h e chromatograph.  The chromatograph used was a  V a r i a n A e r o g r a p h S e r i e s 1700 c o n t a i n i n g d u a l 20 f t . x 1/4 i n c h s t e e l 13X m o l e c u l a r s i e v e columns.  stainless  D e t e c t i o n was made u s i n g WX t h e r m a l  c o n d u c t i v i t y d e t e c t o r s m a i n t a i n e d atl25°C a t a f i l a m e n t c u r r e n t o f  - 72  F i g u r e 12.  -  Schematic diagram of a p p a r a t u s used f o r f l u s h i n g the v o l a t i l e gaseous p r o d u c t s i n t o the gas  chromatograph.  -  73 -  100 ma and the s i g n a l registered on a Westronics, v a r i a b l e speed chart recorder.  Using a column temperature of 55°C and an argon  flow rate of 30 ml a minute. H_, 0 , N„ and CH. were eluted within 30 2 2 2 4 o  minutes a f t e r the s t a r t of f l u s h i n g .  Ethane took much longer,  consequently i t was measured by temperature programming.  A f t e r the  other gas products were detected the column was heated to 130°C at a rate of 20°C per minute.  With a flow rate of 30 ml a minute, i t was  found that over 95% of these v o l a t i l e gases were extracted i n the f i r s t 3 minutes so that t a i l i n g of the peaks was very small. the  However,  bubbling was continued throughout the analysis (except f o r ethane)  to avoid a pressure change and r e s u l t i n g base l i n e d r i f t on terminating the  bubbling.  At the s t a r t of the temperature programming  the four-way  stopcock of the sample c e l l was rotated by 45°, the exterior loop of the  gas chromatograph bypassed, and the Porapak Q "pre-column" back-  flushed.  This allowed another sample to be prepared while C^H^  was being eluted (^ 45 minutes). The "on-line" sample loop was used for monitoring the v a r i a t i o n of the  the detector s e n s i t i v i t y from day to day. During the e l u t i o n of sample gases the sample loop was i n the bypass p o s i t i o n .  After  CH^ had been detected the four-way stopcock of the sample loop was rotated by 90° and the standard gas sample, which was N into the column.  After the ^  2 >  was flushed  standard had been detected, the chromato-  graph was temperature programmed f o r ethane as previously described. The l i n e a r i t y of the detector response and s e n s i t i v i t y towards the various gases were established by i n j e c t i n g known quantities of sample gases using a second sample loop i n place of the i r r a d i a t i o n c e l l .  This  - 74 -  s e c o n d a r y l o o p was f i l l e d w i t h the r e q u i r e d amount o f gas on t h e vacuum l i n e , i t s amount b e i n g measured u s i n g a McLeod gauge.  The  r e s p o n s e o f the d e t e c t o r was l i n e a r f o r a l l the gases o v e r the range studied.  Under the c o n d i t i o n s u s e d , the s e n s i t i v i t y o r r e s p o n s e  2 f a c t o r s (cm /ymole gas) and  12.7,  respectively.  triangulation. The  f o r H„, N„, CH. and C„H, were 59.5, I £. 4 Z D  4.85, 15.2,  The peak a r e a s were measured by manual  A t y p i c a l chromatogram  i s shown i n F i g u r e 13.  l i q u i d p r o d u c t s were a n a l y z e d by f i r s t l y  i n s e r t i n g the n e e d l e  o f a l i q u i d s y r i n g e t h r o u g h the r u b b e r septum o f the sample c e l l t o e x t r a c t a known volume o f i r r a d i a t e d l i q u i d  (25 y£) and t h e n i n j e c t i n g  t h i s sample i n t o a V a r i a n A e r o g r a p h A-90-P2 gas chromatograph. chromatograph used a Porapak Q column m a i n t a i n e d was  Detection  made u s i n g WX t h e r m a l c o n d u c t i v i t y d e t e c t o r s a t 215°C c o u p l e d t o  a Leeds and N o r t h r u p Speedomax c h a r t r e c o r d e r . was  a t 215°C.  This  maintained  a t 170 ma and the h e l i u m  o f 50 m l p e r m i n u t e .  The f i l a m e n t c u r r e n t  c a r r i e r gas h a d a f l o w r a t e 2  The s e n s i t i v i t y f a c t o r s (cm /ymole) f o r w a t e r  and d i m e t h y l s u l f i d e were 17 and 8 r e s p e c t i v e l y . A r e p r e s e n t a t i v e chromatogram i s shown i n F i g u r e 14.  Products  eluting at retention  times g r e a t e r than t h a t f o r DMSO were not o b s e r v e d . decomposition  No t h e r m a l  o f DMSO o c c u r r e d d e s p i t e the h i g h column and d e t e c t o r  t e m p e r a t u r e s used.  argon flow rate 3 0 mls/min column temperature 55°C detector temperature 125°C '2 filament current 100 ma  I Figure  13.  I  0  1  I  20  T y p i c a l chromatograph o b t a i n e d  .  I  1  40 TIME (minutes)  1—//J 1 -L— 60 " 1 0 0 1 2 0  f o r 20 ml DMSO sample c o n t a i n i n g 4 r e c e i v i n g an absorbed dose o f 8 x 10 rads.  0.05 M n i t r o u s o x i d e and  helium flow rate 5 0 mis/min column temperature 215 °C detector temperature 215 T£ filament current 170 ma  to z  O o_  LU Q:  DC  O  hU LU hLU Q  DMSO CH SSCH 3  o Figure 14.  I  5  TIME (minutes)  1  10  3  15  Typical chromatograph obtained after injection of 25 £ of i r r a d i a t e d DMSO sample, dose was 6 Mrad. U  Total absorbed  -  B. 1.  77  PULSE  -  RADIOLYSIS  O u t l i n e o f t h e Technique P u l s e r a d i o l y s i s s t u d i e s o f DMSO and DMSO-l^O m i x t u r e s  performed d u r i n g a s e r i e s o f f i e l d  were  t r i p s t o the P h y s i c s D i v i s i o n o f  the N a t i o n a l R e s e a r c h C o u n c i l i n Ottawa.  As mentioned i n t h e I n t r o d u c t i o n ,  p u l s e r a d i o l y s i s e n a b l e s one t o d e t e c t and o b s e r v e t h e f o r m a t i o n and decay o f t h e r e a c t i v e i n t e r m e d i a t e  species.  The s t u d i e s  reported  a r e a l l concerned w i t h a b s o r p t i o n s p e c t r o s c o p i c measurements.  These  were done s p e c t r o p h o t o m e t r i c a l l y w i t h a l i m i t o f ^ 10 n s e c on t h e time r e s o l u t i o n . A s c h e m a t i c p l a n o f t h e l a y o u t o f t h e a p p a r a t u s w h i c h was used i s shown i n F i g u r e 15.  The e l e c t r o n beam was p a r t i a l l y a b s o r b e d i n a  s m a l l i r r a d i a t i o n c e l l t h r o u g h w h i c h t h e l i g h t beam p a s s e d i n a d i r e c t i o n a t r i g h t a n g l e s t o t h e e l e c t r o n beam.  The t r a n s m i t t e d  light  was then d i r e c t e d , by means o f a s e r i e s o f l e n s e s and m i r r o r s , o u t of t h e i r r a d i a t i o n a r e a t h r o u g h a s m a l l a p e r a t u r e w a l l where i t was s p l i t mirror.  i n the concrete  i n t o two beams by a p a r t i a l l y  reflecting  A f t e r s p l i t t i n g t h e beam, t h e r e s u l t a n t l i g h t beams were  p a s s e d t h r o u g h s u i t a b l e f i l t e r s and f o c u s s e d  onto t h e e n t r a n c e s l i t s  o f t h e monochromators, the o u t p u t s o f w h i c h were o b s e r v e d by p h o t o d i o d e s or p h o t o m u l t i p l i e r s c o u p l e d  t o a dual-beam o s c i l l o s c o p e .  o s c i l l o s c o p e traces corresponding  t o v a r i o u s o s c i l l o s c o p e sweep  speeds and monochromator w a v e l e n g t h s were o b t a i n e d r e c o r d by p h o t o g r a p h i n g t h e o s c i l l o s c o p e s c r e e n . a b s o r p t i o n s p e c t r a c o u l d be c o n s t r u c t e d w a v e l e n g t h s c o u l d be  assessed.  The  f o r a permanent I n t h i s way  and decay r a t e s a t s e l e c t e d  - 78 -  xenon arc lamp  lens  l i g h t shutter pyrex f i l t e r secondary emission monitor  35 MeV l i n e a r accelerator V  irradiation cell lens  mirror  mirro  lens  concrete s h i e l d i n g partially r e f l e c t i n g mirror  concrete s h i e l d i n g  mirror  Corning glass f i l t e r  Corning glass f i l t e r  Bauch and Lomb monochromato  Bauch and Lomb monochromator  .  0 ty  photodetector  photodetector  Tektronix 556 dual-beam oscilloscope Figure 15. Lay-out of the pulse r a d i o l y s i s equipment at the National Research Council r a d i a t i o n laboratory i n Ottawa, Ontario  - 79 2.  R a d i a t i o n Source The s o u r c e of h i g h - e n e r g y e l e c t r o n s was  microwave l i n e a r a c c e l e r a t o r C o u n c i l i n Ottawa.  a 35 MeV  electron  ( l i n a c ) o p e r a t e d by the N a t i o n a l  Research  E l e c t r o n p u l s e w i d t h s o f 10 n s e c and 40 n s e c were  used which d e p o s i t e d about 900 rads and 2200 rads p e r p u l s e r e s p e c t i v e l y . P u l s e - t o - p u l s e v a r i a t i o n s were m o n i t o r e d w i t h a secondary e m i s s i o n monitor  (SEM)  b e h i n d the i r r a d i a t i o n c e l l as shown i n F i g u r e 15.  The  m o n i t o r c o n s i s t e d o f a s e r i e s o f t h i n aluminum f o i l s which c o l l e c t secondary e l e c t r o n s e m i t t e d by the i m p i n g i n g p r i m a r y e l e c t r o n s . t o t a l charge c o l l e c t e d was  measured by a charge i n t e g r a t o r ,  the  The  the  r e a d i n g b e i n g p r o p o r t i o n a l t o the beam c u r r e n t o r the number o f h i g h energy e l e c t r o n s t r a v e r s i n g  the i r r a d i a t i o n  cell.  The m o n i t o r  was  c a l i b r a t e d d a i l y u s i n g an aqueous p o t a s s i u m t h i o c y a n a t e d o s i m e t e r s o l u t i o n saturated with nitrous oxide. peak beam c u r r e n t was  3.  ^ 1  F o r a 10 n s e c p u l s e , the  Amp.  I r r a d i a t i o n C e l l and O p t i c a l D e t e c t i o n System The i r r a d i a t i o n  c e l l used i s shown i n F i g u r e 16.  The  cell  was  p l a c e d i n a r i g i d h o l d e r so t h a t the l i g h t beam passed through the optically silica  flat  so t h a t  p a t h l e n g t h was arc  lamp.  end windows.  The windows were made o f h i g h p u r i t y  they were r e s i s t e n t 1 cm.  to r a d i a t i o n c o l o r a t i o n .  The w h i t e l i g h t  s o u r c e was  ultraviolet  optical  a 900-watt xenon  In o r d e r to p r e v e n t p h o t o c h e m i c a l changes i n the s o l u t i o n  by c o n t i n u o u s i l l u m i n a t i o n from the h i g h i n t e n s i t y f i l t e r was  The  p l a c e d between the c e l l light.  lamp, a p y r e x  and l i g h t source to cut out the  E f f e c t s o f t h i s k i n d were f u r t h e r reduced by u s i n g  F i g u r e 16.  Schematic diagram of i r r a d i a t i o n c e l l used f o r d e o x y g e n a t i o n o f l i q u i d samples used i n the p u l s e r a d i o l y s i s study.  The s p e c t r o s c o p i c c e l l was f i l l e d by t i p p i n g the c e l l h o r i z o n t a l l y  f l u s h i n g w i t h h i g h p u r i t y argon.  after  - 81 -  a r e m o t e l y o p e r a t e d s h u t t e r between t h e lamp and i r r a d i a t i o n c e l l w h i c h was k e p t c l o s e d u n t i l j u s t b e f o r e  the e l e c t r o n p u l s e .  Although the  lamp was u s u a l l y used i n the c o n t i n u o u s mode, a few e x p e r i m e n t s used the lamp under p u l s e d  o p e r a t i o n i n w h i c h t h e c u r r e n t was  increased  -3 briefly  10  The  s e c ) t o produce a h i g h e r  light  intensity.  t r a n s m i t t e d l i g h t beam was s p l i t u s i n g e i t h e r p a r t i a l l y  aluminized  m i r r o r s o r m i r r o r s w h i c h t r a n s m i t t e d o n l y s e l e c t e d wave-  lengths, r e f l e c t i n g a l l others.  The d e s i r e d w a v e l e n g t h s were i s o l a t e d  u s i n g e i t h e r narrow band pass i n t e r f e r e n c e f i l t e r s o r Bauch and Lomb g r a t i n g monochromators. before  Appropriate  Corning colour glass f i l t e r s  placed  t h e monochromator e n t r a n c e s p l i t were used t o e l i m i n a t e s e c o n d -  and h i g h e r - o r d e r  d i f f r a c t e d l i g h t from t h e g r a t i n g s .  F o r the wave-  l e n g t h r a n g e b e l o w 450 nm a P h i l l i p s XP1003 p h o t o m u l t i p l i e r was used. Above 450 nm two d i f f e r e n t p h o t o d i o d e s were used, a S i p h o t o d i o d e (HP 5082-4207) f o r t h e range 450-750 nm and a Ge p h o t o d i o d e F o r d L 4521) f o r t h e range 750-1500 nm.  (Philco-  I n most cases e i t h e r a  93 ohm o r a 50 ohm l o a d r e s i s t o r was used i n t h e p h o t o d e t e c t o r anode circuit.  The v o l t a g e s i g n a l s from t h e p h o t o m u l t i p l i e r o r p h o t o d i o d e  were a m p l i f i e d and d i s p l a y e d on a dual-beam o s c i l l o s c o p e . ( T e k t r o n i x 556)  and photographed u s i n g h i g h speed f i l m ( P o l a r o i d t y p e 410).  h o r i z o n t a l sweep o f the o s c i l l o s c o p e t r a c e was n o r m a l l y the e l e c t r o n p u l s e pick-up placed  The  triggered o f f  a l t h o u g h i t was o c c a s i o n a l l y i n i t i a t e d by an e l e c t r o n  c l o s e t o the SEM chamber.  Because o f t h e dual-beam o s c i l l o s c o p e and s p l i t beam arrangement, i t was p o s s i b l e t o f o l l o w c o n c u r r e n t l y two d i f f e r e n t t r a n s i e n t s .  the absorption  and decay o f  T h i s was a l s o u s e f u l when m e a s u r i n g t h e i r  - 82 -  absorption spectra.  By s p l i t t i n g t h e l i g h t beam and u s i n g one beam  as a v a r i a b l e w a v e l e n g t h and t h e o t h e r a t a f i x e d the s p e c t r a c o u l d be p r o p e r l y n o r m a l i z e d  (reference) wavelength,  t o compensate d i r e c t l y f o r  v a r i a t i o n s i n the p u l s e t o p u l s e a m p l i t u d e  o r any s l i g h t w a n d e r i n g o f  the e l e c t r o n beam.  4.  O s c i l l o s c o p e Measurements The p h o t o d e t e c t o r s  used produce an anode c u r r e n t w h i c h i s p r o p o r t i o n a l  to the l i g h t i n t e n s i t y s t r i k i n g the photocathode.  This current  a v o l t a g e drop a c r o s s the anode l o a d and c o n s e q u e n t l y  creates  the voltage  measured by t h e o s c i l l o s c o p e i s d i r e c t l y p r o p o r t i o n a l t o t h e l i g h t intensity. light  Changes i n v o l t a g e a r e t h e n p r o p o r t i o n a l t o changes i n  intensity. F i g u r e 17 shows a h y p o t h e t i c a l o s c i l l o s c o p e t r a c e i n w h i c h I  i s t h e 100% l i g h t t r a n s m i s s i o n b e f o r e t h e p u l s e , I a t the end o f the p u l s e  (AI t h e l i g h t a b s o r b e d ) .  the l i g h t  transmitted  The absorbance,D,of a  s o l u t i o n is g i v e n by:  D  "  l  o  8io  V t J  (2.6) -  -log  1 0  (1 -  AI/I ) 0  I n t h e e x p e r i m e n t s r e p o r t e d t h e absorbances measured were g e n e r a l l y v e r y s m a l l such t h a t I  >>  A l so t h a t A l / I , t h e f r a c t i o n  was d e t e r m i n e d from t h e o s c i l l o s c o p e t r a c e .  absorption,  T h i s was n o r m a l l y  achieved  by u s i n g the d i f f e r e n t i a l comparator t o o f f s e t t h e d.c. l e v e l so t h a t I  appeared on t h e lower p a r t o f t h e s c r e e n and A* f i l l e d t h e s c r e e n .  0%  tight  lil  CD  < f— _i  CO  transient build up  O >  transient decay  electron pulse 1 0 0 % light Jt  % ABSORPTION (%A) = Al/T X 100 TIME F i g u r e 17.  H y p o t h e t i c a l o s c i l l o s c o p e t r a c e showing the b u i l d up and decay o f t r a n s i e n t a b s o r b i n g The time p r o f i l e of the e l e c t r o n p u l s e i s shown as the d o t t e d  curve.  species.  - 84 When A I / I  Q  i s v e r y s m a l l then i t i s s i m p l y p r o p o r t i o n a l t o D.  I n the  o s c i l l o s c o p e t r a c e s shown, %A r e f e r s t o the % a b s o r p t i o n o r A l / I  5.  Cerenkov  x 100.  Q  Emission  . As mentioned  p r e v i o u s l y , Cerenkov  r a d i a t i o n i s e m i t t e d whenever  h i g h - e n e r g y e l e c t r o n s pass t h r o u g h m a t t e r w i t h a v e l o c i t y g r e a t e r than t h e phase v e l o c i t y o f l i g h t i n t h e medium.  Although the e m i s s i o n  i n t e r f e r r e d w i t h the absorbance measurements, i t was u s e f u l i n d e t e r m i n i n g the time response o f t h e d e t e c t i o n system.  S i n c e the  e m i s s i o n c o i n c i d e s w i t h the time p r o f i l e o f the e l e c t r o n p u l s e and assuming  the e l e c t r o n p u l s e i s s q u a r e , t h e n the r i s e t i m e o f the  d e t e c t i o n system may be o b s e r v e d from the o s c i l l o s c o p e t r a c e . r i s e t i m e i s d e f i n e d as the time i n t e r v a l between t h e 10% and a m p l i t u d e p o i n t s f o r a s t e p v o l t a g e change.  The 90%  F o r the T e t r o n i x 556  dual-beam o s c i l l o s c o p e the i n h e r e n t r i s e t i m e of the a m p l i f i e r i s 9 n s e c , c o n s e q u e n t l y the t o t a l r i s e t i m e o f the o p t i c a l d e t e c t i o n a p p a r a t u s  was  g r e a t e r than t h i s . The Cerenkov systems was a mirror.  l i g h t used f o r measuring  the speed o f the d e t e c t i o n  g e n e r a t e d i n a p i e c e o f S u p r a s i l f u s e d s i l i c a backed The m i r r o r was  up by the o p t i c a l system.  r o t a t e d so t h a t the e m i t t e d l i g h t was p i c k e d F i g u r e 18 shows t y p i c a l o s c i l l o s c o p e  o b t a i n e d f o r the Ge and S i p h o t o d i o d e s . response t i m e s were a l l < 20 n s e c .  traces  I t can be seen t h a t the  However, because the p u l s e i s not  r e a l l y s q u a r e , the a c t u a l r e s p o n s e times a r e u n d o u b t e d l y p r o b a b l y about 15 n s e c .  by  shorter,  L a r g e r l o a d r e s i s t o r s i n c r e a s e d the s i g n a l - t o -  n o i s e r a t i o but l e n g t h e n e d the response time o f the d e t e c t i o n  system.  - 85 -  (a) \ = 1275 nm Ge photodiode 93 ohm load  20 nsec  r 1  \  v  20mv  \J  3«  (b)  X= 1275  nm  6e photodiode 50 ohm load  1  te  20 nsec  (c)  X= 500 Si  nm  photodiode  50 ohm load  20 nsec F i g u r e 18.  Typical oscilloscope detection  t r a c e s • s h o w i n g the response time of the  a p p a r a t u s t o Cerenkov l i g h t  wide e l e c t r o n  pulse.  generated u s i n g a 40 nsec  - 86  T h i s can be seen i n F i g u r e s 18(a) 93 ohm  and 50 ohm  -  and 18(b)  f o r the Ge d e t e c t o r u s i n g a  l o a d r e s i s t o r i n w h i c h the former l o a d  d i s p l a y s a more pronounced t a i l .  G e n e r a l l y the 50 ohm  r e s i s t o r s were used i n a l l e x p e r i m e n t s ,  resistor or 93 ohm  load  e s p e c i a l l y when s t u d y i n g  the  s h o r t - l i v e d t r a n s i e n t s , such as the s o l v a t e d e l e c t r o n , but l a r g e r l o a d r e s i s t o r s were sometimes used when s t u d y i n g l o n g e r - l i v e d t r a n s i e n t s when a b e t t e r s i g n a l - t o - n o i s e r a t i o was A t w a v e l e n g t h s g r e a t e r than 1500  desired.  nm  i t was  shown t h a t the r i s e t i m e  o f the Ge p h o t o d i o d e system i n c r e a s e s n o t i c e a b l y , t a k i n g s e v e r a l hundred nanoseconds to respond c o m p l e t e l y T h i s i n c r e a s e i n r e s p o n s e time was  t o the new  light  level.  a t t r i b u t e d t o the l o n g e r  time  r e q u i r e d f o r the e l e c t r o n s t o d i f f u s e t o the r e v e r s e d - b i a s e d the a b s o r p t i o n c o e f f i c i e n t o f  p-junction.  A t w a v e l e n g t h s above 1500  nm  Ge  decreases r a p i d l y thereby  c a u s i n g the e l e c t r o n s t o be formed deeper i n  the n - j u n c t i o n of the p h o t o d i o d e . The e f f e c t of Cerenkov e m i s s i o n on the a b s o r p t i o n a t 500 nm  of  the t r a n s i e n t s produced i n DMSO u s i n g a 40 n s e c p u l s e i s shown i n F i g u r e 19.  The  pure Cerenkov e m i s s i o n was  o b t a i n e d by  irradiating  the s o l u t i o n w i t h the a n a l y z i n g l i g h t s o u r c e o f f ( F i g . 1 9 ( a ) ) . a c t u a l a b s o r p t i o n t h a t would have been o b s e r v e d i f t h e r e Cerenkov e m i s s i o n  ( F i g . 1 9 ( c ) ) was  o b t a i n e d by a d d i n g the  ( F i g . 1 9 ( a ) ) t o the o b s e r v e d a b s o r p t i o n  (Fig. 19(b)).  was  The no  emission  As shown, the  e f f e c t o f Cerenkov e m i s s i o n on the a b s o r p t i o n maximum i s q u i t e s m a l l . However, when the 10 n s e c p u l s e s were u s e d , the c o n t r i b u t i o n was n e g l i g i b l e and had pulse  absorption.  t o be c o r r e c t e d f o r when c a l c u l a t i n g the end  not of  - 87 -  \  /  (a)  /  (b)  0.95% A t  *•  m I  (O  j  •  •  *  •  S •  • ••  ••  =»  6  20 nsec  F i g u r e 19. E f f e c t o f Cerenkov e m i s s i o n on the a b s o r p t i o n o f t h e t r a n s i e n t s absorbing  a t 500 nm i n pure DMSO.  (a) p u r e  Cerenkov ( a n a l y z i n g l i g h t o f f ) ; (b) o b s e r v e d a b s o r p t i o n ; (c) a b s o r p t i o n t h a t would have been o b s e r v e d h a d t h e r e been no Cerenkov e m i s s i o n .  The d e t e c t o r was a S i p h o t o d i o d e  w i t h a 93 ohm l o a d r e s i s t o r .  The p u l s e w i d t h was 40 n s e c .  - 88 6.  Dosimetry The  dose p e r p u l s e absorbed by the samples was measured u s i n g an  aqueous p o t a s s i u m  thiocyanate chemical d o s i m e t e r . A s the absorbed  dose depended v e r y c r i t i c a l l y on t h e p o s i t i o n o f t h e c e l l on t h e o p t i c a l bench, t h e d o s i m e t r y was c a r r i e d o u t i n t h e same c e l l i n t h e same p o s i t i o n w i t h the same o p t i c a l a l i g n m e n t In  as the o t h e r samples.  t h e r a d i o l y s i s o f aqueous s o l u t i o n s o f p o t a s s i u m  thiocyanate  the h y d r o x y l r a d i c a l r e a c t s r a p i d l y w i t h t h e t h i o c y a n a t e i o n t o produce a t r a n s i e n t absorbing  r a d i c a l s p e c i e s , (CNS)^ , w h i c h has an  a b s o r p t i o n maximum a t 480 nm.  Reactions  (11) and (12) i n d i c a t e t h e  postulated mechanism.^  •OR +  CNS-  CNS~  *~ CNS-  + CNS  (CNS)  +  0R~  (11)  (12)  2  Because the m o l a r e x t i n c t i o n c o e f f i c i e n t o f (CNS)^  i s large  3 - 1 - 1 ^ 500 e  =  7.1 x 10  M  cm  ) and t h e a b s o r p t i o n r e l a t i v e l y l o n g  i t i s p a r t i c u l a r l y u s e f u l as a d o s i m e t e r  solution.  G(-OH) = 2.9 so t h a t f o r a c o n c e n t r a t i o n o f CNS  lived,  I n n e u t r a l water,  s u f f i c i e n t to -3  suppress  a l l r e a c t i o n s o f 'OH e x c e p t  ( 1 1 ) , such as 5 x 10  M, one  w i l l have G(-OH) = G ( ( C N S ) " ) = 2.9. 2  I n N 0 s a t u r a t e d s o l u t i o n s one a l s o has r e a c t i o n (13) HO e + N_0 N_ + 0~ — — * - 0H~ + -OH aq Z Z 2  so t h a t t h e o v e r a l l p r o c e s s e s  l e a d t o r e l a t i o n s h i p (2.7)  (13)  - 89 -  G((CNS)_ ) = GC-OH)  L  +  G(e ) aq (2.7)  = 5.7  where the y i e l d o f h y d r a t e d e l e c t r o n s scavenged i s t a k e n as 2.8. T y p i c a l o s c i l l o s c o p e t r a c e s o f t h e f o r m a t i o n and decay o f ( C N S )  2  -3 a t 500 nm i n a n i t r o u s o x i d e s a t u r a t e d s o l u t i o n o f 5 x 10  M  t h i o c y a n a t e i n w a t e r u s i n g 40 n s e c p u l s e s o f 35 MeV e l e c t r o n s a r e shown i n F i g u r e 20. From the absorbance a t the end o f the p u l s e t h e dose (rads p u l s e ^) absorbed by t h e d o s i m e t e r SEM  s o l u t i o n , f o r a given  r e a d i n g , was o b t a i n e d u s i n g e q u a t i o n 2.8 (compare t o 2.3), 9  „ R, -, (CNS) rHC  0.965 x 10 77—^ (Ge)  =  2  5 ( ) 0  x (absorbance) . , 500 nm , . -1 : rads p u l s e x £ x p (2.8) rr  n  n  m  where Z i s t h e c e l l p a t h l e n g t h (1 cm) and p the d e n s i t y o f t h e -3 dosimeter  s o l u t i o n (1.0 gm cm  ) . The dose p e r p u l s e o f any o t h e r system,  f o r t h e same S E M r e a d i n g , was o b t a i n e d by m u l t i p l y i n g by t h e c o r r e s p o n d ing electron density r a t i o .  F o r o t h e r SEM r e a d i n g s i t was assumed  t h a t t h e dose was p r o p o r t i o n a l t o SEM.  