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UBC Theses and Dissertations

Radiation chemistry of dimethylsulfoxide 1972

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RADIATION CHEMISTRY OF DIMETHYLSULFOXIDE BY TERRY KENNETH COOPER B.Sc., U n i v e r s i t y of B r i t i s h Columbia, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1972 In presenting t h i s thes is i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I fur ther agree that permission f o r extensive copying of t h i s thesis f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representat ives . It i s understood that copying or p u b l i c a t i o n of t h i s thes is f o r f i n a n c i a l gain s h a l l not be allowed without my wri t ten permission. Department The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada ABSTRACT A comprehensive study of the r a d i a t i o n chemistry of di m e t h y l - s u l f o x i d e (DMSO), and DMSO-H20 mix t u r e s , by steady-state y - r a d i o l y s i s , pulse r a d i o l y s i s and low temperature m a t r i x i s o l a t i o n techniques has revealed s e v e r a l s i g n i f i c a n t f e a t u r e s about the t r a n s i e n t i o n i c products of the r a d i o l y s i s . Very s h o r t - l i v e d (T^^ < ^5 nsec), weakly bound s o l v a t e d e l e c t r o n s are formed i n q u i t e h i g h y i e l d s i n these l i q u i d s at room temperature. Solvated e l e c t r o n s i n the b i n a r y DMSO-H^O mixtures e x h i b i t s i n g l e o p t i c a l absorption bands w i t h maxima which vary smoothly w i t h composition from pure water (^ m a x = 720 nm) to pure DMSO (X > 1500 nm). The e l e c t r o n s t a b i l i t y v a r i e s w i t h v max J composition, showing a minimum l i f e t i m e c o i n c i d i n g w i t h DMSO-H^O mixtures which e x h i b i t maximum i n t e r - m o l e c u l a r s t r u c t u r e . In g l a s s y mixtures at 77°K, e l e c t r o n s generated r a d i o l y t i c a l l y or photochemically are not trapped aid s t a b i l i z e d . The p o s i t i v e ions formed from i r r a d i a t e d DMSO are longer l i v e d (T-^^ % ^ ysec) than the solvated e l e c t o n s and have a strong a b s o r p t i o n band centered at X ^ 550 nm. In DMSO-H-0 in c ix z. mixtures t h i s same ab s o r p t i o n band i s formed. The c a t i o n i c y i e l d i s d i r e c t l y p r o p o r t i o n a l to the f r a c t i o n of the t o t a l dose absorbed i n i t i a l l y by DMSO which suggests there i s no charge t r a n s f e r , proton exchange or p o s i t i v e i o n scavenging i n these mixtures. The y i e l d of both e g and c a t i o n s was ̂  30% h i g h e r i n the f u l l y deuterated DMSO than i n the hydrogenated m a t e r i a l i n d i c a t i n g some a l t e r a t i o n i n the e l e c t r o n escape p r o b a b i l i t y due to i s o t o p i c s u b s t i t u t i o n . Most of the above observations are i n t e r p r e t e d i n terms of the p h y s i c a l p r o p e r t i e s of DMSO and i t s aqueous mixtures. The r a d i a t i o n - i i i - yield of free ions is measured to be 1.30 +0.15 which i s in reasonably good accord with a liquid having a dielectric constant of 48. The yield of solvated electrons was ascertained by observing the formation of anthracene anions in solutions of anthracene and corres- ponded very closely to the yield of positive ions determined from the formation of in KBr solutions. The near infrared absorption band of solvated electrons in DMSO indicates a weakly stabilized species which i s consistent with the very poor solvating power for negative ±ms afforded by DMSO due to the aprotic nature of the solvent. On the other hand the lack of charge transfer or proton exchange with Ĥ O shown by the DMSO cation i s consistent with the