UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Nanosecond pulse radiolysis studies. Kenney, Geraldine Anne 1968

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1968_A6_7 K45.pdf [ 5.29MB ]
Metadata
JSON: 831-1.0059843.json
JSON-LD: 831-1.0059843-ld.json
RDF/XML (Pretty): 831-1.0059843-rdf.xml
RDF/JSON: 831-1.0059843-rdf.json
Turtle: 831-1.0059843-turtle.txt
N-Triples: 831-1.0059843-rdf-ntriples.txt
Original Record: 831-1.0059843-source.json
Full Text
831-1.0059843-fulltext.txt
Citation
831-1.0059843.ris

Full Text

NANOSECOND PULSE RADIOLYSIS STUDIES  by GERALDINE ANNE KENNEY L i c e n t i a t e o f t h e R o y a l I n s t i t u t e o f C h e m i s t r y , London 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE  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 conforming t o t h e required standard.  THE UNIVERSITY OF BRITISH COLUMBIA April,  1968  i  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 o f the  requirements  f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y study.  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  t h i s t h e s i s f o r s c h o l a r l y purposes may my  a v a i l a b l e f o r reference  be  Department o r by h i s r e p r e s e n t a t i v e s .  a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n .  Department o f C h e m i s t r y The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, B.C., Canada A p r i l 16, 1968  of  g r a n t e d by the Head o f I t i s understood that  copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l be  copying  and  gain s h a l l  not  ii  Abstract Nanosecond p u l s e r a d i o l y s i s s t u d i e s on the b e h a v i o u r  o f e aq  at h i g h c o n c e n t r a t i o n s as a p r e l i m i n a r y t o the i n v e s t i g a t i o n o f e aq* have shown t h a t c o n t r a r y t o normal c l a s s i c a l homogeneous k i n e t i c s  the  e l e c t r o n decays i n i t i a l l y i n a f i r s t o r d e r manner, moving i n t o second o r d e r decay w i t h i n about 100 nanoseconds a f t e r the e l e c t r o n p u l s e . F u r t h e r i n v e s t i g a t i o n s have shown t h a t f o r a comparable time  after  the p u l s e the d i s t r i b u t i o n o f the a b s o r b i n g s p e c i e s i s not homogeneous thus r e n d e r i n g any c l a s s i c a l k i n e t i c i n t e r p r e t a t i o n i n v a l i d .  Qualitative  c a l c u l a t i o n s on the d u r a t i o n o f the inhomogeneity were performed the e x p e r i m e n t a l The  r e s u l t s are i n r e a s o n a b l e  and  agreement w i t h t h e i r p r e d i c t i o n s ,  f i r s t o r d e r decay t h a t i s observed  i s c o n s i d e r e d t o be  more than a random sequence o f r e a c t i o n s and two p o s s i b l e models are t e n t a t i v e l y p r o p o s e d t o account f o r these  events.  Comparisons are made between t h i s work and o t h e r s i n which inhomogeneity undoubtedly accounts r a t e c o n s t a n t s f o r the p r i m a r y e aq + e aq The  *•  f o r the u n u s u a l l y f a s t b i m o l e c u l a r  decay  + 20H  aq  d e u t e r a t e d e l e c t r o n was  investigated with s i m i l a r  conclusions. The i " aq  r a t e c o n s t a n t s e v a l u a t e d from t h i s work were: k k  e d  2  k . k  2  = 8.80  ± .80 x 1 0  = 5.88  ± 1.20  = 8.14  ± .21 x 1 0  sec"  6  x 10 6  ,.11 .,-1 -1 = ^10 M sec  1 0  M  1  sec"  _1  sec"  1  1  iii  and the b i m o l e c u l a r r a t e c o n s t a n t , determined i n a homogeneous e n v i r o n ment, i s i n good agreement w i t h the accepted probable  l i t e r a t u r e v a l u e s f o r the  r e a c t i o n s i n v o l v i n g e aq i n our system.  iv Table o f Contents Page I.  Interactions of Ionising  Radiation  and M a t t e r  Introduction  1 1  Electromagnetic (i)  .  Rca-.diation  Photoelectric effect  2  (ii)  Compton e f f e c t  3  (iii)  P a i r production  3  Summary  4  Particle (i) (ii) (iii)  Radiation  Elastic collisions  7  Inelastic collisions  7  Bremsstrahlung  7  E l e c t r o n Range  7'  Cerenkov  10  Dose and Y i e l d  1 1  I r r a d i a t i o n o f L i q u i d Water (i)  (ii) (iii)  (a)  Physical  12  (b)  Energy t r a n s f e r e f f e c t s  14  Physicochemical  15  Chemical  17  The H y d r a t e d E l e c t r o n (i) (ii) (iii)  Background  18'  A p h y s i c a l model  20  The f a t e o f the h y d r a t e d e l e c t r o n  27  The Problem  29  V  Page II.  The Technique o f P u l s e R a d i o l y s i s and K i n e t i c S p e c t r o s c o p y The R a d i a t i o n C e l l  32  The E l e c t r o n A c c e l e r a t o r  36  S o l u t i o n s and Flow Techniques  37  The D e t e c t i o n System - O p t i c a l (i) (ii)  . 41  The l a s e r  41  F i l t e r s and l e n s e s  44  The D e t e c t i o n System - E l e c t r o n i c (i) (ii)  45  The p h o t o m u l t i p l i e r  45  O s c i l l o s c o p e and camera  48  The Grounding System III.  49  The Computation o f Data and the R e s u l t s The i n p u t data  52  Computation o f Data  52  V a r i a t i o n of o p t i c a l density with pathlength  56  The f o r m a t i o n  and decay o f e aq a f t e r a 3 Nsec e l e c t r o n p u l s e  The f o r m a t i o n  and decay o f e aq a f t e r a 50 Nsec e l e c t r o n p u l s e .  . . 58 . 63  The e f f e c t s o f m u l t i p l e p u l s i n g  63  The e f f e c t o f H  65  The e f f e c t o f an a l c o h o l  65  The e f f e c t o f oxygen  66  The p i n h o l e experiments  68  The d e u t e r a t e d  73  electron  I n t e r f e r e n c e phenomena  7  ?  vi Page IV.  D i s c u s s i o n and I n t e r p r e t a t i o n o f the (i) (ii)  (iii) (iv) (v)  The b e h a v i o u r  Results  o f e aq at h i g h dose r a t e s  83  A model r e l a t i n g t o the d i s t r i b u t i o n o f the spurs i n space and time  86  C a l c u l a t i o n s and the r e s u l t s  87  The b i m o l e c u l a r decay  91  F i r s t o r d e r decay  92  Epilogue  95  References  97  vii  L i s t o f Tables Page Table I . Table I I .  Rate Constants o f some r a d i c a l r e a c t i o n i n r a d i o l y s e d water. . \B i m o l e c u l a r r a t e c o n s t a n t s f o r the decay o f e aq + e aq  28 30  Table I I I . E x p e r i m e n t a l l y determined r a t e c o n s t a n t s f o r e aq + e aq  61  T a b l e IV.  Data from the p i n h o l e experiments  6p  Table V.  Data f o r the d e u t e r a t e d e l e c t r o n  76  Table V I .  C a l c u l a t i o n s from the s p u r o v e r l a p model  88  viii List of Illustrations Page Diagram 1. Diagram 2. Diagram 3.  Mass 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 energy t r a n s f e r processes i n water  5  Range o f a monoenergetic beam o f e l e c t r o n s i n an absorbing material  9  Sequence o f e v e n t s f o l l o w i n g t h e impact o f a primary p a r t i c l e  13  Diagram 4.  A b s o r p t i o n spectrum o f t h e h y d r a t e d e l e c t r o n . . . .  23  Diagram 5.  Energy models f o r the h y d r a t e d e l e c t r o n  26  Diagram 6.  The a c c e l e r a t o r l a b o r a t o r y  31  Diagram 7.  The p l e x i c e l l and components (photograph)  34  Diagram 8.  The e l e c t r o n i c d e t e c t i o n system (photograph). . . .  40  Diagram 9.  Showing the apparatus i n e x p e r i m e n t a l p o s i t i o n s (photograph) Faraday cup; t y p i c a l p u l s e d waveforms from the  43  two e l e c t r o n tubes  38  Diagram 11.  The f l o w system (photograph)  46  Diagram 12.  The g r o u n d i n g system  50  Diagram 13.  O p t i c a l d e n s i t y as a f u n c t i o n o f p a t h l e n g t h .  Diagram 14.  F i r s t and second o r d e r decay o f e aq  DiagranulO.  ...  57 59  Diagram 15a. F i r s t and Second o r d e r decay o f e d  75  Diagram 15b. Some o s c i l l o s c o p e t r a c e s o f e aq + s o l u t e s  62  Diagram 16.  Second o r d e r decay o f e aq  60  Diagram 17.  The e f f e c t s o f m u l t i p l e p u l s i n g on r a t e c o n s t a n t s and e x p e r i m e n t a l h a l f l i f e o f e aq 64 The e f f e c t s o f s o l u t e s on the e x p e r i m e n t a l h a l f l i f e o f the h y d r a t e d e l e c t r o n 67  Diagram 18. Diagram 19.  O p t i c a l d e n s i t i e s and k i n e t i c d a t a from the p n h o l e experiments  71  ix List of Illustrations  (cont).  Page  Diagram 20.  O s c i l l o s c o p e t r a c e s o f some i n t e r f e r e n c e s i g n a l s . . 78  Diagram 21.  P i c t o r i a l r e p r e s e n t a t i o n o f t h e r e s u l t s o f t h e spur overlap c a l c u l a t i o n s 89  The a s s i s t a n c e o f Mr. D.A. Head i s acknowledged i n Diagrams 1, 2 and 20.  X  Acknowledgment I would l i k e t o thank Dr. D.C.  Walker f o r h i s v a l u e d a d v i c e , p a t i e n c e  and encouragement d u r i n g t h e course o f t h i s r e s e a r c h and i n t h e preparation of this  thesis.  1  Interactions o f I o n i s i n g Radiation with Matter The  a b s o r p t i o n o f h i g h energy r a d i a t i o n s o f e i t h e r p a r t i c l e o r  e l e c t r o m a g n e t i c o r i g i n induces physicochemical processes material.  i o n i s a t i o n , e x c i t a t i o n and a s e r i e s o f  a l l o f which o c c u r i n t h e a b s o r b i n g  target  R a d i c a l s , i o n s and e x c i t e d s p e c i e s produced as a consequence  o f these p r i m a r y e f f e c t s then i n t e r a c t w i t h t h e i r m o l e c u l a r and g i v e r i s e t o t h e s t a b l e c h e m i c a l p r o d u c t s .  Although  termed " i o n i s i n g "  these r a d i a t i o n s a r e not n e c e s s a r i l y r e s t r i c t e d t o i o n i s i n g as w i l l become c l e a r d u r i n g t h i s i n t r o d u c t o r y s e c t i o n .  environment  behaviour  Some o f t h e a s p e c t s  o f t h e p h y s i c a l consequences o f t h e i n t e r a c t i o n s o f r a d i a t i o n and m a t t e r w i l l be d i s c u s s e d as a b a s i s f o r t h e c h e m i c a l r e a c t i o n s t h a t u l t i m a t e l y occur. The  energy o f a r a y o r p a r t i c l e can be w h o l l y o r i n p a r t  t r a n s f e r r e d t o t h e medium through which t h e r a d i a t i o n i s p a s s i n g .  The  mechanisms by which energy i s t r a n s f e r r e d t o , and t h e n - d i s s i p a t e d i n the medium a r e to. some e x t e n t c h a r a c t e r i s t i c o f t h e i n c i d e n t r a d i a t i o n , and t o some e x t e n t dependent on t h e m a t e r i a l i t s e l f .  For electromagnetic  r a d i a t i o n s which w i l l now be d i s c u s s e d t h e r e a r e t h r e e main p r o c e s s e s , each o f which may dominate under s p e c i f i e d  Electromagnetic  circumstances.  radiation  As a photon impinges on t h e s u r f a c e o f a t a r g e t m a t e r i a l t h e r e may be a t r a n s f e r o f energy r e s u l t i n g i n a change o f d i r e c t i o n and energy o f t h e i n c i d e n t photon; a l t e r n a t i v e l y t h e r e may be s i m p l y a r e d u c t i o n i n t h e i n t e n s i t y o f t h e t r a n s m i t t e d r a d i a t i o n i n accordance w i t h Al  = -uIAx  2  where  x i s the thickness o f the absorbing m a t e r i a l , A l the l o s s o f  i n t e n s i t y and u 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 .  F o r a non-monoenergetic  beam o f photons i n c i d e n t on t h e m a t e r i a l HHi1 i x  I = I e T  T  + I e T  -u?x M Z  -u^x + I ;e J T  K  where (I.., u..) c h a r a c t e r i s e s a photon o f energy E... The t h r e e i m p o r t a n t (i) (ii) (iii)  i n t e r a c t i o n s t o consider are  the p h o t o e l e c t r i c e f f e c t t h e Compton e f f e c t p a i r production  and o f t h e s e o n l y one under any g i v e n s e t o f c o n d i t i o n s w i l l c o n t r i b u t e s i g n i f i c a n t l y t o t h e energy exchange p r o c e s s e s . (i)  The p h o t o e l e c t r i c e f f e c t d e s c r i b e s t h e a b s o r p t i o n o f photons  o f low energy, 1 KeV < E.. < 500 KeV, by m a t e r i a l s o f h i g h atomic number, Z, which r e s u l t s i n t h e simultaneous  angular e j e c t i o n o f a photoelectron  whose energy i s g i v e n by E In m e t a l s  pe  = E. - d> J  $ i s t h e work f u n c t i o n , f o r o t h e r media <j> i s g e n e r a l l y thought  o f as t h e b i n d i n g energy b u t i n e i t h e r case may be e q u a l t o o r more than the i o n i s a t i o n p o t e n t i a l o f t h e medium. e j e c t e d a t angles approaching  90° as t h e energy o f t h e photon  The sharp d i s c o n t i n u i t i e s observed v e r s u s atomic  The p h o t o e l e c t r o n s t e n d t o be  on a g r a p h i c a l p l o t o f photon e n e r g i e s  a b s o r p t i o n c o e f f i c i e n t s r e l a t e d t o t h e b i n d i n g energy o f  the e l e c t r o n s i n the d i f f e r e n t s h e l l s .  Vacancies  i n the i n n e r  a r i s i n g from a p h o t o e l e c t r i c p r o c e s s w i l l be f i l l e d by o u t e r e l e c t r o n s ; energy i s conserved 1  decreases.  auger e l e c t r o n s .  shells  shell  w i t h t h e e m i s s i o n o f X-rays o r low-energy  Coherent s c a t t e r i n g o f t h e i n c i d e n t photon by t h e atomic increases  electrons  as t h e e l e c t r o n d e n s i t y o f t h e m a t e r i a l i n c r e a s e s  g e n e r a l l y most pronounced a t low photon e n e r g i e s . o c c u r s i n t h e same energy r e g i o n s  and i s  This s c a t t e r i n g  as t h e p h o t o e l e c t r i c e f f e c t b u t i s  by comparison a weak i n t e r a c t i o n . (ii)  The Compton e f f e c t d e s c r i b e s  partially elastic  collision  o f a photon w i t h an e l e c t r o n d u r i n g which t h e photon i s s c a t t e r e d a diminished  with  energy E and t h e e l e c t r o n r e c o i l s w i t h an i n c r e a s e i n  energy E  r  = E. - E J  As t h e s c a t t e r i n g i s a n g u l a r t h e a b s o l u t e  value o f the r e c o i l  energy E^ i s a f u n c t i o n o f t h e a n g u l a r r e l a t i o n s h i p between t h e i n c i d e n t and  s c a t t e r e d p h o t o n s , and t h e a c c e l e r a t e d E =  electron.  Eo (1 + E o / m c ) ( l - Cos-6-) 2  Q  E r has v a l u e s o f zero ( E r = Eo) and t h e maximum energy i s g i v e n by l e t t i n g 8 ->- 180° Er  = { m a x  Eo  i  } n or /c  1 + 0.25/Eo The Compton e f f e c t dominates r a d i a t i o n i n t e r a c t i o n s between i n c i d e n t photons o f s e v e r a l MeV and m a t e r i a l s  of high electron  density,  and photons o f lower i n c i d e n t e n e r g i e s (20 KeV < E.. < 2MeV) f o r m a t e r i a l s of lower e l e c t r o n d e n s i t y .  I n a medium such as water Compton e f f e c t s  have been observed o v e r t h e range 30 KeV < E.. < 20 MeV. (iii) and  p a i r production  r e f e r s t o t h e appearance o f a p o s i t r o n  e l e c t r o n a t t h e d i s a p p e a r a n c e o f t h e i n c i d e n t photon ( i n t h e v i c i n i t y  o f t h e atomic n u c l e u s ) whose energy i s c o n v e r t e d i n t o t h e k i n e t i c energy  4  and r e s t mass o f t h e s e two p a r t i c l e s .  Photons o f energy l e s s than 1.02 MeV  cannot p a r t i c i p a t e i n t h i s p r o c e s s as t h e r e l a t i o n s h i p Ep + E = E. - 2mc * e j must be s a t i s f i e d .  2  Following the formation o f the p a i r the p o s i t r o n  combines w i t h an e l e c t r o n and two 0.51 MeV Y emitted.  _ r a  y  s  a r e  simultaneously  These a r e r e f e r r e d t o as a n n i h i l a t i o n r a d i a t i o n .  Summary The  atomic c r o s s s e c t i o n s ( o 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  a l l these p r o c e s s e s i n c r e a s e w i t h i n c r e a s i n g e l e c t r o n d e n s i t y o f t h e material involved.  I n a g i v e n medium and a t low photon e n e r g i e s t h e  most i m p o r t a n t e f f e c t i s t h e p h o t o e l e c t r i c phenomenon; a t medium photon e n e r g i e s t h e Compton e f f e c t dominates any o t h e r p r o c e s s e s  and a t h i g h  photon e n e r g i e s p a i r p r o d u c t i o n i s t h e p r e v a i l i n g i n i t i a l energy t r a n s f e r mechanism. The mass 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 s e v a r i o u s p r o c e s s e s i n water have been p l o t t e d a g a i n s t i n c i d e n t photon e n e r g i e s i n diagram I.  The mass a b s o r p t i o n c o e f f i c i e n t i s d e f i n e d as —  . where p i s t h e  -3 d e n s i t y i n grams cm  , and f i s a c o r r e c t i o n f a c t o r f o r s c a t t e r i n g ,  f l u o r e s c e n c e l o s s e s and b r e m s s t r a h l u n g  emission.  The t r u e mass a b s o r p t i o n  c o e f f i c i e n t i s e s s e n t i a l l y t h e sum o f a l l t h e i n d i v i d u a l f o r t h e e f f e c t s d i s c u s s e d above: ±E X — — ii =  p  +  p  +  p  coefficients  (see legend on diagram)  +  p  p  energy c o n t r i b u t i o n s from s c a t t e r e d photons a r e o f t e n n e g l e c t e d t o a f i r s t approximation.  At v e r y h i g h i n c i d e n t photon e n e r g i e s  photonuclear  r e a c t i o n s may o c c u r o f t h e g e n e r a l form (y,n) o r ( y , p ) • t h e r e i s a h i g h  5  l  1  Energy  r  (Mev)  Diagram 1. Mass 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 the v a r i o u s p r o c e s s e s i n water.  6  threshold  f o r these r e a c t i o n s  and one can j u s t i f i a b l y assume t h a t i n  most cases t h e y o f f e r no s i g n i f i c a n t c o n t r i b u t i o n t o t h e t o t a l energy absorbed.  Particle  radiations As a h i g h energy charged p a r t i c l e t r a v e l s through a medium  energy may be t r a n s f e r r e d v i a d i r e c t c o l l i s i o n s and e x c i t a t i o n s  with  the predominant appearance o f i o n i s e d and e x c i t e d atoms and t h e p r o d u c t i o n of r a d i a t i o n .  At lower e n e r g i e s t h e p a r t i c l e s w i l l be i n v o l v e d  i nelastic  s c a t t e r i n g and i n e l a s t i c c o l l i s i o n s t o a more s i g n i f i c a n t degree. The and  i n t e r a c t i o n s o f heavy p a r t i c l e s such as a l p h a p a r t i c l e s  those o f e l e c t r o n s  o f t h e same energy i n any a b s o r b i n g medium can  be o n l y q u a l i t a t i v e l y compared because the e l e c t r o n s v e l o c i t y and as such cause l e s s s p e c i f i c i o n i s a t i o n . h i g h energy e l e c t r o n s  w i l l have a g r e a t e r With a beam o f  as an i n c i d e n t r a d i a t i o n s o u r c e t h e energy t r a n s f e r  p r o c e s s e s t h a t must be c o n s i d e r e d a r e : (i) (ii) (iii)  elastic scattering, deflection i n e l a s t i c s c a t t e r i n g and e x c i t a t i o n emission o f bremsstrahlung r a d i a t i o n .  There w i l l be a s t a t i s t i c a l the  v a r i a n c e i n t h e number o f c o l l i s i o n s between  i n c i d e n t p a r t i c l e and t h e e l e c t r o n s  o f t h e medium and c o n s e q u e n t l y  i n t h e energy t r a n s f e r r e d d u r i n g t h e s e e v e n t s .  T h i s i s i n d i c a t e d by  the ranges o f t h e p a r t i c l e s i n a g i v e n a b s o r b e r , and i n t h e case o f an electron  (which can l o s e a s u b s t a n t i a l amount o f i t s energy i n one  c o l l i s i o n ) t h e s p r e a d i s even more pronounced.  7  (i)  e l a s t i c s c a t t e r i n g arises from t h e d e f l e c t i o n o f t h e  i n c i d e n t e l e c t r o n w i t h o u t l o s s o f energy by t h e coulombic f i e l d s  about  the n u c l e i o f t h e medium; t h e p r o c e s s i s most i m p o r t a n t f o r low energy e l e c t r o n s and m a t e r i a l s  of high electron density.  Any l a r g e angle  d e f l e c t i o n s w i l l be from n u c l e a r s c a t t e r i n g . (ii)  i n e l a s t i c s c a t t e r i n g o c c u r s when t h e i n c i d e n t e l e c t r o n '  i n t e r a c t s w i t h these coulombic f i e l d s o f t h e medium and a l a r g e f r a c t i o n o f t h e energy o f t h e i n c i d e n t e l e c t r o n may be t r a n s f e r r e d i n a s i n g l e collision.  Energy i m p a r t e d t o t h e e l e c t r o n o f t h e medium e x p e r i e n c i n g  a c o l l i s i o n i s o f t e n s u f f i c i e n t t o cause t h i s e l e c t r o n t o t a k e p a r t i n secondary c o l l i s i o n s i t s e l f .  These p r i m a r y and secondary  events  b o t h l e a d t o i o n i s a t i o n and e x c i t a t i o n o f t h e atoms i n t h e a b s o r b i n g m a t e r i a l and a r e t h e dominant p r o c e s s f o r t h e energy l o s s o f mfedl-um energy electrons  <1 MeV below which b r e m s s t r a h l u n g e m i s s i o n i s n o t an i m p o r t a n t  factor. (iii)  B r e m s s t r a h l u n g r a d i a t i o n i s e m i t t e d t o conserve  energy  and momentum r e l a t i o n s h i p s when a h i g h speed e l e c t r o n i s d e a c c e l e r a t e d i n t h e environment  o f a n u c l e u s o f t h e atoms i n t h e a b s o r b i n g m a t e r i a l  and i s predominant  mode o f energy l o s s f o r e l e c t r o n s >10 MeV.  -dE z^Z^ o f l o s s o f energy — 3 — i s p r o p o r t i o n a l t o — — dx 2  The r a t e  where z, Z a r e t h e  m  charges on the p a r t i c l e and n u c l e u s r e s p e c t i v e l y , and m i s t h e mass o f the p a r t i c l e .  The range o f t h e e l e c t r o n The t r a c k o f a charged p a r t i c l e p a s s i n g through a medium w i l l not n e c e s s a r i l y be a s t r a i g h t one, and f o r e l e c t r o n s i n p a r t i c u l a r t h e  8  t r u e range i s much g r e a t e r than t h a t determined e x p e r i m e n t a l l y , t o the a p p r e c i a b l e  and d i v e r s i f i e d s c a t t e r i n g a l r e a d y  the average range i s s t i l l  owing  discussed.  However  dependent on the r a t e o f l o s s o f energy o f  the i n c i d e n t e l e c t r o n -- r e f e r r e d t o as t h e s t o p p i n g power o f the with respect to that p a r t i c l e .  material  I n B e t h e l s f o r m u l a below t h i s l o s s  per u n i t l e n g t h i s r e l a t e d t o b o t h the c h a r a c t e r i s t i c s o f the medium and  the i n c i d e n t p a r t i c l e . J  9  F  -(G£)  4  =  m v E  >—tr-  2  N  .  