F o r DMSO, t h e absorbed doses  f o r 10 n s e c and 40 n s e c p u l s e s were u s u a l l y about 900 rads p u l s e ^ and 2200 r a d s p u l s e ^ r e s p e c t i v e l y . From e q u a t i o n  (2.8) i t can be seen t h a t t h e absorbance o f a g i v e n  t r a n s i e n t i s r e l a t e d t o the p r o d u c t c o e f f i c i e n t ; consequently  o f i t s G v a l u e and e x t i n c t i o n  once t h e absorbed dose o f any sample i s known,  the absorbance can be e x p r e s s e d  i n terms o f Ge.  - 90 -  (a)  4.8% A  100 nsec  (b) 5.6 % A  500 nsec Figure  20.  T y p i c a l oscilloscope traces f o r the formation of (CNS>2 35 MeV  at 500 nm obtained by using a 40 nsec pulse of  electrons on a nitrous oxide saturated s o l u t i o n of -3  5 x 10 ohm  and decay  M thiocyanate  load r e s i s t o r ;  resistor.  i n water.  (a) S i photodiode, 93  (b) photomultiplier, 470 ohm  load  - 91 -  7.  M a t e r i a l s and P u r i f i c a t i o n DMSO (Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade) was used  without  further purification.  Gas chromatography showed i t t o  c o n t a i n l e s s than 0.01% by volume w a t e r and d i m e t h y l  sulfide.  58 P r e v i o u s p u l s e r a d i o l y s i s s t u d i e s on DMSO  showed t h a t t h e r e was no  d i f f e r e n c e i n the o p t i c a l p r o p e r t i e s o r decay r a t e s o f t h e t r a n s i e n t s between those samples s u b j e c t e d t o f r a c t i o n a l d i s t i l l a t i o n o r f r a c t i o n a l c r y s t a l l i z a t i o n , f o l l o w e d by d r y i n g w i t h m o l e c u l a r and d e a e r a t e d  sieves  on a vacuum l i n e and those used d i r e c t l y from t h e  m a n u f a c t u r e r ( F i s h e r S c i e n t i f i c ) and b u b b l e d w i t h h i g h p u r i t y a r g o n . The  deuterated  DMSO (99.5%) was o b t a i n e d from Matheson, Coleman  and B e l l and d r i e d o v e r L i n d e 4A m o l e c u l a r  sieves.  A n a l y s i s by gas  chromatography showed i t t o have no d e t e c t a b l e i m p u r i t y . The w a t e r was t r i p l y d i s t i l l e d by the methods d e s c r i b e d P o t a s s i u m b r o m i d e , s i l v e r n i t r a t e and c o n c e n t r a t e d were a l l a n a l y t i c a l grade o r b e t t e r . a n t h r a c e n e was o b t a i n e d A l l glassware  earlier.  sulfuric acid  The b l u e - v i o l e t f l u o r e s c e n t  from Eastman O r g a n i c  Chemicals.  used was c l e a n e d by the p r o c e d u r e d e s c r i b e d p r e v i o u s l y .  A f t e r each s e r i e s o f e x p e r i m e n t s the i r r a d i a t i o n c e l l s were r i n s e d w e l l w i t h s i n g l y - f o l l o w e d by t r i p l y - d i s t i l l e d w a t e r and d r i e d i n t h e oven u n t i l r e q u i r e d .  A t the end o f the day,  i n the a n n e a l i n g oven and l e f t The  a l l the c e l l s were p l a c e d  overnight.  i r r a d i a t i o n c e l l s c o n t a i n e d about 5 m l o f sample w h i c h was  added by p i p e t t e t h r o u g h the BIO cone shown i n F i g u r e 16.  The samples  were then deoxygenated by b u b b l i n g w i t h h i g h p u r i t y argon f o r a t l e a s t 30 m i n u t e s a f t e r w h i c h the s t o p c o c k s h o r i z o n t a l l y to f i l l  the sidearm  were c l o s e d and the c e l l  c a r r y i n g the s p e c t r o s c o p i c  tipped  cell.  - 92 C.  1.  ELECTRON SPIN RESONANCE  M a t e r i a l s and P u r i f i c a t i o n Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade DMSO was p u r i f i e d  by f r a c t i o n a l d i s t i l l a t i o n as d e s c r i b e d p r e v i o u s l y . t r i p l y d i s t i l l e d by the method d e s c r i b e d e a r l i e r . used were a n a l y t i c a l grade o r b e t t e r . was o b t a i n e d from Canada L i q u i d A i r .  The w a t e r was A l l other chemicals  The argon used f o r d e o x y g e n a t i o n A l l g l a s s w a r e used i n t h e  e x p e r i m e n t s was c l e a n e d as o u t l i n e d p r e v i o u s l y .  2.  R a d i a t i o n Source The i r r a d i a t i o n s o u r c e used was t h e ^ C o Gammacell 220.  a p p r o x i m a t e dose r a t e was 4000 r a d s m i n  1  as e s t i m a t e d by F r i c k e  d o s i m e t r y i n the o t h e r e x p e r i m e n t a l a p p a r a t u s . 5  used i n t h e ESR s t u d y ranged from 1.2 x 10  3.  The  T o t a l absorbed doses 5  t o 9.6 x 10  rads.  Sample P r e p a r a t i o n The samples f o r i r r a d i a t i o n were p r e p a r e d by d r o p p i n g t i n y  s p h e r i c a l drops o f the l i q u i d from t h e c a p i l l a r y t i p o f a p i p e t t e a dewar of l i q u i d n i t r o g e n .  W i t h i n a few seconds t h e l i q u i d  into  drops  f r o z e i n t o n e a r l y s p h e r i c a l b a l l s and dropped t o the bottom o f t h e dewar.  Each b a l l was a p p r o x i m a t e l y 2-3 mm i n d i a m e t e r .  and DMSO,  formed p o l y c r y s t a l l i n e  P u r e DMSO  b a l l s although several binary  m i x t u r e s o f DMSO and w a t e r o f v a r y i n g c o m p o s i t i o n formed c o m p l e t e l y transparent glassy b a l l s .  The s o l i d b a l l s were i r r a d i a t e d i n a s m a l l p y r e x dewar c o n t a i n i n g liquid nitrogen. at  A f t e r t h e i r r a d i a t i o n t h e b a l l s were t r a n s f e r r e d  l i q u i d n i t r o g e n temperature  t o a q u a r t z dewar i n t h e s p e c t r o m e t e r  cavity.  4.  E l e c t r o n S p i n Resonance S p e c t r a A l l e l e c t r o n s p i n resonance  s p e c t r a were taken u s i n g a V a r i a n  A s s o c i a t e s E-3 s p e c t r o m e t e r w h i c h o p e r a t e d a t 9.1 GHz (X-band) and w i t h a 100 kHz f i e l d m o d u l a t i o n .  The magnetic  field  s t r e n g t h and  f i e l d s c a n l i n e a r i t y were c a l i b r a t e d w i t h a p r o t o n - p r o b e gaussmeter and t h e microwave f r e q u e n c y was checked w i t h a H e w l e t t - P a c k a r d model 5255 A d i g i t a l f r e q u e n c y c o u n t e r .  However, t h e microwave power l e v e l s  were r e a d d i r e c t l y from t h e power c o n t r o l d i a l o f t h e s p e c t r o m e t e r and were n o t c a l i b r a t e d . The s p e c t r o s c o p i c s p l i t t i n g f a c t o r s , o r g - v a l u e s , were  determined  u s i n g a f i n e l y powdered sample o f DPPH ( d i p h e n y l p i c r y l h y d r a z y l ) s e a l e d i n a t h i n q u a r t z tube and p l a c e d i n t h e q u a r t z dewar a l o n g w i t h t h e sample b a l l s .  Taking g p p n  = H  2.0036, t h e unknown g - v a l u e s , g , were  c a l c u l a t e d u s i n g e q u a t i o n (2.9)  g  v  =  X  2.0036 ( 1 - |5. )  where AH i s t h e d i f f e r e n c e i n magnetic unknown resonance  (2.9)  M X  f i e l d between t h e c e n t r e o f t h e  and t h a t o f t h e DPPH sample and H^ i s t h e magnetic  f i e l d o f the unknown resonance.  59  - 94 -  5.  P h o t o l y s i s Apparatus An u n f i l t e r e d low p r e s s u r e  mercury lamp (Hanovia #687A45) w i t h  a V y c o r e n v e l o p e was used f o r t h e u l t r a v i o l e t p h o t o l y s i s e x p e r i m e n t s . Generally  t h e p h o t o b l e a c h i n g and p h o t o l y s i s e x p e r i m e n t s were c a r r i e d  out by removing t h e q u a r t z envelope d i r e c t l y against sample b a l l s .  dewar from t h e c a v i t y and p l a c i n g t h e lamp the p o r t i o n o f t h e dewar c o n t a i n i n g t h e  S u r r o u n d i n g t h e dewar and lamp w i t h aluminum f o i l  enhanced t h e p h o t o l y s i n g  light intensity.  were f o l l o w e d by p h o t o l y s i n g  Changes i n t h e e s r s p e c t r a  t h e sample i n t h e c a v i t y .  The c a v i t y g r i d  c u t down t h e l i g h t i n t e n s i t y so t h a t t h e changes were n o t so r a p i d . A 100 w a t t t u n g s t e n lamp was used i n some p h o t o l y s i s e x p e r i m e n t s . These e x p e r i m e n t s were done o u t s i d e  the c a v i t y b u t , because o f t h e  h e a t g i v e n o f f , the lamp was n o t p l a c e d  too close to the quartz  dewar.  - 95 -  CHAPTER I I I STUDIES ON LIQUID DMSO A. 1.  6  ° C o Y-RADIOLYSIS  Gaseous P r o d u c t s The gaseous p r o d u c t s o b t a i n e d under t h e Y  DMSO were h y d r o g e n , methane and ethane.  - r a  d i o l y s i s o f pure  Hydrogen and ethane were  formed w i t h y i e l d s o f G(H_) = 0.20 + 0.01 and G(C„H,) = 0.49 + 0.03, JL  —  /  0  —  w h i c h were independent o f dose up t o a t l e a s t 1.2 x 10^ r a d s as shown i n F i g u r e 21. Methane f o r m a t i o n was n o t l i n e a r w i t h dose and the d a t a suggest t h a t some n o n - v o l a t i l e r a d i a t i o n p r o d u c t was b e i n g b u i l t up w h i c h scavenges t h e p r e c u r s o r o f p a r t o f t h e methane.  This  can be seen i n F i g u r e 22 where t h e v o l a t i l e gases were removed a f t e r each i r r a d i a t i o n and t h e t o t a l a c c u m u l a t e d y i e l d i s p l o t t e d as a f u n c t i o n o f dose.  I n t h e case o f methane, t h r e e d i f f e r e n t s e r i e s were  c a r r i e d o u t ; one i n w h i c h a l l t h e doses were t h e same, a n o t h e r i n w h i c h each dose was d i f f e r e n t and a t h i r d i n w h i c h a s m a l l dose was f o l l o w e d by a l a r g e dose and t h e n f o l l o w e d by the same s m a l l dose. seen i n F i g u r e 22 a l l t h e p o i n t s f a l l on a smooth c u r v e .  As can be From t h e  i n s e t o f F i g u r e 22 i t i s c l e a r t h a t t h e methane y i e l d i s o n l y up t o about 1.8 x 10^ r a d s .  linear  Consequently, a l l scavenging studies w i t h  3.2 2.8  -  2.4 GCX) \0  2.0  ON  0.8  0.4  ^^PQ OCrO-OX)  °  TT  Q -  •  O40  80 DOSE ( r a d x I O " )  120  4  F i g u r e 21.  R a d i a t i o n y i e l d s as a f u n c t i o n o f absorbed dose: •  , C,^  and O > H * 2  by t a k i n g the s l o p e a t v a r i o u s p o r t i o n s of the curve shown i n F i g u r e  The methane curve was 22.  obtained  40r-  90-  U)  jL) O  E Q _J LJ  60-^  > l/) <  o o LLJ  30  < U (J <  160  40 80 120 A C C U M U L A T E D D O S E (rad x 1 0 " ) 4  F i g u r e 22.  A c c u m u l a t e d gas y i e l d s  •  as a f u n c t i o n o f a c c u m u l a t e d  dose:#,A >A > A >  CH^  a  t  various  doses;  - 98 -  v a r i o u s added s o l u t e s were c a r r i e d out w i t h doses l e s s than 1.8 x 10^ r a d s s o t h a t the r e a l e f f e c t o f these s c a v e n g e r s on the methane y i e l d c o u l d be o b s e r v e d .  From the s l o p e a t v a r i o u s p o r t i o n s o f the  curve  the change i n G(CH^) w i t h absorbed dose was o b t a i n e d and t h i s i s shown I t can be seen t h a t G(CH^) f a l l s from 3 . 3 + 0 . 1 t o  i n F i g u r e 21.  2.1 + 0.1 o v e r the dose range s t u d i e d (up t o 1.3 x 10^ r a d s ) . The  o n l y o t h e r r e p o r t on the r a d i o l y s i s o f DMSO i n w h i c h the gaseous  y i e l d s have been measured i s b y K o u l k e s - P u j o  and B e r t h o u ^  i n w h i c h they  r e p o r t t h a t the hydrogen and methane y i e l d s were b o t h i n d e p e n d e n t o f dose.  The y i e l d s , G ( H ) = 0.19 + 0.006 and G ( C H ) = 3.4 + 0.3, 2  4  r e a s o n a b l y w e l l w i t h those o b s e r v e d i n t h i s s t u d y p r o v i d e d a b s o r b e d dose was l e s s than 1.8 x 10~* r a d s . was  agree  their  S i n c e the dose range  n o t s t a t e d , i t i s not p o s s i b l e t o a s c e r t a i n whether o r not the dose  independence o f the methane y i e l d i s i n c o n f l i c t w i t h the r e s u l t s i n t h i s work.  2.  total  Liquid The  No m e n t i o n was made o f ethane b e i n g  obtained  observed.  Products  only l i q u i d products  i d e n t i f i e d i n the Y  - r a  DMSO were d i m e t h y l s u l f i d e and d i m e t h y l d i s u l f i d e .  d i o l y s i s o f pure Other s t u d i e s on t h e  r a d i o l y s i s o f DMSO have quoted d i m e t h y l s u l f i d e , m e t h y l m e r c a p t a n ^ and  d i m e t h y l s u l f o n e ^ as l i q u i d p r o d u c t s  were g i v e n .  but no y i e l d measurements  I n j e c t i o n o f m e t h y l mercaptan as a s t a n d a r d showed t h a t  i t d i d not b e l o n g  t o any o f the u n r e s o l v e d  peaks shown i n F i g u r e 14.  I n t h i s s t u d y , o n l y d i m e t h y l s u l f i d e was o b t a i n e d as a measurable product;  dimethyl d i s u l f i d e  w  a  s  observed only i n t r a c e q u a n t i t i e s .  As shown i n F i g u r e 23, the y i e l d o f ( C H ^ ^ S was dependent on dose as  2  - 99 -  00 X  o E UJ  >  LO  Q Q LU  < ID  U  u <  3 ACCUMULATED F i g u r e 23. Accumulated accumulated  6 D O S E ( r a d s x 10  9 )  d i m e t h y l s u l f i d e y i e l d as a f u n c t i o n o f dose.  The e x t r a p o l a t e d p o r t i o n o f t h e curve  c o r r e s p o n d s t o G(DMS) = 1.2 + 0.2.  - 100 -  it  i n c r e a s e d w i t h accumulated dose.  A possible explanation i s that  some p r o d u c t  i s b e i n g b u i l t up w h i c h y i e l d s d i m e t h y l s u l f i d e upon  radiolysis.  The e x t r a p o l a t e d y i e l d o f d i m e t h y l s u l f i d e was found t o  be G(DMS) = 1 . 2 + 0 . 2 .  3.  Scavenger S t u d i e s The gaseous and l i q u i d p r o d u c t s  observed a r e t h e c h e m i c a l  consequence o f t h e i n t e r a c t i o n o f h i g h - e n e r g y r a d i a t i o n w i t h DMSO. I n o r d e r t o t e s t t h e n a t u r e o f t h e i r p r e c u r s o r s , and i n p a r t i c u l a r the p r i m a r y r e d u c i n g s p e c i e s , v a r i o u s e l e c t r o n and r a d i c a l s c a v e n g e r s were added and t h e e f f e c t o f these s c a v e n g e r s on the c h e m i c a l  yields  observed. Probably  t h e most w i d e l y used s c a v a n g e r f o r s o l v a t e d e l e c t r o n s i n  l i q u i d s i s n i t r o u s o x i d e w h i c h y i e l d s m o l e c u l a r n i t r o g e n upon reduction.  The e f f e c t on t h e gas y i e l d s due t o t h e a d d i t i o n o f v a r i o u s  c o n c e n t r a t i o n s o f n i t r o u s o x i d e t o DMSO i s shown i n F i g u r e 24. The methane y i e l d was found t o d e c r e a s e by about 20% (from 3.3 t o 2.6) w h i l e hydrogen and ethane were u n a f f e c t e d .  As t h e n i t r o u s o x i d e  c o n c e n t r a t i o n was i n c r e a s e d up t o 0.09 M, G(N^) s t e a d i l y i n c r e a s e d w i t h no s i m p l e p l a t e a u as shown i n F i g u r e 25. A t [ ^ 0 ] = 0.054 M, t h e n i t r o g e n y i e l d was independent o f dose up t o a t l e a s t 7.5 x 10^ rads ( F i g u r e 26) s u g g e s t i n g  t h a t whatever t h e n o n - v o l a t i l e p r o d u c t i s  w h i c h i n h i b i t s the methane y i e l d , i t does n o t a f f e c t t h e n i t r o g e n y i e l d . From t h e s l o p e o f t h e l i n e i n F i g u r e 26,  G(N ) = 1.6 + 0.1 a t t h i s 2  concentration. These r e s u l t s suggest t h a t p a r t o f the methane y i e l d a r i s e s from  I  I  li  I  0.02  I  0.04  0.06 [ N  F i g u r e 24.  R a d i a t i o n y i e l d s of CH^ case of CH^,  I  I  (A),  a l l doses were < 1.8  O ]  2  (•)  and H  x 10^  rads.  2  (O)  I  I  1  0-08  I—I  0.10  M as a f u n c t i o n o f the N 0 2  concentration.  I n the  F i g u r e 25.  R a d i a t i o n y i e l d o f n i t r o g e n as a f u n c t i o n of N.O  c o n c e n t r a t i o n i n DMSO.  ACCUMULATED  DOSE  (rad.xlO^)  F i g u r e 26. Accumulated n i t r o g e n y i e l d as a f u n c t i o n o f accumulated dose f o r a 0.054 M s o l u t i o n o f N . O i n DMSO.  - 104  -  some s t r o n g r e d u c i n g p r e c u r s o r .  To t e s t the n a t u r e  s p e c i e s w i t h w h i c h the n i t r o u s o x i d e was s c a v e n g e r s were added t o 0.05  M N^O  of t h i s  chemical  r e a c t i n g , v a r i o u s second  s o l u t i o n s to compete w i t h i t .  on the e f f e c t o f t h e s e s c a v e n g e r s i s shown i n T a b l e I I .  M e t h a n o l and  i s o p r o p a n o l a r e known t o r e a c t r a p i d l y w i t h hydrogen atoms b u t slowly with solvated electrons.  Data  very  On the o t h e r hand, a c e t o n e , s i l v e r  n i t r a t e , c h l o r o f o r m , i o d i n e , carbon t e t r a c h l o r i d e and a c i d a r e known t o react very r a p i d l y w i t h solvated electrons i n other p o l a r s o l v e n t s . Water was  added because DMSO i s v e r y h y g r o s c o p i c  and  t h e r e c o u l d have  been a t r a c e o f w a t e r i n the samples d e s p i t e the s t r i n g e n t p r e c a u t i o n s taken to e l i m i n a t e i t .  As can be seen from T a b l e I I the added w a t e r  had no a p p r e c i a b l e e f f e c t on any of the gas y i e l d s , so t h a t e i t h e r a l l samples c o n t a i n e d enough a d v e n t i t i o u s w a t e r f o r i t s e f f e c t t o be f u l l y f e l t o r w a t e r cannot compete w i t h the p r e c u r s o r s of the gaseous products  a t t h i s c o n c e n t r a t i o n (a. 0.5  % by volume).  W i t h the a s s u m p t i o n t h a t n i t r o u s o x i d e and the second s c a v e n g e r (S) compete f o r the p r e c u r s o r (14) and  (P) o f n i t r o g e n a c c o r d i n g t o r e a c t i o n s  (15)  N0  +  2  P  N k  S  +  P  s  +  products  (14)  products  then  (15)  k ^ o K P ]  G(N ) 2  = G(P)  [ Q  [N 0][P] + 2  kjSHP]  ]  (3.1)  -  TABLE I I .  Second  Radiation yields  105  -  from i r r a d i a t e d  plus  the  tion  g i v e n i n column 2 .  scavenger  second scavenger  Concentration (M)  DMSO c o n t a i n i n g 0 . 0 5 M N 0 2  indicated in  column 1 at  concentra-  G(N )  G(CH )  G(H )  G(C H  2  4  2  2  Water  0.22  1.7  3.1  0.21  0.46  Methanol  0.098  1.7  3.0  0.23  0.42  0.196  1.7  3.0  0.23  0.43  0.196  -  3.6  0.24  0.46  0.103  1.8  2.8  0.26  0.45  0.103  -  3.1  0.25  0.46  0.107  1.7  2.8  0.23  0.45  0.264  1.8  2.6  0.23  0.44  0.00947  1.4  2.6  0.21  0.40  0.0249  1.2  2.4  0.21  0.42  0.0516  0.95  2.3  0.20  0.41  0.102  0.65  2.1  0.19  0.41  0.158  0.57  2.1  0.20  0.41  0.0124  1.2  2.3  0.20  0.36  0.0696  0.68  2.2  0.21  0.41  0.0990  0.63  2.2  0.19  0.39  0.197  0.43  2.1  0.21  0.40  0.304  0.38  1.9  0.20  0.37  0.00652  1.5  0.32  0.22  0.42  0.0208  1.1  0.26  0.21  0.29  0.0300  0.99  0.25  0.22  0.28  0.0941  0.59  0.20  0.20  0.25  0.126  0.45  0.21  0.19  0.26  (a) Isopropanol (a) Acetone  AgN0  CHC1  3  3  Iodine  - 106 -  TABLE I I ( c o n t i n u e d ) Second s c a v e n g e r  cci  Concentration (M)  4  Anhydrous sulfuric  acid  (b)  G(N ) 2  G(CH )  G(H )  G(C H )  4  2  2  0.0179  1.11  2.2  0.21  0.45  0.0490  0.74  1.8  0.21  0.38  0.0685  0.68  1.7  0.21  0.37  0.0740  0.65  1.7  0.20  0.36  0.103  0.45  1.5  0.20  0.36  0.00918  1.4  2.4  0.20  0.41  0.0195  1.2  2.5  0.20  0.45  0.0295  1.1  2.7  0.21  0.45  0.0377  1.0  2.8  0.20  0.47  0.0559  0.80  2.7  0.19  0.47  0.0895  0.68  3.0  0.20  0.50  0.114  0.63  3.0  0.20  0.53  0.137  0.55  3.2  0.20  0.50  0.0718  0.78  2.8  0.23  (a)  No N 0 added  (b)  0.5 M m e t h a n o l added  2  6  - 107 -  where k  g  i s t h e r a t e c o n s t a n t o f P w i t h S,  f o r t h e r e a c t i o n o f P w i t h H^O and G(P) Rearranging  ^ i s the rate  constant  i s the y i e l d o f the p r e c u r s o r .  ( 3 . 1 ) , t h e f o l l o w i n g r e l a t i o n s h i p was o b t a i n e d and a p p l i e d  to the competitions.  GTJp  "  GW  [I  + ^ I S I / ^ O I ^ O ] ]  (3.2)  The d a t a from T a b l e I I a r e p l o t t e d i n F i g u r e s 27 and 28 and appear t o f o l l o w the simple competition r e l a t i o n s h i p . p l o t s the values f o r k / k j j g  and  n  From t h e s l o p e s o f t h e  were o b t a i n e d and a r e g i v e n i n T a b l e I I I  compared w i t h t h e p u b l i s h e d v a l u e s o f these r a t i o s f o r t h e  corresponding  r e a c t i o n s i n water i n which the precursor i s the hydrated  electron, e aq Although  t h e i n t e r c e p t o f t h e s e p l o t s a l l g i v e G(P)  i s not the true precursor y i e l d .  I n f a c t G(N)  = 1.6+0.2  does n o t show a  2  p l a t e a u v a l u e a t h i g h n i t r o u s o x i d e c o n c e n t r a t i o n s w h i c h would correspond G(P)  t o complete s c a v e n g i n g  o f t h e p r e c u r s o r ; thus t h e y i e l d o f  depends on t h e n i t r o u s o x i d e c o n c e n t r a t i o n s used above 0.03 M  (see F i g u r e 2 5 ) .  T h i s i s e v i d e n t from t h e H  +  c o m p e t i t i o n shown i n  F i g u r e 28 i n w h i c h two d i f f e r e n t b u t h i g h v a l u e s N 0 ) o f n i t r o u s o x i d e were used. 2  (0.05 M and 0.07 M  A l t h o u g h b o t h l i n e s have t h e same  s l o p e two d i f f e r e n t i n t e r c e p t s were o b t a i n e d .  For t h i s reason the  n i t r o u s o x i d e c o n c e n t r a t i o n was h e l d c o n s t a n t a t 0.05 M w h i l e t h e c o n c e n t r a t i o n o f t h e second s c a v e n g e r was v a r i e d . F i g u r e s 29 and 30 show t h e e f f e c t o f i o d i n e and hydrogen i o n s on t h e gaseous p r o d u c t s .  A c i d appears t o have no e f f e c t on t h e gas  this  - 108 -  F i g u r e 27.  P l o t of 1/G(N  ) as a f u n c t i o n o f [ S c a v e n g e r ] / [ N O ] .  c o n c e n t r a t i o n was  0.05  t i o n was v a r i e d .  The d a t a was  A,  I_;•  The  NO  M and the second scavenger c o n c e n t r a -  , CHC1-; O , A g  +  and  taken from T a b l e 2. # , •  , acetone.  CCl^;  2.0  concentrations: O  , 0.05  M 1^0; A ,  methanol added t o the a c i d  solution.  0.04 M N 0 2  and  •  , 0.07  M N 0. 2  •  corresponds t o 0.5  M  - 110 -  TABLE I I I . R a t i o o f k / l c  o b t a i n e d f o r t h e p r e c u r s o r o f N- i n DMSO  w i t h v a r i o u s second s c a v e n g e r s of N 0. 2  (S) added t o a 0.05 M s o l u t i o n  Column headed w a t e r r e f e r s t o t h e p u b l i s h e d r a t e  constant r a t i o f o r the hydrated S  k  S  / k  N 0  (  D  M  S  0  2  )  electron.  k  S  / k  N 0 2  <  CC1. 4  0.85 + 0.09  5.4  h  0.60 + 0.06  9.1  0.45 + 0.05  3.6  0.40 + 0.04  5.7  0.25 + 0.03  3.9  CHC1 Ag H  3  +  +  (CH ) CO 3  2  <0.01  1.1  D a t a t a k e n from t h e c o m p i l a t i o n o f r a t e c o n s t a n t s by M. Anbar and P. N e t a , I n t e r . J . A p p l . Rad. I s o t . 18, 493 (1967).  w a t e r  )  a  3.6  -  G C X )  to  I  o  JO—O-O-O  O  008  0.04  O-  0-12 [H SQ ] M 2  F i g u r e 30. R a d i a t i o n y i e l d s o f CH^ (A), A  -O-  (•) and H  0.20  4  2  (O). as a f u n c t i o n o f H  +  concentration.  corresponds t o the y i e l d o f CH, when 0.05 M N~0 was added t o t h e c o r r e s p o n d i n g a c i d s o l u t i o n o f 5 DMSO. I n the case o f CH., a l l doses were < 1.8 x 10 r a d s . 4  - 113 -  y i e l d s , i n c o n t r a s t t o n i t r o u s o x i d e , whereas i o d i n e r e d u c e s t h e y i e l d of ethane and methane w i t h o u t  4.  a f f e c t i n g t h e hydrogen y i e l d .  Discussion A l t h o u g h n i t r o u s o x i d e h a s been w i d e l y used as a s p e c i f i c  s c a v e n g e r f o r s o l v a t e d e l e c t r o n s i n many l i q u i d s , o t h e r s t r o n g l y  reducing  s p e c i e s formed i n t h e r a d i o l y s i s may g i v e r i s e t o t h e n i t r o g e n o b s e r v e d . I n t h e case o f DMSO, s o l v a t e d e l e c t r o n s , hydrogen atoms, f r e e r a d i c a l s (R) o r s o l v e n t r a d i c a l a n i o n s c o u l d y i e l d n i t r o g e n v i a r e a c t i o n s ( 1 7 ) , ( 1 8 ) , o r (19) i f t h e i r a l t e r n a t e f a t e s were c o m p a r a t i v e l y  e  R*  slow.  +  N_0 2  *- N. 2  +  0 s  (16)  +  N 0  *- N  2  +  -OH  (17)  N  2  +  R0»  (18)  s  H-  (16),  +  DMSO~  2  N 0 2  +  N 0  *~ N  2  2  +  DMS0 ~ 2  "  (19)  I n t h i s r e a c t i o n scheme t h e s o l v e n t r a d i c a l a n i o n , DMSO , i s t o be r e g a r d e d as an a n i o n formed e i t h e r by e l e c t r o n attachment o f s o l v a t e d o r f r e e e l e c t r o n s o r i s a d e c o m p o s i t i o n p r o d u c t formed from i t .  In  o r d e r t o e x p l a i n t h e o b s e r v e d r e s u l t s and t o d i f f e r e n t i a t e between the p o s s i b l e p r e c u r s o r s  o f t h e n i t r o g e n , s t r o n g i n f e r e n c e s must be  drawn from t h e c o m p e t i t i o n  studies w i t h the other  scavengers.  