dipolar character of DMSO and i t s strong solvation of positive ions. A variety of free radicals have been observed by electron spin resonance in Y~irradiated polycrystalline DMSO at 77°K amongst which the •CH.j and a sulfur containing radical are distinguished. Under Y-radiolysis liquid DMSO produces H 2 > CH^, and (CH^S with G values of 0.20 + 0.01, 3.3 + 0.1, 0.49 + 0.03 and 1.2 + 0.2 respectively. - i v - TABLE OF CONTENTS Chapter Page I. INTRODUCTION 1 A. I n t e r a c t i o n of High-Energy R a d i a t i o n w i t h Matter 2 1. Electromagnetic r a d i a t i o n 2 2. High-energy e l e c t r o n s 8 3. Range of e l e c t r o n s 13 4. Track e n t i t i e s : spurs, blobs and short t r a c k s 14 B. Chemical Consequences F o l l o w i n g Absorption of High-Energy R a d i a t i o n 17 1. Time s c a l e of events 18 2. Phase-dependent phenomena 21 3. Studies on the chemical events i n condensed phases 22 4. Chemical y i e l d s 24 C. S t a b i l i z e d E l e c t r o n s 26 1. S t a b i l i z a t i o n 26 2. P r o p e r t i e s 27 2.1 O p t i c a l spectrum 28 2.2 E l e c t r o n s p i n resonance 32 2.3 C o n d u c t i v i t y 34 3. Models 35 4. Free i o n y i e l d s 43 D. Binary Mixtures 48 E. Scope of Study 53 - v - Chapter P a § e I I . EXPERIMENTAL 58 A. ^ C o y _ R a d i o l y s i s 58 1. M a t e r i a l s 58 2. R a d i a t i o n source 59 3. Dosimetry 59 4. Sample p r e p a r a t i o n 65 5. Product a n a l y s i s 71 B. Pu l s e R a d i o l y s i s 77 1. O u t l i n e of the technique 77 2. R a d i a t i o n source 79 3. I r r a d i a t i o n c e l l and o p t i c a l d e t e c t i o n system 79 4. O s c i l l o s c o p e measurements 82 5. Cerenkov emission 84 , 6, Dosimetry 88 7. M a t e r i a l s and p u r i f i c a t i o n 91 C. E l e c t r o n Spin Resonance 92 1. M a t e r i a l s and p u r i f i c a t i o n 92 2. R a d i a t i o n source 92 3. Sample p r e p a r a t i o n 92 4. E l e c t r o n s p i n resonance s p e c t r a 93 5. P h o t o l y s i s apparatus 94 - V I - Chapter Page I I I . STUDIES ON LIQUID DMSO 95 A. ^ C o y - R a d i o l y s i s 95 1. Gaseous products 95 2. L i q u i d products 98 3. Scavenger s t u d i e s 100 4. D i s c u s s i o n H 3 B. Pulse R a d i o l y s i s 1 2 2 1. A b s o r p t i o n s p e c t r a i n DMSO and DMSO^ 1 2 2 2. Free i o n y i e l d s 132 3. Geminate i o n scavenging 148 4. D i e l e c t r i c constant and e l e c t r o n s t a b i l i z a t i o n 154 IV. PULSE RADIOLYSIS OF DMSO-WATER BINARY MIXTURES 156 1. Solvated e l e c t r o n s 157 2. DMSO p o s i t i v e ions 169 3. T r a n s i e n t intermediates a t 77°K 173 V. ELECTRON SPIN RESONANCE STUDIES ON DMSO AND DMSO- H 20 MIXTURES AT 77°K .. I 7 7 A. I n t r o d u c t i o n 177 1. B a s i c p r i n c i p l e s of esr 177 2. Amorphous and p o l y c r y s t a l l i n e media 183 B. R e s u l t s and D i s c u s s i o n 187 1. Pure DMSO I 8 7 1.1 U l t r a v i o l e t i r r a d i a t e d I 8 7 1.2 y - I r r a d i a t e d 190 - v i i - Chapter Page V (continued) 2. DMSO-water matrices 193 2.1 y - i r r a d i a t e d p o l y c r y s t a l l i n e Ĥ O 193 2.2 y - i r r a d i a t e d DMSO-H^O mixtures 195 2.3 p h o t o i o n i z a t i o n of K^Fe(CN)^ i n aqueous glas s e s 205 REFERENCES 211 - v i i i - LIST OF TABLES O p t i c a l data f o r e l e c t r o n s s t a b i l i z e d i n l i q u i d media at room temperature R a d i a t i o n y i e l d s from i r r a d i a t e d DMSO c o n t a i n i n g 0.05 M ̂ 0 p l u s the second scavenger i n d i c a t e d i n column 1 at co n c e n t r a t i o n given i n column 2 Ra t i o of k_/k_^ obtained f o r the precursor of N„ i n DMSO w i t h v a r i o u s second scavengers (S) added to a 0.05 M s o l u t i o n of ̂ 0 . Column headed water r e f e r s to the p u b l i s h e