Z [  i _° n  mv  21  Q  _  ( 2  /_ 2  .  g  !  +  g  ^  9 )  l  n  2  +  (1-3 )  + 1 - g  collective  2  + 1_(1 - y/l - g ) ] ergs c m 2  2  - 1  8  v = v e l o c i t y o f i n c i d e n t p a r t i c l e , cm s e c g =  c i s t h e v e l o c i t y o f l i g h t , cm sec ^.  I = mean e x c i t a t i o n p o t e n t i a l o f the atoms, e r g s . N = number o f atoms p e r c.c.  o f stopping material.  e = charge on e l e c t r o n , e s u . m = r e s t mass o f the e l e c t r o n , grams. o • Z = atomic number o f the s t o p p i n g m a t e r i a l . The  dE s t o p p i n g power i s - (-5—) ,, . . , which d i v i d e d by t h e Mx collective r r  6  r  ;  1  d e n s i t y o f the m a t e r i a l p g i v e s t h e mass s t o p p i n g power.  The lower the  i n c i d e n t energy o f the p a r t i c l e the more r e a d i l y w i l l i t r e a c h thermal energies.  I n diagram 2 the number o f i n c i d e n t monoenergetic  electrons  t h a t p e n e t r a t e t o a c e r t a i n p o s i t i o n i n the a b s o r b i n g m a t e r i a l has been p l o t t e d as a f u n c t i o n o f d i s t a n c e .  The t a i l o f the curve r e p r e s e n t s  the d i s t r i b u t i o n o f ranges and the mean range R i s shown w i t h t h e • m 6  extrapolated mental work.  range R > t h e l a t t e r v a l u e b e i n g the one used i n e x p e r i ex  ( I t i s o n l y p o s s i b l e t o determine a maximum range f o r  T  r  01  0-2  1  0-3  r  0-4  Distance Diagram 2.  Range o f a beam o f monoenergetic e l e c t r o n s i n an a b s o r b e r  10  non-monoenergetic beams by means o f a p e n e t r a t i o n s t u d y . ) As t h e e l e c t r o n spends a time i n v e r s e l y v e l o c i t y w i t h i n an i n t e r a c t i o n  d i s t a n c e o f t h e atomic e l e c t r o n s any  changes i n i t s momentum and energy t r a n s f e r this period. the  proportional to i t s  must be a c c o m p l i s h e d w i t h i n  However i n r e l a t d v i s t i c r e g i o n s t h e k i n e t i c energy o f  p a r t i c l e exceeds i t s rest-mass energy (6 -»• 1) and s i g n i f i c a n t 2  variations ionisation  i n t h e In (1-3  ) t e r m l e a d t o an i n c r e a s e i n t h e s p e c i f i c  (-dE/dx) .. p a r a l l e l i n g t h e i n c r e a s e i n v: t h e i n t e r ^collective '  v  r  6  p r e t a t i o n o f t h i s i s based on t h e apparent s h o r t e r time o f c o l l i s i o n ( t h e r e b y i n c r e a s i n g t h e impact parameters) due t o t h e L o r e n t z c o n t r a c t i o n . -dE At n o n - r e l a t i v i s t i c v e l o c i t i e s , (—j—) I T ^decreases as v i n c r e a s e s . ' dx^collective Cerenkov  radiation -13 The  instantaneous (within  10  seconds) e m i s s i o n o f l i g h t  at wavelengths i n t h e I.R., v i s i b l e and U.V. d u r i n g t h e i r r a d i a t i o n o f v a r i o u s media w i t h f a s t charged p a r t i c l e s radiation and  and has s p e c i a l  emission e f f e c t s  significance  i n irradiated  i s c l a s s i f i e d as Cerenkov  i n any s t u d y o f t h e a b s o r p t i o n  liquids.  The i n t e n s i t y  and d u r a t i o n  o f t h e luminescence i s such t h a t i t can mask weak o r v e r y s h o r t  lived  e m i s s i o n o r a b s o r p t i o n and t h e r e f o r e a p p r o p f n t e e x p e r i m e n t a l p r e c a u t i o n s must be t a k e n .  The d u r a t i o n o f t h e e m i s s i o n i s determined by t h e  v e l o c i t y o f t h e charged p a r t i c l e and t h e l e n g t h o f i t s t r a c k b e f o r e i t slows down t o b e l o w - t h r e s h o l d c o n d i t i o n s . r e l a t i v i t i c considerations.  from  I f t h e v e l o c i t y v o f a f a s t charged p a r t i c l e  moving i n a medium o f r e f r a c t i v e light  The e m i s s i o n arises  i n d e x n exceeds t h e phase v e l o c i t y o f  (—) i n t h e same medium, then an e l e c t r o m a g n e t i c shock wave i s  produced at an angle t o t h e t r a c k o f t h e p a r t i c l e as a r e s u l t o f t h e  11  p o l a r i s a t i o n and subsequent r e l a x a t i o n o f t h e m o l e c u l e s i n t h e immediate v i c i n i t y o f the track. The t h r e s h o l d c o n d i t i o n i s t h e r e f o r e ng>>l where g i s the r e l a t i v i s t i c v e l o c i t y \  The angle 6 at which t h e l i g h t  i s emitted i s r e l a t e d to the track d i r e c t i o n Cos 0 = and f o r w a t e r i f n = 1.332, 6 w i l l be 0.751 c o r r e s p o n d i n g t o 265 KeV electrons  f o r t h e t h r e s h o l d p a r t i c l e energy.  Dose and y i e l d The y i e l d o f a s p e c i e s formed o r d e s t r o y e d i n t h e c h e m i c a l p r o c e s s e s i n d u c e d by r a d i a t i o n i s e x p r e s s e d i n terms o f t h e number o f m o l e c u l e s p e r u n i t o f absorbed energy. g (x) Evaluation  _ number o f m o l e c u l e s o f x formed 100 eV absorbed energy  o f t h e absorbed energy i s t h e r e f o r e  c r i t i c a l and a range o f  t e c h n i q u e s are a v a i l a b l e t o measure t h i s energy.  In d o s i m e t r y t h e  common u n i t o f absorbed dose i s t h e r a d . One r a d i s e q u i v a l e n t t o 13 the d e p o s i t i o n o f 100 ergs o r 6.24 x 10  eV p e r gram o f m a t e r i a l .  I r r a d i a t i o n o f l i q u i d water The r a d i a t i o n c h e m i s t r y o f l i q u i d w a t e r i s a r e m a r k a b l y complex s u b j e c t whose t h e o r e t i c a l tenets based on p h y s i c a l and c h e m i c a l e v i d e n c e are n o t f u l l y u n d e r s t o o d even a t the p r e s e n t moment.  To  emphasise t h e c h a n g i n g environment i n t h e v i c i n i t y o f t h e i n c i d e n t charged p a r t i c l e as i t t r a v e l s t h r o u g h t h e l i q u i d medium t h i s  section  12  w i l l be p r e s e n t e d i n t h r e e c h r o n o l o g i c a l s t a g e s f o l l o w i n g t h e impact o f the  primary p a r t i c l e . (i)  (a) p h y s i c a l (b)  10  -18  t o 10  seconds  energy t r a n s f e r e f f e c t s -13  ( i i ) physicochemical 10 _7 (iii)  -16  chemical  >10  -7 t o 10  seconds  seconds  In view o f t h e e x p o n e n t i a l i n c r e a s e i n t h e number o f papers on t h e r a d i a t i o n e f f e c t s i n b i o l o g i c a l l y i m p o r t a n t m o l e c u l e s and i n v i t r o systems where w a t e r p l a y s a major r o l e , i t would be j u s t i f i e d t o complete the  sequence  with 7  (iv)  biochemical, b i o l o g i c a l  >10  as such i s beyond t h e scope o f t h i s t h e s i s .  seconds,even i f d i s c u s s i o n The sequence i s i l l u s t r a t e d  i n diagram 3. ( i ) a.  Stage I - p h y s i c a l  The a b s o r p t i o n o f r a d i a t i o n i s a v e r y f a s t p r o c e s s and does n o t produce a s i m u l t a n e o u s response t h a t can be e x p e r i m e n t a l l y observed o t h e r t h a n Cerenkov r a d i a t i o n .  The p r i m a r y e l e c t r o n ( o r Compton e l e c t r o n  i n t h e case o f i n c i d e n t X o r y v r a y s ) f o l l o w s a l i n e a r t r a c k at h i g h e n e r g i e s b u t i s d e f l e c t e d as i t slows down; secondary i o n i s a t i o n s and 2 e x c i t a t i o n s are produced a l o n g t h i s t r a c k g i v i n g r i s e t o b o t h low (£10 2 and h i g h (>10  eV) secondary e l e c t r o n s .  The l a t t e r , i f s u f f i c i e n t l y  e n e r g e t i c w i l l b r a n c h o f f t o form a n o t h e r s h o r t e r t r a c k r e f e r r e d t o as a § r a y , a l o n g which t h e r e m a i n i n g energy w i l l be d e p o s i t e d . The low energy secondary e l e c t r o n s s u f f e r f u r t h e r d e f l e c t i o n s c a u s i n g i o n i s a t i o n and e x c i t a t i o n i n a d e f i n e d a r e a o f t h e medium. The energy o f t h e p r i m a r y p a r t i c l e i s thus d e p o s i t e d a l o n g these  eV)  13  Impact o f P r i m a r y P a r t i c l e 0 sees 10  sees  10  sees  Physical  Stage  e h i g h energy  e sub  excited  P h y s i c o c h e m i c a l Stage 10  sees  e thermal  10  sees  e  sees  e  10 in"  10  Diagram 3:  7  sees  solvated  reacts m s chemical stage  Sequence o f events a f t e r t h e impact o f the i n c i d e n t p a r t i c l e (41)  14  t r a c k s as a number o f events o r s p W s ; about 60% o f t h e t o t a l energy i s t r a n s f e r r e d t o the medium i n t h i s way.  A spur i s c o n s i d e r e d t o be  an average d e p o s i t i o n o f 100 eV and w i l l be a c l u s t e r o f e l e c t r o n i c a l l y e x c i t e d s p e c i e s , p o s i t i v e i o n s and e l e c t r o n s , a l l o f which now e n t e r the p h y s i c o c h e m i c a l ( i ) b.  stage.  Energy t r a n s f e r e f f e c t s  The d i s t a n c e between t h e spurs i s r e l a t e d t o t h e n a t u r e o f t h e p r i m a r y p a r t i c l e and i s c a l c u l a b l e from a knowledge o f t h e r a t e o f energy t r a n s f e r t o t h e medium.  The l i n e a r energy t r a n s f e r o r L.E.T. f o r m a l l y  e x p r e s s e s t h i s as a r a t e o f energy l o s s o f t h e i n c i d e n t p a r t i c l e i n eV o  p e r A, and p u t s t h e energy l o s s f o r d i f f e r e n t p a r t i c l e s on a q u a n t i t a t i v e basis.  The L.E.T. i n c r e a s e s a l o n g t h e t r a c k as t h e p a r t i c l e slows down  i m p l y i n g t h a t t h e spurs become c l o s e r t o g e t h e r towards t h e end o f a t r a c k - and w i t h h i g h L.E.T. t r a c k s t h e spurs w i l l n e c e s s a r i l y o v e r l a p .  Since  the chemical p r o d u c t s o f r a d i o l y s i s a r e governed by t h e b e h a v i o u r o f the s p e c i e s formed i n t h e s p u r , and t h e s p a t i a l d i s t r i b u t i o n o f t h e spurs by t h e L.E.T. c h a r a c t e r i s t i c s o f t h e p a r t i c l e , L.E.T. e f f e c t s a r e extremely  important.  For a t y p i c a l Co  60  ° y-ray t h e L.E.T. i s 0.02 eV p e r A which i s  s m a l l and t h e spurs w i l l be r e l a t i v e l y i s o l a t e d .  Low energy e l e c t r o n s  have L.E.T. v a l u e s i n c r e a s e d by an o r d e r o f magnitude w h i l e a heavy o  a l p h a p a r t i c l e has a h i g h L.E.T. o f ^ 10 eV p e r A. The model o f s p u r s , t r a c k s and y-rays has been e l a b o r a t e d (6,7) i n an attempt  t o g i v e a more r e a l i s t i c p i c t u r e o f t h e a r b i t r a r i l y  c l a s s i f i e d secondary effects.  e l e c t r o n s i n t h e l i g h t o f t h e s e d i f f e r e n t L.E.T.  15  An e n e r g e t i c secondary than 5000 eV w i l l  e l e c t r o n o r d - e l e c t r o n o f energy g r e a t e r  form i t s own branch t r a c k and because o f t h e low L.E.T.  the spurs w i l l be w e l l s e p a r a t e d , o f the o r d e r 10^ A a p a r t .  As t h e § -  e l e c t r o n l o s e s energy the L.E.T. i n c r e a s e s and at e n e r g i e s below 5000 eV the spurs b e g i n t o o v e r l a p g i v i n g r i s e t o a s h o r t t r a c k .  F i n a l l y as  the ^ - e l e c t r o n approaches e n e r g i e s o f 500 t o 100 eV i t becomes d i f f i c u l t for  the e l e c t r o n or p r o d u c t s o f i t s i o n i s a t i o n power t o move f a r from  t h e i r o r i g i n , thus g i v i n g r i s e t o a d e n s e l y packed a r e a o f i o n i s a t i o n and e x c i t a t i o n w i t h a v i s u a l i s e d pear-shape geometry, o f t e n r e f e r r e d to  as a b l o b .  The secondary  e l e c t r o n s w i t h e n e r g i e s below 100 eV  will  form c l u s t e r s o f i o n i s a t i o n s i n i s o l a t e d s p u r s . As the m o l e c u l a r p r o d u c t s , formed i n i n t r a s p u r r e a c t i o n s o r on the r e c o m b i n a t i o n o f i o n p a i r s w i t h i n the s p u r , d i f f e r i n y i e l d  from  those a t t r i b u t e d t o say r a d i c a l r e a c t i o n s between s p e c i e s d i f f u s i n g o f the s p u r s , i t i s not s u p r i s i n g t h a t v a r i a t i o n s i n the i n t e n s i t y  out and  o r i g i n o f the i o n i s i n g r a d i a t i o n l e a d t o d i f f e r e n t y i e l d s .  (ii) The spur may  Stage I I - p h y s i c o c h e m i c a l c l u s t e r o f i o n i s e d and e x c i t e d s p e c i e s d e s i g n a t e d as a  l o s e energy by c o l l i s i o n and d i s s o c i a t i o n i n t o r a d i c a l s .  Solvent  -12 quenching  o f e x c i t e d s p e c i e s can o c c u r w i t h i n 10  seconds and  metastable  s t a t e s a r e thought t o r e t u r n t o the ground s t a t e by n o n - r a d i a t i v e processes.  As about 30 eV i s expended on average p e r i o n - p a i r  a t y p i c a l spur w i l l c o n t a i n t h r e e i o n - p a i r s or s i x r a d i c a l s .  produced, Energetic  e l e c t r o n s have a l r e a d y been d i s c u s s e d but a s u b - e x c i t e d e l e c t r o n (<10 can become t h e r m a l i s e d and w i t h i n 10  1 1  seconds (the d i e l e c t r i c  eV)  relaxation  16  time o f water) s o l v a t e d . The p o s i t i o n o f the t h e r m a l i s e d e l e c t r o n w i t h r e s p e c t t o the p o s i t i v e p a r e n t i o n must be c a r e f u l l y c o n s i d e r e d -- i f the t h e r m a l of the e l e c t r o n exceeds the a t t r a c t i v e p o t e n t i a l energy r e s u l t i n g the coulombic  energy from  f i e l d o f the c o n c o m i t t a n t p o s i t i v e i o n , the e l e c t r o n  not be r e c a p t u r e d .  will  The net s e p a r a t i o n o f the i o n p a i r due t o random  walk i s c r i t i c a l and f o r aqueous s o l u t i o n s two opposing t h e o r i e s r e l a t i n g to t h e y i e l d s o f m o l e c u l a r p r o d u c t s are based on d i f f e r e n t e v a l u a t i o n s of t h i s parameter.  O b v i o u s l y media o f h i g h d i e l e c t r i c c o n s t a n t s w i l l  p e r m i t t h e e l e c t r o n to t r a v e l f u r t h e r b e f o r e t h e r m a l i s a t i o n . The Samuel-Magee model (8) assumes t h a t r e c a p t u r e does o c c u r i n the coulombic  f i e l d , w h i l e the Lea-Platzman  t o escape b e f o r e t h e r m a l i s a t i o n . evidence  model p e r m i t s t h e e l e c t r o n  C u r r e n t s p e c t r o s c o p i c (10) and  chemical  (11) tend t o f a v o u r the l a t t e r model w h i l e m o d i f i c a t i o n s t o  the former accounts  f o r some d i s c r e p a n c i e s , but t h e r e i s s t i l l  a large  d i f f e r e n c e i n the e s t i m a t e s o f the mean f r e e p a t h o f the e s c a p i n g  electron.  The d i f f u s i o n o f r a d i c a l s b e g i n s i n t h i s s t a g e and the s p u r i s t h e r e f o r e i n c r e a s i n g i n s i z e which r a t h e r i n v a l i d a t e s the  accepted  f i r s t a p p r o x i m a t i o n o f r e g a r d i n g the spur as h a v i n g s p h e r i c a l geometry. U n f o r t u n a t e l y i t i s a l s o d u r i n g t h i s p e r i o d t h a t one r e q u i r e s good v a l u e s for  s p u r s i z e and o v e r l a p i n m e c h a n i s t i c c a l c u l a t i o n s .  I f the s p e c i e s  escapes from the s p u r i t must e v e n t u a l l y r e a c t w i t h another  radical,  s o l u t e o r s o l v e n t m o l e c u l e s , perhaps a t d i f f u s i o n c o n t r o l l e d r a t e s .  A  t h e o r e t i c a l treatment o f such a o n e - r a d i c a l one s o l u t e problem has been g i v e n by Kupperman (12) and extended  t o more complex  systems.  The mechanisms o c c u r i n g d u r i n g the p h y s i o c h e m i c a l s t a g e are  17  H 0  v H 0 . F , p o s s i b l y H 0* +  2  H 0  +  2  2  + H 0  •H 0  2  + OH  +  3  H 0*  »• H + OH  2  e" t h e r m a l  (iii)  2  »• e aquated  Stage I I I - c h e m i c a l  F l u o r e s c e n c e from e l e c t r o n i c a l l y  e x c i t e d s t a t e s may be observed  and, as low energy e l e c t r o n s i n t h e £-ray a r e a w i l l g i v e r i s e t o d i s a l l o w e d e x c i t a t i o n s , phosphorescence on a d e l a y e d time s c a l e .  Unimolecular  r e a c t i o n s i n v o l v i n g a breakdown o f t h e o r i g i n a l m o l e c u l e ,  rearrangements  (H atom m i g r a t i o n ) b i m o l e c u l a r i o n - m o l e c u l e r e a c t i o n s , d i s s o c i a t i v e and n o n - d i s s o c i a t i v e charge t r a n s f e r (depending i o n s formed) a r e a l l p o s t u l a t e d t o account  on t h e s t a b i l i t y o f t h e f o r the r a d i o l y t i c products.  The i r r a d i a t i o n o f l i q u i d water a t pH 7 w i t h a beam o f h i g h energy e l e c t r o n s g i v e s r i s e t o t h e f o l l o w i n g p r o d u c t s ; t h e G v a l u e s a r e in brackets. H 0 2  >-H  H O ( 0 . 7 1 ) , H ( 0 . 6 ) , OH (2.2)  (0.42),  2  e a q ( 2 . 3 ) and (H.OH, H 0 * ) ? 2  The f o l l o w i n g i n t r a spur mechanisms i n t h e p r e v i o u s s t a g e lead t o the product formation f" aq + e" aq  observed:  >• H  + 20H~aq  2  H + H  > H  OH + OH  • H 0  H + OH  »• H 0  H + e aq  >H  2  2  2  2  2  + OH"  I f t h e s p e c i e s d i f f u s e from t h e s p u r t h e y can a l s o r e a c t w i t h s o l v e n t or s o l u t e molecules S  18  H,e" aq, OH  + S  • products.  These a r e t h e main r e a c t i o n s i n t h e i r r a d i a t i o n o f l i q u i d water a l t h o u g h some o t h e r s r e l e v a n t t o t h e b e h a v i o u r o f e x c i t e d w a t e r , i t s e l f a c o n t r o v e r s i a l s p e c i e S ( 1 3 , 1 4 ) , have n o t been  The  hydrated e l e c t r o n (i)  I n 1962 t h e p u b l i c a t i o n o f a paper d e s c r i b i n g an e l e c t r o n  pulse-induced broad absorption was  included.  i n t h e v i s i b l e r e g i o n o f t h e spectrum which  p o s i t i v e l y a t t r i b u t e d t o the hydrated e l e c t r o n , climaxed s e v e r a l years  o f experiments and p r e d i c t i o n s i n d i f f e r e n t l a b o r a t o r i e s and opened up an e n t i r e l y new concept o f p r i m a r y r e a c t i o n mechanisms.  (In  the now c l a s s i c paper o f H a r t and Boag (15,16) n o t o n l y t h i s curious  characterised  s p e c i e S b u t a l s o w i t h t h e t e c h n i q u e o f p u l s e r a d i o l y s i s and  s o l u t e scavenging provided of absolute  retrospect  rate  the f i r s t r e a l basis for the determination  constants).  Perhaps t h e s u c c e s s o f t h e f r e e r a d i c a l h y p o t h e s i s o f Weiss (17) i n a c c o u n t i n g f o r t h e i r r a d i a t i o n p r o c e s s e s i n l i q u i d water was i n p a r t the r e a s o n f o r t h e p r o l o n g e d l a c k o f i n t e r e s t i n t h e secondary e l e c t r o n s . However, i n 1953 Platzman (18) i n a t h e o r e t i c a l paper had q u e s t i o n e d t h e reaction e" . + H_0 thermal 2 u  • H + OH"  on t h e grounds t h a t t h e r e was a s i g n i f i c a n t time d e l a y between t h e r e a c t i o n time and t h e t i m e r e q u i r e d t o u t i l i s e t h e h y d r a t i o n which made t h e r e a c t i o n e n e r g e t i c a l l y f e a s i b l e .  energy  He c o n t i n u e d " . . . f o r  t h i s r e a s o n t h e e l e c t r o n becomes h y d r a t e d . . . . I mean (here) t h a t t h e e l e c t r o n p o l a r i s e s t h e d i e l e c t r i c and i s bound i n a s t a b l e quantum s t a t e to i t  there  i s time f o r t h e h y d r a t i o n  t o t a k e p l a c e , which must as  19  Dr. Onsager s a i d be The  a minimum o f the r e l a x a t i o n time 10  i m p l i c a t i o n s o f Platzman's remarks were not  y e a r s a l t h o u g h S t e i n (19)  i n 1952  had  seconds."  f u l l y r e a l i s e d f o r many  suggested t h a t the h y d r a t e d e l e c t r o n  might be p r e s e n t i n an i r r a d i a t e d methylene b l u e system. By the l a t e f i f t i e s s u f f i c i e n t work had been r e p o r t e d the f a c t t h a t t h e r e had i r r a d i a t e d watery:  t o be two  t h e r e was  d i f f e r e n t reducing  no o t h e r way  species  ferrous-cupric  (Hart  (20))  § Weiss (21)) the p e r o x i d e ( B a r r $ A l l e n (22)) cupric sulphate reducing  e t a l (25)  and D a i n t o n £ Watts (26))  systems was  no  The  longer  included  and the methanol-  s p e c i e s i n i r r a d i a t e d n e u t r a l s o l u t i o n s was § Schwarz (24)  These  the c h l o r o a c e t i c a c i d (Hayon"'  systems (Baxendale £ Hughes ( 2 3 ) ) .  n e g a t i v e charge ( C z a p s k i  in  o f i n t e r p r e t i n g the anomalous  k i n e t i c s i n the v a r i e t y o f aqueous systems i n v e s t i g a t e d . the f o r m i c a c i d and  to e s t a b l i s h '  When the  dominant  shown t o have u n i t  l a t e r c o n f i r m e d by  Collinson  the r o l e o f the e l e c t r o n i n t h e s e  speculative.  transient absorption  r e p o r t e d by Keene (27)  in his  pulse  r a d i o l y s i s experiments i n aqueous s o l u t i o n had been t e n t a t i v e l y a t t r i b u t e d to a h y d r a t e d e l e c t r o n (Matheson (28)) i n view o f the f a c t t h a t absorption  d i d not  the  appear i n s o l u t i o n s c o n t a i n i n g e l e c t r o n s c a v e n g i n g  solutes. The  h y d r a t e d e l e c t r o n i d e n t i f i e d by H a r t § Boag i n t h e i r  s e n s i t i v e c o m b i n a t i o n o f f l a s h s p e c t r o s c o p y and p u l s e r a d i o l y s i s i s known t o be a c h e m i c a l e n t i t y i n i t s own action properties.  r i g h t w i t h d i f f u s i o n and  In the s i x y e a r s s i n c e i t s d i s c o v e r y  now  inter-  the r a t e c o n s t a n t s  f o r some s i x hundred o f i t s r e a c t i o n s w i t h i n o r g a n i c , o r g a n i c  and  bio-  c h e m i c a l systems have been c o m p i l e d ( 2 9 ) ; most o f these are d i f f u s i o n  20  c o n t r o l l e d r a t e s and are among the f a s t e s t r e a c t i o n s known.  