From T a b l e I I i t can be seen t h a t t h e a d d i t i o n o f methanol and  -  isopropanol  114  -  d i d not a l t e r the y i e l d of nitrogen.  Since  these a l c o h o l s  are g e n e r a l l y much b e t t e r hydrogen atom s c a v e n g e r s than i s n i t r o u s o x i d e , t h i s s u g g e s t s t h a t r e a c t i o n (17) does n o t c o n t r i b u t e t o t h e nitrogen y i e l d .  F u r t h e r m o r e , t h e s l i g h t i n c r e a s e i n hydrogen y i e l d ,  w h i c h c a n be p a r t i a l l y a t t r i b u t e d t o t h e y i e l d o b t a i n e d  from t h e d i r e c t  a c t i o n o f t h e r a d i a t i o n on t h e a l c o h o l s , i m p l i e s t h a t t h e hydrogen atoms a r e p r o d u c e d i n l o w y i e l d . I n a s i m i l a r manner, r e a c t i o n (18) can a l s o be d i s r e g a r d e d .  From  T a b l e I I I i t can be seen t h a t i o d i n e competes on an a l m o s t e q u a l basis with nitrous  a i d e f o r the precursor  of nitrogen despite the  f a c t t h a t i o d i n e i s a much b e t t e r r a d i c a l s c a v e n g e r than n i t r o u s I f r e a c t i o n (18) were t h e p r e c u r s o r  oxide.  o f m o l e c u l a r n i t r o g e n one w o u l d  e x p e c t a g r e a t e r r e d u c t i o n i n t h e y i e l d when i o d i n e was added t h a n i s observed. it  T h i s i n f e r e n c e i s f u r t h e r s u p p o r t e d i n F i g u r e 29 where  c a n be seen t h a t i o d i n e v i r t u a l l y e l i m i n a t e s methane, even a t 0.006 M.  T h i s s u g g e s t s t h a t methane a r i s e s from m e t h y l r a d i c a l s w h i c h a b s t r a c t a hydrogen atom from DMSO.  I n t h i s r e g a r d , i t s h o u l d be mentioned  t h a t m e t h y l r a d i c a l s were observed i n y - i r r a d i a t e d p o l y c r y s t a l l i n e DMSO a t 77°K ( t o be d i s c u s s e d  later).  From F i g u r e 29 i t c a n a l s o be seen  t h a t i o d i n e d e c r e a s e s t h e ethane y i e l d s l i g h t l y i m p l y i n g i t i s s i m i l a r l y formed by r a d i c a l r e a c t i o n s . hydrogen y i e l d i s u n a f f e c t e d "molecular process",  that part of  On t h e o t h e r hand, t h e  by i o d i n e i n d i c a t i n g i t i s formed i n a  such as m o l e c u l a r detachment o f hydrogen o r an  i o n - m o l e c u l e o r " h o t " atom r e a c t i o n o c c u r r i n g w i t h i n a few c o l l i s i o n s . S i m i l a r l y about h a l f t h e ethane y i e l d and about 10% o f t h e methane y i e l d may be due t o " m o l e c u l a r  processes".  -  The  inference  115 -  then i s t h a t the r e d u c i n g  a solvated e l e c t r o n o r r a d i c a l anion.  precursor  of nitrogen i s  The r e d u c i n g p r o p e r t i e s and  c h e m i c a l b e h a v i o r o f t h e s e two s p e c i e s may be v e r y s i m i l a r ; perhaps o n l y p u l s e r a d i o l y s i s a b s o r p t i o n r e a l l y d i f f e r e n t i a t e between them.  indeed  spectroscopy o r esr can  As mentioned e a r l i e r t h e a b s o r p t i o n  s p e c t r a o f s o l v a t e d e l e c t r o n s are c h a r a c t e r i z e d by t h e i r b r o a d n e s s and  i n t e n s i t y i n the v i s i b l e and n e a r i n f r a r e d  r e g i o n o f the spectrum.  By c o n t r a s t , r a d i c a l a n i o n s would p r o b a b l y have weak, narrow bands i n the v i s i b l e .  T h i s s i m i l a r i t y i n b e h a v i o r was d e m o n s t r a t e d i n a 46  r e c e n t s t u d y on the r a d i o l y s i s o f l i q u i d formamide p a t t e r n o f r e a c t i v i t y o f the r e d u c i n g  species  t o t h a t o f s o l v a t e d e l e c t r o n s i n o t h e r media.  i n which the  conformed r e m a r k a b l y w e l l However l a t e r  pulse  r a d i o l y s i s s t u d i e s showed t h a t formamide does not form s o l v a t e d electrons with lifetimes w i t h the t h e r m a l i z e d  > 10  ±  L  seconds b u t r a t h e r r e a c t s  rapidly  e l e c t r o n s , presumably t o g i v e r a d i c a l a n i o n s 45  as t h e i r d e c o m p o s i t i o n p r o d u c t s .  These r a d i c a l a n i o n s thus  represent  the " f r e e i o n s " w h i c h r e a c t w i t h the e l e c t r o n s c a v e n g e r s . As i n t h e case o f formamide the c h e m i c a l e v i d e n c e f o r the p r e s e n c e o r absence o f s o l v a t e d e l e c t r o n s i n DMSO i s n o t c o n c l u s i v e . r a d i o l y s i s s t u d y w h i c h w i l l be d e s c r i b e d  l a t e r showed t h a t  e l e c t r o n s a r e formed as the p r i m a r y r e d u c i n g  The p u l s e solvated  s p e c i e s i n DMSO.  However,  t h a t s t u d y a l s o showed t h a t the e l e c t r o n s appear t o r e a c t w i t h t h e s o l v e n t q u i t e r a p i d l y s o t h a t r a d i c a l a n i o n s are p r o b a b l y a l s o i n v o l v e d i n the r e d u c t i o n o f U^O  and o t h e r s c a v e n g e r s a t l o w c o n c e n t r a t i o n s i n  these steady s t a t e experiments. From the d a t a g i v e n i n T a b l e s I I and I I I , i t i s e v i d e n t  that nitrous  - 116  -  o x i d e i s a b e t t e r s c a v e n g e r of r e d u c i n g scavengers.  The  F i g u r e s 27 and  species  r e l a t i v e rate constants  28 and  than the  obtained  other  from the s l o p e s  of  compared w i t h the r e l a t i v e r a t e s o f t h e s e  s c a v e n g e r s w i t h h y d r a t e d e l e c t r o n s show them t o be m a r k e d l y d i f f e r e n t . F u r t h e r m o r e a c e t o n e does n o t compete e f f e c t i v e l y w i t h n i t r o u s although CCl^, I r e s u l t s obtained  2  CHC1 » A g  >  +  3  and H  do.  +  oxide  T h i s i s at variance with  i n the p u l s e r a d i o l y s i s of DMSO c o n t a i n i n g 0.19  the  M 58  acetone i n w h i c h the s o l v a t e d e l e c t r o n was A. p o s s i b l e e x p l a n a t i o n  completely  eliminated.  i s t h a t the acetone r a d i c a l a n i o n ,  undergoes a charge t r a n s f e r w i t h n i t r o u s o x i d e a c c o r d i n g  (CH^COCH^) , to  the  f o l l o w i n g sequence: e  + s  e  + s  N„0 2  >• N_ 2  CH.COCH, 3 3  (CH C0CH ) 3  +  3  Moreover the n e g a t i v e  +  0  (16) s  (CH-COCH.) 3 3  N0  N  2  2  +  °s  (20)  +  C H  3  i o n s o f C C l ^ , 1^ and CHCl^ may  C 0 C H  3  ( > 21  a l s o undergo  charge t r a n s f e r w i t h the n i t r o u s o x i d e so t h a t the a c t u a l r a t e k^ Q,  constant,  c o u l d be a c t u a l l y s m a l l e r t h a n t h a t o b s e r v e d . The  curvature  of G(N ) 2  a t [N 0] 2  > 0.03  M i s d i f f i c u l t to i n t e r p r e t  w i t h o u t knowing the a l t e r n a t e f a t e s of the n i t r o g e n ( s o l v a t e d e l e c t r o n s and absolute  rate constants.  other reducing  precursors  s p e c i e s , i f any)  and  their  A p o s s i b l e i n t e r p r e t a t i o n i s t h a t the n i t r o u s  o x i d e i s s c a v e n g i n g e l e c t r o n s i n s i d e the s p u r .  However t h i s i s  - 117 -  d i s r e g a r d e d because p u l s e r a d i o l y s i s o f a s o l u t i o n o f 0.07 M n i t r o u s 58 o x i d e i n DMSO o n l y scavenged about 80% o f t h e s o l v a t e d e l e c t r o n s . I f one equates the n i t r o g e n y i e l d a t t h e h i g h n i t r o u s o x i d e  concentra-  t i o n s w i t h t h e t o t a l f r e e i o n y i e l d , t h e n G ( f r e e i o n ) = 1.8 + 0 . 2 (see F i g u r e 2 5 ) . However, u s i n g a n t h r a c e n e as a s c a v e n g e r i t was shown by p u l s e r a d i o l y s i s t h a t t h e y i e l d o f s o l v a t e d e l e c t r o n s i s o n l y 1.3 ( t o be d i s c u s s e d s h o r t l y ) .  Perhaps t h e h i g h e r y i e l d i n t h e case o f  n i t r o u s o x i d e i s due t o s c a v e n g i n g o f o t h e r h i g h l y r e d u c i n g s p e c i e s i n addition to solvated electrons. namely t h a t each e s  However, a n o t h e r p o s s i b i l i t y  g i v e s r i s e t o more t h a n one N„ m o l e c u l e . 2  exists, Studies  on o t h e r systems u s i n g n i t r o u s o x i d e have g i v e n h i g h e r n i t r o g e n y i e l d s t h a n t h e t o t a l f r e e i o n y i e l d and i t h a s been p r o p o s e d t h a t 0 may r e a c t f u r t h e r t o produce more n i t r o g e n a c c o r d i n g e  + s  0  Earlier  +  N„0 2  NO  N  0  +  0 s  (16)  +  products  (22)  2  *~ N  to reaction (22).  i t was s u g g e s t e d t h a t most o f t h e methane a r i s e s from  m e t h y l r a d i c a l s w h i c h a b s t r a c t a hydrogen atom from DMSO because t h e methane was v i r t u a l l y e l i m i n a t e d by i o d i n e , an e x c e l l e n t r a d i c a l scavenger.  However, t h e f a c t t h a t n i t r o u s o x i d e can d e c r e a s e t h e  methane y i e l d by 20% s u g g e s t s t h a t i t can a l s o i n t e r f e r e w i t h a minor r e a c t i o n l e a d i n g t o CH^ f o r m a t i o n .  Because no d e c r e a s e i n t h e methane  y i e l d was o b s e r v e d i n t h e p r e s e n c e o f m e t h a n o l and i s o p r o p a n o l , w h i c h are b e t t e r r a d i c a l s c a v e n g e r s than n i t r o u s o x i d e , t h e i n t e r f e r e n c e i s  - 118 -  probably i o n i c i n nature.  This i s s t i l l i n keeping w i t h the i o d i n e  r e s u l t s s i n c e 1^ i s a l s o a good e l e c t r o n s c a v e n g e r as seen from t h e r e l a t i v e r a t e constants i n Table I I I .  I t i s suggested that p a r t o f  t h e methane y i e l d a r i s e s from the d e c o m p o s i t i o n o f the DMSO r a d i c a l a n i o n by the r e a c t i o n sequence shown below.  e  +  DMSO  >- DMSO  (23)  s  DMSO  CR 3  *~ CH 3  +  +  DMSO  products  CH  +  4  (24)  products  (25)  T h i s i s s u p p o r t e d b y t h e f a c t t h a t e l e c t r o n s g e n e r a t e d i n DMSO-water g l a s s e s a t 77°K b y u l t r a v i o l e t p h o t o l y s i s o f K F e ( C N ) ^ undergo r e a c t i o n 4  w i t h the DMSO m o l e c u l e s t o produce m e t h y l r a d i c a l s (see l a t e r ) . N i t r o u s o x i d e , and  other e l e c t r o n scavengers  (except H ) , c o u l d  i n t e r f e r e w i t h t h e methane f o r m a t i o n e i t h e r by s c a v e n g i n g the s o l v a t e d e l e c t r o n b e f o r e i t r e a c t s w i t h the s o l v e n t m o l e c u l e s  t o produce t h e  a n i o n o r by charge t r a n s f e r w i t h the s o l v e n t a n i o n . s t r o n g l y reducing metals  Furthermore,  (M) such as sodium and p o t a s s i u m i n e x c e s s  DMSO decompose the s o l v e n t y i e l d i n g a m i x t u r e o f methane, d i m e t h y l s u l f i d e and the s a l t s o f methane s u l f e n a t e and m e t h y l s u l f i n y l c a r b a n i o n 63 a c c o r d i n g t o t h e r e a c t i o n sequence below.  CR SOCH  3  +  2M  CH SOCH  3  +  CH "M  3  3  *-  3  +  CH SO~M 3  +  +  CH ~M  *- CH SOCH ~M 3  (26)  +  3  2  +  +  CH^  (27)  - 119 CH SOCH  3  +  2M  CH SOCH  3  +  M 0  3  3  *- C H S C H 3  +  3  *• CH SOCH ~M  2  3  (28)  +  2  +  MOH  (29)  R e a c t i o n s (26) and (28) were found t o be about e q u a l w i t h sodium whereas w i t h p o t a s s i u m r e a c t i o n (26) g r e a t l y p r e d o m i n a t e d .  R e a c t i o n s (26) and  (27) may be c o n s i d e r e d somewhat analogous t o r e a c t i o n s (23) t o ( 2 5 ) . I t i s i n t e r e s t i n g t o n o t e t h a t d i m e t h y l s u l f i d e i s a major d e c o m p o s i t i o n p r o d u c t i n a l k a l i m e t a l s o l u t i o n s o f DMSO.  The l a r g e  y i e l d o f d i m e t h y l s u l f i d e , G(DMS) = 1.2 + 0.2, o b s e r v e d i n t h e r a d i o l y s i s o f DMSO may a r i s e d i r e c t l y from the r e d u c t i o n o f t h e s o l v e n t according to r e a c t i o n (30).  CH.SOCH- + e~ 3 3 s  *~ CH.SCH_ 3 3  +  products  (30)  The a d d i t i o n o f 0.5 M A g N 0 w h i c h would r e a d i l y scavenge a l l t h e 3  s o l v a t e d e l e c t r o n s caused no n o t i c e a b l e change i n t h e d i m e t h y l s u l f i d e y i e l d , w h i c h argues a g a i n s t r e a c t i o n ( 3 0 ) .  However, i t i s  p o s s i b l e t h a t t h e s i l v e r atoms produced undergo charge t r a n s f e r w i t h t h e s o l v e n t i n an analogous manner t o r e a c t i o n ( 2 8 ) .  +  Ag  CH S0CH 3  3  ^ - ^ A g ^  +  Ag  + 2  (31)  >• A g 0 2  +  CH SCH 3  3  (32)  T h i s i s s u p p o r t e d by the f a c t t h a t when DMSO was a l l o w e d t o remain i n  - 120 -  contact w i t h a f r e s h l y prepared  s i l v e r m i r r o r f o r s e v e r a l days, a strong  odour o f d i m e t h y l s u l f i d e was o b s e r v e d .  On the o t h e r hand i t i s  p o s s i b l e t h a t DMS does not a r i s e from e^  o r i s a molecular  product  whose p r e c u r s o r cannot be scavenged even a t 0.5 M A g . +  The  a d d i t i o n o f hydrogen i o n s a t c o n c e n t r a t i o n s up t o 0.2 M H  +  d i d n o t a f f e c t the gaseous y i e l d s from p u r e i r r a d i a t e d DMSO (see F i g u r e 30) i n marked c o n t r a s t t o the p r e s e n c e o f n i t r o u s o x i d e . e f f e c t of H The  +  i s i n agreement w i t h the r e s u l t s o f P.ujo and B e r t h o u . ^ ^  s t r o n g e l e c t r o n - d o n a t i n g power o f the s u l f o x i d e group makes DMSO  a p o w e r f u l Lewis base w h i c h i s r e a d i l y p r o t o n a t e d i n accordance  H  The  This  +  w i t h r e a c t i o n (33).  +  experimental  CH SOCH 3  *- ( C H S 0 H C H )  3  3  observations  (33)  +  3  can be e x p l a i n e d i f the r e a c t i o n o f t h e  e l e c t r o n w i t h the p o s i t i v e i o n ( C H S 0 H C H ) 3  DMSO r a d i c a l  by hydrogen i o n s  3  +  has  the same f a t e as t h e  anion.  e~ + (CR* S0HCH ) 3  +  3  (CH^OHCH^  »-CH ' 3  + products  (34)  The p o s i t i v e i o n would compete w i t h n i t r o u s o x i d e f o r the e l e c t r o n and perhaps o t h e r r e d u c i n g s p e c i e s produced i n the r a d i o l y s i s . same time i t would c o u n t e r a c t the methane y i e l d .  A t the  the s l i g h t e f f e c t n i t r o u s o x i d e h a s on  T h i s e f f e c t i s shown i n F i g u r e 30.  The e l e c t r o n s  w h i c h are not scavenged and undergo geminate r e c o m b i n a t i o n  would  - 121  -  produce a h i g h l y e x c i t e d DMSO m o l e c u l e w h i c h would  probably  d i s s o c i a t e a c c o r d i n g to r e a c t i o n ( 3 5 ) .  e~ + DMSO  +  *~ DMSO*  s u l f u r i c a c i d a t 0.2  as the p u l s e r a d i o l y s i s d a t a i n d i c a t e s . c o u l d t h e r e f o r e e x p l a i n why  + products  (35)  M d i d not seem to be i n t e r f e r i n g w i t h  Whereas n i t r o u s o x i d e a t 0.07 geminate r e c o m b i n a t i o n ,  *~ CHy  M may  Reactions  w e l l be d o i n g (34) and  so,  (35)  h i g h a c i d c o n c e n t r a t i o n s had no  effect  on the methane y i e l d . The  r e a c t i o n o f the s o l v a t e d e l e c t r o n w i t h the hydrogen i o n ,  a c c o r d i n g to r e a c t i o n ( 3 6 ) ,  e~ s  +  H  +  *~  H-  (36)  o r w i t h the p r o t o n a t e d DMSO m o l e c u l e  t o produce a hydrogen atom  does not appear t o o c c u r i n v i e w o f the c o m p e t i t i o n between H , +  and m e t h a n o l (see T a b l e I I ) . atom o r i f r e a c t i o n (36) was  ^0  I f r e a c t i o n (34) produced a hydrogen i n c o m p e t i t i o n w i t h r e a c t i o n ( 1 6 ) , then  the hydrogen y i e l d would be i n c r e a s e d by the p r e s e n c e of methanol i n the a c i d i c s o l u t i o n o f DMSO.  No such i n c r e a s e was  observed.  However,  i t i s p o s s i b l e t h a t hydrogen atoms a r e produced b u t t h a t they r e a c t r a p i d l y w i t h DMSO to g i v e non-gaseous p r o d u c t s .  - 122 -  B.  1.  PULSE RADIOLYSIS  A b s o r p t i o n S p e c t r a i n DMSO and DMSO^ P r e v i o u s i n v e s t i g a t i o n s on the p u l s e r a d i o l y s i s o f pure DMSO a t  room t e m p e r a t u r e " * ^ ' h a v e shown t h a t the a b s o r p t i o n s p e c t r a to  the t r a n s i e n t s f a l l i n t o t h r e e main c a t e g o r i e s .  decay c h a r a c t e r i s t i c s and b e h a v i o r  belonging  From t h e i r  towards s p e c i f i c s c a v e n g e r s  were a t t r i b u t e d t o d i f f e r e n t s p e c i e s .  distinct they  The t h r e e p r i n c i p a l b r a n c h e s ,  w i t h t h e i r c h a r a c t e r i s t i c f e a t u r e s , a r e g i v e n as f o l l o w s :  ( i ) a broad,  s t r u c t u r e l e s s a b s o r p t i o n s t r e t c h i n g from the v i s i b l e i n t o the n e a r infrared with a X max  > 1500  nm and h a v i n g a h a l f - l i f e o f ^ 15 n s e c  w h i c h was a t t r i b u t e d t o the s o l v a t e d e l e c t r o n ; ( i i ) a f a i r l y b r o a d band c e n t e r e d a t % 600 nm whose h a l f - l i f e was <\> 1 usee and a t t r i b u t e d t o an o x i d i z i n g s p e c i e s ; and ( i i i ) a b s o r p t i o n bands p r o g r e s s i n g from 400 nm i n t o the n e a r u l t r a v i o l e t r e g i o n b e l o n g i n g  t o a t l e a s t two components  p o s s e s s i n g h a l f - l i v e s > 1 msec and ^ 12 usee. on these  However f u r t h e r s t u d i e s  t r a n s i e n t s , i n p a r t i c u l a r t h a t o f the o x i d i z i n g s p e c i e s and  the s o l v a t e d e l e c t r o n , were w a r r a n t e d i n v i e w o f the f a c t t h a t t h e f r e e i o n y i e l d had n o t been d e t e r m i n e d n o r the i d e n t i t y o f the species.  Unless  stated otherwise,  oxidizing  the a b s o r p t i o n s p e c t r a o f these  and o t h e r t r a n s i e n t s were o b t a i n e d u s i n g e l e c t r o n p u l s e w i d t h s o f 40 n s e c d e p o s i t i n g an average dose o f 2.2 k r a d p e r p u l s e .  The t o t a l  absorbed dose w h i c h any one sample r e c e i v e d was always l e s s than 10"* r a d s . The and  a b s o r p t i o n s p e c t r a o f the o x i d i z i n g s p e c i e s  the s o l v a t e d e l e c t r o n (X > 1500 max and (CD~)_S0 are g i v e n i n F i g u r e 31.  (X  nm) o b t a i n e d i n pure  = 550 nm)  (CH )_S0 51 The t o t a l a b s o r p t i o n was o  300  500  700 900 WAVELENGTH  1100 (nm)  1300  1500  F i g u r e 31. T r a n s i e n t s p e c t r a observed i n ( C I ^ ^ S O and (CD-j^SO. The c i r c l e s refer t o t h e s o l v a t e d e l e c t r o n band c o r r e c t e d f o r the d e t e c t o r response time and the t r i a n g l e s r e f e r t o the DMSO p o s i t i v e i o n , o r o x i d i z i n g s p e c i e s . O and A a r e f o r ( C l ^ ^ S O ; • and A a r e f o r (003)280. Almost a l l d a t a p o i n t s are the mean of a t l e a s t two measurements. The x f o r the & spectrum was e s t a b l i s h e d t o be a t 550 nm from a p r e v i o u s s e t o f experiments. m a x  - 124 -  obtained from the observed absorption peak heights at the end of the pulse.  The bands attributed to the solvated electron i n the deuterated  and undeuterated DMSO had almost completely decayed within 100 nsec; consequently the maximum i n t e n s i t y of the bands centered at 550 nm were deduced by extrapolation to the end of the pulse from times > 100 nsec.  This approximation i s reasonable  since the 550 nm band decays  i n a f i r s t - o r d e r manner with a h a l f - l i f e of 2.3 + 0.2 35). are  jisec (see Figure  T y p i c a l oscilloscope traces of the electron and o x i d i z i n g species shown i n Figures 32 and 33. In order to compare the r e l a t i v e i n t e n s i t y of the bands, i t was  necessary to correct for the decay of the electron during the pulse. By using 10 nsec pulses i t was determined that the electron decays by f i r s t - o r d e r k i n e t i c s with a h a l f - l i f e of 15 + 2 nsec (see Figure 34). Therefore with a 40 nsec pulse, which was used i n the determination of the  absorption spectra, considerable decay of the electron would have  occurred during the pulse.  Furthermore, the time response of the  detection system (oscilloscope and photodiodes) i s comparable  to the  h a l f - l i f e of the electron so that the i n i t i a l absorbance observed i s not  the true end of pulse absorbance.  Using reasonable values f o r the  time constants of the photodiodes and o s c i l l o s c o p e , the electron pulse width and the h a l f - l i f e of the electron, suitable correction factors were computed and applied to the observed absorbance.^  These calculations  show that under the experimental conditions, the maximum electron absorbance observed was only 43% of the absorbance which would have been observed had there been no decay. for  Consequently the observed absorbances  the electron band were m u l t i p l i e d by 2.3 r e l a t i v e to the 550 nm band  data shown i n Figure 31.  - 125. -  (a) \=1300 nm 10 nsec pulse  20 nsec (b)  \ = 1275 nm  U0 nsec pulse 0.82%A  20 nsec F i g u r e 32.  T y p i c a l o s c i l l o s c o p e t r a c e s showing t h e decay o f t h e s o l v a t e d e l e c t r o n i n DMSO.  Both t r a c e s were  obtained  u s i n g a Ge p h o t o d i o d e w i t h a 50 ohm l o a d r e s i s t a n c e , (a) p u l s e w i d t h 10 n s e c ; (b) p u l s e w i d t h 40 n s e c .  -  126  1  V  (a)  A  J >  e  100 nsec  (b)  1.0%  A  F i g u r e 33.  T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of DMSO p o s i t i v e i o n or o x i d i z i n g s p e c i e s a t 550 nm. f a s t i n i t i a l decay i n (a) i s due Both t r a c e s were o b t a i n e d (a)  ohm  The  t o the s o l v a t e d e l e c t r o n .  u s i n g a p u l s e w i d t h of 40 n s e c .  S i p h o t o d i o d e w i t h 93 ohm  m u l t i p l i e r w i t h 470  the  load  load r e s i s t o r ; resistor.  (b) p h o t o -  - 127 -  A t X > 1500 nm the Ge p h o t o d i o d e response i s t o o s l o w t o make measurements on t h e e l e c t r o n band f o r t h e reasons mentioned However, by o b s e r v i n g t h e s t e a d y s t a t e a b s o r p t i o n d u r i n g  earlier.  long 58  i r r a d i a t i o n pulses and InSb  (> 1 usee) u s i n g s l o w InAs ( r i s e - t i m e 2 usee)  ( r i s e - t i m e 100 n s e c ) ^ d e t e c t o r s , i t appears t h a t t h e maximum  e x t i n c t i o n c o e f f i c i e n t f o r the e l e c t r o n band o c c u r s between 1600 and 1800 nm and t h a t Ge i s < 2 5 % l a r g e r than Ge a t 1500 nm. max There a r e s e v e r a l r e a s o n s f o r a s s i g n i n g t h e a b s o r p t i o n band w i t h X > 1500 nm t o t h e s o l v a t e d e l e c t r o n i n DMSO. F i r s t l y , i t i s max 4 -1 -1 e x t r e m e l y broad and i n t e n s e (e > 10 M cm ) , a c h a r a c t e r i s t i c max J  f e a t u r e o f s o l v a t e d e l e c t r o n bands.  S e c o n d l y , t h e band i s e f f i c i e n t l y  removed o r reduced by known e l e c t r o n s c a v e n g e r s , such as O2, ^ 0 , a n t h r a c e n e , + + Ag , a c e t o n e , C C l ^ and H . Moreover, a d d i t i o n o f known p o s i t i v e i o n s c a v e n g e r s such as B r  and w a t e r  h a d no a f f e c t on t h e spectrum.  There i s a l s o e v i d e n c e , w h i c h w i l l be d i s c u s s e d more f u l l y l a t e r , f o r t h e assignment o f t h e band c e n t e r e d a t 550 nm t o t h e DMSO positive ion.  I n the presence o f Br  e l i m i n a t e d and r e p l a c e d by t h e B ^  t h e 550 nm band was c o m p l e t e l y  absorption.  Addition of electron  s c a v e n g e r s i n s u f f i c i e n t c o n c e n t r a t i o n t o j u s t e l i m i n a t e the e l e c t r o n band d i d n o t a f f e c t t h e 550 nm a b s o r p t i o n o r i t s decay r a t e , whereas a d d i t i o n + of H  + and Ag  i n s u f f i c i e n t concentration to interfere with  r e c o m b i n a t i o n (> 0.1 M) i n c r e a s e d i t s absorbance v e r y  geminate  significantly  (by 9 0 % ) . The e v i d e n c e f o r t h e assignment o f t h e s e bands t o t h e s o l v a t e d e l e c t r o n and the p r i m a r y o x i d i z i n g - s p e c i e s i s f u r t h e r s u b s t a n t i a t e d by t h e f a c t t h a t b o t h bands show an e q u i v a l e n t i n c r e a s e upon i s o t o p i c  -  128 -  s u b s t i t u t i o n , s u g g e s t i n g t h a t t h e s p e c i e s r e s p o n s i b l e have a common origin.  