d r a t e constant r a t i o f o r the hydrated e l e c t r o n Summary of data obtained from s t u d i e s on pul s e i r r a d i a t e d DMSO-water mixtures at room temperature. Summary of data obtained from s t u d i e s on Y ' i r r a d i a t e d DMSO-water mat r i c e s at 77°K - i x - LIST OF FIGURES Fig u r e Page 1 Atomic 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 water. Curve A, T o t a l a b s o r p t i o n c o e f f i c i e n t (with coherent s c a t t e r i n g ) ; B, p h o t o e l e c t r i c a b s o r p t i o n c o e f f i c i e n t ; C, Compton c o e f f i c i e n t ( w i t h coherent s c a t t e r i n g ) ; D, Compton c o e f f i c i e n t (without coherent s c a t t e r i n g ) ; -24 2 E, p a i r - p r o d u c t i o n c o e f f i c i e n t . 1 barn = 1 0 cm .. 6 2 Huygens c o n s t r u c t i o n of e l e c t r o n t r a j e c t o r y and r e s u l t i n g Cerenkov waveform 11 3 Schematic p l o t of percentage of energy s p l i t between spurs, blobs and short t r a c k s f o r e l e c t r o n s i n water. 16 4 T h e o r e t i c a l time s c a l e f o r the i n i t i a l processes i n r a d i a t i o n chemistry. The numbers are the negative l o g a r i t h m of time (pt = - l o g t (sec)) 20 5 Schematic r e p r e s e n t a t i o n of e l e c t r o n l o c a l i z a t i o n (shaded region) produced by p o l a r i z a t i o n of the medium. The dotted area represents the v o i d or c a v i t y i n which the e l e c t r o n i s centered 30 6 C o r r e l a t i o n curve of the t r a n s i t i o n energy of the e l e c t r o n i n DMSO and i t s c a v i t y r a d i u s u s i n g the continuous d i e l e c t r i c model w i t h an a d i a b a t i c approximation 39 7 P l o t of G ( f r e e ion) as a f u n c t i o n of the s t a t i c d i e l e c t r i c constant of the medium. Data taken from references 32, 33, 34, 46, 101 46 - x - F i g u r e Page 8 F r i c k e dosimeter r e s u l t s obtained from the r a d i o l y s i s of the s o l u t i o n s i n the i r r a d i a t i o n c e l l used i n t h i s study ... 6 3 9 Pyrex i r r a d i a t i o n c e l l s used f o r deoxygenation of the l i q u i d samples ^6 10 Schematic diagram of vacuum l i n e used f o r degassing the l i q u i d samples and adding n i t r o u s oxide to the samples 68 11 P l o t showing the r e l a t i o n s h i p of the p a r t i a l pressure of n i t r o u s oxide to i t s s o l u b i l i t y i n DMSO at 23°C f o r the two bubbler c e l l s c o n t a i n i n g a medium ( c e l l A) and f i n e ( c e l l B) p o r o s i t y s i n t e r e d d i s k 70 12 Schematic diagram of apparatus used f o r f l u s h i n g the v o l a t i l e gaseous products i n t o the gas chromatograph. 72 13 T y p i c a l chromatograph obtained f o r 20 ml DMSO sample c o n t a i n i n g 0.05 M n i t r o u s oxide and r e c e i v i n g an 4 absorbed dose of 8 x 10 rads 75 14 T y p i c a l chromatograph obtained a f t e r i n j e c t i o n of 25 u£ of i r r a d i a t e d DMSO sample. T o t a l absorbed dose was 6 Mrad 76 15 Lay-out of the pulse r a d i o l y s i s equipment at the N a t i o n a l Research C o u n c i l r a d i a t i o n l a b o r a t o r y i n Ottawa, Ontario 78 - x i - F i g u r e Page 16 Schematic diagram of i r r a d i a t i o n c e l l used f o r deoxy- genation of l i q u i d samples used i n the pulse r a d i o l y s i s study. The s p e c t r o s c o p i c c e l l was f i l l e d by t i p p i n g the c e l l h o r i z o n t a l l y a f t e r f l u s h i n g w i t h high p u r i t y argon 80 17 H y p o t h e t i c a l o s c i l l o s c o p e t r a c e showing the b u i l d up and decay of t r a n s i e n t absorbing s p e c i e s . The time p r o f i l e of the e l e c t r o n pulse i s shown as the dotted curve 83 18 T y p i c a l o s c i