Parallel  t o the i n v e s t i g a t i o n s o f e l e c t r o n - s o l u t e i n t e r a c t i o n s were experiments d e s i g n e d t o produce the h y d r a t e d e l e c t r o n i n d i f f e r e n t ways f o r i t had been i m m e d i a t e l y r e c o g n i s e d  t h a t many r e a c t i o n s p r e v i o u s l y a t t r i b u t e d  t o the hydrogen atom, at a s u i t a b l e pH, may  have been i n t i t a t e d by  the  h y d r a t e d e l e c t r o n , i t s c o n j u g a t e base. At the p r e s e n t time i t i s g e n e r a l l y a c c e p t e d t h a t e~ aq i s the precursor reducing  t o hydrogen whenever water i s r e d u c e d ; as a most p o w e r f u l s p e c i e , e" aq may  be g e n e r a t e d r a d i o l y t i c a l l y ,  photochemically,  e l e c t r o l y t i c a l l y , by c h e m i c a l r e d u c t i o n , from H atoms, by p h o t o - i n d u c e d e l e c t r o n emission  from m e t a l s and  o t h e r media ( 5 ) .  The  i t s reactions studied (ii) The  deuterated  from s t a b l e s o l v a t e d e l e c t r o n s i n e l e c t r o n has  a l s o been p r e p a r e d  and  (30).  A p h y s i c a l model p r e c i s e sequence o f events t h a t l e a d t o the f o r m a t i o n  of  a s o l v e n t s h e a f about the e l e c t r o n and the n a t u r e o f the s t a b l e quantum s t a t e i n which the e l e c t r o n i s then t r a p p e d , d i s c u s s i o n as the e x p e r i m e n t a l Two  e v i d e n c e i s not  t o i n v o k e much  unambiguous.  d i f f e r e n t t h e o r i e s on the immediate f a t e o f the  e l e c t r o n s w i l l be b r i e f l y d i s c u s s e d . energies  continues  thermalised  As an e l e c t r o n approaches t h e r m a l  i t i s s t i l l moving t h r o u g h the water r e l a t i v e l y q u i c k l y  a l t h o u g h i t a f f e c t s p o l a r i s a t i o n o f the water m o l e c u l e s near the  and track  i t i s not l o n g enough i n the r e g i o n t o be t r a p p e d by t h i s p o l a r i s a t i o n ; i f t h e r e a l r e a d y e x i s t s a r e g i o n o f a c c i d e n t a l p o l a r i s a t i o n due  to  the  random t h e r m a l motions o f the m o l e c u l e s themselves then the water d i p o l e s w i l l be p o l a r i s e d t o a more s i g n i f i c a n t degree i n the f i e l d o f the excess  21  electron.  In l i q u i d water b o t h e l e c t r o n i c and o r i e n t a t i o n a l p o l a r i s a t i o n  are i m p o r t a n t , the f i n i t e time a s s o c i a t e d w i t h the l a t t e r due t o m o l e c u l a r rotation. Schiller  The d i e l e c t r i c r e l a x a t i o n time o f water i s 10  seconds.  (31) r e g a r d s t h e d i e l e c t r i c r e l a x a t i o n time i n t h e t r a p p i n g  p r o c e d u r e as the d e c i s i v e paramter.  He assumes t h e t r a c k and spur model  and c o n s i d e r s the time-dependent d i e l e c t r i c p r o p e r t i e s o f the medium w i t h a n o n - c o n s e r v a t i v e e l e c t r i c f i e l d ; the p r o b a b i l i t y o f t r a p p i n g the e l e c t r o n i n c r e a s e s as the r e l a x a t i o n time (T ) i n c r e a s e s .  Investigations  on e l e c t r o n c a p t u r e i n media o f d i f f e r e n t x but s i m i l a r s t a t i c and  optical  d i e l e c t r i c c o n s t a n t s ( l i q u i d w a t e r , s u p e r c o o l e d water and i c e ; v a r i a t i o n o f 10^ i n T ) have s u p p o r t e d h i s i d e a s ( 3 2 ) . The o t h e r model d f e l e c t r o n c a p t u r e i s based on a d i e l e c t r i c c o n s t a n t and a s t a t i c e l e c t r i c f i e l d .  time-independent  Here freeman and Fayadh  (33) use t h e c a v i t i e s p r e s e n t i n the l i q u i d s t r u c t u r e as the  initial  t r a p p i n g c e n t r e s , and suggest t h a t the l i m i t i n g f a c t o r i n the m o b i l i t y o f the e l e c t r o n i s the p h y s i c a l m i g r a t i o n o f t h e s e c a v i t i e s .  Experimental  evidence i s given. At t h e c o n c l u s i o n , on e i t h e r model, the e l e c t r o n i s t r a p p e d i n a p o t e n t i a l w e l l w i t h p a r t i a l o r i e n t a t i o n o f a second and t h i r d  layer  o f water m o l e c u l e s i n the o u t e r s o l v a t i o n s h e a f , a l t h o u g h t h e r m a l a g i t a t i o n w i l l cause t h e s e arrangements t o be time-dependent.  The number o f water  m o l e c u l e s i n the i n n e r s o l v e n t s h e a f c o u l d be from f o u r t o s i x , i t i s not known, and t h e r e may  be a range o f b i n d i n g e n e r g i e s f o r the e l e c t r o n  i n i t s t r a p , o r t r a p s o f d i f f e r e n t depth.  To what e x t e n t t h i s g e n e r a l  p i c t u r e can be r e s p o n s i b l e f o r the broad a b s o r p t i o n spectrum o f the h y d r a t e d e l e c t r o n , and e l e c t r o n s s o l v a t e d i n o t h e r media, i s open t o s p e c u l a t i o n .  22  The a b s o r p t i o n spectrum o f t h e h y d r a t e d  e l e c t r o n (see diagram  °  ;o  4) i s e v i d e n t a t 5000A w i t h a maximum a t 7200A and shows no convergence o  l i m i t a t 5400A.  Trapped e l e c t r o n s i n i c e have t h e same ; \ x '  a n  ma  f e a t u r e l e s s a b s o r p t i o n spectrum although  they a r e i n an o r d e r e d  d matrix  but i t i s i n t e r e s t i n g t o n o t e an i n c r e a s e i n i n t e n s i t y i n t h e n e a r u l t r a v i o l e t r e g i o n has been r e p o r t e d ( 3 4 ) . Y e t i n a n o t h e r study (35) the appearance o f a d i s t i n c t s h o u l d e r i n t h e lower wavelength r e g i o n O  o  ^5500A t o 6000A was observed f o r t h e a b s o r p t i o n s p e c t r a o f some f r o z e n aqueous s o l u t i o n s and c r y s t a l l i n e i c e .  A narrowing  o f the absorption  band was a l s o seen down t o 200°K by these w o r k e r s ; such b e h a v i o u r has not been observed by o t h e r s .  I t may w e l l be t h a t t h e mode o f i n t r o d u c t i o n  o f t h e e l e c t r o n i n t o a s o l u t i o n o r a f r o z e n m a t r i x may account i n p a r t f o r these d i s c r e p a n c i e s .  The assymmetry o f t h e spectrum o f t h e h i g h  energy s i d e o f t h e spectrum o f e" aq has a l s o been observed f o r ammonlated e l e c t r o n s , b u t n e i t h e r s p e c t r a shows, any d i s c e r n i b l e f i n e s t r u c t u r e . Dorfman has found a c o r r e l a t i o n between t h e s t a t i c constants  dielectric  and t h e v a l u e s o f A.max f o r e l e c t r o n s s o l v a t e d i n a s e r i e s o f  alcohols (36).  =10  4  The e x t i n c t i o n c o e f f i c i e n t f o r e~ aq i n t h e r e g i o n o f A max i s - 1 - 1 l i t r e mole cm and thus t h e t r a n s i t i o n c o r r e s p o n d i n g t o t h e  energy of'Xmax must be a l l o w e d .  J o r t n e r (37) p u b l i s h e d v a r i a t i o n a l c a l c u -  l a t i o n s based on a quantum m e c h a n i c a l model which a t t r i b u t e s t h e Xmax t o a (2p -*r I s ) t r a n s i t i o n i n t h e p o t e n t i a l w e l l . as a continuous  He t r e a t e d t h e s o l v e n t  d i e l e c t r i c medium i n which an excess e l e c t r o n has been  caught i n a s e l f - i n d u c e d p o l a r i s a t i o n t r a p . confined to a c a v i t y o f uniform  The e l e c t r o n i s i n c o m p l e t e l y  f i e l d and e x p e r i e n c e s  a decrease i n the  EXTINCTION ( X 10~  4  V  COEFFICIENT  24  coulombic f i e l d as i t wanders o u t s i d e the i n n e r s o l v e n t s h e a f i n t o the b u l k medium.  The parameters  t h a t d e f i n e the t r a p are the s t a t i c  and  o p t i c a l d i e l e c t r i c constants. With h y d r o g e n i c wave f u n c t i o n s he a n t i c i p a t e s the f i r s t  excited  s t a t e t o be a 2p s t a t e , and the (2p <- I s ) t r a n s i t i o n t o o c c u r a t an energy E(hv) = 1.35  eV, the o s c i l l a t o r s t r e n g t h b e i n g f i = 1.1.  The  c a v i t y r a d i u s under these c o n d i t i o n s ' e q u a t e s t o z e r o . E x p e r i m e n t a l l y Amax equates t o 1.75  eV, f i = 0.8  and the r a d i u s o f i n f l u e n c e o f the o  h y d r a t e d e l e c t r o n to be 2.5  t o 3.OA.  The most r e c e n t d e t e r m i n a t i o n i s  o  2.9A may  (38).  On t h i s hydrogen=type model t r e a t m e n t the f i r s t  c o r r e s p o n d to t h r e e q u a r t e r s o f the w e l l depth (diagram 5 ( a ) ) .  Higher t r a n s i t i o n s for  transition  (3p -«- I s ) w i l l be v e r y weak i f they o c c u r at a l l as  a one e l e c t r o n system the sum o f the o s c i l l a t o r s t r e n g t h s has t o be  u n i t y ; the b r o a d a b s o r p t i o n band c e r t a i n l y shows no f i n e s t r u c t u r e but i t may  be masking  a v e r y weak band.  J o r t n e r ' s model encounters more c o m p l i c a t i o n s i n e x p l a i n i n g the n a t u r e o f the h y d r a t e d e l e c t r o n than the ammoniated e l e c t r o n ( 3 7 ) . The temperature dependence o f max f o r e~ aq was p r e d i c t e d t o be -3.3 -3 ' -3 x 10 - dE_ max - -2.2 x 10 (eV p e r degree) and r e c e n t r e s u l t s (38) dT _ 3  r e p o r t a s h i f t o f -2.9-10 systems.  6 Q  eV p e r degree i n Co  y r a d i o l y s e d aqueous  The i n i t i a l o p t i c a l d e n s i t y o f Xmax at 10°C  70% a t 96°C. increasing.  had decreased by  I t appeared t h a t the r a d i u s o f the h y d r a t e d e l e c t r o n  was  However p u l s e r a d i o l y s e d systems i n d i c a t e d no such b e h a v i o u r  i n Xmax o v e r the same temperature range.  Chemical e v i d e n c e o f photo  b l e a c h i n g and p h o t o c o n d u c t i v i t y (39) ambiguously r e l a t e t o the proposed  25  2p upper bound s t a t e o f s o l v a t e d e l e c t r o n s and t h e appearance o f temperature dependent X max f o r o t h e r s o l v a t e d e l e c t r o n s o u t s i d e t h e framework modified  s e t by e" aq and e" :ammn. s t r o n g l y s u g g e s t s t h e need f o r a :  o r a l t e r n a t i v e model.  I t i s not intended to discuss  t h e s e models  i n d e t a i l b u t t o emphasise t h e i m p o r t a n c e o f t h e e x c i t e d s t a t e i n t h e a l t e r n a t i v e theories i n accounting f o r the t r a n s i t i o n s associated the observed A.max.  with  The b r o a d n e s s , asymmetry and l a c k o f f i n e s t r u c t u r e  must a l s o be accounted f o r . The energy o f A max has been "likened? ' t o an i o n i s a t i o n p o t e n t i a l (I ) , t o r e l e a s e o f t h e e l e c t r o n from i t s t r a p . hydration  energy o f t h e e" aq was c a l c u l a t e d t o be 1.72 eV ( 4 0 ) . The  r e l a t i o n s h i p between any p o l a r i s a t i o n energy o f t h e s o l v e n t , I hydration  The  energy AH i s shown i n diagram (5b).  and t h e  V a r i a t i o n i n the s i z e  o f t r a p s , and t h e r m a l motions o f t h e s o l v e n t s h e a f i t s e l f may g i v e r i s e t o t h e broadness o f t h e spectrum (5d).  Even i f t h e t r a p s had  e s s e n t i a l l y t h e same immediate environment i n t h e ground s t a t e  there  c o u l d s t i l l be a wide v a r i a t i o n i n t h i s environment a t e x c i t e d l e v e l s . The l a t t e r may i n f a c t be a continuum analogous t o t h e c o n d u c t i o n band o f a semi c o n d u c t o r i n t o which t h e e l e c t r o n s a r e e x c i t e d by wavelengths l e s s t h a n X.max ( 5 c ) . The p o s s i b i l i t y o f a s y m m e t r i c a l charge t r a n s f e r absorption  i n c o m b i n a t i o n w i t h an i o n i s a t i o n continuuii has a l s o been  proposed i n a d i s c u s s i o n o f t h e s e models i n ( 4 1 ) . I f t h e h y d r a t e d e l e c t r o n were e x c i t e d i . e . p h o t o l y s e d a t s u i t a b l e energies,  i n t o t h i s u n s p e c i f i e d upper s t a t e and i t s b e h a v i o u r  f o l l o w e d t h r o u g h s u i t a b l e means one c o u l d d i s t i n g u i s h between t h e models through t h e changing c o - o r d i n a t e s  o f Amax; an attempt t o e x c i t e t h e  e l e c t r o n from t h i s s t a t e i n t o an even h i g h e r one would a l s o g i v e  valuable  26  Diagram 5.  Model r e l a t i n g E ^ ^ ^  t o the  S t r u c t u r e o'f the H y d r a t e d E l e c t r o n  27  information. The work d e s c r i b e d i n t h i s t h e s i s proved t o be a n e c e s s a r y p r e l i m i n a r y b a s i s f o r experiments  designed to study the nature o f the  e x c i t e d s t a t e o f t h e h y d r a t e d e l e c t r o n , t h e f o r m a t i o n and decay o f which may be f o l l o w e d s p e c t r o s c o p i c a l l y a t wavelengths (iii)  approaching Xmax.  Fate o f t h e h y d r a t e d e l e c t r o n  The p r o c e s s e s t h a t c o n t r i b u t e t o t h e l o s s o f t h e h y d r a t e d e l e c t r o n v a r y i n e f f i c i e n c y w i t h t h e dose r a t e o f t h e i o n i s i n g  radiation,  the pH o f t h e medium, o t h e r s p e c i e s t h a t may be p r e s e n t as i m p u r i t i e s o r scavengers  and t h e temperature o f t h e s o l u t i o n s .  H0  •  2  e" aq, H, OH, H , H ^ ,  H 0 , 0H~ +  3  The f o l l o w i n g r e a c t i o n s may o c c u r d u r i n g and a f t e r t h e i r r a d i a t i o n ; o n l y OH  and H  do n o t r e a c t w i t h e aq.  2  pH (1) e aq + e aq  >- H  (2) e aq + OH (3) e aq + H (4) e aq + H 0 3  +  10.5  4.5-10  • OH" aq  10.5  3.0-10  10  > H~ aq  10.5  2.5-10  10  •  H  + 20H"aq  M-1 Rate (29)M  2  +  H 2  °  2.32'10  4 - 5  9  10  (5) e aq + H 0  > H + OH" aq  8.4  1.6-10  (6) e aq + R 0  ^OH  7  1.23'10  2  + OH" aq  sec -1  1  10  With nanomolar c o n c e n t r a t i o n s o f e aq from low dose r a t e s (50 rads p e r p u l s e ) t h e decay i s a f i r s t o r d e r p r o c e s s ; as t h e dose i n c r e a s e s t h e [e aq] moves t o t h e m i c r o m o l a r r e g i o n and t h e decay now resembles  second o r d e r k i n e t i c s .  At h i g h e r doses t h e h y d r o x y l r a d i c a l s ,  hydroxonium i o n s and h y d r a t e d e l e c t r o n s a r e a l l 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 g i v e c l a s s i c a l second o r d e r k i n e t i c s and t h e r a t e s a r e d i f f u s i o n controlled.  When [e aq] i s i n m i l l i m o l a r c o n c e n t r a t i o n s t h e t e c h n i c a l  28  s i t u a t i o n becomes more complex as t h e h a l f l i f e i s c o n s i d e r a b l y and  reduced  t h e i n t e r p r e t a t i o n o f t h e k i n e t i c s i n a c l a s s i c a l sense appears  ambiguous.  The dominant decay mechanism i s  (1) e aq + e" aq  •H  + 20H aq -  2  but r a d i c a l - r a d i c a l and r a d i c a l - i o n r e a c t i o n s account f o r n e a r l y h a l f o f t h e t o t a l number o f p r o c e s s e s t h a t must be c o n s i d e r e d constant  f o r t h e p r i m a r y decay can be e v a l u a t e d .  before  a rate  Fortunately not a l l  o f t h e s e r e a c t i o n s a r e k i n e t i c a l l y s i g n i f i c a n t under any g i v e n s e t o f experimental  c o n d i t i o n s and w i t h t h e j u d i c i o u s use o f pH and s c a v e n g i n g  s o l u t e s many o f t h e r e a c t i o n s can be a t l e a s t c o n t r o l l e d i f n o t e l i m i n a t e d . The  r a d i c a l r e a c t i o n s a r e l i s t e d i n T a b l e I below  Table I Reaction  pH  H + H  •H  H + OH  •H 0  OH + OH  yH 0  2  H + H 0 2  H 0  +  3  1  2.0  1.0-10  10  3.0  1.2-10  10  2  7.0  4-10  • H 0 + OH  2.1  9-10  7.0  1.43-10  2  2  k" M s e c  2  + OH" —^2H 0 2  Under h i g h dose r a t e c o n d i t i o n s t h e s p e c i e s that could appreciably are t h e h y d r o x y l  9  7  11  i n n e u t r a l water  i n t e r f e r e w i t h t h e "pure" b i o m o l e c u l a r  r a d i c a l and t h e hydrogen atom.  1  decay  I n t h e presence o f a  c a l c u l a t e d amount o f methanol ( o r h i g h e r a l c o h o l ) t h e hydroxy r a d i c a l s w i l l be removed w i t h t h e hydrogen atoms; i n b o t h i n s t a n c e s t h e r e l a t i v e l y i n e r t CH 0H r a d i c a l i s produced. 2  (7)  H + CH 0H  • CH 0H + H  (8)  OH + CH OH  • CH 0H + H 0  3  o  2  2  o  o  Z  I  K  = 1.7-10  6  M sec'  1  y  K  = 5.1-10  8  M" sec"  1  D  _ 1  1  o  In a d d i t i o n common i m p u r i t i e s such as d i s s o l v e d 0  2  or C0 i n 2  29  the l i q u i d w i l l  have a p p r e c i a b l e e f f e c t s on t h e i n i t i a l r a t e o f decay  o f t h e h y d r a t e d e l e c t r o n and must be removed as much as p o s s i b l e . (9)  OH + C 0  3  (10) e aq + 0 The ion  absorbs  2  • C0~  pH 11, K  t 0~  pH 7, K  reaction with C0 o  at 6000A.  3  = 3.0"-10 M 8  g  = 2.10  1 0  M  _ 1  _ 1  sec  sec  _ 1  _ 1  i s p a r t i c u l a r l y u n d e s i r a b l e as t h e CO^  P e r o x i d e s and o t h e r r a d i o l y t i c p r o d u c t s a r e  dealt w i t h according t o the p r e v a i l i n g experimental c o n d i t i o n s . In many i n s t a n c e s however t h e e f f e c t i v e c o n c e n t r a t i o n o f competing s p e c i e s does n o t decrease The  as r a p i d l y as t h e c o n c e n t r a t i o n o f e aq ( 3 0 ) .  Problem The predominant decay mechanism o f t h e h y d r a t e d e l e c t r o n i n  n e u t r a l w a t e r under h i g h dose r a t e c o n d i t i o n s i s (1)  e aq + e aq  ,•  > (e)  2  aq  >- H^ + 20H  aq  and would be d e s c r i b e d i n a c l a s s i c a l sense as a b i m o l e c u l a r decay. 2(The t r a n s i e n t (e)„ 2  , specieShas been i d e n t i f i e d i n s e v e r a l o t h e r solv  systems, l i q u i d , g l a s s y and s o l i d m a t r i c e s (34,42,43,44) g e n e r a l l y through t h e a b s o r p t i o n o r e . s . r . s p e c t r a . )  The r a t e c o n s t a n t f o r t h i s  r e a c t i o n i s d i f f u s i o n c o n t r o l l e d b u t t h e l i t e r a t u r e c o n t a i n s an abundance of data that f i t s n e i t h e r a f i r s t treatment. to  o r d e r n o r a second o r d e r k i n e t i c  There would appear t o be a t r a n s i t i o n from one mode o f decay  another w i t h i n a v e r y s h o r t time and thus t h e d a t a n o t u n e x p e c t e d l y  shows v a r i a t i o n s a c c o r d i n g t o the r a d i a t i o n s o u r c e , t h e d u r a t i o n and i n t e n s i t y o f p u l s e d r a d i a t i o n and t h e speed w i t h which t h e decay can then be f o l l o w e d .  The d i f f e r e n t v a l u e s f o r k'^ were a l l w i t h i n an o r d e r  o f magnitude (see T a b l e I I : b e l o w ) u n t i l t h e s t a r t l i n g l y h i g h v a l u e o f  30  3.2  10  1 1  M  1  sec  1  was  r e p o r t e d by K l e i n and Warner (45) i n 1966;  this  i s the f a s t e s t r e a c t i o n i n the l i q u i d phase e v e r p u b l i s h e d , and they suggest t h a t the model  used t o e x p l a i n the r a d i a t i o n c h e m i s t r y o f  w a t e r at low dose r a t e s i s inadequate at v e r y h i g h dose r a t e s such as they had employed.  Other v a l u e s f o r  and r e c e n t l y c a l c u l a t e d are  g i v e n below. Table I I Reference  Author  KM  Technique  Dose Rate  45  Klein.Warner  3.2.10  p.r.  ,9 5.5 ± 0.75.10 p . r .  ^10 eVA >8 sec~l 19 -1 ^10 eV/l >8 -1 sec high 7-14  38  Gottschall.Hart  6.3 ± 1.10  46  Matheson.Rabani  47  Dorfman.Taub  <7.0.10 9 6.0.10  'high 12 24 -1 ^10 eV/£_J pD13.4  30  Hart.Fielden_  1  sec  1  11  9  9  Co  60  p.r. p.r.  2 5  y  for(ed+ed) In o r d e r t o examine the' e aq  pH _ 1  sec e aq mechanism i t i s n e c e s s a r y  t o have an a c c u r a t e p r o f i l e o f the ground s t a t e b e h a v i o u r o f the h y d r a t e d e l e c t r o n , t h a t i s the decay o f t h i s s p e c i e s at h i g h c o n c e n t r a t i o n s . Experiments were t h e r e f o r e d e s i g n e d t o f o l l o w t h e f o r m a t i o n and o f the h y d r a t e d e l e c t r o n at r a d i a t i o n i n t e n s i t i e s a f a c t o r o f 10 those employed by K l e i n and Warner.  decay 2 above  31  accelerator c o n t r o l console • plexicell laser lead screens f l o w system l e a d box photomultiplier lens, f i l t e r , i r i s j u n c t i o n box power s u p p l y t o H oscilloscope copper buss b a r , ground s t a k e  Diagram 6.  The a c c e l e r a t o r L a b o r a t o r y  32  The Technique o f P u l s e R a d i o l y s i s , arid K i n e t i c The  f o r m a t i o n and decay o f the h y d r a t e d e l e c t r o n produced  d u r i n g the p u l s e r a d i o l y s i s o f aqueous media was spectrophotometric technique. ing  Spectroscopy  f o l l o w e d by a k i n e t i c  A Helium-Neon l a s e r was  used as a m o n i t o r -  l i g h t source and t r a n s i e n t o p t i c a l a b s o r p t i o n s produced d u r i n g  a f t e r the p u l s e were d e t e c t e d by a p h o t o m u l t i p l i e r , d i s p l a y e d on  and  an  o s c i l l o s c o p e and photographed. A s c h e m a t i c diagram o f the a c c e l e r a t o r l a b o r a t o r y i n which these experiments  were c a r r i e d out i s shown i n diagram 6; d e t a i l s o f the  equipment are l i s t e d i n the f o l l o w i n g s e c t i o n s and i n diagrams 7, 8 and  The  Irradiation  Cell  In d e s i g n i n g the i r r a d i a t i o n c e l l s e v e r a l i m p o r t a n t f a c t o r s to be  9.  had  considered: (i)  the window o f the c e l l s h o u l d be o f s u i t a b l e m a t e r i a l and  t h i c k n e s s t o a l l o w the maximum number o f e l e c t r o n s i n the p u l s e d beam to p e n e t r a t e the s o l u t i o n . (ii)  I f too deep a volume o f s o l u t i o n i s i r r a d i a t e d the e l e c t r o n s  w i l l p e n e t r a t e t o o n l y a c e r t a i n depth and thus space charges may (iii)  I t was  appear.  n e c e s s a r y t o remove the i r r a d i a t e d s o l u t i o n  and  r e p l e n i s h the c e l l q u i c k l y between the p u l s e s . (iv)  A v a r i a b l e p a t h l e n g t h was  o f the assumptions i n r e l a t i n g charges  d e s i r a b l e i n view o f the n a t u r e  i n o p t i c a l d e n s i t y t o charges i n  the c o n c e n t r a t i o n o f the a b s o r b i n g s p e c i e . (v)  The  w i d t h o f the i r r a d i a t i o n c e l l s h o u l d be comparable t o  33  the w i d t h o f t h e l a s e r beam i n o r d e r t o d e t e c t s i m u l t a n e o u s l y events i n c a l l areas o f t h e c e l l . The f i n a l d e s i g n o f t h e i r r a d i a t i o n c e l l i n i t s s u p p o r t i n g framework i s shown i n diagram 7.  