As shown i n F i g u r e 31, Ge i s % 30% l a r g e r f o r t h e d e u t e r a t e d  DMSO than i t i s f o r t h e u n d e u t e r a t e d .  S i n c e i t i s u n l i k e l y b o t h bands  w o u l d have e x a c t l y t h e same change i n e x t i n c t i o n c o e f f i c i e n t s i n g o i n g from t h e d e u t e r a t e d m a t e r i a l t o t h e u n s u b s t i t u t e d , i t i m p l i e s t h a t t h e change a r i s e s from a G v a l u e e f f e c t .  T h i s suggests then t h a t b o t h  p r i m a r y s p e c i e s have an e q u a l p r o b a b i l i t y o f s u r v i v i n g geminate s p u r reactions. of  A s i m i l a r e f f e c t occurs i n water i n which  the f r a c t i o n  i o n s w h i c h escape geminate r e c o m b i n a t i o n and become s o l v a t e d i s 67 6 8  s u b s t a n t i a l l y l a r g e r i n the deuterated m a t e r i a l .  '  B^O and IL^O  have i d e n t i c a l b u l k d i e l e c t r i c c o n s t a n t s so t h a t t h e i n c r e a s e i n y i e l d cannot be a s i m p l e d i e l e c t r i c c o n s t a n t e f f e c t .  What i t does  s u g g e s t i s t h a t t h e r e i s a w i d e r range o f i n i t i a l s e p a r a t i o n d i s t a n c e s between t h e t h e r m a l i z e d e l e c t r o n and i t s p a r e n t p o s i t i v e i o n i n D^O than i n 1^0.  I t i s known t h a t D^O has a s l o w e r d i e l e c t r i c  relaxation  time t h a n rl^O; c o n s e q u e n t l y t h e t h e r m a l e l e c t r o n s w i l l have t o t r a v e l f u r t h e r b e f o r e they become s o l v a t e d by o r i e n t a t i o n a l  polariza-  t i o n and hence w i l l have a h i g h e r p r o b a b i l i t y o f e s c a p i n g geminate recombination.  On t h e o t h e r hand, i t i s p o s s i b l e t h a t t h e m a t e r i a l  c o n t a i n i n g the h e a v i e r i s o t o p e i s l e s s e f f i c i e n t i n i t s i n e l a s t i c s c a t t e r i n g o f t h e " s u b e x c i t a t i o n e l e c t r o n " s o t h a t , on t h e a v e r a g e , the e l e c t r o n g e t s f u r t h e r away from i t s concomitant  partner before i t  becomes t h e r m a l i z e d . The decay r a t e s o f b o t h t h e e l e c t r o n and p o s i t i v e i o n were t h e same w i t h i n e x p e r i m e n t a l e r r o r i n the d e u t e r a t e d DMSO as they were i n the p r o t o n a t e d m a t e r i a l .  I n b o t h cases t h e decay o f t h e e l e c t r o n b e i n g  - 129 -  n e a r l y 100 times f a s t e r .  Both s p e c i e s decay by f i r s t - o r d e r  as shown i n F i g u r e s 34 and 35; t h e pseudo f i r s t - o r d e r r a t e  kinetics, constants  f o r t h e e l e c t r o n and p o s i t i v e i o n a r e (4.8 + 0 . 1 ) x l O ^ s e c (3.0 + 0.1) x 10~*sec  L  respectively.  L  and  This suggests both t r a n s i e n t s  r e a c t w i t h t h e s o l v e n t o r e l s e w i t h some r e s i d u a l i m p u r i t y .  However  a d d i t i o n o f % 0.1 M d i m e t h y l s u l f i d e and w a t e r , w h i c h were t h e o n l y d e t e c t a b l e i m p u r i t i e s , showed no o b s e r v a b l e  i n c r e a s e i n decay r a t e s .  I t i s thought t h a t t h e e l e c t r o n decays by r e a c t i n g w i t h t h e s o l v e n t medium f o r r e a s o n s w h i c h have a l r e a d y been mentioned.  Although the  DMSO r a d i c a l a n i o n has been o b s e r v e d t o have an a b s o r p t i o n maximum a t 350 nm i n i r r a d i a t e d aqueous a l k a l i n e s o l u t i o n s (pH > 1 0 ) c o n t a i n i n g 69 0.7 M DMSO,  i t was n o t o b s e r v e d i n t h e p r e s e n t  study.  Ifi t i s  formed, i t i s p o s s i b l e t h a t i t i s v e r y u n s t a b l e and decays f a s t e r than the response time o f t h e d e t e c t i o n  apparatus.  I t i s also possible  t h a t i t s e x t i n c t i o n c o e f f i c i e n t i s t o o s m a l l t o be o b s e r v e d . e x p e r i m e n t a l c o n d i t i o n s used i n t h i s s t u d y o f an a b s o r p t i o n i s g i v e n by G e ^ 1000 p r o v i d i n g , o f c o u r s e , t h a t the mean l i f e  Under t h e  the l i m i t o f d e t e c t a b i l i t y  time o f t h e s p e c i e s i s l o n g  compared t o t h e p u l s e l e n g t h and response t i m e o f t h e d e t e c t i o n apparatus. 750 M ^cm  F o r a G v a l u e o f 1.3 t h i s imposes an upper l i m i t o f x  on t h e e x t i n c t i o n c o e f f i c i e n t o f t h e s o l v e n t a n i o n .  DMSO p o s i t i v e i o n can presumably r e a d i l y undergo i o n - m o l e c u l e w i t h t h e s o l v e n t o r spontaneous u n i m o l e c u l a r  decomposition.  The reactions  I t should  64 be n o t e d t h a t K o u l k e s - P u j o  et a l .  found t h a t the o x i d i z i n g  species  decayed by a s e c o n d - o r d e r r e a c t i o n w h i c h had a f i r s t h a l f - l i f e o f 0.36  usee.  T h i s was p r o b a b l y  due t o r e a c t i o n w i t h a n e g a t i v e i o n  -  -2.00  -  -2.20  -  130 -  -2.40-  -2.60  -  -2.80 TIME (nsec) ure 34. F i r s t - o r d e r decay p l o t o f t h e s o l v a t e d e l e c t r o n i n DMSO taken a t 1300 nm.  The p u l s e w i d t h was 10 n s e c , g i v i n g an absorbed  dose o f 900 rads p e r p u l s e .  The decay was measured u s i n g a  Ge p h o t o d i o d e w i t h a 50 ohm l o a d  resistor.  - 131  -  -1.90  -2.10  -2.30  d  c  o -2.50  -2-70  -2.90 F i g u r e 35.  2.0  TIME  4.0  (jisec) F i r s t - o r d e r p l o t of the decay o f the DMSO p o s i t i v e i o n . was  measured a t 550 nm  load r e s i s t o r .  The  The  u s i n g the p h o t o m u l t i p l i e r w i t h a 470  p u l s e w i d t h was  dose o f 2200 rads p e r p u l s e .  decay ohm  40 n s e c , g i v i n g an absorbed  - 132 -  p r o d u c e d from the e l e c t r o n decay.  T h e i r dose r a t e was n e a r l y 60 t i m e s  t h a t used i n t h i s s t u d y (3 x 10"^ r a d s s e c  X  compared t o 5 x 1 0 ^  r a d s s e c ) so t h a t t h e " i n s t a n t a n e o u s " c o n c e n t r a t i o n o f t r a n s i e n t X  ionic  s p e c i e s would be much h i g h e r i n t h e i r s t u d y .  2.  Free Ion Y i e l d s A n t h r a c e n e has been w i d e l y used as a s c a v e n g e r o f s o l v a t e d e l e c t r o n s  and r a d i c a l  a n i o n s because o f i t s h i g h r e a c t i v i t y and t h e i n t e n s e  4 -1 -1 a b s o r p t i o n (e = 10 M cm ) o f i t s r a d i c a l a n i o n i n t h e v i s i b l e max r e g i o n o f the spectrum ( *  m a x  ^ 720 nm).  I n t h i s s t u d y a n t h r a c e n e was  added t o DMSO a t v a r i o u s c o n c e n t r a t i o n s up t o i t s s o l u b i l i t y l i m i t , 0.02 M.  The s o l u t i o n s were p r e p a r e d j u s t p r i o r  to irradiation  since  i t was o b s e r v e d t h a t s o l u t i o n s c o n t a i n i n g a n t h r a c e n e "aged" on s t a n d i n g i n t h e f l u o r e s c e n t room l i g h t i n g f o r a day o r more, p r o b a b l y p r o d u c i n g the  a n t h r a c e n e p h o t o d i m e r , w h i c h caused a l o n g l i v e d t r a n s i e n t t o be  formed i n t h e r e g i o n 450-800 nm upon i r r a d i a t i o n . I n t a k i n g t h e a b s o r p t i o n s p e c t r u m o f t h e r a d i c a l a n i o n (A ) and e v a l u a t i n g i t s a b s o r b a n c e , i t was n e c e s s a r y t o make c o r r e c t i o n s f o r t h e DMSO p o s i t i v e  i o n and t h e s o l v a t e d e l e c t r o n , b o t h o f w h i c h absorb t o  some e x t e n t i n the same r e g i o n .  The a b s o r p t i o n s p e c t r a o f t h e s e l a t t e r  two t r a n s i e n t s h a d a l r e a d y been d e t e r m i n e d and c o n s e q u e n t l y t h e c o r r e c t i o n s were r e l a t i v e l y easy t o a p p l y because A  does n o t absorb  a t 550 nm ( t h e DMS0 peak) n o r a t 1275 nm (where e absorbs s +  strongly).  The c o n t r i b u t i o n s o f these two s p e c i e s a t each w a v e l e n g t h were s i m p l y s u b s t r a c t e d from the t o t a l o b s e r v e d end o f p u l s e absorbance.  The  s p l i t - b e a m o p t i c a l method was used t o n o r m a l i z e t h e t h r e e s p e c t r a .  - 133 -  The spectrum of A  obtained i n this manner for 0.02 M anthracene i n  DMSO i s shown i n Figure 36, where i t can be seen that X = 750 nm. ' max &  Figures 37 and 38 show the oscilloscope traces at 750 nm f o r 0.001, 0.005, 0.01 and 0.02 M anthracene i n DMSO.  For a l l traces shown,  the contribution from the p o s i t i v e ion decreases only s l i g h t l y over the time period shown since i t s h a l f - l i f e i s > 2 ysec. concentration,  At the lowest  the contribution made by the short l i v e d solvated  electron can r e a d i l y be observed. From these traces i t can be deduced that the absorbance due to A  consists of two components, one of which builds up during the pulse  and i s larger at the higher concentrations  and the other which builds  i n a f t e r the pulse with a rate of growth which increases with the anthracene concentration.  Furthermore, i t i s clear from Figure  37(b)  that the slow component builds i n at a much slower rate than the electron decays, which strongly suggests that the anthracene i s scavenging both the solvated electron and the reducing species r e s u l t i n g from the electron decay, either the DMSO r a d i c a l anion or i t s decomposition product.  e  +  A  Thus the savenging reactions may be written  *~ A  (37)  s e  S  s  + S  (38)  + A  (39)  as  CO I  o  \—  X  u>  O  500 F i g u r e 36.  600  700 800 WAVELENGTH (nm)  900  End o f p u l s e a b s o r p t i o n spectrum of the anthracene r a d i c a l a n i o n o b t a i n e d from a DMSO s o l u t i o n 0.02 M i n anthracene a f t e r absorbances due t o the e l e c t r o n and o x i d i z i n g s p e c i e s had been s u b t r a c t e d .  - 135  (a) X = 750 nm no anthracene added  100 nsec  (b)  X  = 750 nm  0.001 M anthracene 0.97% A IT  200 nsec  (O \ = 750 nm 0.005 M  anthracene  2.1% A  F i g u r e 37.  100 nsec T y p i c a l o s c i l l o s c o p e t r a c e s showing t h e decay o f t h e e l e c t r o n and b u i l d up o f t h e a n t h r a c e n e r a d i c a l a n i o n a t 750 nm.  (a) no  a n t h r a c e n e added; (b) 0.001 M a n t h r a c e n e i n DMSO; ( c ) 0.005 M a n t h r a c e n e i n DMSO.  -  Figure 3 8 .  Typical oscilloscope  136  -  traces showing the b u i l d up and decay of  the anthracene r a d i c a l anion at 750 nm. i n DMSO; (b) 0.02 M anthracene i n DMSO.  (a) 0.01 M anthracene  - 137  -  where S i s the s o l v e n t medium and decomposition  pulse The  i s the r a d i c a l a n i o n o r i t s  product.  F i g u r e 39 shows how end o f p u l s e  S  the absorbance observed i m m e d i a t e l y  at  the  (curve 2) and the maximum absorbance o b s e r v e d a f t e r  the  ( c u r v e 1) of A~ a t 750 nm v a r y w i t h the a n t h r a c e n e c o n c e n t r a t i o n .  immediate absorbances were c o r r e c t e d f o r the p o s i t i v e i o n and e l e c t r o n  c o n t r i b u t i o n s a t 750 nm by m e a s u r i n g t h e i r r e s p e c t i v e absorbances a t 550  nm  and  1275  according to  nm and a p p l y i n g the a p p r o p r i a t e c o r r e c t i o n f a c t o r  £759^550 ^ 75c/ 1275" an<  m o n i t o r e d a t 1275  e  e  e  l  e c t r o n  absorbance  nm u s i n g the s p l i t - b e a m o p t i c a l method.  was  The maximum  absorbances were o b t a i n e d by e x t r a p o l a t i n g the d e c a y i n g p o r t i o n o f absorbance back t o the end o f the p u l s e .  the  The decay o f the p o s i t i v e  i o n , o b t a i n e d at 550 nm and u s i n g the c o r r e c t i o n p r o c e d u r e d e s c r i b e d above, was  added t o the o b s e r v e d decay a t 750 nm so t h a t the t r u e A  b u i l d up and decay c o u l d be The  obtained.  e f f e c t o f the a n t h r a c e n e on the y i e l d s o f the p o s i t i v e i o n  ( c u r v e 3) and the s o l v a t e d e l e c t r o n s ( c u r v e 4) are a l s o shown i n F i g u r e 39.  The  t o Ihe s c a v e n g i n g otherwise  s l i g h t i n c r e a s e i n the p o s i t i v e i o n y i e l d may  be  due  o f e l e c t r o n s or o t h e r r e d u c i n g s p e c i e s w h i c h were  doomed t o r e c o m b i n a t i o n  w i t h the c a t i o n .  f a c t t h a t the y i e l d o f the p o s i t i v e i o n was  Nevertheless,  the  not d e c r e a s e d n o r was i t s  decay r a t e a p p r e c i a b l y i n c r e a s e d , s u g g e s t s t h a t p a r t o f the absorbance a t t r i b u t e d to A A , +  cannot be due  t o the p r e s e n c e o f a n t h r a c e n e c a t i o n s ,  as has been s u g g e s t e d i n some o t h e r s y s t e m s . ^  The  anthracene  c a t i o n i s b e l i e v e d t o have a v e r y s i m i l a r e x t i n c t i o n c o e f f i c i e n t  and  a b s o r p t i o n spectrum as i t s a n i o n c o u n t e r p a r t and i s formed t h r o u g h  15  Figure 39.  Graph shewing the scavenging of solvated electrons i n pure DMSO by anthracene and the formation of anthracene r a d i c a l anions. 9, absorbance at 750 nm due to A~ immediately at the end of the pulse; O > maximum i n the absorbance at 750 nm due to A a f t e r the pulse; A , absorbance due to the p o s i t i v e ions at 550 nm; B , absorbance due to solvated electrons at 1275 nm (not corrected f o r decay during the pulse nor for the detector response time). -  /  .  "  '  - 139 -  charge t r a n s f e r w i t h t h e p r i m a r y o x i d i z i n g s p e c i e s i n some media.  34 64 I n t h e p r e v i o u s s t u d i e s on DMSO, c o n c e n t r a t i o n o f 0.005 M.  '  a n t h r a c e n e was used a t a  As can be seen from F i g u r e 39, a n t h r a c e n e a t  0.005 M reduced t h e o b s e r v a b l e y i e l d o f e  g  t o about 50% o f i t s y i e l d  i n t h e pure system a l t h o u g h i t then scavenged the decay p r o d u c t o f most o f t h e s e unscavenged coefficient of A  34 64  electrons.  By assuming the e x t i n c t i o n  4-1-1  atX (750 nm)to be 1.0 x 10 M max  cm  , as used '  previously, ' i t can be c a l c u l a t e d t h a t the y i e l d i m m e d i a t e l y a t the end o f t h e p u l s e i s G(A ) = 0.6. T h i s then i n c r e a s e d t o G(A ) = 1.1 + 0.1 due t o t h e s l o w e r component o f t h e s c a v e n g i n g p r o c e s s .  34 Hayon's  v a l u e o f 1.62 i s much l a r g e r than t h i s b u t i s p r o b a b l y i n  e r r o r because i t was n o t c o r r e c t e d f o r the s i g n i f i c a n t absorbance b y t h e p o s i t i v e i o n s a t the w a v e l e n g t h used (720 nm) t o d e t e r m i n e G(A ) . Indeed a d d i t i o n o f t h e two absorbances a t t h i s w a v e l e n g t h g i v e s an " e f f e c t i v e " G o f 1.4 + 0.1, assuming ( / 5 5 o °** e  e  =  8 f  o  r t  h  e  D  M  S  0  7 2  64 positive ion.  On t h e o t h e r hand K o u l k e s - P u j o e t a l .  obtained a  much l o w e r y i e l d , G(A ) = 0.64, w h i c h i s more d i f f i c u l t t o r e c o n c i l e w i t h the data obtained i n t h i s study.  T h e i r y i e l d was o b t a i n e d b y i r r a d -  i a t i n g a s o l u t i o n o f 0.1 M i n B r ( t o scavenge t h e o x i d i z i n g s p e c i e s ) and 0.005 M i n a n t h r a c e n e . 0.1 M B r  When a DMSO s o l u t i o n w h i c h c o n t a i n e d  and 0.02 M a n t h r a c e n e was i r r a d i a t e d , t h e a n t h r a c e n e a n i o n  y i e l d was reduced by about 40%, t o G(A ) = 0.8, from what i t was w i t h o u t t h e a d d i t i o n o f B r , d e s p i t e t h e f a c t t h a t t h e e l e c t r o n was completely eliminated.  However t h e c h e m i s t r y must be r a t h e r d i f f e r e n t  i n t h i s m i x t u r e because a w h i t e p r e c i p i t a t e was produced a t the end o f the i r r a d i a t i o n .  F o r t h e reasons mentioned e a r l i e r , however, t h i s  d e c r e a s e i n absorbance a t 750 nm i n t h e p r e s e n c e o f B r i s thought n o t  - 140 -  t o a r i s e from the p r e s e n c e o f A  +  c o n t r i b u t i n g t o the 750 nm a b s o r p t i o n .  Thus the y i e l d o f f r e e i o n s i n DMSO, as o b t a i n e d from G(A ) a t 0.02  M, i s c a l c u l a t e d t o be 1.3 + 0.15, —  M ^"cm  assuming  4 e (A ) = 1.0 x 10 max  A t 0.02 M a n t h r a c e n e , the s o l v a t e d e l e c t r o n y i e l d was  r e d u c e d by about 90%, as shown i n F i g u r e 39, the r e m a i n i n g 10% b e i n g scavenged  as the s o l v e n t a n i o n s o r t h e i r d e c o m p o s i t i o n p r o d u c t .  However, even a t t h i s c o n c e n t r a t i o n the t o t a l y i e l d o f A  i s not  e n t i r e l y i n d e p e n d e n t o f the a n t h r a c e n e c o n c e n t r a t i o n , as e v i d e n c e d by t h e lack o f a good s c a v e n g i n g p l a t e a u .  Perhaps a t 0.02 M some i n t r a -  spur scavenging occurs. From G(e ) = 1.3 one c a l c u l a t e s t h a t e , ^ f o r the s o l v a t e d s 1500 nm Crt  e l e c t r o n band i n DMSO i s about 14,000 M "*"cm \ DMSO i s ^25% g r e a t e r than Ge, . , then e . 1500 nm max crvr  I f Ge for e i n max s i s presumably o f t h e r  J  o r d e r o f 17,000 M^cm" . 1  I n v i e w o f the d i f f e r e n c e i n the f r e e i o n y i e l d o f r e d u c i n g s p e c i e s o b t a i n e d from the n i t r o u s o x i d e s t e a d y s t a t e e x p e r i m e n t s and a n t h r a c e n e p u l s e r a d i o l y s i s e x p e r i m e n t s , i t was i m p e r a t i v e t h a t the y i e l d o f t h e p o s i t i v e i o n ( o r p r i m a r y o x i d i z i n g s p e c i e s ) be d e t e r m i n e d .  Br ions  have been used i n aqueous systems as a scavenger f o r o x i d i z i n g s p e c i e s such as t h e 'OH r a d i c a l s and p o s s i b l y a l s o ^ 0 * . c o n v e n i e n t because the t r a n s i e n t p r o d u c t , the B ^  I t i s very  i o n , has a s t r o n g  72—76 o p t i c a l a b s o r p t i o n around 360 nm.  C o n s e q u e n t l y KBr was s e l e c t e d  as a p o s i t i v e i o n s c a v e n g e r i n DMSO and used a t v a r i o u s c o n c e n t r a t i o n s r a n g i n g from 0.001  t o 0.1 M.  A t [Br ]> 0.01 M the a b s o r p t i o n band c e n t e r e d a t 550 nm was c o m p l e t e l y e l i m i n a t e d i n d i c a t i n g complete s c a v e n g i n g o f the f r e e p o s i t i v e  - 141 -  i o n s and a new a b s o r p t i o n band c e n t e r e d a t 375 nm was p r o d u c e d . a c c o u n t o f i t s s i m i l a r i t y t o t h e Br,, was  a t t r i b u t e d to this species.  spectrum i n w a t e r the new band  The s p e c t r u m a t t r i b u t e d t o t h e B r  i o n a t two c o n c e n t r a t i o n s o f B r  2  i s shown i n F i g u r e 40. The i n s e t o f  F i g u r e 40 shows t h e absorbance o f B r concentration of Br presented  On  a t 375 nm as a f u n c t i o n o f t h e  2  on a s e m i - l o g p l o t .  Even a t the  highest  c o n c e n t r a t i o n s t u d i e d , [ B r ] = 0.1 M, t h e e l e c t r o n a b s o r p t i o n band was u n a l t e r e d which suggests t h a t the Br geminate r e c o m b i n a t i o n .  i o n d i d not i n t e r f e r e w i t h  The c o n c e n t r a t i o n independent v a l u e o f  Ge,,-,,. = 15 x 1 0 i o n s (100 eV^hf^cnT J/D nm  f o r [ B r ~ ] > 0.01 M was  i d e n t i c a l t o t h a t r e p o r t e d by K o u l k e s - P u j o  et a l .  3  1  64  that the e max value f o r B r G(Br  2  I f one assumes  f o r B r _ i n DMSO i s t h e same as t h e r e c e n t l y r e p o r t e d 2 2  i n water,^namely 1.2 x 10^M "'"cm \  ) = 1.3+0.1.  t h e n one c a l c u l a t e s  T h i s v a l u e s h o u l d be compared t o t h a t  obtained  by Kemp e t a l . ^ ^ i n w h i c h t h e p o s i t i v e i o n s were scavenged u s i n g 0.01  M TMPD ( N , N , N * , N ' - t e t r a m e t h y l - p - p h e n y l e n e d i a m i n e ) .  G(DMS0 ) = +  1.7 was c a l c u l a t e d from the y i e l d o f TMPD w h i c h i s somewhat l a r g e r +  than the v a l u e o b t a i n e d  here.  S e v e r a l i n t e r e s t i n g f e a t u r e s a r o s e from the s t u d y o f the K B r s o l u t i o n s i n DMSO.  From t h e o s c i l l o s c o p e t r a c e s i n F i g u r e 41 i t can  be seen t h a t a t [Br ]= 0.01 o r 0.001 M, t h e a b s o r p t i o n a t 365 nm d i d n o t grow i n d u r i n g t h e 40 n s e c p u l s e .  Instead a s h o r t - l i v e d t r a n s i e n t  a b s o r p t i o n was o b s e r v e d whose spectrum i s c e n t e r e d a t 450-500 nm (see F i g u r e 4 0 ) .  T h i s t r a n s i e n t c o u l d n o t be c o n f u s e d w i t h t h e s o l v e n t  c a t i o n because i t s a b s o r p t i o n was almost zero a t 650 nm i n a 0.001 M K B r  WAVELENGTH F i g u r e 40.  A b s o r p t i o n s p e c t r a f o r KBr s o l u t i o n s i n DMSO. r e f e r s t o (he t r a n s i e n t p r e c u r s o r o f B ^ solution; H  , t r a n s i e n t p r e c u r s o r o f Br^  Br " a g a i n s t l o g ( [ B r ~ ] / M ) .  .A,  B^  (nm)  S o l i d c u r v e r e f e r s t o Br2  s p e c t r a ; the d o t t e d  from 0.1 M KBr s o l u t i o n ; #  at 0.01 M KBr.  , B^  curve  from 0.01 M KBr  The i n s e t i s a p l o t o f Ge a t 375 nm f o r  143"  r  -  (a) 365 nm  X=  0.1 M  —^  KBr  17% A  9»  «£  (b)  X=  365 nm  0.01 M KBr 1.4% A  J &  e  (c)  \ = 365 nm 0.001 M KBr  2.0% A  1 ^.sec Figure 41.  T y p i c a l o s c i l l o s c o p e t r a c e s showing the b u i l d up and decay o f Br  2  a t 365 nm i n pure DMSO.  m u l t i p l i e r w i t h a 470 ohm  D e t e c t i o n made u s i n g the p h o t o -  load r e s i s t o r .  (b) 0 . 0 1 M K B r ; (c) 0 . 0 0 1 M KBr.  (a) 0 . 1 M K B r ;  - 144 -  solution.  A t t h i s c o n c e n t r a t i o n , t h e t r a n s i e n t decayed w i t h a h a l f -  l i f e o f ^0.8 usee, w h i c h was a p p r o x i m a t e l y Br  2  a b s o r p t i o n a t 365 nm grew i n .  4 1 ( c ) , was observed 42.  t h e same r a t e a t w h i c h t h e  T h i s b u i l d up, shown i n F i g u r e  t o f o l l o w f i r s t - o r d e r k i n e t i c s as shown i n F i g u r e  I n t h i s p l o t D^ i s t h e absorbance o f B r ^ a t 365 nm a t time t a f t e r  t h e p u l s e and D CO  b u i l d up.  i s the maximum absorbance o f Br„ 2  attained a f t e r the  S i n c e t h e b u i l d up and decay o f B r ^ a t 0.001 M K B r were  not w e l l s e p a r a t e d i n t i m e , D  m  was o b t a i n e d b y e x t r a p o l a t i n g t h e decay  c u r v e back t o t h e end o f t h e p u l s e and t a k i n g t h e v a l u e o f t h e absorbance on t h e e x t r a p o l a t i o n c o r r e s p o n d i n g t o time t . The pseudo f i r s t - o r d e r r a t e c o n s t a n t o b t a i n e d from t h i s p l o t , k = (8.7 + 0 . 3 ) 5 —1 8 x 10 s e c , y i e l d s a s e c o n d - o r d e r r a t e c o n s t a n t o f (8.7 + 0 . 3 ) x 10 M ^sec  L  f o r <he r e a c t i o n between t h e t r a n s i e n t and B r  t o form t h e  Br2 i o n .  I t i s p r o p o s e d t h a t t h e decay o f DMS0 and t h e f o r m a t i o n o f B r +  goes v i a a two s t e p p r o c e s s , a s  2  shown b e l o w , w i t h t h e second s t e p  being rate determining.  DMS0  +  +  Br"  [DMS0  +  Br~] — — * - B r ~ + 2  DMSO (40)  The t r a n s i e n t i n t e r m e d i a t e c e n t e r e d a t 450-500 nm i s a t t r i b u t e d t o t h e charge t r a n s f e r complex [DMS0 ....Br ] . +  The t r a n s i e n t complex i s observed  a t t h e end o f t h e 40 n s e c p u l s e  even i n 0.001 M K B r , as shown i n F i g u r e 43, w h i c h s u g g e s t s h a l f - l i f e o f t h e DMSO p o s i t i v e i o n i s < 1 0 Scavenging  -  1  1  that the  seconds i n 0.1 M K B r .  on t h i s time s c a l e s h o u l d a f f e c t geminate r e c o m b i n a t i o n and  145  -  -1.90  0.4 F i g u r e 42.  0.8  1.2 T I M E tyisec)  P l o t showing f i r s t - o r d e r b u i l d up of Br^ DMSO s o l u t i o n 0.001  M i n KBr.  (Figure 41(c)) f o r  - 146 -  (a)  X  = 400 nm  0.1 M KBr ^  ^  ^  ^  ^  ^  1.4% A  1 yisec  \  (b)  X = 450 nm  \ \  0.01 M KBr  1.0% A ii  *> s>  £ (c)  X = 450 nm 0.001 M KBr 1.9% A  F i g u r e 43.  