l l o s c o p e t r a c e s showing the response time of the d e t e c t i o n apparatus to Cerenkov l i g h t genera- ted using a 40 nsec wide e l e c t r o n pulse 85 19 E f f e c t of Cerenkov emission on the a b s o r p t i o n of the t r a n s i e n t s absorbing at 500 nm i n pure DMSO. (a) pure Cerenkov ( a n a l y z i n g l i g h t o f f ) ; (b) observed a b s o r p t i o n ; (c) a b s o r p t i o n that would have been observed had there been no Cerenkov emission. The detector was a S i photodiode w i t h a 93 ohm load r e s i s t o r . The pulse w i d t h was 40 nsec 87 20 T y p i c a l o s c i l l o s c o p e t r a c e s f o r the formation and decay of (CNS^ at 500 nm obtained by u s i n g a 40 nsec pulse of 35 MeV e l e c t r o n s on a n i t r o u s oxide -3 s a t u r a t e d s o l u t i o n of 5 x 10 M thiocyanate i n water, (a) S i photodiode, 93 ohm load r e s i s t o r ; (b) photo- m u l t i p l i e r , 470 ohm load r e s i s t o r 90 - x i i - Figure Page 21 R a d i a t i o n y i e l d s as a f u n c t i o n of absorbed dose: Q, ^2^6 a n <^ 0> H 2 * ^ e I n e t ' i a n e c u r v e w a s obtained by t a k i n g the slope at v a r i o u s p o r t i o n s of the curve shown i n Figure 22 22 Accumulated gas y i e l d s as a f u n c t i o n of accumulated dose: 9 , A , £\ , , CH. at v a r i o u s doses: Q , C_H,; O, H 2 23 Accumulated dimethyl s u l f i d e y i e l d as a f u n c t i o n of accumulated dose. The e x t r a p o l a t e d p o r t i o n of the curve corresponds to G(DMS) = 1.2 + 0 . 2 24 R a d i a t i o n y i e l d s of CH 4 (A), C ^ (•) and H 2 (O) as a f u n c t i o n of the ̂ 0 c o n c e n t r a t i o n . In the case of CH^, a l l doses were < 1.8 x 10^ rads 96 97 99 101 102 25 R a d i a t i o n y i e l d of n i t r o g e n as a f u n c t i o n of ^ 0 co n c e n t r a t i o n 26 Accumulated n i t r o g e n y i e l d as a f u n c t i o n of accumulated dose 103 27 P l o t of 1/G(N 2) as a f u n c t i o n of [Scavenger]/[N 20]. The N 20 c o n c e n t r a t i o n was 0.05 M and the second scavenger c o n c e n t r a t i o n was v a r i e d . The data was taken from Table I I . © , C C l ^ ; A, I 2 ; B, CHC1 3; O > A g + and D , acetone. 108 28 P l o t of 1/G(N ) as a f u n c t i o n of [H +]/[N 20] i n which O > D correspond to d i f f e r e n t N 20 concentra- t i o n s : O, 0.05 M NO; A, 0.04 M N 20 and • , 0.07 M NT20. © Corresponds to 0.5 M methanol added to the a c i d s o l u t i o n 109 - x i i i - Figure Page 29 . R a d i a t i o n y i e l d s of CH^ (A), (•) , and H 2 (O) as a f u n c t i o n of co n c e n t r a t i o n . The doses were < 5 x 10 4 rads I l l 30 R a d i a t i o n y i e l d s of CH 4 (A), C ^ (•) and H 2 (O) as a f u n c t i o n of H + c o n c e n t r a t i o n . corresponds to the y i e l d of CH, when 0.05 M N.O was added to the 4 2 corresponding a c i d s o l u t i o n of DMSO. In the case of CH. , a l l doses were < 1.8 x 10^ rads 112 4 31 Tran s i e n t s p e c t r a observed i n (CH^) 2S0 and (CD,j) 2S0. The c i r c l e s r e f e r to the s o l v a t e d e l e c t r o n band c o r r e c t e d f o r the detector response time and the t r i a n g l e s r e f e r to the DMSO p o s i t i v e i o n , or o x i d i z i n g s p e c i e s . O and A are f o r (CH^) 2S0; 9 and are f o r (CD.j) 2S0. Almost a l l data p o i n t s are the mean of at l e a s t two measurements. The X f o r the A spectrum max was e s t a b l i s h e d to be at 550 nm from a previous s e t of experiments 123 32 T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the so l v a t e d e l e c t r o n i n DMSO. Both t r a c e s were obtained u s i n g a Ge photodiode w i t h a 50 ohm load r e s i s t a n c e . (a) pulse width 10 nsec; (b) pulse width 40 nsec 125 33 T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the