The s u p p o r t was c o n s t r u c t e d from 25 mm  t h i c k p o l i s h e d p l e x i g l a s s and thus t h e i r r a d i a t i o n c e l l has been c a l l e d the p l e x i c e l l . high.  The p l e x i c e l l i s 11.2 mm i n l e n g t h and stands 10.4 mm  An upper s e c t i o n o f t h e p l e x i g l a s s b l o c k was removed, the a r e a o f  the r e s u l t i n g space b e i n g a p p r o x i m a t e l y t h a t o f t h e e l e c t r o n tube window i n the a c c e l e r a t o r .  A h o l e was d r i l l e d through t h e r e m a i n i n g p l e x i g l a s s  e i t h e r s i d e o f t h e space, i n t o which a l o n g t h i n g l a s s tube c o u l d be inserted.  A r e c e s s f o r an end window and a p o r t were a l s o machined a t  each end o f t h e b l o c k as can be seen i n t h e diagram. The t h i n t r a n s p a r e n t 100 mm l o n g g l a s s tube o f 1.5 mm  internal  d i a m e t e r and 0.2 mm t h i c k w a l l s a c t e d as t h e i r r a d i a t i o n c e l l p r o p e r a l o n g which t h e l a s e r beam was d i r e c t e d .  ( A f t e r s e v e r a l pulses the glass  d i s c o l o u r e d due t o t h e p r e s e n c e o f t r a p p e d e l e c t r o n s , and so t h e tubes were r e g u l a r l y r e p l a c e d .  To ensure u n i f o r m i t y i n t h i c k n e s s and t r a n s -  parency i n a l l t h e experiments a s u p p l y o f KIMAX (U.S.A.) c a p i l l a r y tubes was p u r c h a s e d and t h e f i n a l s p e c i f i c a t i o n s o f t h e p l e x i c e l l m o d i f i e d t o f i t these tubes.) set  The g l a s s tube was r e t a i n e d f i r m l y i n p o s i t i o n by a  o f aluminium p l a t e s a t b o t h ends o f t h e p l e x i c e l l ; t h e s e a l s o s u p p o r t e d  the windows and t h e i r r u b b e r b a c k i n g d i s c s ensured t h e c e l l t o be water t i g h t when a g e n t l e p r e s s u r e was a p p l i e d t o t h e p l a t e screws. The windows o f t h e c e l l were p o l i s h e d 15 mm d i a m e t e r pyrex d i s c s . The g l a s s tube reached t o w i t h i n 5 mm o f e i t h e r window; a s m a l l e r d i s t a n c e between them l e d t o t h e t r a p p i n g o f gas bubbles o f t e n i n t r o d u c e d i n t h e  34  Diagram 7.  The P l e x i c e l l and Components  35  f l o w o f t h e sample s o l u t i o n s and o t h e r w i s e e a s i l y removed.  Fitted plexi-  g l a s s p o r t s o f 6.7 mm d i a m e t e r t a p e r i n g t o 2.5 mm d i a m e t e r j u s t above t h e ends o f t h e g l a s s tube c o n t r o l l e d t h e f l o w o f s o l u t i o n through t h e p l e x i c e l l . A b l o c k o f aluminium 9.6 mm t h i c k and 4.5 mm by 4.1 mm h i g h was machined t o f i t i n t o t h e empty s e c t i o n i n t h e p l e x i c e l l . 1 mm wide s l i t  The 20 mm by  c u t along t h e c e n t r e o f t h e b l o c k was a l i g n e d w i t h t h e  g l a s s t u b e , and two s m a l l doors o f 0.8 mm aluminium s l i d along a r e c e s s (in the block) The  f a c i n g t h e tube. aluminium b l o c k had a t w o - f o l d purpose.  Firstly i t  represented  the v a r i a b l e p a t h l e n g t h f o r t h e irradiations as i t was i n s e r t e d on t h e f r o n t side o f the p l e x i c e l l f a c i n g the a c c e l e r a t o r .  When t h e aluminium doors  were f u l l y c l o s e d t h e t o t a l e l e c t r o n beam was absorbed by them. the s l i t i n t h e b l o c k s e r v e d t o c o l l i m a t e t h e e l e c t r o n beam.  Secondly  Previous  s t u d i e s u s i n g the whole o f t h e e l e c t r o n beam t o i r r a d i a t e t h e s o l u t i o n s i n t h e p l e x i c e l l had shown some v e r y c u r i o u s v a r i a t i o n s i n t h e l i g h t intensity The  that completely  masked the a b s o r p t i o n  s i g n a l under i n v e s t i g a t i o n .  p r o b a b l e o r i g i n o f t h e s e w i l l be d i s c u s s e d b r i e f l y e l s e w h e r e , b u t  t h e i r e l i m i n a t i o n was o n l y p o s s i b l e by r e s t r i c t i n g t h e e l e c t r o n beam t o an a r e a comparable t o t h e i r r a d i a t i o n tube.  For a while the r e s t r i c t o r  was a c i r c u l a r p l a t e o f 0.8 mm t h i c k aluminium c o n t a i n i n g a s l i t o f the r e q u i r e d p a t h l e n g t h t h a t c o u l d be a t t a c h e d accelerator.  t o the f r o n t f a c e o f t h e  However t h i s s l i t d i d n o t d e f i n e t h e p a t h l e n g t h  with  s u f f i c i e n t a c c u r a c y due t o t h e undetermined d i v e r g e n c e o f t h e e l e c t r o n s between t h e s l i t and t h e g l a s s t u b e ; t h e c o n c e n t r a t i o n s observed r a t e constants  o f e aq and  c a l c u l a t e d from these e a r l y s t u d i e s were t o g r e a t l y  i n f l u e n c e the design o f l a t e r experiments.  36  Three h o l e s d r i l l e d i n t o t h e lower p a r t o f t h e p l e x i c e l l p e r m i t t e d screw attachment o f t h e c e l l d i r e c t l y t o t h e f r o n t o f the a c c e l e r a t o r , o r t o a b r a s s support  ( o r t h e o p t i c a l bench) on which t h e  p l e x i c e l l c o u l d be r o t a t e d i n any d i r e c t i o n .  I n a l l t h e experiments  d i s c u s s e d h e r e the p l e x i c e l l was a t t a c h e d t o t h e Febetron reduced t h e h i g h n o i s e l e v e l on t h e s i g n a l s r e c o r d e d ;  as t h i s markedly  the d i r e c t grounding  o f t h e c e l l when i n c o n t a c t w i t h the machine and t h e reduced a i r space between t h e window o f t h e e l e c t r o n tube and the i r r a d i a t i o n c e l l b o t h c o n t r i b u t e d t o t h e improved s i g n a l t o n o i s e  probably  ratio.  The E l e c t r o n A c c e l e r a t o r In these experiments t h e source o f h i g h energy i o n i s i n g r a d i a t i o n was a p u l s e d e l e c t r o n a c c e l e r a t o r manufactured by F i e l d Corporation  (Oregon).  The Febetron  which o p e r a t e s  Emission  on t h e f i e l d  emission  p r i n c i p l e produced s i n g l e i n t e n s e p u l s e s o f nanosecond d u r a t i o n o f 0.5 MeV electrons. In t h e i n i t i a l stages o f t h i s work t h e Febetron model 701-2660 p u l s e r and 5235 e l e c t r o n tube were employed t o produce a 50 nanosecond p u l s e o f 0.52 MeV e l e c t r o n s .  The peak beam c u r r e n t observed at t h e  window o f t h i s e l e c t r o n tube was 1000 amperes.  The energy o f t h e e l e c t r o n s  i n the p u l s e can be v a r i e d a c c o r d i n g t o t h e D.C. c h a r g i n g v o l t a g e ; t h e charging  c i r c u i t o f the Febetron  principle.  operates  on t h e Marx Surge C i r c u i t  The impedance o f t h e e l e c t r o n tube was f a i r l y  the beam c u r r e n t v a r i e d w i t h t h e c h a r g i n g v o l t a g e .  constant  thus  The beam c u r r e n t and  p u l s e shape ( w i t h a h a l f w i d t h o f ^20 nanoseconds) were r e p r o d u c i b l e to w i t h i n a few p e r c e n t  (a = ±3%) under i d e n t i c a l c h a r g i n g c o n d i t i o n s .  37  At peak performance the Febetron  gave a 50 nanosecond p u l s e o f 0.52  MeV  19 e l e c t r o n s amounting t o a t o t a l d e p o s i t i o n o f energy o f 'vlO  eV.  The m a j o r i t y o f experiments were c a r r i e d out w i t h the 3 nanosecond p u l s e which was  o b t a i n e d by a t t a c h i n g the model 2770 p u l s e  shortner  t o the Model 2660 p u l s e r case and r e p l a c i n g the e l e c t r o n tube w i t h a s m a l l e r model, no 5510. p u l s e o f 0.5  At peak performance t h i s assembly gave a 3 nanosecond  e l e c t r o n s and a beam c u r r e n t o f ^1000 amperes. The 19 t o t a l energy d e p o s i t e d was ^10 eV but the dose r a t e was i n c r e a s e d t o 26 1 1 26 1 5 x 10 eV gram sec compared t o 10 eV gram * sec w i t h the long  pulse.  The  MeV  r e p r o d u c i b i l i t y was  w i t h i n 5%, but w i t h b o t h e l e c t r o n tubes  the g r e a t e r the number o f p u l s e s i n excess o f the mean " l i f e t i m e " o f the t u b e , the worse the r e p r o d u c i b i l i t y o f t h e i r quoted c h a r a c t e r i s t i c s . T y p i c a l p u l s e shapes are shown i n f i g u r e 10a. The Cup  e l e c t r o n beam c u r r e n t was measured w i t h an a p e r t u r e d  shown i n f i g u r e 10b.  The  Faraday  cup c o n t a i n s a T § M Research Products  Model  GR-1-05 c u r r e n t v i e w i n g r e s i s t o r (C.V.R.) w i t h an impedance o f 0.0507 ohms. The  v o l t a g e p u l s e across the C.V.R. was  f e d i n t o the v e r t i c a l d i s p l a y o f  a T e k t r o n i x model 454 o s c i l l o s c o p e v i a doubly s h i e l d e d RG58 c a b l e coupled t o a 50 ohm  coaxial  t e r m i n a t o r at the i n p u t t o the o s c i l l o s c o p e .  S o l u t i o n s and Flow Techniques Laboratory  d i s t i l l e d w a t e r was  r e d i s t i l l e d from a c i d i f i e d p o t a s s i u m  permangate s o l u t i o n and kept i n a r e s e v o i r f l a s k under an atmosphere o f Helium gas.  ( b u i l t i n t o the f l o w system)  S o l u t i o n s prepared  containing other  s o l u t e s were always made up from t h i s s u p p l y o f doubly d i s t i l l e d water.  38  50 n s e c s / d i v ( h o r i z ) t y p i c a l pulsed wave forms o b s e r v e d from the e l e c t r o n i r r a d i a t i o n tube.  J  U  Diagram 10.  10 n s e c s / d i v  ' I V - I M  (a)  (b)  (horiz)  39  The 0.26 M i s o p r o p y l a l c o h o l s o l u t i o n was always  freshly  p r e p a r e d from a n a l a r B.D.H. reagent w i t h o u t f u r t h e r p u r i f i c a t i o n .  A  s u p p l y o f 0.0025 M and 0.1 M H^SO^ s o l u t i o n s were a l s o made up from a n a l a r B.D.H. a c i d , and a f t e r d e g a s s i n g k e p t under an i n e r t atmosphere o f h e l i u m i n a second r e s e v o i r f l a s k i n t h e f l o w system. These r e s e v o i r s were 2 l i t r e t h r e e necked f l a s k s w i t h i n d i v i d u a l f l o w l i n e s t o t h e i r r a d i a t i o n c e l l t h a t c o u l d be s e a l e d when not i n use.  Each f l a s k was connected t o a common threeway s t o p c o c k which  c o n t r o l l e d t h e f l o w o f h e l i u m gas through t h e f l a s k s ; an arrangement o f t r a p s and secondary s t o p c o c k s made improbable any a c c i d e n t a l f i l l i n g o f the  gas l i n e w i t h s o l u t i o n .  Helium gas was b u b b l e d v i g o r o u s l y t h r o u g h  the  s o l u t i o n s v i a a f r i t t e r e d g l a s s o v a l t o ensure e f f i c i e n t d e g a s s i n g ;  the  oxygen c o n t e n t o f t h e s o l u t i o n s was ^1 ppm, i . e . t h a t c o n t a i n e d i n  the  gas i t s e l f .  The d e g a s s i n g took p l a c e f o r s e v e r a l hours when t h e  r e s e v o i r s were c o m p l e t e l y r e f i l l e d w i t h f r e s h s o l u t i o n s , o t h e r w i s e f o r 30 minutes b e f o r e t h e experiments began.  (When i s o p r o p y l a l c o h o l  solutions  were used d e g a s s i n g was g e n t l e and o n l y f o r 10 minute p e r i o d s due t o t h e h i g h vapour p r e s s u r e o f t h e a l c o h o l . ) to the  D u r i n g t h i s time t h e h e l i u m escaped  t h e atmosphere t h r o u g h an open s t o p c o c k i n t h e f l a s k ; t h e s o l u t i o n i n p l e x i c e l l was r e p l a c e d w i t h a f r e s h volume o f l i q u i d by c l o s i n g  this  s t o p c o c k , b r i e f l y p r e s s u r i s i n g t h e f l a s k and then opening t h e a p p r o p r i a t e flow l i n e t o the c e l l .  The s u r p l u s l i q u i d was c o l l e c t e d through another  l i n e on t h e e x i t p o r t o f t h e c e l l t h a t l e d i n t o a r e s i d u e f l a s k .  The  temperature o f a l l t h e s e s o l u t i o n s was 19° ±1°C. Unless s t a t e d o t h e r w i s e i n t h e t e x t t h e s o l u t i o n was r e p l e n i s h e d i n t h e i r r a d i a t i o n c e l l a f t e r each p u l s e . the. f^low system is-shown i n diagram 11.  A prctunei  representation o f  Diagram 8. The E l e c t r o n i c D e t e c t i o n System  41  The  Detection The  System - O p t i c a l l o c a t i o n s o f t h e l a s e r and p h o t o m u l t i p l i e r w i t h  respect  t o t h e p l e x i c e l l and t h e a c c e l e r a t o r a r e shown i n diagram 6. (i) The  The l a s e r a n a l y s i n g l i g h t s o u r c e was a S p e c t r a p h y s i c s model 130C  D.C. e x c i t e d Helium-Neon l a s e r , t h a t e m i t t e d  a c o n t i n u o u s p a r a l l e l beam  o  at 6328A. The  The beam w i d t h was 1.4 mm and the maximum o u t p u t power 1 m w a t t .  i n t e n s i t y o f t h e beam c o u l d be v a r i e d ; t h e l a s e r was o p e r a t e d under  c o n d i t i o n s o f minimum " n o i s e " which p r o v e d t o be j u s t below t h e maximum o u t p u t power.  The r e a s o n f o r t h i s i s g i v e n below.  S m a l l i r r e g u l a r i t i e s on the i n s i d e o f t h e plasma tube bore and the e x i t a p e r t u r e  t e n d t o d i f f r a c t some o f t h e l i g h t away from t h e p r i n c i p a l  a x i s , and a l t h o u g h t h e l o s s i s so s m a l l as t o n o t n o t i c e a b l y a f f e c t t h e power o u t p u t , t h e 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 beam v a r i e s w i t h from t h e a p e r t u r e . aperture  distance  I n t h i s case t h e i r r a d i a t i o n c e l l was 90 cm from t h e  and t h e i n t e n s i t y s p r e a d was s t i l l  contained  w i t h i n 2 mm, t h e  d i a m e t e r o f t h e g l a s s i r r a d i a t i o n tube; t h e r e was no a p p r e c i a b l e  loss of  i n t e n s i t y as the beam p a s s e d through t h e s o l u t i o n b u t the d i f f r a c t i o n was enhanced t o a s m a l l e x t e n t . box  At the e n t r a n c e t o t h e p h o t o m u l t i p l i e r  some 3 metres away t h e main beam was s t i l l  2 mm i n diameter b u t t h e  d i f f r a c t e d l i g h t was i r r e g u l a r l y s c a t t e r e d about i t .  An a d j u s t a b l e non  r e f l e c t i n g i r i s was mounted on t h e o p t i c a l bench i m m e d i a t e l y b e f o r e t h e f o c u s s i n g l e n s and i n t h i s way t h e s c a t t e r e d l i g h t was e l i m i n a t e d . An i n e v i t a b l e degree o f i n s t a b i l i t y i n the l a s e r and 60 Hz f l u c t u a t i o n s i n t h e i n t e n s i t y from t h e main power l i n e s were d e t e c t e d on  42  the p h o t o m u l t i p l i e r and added t o the " n o i s e " on the a b s o r p t i o n and decay t r a c e o f the h y d r a t e d e l e c t r o n . extremely  I t was  p o s s i b l e as w e l l t o p i c k up  f a s t and r e p r o d u c i b l e s i g n a l s o f %200  i n the l a s e r beam.  T h i s was  M Hz frequency  a t t r i b u t e d t o one o f the  and  additional  resonance f r e q u e n c i e s i n the o s c i l l a t i o n as t h i s l a s e r does not on a " s i n g l e - m o d e " t e c h n i q u e .  The  resonant  higher  operate  f r e q u e n c i e s are spaced f =  c_ , 2c, 3c .. . where c = speed o f l i g h t and L i s the o p t i c a l 2L 2L 2L  cavity  length.  which  In t h i s system the o p t i c a l c a v i t y l e n g t h i s fv30 cm)  could correspond  t o the h i g h frequency  observed.  In a d d i t i o n these  f r e q u e n c i e s are p e r t u r b e d by s m a l l amounts a c c o r d i n g t o how  f a r the  resonance i s from the c e n t r e o f the Neon resonance and any s l i g h t changes i n the c a v i t y l e n g t h (due p r i n c i p a l l y to thermal e f f e c t s ) . b a t i o n f r e q u e n c i e s can a l s o be d e t e c t e d i n the I K  These p e r t u r -  Hz t o 100 K Hz  w i l l be p r e s e n t s i m u l t a n e o u s l y and are not i n phase.  range,  As a r e s u l t  main s i g n a l from the l a s e r on a wide-band o s c i l l o s c o p e has  the  superimposed  n o i s e l i k e t r a c e s which o n l y d i s a p p e a r when these c a v i t y resonances become s y m m e t r i c a l  about the Neon resonance l i n e and, through  s e l f - p h a s e l o c k i n g , go to z e r o .  The  an apparent  e x t e n t t o which t h i s r i p p l e on  the  l a s e r s i g n a l i n t e r f e r e d w i t h the t r a n s i e n t a b s o r p t i o n s i g n a l s i s c o n s i d e r e d i n the r e s u l t s and d i s c u s s i o n . . In  the experiment d e s i g n e d t o f o l l o w the f o r m a t i o n o f  absorbing  s p e c i e s i n d i f f e r e n t areas i n the i r r a d i a t i o n c e l l , the l a s e r beam passed a l o n g the whole tube as u s u a l and the changes i n i n t e n s i t y a c r o s s diameter o f the l a s e r beam i t s e l f were m o n i t o r e d . ing  T h i s was  the  done by  align-  a t h i n b r a s s r e s t r i c t i o n p l a t e c o n t a i n i n g f o u r p i n h o l e s o f 0.031,  (9.020, 0.0225 and 0.0135 (thousandths  o f an i n c h ) bore p a r a l l e l t o the  Diagram 9.  Showing t h e apparatus  i n experimental  positions  44  l a s e r beam, i n a p o s i t i o n before the i r i s .  on t h e o p t i c a l bench a f t e r t h e p l e x i c e l l and  The r e s t r i c t i o n p l a t e was mounted on a s t a n d which had  a f i n e l a t e r a l adjustment; thus t h e p i n h o l e o f c h o i c e c o u l d be moved a c r o s s t h e w i d t h o f t h e l a s e r beam and e f f e c t i v e l y " s c a n " t h e i r r a d i a t i o n cell. (ii)  F i l t e r s and Lenses o  A 6328A ( B a i r d - A t o m i c  I n c . i n t e r f e r e n c e type B l l w i t h <10%  t r a n s m i s s i o n o u t s i d e t h e narrow band pass) f i l t e r t h a t t r a n s m i t t e d about 60% o f t h e i n c i d e n t l a s e r beam was i n c o r p o r a t e d i n t o t h e o p t i c a l ments as shown i n diagram 6.  The reasons were t w o f o l d .  arrange-  The h i g h  intensity  Cerenkov e m i s s i o n occurs a t a l l wavelengths and a l t h o u g h t h e p l e x i c e l l had been b l a c k e n e d  on i t s o u t e r s u r f a c e s t o p r e v e n t  the r e f l e c t i o n or  t r a n s m i s s i o n o f the Cerenkov l i g h t t h e c e l l windows were open and q u i t e transparent.  I t was n o t e x p e c t e d however t o observe a s i g n i f i c a n t  at 90° t o t h e e l e c t r o n beam.  intensity  The second reason f o r u s i n g a r e d f i l t e r  concerns the r e l a t i v e s e n s i t i v i t y o f the p h o t o m u l t i p l i e r t o d i f f e r e n t wavelengths.  The p h o t o m u l t i p l i e r i n use had a h i g h e r r e l a t i v e s e n s i t i v i t y o  to s h o r t e r wavelengths w i t h a maximum response at ^3500A. fluctuation  A small  i n i n t e n s i t y a t a low wavelength would c o r r e s p o n d  to a large  change i n i n t e n s i t y i n t h e r e d and might t h e r e f o r e f a l s i f y t h e h e i g h t and shape o f t h e s i g n a l  recorded.  The n e u t r a l d e n s i t y f i l t e r s used p e r i o d i c a l l y t o t e s t t h e l i n e a r i t y o f t h e system were 1% and 10%. ference  These were B a i r d - A t o m i c  inter-  filters. Complete a b s o r p t i o n o f t h e l a s e r beam gave a s i g n a l o f o n l y  ^25 mv from t h e p h o t o m u l t i p l i e r o p e r a t i n g a t 550 v o l t s . the e l e c t r o n i c  The d e t a i l s o f  d e t e c t i o n system a r e g i v e n l a t e r t o g e t h e r w i t h t h e reasons  45  f o r t h i s small s i g n a l .  U l t i m a t e l y the b e s t s i g n a l t o n o i s e r a t i o  (set  by the l i m i t s o f the maximum p e r m i s s i b l e g a i n i n the p h o t o m u l t i p l i e r the n o i s e o b s e r v e d at h i g h e r o p e r a t i n g s i g n a l ) was A 8.5  mm  voltages  used t o a m p l i f y the  and small  o b t a i n e d by f o c u s i n g the l a s e r beam onto the photocathode.  f o c a l length polished quartz  l e n s was  mounted on a s u p p o r t  capable o f f i n e l a t e r a l and v e r t i c a l adjustment and o r i e n t a t e d t o g i v e the b e s t p o s s i b l e s i g n a l from the l a s e r beam.  Normally readjustment  u n n e c e s s a r y as the geometry o f a l l the equipment was a f t e r any  The  even  d i s p l a c e m e n t o r m o d i f i c a t i o n s t o the system.  Detection (i)  System - E l e c t r o n i c The  photomultiplier  A R.C.A. V i c t o r 1P28 and a 100  reproducible  was  ohm  p h o t o m u l t i p l i e r w i t h a S5 s p e c t r a l response  l o a d r e s i s t a n c e was  i n t e n s i t y o f the t r a n s m i t t e d  used t o f o l l o w the v a r i a t i o n s i n the  l a s e r beam.  The  l o a d r e s i s t a n c e and r e s i s t o r s  i n the dynode c h a i n were f i t t e d i n t o a compact m e t a l base and the whole assembly was  housed i n a copper box  19 cm h i g h by  12 cm by  17 cm t h a t  been l i n e d w i t h %" t h i c k l e a d ( t o p r o t e c t the p h o t o m u l t i p l i e r from radiation).  The  l i d o f the box  contained eight small holes that  had  X-  provided  a minimum amount o f v e n t i l a t i o n but because o f the a c t u a l p o s i t i o n o f  the  phototube no d i f f u s e l i g h t c o u l d be p i c k e d up and a m p l i f i e d w i t h the s i g n a l . The  ambient temperature i n the box  rose appreciably  a f t e r prolonged  o f the p h o t o m u l t i p l i e r and t h e r e f o r e the d u r a t i o n o f experiments was  use kept  w i t h i n a r e a s o n a b l e time and the p h o t o m u l t i p l i e r p e r i o d i c a l l y checked f o r any marked i n c r e a s e i n dark c u r r e n t . a constant  A m o d i f i c a t i o n t o the h o u s i n g t o  provide  temperature c o o l i n g system has been d e s i g n e d and w i l l be i n use  46  Diagram 11.  The Flow System  47  f o r t h e n e x t experiments i n t h i s p r o j e c t .  Under t h e p r e s e n t c o n d i t i o n s  the temperature i n s i d e t h e copper box was 21° ±1°C. A p i n h o l e o f 1.5 mm d i a m e t e r was d r i l l e d through t h e f r o n t f a c e o f the s h i e l d i n g box thus a l l o w i n g l i g h t f o c u s e d through t h e p i n h o l e t o f a l l on t o t h e photocathode.  