T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay o f B r ^ and i t s transient precursor.  The f a s t i n i t i a l decay i n (b) and ( c )  i s a t t r i b u t e d t o the t r a n s i e n t complex.  D e t e c t i o n was made  u s i n g t h e p h o t o m u l t i p l i e r w i t h a 470 ohm l o a d r e s i s t o r .  - 147 thereby  i n c r e a s e the y i e l d o f e ; however no such i n c r e a s e was observed. g  T h i s i m p l i e s t h a t the e l e c t r o n can r e a c t w i t h the complex d u r i n g an i n t r a - s p u r decay p r o c e s s  i n the same manner as i t does w i t h t h e  p o s i t i v e i o n according t o r e a c t i o n (41),  [DMSO  +  Br~] +  e~  DMSO* +  Br~  (41)  * where DMSO  i s an e x c i t e d DMSO m o l e c u l e .  From the s e c o n d - o r d e r r a t e — 8  c o n s t a n t f o r the r e a c t i o n between t h i s complex and B r , 8.7 x 10 M ''"sec \  the h a l f - l i f e o f t h i s complex i s e v a l u a t e d t o be ^ 8 n s e c  i n 0.1 M KBr.  T h i s i s too l o n g t o a f f e c t the r e c o m b i n a t i o n  the spur e l e c t r o n given i n equation e  g  i s expected  (41); consequently  t o be unchanged, as o b s e r v e d .  the y i e l d o f TSr^  with  the y i e l d o f  However, the f a c t t h a t  h a s d i m i n i s h e d by about 30% a t 0.001  M implies that  the charge t r a n s f e r complex has an a l t e r n a t i v e f a t e , perhaps d i s s o c i a t i o n back i n t o the B r i o n and the p o s i t i v e i o n .  There i s a t r a c e o f  some t r a n s i e n t s p e c i e s i n the w a v e l e n g t h r e g i o n where the p o s i t i v e ion  a b s o r b s w h i c h i s too l o n g - l i v e d t o be e i t h e r the e l e c t r o n o r the  t r a n s i e n t complex.  T h i s s p e c i e s c o u l d be the p o s i t i v e i o n i t s e l f  o r e l s e i t must be the a t t r i b u t e d t o y e t another  transient species.  So f a r i t h a s been assumed t h a t the o x i d i z i n g s p e c i e s  observed  to have an a b s o r p t i o n band c e n t e r e d a t 550 nm i s the DMSO p o s i t i v e ion.  However, the r e s u l t s a r e c o m p a t i b l e w i t h t h i s t r a n s i e n t b e i n g  some o t h e r o x i d i z i n g s p e c i e s , perhaps e i t h e r a f r e e r a d i c a l o r o t h e r c a t i o n p r o d u c e d by the r a p i d d e c o m p o s i t i o n  o f DMS0 .  i t seems c l e a r t h a t G ( o x i d i z i n g s p e c i e s ) = 1.3, ° the 550 nm band i s 3500 M~ cm~ . 1  1  +  I n any e v e n t  i n w h i c h case c  for max  - 148 -  Thus, from the p u l s e r a d i o l y s i s s t u d i e s , i t t r a n s p i r e s t h a t the y i e l d of primary  reducing species (  e g  ) and o x i d i z i n g s p e c i e s  ( p r o b a b l y DMSO ) a r e i d e n t i c a l , b e i n g 1.3 + 0 . 1 . +  T h i s y i e l d can  hence be e q u a t e d t o the f r e e i o n y i e l d i n DMSO.  3.  Geminate I o n S c a v e n g i n g A t h i g h c o n c e n t r a t i o n s o f I^SO^,which i s an e f f i c i e n t e l e c t r o n  scavenger»the e l e c t r o n band was c o m p l e t e l y e l i m i n a t e d but the band c e n t e r e d a t 550 nm was found t o be i n c r e a s e d m a r k e d l y . a t t r i b u t e d t o the s c a v e n g i n g  This i s  o f e l e c t r o n s w h i c h were o t h e r w i s e doomed  t o geminate r e c o m b i n a t i o n w i t h t h e i r concomitant the p o s i t i v e i o n y i e l d i s i n c r e a s e d .  partner;  consequently  The i n c r e a s e i n absorbance f o r  a DMSO s o l u t i o n c o n t a i n i n g 0.2 M H^SO^ i s shown i n F i g u r e 44 and i s 64 s i m i l a r t o t h a t r e p o r t e d p r e v i o u s l y u s i n g 0.1 M I^SO^.  In addition,  a l o n g - l i v e d t r a n s i e n t h a v i n g an a b s o r p t i o n c e n t e r e d a t 450 nm w i t h 3 - L -1 -1 Ge ^ 10 m o l s . (100 eV) M cm at i t s X was a l s o o b s e r v e d . This max t r a n s i e n t i s thought t o be the s u l f a t e r a d i c a l a n i o n , SO^ , w h i c h i s known t o be formed i n the p u l s e r a d i o l y s i s o f aqueous s u l f u r i c  acid  78—80 systems.  I n the aqueous system i t i s thought t o a r i s e from  'OH  +  HSO. 4  >- H„0 + 2  SO. 4  (42)  I n DMSO i t may a r i s e from the m e t h y l r a d i c a l s a c c o r d i n g t o r e a c t i o n (43) s i n c e the m e t h y l r a d i c a l y i e l d i s h i g h (G(CH *) > 3 ) . 3  'CH_  + 3  HSO. 4  *- CH. + 4  SO. 4  (43)  149 -  10r  400  500 600 W A V E L E N G T H (nm)  700  F i g u r e 44. End o f p u l s e a b s o r p t i o n spectrum o f the DMSO p o s i t i v e i o n [0] i n 0.2 M H„S0.. 2 4  The d o t t e d l i n e r e f e r s t o t h e absorbance o f t h e  p o s i t i v e i o n i n pure DMSO.  X r e f e r s t o t h e l o n g - l i v e d SO^  i n t e r m e d i a t e produced i n t h e a c i d s o l u t i o n .  - 150 -  When c o r r e c t i o n s f o r the decay o f t h i s s u l f a t e r a d i c a l a n i o n were a p p l i e d , i t was n o t e d  t h a t the DMSO p o s i t i v e i o n decays b y a f i r s t -  o r d e r p r o c e s s w i t h almost the same h a l f - l i f e constant  (2.5 usee) and r a t e  (2.7 x 10~*sec ) as i t d i d i n p u r e DMSO (see F i g u r e X  45).  T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay o f the p o s i t i v e i o n and the l o n g - l i v e d SO^ The  r a d i c a l anion are presented  a b s o r p t i o n spectrum o b t a i n e d f o r  + M Ag i s shown i n F i g u r e 47.  0.5  i n F i g u r e 46.  a DMSO s o l u t i o n c o n t a i n i n g  By a n a l o g y w i t h aqueous systems  the a b s o r p t i o n maximum a t X < 330 nm i s a t t r i b u t e d t o the  81—83  silver  atom formed by t h e r e a c t i o n  e~  +  Ag  »- Ag°  +  (44)  However a t X > 350 nm i t appears t h a t t h e r e i s a b s o r p t i o n from o t h e r s p e c i e s i n a d d i t i o n t o DMS0 , p o s s i b l y one c e n t e r e d a t 400-500 nm. +  In  c o n t r a s t t o the H  +  system d i s c u s s e d above, the absorbances i n t h e  range 320-700 nm were a l l much l o n g e r l i v e d w i t h f i r s t > 100 usee. order. Ag^ , +  half-lives  The decay k i n e t i c s were n e i t h e r f i r s t - o r d e r n o r  Although  a  v a r i e t y o f s i l v e r i o n adducts,  second-  such as A g  + 2  and  a r e known t o be formed i n the p u l s e r a d i o l y s i s o f aqueous  s i l v e r s o l u t i o n s , they a r e not known t o absorb a t X > 400 nm.  It i s  p o s s i b l e t h a t the peak c e n t e r e d a t 400-500 nm i s due t o a complex o f the DMS0  +  i o n and a s i l v e r atom o r o t h e r s i l v e r a g g r e g a t e s .  absorbances were not w e l l s e p a r a t e d  Because t h e  i n t i m e , i t i s not p o s s i b l e t o s t a t e  whether the o x i d i z i n g s p e c i e s r e s p o n s i b l e f o r the 550 nm band i n pure DMSO e x i s t e d as such i n these s i l v e r s o l u t i o n s .  - 151 -  -1.80  -2.00  -2.20  6 o o  o -2.40  -2.60  -2.80  _L  2.0  1  JL  4.0 6.0 T I M E (^sec)  8.0  F i g u r e 45. F i r s t - o r d e r decay p l o t o f the DMSO p o s i t i v e i o n i n the p r e s e n c e o f 0.2 M H^SO^.. m u l t i p l i e r with  Decay measured a t 550 nm u s i n g t h e p h o t o a 470 ohm l o a d  resistance.  - 152 -  1  X  t  %A  *****  i  >  = 650 nm  ****  «s sec  X=450 nm  2^sec Figure  46.  T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay o f t h e DMSO p o s i t i v e i o n i n t h e p r e s e n c e o f 0.2 M ^ S O ^ a t 650 nm and 450 nm.  The l o n g e r - l i v e d . S O ^  a t 450 nm.  t r a n s i e n t i s r e a d i l y observed  - 154-  4.  D i e l e c t r i c C o n s t a n t and E l e c t r o n The  Stabilization  y i e l d o f f r e e i o n s i n DMSO, G ( f r e e i o n ) = 1.3 + 0.1,  t o be c o n s i s t e n t w i t h i t s r e l a t i v e l y h i g h d i e l e c t r i c c o n s t a n t e m p i r i c a l r e l a t i o n s h i p shown i n F i g u r e 7. s t a t i c d i e l e c t r i c constant i o n y i e l d i n DMSO.  appears and t h e  T h i s would s u g g e s t t h a t t h e  i s the major f a c t o r d e t e r m i n i n g  the  free  On the o t h e r hand, the a b s o r p t i o n maximum o f t h e  s o l v a t e d e l e c t r o n i s a t X > 1500 t r a n s i t i o n energy < 0.8 eV.  nm, c o r r e s p o n d i n g t o an o p t i c a l  I n t h i s r e s p e c t DMSO behaves l i k e a  s a t u r a t e d h y d r o c a r b o n towards e l e c t r o n s o l v a t i o n and s u g g e s t s t h a t t h e s o l v a t i n g power o f the medium p l a y s the dominant r o l e i n e l e c t r o n stabilization.  The n a t u r e o f t h e s e weak i o n - d i p o l e f o r c e s s o l v a t i n g  the e l e c t r o n may be due t o the i n a b i l i t y o f the DMSO m o l e c u l e s t o a l i g n t h e i r d i p o l e s f o r maximum i n t e r a c t i o n w i t h the e l e c t r o n o r due to a l a r g e c a v i t y r a d i u s .  I n p o l a r p r o t i c media, the p o s i t i v e charge  i s l o c a l i z e d on the hydrogen atom and i s a b l e t o f i t more c l o s e l y about the n e g a t i v e  c e n t r e whereas i n DMSO the p o s i t i v e charge due t o  the S -> 0 d i p o l e i s s h i e l d e d by the two m e t h y l groups.  Furthermore,  the b u l k y m e t h y l groups w i l l p r e v e n t an optimum o r i e n t a t i o n o f t h e d i p o l e s around the e l e c t r o n , as w e l l as p r o d u c i n g a l a r g e v o i d , so t h a t the p o l a r i z a t i o n p o t e n t i a l w o u l d be c o n s i d e r a b l y t h e r e was no s t e r i c h i n d e r a n c e .  l e s s than i f  I t i s a l s o i n t e r e s t i n g t o note that o  the c o r r e l a t i o n curve i n F i g u r e 6 g i v e s a c a v i t y r a d i u s > 3.3 A f o r t h e e l e c t r o n i n DMSO based on hv max The are not  < 0.8 eV.  d a t a s u g g e s t , t h e r e f o r e , t h a t the e l e c t r o s t a t i c i n t e r a c t i o n s the same i n s o l v e n t s o f e q u i v a l e n t  d i e l e c t r i c c o n s t a n t and  t h a t e l e c t r o n s t a b i l i z a t i o n o c c u r s t h r o u g h s p e c i f i c i n t e r a c t i o n s as  - 155  -  governed by the m i c r o s c o p i c p r o p e r t i e s o f the medium. are s t i l l  The  electrons  t o be r e g a r d e d as b e i n g s t a b i l i z e d by i o n - d i p o l e i n t e r a c t i o n s ,  b u t the e x t e n t of t h i s i n t e r a c t i o n i s r e l a t e d t o the i n t r i n s i c p r o p e r t i e s o f t h e s o l v e n t , such as the m o l e c u l e shape, s i z e , type o f f u n c t i o n a l g r o u p s , d i p o l e moment and so f o r t h , and n o t s o l e l y t o the m a c r o s c o p i c d i e l e c t r i c c o n s t a n t o f the  continuum.  - 156 -  CHAPTER I V PULSE RADIOLYSIS OF DMSO-WATER BINARY MIXTURES I n the p r e v i o u s  c h a p t e r i t was shown t h a t the e l e c t r o n i n DMSO  i s v e r y w e a k l y t r a p p e d as i n d i c a t e d by i t s a b s o r p t i o n band i n t h e near i n f r a r e d w i t h a X max  > 1500  nm.  On the o t h e r hand, p o l a r p r o t i c  s o l v e n t s , such as w a t e r and the a l c o h o l s , a r e c h a r a c t e r i z e d b y e l e c t r o n a b s o r p t i o n bands i n the v i s i b l e r e g i o n . with t h e i r a b i l i t y to solvate negative  i o n s through s t r o n g i o n -  d i p o l e i n t e r a c t i o n s and hydrogen b o n d i n g . as b e i n g  This i s i n keeping  The e l e c t r o n i s r e g a r d e d  s t a b i l i z e d t h r o u g h i n t e r a c t i o n w i t h s e v e r a l m o l e c u l e s so  t h a t s t u d i e s on the a b s o r p t i o n  s p e c t r a , and y i e l d s , o f e l e c t r o n s  s o l v a t e d i n b i n a r y m i x t u r e s are o f i n t e r e s t , p a r t i c u l a r l y when t h e extent  and manner o f s o l v a t i o n d i f f e r markedly i n the  components.  individual  As mentioned i n the I n t r o d u c t i o n , s t u d i e s have been  a t t e m p t e d on b i n a r y m i x t u r e s o f a l c o h o l s o r w a t e r w i t h a p r o t i c 40 41 39 43 44 hydrocarbons ' or ethers. ' ' However t h e s e were n o t i d e a l c o m b i n a t i o n s s i n c e t h e y formed V e r y inhomogeneous m i x t u r e s on t h e microscopic  scale.  T h i s r e s u l t e d i n a g g r e g a t e s o f the p o l a r h y d r o x y l i c  s o l v e n t a c t i n g as s c a v e n g i n g c e n t e r s  f o r e l e c t r o n s w h i c h would  o t h e r w i s e have been l o s t t h r o u g h geminate r e c o m b i n a t i o n i n t h e a p r o t i c component.  C o n s e q u e n t l y , b o t h the y i e l d and o p t i c a l p r o p e r t i e s o f t h e  s o l v a t e d e l e c t r o n s were dominated by the p o l a r component.  - 157 -  DMSO would appear t o be a model h y d r o c a r b o n - l i k e s t u d y i n g such b i n a r y m i x t u r e s .  solvent for  U n l i k e the a p r o t i c s o l v e n t s mentioned  above i t h a s a h i g h f r e e i o n y i e l d , comparable t o t h a t o f the h y d r o x y l i c solvents.  As a r e s u l t , t h e s c a v e n g i n g o f geminate i o n s by p o l a r  a g g r e g a t e s s h o u l d not be a dominant f a c t o r . completely mixtures  F u r t h e r m o r e , DMSO i s  m i s c i b l e w i t h w a t e r i n a l l p r o p o r t i o n s ; i n f a c t DMSO-water  show more i n t e r m o l e c u l a r s t r u c t u r e t h a n i s p r e s e n t i n  e i t h e r o*f t h e pure il i• q u i-A d s . »~92 8Z  1.  Solvated  Electrons  I n DMSO-water b i n a r y m i x t u r e s o b s e r v e d f o r w a v e l e n g t h s > 400 nm. band ( n/2 T  was  two e a s i l y r e s o l v a b l e bands were As i n p u r e DMSO, the l o n g e r - l i v e d  ^ 2 usee) was a t t r i b u t e d t o the DMSO p o s i t i v e i o n and  centered  a t 550 nm f o r a l l m i x t u r e s .  The o t h e r band was a s s i g n e d  t o the s o l v a t e d e l e c t r o n and was v e r y s h o r t - l i v e d ( t - , ^ 25 n s e c ) . <  The  X max  o f the e l e c t r o n band s h i f t e d from t h a t i n p u r e w a t e r (X = max  720 nm) t o t h a t i n p u r e DMSO (X > 1500 max  nm) w i t h i n c r e a s i n g DMSO  r  concentration. observed.  I n each m i x t u r e  o n l y a s i n g l e e l e c t r o n band was  The l a c k o f " s h o u l d e r s " o r b r o a d e n i n g i n the bands suggest  t h a t t h e s e s p e c t r a a r e n o t the r e s u l t o f a s i m p l e c o m b i n a t i o n o r o v e r l a p of the e and e „ a b s o r b a n c e s . To prove t h i s a s e r i e s o f aq DMSO W O r t  r  r e p r e s e n t a t i v e s p e c t r a a t v a r i o u s c o n c e n t r a t i o n s were  constructed  based on s i m p l e band o v e r l a p but the r e s u l t a n t s p e c t r a d i d not a t a l l w i t h the o b s e r v e d s p e c t r a .  agree  Furthermore, simple combination o f  the two s p e c t r a would r e q u i r e f r e e " i c e b e r g s " o f DMSO and w a t e r m o l e c u l e s i n the m i l i e u w h i c h i s u n l i k e l y i n v i e w o f the s t r o n g molecular  i n t e r a c t i o n s e x h i b i t e d by the two components.  inter-  - 158 -  F i g u r e 48 shows the s p e c t r a f o r the s o l v a t e d e l e c t r o n i n mixtures  c o n s i s t i n g o f 0, 0.20,  f r a c t i o n DMSO.  0.28,  0.43,  0.72,  0.93 and 1.0 mole  The s p e c t r a were a l l taken u s i n g 40 n s e c p u l s e s .  They have n o t been c o r r e c t e d f o r decay d u r i n g the p u l s e nor f o r t h e r e s p o n s e time o f t h e d e t e c t i o n a p p a r a t u s ;  b u t t h e c o n t r i b u t i o n s from  the DMSO p o s i t i v e i o n s were deduced by e x t r a p o l a t i o n t o t h e end o f the p u l s e from times  > 100 n s e c when the e l e c t r o n band had f u l l y  d e c a y e d , and t h e s e have been s u b t r a c t e d . hydrated  The a b s o r p t i o n band o f t h e  4 - l - I -1 e l e c t r o n , w i t h Ge = 4.3 x 10 mols. (100 eV) M. cm a t -  \nax The  nm), c o r r e s p o n d s c l o s e l y t o the p u b l i s h e d s p e c t r u m o f  a  q'  s o l v a t e d e l e c t r o n decay f o l l o w e d good f i r s t - o r d e r k i n e t i c s i n a l l  binary mixtures. and  e  93  I n pure w a t e r the k i n e t i c s were a m i x t u r e  s e c o n d - o r d e r and were not a n a l y z e d  l i f e t i m e was > 10 ^ s e c .  f u r t h e r although  Decay p l o t s f o r e  10 n s e c p u l s e s and f o l l o w e d a t X > 1000  g  of f i r s t -  the mean  were o b t a i n e d  using  nm where t h e c o n t r i b u t i o n o f  the p o s i t i v e i o n was n e g l i g i b l e f o r a l l m i x t u r e s .  As i n pure DMSO, t h e  s o l v a t e d e l e c t r o n i s thought t o decay b y r e a c t i n g w i t h DMSO. f i r s t - o r d e r rate constants (column 6) i n T a b l e IV.  thus o b t a i n e d a r e e x p r e s s e d  The pseudo  as h a l f - l i v e s  the observed X , max the w i d t h - a t - h a l f - h e i g h t i n energy u n i t s , AW, and t h e o b s e r v e d end o f p u l s e a b s o r b a n c e , Ge (obs) o f t h e s o l v a t e d e l e c t r o n band f o r each max o f the m i x t u r e s .  This t a b l e a l s o records  A s e c o n d - o r d e r r a t e c o n s t a n t was c a l c u l a t e d f o r  each m i x t u r e by c o m b i n i n g the observed f i r s t - o r d e r r a t e c o n s t a n t the b u l k c o n c e n t r a t i o n o f DMSO.  with  These are g i v e n i n column 7 o f  T a b l e I V and are o b s e r v e d t o be f a i r l y c o n s t a n t  a t 4.5 x 10^ M "'"sec "*".  T h i s i s somewhat h i g h e r than t h a t f o r pure DMSO and f o r t h e d i l u t e  30  —  / -  /A  /  //  A /0.20\\  LU <J  2° x0.3  \V  / •  *• \  1 ;  —  H  . \  / / 0.28 \ \ #• *. \ / • • •• • \ /: • \ /• / • • ••\ .•\\• \ #. •  -  <  //  g 10  DMSO 0.93  / 0.43  to CD <  •-.....0.72  1 400 Figure 48.  ^  ,  1  .  600  1  .  1  .  1  800  1 0 0 0 1200 WAVELENGTH (nm)  1400  Absorption spectra of solvated electrons i n DMS0-H 0 mixtures; 0, 0.20, 0.28, 0.43, 0.72, 0.93 and 1.0 2  mole fraction DMSO.  Data points were obtained at 50 nm i n t e r v a l s .  multiplied by a factor of 0.3 r e l a t i v e to the others.  The data for pure water have been  TABLE IV. Summary of data obtained from studies on pulse i r r a d i a t e d DMSO-water mixtures at room temperature.  Mole Fraction DMSO  D s  3  X max (nm)  AW (eV)  0  78  720  0.88  0.017  78  —  —  0.20  75  750  0.76  0.28  73  800  0.83  0.43  67  900  0.72  57  0.93 1.00  k (calc)  ^ max(obs) (x 10" ) 3  b  2  G  (nsec)  Ge (corr)° max  , -1 -1, (M sec ) M  .—  (x 10~ ) 3  43.7  >10  —  340  2.9 x 10  6  11.0  22  4.5 x 10  6  20.5  7.22  19  4.4 x 10  6  14.8  0.97  4.24  15  4.5 x 10  6  10.2  1000  0.86  3.85  12  4.6 x 10  6  11.2  48  1350  >0.65  5.85  —  —  46  >1500  __  Data taken from references (84,85)  >7.44  3  14.5  3.4 x 10  43.7  —  — 6  >18.9  for mixture temperature of 25°C.  Calculated from the observed pseudo-first order rate constant using bulk concentration of DMSO. Corrected f o r decay during the pulse and response time of the detection system.  ON  o  -  161 -  s o l u t i o n , 0 . 0 1 7 mole f r a c t i o n DMSO ( 0 . 7 M i n DMSO). i n t h i s l a t t e r m i x t u r e , 2 . 9 x 1 0 ^ M ^"sec \  The r a t e c o n s t a n t  i s h i g h e r than t h e  p u b l i s h e d v a l u e s f o r the r e a c t i o n o f DMSO w i t h h y d r a t e d e l e c t r o n s a t c o n c e n t r a t i o n s up to and i n c l u d i n g 0 . 7 M DMSO i n w a t e r  (k = 1 . 6 x  6.,-l - 1 . 69,94 10 M s e c ) . i n  The o b s e r v e d v a l u e s , G E ( o b s ) , were c o r r e c t e d f o r decay d u r i n g max t h e p u l s e and response t i m e o f t h e o p t i c a l d e t e c t i o n system u s i n g t h e same c o m p u t a t i o n a l p r o c e d u r e s mentioned p r e v i o u s l y . Ge  max  The v a l u e s o f  (obs) and Ge ( c o r r ) f o r e a r e p l o t t e d as a f u n c t i o n o f t h e max s r  mole f r a c t i o n DMSO i n F i g u r e 49 and b o t h a r e seen t o pass t h r o u g h a minimum, the c o r r e c t e d one b e i n g more pronounced.  T h i s minimum may  a r i s e from a change i n G, e o r a c o m b i n a t i o n o f b o t h .  I t i s believed  t h a t t h e minimum a r i s e s from a G v a l u e e f f e c t f o r two r e a s o n s . the v a l u e o f e  max  Firstly,  was e s t i m a t e d i n the p r e v i o u s c h a p t e r t o be  % 1 7 , 0 0 0 M "'"cm ^ w h i c h i s s i m i l a r t o e max  = 1 8 , 5 0 0 M "'"cm "*" f o r  95  e  a t 720 nm.  Consequently  the p u r e components have v e r y  similar  c o e f f i c i e n t s s o t h a t i t seems u n l i k e l y t h a t the m i x t u r e w i l l show t h e s o r t o f minimum g i v e n i n F i g u r e 4 9 .  T h i s i s f u r t h e r s u p p o r t e d by  t h e f a c t t h a t AW i s f a i r l y c o n s t a n t f o r a l l m i x t u r e s . s t r e n g t h o f an a b s o r p t i o n band, f  > i s r e l a t e d t o the i n t e g r a t e d  m o l a r e x t i n c t i o n c o e f f i c i e n t e by the f  mn  =  4.32 x 10~ F / 9  / ^ l  The o s c i l l a t o r  relationship^ e d ai  (4.1)  where the i n t e g r a t i o n extends o v e r the e n t i r e band r e l a t e d t o t h e t r a n s i t i o n from the s t a t e n-«-m and F i s a c o r r e c t i o n f a c t o r n e a r  unity  - 162 -  78 I  1  O F i g u r e 49.  75 f—  •  i  68  61 i  1  i  I  i  1  52 1  46 1  i—I—.—I  0.2 0.4 0.6 0 . 8 1.0 M O L E F R A C T I O N DMSO  P l o t o f t h e v a l u e s o f Ge  f o r the s o l v a t e d e l e c t r o n absorption max  bands p r e s e n t e d  r  i n F i g u r e 48 as a f u n c t i o n o f t h e mole f r a c t i o n  DMSO f o r t h e DMSO-l^O m i x t u r e s .  The n o n - l i n e a r a x i s showing t h e  change as a f u n c t i o n o f s t a t i c d i e l e c t r i c c o n s t a n t mixture  of the bulk  i s shown on t h e t o p a b s c i s s a . O » a c t u a l o b s e r v e d  absorbance peak h e i g h t s . •  , c o r r e c t e d f o r decay d u r i n g t h e  p u l s e and response time o f t h e d e t e c t o r .  - 163  -  r e l a t e d t o the r e f r a c t i v e i n d e x of the medium w h i c h c o n t a i n s absorbing  species.  Ef. x  =  the  For a s i n g l e e l e c t r o n  1  (4.2)  where Ihe summation e x t e n d s o v e r a l l a b s o r p t i o n s i n v o l v i n g the e l e c t r o n . F o r most s o l v a t e d e l e c t r o n bands the o s c i l l a t o r s t r e n g t h i s ^ 0.7  + 0.2  w h i c h i m p l i e s t h a t t r a n s i t i o n s o t h e r t h a n (2p «- I s ) must  be v e r y weak i f they o c c u r a t a l l .  Assuming a G a u s s i a n shaped  absorption  band f o r the e l e c t r o n , to ' e d a) co, /  -  e  max  AW  (4.3)  so t h a t (4.1) becomes  f  mn  -  (constant) e  max  AW  (4.4)  By a n a l o g y w i t h o t h e r s o l v a t e d e l e c t r o n bands the o s c i l l a t o r i s not e x p e c t e d t o change a p p r e c i a b l y f o r the m i x t u r e s , AW o b s e r v e d t o be c o n s t a n t and hence the e  s h o u l d remain  strength  is reasonably  max constant.  U n f o r t u n a t e l y s u i t a b l e e l e c t r o n scavengers,  a r e n o t s o l u b l e enough i n t h e s e b i n a r y m i x t u r e s  such as a n t h r a c e n e ,  to g i v e an e l e c t r o n y i e l d  measurable by p u l s e r a d i o l y s i s so t h a t t h i s i n f e r e n c e c o u l d n o t be I t s h o u l d be n o t e d t h a t t h e o b s e r v e d minimum may an e x p e r i m e n t a l  artifact.  One  verified.  a l s o a r i s e from  o f the p a r a m e t e r s used i n the p r o c e d u r e  f o r c o r r e c t i n g the o b s e r v e d absorbance i s the e l e c t r o n h a l f - l i f e . S i n c e t h i s minimum a l s o c o i n c i d e s a p p r o x i m a t e l y  w i t h the f a s t e s t  - 164 -  decaying electrons  (see T a b l e I V ) i t i s p o s s i b l e t h a t t h e measured  decay r a t e may have o v e r - e s t i m a t e d t h e h a l f - l i f e s l i g h t l y , p r o d u c i n g a more pronounced  minimum.  However, t h e second o r d e r r a t e c o n s t a n t s  a r e a l l r e a s o n a b l y c o n s t a n t i n t h i s r e g i o n w h i c h would s u g g e s t  that  t h i s cannot be t h e r e a s o n f o r t h e minimum. I t i s s u g g e s t e d t h a t t h i s minimum i n t h e s o l v a t e d e l e c t r o n y i e l d , i f i t i s a r e a l e f f e c t , a r i s e s n o t from a d e c r e a s e i n t h e f r e e i o n y i e l d b u t r a t h e r from an enhanced r e a c t i o n o f t h e t h e r m a l e l e c t r o n s prior to solvation.  There a r e s e v e r a l r e a s o n s f o r t h i s  conclusion.  The b u l k d i e l e c t r i c c o n s t a n t f o r t h e m i x t u r e s changes m o n o t o n i c a l l y w i t h c o m p o s i t i o n , from D = 78 (water) t o D = 4 6 (DMSO), e x h i b i t i n g s s no minimum.  C o n s e q u e n t l y one would n o t e x p e c t t h e f r e e i o n y i e l d t o  pass t h r o u g h a minimum.  F u r t h e r m o r e , i f t h e minimum a r o s e from a  f r e e i o n y i e l d e f f e c t , t h e n t h e y i e l d o f DMSO p o s i t i v e i o n s s h o u l d have shown a s i m i l a r e f f e c t i n t h e s e m i x t u r e s .  I n f a c t they d i d n o t , as  w i l l be shown l a t e r , s o t h a t i f t h i s minimum i s t o be a t t r i b u t e d t o a change i n t h e free i o n y i e l d i t must a r i s e s o l e l y from t h e c o n t r i b u t i o n of w a t e r t o t h e t o t a l f r e e i o n y i e l d o f e  g  i n the mixtures.  However,  i t i s u n r e a s o n a b l e t o suppose t h a t w a t e r c o u l d a f f e c t t h e y i e l d so m a r k e d l y a t > 0.8 mole f r a c t i o n DMSO where l e s s t h a n 10% o f t h e p r i m a r y i o n i z a t i o n events i n v o l v e water molecules. I t i s o b s e r v e d t h a t t h e l i f e t i m e o f the s o l v a t e d e l e c t r o n i n t h e s e m i x t u r e s p a s s e s t h r o u g h a s i m i l a r minimum t o t h e Ge v a l u e s and t h i s suggests that e pure DMSO.  g  r e a c t f a s t e r w i t h DMSO i n t h e m i x t u r e s t h a n i n  I f the p r e c u r s o r e l e c t r o n s of e  g  a r e a l s o more r e a c t i v e  i n t h e s e m i x t u r e s , t h e n t h i s would account f o r t h e minimum i n t h e  - 165 -  94  solvated electron y i e l d . Recently Koulkes-Pujo e t a l . reported the e f f e c t o f d i l u t e s o l u t i o n s o f DMSO on t h e h y d r a t e d e l e c t r o n up t o 3 . 5 M i n DMSO ( 0 . 0 7 7 mole f r a c t i o n DMSO)  and observed an i n c r e a s i n g  v a l u e o f k/G w i t h i n c r e a s i n g DMSO c o n c e n t r a t i o n .  In this  expression  k i s the r a t e constant f o r the r e a c t i o n of e + DMSO ( 1 . 6 x 1 0 ^ aq M ''sec *) and G i s t h e a p p a r e n t y i e l d o f e ^ . t r e a t e d as a c o n s t a n t  i n t h i s s t u d y so t h a t the i n c r e a s e i n k/G  c o r r e s p o n d s t o a d e c r e a s e i n G. the a u t h o r s  The r a t e c o n s t a n t i s  T h i s b e h a v i o r was c o n t r a r y t o what  observed w i t h other e l e c t r o n scavengers,  such as N^O, H  +  and CH^Cl, i n w h i c h t h e G v a l u e i n c r e a s e d due t o s p u r p e n e t r a t i o n . T h i s phenomena i n DMSO was a t t r i b u t e d t o t h e u n h y d r a t e d e l e c t r o n s b e i n g scavenged b e f o r e they c o u l d become s o l v a t e d , t h e r e b y a d e c r e a s e i n G.  causing  These r e s u l t s a r e e n t i r e l y c o n s i s t e n t w i t h t h e  observations i n t h i s study.  A d e c r e a s e d s o l v a t e d e l e c t r o n y i e l d (G)  and/or an i n c r e a s e d decay r a t e (k) as t h e c o n c e n t r a t i o n o f DMSO i s i n c r e a s e d i n t h i s mole f r a c t i o n r e g i o n would g i v e an i n c r e a s e d k/G.  e  s  I t i s i n t e r e s t i n g t h a t t h i s r e g i o n o f minimum absorbance due t o o c c u r s where t h e s e DMSO-water m i x t u r e s show maximum i n t e r m o l e c u l a r  s t r u c t u r e as s i g n i f i e d by a l a r g e v i s c o s i t y i n c r e a s e , n e g a t i v e  heat  84-92  o f m i x i n g and c o n s i d e r a b l e volume c o n t r a c t i o n .  Furthermore,  measurements on t h e s p i n - l a t t i c e  (T^) r e l a x a t i o n  (T^) and t r a n s v e r s e  times i n d i c a t e a minimum i n m o l e c u l a r m o b i l i t y around 0 . 3 5 mole 89  f r a c t i o n DMSO.  Perhaps t h i s i n c r e a s e d m o l e c u l a r  structure provides  f o r a lower a c t i v a t i o n energy f o r the r e a c t i o n o f t h e e l e c t r o n , b o t h q u a s i - f r e e and s o l v a t e d , w i t h DMSO.  The l i f e t i m e s o f these  f r e e e l e c t r o n s may be f u r t h e r reduced by t h e i n c r e a s e d  quasi-  molecular  - 166 -  r e l a x a t i o n time.  Those e l e c t r o n s w h i c h a r e i n i t i a l l y t r a p p e d i n l e s s  t h a n optimum v o i d s may r e a c t w i t h the medium b e f o r e they become f u l l y solvated. I n F i g u r e 50, the v a l u e s o f the photon energy ( i n cm ) a t t h e x  a b s o r p t i o n band maxima o f the m i x t u r e s ,  , are p l o t t e d a g a i n s t max t h e i r respective bulk d i e l e c t r i c constant. The d i e l e c t r i c c o n s t a n t 8  d a t a f o r the DMSO-water b i n a r y m i x t u r e s was o b t a i n e d  from t h e  I t i s i n t e r e s t i n g , b u t perhaps f o r t u i t o u s , t h a t t h i s p l o t i s l i n e a r , w i t h i n experimental  literature. approximately  e r r o r , o u t t o 0.9 mole f r a c t i o n DMSO. The  a b s o r p t i o n band w i d t h - a t - h a l f - h e i g h t , AW, i s f a i r l y c o n s t a n t  despite  t h e widely d i f f e r e n t s o l v a t i o n e n e r g i e s p r o v i d e d by DMSO and w a t e r separately.  This behavior  i s c o n s i s t e n t w i t h the continuum  o r semi-  continuum model i n w h i c h the e l e c t r o n can sample the environment o f t h e mixture.  I f t h e e l e c t r o n was t r a p p e d by s o l v a t i o n w i t h o n l y a s m a l l  number of m o l e c u l e s t h e n the a b s o r p t i o n bands c o u l d be e x p e c t e d t o be much b r o a d e r .  The s h i f t i n a b s o r p t i o n from the n e a r i n f r a r e d  DMSO) t o the v i s i b l e  (pure  (pure w a t e r ) w i t h i n c r e a s i n g w a t e r c o n t e n t may  be due t o e i t h e r a d e c r e a s e i n t h e c a v i t y r a d i u s o r t o an i n c r e a s e i n t  e l e c t r o n - d i p o l e i n t e r a c t i o n o r a combination a plot of E A  of both.  F i g u r e 51 shows  a g a i n s t the mole f r a c t i o n o f w a t e r i n the m i x t u r e .  From  max t h i s c o r r e l a t i o n curve and F i g u r e 50 i t can be seen t h a t n e i t h e r DMSO n o r w a t e r dominate the o p t i c a l p r o p e r t i e s o f the a b s o r p t i o n band and s u g g e s t s t h a t e l e c t r o n s o l v a t i o n o c c u r s by a c o m b i n a t i o n  of strong  ( w a t e r ) and weak (DMSO) i o n - d i p o l e i n t e r a c t i o n s , the e x t e n t o f w h i c h i s governed l a r g e l y by t h e c a v i t y r a d i u s t h r o u g h t h e c o m p o s i t i o n mixtures.  o f the  - 167 -  M O L E FRACTION DMSO 1.0 j  F i g u r e 50.  0.87  0.61  0.34  0.0  1  1  1  1  P l o t o f t h e photon energy o f the a b s o r p t i o n band maximum f o r the s o l v a t e d e l e c t r o n i n t h e DMSO-I^O m i x t u r e s  a g a i n s t the  bulk s t a t i c d i e l e c t r i c constant o f the mixtures The n o n - l i n e a r a x i s showing t h e c o r r e s p o n d i n g DMSO i s shown i n t h e upper a b s c i s s a .  ( a t 25°C).  mole f r a c t i o n  - 168 -  46 h-  52  61 —f-  68  75  78 —l  E u  b X  o E  M O L E FRACTION H 0 2  F i g u r e 51. P l o t o f t h e photon energy o f t h e a b s o r p t i o n band maximum f o r the s o l v a t e d e l e c t r o n i n t h e DMSO-H^O m i x t u r e s mole f r a c t i o n o f w a t e r . corresponding  against the  The n o n - l i n e a r a x i s showing t h e  bulk d i e l e c t r i c constants of the mixtures i s  shown i n t h e upper a b s c i s s a .  - 169 -  Thus the d a t a are c o m p a t i b l e mixtures  .  3  o f ammonia,  8  .  ,  .  w i t h t h a t o f aqueous b i n a r y .  .  ethylenediamme  3 8  .  ,  .  ,  and a l c o h o l s  3 7 , 4 7 , 4 8  ,  ,  .  ,  i n which  the s t a b i l i z e d e l e c t r o n s have a b s o r p t i o n band maxima and h a l f - w i d t h s i n t e r m e d i a t e between t h o s e o f the pure l i q u i d s .  I n these  mixtures  the e l e c t r o n s see a p o l a r i z e d medium i n w h i c h the i n t e r a c t i o n energy i s dependent upon the p r o p e r t i e s o f the continuum. water mixtures  are a t v a r i a n c e w i t h m i x t u r e s v  A  v  and a p r o t i c n o n - p o l a r h y d r o c a r b o n s The  4 0 , 4 1  r e a s o n f o r t h i s i s t h a t the t h e r m a l  However the results in DMSO-  o f o t h e r h y d r o x y l i c media , . » , , . ,  or s l i g h t l y polar  „ .  ethers.  3 9 , 4 3 , 4 4  e l e c t r o n s do not s e e a  d i e l e c t r i c continuum but r a t h e r a random d i s t r i b u t i o n o f s t r o n g l y and w e a k l y p o l a r i z a b l e a g g r e g a t e s \ c o n s e q u e n t l y the o p t i c a l p r o p e r t i e s o f the s t a b i l i z e d e l e c t r o n s are dominated b y the p o l a r component. I t w o u l d appear then t h a t f o r media w h i c h a r e c o m p l e t e l y i n a l l proportions  the y i e l d and o p t i c a l p r o p e r t i e s o f  e l e c t r o n s depend upon the mean b u l k p r o p e r t i e s o f the regardless  miscible  stabilized  mixture,  o f the s o l v a t i o n power e i t h e r component has f o r t h e  electrons.  2.  DMSO P o s i t i v e Ions The  a b s o r p t i o n band c e n t e r e d  a t 550 nm, and a t t r i b u t e d t o the DMSO  p o s i t i v e i o n , d i d not change i n p o s i t i o n o r shape but d i d i n c r e a s e i n magnitude as the DMSO c o n c e n t r a t i o n was i n c r e a s e d .  F i g u r e 52(a)  shows  t h i s absorbance change over the c o m p o s i t i o n  range 0.20 t o 1.00 mole  f r a c t i o n DMSO.  a f t e r the a b s o r p t i o n due  These s p e c t r a were o b t a i n e d  to the s o l v a t e d e l e c t r o n had decayed ( l i f e t i m e < 100 nsec) and were c o r r e c t e d t o zero time b y e x t r a p o l a t i o n back t o the end o f the p u l s e ( t h e  half-  - 170 -  F r a c t i o n of Dose a b s o r b e d by D M S O F i g u r e 52.  (a) A b s o r p t i o n s p e c t r a a t t r i b u t e d produced i n DMS0-H 0 m i x t u r e s . 2  t o t h e DMSO p o s i t i v e i o n  Curve 6 i s pure DMSO; 5, 0.93;  4, 0.72; 3, 0.43; 2, 0.28; and 1, 0.20 mole f r a c t i o n DMSO. (b) Peak absorbance ( a t 550 nm) o f the bands shown i n (a) p l o t t e d a g a i n s t t h e f r a c t i o n o f dose absorbed i n i t i a l l y by DMSO.  - 171 -  life  f o r t h e p o s i t i v e i o n <v 2 usee f o r a l l m i x t u r e s ) .  absorbance  The change i n  o f t h e band a t 550 nm as a f u n c t i o n o f t h e dose i n i t i a l l y  absorbed by t h e DMSO i n t h e m i x t u r e (as g i v e n by i t s e l e c t r o n  fraction  and a p p r o x i m a t e l y e q u a l t o i t s volume %) i s shown i n F i g u r e 5 2 ( b ) . I t can be seen t o be r e a s o n a b l y l i n e a r o v e r t h e range s t u d i e d . The d a t a g i v e n i n F i g u r e 52, when c o u p l e d w i t h t h e f a c t t h a t t h e l i f e t i m e o f DMS0  +  i s unaltered, strongly  indicates that the cation  i s u n a f f e c t e d by t h e w a t e r c o n t e n t , even a t 0.80 mole f r a c t i o n w a t e r . T h i s i m p l i e s t h a t t h e DMSO p o s i t i v e i o n does n o t undergo p r o t o n o r charge t r a n s f e r w i t h t h e w a t e r m o l e c u l e s n o r i s t h e r e any exchange of p r i m a r y o x i d i z i n g s p e c i e s between t h e two components. n o t e d t h a t v a r i o u s p r o t o n a c c e p t o r s , such as ammonia  97  I t s h o u l d be  98 and e t h a n o l ,  have been used i n n o n p o l a r a p r o t i c media as scavengers o f p o s i t i v e i o n s ; b u t t h e above  d a t a s u g g e s t t h a t r e a c t i o n (45) does n o t o c c u r i n  DMSO-water b i n a r y m i x t u r e s .  (CH ) S0 3  2  +  +  H 0  CH SOCH  2  3  2  +  H 0  +  3  (45)  However t h e r e i s an i m p o r t a n t d i f f e r e n c e between n o n p o l a r a p r o t i c media, such as c y c l o h e x a n e , and DMSO.  I n DMSO c a t i o n s a r e s t r o n g l y s o l v a t e d  whereas they a r e e x t r e m e l y u n s t a b l e i n t h e n o n p o l a r a p r o t i c s o l v e n t s and hence w i l l r e a d i l y undergo p r o t o n t r a n s f e r .  S i n c e b o t h DMSO and  w a t e r r e a d i l y s o l v a t e c a t i o n s , t h e r e i s no " d r i v i n g f o r c e " f o r t h e i o n i z e d s p e c i e s t o undergo charge o r p r o t o n In v i e w o f t h e f a i r l y of  transfer.  large difference i n i o n i z a t i o n  99 DMSO (8.85 eV) and w a t e r  100 (12.6 e V ) ,  potential  i t i s s u r p r i s i n g that the  - 172 -  y i e l d o f DMSO p o s i t i v e i o n s i s independent o f the w a t e r c o n t e n t . w o u l d have e x p e c t e d  charge o r i n t e r m o l e c u l a r energy t r a n s f e r t o o c c u r  from the w a t e r t o the DMSO m o l e c u l e s . by t h e h i g h - e n e r g y  One  The i n i t i a l d e p o s i t i o n o f energy  electrons r e s u l t s i n h i g h l y e x c i t e d molecules,  e i t h e r through d i r e c t e l e c t r o n i c e x c i t a t i o n o r e x c i t a t i o n produced b y i o n n e u t r a l i z a t i o n (geminate r e c o m b i n a t i o n ) .  S i n c e these s o l u t i o n s a r e  c o m p l e t e l y homogeneous, e x c i t a t i o n t r a n s f e r between the e x c i t e d w a t e r molecules  and n e i g h b o u r i n g DMSO m o l e c u l e s  l a t t e r being i o n i z e d . suggests  c o u l d have r e s u l t e d i n t h e  N e v e r t h e l e s s , the l i n e a r i t y o f F i g u r e  52(b)  t h a t the y i e l d o f DMSO p o s i t i v e i o n s i n the m i x t u r e i s s i m p l y  proportional to i t s electron fraction.  I t would a l s o appear t h a t t h e  y i e l d o f f r e e i o n s ( t h o s e e l e c t r o n s w h i c h escape geminate  recombination)  i s i n d e p e n d e n t o f t h e b u l k d i e l e c t r i c c o n s t a n t o f the medium. D i l u t i o n o f DMSO by w a t e r (up t o 50% by volume) causes the b u l k d i e l e c t r i c c o n s t a n t t o change from 46 t o 75 ( a t 25°C), y e t t h e f r e e i o n y i e l d o f DMSO, as g i v e n b y t h e y i e l d o f DMSO p o s i t i v e i o n s , i s n o t a p p r e c i a b l y changed.  Perhaps t h e t h e r m a l i z a t i o n p a t h o f the low energy  e l e c t r o n i s s h o r t e n e d by the " t i g h t e r " DMSO-water s t r u c t u r e b u t t h e i n c r e a s e d coulombic dielectric  a t t r a c t i o n energy i s compensated f o r by t h e h i g h e r  constant.  I t s h o u l d be mentioned t h a t i n a m i x t u r e c o n s i s t i n g o f 0.72 mole f r a c t i o n DMSO, the a b s o r p t i o n a t 550 nm was i n c r e a s e d a p p r o x i m a t e l y 90% by t h e p r e s e n c e eliminated. to scavenging  of  0.5 M A g and the e l e c t r o n band +  completely  As i n the case o f p u r e DMSO, t h i s i n c r e a s e i s a t t r i b u t e d o f t h o s e e l e c t r o n s w h i c h were o t h e r w i s e doomed t o geminate  r e c o m b i n a t i o n w i t h t h e i r concomitant  partner.  - 173 -  3.  Transient The  Intermediates  a t 77°K  p u l s e r a d i o l y s i s o f a DMSO-water g l a s s a t 77°K was u n d e r t a k e n  f o r two r e a s o n s .  F i r s t l y , e l e c t r o n s p i n resonance s t u d i e s on y - i r r a d i a t e d  aqueous DMSO g l a s s e s d i d n o t i n d i c a t e  the p r e s e n c e o f t r a p p e d  ( t o be d i s c u s s e d i n the n e x t c h a p t e r ) . may  have been v e r y s h o r t  have decayed c o m p l e t e l y  electrons  However t h e i r n a t u r a l l i f e t i m e  ( l e s s than a few m i n u t e s ) s o t h a t t h e y would b e f o r e e s r measurements c o u l d be t a k e n .  I t was  hoped t h a t the p u l s e r a d i o l y s i s o f a g l a s s y m i x t u r e w o u l d show whether or not trapped rate.  e l e c t r o n s a r e formed and i f s o , t o measure t h e i r decay  Secondly, a purple-coloured  y - i r r a d i a t i o n o f the g l a s s e s . was  the DMSO p o s i t i v e  i n t e r m e d i a t e was o b s e r v e d upon  I t was s u s p e c t e d t h a t t h i s t r a n s i e n t  i o n but o n l y by r e c o r d i n g i t s o p t i c a l spectrum  c o u l d t h i s s u p p o s i t i o n be v e r i f i e d . An aqueous g l a s s c o n t a i n i n g 0.39 mole f r a c t i o n DMSO was p r e p a r e d by r a p i d l y p l u n g i n g  the o p t i c a l c e l l c o n t a i n i n g the m i x t u r e  dewar w i t h o p t i c a l windows f i l l e d w i t h l i q u i d n i t r o g e n .  into a  The m i x t u r e  had been p r e v i o u s l y deoxygenated by b u b b l i n g w i t h h i g h p u r i t y A t the end o f the e x p e r i m e n t b o t h the DMSO-water g l a s s and the were c r a c k e d .  argon. cell  I t i s not known whether the c e l l c r a c k e d upon c o o l i n g  down, warming up o r d u r i n g the e x p e r i m e n t . would probably  prevent contamination  However the " s p l i n t e r  by a i r (oxygen) s i n c e the  cell  was  immersed i n l i q u i d n i t r o g e n .  The  r e s t o f the o p t i c a l d e t e c t i o n system was the same as d e s c r i b e d  previously.  crack"  The o p t i c a l p a t h l e n g t h was 2 mm.  An e l e c t r o n p u l s e w i d t h o f 500 nsec was used w h i c h gave 4  an a p p r o x i m a t e dose o f 10  rads p e r p u l s e .  u s i n g an aqueous KCNS s o l u t i o n  D o s i m e t r y was p e r f o r m e d  as p r e v i o u s l y d e s c r i b e d e x c e p t the  solution  - 174 -  was  s a t u r a t e d w i t h a i r r a t h e r than n i t r o u s o x i d e .  was  t a k e n as 2.9.  As a r e s u l t , GCCNS)^  The d o s i m e t r y was o n l y approximate because t h e  dewar d i d not c o n t a i n an a p p r o p r i a t e medium (such as methanol) t o t a k e a c c o u n t o f the s c a t t e r i n g o f the h i g h energy e l e c t r o n beam by t h e liquid nitrogen.  The r e a s o n f o r t h i s was t h a t the c e l l was c r a c k e d  and  contamination  o f the d o s i m e t e r  The  a c t u a l dose a b s o r b e d by the g l a s s y sample was p r o b a b l y h i g h e r f o r  s o l u t i o n would have r e s u l t e d .  a g i v e n SEM r e a d i n g than t h a t c a l c u l a t e d from the d o s i m e t e r  results  b e c a u s e the c e l l i n l i q u i d n i t r o g e n would be i n e l e c t r o n i c e q u i l i b r i u m ( e l e c t r o n s s c a t t e r e d out o f the c e l l compensated by those s c a t t e r e d i n t o the c e l l i n dense s u r r o u n d i n g s ) . r e a d i n g s , when e x p r e s s e d  C o n s e q u e n t l y the absorbance  as Ge, a r e p r o b a b l y an upper l i m i t .  g l a s s y sample was p h o t o b l e a c h e d  The  between p u l s e s and the t o t a l absorbed  dose was < 3.5 x 10"* r a d s . The  s p e c t r u m observed  i n F i g u r e 53.  a t the end o f the 500 n s e c p u l s e i s shown  Only two d i s t i n c t bands were o b s e r v e d ,  550-600 nm and the o t h e r a t X max  < 400 nm.  one c e n t e r e d a t  The a b s o r p t i o n band o f r  the DMSO p o s i t i v e i o n i n l i q u i d DMSO was n o r m a l i z e d  t o the spectrum o f  the g l a s s y sample a t 600 nm and i s shown as the d o t t e d l i n e i n F i g u r e 53.  I t can be seen t h a t the o b s e r v e d  spectrum d e v i a t e s from t h e  p o s i t i v e i o n band a t X > 700 nm s u g g e s t i n g t h a t a n o t h e r absorbing i n t h i s region.  intermediate i s  T h i s i s p r o b a b l y due t o the t r a n s i e n t  a b s o r p t i o n by the s i l i c a windows ( S u p r a c i l ) o f t h e c e l l due t o e l e c t r o n bombardment a t 77°K. for  two r e a s o n s .  I t i s not b e l i e v e d t o be due t o the t r a p p e d e l e c t r o n  F i r s t l y , the t r a p p e d e l e c t r o n s h o u l d have an a b s o r p t i o n  maximum a t ^ 800 nm on the b a s i s o f the s p e c t r a g i v e n i n F i g u r e 48 f o r  40r-  300 F i g u r e 53.  500  700  900 1100 WAVELENGTH (nm)  1300  1500  A b s o r p t i o n spectrum o f t r a n s i e n t s produced by the p u l s e r a d i o l y s i s o f a DMSO-l^O g l a s s (39 mole % DMSO) a t 77°K.  The p u l s e w i d t h was 500 n s e c , the dose p e r p u l s e b e i n g ^ 10 k r a d . The d o t t e d  t h a t o f the DMSO p o s i t i v e i o n i n pure DMSO n o r m a l i z e d a t 600 nm.  curve  - 176 -  the l i q u i d m i x t u r e s  (assuming no s p e c t r a l s h i f t ) whereas t h e observed  a b s o r p t i o n i n t h e r e g i o n 600-1100 nm showed a v e r y b r o a d c o n t i n u o u s t a i l w i t h no d i s c o n t i n u i t y o r the p r e s e n c e o f any " s h o u l d e r s " . S e c o n d l y , t h e a b s o r p t i o n maximum a t 500-600 nm was c a l c u l a t e d t o be Ge < 6000 mols. (100 eV)  '"cm ^ w h i c h seems t o be t o o s m a l l f o r a  s t a b l e trapped e l e c t r o n . The  end o f p u l s e o s c i l l o s c o p e t r a c e s showed no change i n  absorbance w i t h t i m e s u g g e s t i n g had  t h a t any t r a p p e d  or solvated electrons  r e a c t e d by t h e end o f t h e p u l s e and hence t h a t t h e i r l i f e t i m e was  l e s s than 10 ^ s e c .  T h i s i s c o n s i s t e n t w i t h t h e e s r s t u d i e s on y-  i r r a d i a t e d g l a s s y DMSO-water m i x t u r e s s i g n a l c o u l d be d e t e c t e d .  a t 77°K i n w h i c h no t r a p p e d  electron  Even when t h e e l e c t r o n s were g e n e r a t e d i n  the g l a s s a t 77°K i n t h e s p e c t r o m e t e r c a v i t y by t h e u l t r a v i o l e t p h o t o l y s i s o f K^Fe(CN)g, t h e e l e c t r o n s were u n s t a b l e  and decayed  immediately, producing  - i r r a  t h e same e s r s p e c t r u m as t h e Y  T h i s w i l l be d i s c u s s e d more f u l l y i n t h e n e x t  diated  glass.  chapter.  I t i s w o r t h n o t i n g t h a t i n many o t h e r p o l a r and n o n p o l a r low t e m p e r a t u r e g l a s s e s e l e c t r o n s can be t r a p p e d  and s t a b i l i z e d  indefinitely.  I t i s s u g g e s t e d t h e n t h a t t h e a c t i v a t i o n energy i s v e r y low f o r t h e r e a c t i o n o f t h e e l e c t r o n w i t h a DMSO m o l e c u l e f o r m i n g  a part of i t s  s o l v e n t cage so t h a t r e a c t i o n r e a d i l y p r o c e e d s even a t 77°K.  - 177 -  CHAPTER V ELECTRON SPIN RESONANCE STUDIES ON DMSO AND DMSO-H 0 MATRICES AT 77°K 2  A.  INTRODUCTION  * 1.  B a s i c P r i n c i p l e s o f ESR Use  o f t h e t e c h n i q u e o f e l e c t r o n s p i n resonance ( e s r ) i s r e s t r i c t e d  t o t h o s e m o l e c u l e s o r atomic s p e c i e s  containing unpaired electrons.  Because o f i t s charge and i n t r i n s i c a n g u l a r momentum, o r s p i n , the e l e c t r o n h a s a m a g n e t i c moment a s s o c i a t e d it.  A c c o r d i n g t o quantum t h e o r y ,  with  a s i n g l e e l e c t r o n can s p i n i n  e i t h e r o f two d i r e c t i o n s as g i v e n b y t h e quantum numbers M (a s p i n ) o r M  g  = -1/2 (£ s p i n ) .  g  = +1/2  I n t h e absence o f any e x t e r n a l  m a g n e t i c f i e l d t h e e l e c t r o n h a s no p r e f e r e n c e f o r an a o r g s p i n s i n c e they a r e o f e q u a l energy.  When an e x t e r n a l m a g n e t i c f i e l d i s  a p p l i e d t o t h e p a r a m a g n e t i c system, t h i s degeneracy i s removed.  