Between experiments t h e photocathode was  p r o t e c t e d from t h e l a s e r beam by c o v e r i n g t h e p i n h o l e . The 1P28 has f a s t time r e s o l u t i o n c h a r a c t e r i s t i c s t h e anode p u l s e r i s e time o f 10  -8  v o l t a g e s employed.  -9 seconds and time s p r e a d o f 2 x 10 seconds a t t h e The minimum l e n g t h o f RG71/B/U c o a x i a l c a b l e (used t o  match impedance w i t h t h e l o a d r e s i s t a n c e ) n e c e s s a r y t o t r a n s m i t t h e s i g n a l t o t h e o s c i l l o s c o p e was I B " ; t h e c a b l e was doubly s h i e l d e d and f e d t h e s i g n a l through a 93 ohm T e k t r o n i x t e r m i n a t o r i n t o t h e v e r t i c a l  amplifier  input o f the o s c i l l o s c o p e . The maximum c u r r e n t t h a t t h e dynode c h a i n c o u l d s u s t a i n was 10 m amps b u t t o a v o i d s a t u r a t i o n e f f e c t s and problems o f non l i n e a r i t y i n the response o f the p h o t o m u l t i p l i e r tube a w o r k i n g v o l t a g e o f 550 v o l t s from a power s u p p l y (5.5 mA D.C. a c r o s s t h e c h a i n ) was n o r m a l l y used and the s i g n a l s a t t e n u a t e d on t h e o s c i l l o s c o p e .  For very small v a r i a t i o n s i n  i n t e n s i t y o f t h e l a s e r beam and t h e " p i n h o l e e x p e r i m e n t s " t h e v o l t a g e was r a i s e d t o a maximum o f 750 V.  At v e r y low v o l t a g e s i . e . low c u r r e n t  a l o n g t h e dynode r e s i s t o r c h a i n , f l u c t u a t i o n s i n t h e dynode v o l t a g e s conr t r i b u t e d towards t h e n o n - l i n e a r b e h a v i o u r o f t h e tube; a t v o l t a g e s above 750 V t h e l e v e l o f t h e n o i s e on t h e s i g n a l , which had c o n t r i b u t i o n s from dark c u r r e n t s i n t h e t u b e , a m p l i f i e d l a s e r r i p p l e , and feedback was u n a c c e p t a b l e .  effects  I n a d d i t i o n d u r i n g t h e experiments t h e r e were adverse  changes i n t h e dark c u r r e n t and n o i s e p u l s e s due t o t h e i n t e n s e i o n i s i n g  48  r a d i a t i o n and f i e l d s g e n e r a t e d by the e l e c t r o n p u l s e .  However these  e f f e c t s were reduced t o as low a l i m i t as p o s s i b l e by the l e a d s c r e e n i n g i n the copper box,  the r a d i a t i o n s h i e l d s and the double s h i e l d i n g on a l l  the t r a n s m i s s i o n l i n e s , a l t h o u g h t h i s l e v e l was  by no means  completely  satisfactory. The p h o t o m u l t i p l i e r was v o l t a g e D.C.  powered by a F l u k e model 412B  power s u p p l y whose o u t p u t was  p e r h o u r o r ±0.02% p e r day's o p e r a t i o n .  high  s t a b i l i s e d t o w i t h i n ±0.005%  The power s u p p l y was  connected  t o the p h o t o m u l t i p l i e r w i t h c o a x i a l doubly s h i e l d e d t r a n s m i s s i o n c a b l e .  (ii)  O s c i l l o s c o p e and  camera  The T e k t r o n i x model 454 wide band p a t h o s c i l l o s c o p e was  used t o  d i s p l a y the v a r i a t i o n s i n the i n t e n s i t y o f the l a s e r beam ( i . e . the change i n anode c u r r e n t a c r o s s the l o a d r e s i s t a n c e i n the p h o t o m u l t i p l e r )  during  and a f t e r the e l e c t r o n p u l s e from the a c c e l e r a t o r as a time dependent voltage signal. was  2.6  The  r i s e time o f the o s c i l l o s c o p e under these  nanoseconds. The  i n p u t power l i n e from the mains 110 v o l t s s u p p l y was  through a r a d i o f r e q u e n c y scope.  l i n e f i l t e r a t t a c h e d t o the back o f the  t r a c e on the s c r e e n was  s i g n a l and a l l low frequency the 100%  filtered oscillo-  As a l r e a d y mentioned the s i g n a l from the p h o t o m u l t i p l i e r was  t e r m i n a t e d w i t h 93 ohms at the ircput on channel The  conditions  l i g h t l e v e l was  t r i g g e d by an A.C.  1 o f the o s c i l l o s c o p e . p o s i t i v e f l u c t u a t i o n i n the  s i g n a l s were r e j e c t e d .  recorded  immediately  In each experiment  a f t e r the p u l s e by m a n u a l l y  chopping the l a s e r beam (at a p p r o x i m a t e l y  1 K Hz)  and r e c o r d i n g the  o f the s i g n a l and subsequent f a l l t o 100%  transmission  again.  rise  49  A T e k t r o n i x Type C-40 ( p o l a r o i d ) camera s u p p l i e d w i t h attachments f o r t h e o s c i l l o s c o p e was used t o r e c o r d t h e t r a c e s on p o l a r o i d p o l a s c o p e type 410 f i l m which i s p a r t i c u l a r l y s u i t a b l e f o r e x t r a - h i g h speed  (rated  as 10,000 ASA e q u i v a l e n t ) photography and low l e v e l l i g h t s o u r c e s such as t h e t r a c e i n t e n s i t i e s a v a i l a b l e w i t h t h e f a s t e s t sweeprate on t h e o s c i l l o scope .  The Grounding System I t was i m p e r a t i v e t o have a good h i g h f r e q u e n c y g r o u n d i n g system w i t h a l l the equipment grounded e f f e c t i v e l y t o one p o i n t as o t h e r w i s e t h e i n t e n s e magnetic and e l e c t r i c f i e l d s produced by t h e a c c e l e r a t o r d u r i n g the e l e c t r o n p u l s e c o m p l e t e l y i n t e r f e r e d w i t h any measurements.  The  power s u p p l y , o s c i l l o s c o p e and p h o t o m u l t i p l i e r were a l l grounded t o one a n o t h e r t h r o u g h t h e o u t e r s h i e l d i n g on t h e doubly s h i e l d e d c o a x i a l  cable.  However i t a l s o appeared n e c e s s a r y t o ground t h e n o i s e f i l t e r on t h e o s c i l l o s c o p e s e p a r a t e l y , t o t a k e a ground l e a d from t h e p h o t o m u l t i p l i e r box t o t h e "common" ground p o i n t and t o remove a l l the equipment the mains e a r t h by f l o a t i n g t h e p l u g s i n a common j u n c t i o n box.  from It i s  p r o b a b l y more i n f o r m a t i v e t o i l l u s t r a t e t h e complete g r o u n d i n g scheme i n a diagram (see diagram 12).  The ground s t a k e was a t t a c h e d t o a l o n g  copper b a r t o which i n d i v i d u a l g r o u n d i n g tapes were b o l t e d . D u r i n g t h e experiments i t was d i s c o v e r e d t h a t many o f t h e long t h i c k g r o u n d i n g tapes and u n s h i e l d e d c a b l e s were m e r e l y a c t i n g as a t t e n n a e , so a l l g r o u n d i n g t a p e s were k e p t t o a m i n i m a l s i z e and no c a b l e remained unshielded. The l a s e r was a l s o a t t a c h e d t o a f l o a t i n g p l u g and grounded then  A' B C D E H L K M  Diagram -12...  The Grounding System  accelerator c o n t r o l console plexicell laser lead screens pho.tomul.tiplier oscilloscope power s u p p l y t o H copper buss b a r , . ground s t a k e  51  to the copper b a r a p p r o x i m a t e l y t h r e e f e e t from the grounding s t a k e . p i e c e o f %" l e a d was  f o l d e d around the s i d e o f the l a s e r n e a r t o the  a c c e l e r a t o r t o p r e v e n t any adverse e f f e c t s on the l a s e r output and There was  A  a t h r e e - s i d e d l e a d box i m m e d i a t e l y b e h i n d the p l e x i c e l l  ripple. which  i n the absence o f t h e c e l l o r o t h e r attachment on the f r o n t o f the a c c e l e r a tor  a c t e d as a s i n k f o r the r a d i a t i o n  produced.  As the alignment o f the l a s e r , p l e x i c e l l , p h o t o m u l t i p l i e r was markedly  f o c u s i n g l e n s and  c r i t i c a l , any v i b r a t i o n o f t h e s e p i e c e s o f equipment  a f f e c t e d the s i g n a l , and o f t e n heavy " n o i s e " on the s i g n a l  c o u l d be t r a c k e d back t o i m p e r f e c t a l i g n m e n t .  The o t h e r f a c t o r s to  c o n s i d e r were the o c c u r r e n c e o f ground l o o p s from doubly grounding some o f t h e e l e c t r o n i c equipment and the i n e v i t a b l e p i c k u p from the mains o f h i g h f r e q u e n c y s i g n a l s from machines i n nearby  laboratories.  52  The  Computation o f Data and t h e R e s u l t s  The  Input  Data T h i s c o n s i s t e d o f t h e r e c o r d e d a b s o r p t i o n s i g n a l as a f u n c t i o n  o f time d u r i n g t h e decay o f t h e h y d r a t e d e l e c t r o n .  The measurements  were made on t h e photographs o f t h e o s c i l l o s c o p e t r a c e s u s i n g a M i c r o s c a l e w i t h 0.1 mm d i v i s i o n s , and r e a s o n a b l e the next d e c i m a l p l a c e . 14.8  A typical i n i t i a l  ± .05 mm ( f o r a 2.5 mm p a t h  e s t i m a t e was p o s s i b l e f o r a b s o r p t i o n s i g n a l would be  l e n g t h ) d e c r e a s i n g t o ^10 mm a f t e r  150 nanoseconds, w i t h a maximum p o s s i b l e a b s o r p t i o n c o r r e s p o n d i n g t o 30 mm.  Measurements were made every 5, 10 o r 20 nanoseconds t h e time  i n t e r v a l depending on t h e sweeprate o f t h e o s c i l l o s c o p e , and sometimes e s t i m a t e s o f t h e t r u e h e i g h t o f t h e s i g n a l had t o be made due t o t h e noise l e v e l .  Because o f t h i s u n c e r t a i n t y a f t e r 100 o r 200 nanoseconds,  i t was o f t e n d i f f i c u l t t o determine a n y t h i n g b u t an approximate h a l f l i f e f o r e aq under t h e e x p e r i m e n t a l  c o n d i t i o n s and t h e r e f o r e a comparison  has been made between t h e d i f f e r e n t r e s u l t s t h a t r e l a t e s t h e % decrease i n t h e a b s o r p t i o n s i g n a l at a g i v e n time a f t e r t h e p u l s e .  I n those t r a c e s  where t h e n o i s e and l a s e r i n t e r f e r e n c e (see s e c t i o n I I ) were more than an annoying w i d t h on t h e s i g n a l t h e e aq decay was n o t a n a l y s e d .  Attempts  were made t o e n l a r g e t h e p o l a r o i d photographs i n t o b o t h n e g a t i v e s and p r i n t s f o r e a s i e r working but the i n e v i t a b l e d i s t o r t i o n s i n the s c a l e and on t h e t r a c e a l t h o u g h s m a l l were s u f f i c i e n t t o outweigh any v i s u a l advantages.  Computation o f Data The  a n a l y s i s o f t h e r e s u l t s c o l l e c t e d i n t h e l o n g p u l s e (50 Nsec)  experiments i n d i c a t e d t h a t t h e d a t a c o u l d n o t be s i m p l y matched t o  53  e i t h e r a f i r s t o r second o r d e r k i n e t i c t r e a t m e n t .  In a d d i t i o n the  d u r a t i o n o f the p u l s e meant t h a t t h e h y d r a t e d e l e c t r o n s were r e a c t i n g b e f o r e the e l e c t r o n p u l s e (and thus the f o r m a t i o n o f t h e h y d r a t e d e l e c t r o n s ) was f i n i s h e d .  With t h e s h o r t (3 Nsec) e l e c t r o n p u l s e  the l a t t e r p r o b l e m was i n s i g n i f i c a n t b u t t h e k i n e t i c s were not s i m p l i f i e d . The r e s u l t s  from these experiments  were t h e r e f o r e c o n s i d e r e d from t h r e e  independent approaches, c o v e r i n g the d i f f e r e n t mechanisms f e a s i b l e i n t h i s system. (i)  Two s i g n i f i c a n t s i m u l t a n e o u s  processes  o c c u r r i n g immediately  a f t e r the p u l s e . (ii)  One s i g n i f i c a n t p r o c e s s o n l y o c c u r r i n g a f t e r t h e p u l s e .  (iii)  Two s i g n i f i c a n t p r o c e s s e s  occurring consecutively after  the p u l s e . Let x be the a b s o r b i n g s p e c i e ( i e . t h e h y d r a t e d e l e c t r o n ) and y any o t h e r s p e c i e s u b s e q u e n t l y  formed i n t h e system i n s i g n i f i c a n t  c o n c e n t r a t i o n s ; k^ i s a f i r s t o r d e r r a t e c o n s t a n t and  a second o r d e r  rate constant. (i)  I f s e v e r a l processes  a r e o c c u r r i n g s i m u l t a n e o u s l y an  e x p r e s s i o n can be d e r i v e d r e l a t i n g t h e change i n c o n c e n t r a t i o n o f x t o these r e a c t i o n r a t e s and the c o n c e n t r a t i o n s o f any o t h e r s p e c i e s i n v o l v e d . From an i n i t i a l s t u d y o f t h e d a t a i t was c l e a r t h a t i f t h e r e were two simultaneous  p r o c e s s e s i n t h i s system they would be f i r s t  and second  o r d e r type r e a c t i o n s and so the f o l l o w i n g e q u a t i o n d e s c r i b e s t h i s -d[x] dt  = k [x] + k [ x ]  -1_ . d [x] [x] ~dt~ plot of  1 d[x] [x] dt  = k  + k [x] ?  2  case:  ( o r k xy) which on rearrangement shows t h a t a  1  a g a i n s t [x] s h o u l d g i v e a s t r a i g h t  l i n e o f s l o p e - k„ 2  54  and i n t e r c e p t k^.  A f u r t h e r rearrangement g i v e s m a t h e m a t i c a l l y  p l o t but an i n t e r e s t i n g check on the way -dfeix dt In  = k 1  + k  the d a t a f i t s the  identical  treatment.  [] X  1  the event o f o n l y one p r o c e s s  o c c u r i n g the a p p r o p r i a t e k  w i l l become i n s i g n i f i c a n t and the term w i l l equate t o z e r o .  The  para-  meters e v a l u a t e d from t h e s e p l o t s were then f i t t e d t o a t h i r d degree polynomial. The  assumption t h a t o p t i c a l d e n s i t y c o u l d be l i n e a r l y  to the c o n c e n t r a t i o n o f the a b s o r b i n g O.D.  related  specie  = c. e l  e  extinction  coefficient  % p a t h l e n g t h i n cms., was  i n v e s t i g a t e d e x p e r i m e n t a l l y and the r e s u l t s w i l l be d i s c u s s e d The  o p t i c a l , d e n s i t y was  c o n s i d e r e d as the v a r i a b l e , and  two e x p r e s s i o n s below used t o s e p a r a t e the two simultaneous -1 O.D.  d O.D. dt  -dJto O.D. dt  = k  = k  + k  + k  2  later. the  processes  O.D. el  2  O.D. el  up t o 200 Nseconds a f t e r t h e p u l s e . :  (ii)  I f t h e r e were o n l y one s i g n i f i c a n t p r o c e s s  o c c u r r i n g then  a d e t a i l e d g r a p h i c a l a n a l y s i s o f the changes i n o p t i c a l d e n s i t y ( i n the f i r s t 100 Nseconds o r so a f t e r the p u l s e ) s h o u l d i n d i c a t e the r e a c t i o n order. F i r s t order process:  Jinx = k T +  constant  A p l o t o f ln(0.D.) a g a i n s t t s h o u l d g i v e a s t r a i g h t l i n e o f s l o p e k . el ~" Second o r d e r p r o c e s s : 1_ = k x + c o n s t a n t 2  x  A p l o t o f 1_ O.D.  against x should give a s t r a i g h t l i n e of slope  k  2. eT  The  55  k„ v a l u e was  1_  c a l c u l a t e d from  - 1_  O.D.  O.D. T  (iii)  O  = _2_ eT  I f the mode o f decay o f the h y d r a t e d e l e c t r o n changes  a f t e r a c e r t a i n time and, f o r example, b o t h f i r s t  and second  order  p r o c e s s e s are i n v o l v e d i n sequence, then the b e h a v i o u r o f the s p e c i e still  may  period.  be s p e c i f i c a l l y c l a s s i f i e d b e f o r e and a f t e r the t r a n s i t i o n Here one must a l s o i n c l u d e the p o s s i b i l i t y t h a t t h e  two  mechanisms are o f the same r e a c t i o n o r d e r but o c c u r w i t h s i g n i f i c a n t l y d i f f e r e n t rate constants.  Any changes i n the s l o p e s observed from the  t r e a t m e n t i n ( i i ) s h o u l d o c c u r at a p p r o x i m a t e l y the same time  after  the p u l s e and i n the same d i r e c t i o n . The u n d e r l y i n g assumption  i n t h e s e approaches  i s that c l a s s i c a l  k i n e t i c t h e o r y can be used t o d e s c r i b e the b e h a v i o u r o f a system whose p r i n c i p a l a c t i v e s p e c i e has been consumed by the p r o d u c t s o f i t s own i n i t i a l r e a c t i o n s i n about 200  nanoseconds, and can be scavenged  by  a d d i t i v e s o f s u i t a b l e c o n c e n t r a t i o n s i n o n l y a few nanoseconds. The d a t a was  f e d i n t o a computer; the  programme c o n t a i n i n g the t r e a t m e n t s f o r ( i ) and  least-squares-fit ( i i ) was  a l s o designed  to p r o v i d e i n f o r m a t i o n f o r ( i i i ) by s y s t e m a t i c a l l y e l i m i n a t i n g the d a t a p o i n t s up t o a g i v e n time a f t e r the p u l s e and r e c o r d i n g the v a r i a t i o n s i n the r a t e c o n s t a n t s and the q u a l i t y o f the  fit.*  A l l the l o n g p u l s e d a t a had been a n a l y s e d assuming ( i i ) i t was  from t h e s e graphs t h a t t h e f i r s t  apparent k^ and  and  v a l u e s were  c a l c u l a t e d ; t h e s e were;.-a good i n d i c a t i o n o f t h e o r d e r o f magnitude t o expect on t h e s h o r t p u l s e d a t a some o f which was  also analysed i n t h i s  * The a u t h o r g r a t e f u l l y acknowledges the a s s i s t a n c e o f Mr. S.C. i n p r e p a r i n g t h e programme.  Wallace  56  way  as a p r e c a u t i o n a g a i n s t e r r o r s i n the i n p u t d a t a o r computer programme. The r e s u l t s o f each o f the s e r i e s o f experiments w i l l now  be  given i n r e l a t i o n t o the three treatments.  V a r i a t i o n s o f o p t i c a l density w i t h path Path l e n g t h s o f 1.0 mm, 5.5 mm were used.  1.5  length  mm,  2.7 mm,  3.5 mm,  5.0  mm  and  When i t became c l e a r t h a t the l a r g e r p a t h l e n g t h s were  g i v i n g r i s e t o o p t i c a l d e n s i t i e s (O.D.) t h a t d e v i a t e d from the a n t i c i p a t e d v a l u e s ( d e r i v a t i v e s from Beer's law) then attempts were made t o look at O.D.  at even s h o r t e r p a t h l e n g t h s .  However the t e c h n i c a l problems  a s s o c i a t e d w i t h the d e t e c t i o n and a m p l i f i c a t i o n o f the s i g n a l p r e v e n t e d such e x p e r i m e n t s .  Two  p o s s i b l e reasons f o r t h i s d e v i a t i o n are (a) the  e f f e c t i v e p a t h l e n g t h o f a b s o r b i n g s p e c i e s was of t h e s l i t s  due t o an undetermined  g r e a t e r than t h e w i d t h  d i v e r g e n c e o f the e l e c t r o n beam, a  more s i g n i f i c a n t e r r o r at l o n g e r p a t h l e n g t h s ; and (b) t h e d i s t r i b u t i o n o f t h e e l e c t r o n beam i t s e l f of  radiali  i s such t h a t i f the c e n t r e p o i n t  the s l i t s d i d n o t c o i n c i d e w i t h t h a t o f the e l e c t r o n beam then t h e r e  may be d i f f e r e n t numbers o f a b s o r b i n g s p e c i e s a l o n g the p a t h  length,  a r i s i n g from a s p a t i a l v a r i a t i o n i n the energy d e p o s i t e d i n the medium. The d a t a i s summarised i n diagram 13. O.D.  The v a r i a t i o n s i n  observed at the same p a t h l e n g t h may be i n p a r t due t o (b) above  and a l s o t o the f a c t t h a t r e c e n t l y f l u c t u a t i o n s i n the output o f the e l e c t r o n a c c e l e r a t o r have been observed i n t h i s l a b o r a t o r y . of  The  energy  the e l e c t r o n beam v a r i e d w i t h the a c t u a l c h a n g i n g c o n d i t i o n s o f the  a c c e l e r a t o r and the time i n t e r v a l between p u l s e s t o a more s i g n i f i c a n t degree than the manufacturers  claimed.  The p u l s e t o p u l s e r e p r o d u c i b i l i t y  57  .0 .5 Diagram 13.-  1.5  2.5  3.5  45  5.5  O p t i c a l d e n s i t y as a f u n c t i o n o f path .length  mm  58  has now been improved by changing a l l the n i t r o g e n gas i n the h i g h p r e s s u r e chambers t o oxygen.  The  f o r m a t i o n and decay o f e aq a f t e r a 5 Nsec e l e c t r o n p u l s e T h i s s e c t i o n r e f e r s t o the d a t a c o l l e c t e d on the decay o f e aq  a f t e r a s i n g l e e l e c t r o n p u l s e at d i f f e r e n t p a t h l e n g t h s . p l o t s are g i v e n i n diagrams  14, 15,16 which show t h e f i r s t  Some t y p i c a l and  second  o r d e r t r e a t m e n t s and the apparent t r a n s i t i o n from one t o the o t h e r at about 80 ± 10 nanoseconds i n each case.  U n i m o l e c u l a r and b i m o l e c u l a r  decay p r o c e s s e s might have been o c c u r r i n g s i m u l t a n e o u s l y w i t h one mode d o m i n a t i n g the o t h e r at d i f f e r e n t times a f t e r t h e p u l s e , but none o f the data f i t t e d treatment  (i).  In some cases the t r a n s i t i o n p e r i o d i n  the f i r s t o r d e r p l o t s was  then f o l l o w e d by a l o n g l i n e a r p o r t i o n which  might i t s e l f be i n d i c a t i v e o f a n o t h e r f i r s t o r d e r decay o c c u r r i n g at a different rate.  The s l o p e o f t h i s l i n e was  l i n e a r t o about 150 nanoseconds  and gave a r a t e c o n s t a n t s l o w e r by a f a c t o r o f 3. The r a t e o f decay o f e aq was  f i r s t t a b u l a t e d from t h o s e  experiments i n which i t had been p o s s i b l e t o c o l l e c t d a t a every 10 nanoseconds and i n most cases e v e r y 5 nanoseconds. the f i r s t o r d e r r a t e c o n s t a n t  as c a l c u l a t e d from a p l o t o f l o g (O.D.) 6  a g a i n s t time was  8.80  ± .8 x 10  The average v a l u e f o r  —1 sec  .  With d a t a t a k e n i n o t h e r  experiments o n l y e v e r y 20 nanoseconds t h e r e were not as many p o i n t s on the 'graph up t o the 80 nanosecond t r a n s i t i o n p e r i o d and i t was t h a t t h i s average k^ v a l u e was The p l o t s o f  1_ O.D.  apparent  s l i g h t l y l o w e r , v i z . 5.7 ± .8 x 10^ sec  a g a i n s t time were l i n e a r a f t e r about 80  nanoseconds i n a l l the e x p e r i m e n t s , and i r r e s p e c t i v e o f the time i n t e r v a l s i n which the d a t a was  c o l l e c t e d the v a l u e s f o r k„ were i n r e a s o n a b l e  8l  J_  path length @ 1 mm © 2 5 mm © 1.5 mm  /  0  / P  7 6  /  ft  d  %  O.D.  —•V  9 '  0  .0  3<K^ 2 1  Log fQD el y O ^  4.1  0  o  ft'  3.9  0  o 1 mm © 2.7 mm © 1.5 mm  3.7' .0  3.5  0  50  100  150  200  250  Time (nsccs) Diagram 14.  Some f i r s t and second o r d e r p l o t s f o r t h e decay o f t h e hydrated electron  60  0  20  Diagram 16.  60  Time  (nsecs)180  Second o r d e r p l o t s f o r t h e decay o f e"  220  260  61  agreement w i t h an average v a l u e o f 5.88 ± 1.2  x 10^  M ^ sec ^.  I n d i v i d u a l e x p e r i m e n t a l r e s u l t s are compared i n Table ( I I I ) Table I I I .. path length  , k^  ,-6 10  1f  -1 sec  , k  in 2  -10 10  ..-1 M  -1 sec  cm Data every 5, 10 nanoseconds 0.1  7.17  ± .15  4.49  +  .07  0.15  8.38  ± .02  6.45  ±  .23  0.15  8.26  ± .13  6.11  ±  .11  0.35  9.56  ± .54  6.00  ±  .42  0.55  8.19  ± .07  insufficient  data  0.55  8.19  ± .09  insufficient  data  Data every 20 nanoseconds o r l o n g e r 0.1  6.72  ± .14  4.25  ±  .23  0.1  5.73 ± .42  4.14  ±  .09  0.1  5.18  ± .29  5.23  ±  .26  0.27  4.92  ± .02  8.20  ±  .33  0.27  6.06  ± .00  6.60  ±  .11  0.27  insufficient data 5.30 ± .45  7.49  ±  .29  0.55  insufficient  data  The e v i d e n c e s t r o n g l y suggests t h a t f o r the f i r s t  80 nanoseconds  at l e a s t t h e e aq decays i n a 1st o r d e r p r o c e s s ; t h i s i s a p u z z l i n g concept i n view o f the g e n e r a l l y a c c e p t e d b i m o l e c u l a r n a t u r e o f the decay o f e aq i n i r r a d i a t e d t o be a t r a n s i t i o n halflives.  The  n e u t r a l water.  A f t e r t h i s p e r i o d t h e r e appears  t o 2nd o r d e r b e h a v i o u r which h o l d s over s e v e r a l  apparent  l i n e a r i t y o f some f i r s t o r d e r p l o t s a f t e r  and up t o about 150 nanoseconds w i t h an average k^ v a l u e o f 2.62  80  ± 1 x  62  e , 1 pulse. Q q  20mV/div.  I'Vl rrti i  100 nsocs/div. ,  2 pulses. .50mV/div.  •v,ft  50nsecs/div.  aq  10 mV/div. 50 nsecs/div.  eaq+ R-OH 20mV/div. ... .•• p  50 nsecs/div. Diagram 15 ( b ) . O s c i l l o s c o p e t r a c e s showing' t h e e aq decay i n the p r e s e n c e o f d i f f e r e n t s o l u t e s .  63  10  sec  may o r may n o t be a r e a l e f f e c t : t h i s might s i m p l y be t h e  b e g i n n i n g o f a v e r y l o n g slow curve which would o n l y be n o t i c e d i f d a t a up t o s e v e r a l hundred nanoseconds c o u l d be t a k e n at s m a l l time intervals.  The l e v e l o f n o i s e  ( t h a t masked t h e a b s o r p t i o n s i g n a l ) on  the t r a c e s was t o o great t o p e r m i t a c c u r a t e measurements up t o t h i s stage.  Formation  and decay o f e aq a f t e r a 50 Nsec e l e c t r o n p u l s e These experiments were c a r r i e d out i n a d i f f e r e n t l a b o r a t o r y  b u t w i t h e s s e n t i a l l y t h e same equipment.  The/important d i f f e r e n c e s a r e  the l e n g t h o f t h e p u l s e , d u r i n g which time the e aq i s formed and b e g i n s to  decay and may p o s s i b l y reach a steady s t a t e c o n c e n t r a t i o n , and t h e  p a t h l e n g t h o f t h e c e l l which was always 7 mm. The  decay o f e aq was shown 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  over about 100 nanoseconds, and a h a l f l i f e The  The  o f about 130 nanoseconds.  r a t e c o n s t a n t s a r e g i v e n as k  =  1.91 ± .07 x 1 0  k  =  •11 ^10  2  7  sec"  1  ..-1 -1 M sec  effects of multiple pulsing The  decay o f e aq was b i m o l e c u l a r a f t e r s e v e r a l p u l s e s and  the r a t e s i n c r e a s e d s i g n i f i c a n t l y .  Although  some f i r s t o r d e r c h a r a c t e r  was s t i l l e v i d e n t i n the d a t a a f t e r t h r e e o r f o u r p u l s e s t h e i n c r e a s e i n r a t e s was such as t o i n d i c a t e pseudo f i r s t o r d e r b e h a v i o u r  i n the  presence o f r e l a t i v e l y high concentrations o f r a d i o l y t i c products. average h a l f l i f e  o f [e aq] was reduced by about a f a c t o r o f 2 a f t e r  The  64  16 14 12 K 8 6 4 2  1  ,  J  i  i  1 2 3 4 Pulses (a)  1 2 3 4  t  i  - -6 -1 x 10 s e c - 1 0 ..-1 -1 x 10 M sec 1n  i n  i  8 9 10  Pulses 8 .9 10 ( b)  Diagram 17. (a) -The e f f e c t s o f m u l t i p l e p u l s i n g on (a) r a t e c o n s t a n t s and k^, and (b) time t a k e n f o r t h e i n i t i a l a b s o r p t i o n s i g n a l t o t o reduce by a f a c t o r o f 2  65  10 p u l s e s .  A t y p i c a l p l o t from a m u l t i p l e - p u l s e experiment i s shown  i n diagram 17 where the r e l a t i v e changes i n of  and k^, and the h a l f l i f e  e aq are i n c l u d e d as- a f u n c t i o n o f the number o f p u l s e s f o r two  d i f f e r e n t p a t h l e n g t h s , 1.0 mm  The E f f e c t o f H  and 2.6  mm.  +  The a d d i t i o n o f a c i d t o the aqueous system markedly reduced the  l i f e t i m e o f the h y d r a t e d e l e c t r o n .  and 5 x 10 ^ M H  +  With a p a t h l e n g t h o f 3.6  mm  from s u l p h u r i c a c i d the a b s o r p t i o n s i g n a l had d i s a p p e a r e d  i n about 40 nanoseconds  (whereas the n e u t r a l w a t e r c o n t r o l i n d i c a t e d o n l y  a 17.5% decrease d u r i n g the same time i n t e r v a l ) . From the p o i n t s a v a i l a b l e f o r the e aq decay i n a c i d s o l u t i o n the  r e a c t i o n was unambiguously  concentration of H  +  f i r s t o r d e r ; t h i s i s e x p l a i n e d as the  exceeds t h a t o f e aq (as c a l c u l a t e d from the i n i t i a l 2  o p t i c a l d e n s i t y ) by 10  and thus pseudo f i r s t o r d e r k i n e t i c s might be  anticipated. e aq + H  +  >• H + OH aq 7  The r a t e c o n s t a n t was c a l c u l a t e d t o be 7.4 ± .5 x 10  -1 sec  .  The  c o n t r o l sample d a t a showed a f i r s t o r d e r decay (k = 9.46 ± .16 x 10^ sec ^) up t o about 80 nanoseconds take o v e r .  and then a second o r d e r p r o c e s s appeared t o  The p r o b a b l e reasons f o r t h i s e f f e c t commonly seen i n  n e u t r a l w a t e r w i l l be d i s c u s s e d i n p a r t IV. The e f f e c t o f an a l c o h o l I s o p r o p y l a l c o h o l r e a c t s r e a d i l y w i t h OH and H r a d i c a l s  and  66  t h e r e f o r e should increase the l i f e t i m e o f the hydrated e l e c t r o n . Whereas i n t h e c o n t r o l sample t h e i n i t i a l a b s o r p t i o n s i g n a l had d e c r e a s e d by 42% 100 Nsecs a f t e r t h e e l e c t r o n p u l s e , t h e s i g n a l from t h e a l c o h o l i c s o l u t i o n had o n l y d e c r e a s e d by 20%.  Again d a t a from  t h e c o n t r o l sample f i t t e d a f i r s t o r d e r t r e a t m e n t up t o about 75 nanoseconds.  The d a t a from t h e a l c o h o l experiments gave good second o r d e r  p l o t s o v e r t h e complete time p e r i o d examined (140 nanoseconds) and w i t h one e x c e p t i o n t h e f i r s t o r d e r t r e a t m e n t s c o u l d be i g n o r e d .  The  reasons for. the e x c e p t i o n are n o t known -- t h e p o i n t s were not s c a t t e r e d . In  t r e a t m e n t ( i ) a l t h o u g h t h e f i t s were n o t s a t i s f a c t o r y t h e  e v a l u a t e d r a t e c o n s t a n t s were o f t h e c o r r e c t o r d e r o f magnitude.  The  k^ c a l c u l a t e d from t r e a t m e n t ( i i ) was 3.8 ± .1 x l O ^ M s e c . 1  1  The e f f e c t o f oxygen The v a r i a t i o n i n t h e r a t e o f decay o f e aq i n the presence o f d i f f e r e n t c o n c e n t r a t i o n s o f d i s s o l v e d oxygen was f o l l o w e d d u r i n g t h r e e s e r i e s o f e x p e r i m e n t s ; t h e f i r s t s e r i e s used l i q u i d water  containing  " a t m o s p h e r i c " oxygen (1/5 atmos), t h e second employed oxygen s a t u r a t e d solutions  (1 atmos) and t h e t h i r d a c o n t r o l s o l u t i o n degassed  u s u a l manner.  A 3.6 mm p a t h l e n g t h was used.  i n the  The d a t a from t h e decay  of  e aq i n t h e c o n t r o l s o l u t i o n i n i t i a l l y f i t t e d a 1° o r d e r t r e a t m e n t  to  about 90 nanoseconds and t h e decay i n t h e oxygen s a t u r a t e d s o l u t i o n s  f o l l o w e d t h e same p a t t e r n .  I t i s suggested t h a t pseudo f i r s t  order  k i n e t i c s a r e g i v e n i n t h e l a t t e r case because o f t h e much h i g h e r c o n c e n t r a t i o n o f oxygen i n r e l a t i o n t o t h e h y d r a t e d e l e c t r o n .  The  a t m o s p h e r i c oxygen samples s u p p o r t e d a second o r d e r decay f o r t h e h y d r a t e d  67  control  Oxygen Alcohol Acid Diagram 18. The t i m e t a k e n f o r t h e i n i t i a l a b s o r p t i o n s i g n a l o f e~ aq t o decrease by a f a c t o r o f 2 i n t h e presence o f d i f f e r e n t s o l u t e s .  68  e l e c t r o n which was t o be expected a l t h o u g h t h e c u r v e s on t h e f i r s t o r d e r p l o t s had l o n g l i n e a r p o r t i o n s t o which t h e computer f i t t e d a r a t e constant.  Data up t o 120 nanoseconds used f o r t h e second o r d e r p l o t s .  i  "I k^ s e c C o n t r o l sample  8.75  6  7 (1.03 * .04 x 10 ) 7 1.28 .01 x 10  atomospheric C>  2  saturated 0  .47 x 1 0  ±  k  ;  ±  2  2  M sec  8.43 ± .25 x 10 1.12 ± .25 x 10 1.45 * .35 x 10  The e f f e c t s o f t h e s e s o l u t e s a r e compared i n diagram 18. The p i n h o l e experiments In t h e s e experiments a s t u d y was made o f t h e d i f f e r e n t  initial  o p t i c a l d e n s i t i e s and r a t e c o n s t a n t s observed f o r t h e e aq decay a t v a r y i n g p o s i t i o n s across the width o f the r a d i a t i o n tube.  Intuitively  one would expect a h i g h e r c o n c e n t r a t i o n o f r e a c t i n g s p e c i e s i n t h e side o f the c e l l  t h r o u g h which t h e e l e c t r o n beam f i r s t p e n e t r a t e s as  i t i s here t h a t t h e m a j o r i t y o f secondary i o n i s a t i o n s w i l l o c c u r . p i n h o l e r e s t r i c t o r p l a t e has a l r e a d y been d e s c r i b e d and t r i a l  The  experiments  i n d i c a t e d t h a t p i n h o l e no. 2 (0.0225 thouO was t h e most s a t i s f a c t o r y compromise between an adequate i n t e n s i t y f o r t h e emerging  l a s e r beam  and t h e r a t i o o f t h e d i a m e t e r o f t h e p i n h o l e t o t h a t o f t h e beam.  The  p o s i t i o n s were r e c o r d e d as n e a r , c e n t r e and f a r (see diagram 1 9 ) .  Great  c a r e was t a k e n t o scan o n l y t h e main p o r t i o n and a v o i d t h e d i f f r a c t e d and s c a t t e r e d p e r i p h e r y o f t h e l a s e r beam.  Attempts t o reproduce these  p o s i t i o n s on d i f f e r e n t o c c a s i o n s by v i s u a l s e t t i n g s ( w i t h t h e a i d o f the  f i n e l a t e r a l adjustment on t h e base o f t h e p l a t e ) were r e a s o n a b l y  s u c c e s s f u l as i n d i c a t e d by t h e good agreement between t h e s e r i e s o f experiments.  69  The d a t a i s t a b u l a t e d below.  I n t h e near p o s i t i o n about 86%  of t h e l i g h t was i n i t i a l l y absorbed, about 66% i n c e n t r e and o n l y about 43% a t t h e f a r s i d e .  These a r e average p e r c e n t a g e s o v e r a l l e x p e r i -  ments and do n o t v a r y more t h a n ±5%.  The&I<_Q i s t h e decrease i n t h e  a b s o r p t i o n s i g n a l a f t e r 50 nanoseconds e x p r e s s e d as a p e r c e n t a g e o f t h e i n i t i a l absorption s i g n a l ; although the l a t t e r v a r i e s considerably a c r o s s t h e tube ( i n d i c a t i n g a s u b s t a n t i a l c o n c e n t r a t i o n g r a d i e n t ) t h i s v a l u e does n o t .  D e s p i t e t h e c o n c e n t r a t i o n g r a d i e n t i t appears as  though t h e r a t e o f l o s s o f t h e a b s o r b i n g s p e c i e s i s n o t v e r y d i f f e r e n t a c r o s s t h e tube which i s perhaps a q u a l i t a t i v e statement o f f i r s t  order  behaviour.  T a b l e IV Position  I n i t i a l O.D. % I , , O.D. a t 90 TNsecs.  A V  A I _ . k,sec 5 0  -1  -1 -1 k„M' sec  1  2  NEAR  .89 ± .15  .34  86%  21%  7 1.01 ± .04 x 10  7.02  CENTRE  .47 ± .06  .23  66%  28%  8.73 ± 1.3 x 1 0  8.03 ± 1.7 x 1 0  pAR  .25  WHOLE BEAM  .95  ±  .04 ,-•  .16 .32  43% 85%  23% poor f i t 25%  6.10  6  6  ±.03 x 10  1.36 ± .12 x 1 0 7.10  10  A d e t a i l e d e x a m i n a t i o n o f f i r s t and second o r d e r p l o t s  (with  p o i n t s every 5 nanoseconds) o f a l l t h e d a t a p r o v i d e d no obvious c o n c l u s i o n s c o n c e r n i n g t h e k i n e t i c b e h a v i o u r o f e aq i n t h e s e s p e c i f i c areas. In t h e near p o s i t i o n s t h e f i r s t o r d e r p l o t s were c o n s i s t e n t l y good f o r about 40 nanoseconds a f t e r t h e p u l s e and t h e n t h e y t a i l e d o f f ;  10 1 0  1 1  70  at  approximately  t h e same time t h e second o r d e r p l o t s became l i n e a r  and c o n t i n u e d so  f o r the emainder o f the absorption s i g n a l f o l l o w e d , r  which was n o r m a l l y  100 nanoseconds.  s l o p e a d r a m a t i c one.  I n no i n s t a n c e was t h e change i n  ( I n t a k i n g d a t a from o s c i l l o s c o p e t r a c e s  every  f i v e nanoseconds one i s l i m i t e d by t h e d u r a t i o n and n o i s e l e v e l on t h e t r a c e : on photographs c o v e r i n g s e v e r a l hundred nanoseconds r e s o l u t i o n o f l e s s than 10 nanoseconds becomes v e r y u n c e r t a i n ) .  T h i s appears  t o be a r e a l e f f e c t , n o t an a r t e f a c t due t o say some l i m i t a t i o n o f t h e d e t e c t i o n system o r w i d e l y s c a t t e r e d p o i n t s . t a b l e a r e averaged over a l l e x p e r i m e n t s ,  The r a t e s g i v e n i n t h e  and have an a b s o l u t e  value  w i t h i n framework o f t h e p i n h o l e e x p e r i m e n t s b u t a r e i n t h e f i r s t for  comparison o n l y w i t h t h e f u l l beam e x p e r i m e n t s .  the u n i m o l e c u l a r to  The agreement f o r  (now r e f e r e d t o as k^) and b i m o l e c u l a r (now r e f e r r e d  as k^) r a t e c o n s t a n t s were good, and b o t h k^ and k^ r e p r e s e n t  fast reactions. In  instance  very  Treatment ( i ) d i d n o t g i v e any u s e f u l r e s u l t s .  t h e c e n t r e p o s i t i o n t h e graphs were almost as w e l l matched  as those o f t h e n e a r p o s i t i o n s b u t i t was more d i f f i c u l t t o judge t h e slopes.  I t d i d n o t appear t h a t two mechanisms were o c c u r r i n g s i m u l t a n e o u s l y  as treatment  ( i ) gave v e r y p o o r r e s u l t s .  The o n l y c l e a r f a c t was t h a t  the d a t a c o u l d f i t b o t h f i r s t and second o r d e r t r e a t m e n t s and t h i s i n i t s e l f was c o n f u s i n g .  quite  adequately,  A s u b t l e change i n s l o p e appeared  i n some o f second o r d e r p l o t s i n d i c a t i n g l i n e a r i t y above ^35 nanoseconds ( a f t e r t h e p u l s e ) when t h e i n i t i a l a b s o r p t i o n s i g n a l had decreased by some 15%, b u t t o what e x t e n t i f any t h i s i s a change i n s l o p e i s due to  a change i n mechanism i s open t o s p e c u l a t i o n .  The f i r s t o r d e r p l o t s  71  AOD = (OD  far  centre near  ©  0  e  "  i  I  i  J  1  1  1  1  10  1  i  t=9()  )  Diagram 19. The f i r s t o r d e r decay o f e aq i n t h e p i n h o l e e x p e r i m e n t s .  o  T  - OD  t  i  1  1  t  1  i  20 Time (nsecs) 50  1  i  1  r  t  6.0  70  72  were l i n e a r as f a r as they were f o l l o w e d , t o 65 nanoseconds; the reasons for  t h i s s e e m i n g l y s h o r t d a t a - p e r i o d have a l r e a d y been g i v e n .  and k^  e v a l u a t e d from these s l o p e s  (k^  The  a f t e r 35 nanoseconds) are  again k^  h i g h but a l t h o u g h the average v a l u e s are s l i g h t l y lower than the k^, ( n e a r ) , i n d i v i d u a l v a l u e s are i n the same range, and w i t h i n e r r o r can be c o n s i d e r e d  experimental  constant.  F i n a l l y the d a t a from the f a r p o s i t i o n was  compared t o t h a t  d i s c u s s e d above and t h i s time g r a p h i c a l a n a l y s i s i n d i c a t e d r a t h e r p o o r f i r s t o r d e r and r e a s o n a b l e mative Iher.e.-'too:.  second o r d e r f i t s .  Treatment ( i ) was  uninfor-  However a n o t h e r c u r i o u s f e a t u r e appeared i n the absorp-  t i o n and decay s i g n a l s o f most o f these experiments.  The  i n i t i a l 25 ±  5 nanoseconds were c h a r a c t e r i s e d by a p l a t e a u a f t e r which the decay o f the a b s o r b i n g s p e c i e was  bimolecular.  In view o f the r e l a t i v e l y weak  a b s o r p t i o n o b s e r v e d i n the f a r p o s i t i o n and thus the p o o r s i g n a l t o noise r a t i o i t i s not at p r e s e n t p o s s i b l e t o d e c i d e u n e q u i v o c a l l y whether the p l a t e a u was  i n f a c t an e x t r e m e l y  slow decay whose s l o p e c o u l d not  be  determined at the l e v e l s o f s e n s i t i v i t y employed, o r whether i t r e p r e s e n t e d a time " e q u i l i b r i u m " i n t h a t f a r r e g i o n o f the r a d i a t i o n c e l l . A g a i n i t i s f e l t t h a t t h i s i s a genuine r e s u l t and not a t e c h n i c a l a r t e f a c t as i t was  r e p r o d u c i b l e on d i f f e r e n t o c c a s i o n s when the components  o f the d e t e c t i o n and m o n i t o r i n g system were i n d i v i d u a l l y checked. second o r d e r r a t e c o n s t a n t was  determined t o be  the o t h e r k^  1.36  f o r the d i s a p p e a r a n c e o f the a b s o r b i n g  The specie  ± .12 x 10^^ which i s s i g n i f i c a n t l y f a s t e r than  values.  In summary the p i n h o l e experiments l e a d t o t h r e e  conclusions.  F i r s t t h a t t h e r e i s a marked change i n o p t i c a l d e n s i t y across the depth  73  of  the i r r a d i a t e d volume o f l i q u i d which i n terms o f t h e c o n c e n t r a t i o n -4  of  the h y d r a t e d e l e c t r o n means about a 70% decrease from 2.9 x 10  at  the n e a r edge t o 8 x 10 ^ M at t h e f a r edge.  M  S e c o n d l y the k i n e t i c s are  complex even i n t h e r e s t r i c t e d r e g i o n s o f the i r r a d i a t i o n l i q u i d t h a t were s t u d i e d ; o n l y i n the f a r r e g i o n do the k i n e t i c s appear t o be second order, while a preference f o r f i r s t order behaviour i s i n d i c a t e d else7 where.  T h i r d l y the r a t e c o n s t a n t s are h i g h , k^ b e i n g o f the o r d e r 10  and k^ v a r y i n g from 0.70  t o 1.36 !x ''lOsecM"} sec  -1 sec  1  The v a r i a t i o n s i n k^ w i l l be averaged i n a f u l l beam s t u d y and t h u s i l l u s t r a t e the hazards o f making d e t a i l e d k i n e t i c  interpretations  on systems w i t h an inhomogeneous d i s t r i b u t i o n s o f r e a c t i n g s p e c i e s . The d e u t e r a t e d e l e c t r o n (i)  The f o r m a t i o n and decay o f the d e u t e r a t e d e l e c t r o n were  a l s o s t u d i e d w i t h the f o l l o w i n g m o d i f i c a t i o n s t o t h e system.  A 15 mwatt  s p e c t r a p h y s i c s model 124 H e l i u m Neon l a s e r r e p l a c e d the lm watt l a s e r and the beam emerging from the window o f the p l e x i c e l l was d i r e c t e d on to  t h e photocathode w i r e s i n the p h o t o m u l t i p l i e r tube now  copper h o u s i n g (and t h e r e f o r e f u l l y exposed t o the beam). of  out o f i t s The a p e r t u r e  the i r i s some 10 cms away from t h e p h o t o m u l t i p l i e r was a d j u s t e d t o remove  a l l the s c a t t e r e d l i g h t around t h e main beam, and a l l the experiments were c a r r i e d out i n a darkened room.  The l e n s was  o m i t t e d from the a l i g n m e n t .  The v e r y h i g h f r e q u e n c y s i g n a l s observed from t h e 1 m watt l a s e r were not u n e x p e c t e d l y p r e s e n t ( i n view o f t h e i r p r o b a b l e o r i g i n ) i n t h i s more p o w e r f u l l a s e r , b u t o p e r a t i n g t h e p h o t o m u l t i p l i e r at 500 v o l t s gave a S:N  r a t i o o f 150:1 under s t e a d y s t a t e c o n d i t i o n s and about 25:1 d u r i n g and  74  immediately  a f t e r the e l e c t r o n pulse.  by t h e l i g h t i n t e n s i t y  The p h o t o m u l t i p l i e r was n o t s a t u r a t e d  ( a l t h o u g h t h e concept o f s h i n i n g such a l a s e r  at a p h o t o m u l t i p l i e r i s n o t t o be t o o r e a d i l y accepted) 'under t h e w o r k i n g conditions. to  The apparent improvement i n t h e S:N r a t i o can be a t t r i b u t e d  t h e lower w o r k i n g v o l t a g e on t h e p h o t o m u l t i p l i e r and t h e i n c r e a s e i n  a v a i l a b l e l i g h t i n t e n s i t y which p e r m i t t e d a h i g h e r a t t e n u a t i o n t o be used i n r e c o r d i n g t h e s i g n a l s . (ii)  U n f o r t u n a t e l y t h e l i m i t e d s u p p l y o f D^O  a v a i l a b l e was i n s u f f i c i e n t f o r d e g a s s i n g previously described. s t e r i l i s e d polythene  immediately  t o t a k e p l a c e i n t h e manner  A 50 ml sample o f D^O (Merck.S§D.) i n a s e a l e d , c o n t a i n e r was v i g o r o u s l y degassed w i t h h e l i u m  supplied  t h r o u g h a f r i t t e r e d g l a s s o v a l and e s c a p i n g v i a a temporary opening. t h i r t y minutes o f s h a k i n g and d e g a s s i n g  After  t h e g a s - l i n e on t h e c o n t a i n e r  was q u i c k l y exchanged f o r a s h o r t f i t t e d n o z z l e .  The l a t t e r s l i p p e d  i n t o t h e f i l l i n g p a r t o f t h e p l e x i c e l l and t h u s t h e 0^0 was i n t r o d u c e d by p r e s s u r i s i n g the container.  When t h e c e l l was f u l l and t h e r e were  no gas b u b b l e s p r e s e n t t h e n o z z l e was removed and b o t h p a r t s s e a l e d . The  oxygen content  o f t h e D2O was c e r t a i n l y l e s s than atmospheric  but g r e a t e r than t h a t e s t i m a t e d  f o r t h e l i q u i d w a t e r experiments.  The  e f f e c t o f oxygen on t h e decay o f e aq has been s t u d i e d and from t h i s i t is  f e l t that the probable  amount o f oxygen p r e s e n t would n o t g r e a t l y e f f e c t  the o b s e r v e d r a t e c o n s t a n t s accurate determinations  f o r t h e decay o f e^, b u t n e v e r t h e l e s s more  can be made.  The p i n h o l e experiment was  w i t h D 0, b u t h e r e t h e p r e s e n c e o f oxygen and r a d i a t i o n p r o d u c t s 2  to  repeated owing  t h e n e c e s s i t y o f r e p e t i t i v e p u l s i n g i n many i n s t a n c e s p e r m i t s o n l y a  76  q u a l i t a t i v e statement (iii)  o f the r e s u l t s .  