The  e l e c t r o n w i l l have l o w e r energy i f i t s s p i n m a g n e t i c moment i s a l i g n e d so as t o be i n t h e d i r e c t i o n o f t h e a p p l i e d f i e l d r a t h e r than i t , which i s the only other allowed  orientation.  The energy  against separation  between t h e two s p i n s t a t e s i s p r o p o r t i o n a l t o t h e p r o d u c t o f a constant,  8 , (the e l e c t r o n i c Bohr magneton) and t h e s t r e n g t h  P r e p a r e d from r e f e r e n c e s  59 and 104.  of the  - 178 -  e x t e r n a l m a g n e t i c f i e l d , H.  The p r o p o r t i o n a l i t y c o n s t a n t ,  r e f e r r e d t o as t h e s p e c t r o s c o p i c represents  g, i s  s p l i t t i n g f a c t o r , o r g - f a c t o r , and  t h e r a t e o f d i v e r g e n c e o f t h e m a g n e t i c energy l e v e l s w i t h  the m a g n e t i c f i e l d . electromagnetic  I f the paramagnetic species  r a d i a t i o n possessing  i s i r r a d i a t e d with  energy, hv, equal t o the  s e p a r a t i o n between t h e two energy l e v e l s , t h e u n p a i r e d e l e c t r o n w i l l absorb energy and " f l i p o v e r " t o t h e s t a t e o f h i g h e r  energy i n w h i c h  i t s s p i n m a g n e t i c moment i s a l i g n e d a n t i - p a r a l l e l t o t h e m a g n e t i c T h i s resonance c o n d i t i o n i s d e s c r i b e d  hv  =  by e q u a t i o n  field.  (5.1).  ge3H  (5.1)  F o r a f r e e e l e c t r o n g = 2.0023 (the d e v i a t i o n from t h e i n t e g r a l number being a r e l a t i v i s t i c electron).  c o r r e c t i o n f o r the o r b i t a l v e l o c i t y of the  The term f r e e e l e c t r o n o r f r e e s p i n r e f e r s t o an u n p a i r e d  e l e c t r o n h a v i n g s p i n a n g u l a r momentum b u t no o r b i t a l a n g u l a r momentum (such as t h a t p o s s e s s e d by an e l e c t r o n i n an s o r b i t a l ) . t h i s condition i s r a r e l y r e a l i z e d f o r organic  Although  free r a d i c a l s , the  o r b i t a l a n g u l a r momentum a s s o c i a t e d w i t h a p o r d o r b i t a l e l e c t r o n i s u s u a l l y qienched so t h a t t h e g - f a c t o r s  a r e near that of f r e e e l e c t r o n s .  I n p r i n c i p l e e l e c t r o n s p i n resonance a b s o r p t i o n s by v a r y i n g  the magnetic f i e l d  combination o f both.  may be produced  (H), t h e r a d i a t i o n f r e q u e n c y (v) o r a  I n p r a c t i c e , however, t h e f r e q u e n c y i s u s u a l l y  f i x e d and t h e f i e l d s l o w l y v a r i e d s i n c e t h e k l y s t r o n o s c i l l a t o r w h i c h g e n e r a t e s t h e microwave r a d i a t i o n i s t u n a b l e o n l y o v e r a v e r y narrow range. F o r t h e X-band s p e c t r o m e t e r used i n t h i s s t u d y t h e microwave f r e q u e n c y was  about 9.1 GHz so t h a t t h e r e q u i r e d m a g n e t i c f i e l d f o r t h e resonance  - 179  -  c o n d i t i o n o f a f r e e e l e c t r o n o c c u r r e d around 3200 gauss. Thus s t u d i e s i n e s r depend upon the a b s o r p t i o n  of microwave  energy between non-degenerate s p i n s t a t e s o f a p a r a m a g n e t i c  species.  However the p r o b a b i l i t i e s of upward ( s t i m u l a t e d a b s o r p t i o n ) and  downward  ( s t i m u l a t e d emission) t r a n s i t i o n s are equal;  condition  c o n s e q u e n t l y the  of:• resonance i s dependent upon t h e r e b e i n g a d i f f e r e n c e i n between t h e s e two  states.  The  population  r a t i o of paramagnetic species  with  t h e i r s p i n s a l i g n e d i n the d i r e c t i o n of the a p p l i e d m a g n e t i c (N ) t o those a l i g n e d a g a i n s t i t (N ) +  field  a t temperature T i s g i v e n by  the  Boltzman d i s t r i b u t i o n .  N /N~ +  S i n c e ggH  =  << kT f o r t e m p e r a t u r e s above a few  exponential  degrees a b s o l u t e ,  f a c t o r i s c l o s e t o u n i t y so t h a t the e x c e s s  i n the ground s t a t e i s v e r y s m a l l H = 3000 g a u s s ) . increased,  (5.2)  exp(-gBH/kT)  (^ 0.07%  f o r g = 2, T = 300°K and  The  appreciably  l o w e r s p i n s t a t e s w i l l become e q u a l l y p o p u l a t e d  so t h a t t h e r e w i l l be no net energy a b s o r p t i o n resonance s i g n a l .  population  I f the r a d i a t i o n f i e l d a t resonance i s  the upper and  and  therefore  s p i n system i s t h e n s a i d t o be power  T h i s s a t u r a t i o n i s c o u n t e r b a l a n c e d by r e l a x a t i o n p r o c e s s e s , c h a r a c t e r i z e d by  the  the s p i n - l a t t i c e r e l a x a t i o n time (T^) and  no saturated. as spin-spin  r e l a x a t i o n time ( T ^ ) , w h i c h tend t o r e s t o r e t h e r m a l e q u i l i b r i u m .  In  the f o r m e r p r o c e s s the s p i n system i n t e r a c t s w i t h the medium o r l a t t i c e by d o n a t i n g i t s excess energy t o the v i b r a t i o n a l and r o t a t i o n a l modes of the s u r r o u n d i n g m o l e c u l e s .  Rapid d i s s i p a t i o n of t h i s excess  - 180  s p i n energy ( s h o r t T^)  i s e s s e n t i a l i f the p o p u l a t i o n  s p i n s t a t e s i s t o be m a i n t a i n e d . i s g r e a t e s t a t low absorption,  -  d i f f e r e n c e of  Although t h i s population  temperatures, therefore a l l o w i n g a  the  difference  stronger  the s p i n - l a t t i c e r e l a x a t i o n p r o c e s s i s l e s s e f f i c i e n t .  C o n s e q u e n t l y f r e e r a d i c a l s a r e o f t e n e a s i l y power  saturated  at  low  temperatures. Spin-spin  r e l a x a t i o n i n v o l v e s m a g n e t i c i n t e r a c t i o n s between  u n p a i r e d e l e c t r o n s and  s u r r o u n d i n g m a g n e t i c d i p o l e s , such as  other  u n p a i r e d e l e c t r o n s o r m a g n e t i c n u c l e i n a t i v e t o the l a t t i c e . i n t e r a c t i o n s are n o t energy d i s s i p a t i n g and  t h e r e f o r e do n o t  d i r e c t l y i n r e t u r n i n g the s p i n system t o e q u i l i b r i u m . s p i n - l a t t i c e t r a n s i t i o n s described s p i n process brings t r a n s i t i o n to the  above may  the  These contribute  However the  be enhanced i f the  spin-  the e x c e s s e n e r g y t o a p o s i t i o n f o r a p r o p i t i o u s lattice.  These s p i n - s p i n t r a n s i t i o n s are i m p o r t a n t i n a n o t h e r sense i n t h a t t h e y cause b r o a d e n i n g o f the a b s o r p t i o n  lines.  The  t o t a l magnetic  f i e l d e x p e r i e n c e d by a p a r t i c u l a r s p i n system w i l l i n c l u d e  contributions  f r o m i t s n e i g h b o u r s as w e l l as the a p p l i e d m a g n e t i c f i e l d ;  consequently  the resonance t r a n s i t i o n s w i l l o c c u r o v e r a range o f c o r r e s p o n d i n g t o the v a r i a t i o n s i n l o c a l f i e l d .  frequencies  I n l i q u i d media  m o l e c u l e s undergo r a p i d random m o t i o n so t h a t these i n d u c e d d i p o l a r f i e l d s are s u b j e c t e d  to extensive  time a v e r a g i n g .  As  a r e s u l t the  l i n e s are n a r r o w e r than those i n the c o r r e s p o n d i n g s o l i d s t a t e where the p a r a m a g n e t i c s p e c i e s  are p r e v e n t e d from r o t a t i o n a l o r t r a n s l a t i o n a l  motion. A s p e c i a l case of s p i n - s p i n i n t e r a c t i o n may  o c c u r between  the  - 181 -  u n p a i r e d e l e c t r o n and n u c l e a r s p i n s w i t h i n t h e same atom o r m o l e c u l e . This i n t e r a c t i o n r e s u l t s , not i n l i n e broadening, resolved hyperfine structure.  but r a t h e r i n  J u s t l i k e the e l e c t r o n s p i n , the  nuclear s p i n i s quantized having 2 1 + 1  s t a t e s of e q u a l energy f o r  a n u c l e u s o f s p i n I . When a m a g n e t i c f i e l d i s a p p l i e d , t h e s e d e g e n e r a t e s t a t e s a r e s p l i t and t h e n u c l e a r m a g n e t i c moment forms t h e 21+1  allowed o r i e n t a t i o n s w i t h respect t o the f i e l d d i r e c t i o n .  Under  the c o n d i t i o n s o f an e s r e x p e r i m e n t (H <\» 3000 gauss, v ^ 9.0 GHz) a l l 2 1 + 1 n u c l e a r moment o r i e n t a t i o n s may be c o n s i d e r e d as e q u a l l y probable  s i n c e the d i f f e r e n c e i n p o p u l a t i o n o f the n u c l e a r s u b - l e v e l s  i s s e v e r a l o r d e r s o f magnitude l e s s than t h e c o r r e s p o n d i n g e l e c t r o n spin states.  T h i s r e s u l t s i n each o f t h e e l e c t r o n s p i n s t a t e s b e i n g  further s p l i t into 2 1 + 1  sub-levels of equal separation.  S i n c e the  n u c l e a r s p i n s a r e u n a f f e c t e d by t h e o s c i l l a t i n g microwave f i e l d w h i c h causes t h e e l e c t r o n i c t r a n s i t i o n s  ( t h e resonance f r e q u e n c y  of a proton,  f o r example, i n a f i e l d o f 3000 gauss i s about 14 MHz), t h e s e l e c t i o n r u l e s f o r t h e e s r t r a n s i t i o n s a r e AM^ = + 1 and AM^ = 0. s p e c t r u m w i l l t h e r e f o r e c o n s i s t o f 21 + 1 l i n e s .  The e s r  O f t e n t h e r e a r e groups  o f n u c l e i i n c h e m i c a l l y and m a g n e t i c a l l y e q u i v a l e n t p o s i t i o n s , i n w h i c h case they a c t t o g e t h e r t o g i v e a s p l i t t i n g c h a r a c t e r i z e d by t h e i r t o t a l s p i n j_l£'  Here n^ i s t h e number o f e q u i v a l e n t n u c l e i  with nuclear spin I..  The combined i n t e r a c t i o n produces 2 n . I . + 1  n  l i n e s o f e q u a l s e p a r a t i o n , t h e i n t e n s i t i e s o f w h i c h can be i d e n t i f i e d w i t h the c o e f f i c i e n t s of the a p p r o p r i a t e multinominal The  expansion.  r e s u l t i n g l i n e separation i s c a l l e d the hyperfine s p l i t t i n g .  A  much more c o m p l i c a t e d s i t u a t i o n a r i s e s when t h e e l e c t r o n i n t e r a c t s w i t h  - 182 -  more than one n o n - e q u i v a l e n t In  magnetic n u c l e i .  s o l i d s and h i g h l y v i s c o u s media, i n w h i c h t h e p a r a m a g n e t i c  c e n t r e s a r e n o t f r e e t o tumble, t h e h y p e r f i n e s p l i t t i n g s w i l l depend upon the r e l a t i v e o r i e n t a t i o n o f the l a t t i c e and a p p l i e d m a g n e t i c  field.  The m a g n e t i c d i p o l e - d i p o l e i n t e r a c t i o n s between t h e u n p a i r e d e l e c t r o n and n u c l e a r moments have a d i r e c t i o n a l c h a r a c t e r a s s o c i a t e d w i t h them so t h a t t h e e f f e c t i v e m a g n e t i c f i e l d f e l t by t h e e l e c t r o n w i l l be anisotropic.  T h i s i s a c c o u n t e d f o r by e x p r e s s i n g t h e s p l i t t i n g s i n  terms o f a h y p e r f i n e t e n s o r A.  I f the paramagnetic species i s r a p i d l y  r e o r i e n t i n g , the a n i s o t r o p i c p a r t w i l l average t o z e r o and t h e s p l i t t i n g s w i l l a r i s e s o l e l y from t h e i s o t r o p i c c o u p l i n g contact The  (Fermi  interaction). g - f a c t o r may s i m i l a r l y show a n i s o t r o p i c b e h a v i o u r .  unpaired e l e c t r o n possesses  o r b i t a l angular  w i t h t h e s p i n a n g u l a r momentum.  I f the  momentum i t w i l l  ccuple  Because o f t h e o r i e n t a t i o n a l  dependence o f t h i s s p i n - o r b i t c o u p l i n g , i t w i l l be e a s i e r t o make t h e e l e c t r o n r e v e r s e i t s s p i n when t h e m a g n e t i c f i e l d i s a p p l i e d i n c e r t a i n d i r e c t i o n s as compared t o o t h e r s .  This anisotropy r e s u l t s i n the  p a r a m a g n e t i c system r e s o n a t i n g a t d i f f e r e n t m a g n e t i c f i e l d s . t h i s reason the g - f a c t o r i s o f t e n expressed  For  as a t e n s o r i n w h i c h t h e  p r i n c i p a l values are g , g and g . I n most cases t h e o f f - d i a g o n a l xx yy zz elements a r e s m a l l and may be n e g l e c t e d so t h a t , t o a good a p p r o x i m a t i o n , g °xx  =  g > g = g and g = g • °x °yy °y °zz z  Here x , y, and z r e f e r t o t h e  l a b o r a t o r y f i x e d axes and a r e r e l a t e d t o the c r y s t a l o r m o l e c u l a r f i x e d axes by a s u i t a b l e c o o r d i n a t e t r a n s f o r m a t i o n m a t r i x ( n o t n e c e s s a r i l y t h e same f o r t h e g and A t e n s o r s ) .  In a l i q u i d or s l i g h t l y  - 183 -  v i s c o u s s o l i d t h e o b s e r v e d g - v a l u e s may be c o n s i d e r e d i s o t r o p i c s i n c e the 8  yy  m o l e c u l e s w i l l be t u m b l i n g r a p i d l y and randomly; hence g = 1/3(g +  g  zz '  2.  Amorphous and P o l y c r y s t a l l i n e In  are  )  Media  amorphous o r p o l y c r y s t a l l i n e s o l i d s the p a r a m a g n e t i c s p e c i e s  o r i e n t e d randomly i n t h e m a t r i x .  I f t h e medium t e m p e r a t u r e i s  s u f f i c i e n t l y l o w , r o t a t i o n a l and t r a n s l a t i o n a l m o t i o n s w i l l h i n d e r e d so t h a t the time a v e r a g i n g p r o p e r t i e s o f f l u i d lost.  be  systems a r e  Thus the o b s e r v e d resonance w i l l be a sum o f the i n d i v i d u a l  r e s o n a n c e s o f the randomly o r i e n t e d p a r a m a g n e t i c c e n t r e s .  In t h i s  case the spectrum w i l l be governed by the a n i s o t r o p y o f the g - f a c t o r and h y p e r f i n e s p l i t t i n g as w e l l as any b r o a d e n i n g due t o d i p o l e - d i p o l e interactions.  T h i s r e s u l t s i n a l e s s d e t a i l e d p i c t u r e of the s t r u c t u r e  and e l e c t r o n i c d i s t r i b u t i o n o f the r a d i c a l than i f i t was p r e s e n t i n a single crystal or solution. of the  However, s i n c e the p r i m a r y o b j e c t i v e  t h i s and o t h e r s t u d i e s i n r a d i a t i o n c h e m i s t r y i s s i m p l y t o i d e n t i f y p a r a m a g n e t i c s p e c i e s i n v o l v e d i n the c h e m i c a l p r o c e s s e s , s t u d i e s  on t h e amorphous o r p o l y c r y s t a l l i n e s t a t e a r e u s u a l l y p e r f o r m e d the  since  e x p e r i m e n t a l p r o c e d u r e and m a t h e m a t i c a l a n a l y s i s a r e l e s s  complicated. S p e c t r a l l i n e shapes have been computed f o r p a r a m a g n e t i c s p e c i e s randomly t r a p p e d i n s o l i d m a t r i c e s . occurring l i n e  Two  of the most commonly  shapes a r e shown i n F i g u r e 54 f o r r a d i c a l s  exhibiting  +  - 184 no h y p e r f i n e s t r u c t u r e . for g  X  a  radical  and g  zz  possessing  = g u  F i g u r e 5 4 ( a ) shows the resonance a b s o r p t i o n an a x i a l l y symmetric g - t e n s o r where g ^ = g ^ =  whereas F i g u r e 5 4 ( b ) r e f e r s t o a  a s y m m e t r i c g - t e n s o r where g^ < g^ < g-j.  completely  The a p p l i e d m a g n e t i c  i s t a k e n as b e i n g a l i g n e d a l o n g t h e z a x i s .  field  An example o f a p a r a -  m a g n e t i c system h a v i n g an a x i a l l y symmetric g and A t e n s o r w i t h t h e same a x i s o f symmetry f o r a s i n g l e n u c l e u s w i t h n u c l e a r s p i n I = 1/2 i s shown i n F i g u r e 55.  I n t h i s spectrum the a n i s o t r o p i c h y p e r f i n e  s p l i t t i n g constants A  and A ^ a r e >> 8H(g^ - g ^ ) .  (|  The c e n t r a l  d o t t e d p o r t i o n i s t h e t h e o r e t i c a l spectrum i n t h e absence o f t h e hyperfine  coupling.  These t h r e e cases r e p r e s e n t t h e optimum c o n d i t i o n s t o be e x p e c t e d when d e a l i n g w i t h p o l y c r y s t a l l i n e and amorphous s o l i d s . matrices  the s p e c i f i c f e a t u r e s corresponding  I n some  t o t h e t e n s o r components  o f A and g a r e l o s t due t o e x c e s s i v e l i n e b r o a d e n i n g .  This i s  e s p e c i a l l y t r u e f o r r a d i c a l s t r a p p e d i n h y d r o c a r b o n m a t r i c e s a t low t e m p e r a t u r e s where t h e d i p o l a r b r o a d e n i n g due t o t h e p r o t o n s g r e a t e r t h a n BH(g  it  l i n e s approximately corresponding  - g ) and (A - A j_ li j.  i s often  The b r o a d e n i n g r e s u l t s i n  G a u s s i a n i n shape w i t h t h e peak-to-peak s e p a r a t i o n  f a i r l y c l o s e l y t o t h e i s o t r o p i c component o f t h e  hyperfine s p l i t t i n g constant.  Generally the l i n e widths  f o r the a l i p h a t i c  r a d i c a l s a t 77°K a r e about 10-15 gauss b u t can be n a r r o w e r i f t h e r a d i c a l i s f r e e t o r o t a t e o r tumble w i t h i n t h e t r a p p i n g s i t e and thereby average out t h e d i p o l a r i n t e r a c t i o n s . O f t e n t h e broadening can be r e d u c e d s i g n i f i c a n t l y by u s i n g d e u t e r a t e d m o l e c u l e s .  Since the nuclear  moment o f d e u t e r i u m i s s m a l l e r than t h a t o f hydrogen, t h e h y p e r f i n e  - 185 -  F i g u r e 54.  T h e o r e t i c a l e s r l i n e shapes f o r (a) a x i a l l y symmetric and (b) c o m p l e t e l y  asymmetric g t e n s o r s .  The upper c u r v e s r e f e r  t o a b s o r p t i o n s p e c t r a of the p a r a m a g n e t i c s p e c i e s .  The l o w e r  curves r e f e r to the e x p e r i m e n t a l l y observed f i r s t d e r i v a t i v e spectra.  (Adapted from F i g u r e s 9.3 and 9.4, pages  r e f e r e n c e 104).  324-325,  Figure 55.  T h e o r e t i c a l f i r s t d e r i v a t i v e e s r spectrum f o r a paramagnetic s p e c i e s w i t h S = 1 / 2 , w i t h a x i a l l y asymmetric g and A t e n s o r s .  I = 1/2  The c e n t r a l d o t t e d p o r t i o n i s the t h e o r e t i c a l  spectrum i n the absence of the h y p e r f i n e i n t e r a c t i o n s . (Adapted from F i g u r e 9 . 7 , page 3 2 7 , reference  104).  and  - 187 -  i n t e r a c t i o n s and l i n e w i d t h s w i l l be r e d u c e d compared t o t h e u n s u b s t i t u t e d medium. In  t h e p r e c e d i n g d i s c u s s i o n i t has been assumed t h a t the resonance  a b s o r p t i o n i s due t o a s i n g l e p a r a m a g n e t i c s p e c i e s randomly o r i e n t e d i n the  matrix.  G e n e r a l l y t h i s i s not the case.  O f t e n the s p e c t r u m i s f u r t h e r  c o m p l i c a t e d by the appearance o f o t h e r r a d i c a l s r e s o n a t i n g i n the same r e g i o n s o t h a t i d e n t i f i c a t i o n i s n o t always p o s s i b l e .  This i s p a r t i c u l a r l y  t r u e i f the medium has been exposed t o i o n i z i n g r a d i a t i o n , such as X- o r y - r a y s , w h i c h a r e n o t s e l e c t i v e i n the. t y p e s o f r a d i c a l s they p r o d u c e . I n many systems s p e c t r a l a n a l y s i s may be s i m p l i f i e d by u s i n g a Hg lamp as t h e radiation  source.  S i n c e u l t r a v i o l e t p h o t o l y s i s i s more s e l e c t i v e i n  c l e a v i n g m o l e c u l a r bonds, o n l y c e r t a i n r a d i c a l s w i l l be p r o d u c e d .  B. 1.  P u r e DMSO  1.1  Ultraviolet  RESULTS AND DISCUSSION  Irradiated  The r a d i c a l s produced when p o l y c r y s t a l l i n e DMSO i s p h o t o l y z e d a t 77°K w i t h u l t r a v i o l e t l i g h t e x h i b i t a b r o a d , asymmetric spectrum c o n s i s t i n g o f seven r e s o l v a b l e l i n e s c e n t e r e d n e a r g = 2.004 as shown i n F i g u r e 56.  S i n c e the V y c o r e n v e l o p e i s opaque b e l o w 220 nm, t h e  e m i t t e d l i g h t from the l o w - p r e s s u r e mercury resonance a r c i s c h i e f l y 253.7 nm c o r r e s p o n d i n g t o an energy of = 110 k c a l mole DMSO a b s o r b s s t r o n g l y below 260 nm s u g g e s t i n g t h a t the f i r s t  \  electroni-  c a l l y e x c i t e d s t a t e i s comparable t o the energy o f t h i s mercury l i n e . I f d i s s o c i a t i o n o c c u r s by i n t e r n a l c o n v e r s i o n from the l o w e s t e l e c t r o n i c a l l y  - 188 excited  s t a t e , then t h e p r o b a b i l i t y o f r a d i c a l f o r m a t i o n w i l l  the weakest bond.  favour  I n DMSO t h e C-S bond has a mean d i s s o c i a t i o n  energy D o f ^ 50 k c a l / m o l e  1  as compared t o D(S-O) and D(C-H) w h i c h  a r e 86 and 90-100 k c a l mole * r e s p e c t i v e l y .  S i n c e t h e C-S bond i s  a p p r e c i a b l y weaker than t h e r e s t one would e x p e c t CH^SO and 'CH^ r a d i c a l s to p r e d o m i n a t e i n t h e p r i m a r y p h o t o l y t i c p r o c e s s . The spectrum shown i n F i g u r e 56 The  tends t o c o n f i r m t h i s s u p p o s i t i o n .  q u a r t e t , c e n t e r e d a t g • 2.003 and h a v i n g  a hyperfine  splitting  o f 22 + 1 gauss (as measured from t h e d e r i v a t i v e m a x i m a ) , i s t h a t e x p e c t e d f o r an u n p a i r e d e l e c t r o n a protons.  i n t e r a c t i n g w i t h three  The l i n e w i d t h , AH, as measured between t h e peaks o f t h e  d e r i v a t i v e ' m a x i m u m and minimum, i s a p p r o x i m a t e l y that the methyl r a d i c a l i s tumbling The  equivalent  6 t o 7 gauss i n d i c a t i n g  f r e e l y i n t h e DMSO m a t r i x a t 77°K.  i n t e n s i t y d i s t r i b u t i o n o f t h e resonance l i n e s o f t h e q u a r t e t , as  measured by t h e peak-to-peak a m p l i t u d e s ,  agrees w i t h t h e t h e o r e t i c a l  r a t i o o f 1:3:3:1. The  asymmetric t h r e e l i n e spectrum a s c r i b e d t o t h e CH^SO r a d i c a l s  i s i n d i c a t i v e o f t h e resonance p a t t e r n o b s e r v e d f o r o t h e r r a d i c a l s i n u l t r a v i o l e t and y - i r r a d i a t e d  sulfur  thiols, sulfides,  disulfides  and s u l f o n e s where t h e a n i s o t r o p y i n t h e g - f a c t o r i s a t t r i b u t e d strong spin-orbit  coupling of the unpaired e l e c t r o n  sulfur a t o m s . S i n c e  maximum, z e r o and minimum p o i n t s i n t h e d e r i v a t i v e  1.988, g  l o c a l i z e d on t h e  t h e s u l f u r resonance shows seven p o i n t s  of i n f l e c t i o n , t h e three p r i n c i p l e g - f a c t o r s w i l l  54(b)).  correspond curve  to the  (see  The g - f a c t o r s o b t a i n e d i n t h i s manner a r e as f o l l o w s : 9  t o the  = 2.004 and g_ = 2.017.  Figure g^ =  No r e s i d u a l h y p e r f i n e s p l i t t i n g was  g  3  - 2.017 g  2  = 2.004  1.988  c  v.  L  10 gauss  H  J  L •CH e 56.  3  Electron spin resonance spectrum obtained after the u l t r a v i o l e t photolysis at 77°K.  of p o l y c r y s t a l l i n e DMSO  The arrows correspond to the asymmetric g-factors of the s u l f u r r a d i c a l CH^SO.  methyl r a d i c a l quartet i s indicated by the s t i c k plot.  The  - 190 observed suggesting  t h a t the u n p a i r e d  e l e c t r o n i s l o c a l i z e d mainly  on  the s u l f u r atom and i s not i n t e r a c t i n g w i t h the B p r o t o n s o f the m e t h y l group.  I t i s p o s s i b l e , however,  t h a t the s p l i t t i n g i s t o o s m a l l t o  be r e s o l v e d due t o the d i p o l a r b r o a d e n i n g . 1.2  Y~I  r r a  diated  F i g u r e 57 shows the e s r s p e c t r u m o b t a i n e d when t h e p o l y c r y s t a l l i n e DMSO b a l l s were Y sample  _ i r  radiated  i n t h e d a r k a t 77°K.  U n l i k e the photolyzed  ( F i g u r e 5 6 ) , t h e resonance s p e c t r a o f the r a d i c a l s p r o d u c e d were  poorly resolved.  The two d e r i v a t i v e peaks on the h i g h and low f i e l d  p o r t i o n o f t h e spectrum c o r r e s p o n d t o t h e resonance p o s i t i o n s of t h e o u t e r l i n e s o f the m e t h y l r a d i c a l q u a r t e t . s u l f u r p a t t e r n observed i n the photolyzed  However t h e asymmetric sample cannot be p o s i t i v e l y  a s s i g n e d because o f t h e o v e r l a p p i n g resonance of o t h e r p a r a m a g n e t i c species. When t h e microwave power was i n c r e a s e d above 1.0 mW, d i f f e r e n t paramagnetic species present  i n the Y  c e n t e r e d n e a r g = 2.006 began t o s a t u r a t e .  _ i r r a  diated  one o r more sample  and  T h i s i s e v i d e n t from F i g u r e  57(b) f o r which the microwave power i s 10 mW.  Superimposed upon the  r e s o n a n c e p a t t e r n shown i n F i g u r e 57 i s a b r o a d asymmetric s i n g l e t a t g = 2.007 w i t h a l i n e w i d t h o f <\» 20 gauss.  centered  T h i s s i n g l e t i s n o t due t o  t r a p p e d e l e c t r o n s because the a b s o r p t i o n i s n o t s a t u r a t e d a t h i g h power (160 mW)  and i t s g - f a c t o r i s g r e a t e r than t h e f r e e s p i n v a l u e .  I t i s w o r t h n o t i n g t h a t the r a d i c a l s produced i n the samples by y - i r r a d i a t i o n and u l t r a v i o l e t p h o t o l y s i s were s t a b l e and showed no s i g n s o f decay when the i r r a d i a t e d samples were k e p t i n the dark i n l i q u i d nitrogen.  Even a f t e r s e v e r a l days the resonance p a t t e r n s and  - 191 -  (b)  F i g u r e 57.  E l e c t r o n s p i n resonance s p e c t r a o f y - i ^ r a d i a t e d DMSO.  The sample  was i r r a d i a t e d i n the d a r k a t 77"K t o a t o t a l a b s o r b e d dose o f 0.72 Mrad. 10 mW.  g  (a) microwave power 0.44 mW;  D p p H  = 2.0036.  (b) microwave power  - 192  F i g u r e 58.  -  E l e c t r o n s p i n resonance s p e c t r a o f y i _  r r a  diated  DMSO a f t e r  b l e a c h i n g i r r a d i a t e d sample w i t h u l t r a v i o l e t l i g h t f o r f o r t y minutes ( i n spectrometer c a v i t y ) .  Sample y i r r a d i a t e d  a t 77°K  i n t h e dark t o a t o t a l a b s o r b e d dose o f 0.72 Mrad. (a) microwave power 0.52 mw;  (b) microwave power 10  mW.  - 193 -  i n t e n s i t i e s were unchanged.  When t h e y i r r a d i a t e d sample was p h o t c b l e a c h e d  w i t h u l t r a v i o l e t l i g h t , t h e spectrum shown i n F i g u r e 58 was o b s e r v e d and i s comparable  t o t h a t found f o r d i r e c t u l t r a v i o l e t p h o t o l y s i s and g i v e n i n F i g u r e  56. The s p e c t r a l change d i d n o t a r i s e from s i m p l e p h o t o l y s i s o f t h e DMSO m o l e c u l e s s i n c e the m e t h y l r a d i c a l c o n c e n t r a t i o n was unchanged (as measured by t h e o u t e r peaks o f t h e q u a r t e t ) . F u r t h e r m o r e , t h e sample was p h o t o b l e a c h e d i n t h e e s r c a v i t y f o r o n l y 40 minutes w h i c h , on t h e g a i n s e t t i n g used, would n o t have caused any o b s e r v a b l e change i n t h e spectrum due t o t h e d i r e c t p h o t o l y s i s o f DMSO.  On t h e o t h e r hand, t h e b r o a d s i n g l e t was r e d u c e d  i n i n t e n s i t y and t h e s a t u r a t i o n  e f f e c t s a t h i g h microwave  power  (> 1.0 mW) were l e s s pronounced s u g g e s t i n g t h a t t h e o b s e r v e d resonance change was due t o t h e p a r t i a l b l e a c h i n g o f t h e s e two p a r a m a g n e t i c s p e c i e s .  2.  DMSO-Water M a t r i c e s  2.1  y - I r r a d i a t e d P o l y c r y s t a l l i n e H^O In order t o analyze the e s r s p e c t r a of b i n a r y m i x t u r e s , i t i s  e s s e n t i a l t o have a d e t a i l e d knowledge  o f t h e type and s t a b i l i t y o f  t h e resonance p a t t e r n i n d u c e d i n each o f t h e pure components. resonance s p e c t r u m o f Y  - i r  The  * " a d i a t e d p o l y c r y s t a l l i n e w a t e r has been  e x t e n s i v e l y s t u d i e d and t h e l i n e a s s i g n m e n t s a r e r e a s o n a b l y w e l l understood.The  spectrum o b s e r v e d i n the p r e s e n t s t u d y i s shown i n F i g u r e  59 and agrees w i t h t h a t o b s e r v e d by o t h e r a u t h o r s .  The "water r e s o n a n c e "  i s c h a r a c t e r i z e d by a d i s t i n c t d o u b l e t s p l i t by about 40 gauss w i d t h ^ 12 gauss) and c e n t e r e d around g = 2.008.  (line  The b r o a d "hump" t o  20 gauss  Figure 59.  Electron spin resonance spectrum of y - i r r a d i a t e d p o l y c r y s t a l l i n e i c e at 77°K. Resonance pattern corresponds to that of the "OH  radical.  - 195 -  the l o w f i e l d s i d e o f t h e d o u b l e t had p r e v i o u s l y been a t t r i b u t e d t o such p a r a m a g n e t i c s p e c i e s as R^o"*", H^O  o r RG^*.  However subsequent  i n v e s t i g a t i o n s have shown t h a t t h e e n t i r e e s r s p e c t r u m may be a s s i g n e d 115 t o an a n i s o t r o p i c *0H r a d i c a l . Only t h e h y d r o x y l r a d i c a l i s o b s e r v e d b y e s r i n y - i r r a d i a t e d i c e a t 77 C. P  O t h e r p a r a m a g n e t i c s p e c i e s produced i n t h e r a d i o l y s i s a r e  e i t h e r t o o u n s t a b l e and i m m e d i a t e l y r e a c t w i t h t h e m a t r i x (H* atoms) o r t h e i r y i e l d i s t o o low t o be o b s e r v e d ( e 2.2  t >  H02*)»  y - I r r a d i a t e d DMSO-H^O M i x t u r e s E l e c t r o n s p i n resonance s t u d i e s were made on a s e r i e s o f t e n  m i x t u r e s r a n g i n g from 0.01 t o 0.89 mole f r a c t i o n DMSO. T a b l e V g i v e s t h e m i x t u r e s s t u d i e d and t h e type o f m a t r i x (amorphous o r p o l y c r y s t a l l i n e ) formed i n each system when "shock c o o l e d " i n l i q u i d n i t r o g e n by t h e " b a l l technique".  Only two m i x t u r e s c o r r e s p o n d i n g t o 0.20 and  0.39 mole f r a c t i o n DMSO formed g l a s s y (amorphous)  balls.  The sample  b a l l s were y - i r r a d i a t e d i n t h e d a r k t o a t o t a l absorbed dose o f 0.96 Mrad.  A l l p h o t o l y s i s o r p h o t o b l e a c h i n g e x p e r i m e n t s were p e r f o r m e d w i t h  the dewar i n t h e s p e c t r o m e t e r c a v i t y . When u s i n g t h e u l t r a v i o l e t  light  s o u r c e (Hg l a m p ) , t h e b l e a c h i n g t i m e was always l e s s than 20 m i n u t e s ; c o n s e q u e n t l y the o b s e r v e d changes  i n t h e spectrum were n o t due t o t h e  p h o t o l y t i c d e c o m p o s i t i o n o f the DMSO m o l e c u l e s . Two d i s t i n c t resonance p a t t e r n s were o b s e r v e d f o r t h e c o m p o s i t i o n range s t u d i e d and were n o t a s i m p l e c o m b i n a t i o n o f t h e pure components. A t h i g h DMSO c o n c e n t r a t i o n s (0.89, 0.80, 0.69 and 0.67 mole f r a c t i o n DMSO) t h e y - i r r a d i a t e d m a t r i c e s produced resonance s p e c t r a s i m i l a r t o  - 196 -  TABLE V.  Summary o f d a t a o b t a i n e d  from. s t u d i e s on Y  - l r r a  diated  DMSO-water  m a t r i c e s a t 77°K. Mole Fraction DMSO  Volume fraction DMSO  1.0  1.0  Matrix a t 77°K  Sample c o l o u r after y-irradiation  polycrystalline  yellow tinge  Radicals observed  .X, 'SX.-CH^ (DMS0)  0.97  0.89  polycrystalline  yellow tinge  0.94  polycrystalline  yellow tinge  3  0.90  polycrystalline  purple  tinge  3  0.89  polycrystalline  purple tinge  +  •X, »SX, * C H (DMS0)  0.67  +  •X,«SX,-CH , (DMS0)  0.69  +  .X,'SX,«CH , (DMS0)  0.80  0  3>  +  •X,-SX,'CH , 3  (DMS0)  +  0.39  0.71  amorphous  purple  •SX,'CH ,(DMS0)  +  0.20  0.50  amorphous  purple  •SX,'CH ,(DMS0)  +  0.11  0.33  polycrystalline  purple tinge  •SX,*CH ,(DMS0)  +  0.06  0.20  polycrystalline  purple tinge  •SX,'CH ,(DMS0)  +  0.03  0.10  polycrystalline  colourless  •SX,*CH ,(DMS0) ,'0H  0.01  0.04  polycrystalline  colourless  •SX,'CH ,(DMS0) ,-OH  Volume f r a c t i o n a p p r o x i m a t e l y  3  3  3  3  +  3  +  3  e q u a l t o f r a c t i o n dose absorbed  initially  by DMSO i n m i x t u r e . P u r p l e c e n t r e t u r n e d y e l l o w upon b l e a c h i n g w i t h u l t r a v i o l e t and visible  light.  •X = p a r a m a g n e t i c s p e c i e s c e n t e r e d a t g = 2.006, r e a d i l y power s a t u r a t e d above 1.0 mW and p h o t o b l e a c h e d w i t h u l t r a v i o l e t l i g h t . •SX= asymmetric s u l f u r r a d i c a l w i t h g^ = 1.988, g  3  = 2.017.  = 2.004 and  -  197 -  those o b s e r v e d f o r pure DMSO. tinge after Y ~ The  i r r a  The f i r s t  two m i x t u r e s had a y e l l o w  d i a t i o n whereas the l a t t e r two had a p u r p l e  tinge.  paramagnetic species r e s p o n s i b l e f o r the s a t u r a t i o n e f f e c t s a t  low microwave power ( h e r e a f t e r c a l l e d although t h e i r w a t e r content  *X f o r b r e v i t y ) were s t i l l  c o n t r i b u t i o n t o t h e o v e r a l l spectrum d i m i n i s h e d increased.  as t h e  T h i s i s i n d i c a t e d i n F i g u r e 60 i n which the  r e g i o n o f t h e spectrum where *X r e s o n a t e s upon power s a t u r a t i o n (pure DMSO) as w i t h mixtures).  present  shows t h e same b e h a v i o u r a d d i t i o n o f water  The r e m a i n i n g p a r t o f the e s r s p e c t r a o f these  m i x t u r e s a r e c o m p a t i b l e w i t h F i g u r e 57 (pure DMSO).  (DMSO-water four  When the i r r a d i a t e d  samples were p h o t o b l e a c h e d f o r 20 minutes w i t h u l t r a v i o l e t l i g h t , the b r o a d asymmetric s i n g l e t  and *X d i s a p p e a r e d  c o l o u r was r e p l a c e d by a y e l l o w by  a s l i g h t increase  tinge.  completely  and the  purple  The b l e a c h i n g was accompanied  20%) i n the m e t h y l r a d i c a l y i e l d .  These  f e a t u r e s a r e shown i n F i g u r e 61 f o r 0.80 mole f r a c t i o n DMSO and a r e r e p r e s e n t a t i v e o f the o t h e r and  The m e t h y l r a d i c a l  t h e asymmetric " s u l f u r p a t t e r n " a r e c l e a r l y e v i d e n t  photobleaching The and  three mixtures.  ( F i g u r e s 61(b)  quartet  after  and ( c ) ) .  remaining y - i r r a d i a t e d  mixtures  (0.39, 0.20, 0.11, 0.06, 0.03  0.01 mole f r a c t i o n DMSO) gave w e l l - r e s o l v e d e s r s p e c t r a showing  the b r o a d asymmetric s i n g l e t , the m e t h y l r a d i c a l q u a r t e t .  the c h a r a c t e r i s t i c  " s u l f u r p a t t e r n " and  However t h e r e was no e v i d e n c e o f "X as  i n d i c a t e d by the power s a t u r a t i o n b e h a v i o u r o f the s p e c t r a . T y p i c a l e s r s p e c t r a o f these m i x t u r e s a r e g i v e n f r a c t i o n DMSO. least  T r a c e s o f the h y d r o x y l  concentrated  DMSO m i x t u r e s  (0.03  i n F i g u r e 62 f o r 0.20 mole  r a d i c a l were o b s e r v e d i n the and 0.01 mole f r a c t i o n DMSO),  (a) F i g u r e 60.  Resonance p a t t e r n showing b e h a v i o u r of *X w i t h (a) i n c r e a s i n g c o m p o s i t i o n (microwave power 0.42 power (pure DMSO).  mW)  and  (b) i n c r e a s i n g microwave  Numbers c o r r e s p o n d i n g t o s p e c t r a on l e f t  t o mole f r a c t i o n DMSO.  water  The arrows r e f e r t o g  .  refer  F i g u r e 61.  E l e c t r o n resonance s p e c t r a o f p o l y c r y s t a l l i n e y - i r r a d i a t e d  DMSO-  w a t e r m i x t u r e (0.80 mole f r a c t i o n DMSO) a t 77°K. (a) microwave power 0.42 mW; (b) a f t e r b l e a c h i n g w i t h u l t r a v i o l e t l i g h t f o r 20 m i n u t e s , microwave power 0.42 mW; ( c ) same as ( b ) , microwave power  - 200 -  F i g u r e 62. E l e c t r o n s p i n resonance s p e c t r a o f y - i r r a d i a t e d DMSO-water g l a s s (0.20 mole f r a c t i o n DMSO) a t 77°K. The " s u l f u r p a t t e r n " and m e t h y l r a d i c a l q u a r t e t a r e r e a d i l y o b s e r v e d , (a) microwave power 0.42 (b) microwave power 10 mW.  mW;  - 201 -  e s p e c i a l l y a t h i g h power, and the asymmetric s i n g l e t was not as pronounced (see F i g u r e 6 3 ) .  The i r r a d i a t e d sample b a l l s o f these  l a t t e r two m i x t u r e s were c o l o u r l e s s whereas the o t h e r f o u r samples were s l i g h t l y p u r p l e .  The p u r p l e c o l o u r and b r o a d asymmetric s i n g l e t  were r e a d i l y p h o t o b l e a c h e d w i t h u l t r a v i o l e t and v i s i b l e l i g h t and the r e s u l t i n g s p e c t r a were i d e n t i c a l t o t h a t shown i n F i g u r e  61(b).  The m e t h y l r a d i c a l c o n c e n t r a t i o n showed a s i m i l a r i n c r e a s e upon b l e a c h i n g and the b a l l s had a y e l l o w  tinge.  I t i s i n t e r e s t i n g t h a t the h y d r o x y l r a d i c a l s were o n l y o b s e r v e d a t the h i g h e s t water c o n c e n t r a t i o n s  (0.97  and 0.99 mole f r a c t i o n w a t e r )  d e s p i t e the f a c t t h a t t h e i r y i e l d i n Y - i r r a d i a t e d f a i r l y h i g h (G(-OH) = 0 . 8 ) . ^ " ^  i c e a t 77°K i s  E l e c t r o n s p i n resonance s t u d i e s on  t h e l i q u i d s t a t e have shown t h a t *0H r a d i c a l s r e a c t a t a d i f f u s i o n c o n t r o l l e d r a t e w i t h DMSO t o produce m e t h y l r a d i c a l s by c l e a v a g e o f 116—118 the C-S bond.  The mechanism p r o p o s e d i s g i v e n by e q u a t i o n  0  II  "OH + CH-SCH. J J  I  0  -  [CH--S-CHJ J | J OH  (46).  0  II  CH.S0H + 'CHj  (46)  J  I t i s s u g g e s t e d h e r e t h a t 'OH r a d i c a l s formed i n the r a d i o l y s i s o f the DMSO-water m i x t u r e s i n f e r e n c e i s supported  react  w i t h DMSO even a t 77°K.  by the o b s e r v a t i o n t h a t h y d r o x y l  g e n e r a t e d by t h e p h o t o d e c o m p o s i t i o n o f hydrogen p e r o x i d e  This  radicals reacted with  DMSO t o g i v e an e s r s p e c t r u m composed o f a m e t h y l r a d i c a l q u a r t e t and a b r o a d , u n i d e n t i f i e d s i n g l e t (AH ^ 40 gauss) c e n t e r e d n e a r g = 2.008.  The g l a s s y sample was p r e p a r e d  DMSO and 15% hydrogen p e r o x i d e  from e q u a l p a r t s (by volume)  and p h o t o l y s e d  a t 77°K w i t h u l t r a v i o l e t  - 202 -  F i g u r e 63.  Electron  s p i n resonance s p e c t r a o f y - i r r a d i a t e d  polycrystalline  DMSO-water m i x t u r e (0.01 mole f r a c t i o n DMSO) a t 77°K. wave power 0.42 mW;  (b) microwave power 10 mW.  (a) m i c r o -  The low f i e l d  "hump" and d o u b l e t o f the 'OH r a d i c a l s a r e e v i d e n t a t 10 mW power (see F i g u r e 5 9 ) .  - 203 -  light.  Another p o s s i b i l i t y i s that energy and charge transfer from  the water to the DMSO molecules might be occurring i n these mixtures at 77°K.  Although these processes i n the l i q u i d state were  discounted  on the basis of the pulse r a d i o l y s i s data discussed e a r l i e r , these transfer processes may be quite e f f i c i e n t i n the s o l i d state at low temperature. The esr spectra indicate there are at least four d i s t i n c t paramagnetic species  ( i n addition to the *0H r a d i c a l ) produced by the  y - i r r a d i a t i o n of DMSO-water mixtures and trapped i n the matrices at 77°K.  The methyl and asymmetric s u l f u r r a d i c a l s are r e a d i l y i d e n t i f i e d  by t h e i r c h a r a c t e r i s t i c spectra and, as w i l l be shown i n the next section, are formed primarily by the reaction of electrons with DMSO. The identities of the other two paramagnetic e n t i t i e s are less c e r t a i n . As mentioned already  *X i s only present i n the p o l y c r y s t a l l i n e  media of high DMSO composition and i t s contribution to the resonance pattern decreases as the water content increases  (see Figure  60).  The r a d i c a l i s e a s i l y power saturated, photobleached with v i s i b l e or u l t r a v i o l e t l i g h t and i s centered at g = 2.006.  Since a l l known  trapped electrons have i s o t r o p i c g-values less than the free spin 22 value  (g = 2.0023),  i t i s u n l i k e l y that *X i s a trapped electron.  However this cannot be taken as d i s t i n c t trapped electrons.  evidence f o r the absence of  Theoretical studies indicate that electrons may  be bound i n the f i e l d of a stationary dipole moment provided 119 strength of the dipole moment i s greater than 1.6 D.  the  DMSO has a  f a i r l y large dipole moment (4.3 D) so that i t i s t h e o r e t i c a l l y possible for electrons to be trapped i n c r y s t a l l i n e DMSO at low temperatures  - 204 -  where the d i p o l e s a r e n o t f r e e t o r o t a t e .  The c r y s t a l s t r u c t u r e o f  DMSO has been r e p o r t e d and t h e d a t a i n d i c a t e a s t r o n g c o u p l i n g between a p a i r o f DMSO m o l e c u l e s i n which t h e i r d i p o l e s a r e a l i g n e d i n 120 opposite d i r e c t i o n s .  Perhaps  e l e c t r o n s are i n i t i a l l y  these s t r o n g d i p o l a r f i e l d s b u t immediately  trapped i n  form DMSO r a d i c a l  anions  due to t h e p r e s e n c e o f v a c a n t l o w - l y i n g d o r b i t a l s p r e s e n t on t h e s u l f u r atoms.  Thus the DMSO r a d i c a l a n i o n o r i t s decomposition p r o d u c t c o u l d  be r e s p o n s i b l e f o r t h e resonance  c h a r a c t e r i s t i c s o f *X.  of these r a d i c a l a n i o n s i s governed by t h e d i p o l a r  I f the s t a b i l i t y  interactions  a s s o c i a t e d w i t h the DMSO m o l e c u l e s , then t h e a d d i t i o n o f w a t e r may lower t h e r a d i c a l s a c t i v a t i o n energy strong dipolar f i e l d s .  f o r r e a c t i o n by b r e a k i n g up t h e s e  T h i s c o u l d e x p l a i n why *X d e c r e a s e s as the  water c o n c e n t r a t i o n i n c r e a s e s . The b r o a d asymmetric mixtures i s a t t r i b u t e d the e s r spectrum  s i n g l e t observed i n pure DMSO and i n a l l b i n a r y  t o DMSO p o s i t i v e i o n s f o r two r e a s o n s .  Firstly,  o b t a i n e d when e l e c t r o n s were g e n e r a t e d i n a DMSO-water g l a s s  (0.20 mole f r a c t i o n DMSO) by the p h o t o i o n i z a t i o n o f K.Fe(CN),(see is  comparable t o t h a t o f t h e c o r r e s p o n d i n g y - i r r a d i a t e d g l a s s  except f o r the absence T h i s absence  o f the b r o a d asymmetric  below)  ( F i g u r e 62)  s i n g l e t and p u r p l e c o l o u r .  suggests t h a t the s i n g l e t i s due t o an o x i d i z e d  paramagnetic  s p e c i e s produced by t h e i o n i z i n g r a d i a t i o n . Secondly, the p u r p l e c e n t r e and s i n g l e t show t h e same b e h a v i o u r towards p h o t o b l e a c h i n g i n f e r r i n g  they  c o r r e s p o n d t o t h e same s p e c i e s . I t was shown i n t h e p r e v i o u s c h a p t e r t h a t the a b s o r p t i o n spectrum i o n s . The absence  of a  p u r p l e c e n t r e i s due t o the DMSO p o s i t i v e  o f a d i s t i n c t p u r p l e t i n g e a t the h i g h e s t DMSO c o m p o s i t i o n s  (1.0, 0.89 and 0.80 mole f r a c t i o n DMSO) may be a y i e l d e f f e c t .  I f the  r e a c t i v i t y o f the t h e r m a l i z e d e l e c t r o n s i s enhanced by t h e p r e s e n c e o f water,  as suggested by s t u d i e s on the l i q u i d m i x t u r e s , then the  - 205 f r a c t i o n o f DMSO p o s i t i v e i o n s e s c a p i n g  recombination  i n the p o l y -  c r y s t a l l i n e s t a t e i s expected t o increase w i t h i n c r e a s i n g water composition. The  i n c r e a s e i n m e t h y l r a d i c a l s upon p h o t o b l e a c h i n g  the y - i r r a d i a t e d  samples c o u l d be a t t r i b u t e d t o t h e p h o t o d e c o m p o s i t i o n o f t h e DMSO p o s i t i v e i o n according to equation  (CH SOCH ) 3  +  +  3  hv  *-  (47).  CH * 3  +  CH S0  +  3  Unfortunately the e s r spectra corresponding  (47)  t o t h e p o s i t i v e i o n s and  m e t h y l r a d i c a l s a r e n o t w e l l - r e s o l v e d s o t h a t i t i s i m p o s s i b l e t o see if  2.3  t h e decay and b u i l d up o f t h e r e s p e c t i v e r a d i c a l s a r e comparable.  P h o t o i o n i z a t i o n o f K^Fe(CN)^ i n Aqueous G l a s s e s E l e c t r o n s may be g e n e r a t e d i n a system b y e x p o s i n g  t h e sample t o  i o n i z i n g r a d i a t i o n b u t they can a l s o be formed by p h o t o l y s i s o f s e v e r a l s o l u t e s a t w a v e l e n g t h s w i t h i n t h e i r c h a r g e - t r a n s f e r - t o - s o l v e n t bands. The  l a t t e r process  i s p a r t i c u l a r l y u s e f u l when s t u d y i n g t h e r e a c t i o n s o f  e l e c t r o n s i n s o l i d m a t r i c e s a t l o w t e m p e r a t u r e s by e s r s i n c e resonance i n t e r f e r e n c e by o t h e r r a d i c a l s produced b y i o n i z i n g r a d i a t i o n may be eliminated.  Ferrocyanide  i o n s a r e o f t e n used as a r e d u c i n g s o l u t e s i n c e  they a r e e a s i l y p h o t o i o n i z e d by u l t r a v i o l e t l i g h t a t 254 nm a c c o r d i n g 121 to equation (48). Fe(CN)  4 _ 4  +  hv  Fe(CN) ~ 3  6  +  e~  (48)  I n t h i s study, e l e c t r o n s were g e n e r a t e d a t 77°K b y p h o t o l y s i n g w i t h a l o w p r e s s u r e mercury resonance lamp g l a s s y samples c o n t a i n i n g 0.01 M  - 206 potassium ferrocyanide i n the spectrometer cavity. Two  d i f f e r e n t glassy systems were investigated.  One consisted  of an aqueous DMSO glass (0.20 mole f r a c t i o n DMSO) and the other an aqueous a l k a l i n e glass (8 M sodium hydroxide) to which DMSO was added as a scavenger (1.0 M DMSO).  Electrons s t a b i l i z e d i n aqueous a l k a l i n e glasses  at 77°K have been shown previously to e x h i b i t an intense blue colour (X  m a x  = 580 nm) and narrow esr s i n g l e t (AH ^ 16 gauss) centered at g =  2.0006."'"''""' Figure 65(a) shows the esr s i n g l e t obtained i n the  present  study by the u l t r a v i o l e t photolysis of the ferrocyanide ion i n an aqueous 8 M sodium hydroxide glass at 77°K. were bright blue i n colour.  The sample b a l l s a f t e r photolysis  However trapped electrons were not observed  i n e i t h e r the DMSO-water glass (Figure 64) or the a l k a l i n e glass containing 1.0 M DMSO (Figure 65(b)) upon photolysis, as indicated by the absence of an esr s i n g l e t or colour centre i n these samples.  total The esr spectrum  of the DMSO-water glass i s i d e n t i c a l to the y - i r r a d i a t e d sample (Figure 62) except f o r the absence of the broad s i n g l e t attributed to DMS0 . The +  resonance pattern of the a l k a l i n e glass (Figure 65(b)) i s s i m i l a r to Figures 62 and 64 except that the " s u l f u r pattern" appears to be more prominant.  Similar spectra were obtained when an a l k a l i n e 8 M sodium  hydroxide glass containing 1.0 M DMSO was Y  - i r r a  diated  at 77°K to an  absorbed dose of 0.2 Mrad (Figure 66(b)).  In this case not a l l the  electrons were scavenged, as indicated by the blue tinge of the i r r a d i a t e d sample.  Subsequent photobleaching  with u l t r a v i o l e t l i g h t eliminated  the blue colour and caused a s l i g h t (^10-20%) increase i n the methyl r a d i c a l resonance (Figure 66(c)).  It was not possible to discern i f the  " s u l f u r pattern" changed i n i n t e n s i t y with bleaching because of the  Figure 64.  Electron spin resonance spectrum obtained after photoionization at 77°K of 0.01 M K^Fe(CN) i n 0.20 mole fraction DMSO-water glass (compare to Figure 62(a)).  - 208 -  (a)  I  H  I  10 gauss  Figure  >  65. E l e c t r o n s p i n resonance s p e c t r a o b t a i n e d of 0.01 M K F e ( C N ) 4  6  f o r the p h o t o i o n i z a t i o n  i n 8 M NaOH g l a s s a t 77°K.  added; (b) 1.0 M DMSO p r e s e n t  i n glass.  (a) no DMSO  F i g u r e 66.  E l e c t r o n s p i n resonance s p e c t r a o b t a i n e d Mrad) o f 8 M NaOH g l a s s a t 77°K.  (0.24  (a) no DMSO added; (b) 1.0 M  DMSO added; (c) a f t e r p h o t o b l e a c h i n g l i g h t f o r 20 m i n u t e s .  for y-irradiation  (b) w i t h u l t r a v i o l e t  - 210 -  decay o f t h e u n d e r l y i n g e l e c t r o n The  singlet.  e s r d a t a shown i n F i g u r e s 64, 65 and 66 thus s t r o n g l y  suggest  t h a t e l e c t r o n s r e a c t w i t h DMSO i n g l a s s y s o l i d s a t 77°K, and t h a t t h e reaction  produces m e t h y l and s u l f u r  radicals.  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