The decay o f e^ was undoubtedly  first  order f o r the  i n i t i a l 90 nanoseconds a f t e r t h e p u l s e u s i n g the f u l l beam b u t t h e d e v i a t i o n from t h e l i n e a r p l o t was n o t v e r y pronounced even at 120 nanoseconds; second o r d e r t r e a t m e n t  gave no s a t i s f a c t o r y f i t s a l t h o u g h  approximate e s t i m a t e o f t h e b i m o l e c u l a r r a t e c o n s t a n t was as t h e f i t improved. tially  1 1  M  The decay o f e^ a f t e r 2 c o n s e c u t i v e p u l s e s  f o l l o w e d second o r d e r k i n e t i c s .  indicate  10  am  1  sec  1  1  essen-  In n e i t h e r case d i d treatment ( i )  two s i m u l t a n e o u s l y o c c u r r i n g p r o c e s s e s . The p i n h o l e experiments  a g a i n showed a marked c o n c e n t r a t i o n  g r a d i e n t a c r o s s t h e depth o f t h e i r r a d i a t e d volume b u t no attempt w i l l be made t o d i f f e r e n t i a t e between t h e p r o c e s s e s areas f o r reasons but c u r i o u s l y  The A I ^ Q v a l u e s v a r i e d l i t t l e i n these  g i v e n above.  decreased  a f t e r two p u l s e s .  few comparisons a r e made below.  F u l l beam 1 pulse F u l l beam 2 pulses  O.D. -i-67 ±.16  6  7  ± > 1 6  A l  T h i s may be an a r t e f a c t .  A  50  30%  20%  V ^1  s  e  c  ^  8.14 ± .21 x 1 0  (10 ) 7  ^2 6  M  s  e  c  _  1  U  9.88 ± .50 x 1 0  14%  (10 ')  (3.10 )  CENTRE  1.8 ± .1  13%  (10 ) 6  (4.10 )  FAR  .77 ± .06  11%  poor f i t  (6.10 )  6  ^  (10 )*  It= 0  NEAR  areas,  A p a t h l e n g t h o f 0.25 mm was used.  Table Region  o c c u r r i n g i n the d i f f e r e n t  1 0  1 0  1 0  *the d a t a i n b r a c k e t s a r e o n l y i n c l u d e d t o show t h e r e l a t i v e r a t e s t h a t were i n d i c a t e d and i m p l y n o t h i n g more.  1 0  77  In summary the decay o f the d e u t e r a t e d  e l e c t r o n appears 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 f o r about 90 nanoseconds a f t e r the p u l s e then tends towards second o r d e r b e h a v i o u r  as was  and  observed f o r the  hydrated e l e c t r o n .  I n t e r f e r e n c e phenomena The  d e t e c t i o n of appreciable f l u c t u a t i o n s i n high  a f t e r the e l e c t r o n p u l s e induced  a b s o r p t i o n , f o r some w h i l e  intensity prevented  any a c c u r a t e measurements on the r e a l a b s o r p t i o n and decay s i g n a l o f the hydrated  electron.  At t h i s stage the p l e x i c e l l o n l y was  no e l e c t r o n beam r e s t r i c t o r b l o c k o r g r o u n d i n g p l a t e .  b e i n g used w i t h  The f l u c t u a t i o n s  sometimes amounted t o 70% o f t h e . i n i t i a l a b s o r p t i o n , were q u i t e r e p r o d u c i b l e and l a s t e d s e v e r a l m i c r o s e c o n d s . photolysed  The p o s s i b i l i t y t h a t e aq* was  being  i n s i g n i f i c a n t q u a n t i t i e s by the h i g h i n t e n s i t y o f the l a s e r  beam and on r e t u r n i n g t o the ground s t a t e proceeded t o absorb once more c o u l d not be r u l e d o u t .  However i n experiments under i d e n t i c a l c o n d i t i o n s  but w i t h a c i d i f i e d w a t e r (0.1 M H ) +  the e aq a b s o r p t i o n c o m p l e t e l y  disap-  p e a r e d as e x p e c t e d but the i n t e n s i t y f l u c t u a t i o n s remained as s t r o n g c h a r a c t e r i s t i c as i n the n e u t r a l water experiments. r e p r o d u c e d i n diagram (20) i n c l u d i n g one  The  and  t r a c e s are  showing l a s e r i n t e r f e r e n c e .  Other  p o s s i b l e a b s o r b i n g s p e c i e s were a l s o r u l e d out. Because o f the i n t e r f e r i n g but r e p r o d u c i b l e n a t u r e o f f l u c t u a t i o n s a s y s t e m a t i c study was  made t o t r y and d i s c o v e r t h e i r o r i g i n  and t o e l i m i n a t e them ( i f not c o m p l e t e l y ) 500 nanoseconds on the t r a c e s .  these  at l e a s t from the  first  For convenience t h e s e s i g n a l s w i l l  be c a l l e d "waves" as t h i s b e s t d e s c r i b e s t h e i r i r r e g u l a r u n d u l a t i n g  now  No restrictor top-- H 0 bottom: acid Long pulse  plate  2  200 nsec/div.  Beam restrictor and A l block in place Short pulse - h-LO 10mV/div.  T y p i c a l high laser signal  frequency , 20mV/div.  •  TP 100nsec/div. *****  A)  r  uu  10 nsec/div. Diagram 20.  O s c i l l o s c o p e traces o f the Interference  Signals  79  appearance. There were t h r e e p o s s i b l e g e n e r a l causes f o r t h e s e waves (i)  e l e c t r i c a l ; r i n g i n g i n the p h o t o m u l t i p l i e r o r r e f l e c t i o n s  i n the g r o u n d i n g system, space charge e f f e c t s , mismatch o f t r a n s m i s s i o n lines. (ii)  o p t i c a l ; m a l f u n c t i o n o f the l a s e r , sudden  localised  changes i n r e f r a c t i v e i n d e x i n the l i q u i d , s c a t t e r i n g o f l i g h t . (iii)  p r e s s u r e waves; a c r o s s the i r r a d i a t i o n c e l l a f t e r the  impact o f the e l e c t r o n p u l s e , shock wave t h r o u g h the a i r space  between  the e l e c t r o n tube window and t h e c e l l . The r e s u l t s o f the s t u d y w i l l be summarised b r i e f l y ; u l t i m a t e l y i t was o n l y p o s s i b l e t o d e l a y the onset and reduce the magnitude o f the waves, the p r e c i s e n a t u r e o f t h e i r o r i g i n r e m a i n i n g u n c e r t a i n .  In as much as  they were n o t the r e s u l t o f c h e m i c a l a c t i v i t i e s o f e aq and the a b s o r p t i o n and decay s i g n a l s were f i n a l l y r e s o l v e d the p r o b l e m remained a t e c h n i c a l one i n the c o n t e x t o f t h i s r e s e a r c h .  I t would be e x t r e m e l y i n t e r e s t i n g  however t o pursue t h e i r o r i g i n as a p r i m a r y o b j e c t i v e . (i)  electrical:  i n the absence o f the p l e x i c e l l but o t h e r w i s e  i d e n t i c a l c o n d i t i o n s t h e waves c o u l d not be observed.  S e v e r a l gf'ound  l o o p s were d e l i b e r a t e l y i n c o r p o r a t e d t o see i f the f r e q u e n c y o r appearance o f t h e waves c o u l d be a l t e r e d i n the p r e s e n c e o f the p l e x i c e l l , but w i t h negative r e s u l t s .  The p h o t o m u l t i p l i e r was checked f o r s a t u r a t i o n e f f e c t s  and o v e r s h o o t ; t h e waves d i d not reduce o r d i s a p p e a r as the p h o t o m u l t i p l i e r o p e r a t i n g v o l t a g e was i n c r e a s e d and t h e r e f o r e space charges as w e l l were r u l e d out.  I t appeared t h a t t h i s approach would not l e a d t o the o r i g i n  o f the waves.  80  (ii)  optical:  i f the l a s e r were s w i t c h e d o f f (but remained  i n the c i r c u i t ) and t h e acceleratJ>Ja f i r e d under normal  experimental  c o n d i t i o n s the waves d i s a p p e a r e d , t h e r e f o r e no e m i s s i o n was c o n t r i b u t i n g to  t h e i r shape and the v a l i d response  I f the l a s e r was  o f the p h o t o m u l t i p l i e r was  confirmed.  s w i t c h e d on but the p l e x i c e l l emptied o f a l l s o l u t i o n  the waves a g a i n were not observed.  On s u b s t i t u t i n g a 2 mm d i a m e t e r  glass  :iod w i t h p o l i s h e d ends f o r the g l a s s i r r a d i a t i o n tube i n the c e l l an  absor-  p t i o n s i g n a l and permanent change i n o p t i c a l d e n s i t y o c c u r r e d a f t e r 1 ^.second (due to the presence  o f t r a p p e d e l e c t r o n s ) but the h e i g h t and shape o f  the s i g n a l i n d i c a t e d t h a t t h e r e were no waves superimposed d u r i n g t h e i n i t i a l 500 nanoseconds. S c a t t e r i n g o f the l a s e r beam.by d u s t - p a r t i c l e s i n s o l u t i o n o r m i c r o s p l i n t e r s o f g l a s s t h a t may s e v e r a l e l e c t r o n p u l s e s was  be p r e s e n t i n the i r r a d i a t i o n tube a f t e r  u n l i k e l y t o cause such enormous f l u c t u a t i o n s  i n the i n t e n s i t y ; t h e waves remained u n a f f e c t e d by changing  radiation  tubes and u s i n g f r e s h s o l u t i o n s . The waves t h e r e f o r e were a f u n c t i o n o f the l i q u i d and i t seemed p r o b a b l e t h a t t h e y arose from the sudden l o c a l i s e d changes i n r e f r a c t i v e index t h a t occurred immediately i r r a d i a t i o n tube. led  The  a f t e r the p u l s e a l o n g the l e n g t h o f t h e  r a d i a l d i s t r i b u t i o n o f t h e e l e c t r o n beam i t s e l f  t o a f a r h i g h e r c o n c e n t r a t i o n of e l e c t r o n s i n the centre o f the  i r r a d i a t i o n tube i n comparison t o e i t h e r end o f the tube.  However t h i s  c o u l d not be the complete e x p l a n a t i o n as the waves c o n t i n u e d t o appear q u i t e s t r o n g l y f o r tens o f microseconds a f t e r the p u l s e and any i.nhomogeneity would be smoothed out w i t h i n a  microsecond.  81  (iii) powerful  pressure  waves:  t h e impact o f t h e e l e c t r o n beam i s  -- i n t h e 2 MeV F e b e t r o n 30 nanosecond e l e c t r o n p u l s e o f 1.8  MeV e l e c t r o n s h a t t e r s any g l a s s windows on t h e c e l l s and s t a i n l e s s steerl must be used (48.) .  I t would n o t be s u r p r i s i n g t h e r e f o r e t o have a  p r e s s u r e wave r e f l e c t i n g back and f o r t h a c r o s s t h e d i a m e t e r o f t h e i r r a d i a tion cell.  This pressure  wave c o u l d a r i s e from t h e energy t r a n s f e r  between the g l a s s i r r a d i t i o n tube and the l i q u i d a f t e r t h e impact o f t h e e l e c t r o n beam, o r c o u l d be t r a n s m i t t e d from a shock wave moving b e h i n d t h e e l e c t r o n beam (as i t emerges from t h e e l e c t r o n - t u b e window towards t h e g l a s s tube) a c r o s s t h e e n t i r e p l e x i c e l l .  A n o t h e r p o s s i b i l i t y f o r an  i n t e r n a l p r e s s u r e wave has been d e s c r i b e d as ' m i c r o c a v i t a t i o n ' ( 4 9 ) ; i t i s s u g g e s t e d t h a t t h e r e i s a sudden i n c r e a s e i n volume when t h e e l e c t r o n s f i n a l l y become s o l v a t e d (10  1 1  seconds) and t h i s amounts t o p o c k e t  "explosions" i n the l i q u i d . A p r e s s u r e wave w i l l g i v e r i s e t o changes i n r e f r a c t i v e and thus i t i s d i f f i c u l t t o s e p a r a t e  index  t h e two i d e a s e x p e r i m e n t a l l y .  That  t h e r e was a s i g n i f i c a n t impact on t h e r a d i a t i o n tube was shown by f i l l i n g the p l e x i c e l l w i t h water c o n t a i n i n g a v e r y s m a l l q u a n t i t y o f o i l .  The  l a t t e r s e t t l e d t o t h e bottom o f t h e tube and d i d not i n t e r f e r e w i t h the l a s e r beams. recorded  On p u l s i n g t h i s s o l u t i o n t h e same s o r t o f wave s i g n a l s were  w i t h t h e e aq a b s o r p t i o n ; b u t p r o j e c t i n g t h e t r a n s m i t t e d l a s e r  beam on t o a s c r e e n one c o u l d even see t h e changes i n i n t e n s i t y as t h e o i l d r o p l e t s moved i n t o t h e p a t h o f t h e beam.  The same e f f e c t cOuld be  produced by g e n t l y t a p p i n g t h e tube i n t h e p l e x i c e l l t o mix t h e l i q u i d s . Other l i q u i d s w i t h d i f f e r e n t v i s c o s i t i e s and non H-bonded  82  s o l v e n t s were examined i n the p l e x i c e l l and l a t e r r a d i a t i o n tubes o f up t o 1.5  cm i n d i a m e t e r i n i n d i v i d u a l s u p p o r t i n g frameworks used t o c o n t a i n  these  l i q u i d s and i n v e s t i g a t e the p r o p e r t i e s o f the waves ait'some d i s t a n c e  from the f o r m a t i o n o f the h y d r a t e d  electron.  In these areas the waves  were v e r y weak and s t r u c t u r e l e s s s i g n a l s as one might a n t i c i p a t e from a d i s s i p a t e d p r e s s u r e wave, but i n the l a r g e r tubes the problem o f space charges c o u l d no l o n g e r be i g n o r e d . amplitude  and frequency (iv)  summary:  In s o l v e n t s o f h i g h v i s c o s i t y  the  o f the waves were c o n s i d e r a b l y reduced. on the assumption t h a t these waves were phenomenon  a r i s i n g from sudden ( i n t e r n a l o r e x t e r n a l ) p r e s s u r e o r  concentration  changes i n the l i q u i d i n the c e l l attempts were made t o reduce the a i r space between the p l e x i c e l l and the e l e c t r o n - t u b e window and to the e l e c t r o n beam.  The  collimate  most s a t i s f a c t o r y t e c h n i c a l arrangements have  been d e s c r i b e d i n the e x p e r i m e n t a l  s e c t i o n a l t h o u g h the b e g i n n i n g o f a  much weaker wave s i g n a l c o u l d s t i l l be observed a f t e r about 600  nanoseconds.  83  D i s c u s s i o n and (i)  I n t e r p r e t a t i o n o f the  Results  The b e h a v i o u r o f e aq at h i g h dose r a t e s The  aim o f t h i s r e s e a r c h was  to provide  a f i r m b a s i s f o r the  s t u d y c o f e aq* and t h e r e f o r e i n v e s t i g a t e d the decay o f e aq produced i n high concentrations  by an e l e c t r o n p u l s e , the i n t e n s i t y o f which gave  2 dose r a t e s ^10 The  above those used i n p r e v i o u s i n v e s t i g a t i o n s . d i s a p p e a r a n c e o f e aq o v e r a p e r i o d o f about 70 t o  80  nanoseconds a f t e r the 3 nanosecond e l e c t r o n p u l s e has been shown t o f o l l o w a f i r s t o r d e r decay; t h e r e i s s u b s e q u e n t l y a slow t r a n s i t i o n towards b i m o l e c u l a r b e h a v i o u r and t h i s appears t o be f i r m l y e s t a b l i s h e d - i n about 100 t o 110 nanoseconds a f t e r the p u l s e .  I t i s worth n o t i n g that t h i s  t r a n s i t i o n i s . c o n t r a r y t o normal c l a s s i c a l homogeneous k i n e t i c s where a t r a n s i t i o n from l r \ d -»- Isd.  o r d e r would be e x p e c t e d as the  o f the r e a c t i v e s p e c i e s d e c r e a s e s . for  The  concentration  s e c o n d - o r d e r decay can be  followed  s e v e r a l hundred nanoseconds a f t e r the p u l s e a l t h o u g h ' b y t h i s s t a g e  the r e a c t i o n s are not e x c l u s i v e l y due  t o the c o m b i n a t i o n o f two  hydrated  e l e c t r o n s but g e n e r a l l y t o r e a c t i o n s w i t h the p r o d u c t s o f r a d i o l y s i s . The the system was  p i n h o l e experiments however demonstrated v e r y c l e a r l y t h a t not homogeneous d u r i n g the f i r s t 90 nanoseconds  t h a t even i n l o c a l i s e d r e g i o n s o f the i r r a d i a t e d volume the  and  kinetics  are n e i t h e r f i r s t n o r second o r d e r i n the c l a s s i c a l sense but are more complex. The  p a t t e r n o f b e h a v i o u r observed f o r the d e u t e r a t e d  electron  f o l l o w e d the same time sequence and a g a i n the p i n h o l e e x p e r i m e n t s demons t r a t e d t h a t the d i s t r i b u t i o n o f a b s o r b i n g  s p e c i e s was  inhomogeneous f o r  84  a s i g n i f i c a n t i n t e r v a l a f t e r the p u l s e . The  first  and second o r d e r r a t e c o n s t a n t s e v a l u a t e d a t a p p r o p r i a t e  times a f t e r the p u l s e are t a b u l a t e d below t o g e t h e r w i t h some second o r d e r r a t e c o n s t a n t s p u b l i s h e d f o r these r e a c t i o n s . Table e" aq  y ?  e" aq + e" aq  y H  2  VII, .'  + 20H"  k' = 8.80  ± .8 x 1 0  k  = 5.88  ± 1.2  = 1.22  ± .1 x 10  2  = 3.2 ed i d + ed  x 10  M"  U  M  1 0  10  M  ± .21 x 1 0  y D„ + 20D" 2  k, = ^lO  Nfsec" *  6  sec" *'  _1  1  -1  sec"  1  k" = 8.14  1.20  1  x 10  y ?  11  sec" *  6  sec  -1**  (46)  1  sec" * 1  1  ± .1 x 1 0  M  1 0  sec"  1  1  (30)  * t h i s work **average v a l u e from s e v e r a l l a b o r a t o r i e s :  see t a b l e I I  The b i m o l e c u l a r r a t e c o n s t a n t determined .in t h i s work i s a factor  o f ^5 h i g h e r than t h a t p r e v i o u s l y r e p o r t e d and a f a c t o r  lower than K l e i n § Warner's r e s u l t s ; a f i g u r e  approaching  o f ^5  t h e i r value  was  e s t i m a t e d f o r the decay i n the f a r r e g i o n s o f the i r r a d i a t i o n tube where the d i s t r i b u t i o n was tions  certainly  inhomogeneous and  (as l a t e r  show) would remain so f o r s e v e r a l hundred nanoseconds.  <have made the p r i o r assumption o f homogeneous k i n e t i c s  calcula-  These  authors  i n t h e i r system  -8 on the b a s i s o f t h e mean l i f e t i m e o f a spur b e i n g ^10  seconds ( 2 ) .  I t would appear t h a t the changes i n o p t i c a l d e n s i t y t h e y i n i t i a l l y were averages o f q u i t e v a r i e d changes a c r o s s t h e i r r a d i a t i o n t o t h e inhomogeneous d i s t r i b u t i o n  o f e aq.  cell  recorded due  85  In c o n t r a s t , the b e h a v i o u r  o f e aq i n the presence o f  s p e c i e s d i d not show any u n p r e d i c t a b l e t r e n d s . i n the system was  scavenging  The presence o f e aq  c o n f i r m e d by p u l s i n g an a c i d i f i e d aqueous s o l u t i o n i n  which no a b s o r p t i o n at 6328 A was  detected.  Under t h e s e c o n d i t i o n s the  h y d r a t e d e l e c t r o n i s c o n v e r t e d t o a hydrogen atom too q u i c k l y f o r e aq to be o b s e r v e d ,  w h i l e at a s l i g h t l y h i g h e r pH t h e a b s o r p t i o n c o u l d be  observed v e r y b r i e f l y b e f o r e t h i s c o n v e r s i o n was a l c o h o l s o l u t i o n s the e x p e r i m e n t a l h a l f l i f e  complete.  In i s o p r o p y l  (time t a k e n f o r the  initial  a b s o r p t i o n s i g n a l t o f a l l t o h a l f i t s v a l u e ) i n c r e a s e d as the a l c o h o l removed H and OH r a d i c a l s b e f o r e they c o u l d r e a c t w i t h e aq and its  concentration.  ambiguous.  The  The  deplete  decay was b i m o l e c u l a r a l t h o u g h e a r l y stages were  e f f e c t o f d i s s o l v e d oxygen i n the system was  t o decrease  the h a l f - l i f e o f e aq but even i n t h e oxygen s a t u r a t e d s o l u t i o n s , the e aq would have t o move from the spur i n t o the b u l k medium b e f o r e r e a c t i n g w i t h the oxygen, as the s o l u t e c o n c e n t r a t i o n i s s t i l l The the f i r s t  o n l y miHi>molar.  fundamental problem t h a t has been r a i s e d by the events  100 nanoseconds o r so a f t e r the e l e c t r o n p u l s e i s one  of  inhomogeneity, and one t h a t i n v a l i d a t e s the i n t e r p r e t a t i o n o f the from the c l a s s i c a l k i n e t i c v i e w p o i n t .  In o r d e r t o c a l c u l a t e the  of  data time  l a p s e between the end o f the e l e c t r o n p u l s e and the o v e r l a p o f the spurs, t h a t i s homogeneous d i s t r i b u t i o n o f the h y d r a t e d c a l c u l a t i o n s were made.  e l e c t r o n , the f o l l o w i n g  86  (ii)  A model r e l a t i n g t o t h e d i s t r i b u t i o n o f t h e spurs i n space and time Assume a s i m p l e p h y s i c a l model o f the i r r a d i a t e d volume o f  l i q u i d i m m e d i a t e l y a f t e r t h e p u l s e , w i t h s p l ^ s randomly d i s t r i b u t e d i n s m a l l l o c a l i s e d a r e a s , b u t a marked degree o f rinhomo.igeneity a l o n g t h e c r o s s s e c t i o n o f t h e volume through which t h e l a s e r beam i s p a s s i n g . U t i l i s i n g t h e d a t a from t h e p i n h o l e experiments  (see T a b l e IV) t h e o p t i c a l  d e n s i t y r e c o r d e d a t t = 0 nanoseconds i s t a k e n as p r o p o r t i o n a l t o t h e concentration o f the absorbing specie i n a p a r t i c u l a r region o f the i r r a d i a t e d volume and from t h i s c o n c e n t r a t i o n t h e energy d e p o s i t e d i n t h a t r e g i o n can be e s t i m a t e d .  From t h e l a t t e r , t h e number o f spurs i s  c a l c u l a b l e and t h e r e f o r e t h e mean volume space o c c u p i e d by a spur. o  The i n i t i a l radius* o f a spur a f t e r t h e p u l s e i s b e l i e v e d t o be^20 A (12) and i f one c o n s i d e r s t h i s s p u r t o be c e n t r a l l y l o c a t e d i n t h e mean volume space by r e g a r d i n g t h e two volumes as c o n c e n t r i c spheres t h e n i t i s p o s s i b l e t o determine  t h e d i s t a n c e between t h e s u r f a c e o f t h e s p u r and t h e l i m i t s  o f the volume i n which t h e s p u r must be s t a t i s t i c a l l y p r e s e n t .  The time  t a k e n f o r t h e s p u r t o d i f f u s e t o t h e l i m i t s o f t h e mean volume space i s then c a l c u l a t e d .  As a l l t h e spheres r e p r e s e n t i n g t h e mean volume space  are i n t h r e e d i m e n s i o n a l c o n t a c t w i t h n e i g h b o u r i n g volumes t h e time taken f o r t h i s d i f f u s i o n i s t h e time l a p s e b e f o r e s p u r o v e r l a p c r e a t e s a homogeneous d i s t r i b u t i o n o f h y d r a t e d e l e c t r o n s and r a d i c a l s .  The a p p r o x i -  mations n e c e s s a r y a r e t h a t t h e s p u r i s s p h e r i c a l and d i f f u s e s w i t h s p h e r i c a l geometry and t h a t t h e volume o f l i q u i d can be r e p r e s e n t e d as an a r e a o f c l o s e packed spheres i n each o f which t h e r e i s u n i t p r o b a b i l i t y o f f i n d i n g a spur.  There i s a p p r o x i m a t e l y one t h i r d f r e e volume space n o t accounted  87  f o r by l a t t e r assumption and so t h i s was i n c o r p o r a t e d i n t o another set  o f c a l c u l a t i o n s t o g i v e an upper l i m i t t o the r e s u l t s ; the u l t i m a t e  d i f f e r e n c e i n the time n e c e s s a r y f o r o v e r l a p was not v e r y l a r g e , ^10%.  (iii) The  C a l c u l a t i o n s and the r e s u l t s  f o l l o w i n g c o n s t a n t s were used:  Ge aq  =  2.6  1 spur  =, 100  eV  e  = 1.2  x  10  4  £, Path l e n g t h ( f o r t h i s experiment) 2.6 No  mm  = 6.025 x 1 0 . _ , -5 2 -1 = 4.7 x 10 cm sec 2 3  „ D— aq e The c o n c e n t r a t i o n o f e aq i n a s p e c i f i c r e g i o n i s 0D_ n  n  _  ei  I f a d e p o s i t i o n o f 100 eV g i v e s r i s e t o 2.6 m o l e c u l e s o f e aq, then the t o t a l energy d e p o s i t e d p e r mL t h a t a r e a i s E. E ( e V m L ) = 100 eV  .  _1  %  [e aq]  aq  10  . No  3  The numbers o f s p u r s , C^ i s g i v e n by C  = E_ 100  S  spurs mL  1  The volume o c c u p i e d by 1 s p u r i s 3 1  cm  = t o a sphere o f r a d i u s r .  c~  S  s  r  = (.3 S  ) 4.H.C  A s p u r has r a d i u s r  Q  1  /  3  x 10  8  A  s at time t ; the r a d i a l d i f f e r e n c e between the s p u r Q  and volume o f l i q u i d i t o c c u p i e s i n t h i s r e g i o n at t  Q  i s (f- - r ) . s  88  The time t t a k e n f o r a s p u r t o d i f f u s e t h a t d i s t a n c e (r  1/2 (Dt)  - r ) = s o  1 / Z  The r e s u l t s o f t h e c a l c u l a t i o n s p e r t a i n i n g t o t h r e e d i f f e r e n t areas i n t h e i r r a d i a t e d l i q u i d u s i n g d a t a from t h e p i n h o l e experiments are t a b u l a t e d below.  The upper l i m i t t * on t h e time i s t h a t  calculated  w i t h t h e a d d i t i o n o f (:I ) t o t h e mean volume space a v a i l a b l e . 3C s  Similar  c a l c u l a t i o n s u s i n g t h e o p t i c a l d e n s i t i e s r e c o r d e d f o r t h e whole o f t h e a r e a o f t h e i r r a d i a t e d l i q u i d l e d t o o v e r l a p times r a n g i n g from 30 t o T a b l e VI t Nsecs  t*  1 6  153  39  43  3.46 x 1 0  1 6  190  62  76  1.84 x 1 0  1 6  558  503  538  [e aq]M  NEAR  2.85 x 1 0 ~  4  6.57 x 1 0  CENTRE  1.50 x 1 0 ~  4  FAR  8.0 x 1 0 "  90 nanoseconds.  C  5  o  r A s  Region  s  ml"  1  Nsecs  However i n these i n s t a n c e s one i s w o r k i n g at an average  s i t u a t i o n and t h e time t a k e n f o r t h e a c t u a l d i s t r i b u t i o n o f t h e spurs t o be t r u l y homogeneous may be q u i t e d i f f e r e n t .  The r e s u l t s o f t h e c a l c u l a -  t i o n s are shown p i c t o r i a l l y i n diagram 21. These c a l c u l a t i o n s show t h a t about 50% o f t h e energy d e p o s i t e d i n spurs (not a l l t h e energy i s t r a n s f e r r e d t o t h e medium v i a s p u r s ) may be found w i t h i n a v e r y l o c a l i s e d r e g i o n where t h e e l e c t r o n p u l s e f i r s t p e n e t r a t e s , some 30% a t a g r e a t e r d i s t a n c e away and about 1.5 mm away from t h e dominant s i t e o f i o n i s a t i o n and e x c i t a t i o n about 15%.  These are  o n l y g e n e r a l f i g u r e s b u t t h e l a s t one may e x p l a i n t h e appearance o f \ a p l a t e a u on t h e a b s o r p t i o n and decay t r a c e s t a k e n i n t h e f a r r e g i o n o f the tube.  The spurs here are so i s o l a t e d t h a t t h e e aq cannot  react  89  0 0 0 o  normalised d i s t r i b u t i o n of spurs ( i m m e d i a t e l y a f t e r t h e e l e c t r o n pulse) i n the l a s e r beam.  o o O  Q  o  0 o  D i f f u s i o n o f spurs i n t o b u l k volume i n 40 nanoseconds s t i l l l e a v e s r e g i o n s o f inhomogeneity at f a r s i d e o f r a d i a t i o n c e l l .  near • centre • far Diagram 21.  P i c t o r a l R e p r e s e n t a t i o n o f t h e spur o v e r l a p calculations.  9(X>  except (assuming the b i m o l e c u l a r o r r a d i c a l i n the same s p u r .  decay w i t h i t s e l f ) w i t h another e aq  As the s p u r i s d i f f u s i n g i n time the  p r o b a b i l i t y of i n t r a spur r e a c t i o n s t e a d i l y decreases, although random d i s t r i b u t i o n o f spurs may  the  give r i s e to a greater extent  of  r e a c t i o n t h a n i s p r e d i c t e d by assuming r e g u l a r i n t e r s p u r d i s t a n c e s . I t was  stated i n part I I I that d e t a i l e d graphical analysis  showed c o n s i s t e n t l y good f i r s t o r d e r p l o t s f o r the decay o f e aq i n the n e a r r e g i o n up t o 40 nanoseconds and  a f t e r t h a t time the second  p l o t s became l i n e a r w i t h i n the margin o f e x p e r i m e n t a l  error.  Even f o r t u i t o u s  r e s u l t s are e n c o u r a g i n g as the t i m e c a l c u l a t e d f o r the spurs t o i n t h i s r e g i o n was tabulated before the f i r s t  39 t o 43 Nsecs.  The  o r d e r decay f o r the c e n t r e p o s i t i o n was  l i n e a r up t o 65 nanoseconds  at t h i s t i m e ; thus i t would not  be p o s s i b l e t o see a d e f i n i t e change in::slope as i n s u f f i c i e n t available.  is s t i l l  The  stated that  In the l i g h t o f these c a l c u l a t i o n s the  spurs were i n the p r o c e s s o f o v e r l a p p i n g  was  overlap  d a t a had been s c r u t i n i s e d and  t h e s e c a l c u l a t i o n s were p e r f o r m e d and i t was  the l e n g t h o f the d a t a p e r i o d .  order  data  " s u b t l e change" i n the s l o p e around 35 nanoseconds  open t o s p e c u l a t i o n . S i m i l a r l y i t i s not s u r p r i s i n g t h a t the graphs from the  r e g i o n were s l i g h t l y d i f f e r e n t i n r e l a t i o n t o these o t h e r s , and 11 very f a s t bimolecular  r a t e constant  (>10  -1 M  far  the  -1 sec  ) must be  t o be an u n r e a l i s t i c e v a l u a t i o n because i n c o n v e r t i n g o p t i c a l  considered density  t o c o n c e n t r a t i o n homogeneity has been assumed. I n c o n c l u s i o n t h e s e c a l c u l a t i o n s ( i n s p i t e o f the  reasonable  arguments a g a i n s t the a p p r o x i m a t i o n s but the l a c k o f e q u a l l y r e a s o n a b l e  91  a l t e r n a t i v e s ) appear t o be q u a l i t a t i v e l y s u c c e s s f u l i n e x p l a i n i n g t h e inhomogeneity  observed i n r e g i o n s o f t h e i r r a d i a t e d volume and i n  p r o v i d i n g an a c c e p t a b l e b a s i s f o r t h e change i n s l o p e t h a t appears i n a l l t h e f i r s t o r d e r p l o t s f o r t h e decay o f t h e h y d r a t e d e l e c t r o n , (iv)  The b i m o l e c u l a r decay A r a t e c o n s t a n t o f (5.88 i l ^ l O ^ M '''sec 1  for  a second  has been  1  determined  o r d e r decay a f t e r t h e i n i t i a l 50 nanoseconds  .  w i l l be s i g n i f i c a n t amounts o f o t h e r r a d i c a l s and i o n s a t t h i s  There  stage  and so t h e decay w i l l n o t be s o l e l y due t o e~ aq + e aq but w i l l  ^  >- H  ^2 —>  OH  + 20H" ^  2  = 9.10 M 9  sec"  _ 1  1  also include — e»aq + OH — + e aq + H^O  *—•  K  aq  H + OH  ^  k  = 3.10 3  10  M  = 2.32.10  -1  -1 sec  1 0 - 1 - 1 M sec  and c o n t r i b u t i o n s from OH w i l l decrease r a p i d l y due t o OH + OH  —  »•  H 0 2  k  2  = 4 x 10 M 9  4  _ 1  sec"  1  As t h e G-values f o r a l l t h e s e o t h e r s p e c i e s a r e a p p r o x i m a t e l y t h e same one can assume t h a t t h e i r i n i t i a l c o n c e n t r a t i o n s a r e t h e same and t h e e x p r e s s i o n f o r t h e l o s s o f e aq -d(e~ aq) dt  = k [e a q ] + k [e aq] [OH]  + k [e aq] [H 0 ] * s  2  +  can be s i m p l i f i e d t o = (k _ + ]  k  2  + k ) 3  [e~ a q ]  2  By a d d i n g t h e v a l u e s f o r t h e r a t e c o n s t a n t s one has a composite r a t e c o n s t a n t f o r t h e b i m o l e c u l a r decay o f G ^ . I O ^ M 1  e x p e r i m e n t a l l y determined  v a l u e o f S.88.10 ^ M 1  1  sec  1  1  sec  1  and t h e a v e r a g i n g  i s i n good agreement.  A l h t o u g h n o t as much i n f o r m a t i o n i s a v a i l a b l e f o r e^, t h e same e x p l a n a t i o n can be o f f e r e d t o e x p l a i n t h e b i m o l e c u l a r r a t e c o n s t a n t t o ' v l O M  _ 1  sec  - 1  as i n a l l o f i t s r e a c t i o n s s t u d i e d (except f o r e^ + D 0) e^ has  r a t e c o n s t a n t s v e r y s i m i l a r t o "e aq.  2  11  92  (v)  F i r s t o r d e r decay Although  the k i n e t i c s i t u a t i o n i s f a r from i d e a l , t h e r e must .  be more t h a n a c a s u a l sequence o f events t h a t g i v e s r i s e t o the c o n s i s t e n t l y observed  f i r s t o r d e r decay.  And  i f t h e r e were the events would  be t a k i n g p l a c e i n the spurs themselves which i m m e d i a t e l y  t r a n s f e r s the  s i t u a t i o n i n t o an environment about which l i t t l e i s known and much i s discussed.  (Even the second o r d e r p r o c e s s might be the o v e r a l l p i c t u r e  o f two f i r s t o r d e r p r o c e s s e s  though against a l l p u b l i s h e d work t h i s i s  doubtful). T h e r e f o r e two p o s s i b l e mechanisms t h a t c o u l d g i v e r i s e t o f i r s t o r d e r decay i n the c l a s s i c a l sense w i l l be o u t l i n e d b r i e f l y . The  time c a l c u l a t e d f o r o v e r l a p i s an average parameter  because i n the r e a l case t h e r e may  a t any time be a s i g n i f i c a n t number  o f spurs o v e r l a p p i n g due t o the p a r t i t i o n o f the energy o f the i n c i d e n t e l e c t r o n i n t o b l o b s , s h o r t t r a c k s and s p u r s .  primary  For a 0.5  MeV  e l e c t r o n about 64% o f the p r i m a r y energy f i n d s i t s e l f i n i s o l a t e d but t h e r e are s u b s t a n t i a l c o n t r i b u t i o n s from b l o b s (24%) .  The  spurs  (12%) and s h o r t t r a c k s  c h e m i c a l e f f e c t s t h a t t a k e p l a c e i n these areas o c c u r i n  i s o l a t i o n a l t h o u g h , f o r example, a t the end o f a c y l i n d r i c a l t r a c k t h e r e may  be q u i t e a h i g h l o c a l c o n c e n t r a t i o n o f r a d i c a l s p e c i e s .  These s p e c i e s are s t i l l  t e m p o r a r i l y i s o l a t e d from the o t h e r  o c c u r r i n g i n the medium, and may spur.  The  events  r e a c t w i t h each o t h e r w i t h i n the  r a d i c a l s p e c i e s thus d i s t r i b u t e d i n the l i q u i d  the e l e c t r o n p u l s e may  d i f f u s e t o g e t h e r t o form  a t r a n s i e n t i n t e r m e d i a t e whose o c c u r r e n c e  after  - 'encounter-paiis',  depends on the p r o x i m i t y o f  93  the s p e c i e s species  and t h e encounter t i m e .  The r a t e o f d i s a p p e a r a n c e o f t h i s  depends on two f a c t o r s ; t h e s e a r e ( i ) t h e f r e q u e n c y o f p a i r  e n c o u n t e r w i t h i n t h e s p u r and ( i i ) t h e number o f p a i r s , t h e r e f o r e t h e number o f spurs i n t h e p a t h o f t h e l a s e r beam.  T h i s p r o b a b i l i t y o f an  event t a k i n g p l a c e would g i v e r i s e t o an o v e r a l l f i r s t  order.  Suppose f o r i n s t a n c e t h a t t h e r e were a c e r t a i n p r o b a b i l i t y o f two h y d r a t e d e l e c t r o n s  f o r m i n g such a t r a n s i e n t i n t e r m e d i a t e  e i t h e r r e l a x i n t o the free species f a m i l i a r r a d i o l y t i c products.  which  again or react q u i c k l y to give the  I n t h e l a t t e r case t h e c o n c e n t r a t i o n  the h y d r a t e d e l e c t r o n would be d e c r e a s i n g o f the intermediate.  of  at a r a t e p r o p o r t i o n a l t o the k  rate o f formation  could  Perhaps e aq + e aq  l (e)  N v k  - l  2aq 2  i s e s t a b l i s h e d and t h e p r o b a b i l i t y o f f u r t h e r r e a c t i o n i n t h e spur i s zero  order; (e) "aq 2  ^  >H  g  + 20H~ aq  o r , i f t h e e n c o u n t e r - p a i r r e l a x e s t o g i v e two h y d r a t e d e l e c t r o n s , 2e~ aq But  — •  H  2  + 20H~ aq.  as t h e number o f spurs i s r e f l e c t e d i n t h e o p t i c a l d e n s i t y measurements,  the r a t e o f l o s s o f e aq as measured e x p e r i m e n t a l l y  by AOD w i l l dt  give  rise  t o a f i r s t o r d e r decay. The d i m i n i s h i n g c o n c e n t r a t i o n  o f e aq and t h e r a p i d l y expanding  spur (due t o t h e d i f f u s i o n o f s p e c i e s w i t h i n t h e spur) l i m i t t h e time up t o which t h e s e i n t r a s p u r encounters have any s i g n i f i c a n c e .  When t h e spurs  have o v e r l a p p e d then t h e dominant decay i s through i n t e r s p u r mechanisms. — 2I f an e q u i l i b r i u m between e aq and t h e ( e ) d i d e x i s t then the k term may be l a r g e enough t o observe a t r u e e q u i l i b r i u m e x p e r i m e n t a l l y 2  when t h e number o f e n c o u n t e r p a i r s ( o r spurs) i s r e l a t i v e l y s m a l l , and  94  t h i s i s p o s s i b l y a n o t h e r r e a s o n b e h i n d t h e p l a t e a u observed i n t h e f a r r e g i o n i n the p i n h o l e experiments. There a r e many c o n s i d e r a t i o n s t o an approach o f t h i s k i n d t h e Thermodynamics o f t h e r e a c t i o n sequence and t h e magnitude o f t h e force f i e l d s i n the l i q u i d  (which can produce a p p r e c i a b l e changes i n t h e  s t r u c t u r e i n t h e l i q u i d ) b e i n g two o f b a s i c  importance.  Whereas above t h e f o r m a t i o n o f an e n c o u n t e r - p a i r was proposed, an a l t e r n a t i v e i s t h e r a p i d f o r m a t i o n i n t h e spur o f a c o r r e l a t e d e aq + H^O* H 0 . e aq  » H^0  +  +  3  f  HO  ion-pair,  e aq + H  which t h e n b r e a k e s up t o g i v e a hydrogen atom and w a t e r .  I f the only  r e a s o n f o r t h e decay o f t h e s p e c i e s i n t h e spur i s t h r o u g h such a mechanism (analogous t o e l e c t r o n - h o l e p a i r s ) t h e n t h e s e p a r a t i o n and coulombic a t t r a c t i o n s between t h e i o n s i s o f c r i t i c a l i m p o r t a n c e .  The i o n i n  d i f f u s i n g away from i t s o r i g i n w i l l see a time dependent coulombic  field  due t o t h e s p a t i a l d i s t r i b u t i o n o f t h e o t h e r i o n s i n t h e s p u r , and i f t h i s energy i s comparable  t o t h e r m a l e n e r g i e s i t i s u n l i k e l y such i o n  p a i r combinations could occur. are  [ S i m i l a r menomer and dimer s p e c i e s t h a t  e s s e n t i a l l y i o n - p a i r s have been p o s t u l a t e d f o r e l e c t r o n s i n  ammonia ( 5 0 ) ] .  The c o n t r o v e r s i e s a s s o c i a t e d w i t h t h e r e c a p t u r e o f  e l e c t r o n s a r e fundamental problems  i n t h i s f i e l d o f s t u d y (10,11).  Work p u b l i s h e d on t h e r e c o m b i n a t i o n o f t r a p p e d e l e c t r o n s i n g l a s s e s and f r o z e n m a t r i c e s has a l s o shown f i r s t o r d e r decay b e h a v i o u r and s u g g e s t i o n s have been made t h a t t h e d i s a p p e a r a n c e o f t h e e l e c t r o n i s through geminate r e c o m b i n a t i o n w i t h t h e p o s i t i v e i o n s (14) .  According  95  to  t h e d i s t r i b u t i o n o f t h e i o n s b o t h f i r s t and second o r d e r decays  c o u l d be observed dependent.  and t h e r e c o m b i n a t i o n times were markedly  S i m i l a r work (42,43) showed a temperature  temperature  dependent f i r s t  g o i n g i n t o o v e r a l l second o r d e r decay i n g l a s s e s as t h e temperature and t h e e l e c t r o n s became f r e e t o d i f f u s e .  order  rose  T h i s was t a k e n as evidence f o r  different kind of traps. R e c e n t l y (51) t h e e l e c t r o n - h o l e p a i r has a g a i n been d i s c u s s e d i n terms o f an e x c i t o n v i z [(H^O) ''''(H^O) ] , two o f which u l t i m a t e l y +  g i v e r i s e t o m o l e c u l a r p r o d u c t s o f hydrogen, water and hydrogen p e r o x i d e . A l t h o u g h t h e p r o p o s a l i n t h e main r e l a t e s t o e x p e r i m e n t a l r e s u l t s i n i c e , Weiss r e g a r d s t h e s i t u a t i o n i n p u l s e r a d i o l y s e d water i n t h e nanosecond r e g i o n as analogous, (vi)  Epilogue As a b a s i s f o r s t u d y on t h e n a t u r e o f e* aq t h i s work p r o v i d e d  some unexpected b u t n o t s u r p r i s i n g c o n c l u s i o n s r e l a t i n g t o t h e inhomogeneous d i s t r i b u t i o n o f t h e s p e c i e s whose k i n e t i c b e h a v i o u r was under investigation.  I f t h e i n i t i a l decay mechanism i s a r e a l f i r s t  order  p r o c e s s t h e p r e c i s e sequence o f events i s open t o s p e c u l a t i o n a l t h o u g h an e q u i l i b r i u m o r charge c o r r e l a t i o n model might be c o n s i d e r e d . b i m o l e c u l a r r a t e c o n s t a n t determined  The  was a f a c t o r o f 5 lower than  Klein  2 and Warner's result d e s p i t e t h e 10  i n c r e a s e i n t h e dose r a t e .  The inhomo-  g e n e i t y o f t h e system adds some u n c e r t a i n t y t o even t h i s d e t e r m i n a t i o n and undoubtedly  accounts  f o r t h e i r high value.  I t i s apparent  that i n  s t u d y i n g r e a c t i o n s o c c u r r i n g i n tens o f nanoseconds one cannot j u s t i f i a b l y c o n v e r t o p t i c a l d e n s i t i e s i n t o c o n c e n t r a t i o n s on which t h e b i m o l e c u l a r r a t e c o n s t a n t s depend.  Because o f t h e e x i s t e n c e o f l o c a l i s e d areas o f  96  v e r y h i g h c o n c e n t r a t i o n s o f r e a c t i v e s p e c i e s t h e Beer-Lambert r e l a t i o n s h i p which a p p l i e s t o homogeneous and continuous t h e l e s s t h e i n i t i a l mode o f b e h a v i o u r  systems b r e a k s down.  Never-  and h a l f l i f e o f e aq i n our system  i n t h e absence o f any p h o t o l y s i n g l i g h t has been e s t a b l i s h e d and t h e next experiments w i l l be t o look a t t h e b e h a v i o u r the h y d r a t e d e l e c t r o n .  o f e* aq by f l a s h i n g p h o t o l y s i n g  97  References 1.  I.V. V e r e s h c h i n s k i i § A.K. P i k a e v , I n t r o d u c t i o n t o R a d i a t i o n C h e m i s t r y , E n g l i s h t r a n s l a t i o n from I s r e a l Program f o r S c i e n t i f i c t r a n s l a t i o n s , J e r u s a l e m (1964).  2.  J.W.T. S p i n k s § R.J. Woods, An I n t r o d u c t i o n t o R a d i a t i o n C h e m i s t r y , John W i l e y $ Sons, N.Y. (1964).  3.  A.E.  4.  R.F. Gould ( E d i t o r ) S o l v a t e d E l e c t r o n , Am. Washington (1965) .  5.  D.C.  6.  A Mozumder § J.L. Magee, Rad.  Res., 28_, 203-214, (1964).  7.  A. Mozumder $ J.L. Magee, Rad.  Res., 28_, 215-231 (1964).  8.  A.H.  9.  R.L. P l a t z m a n , P h y s i c a l § Chemical Aspects o f B a s i c Mechanisms i n R a d i o b i o l o g y , Pub. no. 305, 22 (U.S./NRC) (1953).  Moelwyn-Hughes, P h y s i c a l C h e m i s t r y , Pergamon P r e s s , O x f o r d Chem. Soc.  (1961)  Publication,  Walker, i n p r e s s .  Samuel $ J.L. Magee, J . Chem. Phys., 21_, 1080  (1955).  10.  L.H.  Gray, J . Chim. Phys. 48, 172. (1951).  11.  J.L. Magee. P r o c . I n f . Conf., H i g h l a n d Park I l l i n o i s 305, 22 (1953).  12.  A. Kupperman, D i f f u s i o n K i n e t i c s i n R a d i a t i o n Chemistry.  13.  E.A.  14.  P r o c . Vth I n f . Conf. on the R a d i a t i o n C h e m i s t r y o f water AEC C00-38-519.  15.  J . Boag § E . J . H a r t , N a t u r e , 197, 45  16.  E . J . H a r t § J . Boag, J.A.C.S. 84_, 4090 (1962).  17.  J . Weiss, N a t u r e , 153, 748  18.  R.L. P l a t z m a n , P r o c . I n f . Conf., H i g h l a n d Park I l l i n o i s , N.A.S. 305, 22 (1953).  19.  G. S t e i n , D i s c . Farad. S o c ,  20.  E . J . H a r t , J.A.C.S., 76, 4312  Shaede, M.Sc.  Thesis  N.A.S.,  (1967). no  (1963).  (1944).  12_, 227 (1954).  (1952).  98  21.  E. Hayon $ J . Weiss. P r o c . I n t e r n . Conf. o f P e a c e f u l uses o f Atomic Energy (2) 29_, 80 (1952).  22.  N.F. B a r r $ A.D. A l l e n , J . Phys. Chem., 63_, 628 (1959).  23.  J.H. Baxendale  24.  G. C z a p s k i £ H.A. Schwarz, J . Phys. Chem., 66_, 471 (1962).  25.  E. C o l l i n s o n , F.S. D a i n t o n , D.R. Smith § S. Tazuke, P r o c . Chem. Soc. 140 (1962).  26.  $ G. Hughes, Z. Phys. Chem. ( F r a n k f u r t ) L4, 306, (1958).  .F.S. D a i n t o n £ W.S. Watt, P r o c . Roy. Soc. 275A, 447 (1963).  27.  J.P. Keene, Nature 197, 47 (1963).  28.  M.S. Matheson, Ann. Rev. Phys. Chem. 1_3, 77 (1962).  29.  M. Anbar $ P. N e t a , I n t . J . o f App. Rad. $ I s o t o p e s 18, 493 (1967).  30.  E . J . H a r t $ E.M. F i e l d e n , J . Phys. Chem. 72_, 577 (1968).  31.  R. S c h i l l e r , J . Chem. Phys. 47, 2278 (1967).  32.  Ibid  2281 (1967).  33.  G.R. Freeman § J.M. Fayadh, J . Chem. Phys. 43 86 (1965).  34.  J.E. Bennet, B. M i l e £ A. Thomas, J . Chem. Soc. A 1967, 1394.  35.  V.N. S h u b i n , V.A. Zhigunov, V . I . Z o l o t a r e v s k y £ P . I . D o l i n , Nature 212, 1002 (1966).  36.  L.M. Dorfman (ed. R. G o u l d ) , S o l v a t e d E l e c t r o n , opus c i t .  37.  J . J o r t n e r , Rad. Res. S u p p l . no. 4, 24-34 (1964).  38.  W.C. G o t t s c h a l l £ E . J . H a r t , J . Phys. Chem. 71_, 2102 (1967).  39.  R.L. P o t t e r , R.G. Shares $ J.L. Dye, J . Chem. Phys. 35, 1907 (1961).  40.  J.H. Baxendate,  41. 42.  D.C. W a l k e r , Quart. Rev. Chem. Soc. 1967 (XXI no. 1. F.S. D a i n t o n , G.A. Salmar § J . T e p l e , P r o c . Roy. Soc. (1965) 286A, 27. I b i d , 319.  43.  Rad. Res. S u p p l e , no. 4, 139 (1964).  99  44.  D. S c h u l t e - F r o h l i n d e § K. E i b e n , Z. N a t u r - f o r s c h . 17a, 445 (1962).  45.  N. K l e i n £ J . Warner, U.S. Army N u c l . Def. Lab., Doc AD634693  46.  M. S. Matheson $ J . R a b a n i , J . Phys. Chem. 69, 1324 (1965).  47.  L.M. Dorfman S I.A. Taub, J.A.C.S., 85, 2370  48.  A.W. Boyd, P e r s o n a l  communication.  49. . L.M. Dorfman, P e r s o n a l  communication.  50.  W. J o l l y , N.A.T.O. Adv. Study, I n s t . (see a l s o r e f e r e n c e 4)  51.  J . J . Weiss, N a t u r e ,  References  1-4  (1963).  (Hamilton,.Ontario)  1967  215, 151 (1967).  were used f o r g e n e r a l background i n f o r m a t i o n .  (1965).  Corrections Page 16  Line 12  Correction i n c l u d e r e f . (9) a f t e r  Platzman  97  r e f . 12  A. Kupperman (ed. M. H a i s s i n s k y ) . The Chemical § B i o l o g i c a l Action of Radiations, v o l . V Publ. Academic P r e s s 1961.  98  r e f . 40  J.H. Baxendale  98  r e f . 42  G.A.  Salmon  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0059843/manifest

Comment

Related Items