DMSO p o s i t i v e i o n or o x i d i z i n g species at 550 nm. The f a s t i n i t i a l decay i n (a) i s due to the s o l v a t e d e l e c t r o n . Both t r a c e s were obtained using a pulse width of 40 nsec. (a) S i photodiode w i t h 93 ohm load r e s i s t o r ; (b) p h o t o m u l t i p l i e r w i t h 470 ohm load r e s i s t o r 126 - x i v - Figure • Page 34 F i r s t - o r d e r decay p l o t of the solv a t e d e l e c t r o n i n DMSO taken at 1300 nm. The pulse width was 10 nsec, g i v i n g an absorbed dose of 900 rads per pulse. The decay was measured using a Ge photodiode w i t h a 50 ohm load r e s i s t o r 130 35 F i r s t - o r d e r p l o t of the decay of the DMSO p o s i t i v e i o n . The decay was measured at 550 nm using the photo- m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r . The pulse was 40 nsec, g i v i n g an absorbed dose of 2200 rads per pulse ^31 36 End of pulse a b s o r p t i o n spectrum of the anthracene r a d i c a l anion obtained from a DMSO s o l u t i o n 0.02 M i n anthracene a f t e r absorbances due to the e l e c t r o n and o x i d i z i n g species had been sub t r a c t e d 134 37 T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the e l e c t r o n and b u i l d up of the anthracene r a d i c a l anion at 750 nm. (a) no anthracene added; (b) 0.001 M anthracene i n DMSO; (c) 0.005 M anthracene i n DMSO. I 3 5 38 T y p i c a l o s c i l l o s c o p e t r a c e s showing the b u i l d up and decay of the anthracene r a d i c a l anion at 750 nm. (a) 0.01 M anthracene i n DMSO; (b) 0.02 M anthracene i n DMSO I 3 6 - X V - Figure Page 39 Graph showing the scavenging of s o l v a t e d e l e c t r o n s i n pure DMSO by anthracene and the formation of anthracene r a d i c a l anions. © , absorbance at 750 nm due to A immediately at the end of the p u l s e ; O » maximum i n the absorbance at 750 nm due to A a f t e r the p u l s e ; A » absorbance due to the p o s i t i v e ions at 550 nm; B , absorbance due to s o l v a t e d e l e c t r o n s at 1275 nm (not co r r e c t e d f o r decay during the pulse nor f o r the detector response time) 138 40 A b s o r p t i o n s p e c t r a f o r KBr s o l u t i o n s i n DMSO. S o l i d surve r e f e r s to Br^ s p e c t r a ; the dotted curve r e f e r s to the t r a n s i e n t precursor of B r 2 • <^, B r 2 from 0.1 M KBr s o l u t i o n ; # , B r 2 ~ from 0.01 M KBr s o l u t i o n ; 13 , t r a n s i e n t p r e c u r s o r of B r 2 at 0.01 M KBr. The i n s e t i s a p l o t of Ge at 375 nm f o r B r 2 a g a i n s t l o g ([Br~]/M) 142 41 T y p i c a l o s c i l l o s c o p e t r a c e s showing the b u i l d up and decay of B r 2 at 365 nm i n pure DMSO. De t e c t i o n made using the p h o t o m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r . (a) 0.1 M KBr; (b) 0.01 M KBr; (c) 0.001 M KBr 143 42 P l o t showing f i r s t - o r d e r b u i l d up of B r 2 (Figure 41(c)) f o r DMSO s o l u t i o n 0.001 M i n KBr 145 - x v i - Figu r e Page 43 T y p i c a l o s c i l l o s c o p e traces showing the decay of B r 2 and i t s t r a n s i e n t p r e c u r s o r . The f a s t i n i t i a l decay i n (b) and (c) i s a t t r i b u t e d to the t r a n s i e n t complex. Detection was made using the p h o t o m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r 44 End of pulse a b s o r p t i o n spectrum of the DMSO p o s i t i v e i o n i n 0.2 M H„S0.. The dotted l i n e r e f e r s to the L 4 absorbance of the p o s i t i v e i o n i n pure DMSO. X r e f e r s to the l o n g - l i v e d yS0^ intermediate produced i n the a c i d s o l u t i o n L ^ 45 F i r s t - o r d e r decay p l o t of the DMSO p o s i t i v e i o n i n the presence of 0.2 M H^SO^. Decay measured at 550 nm using the p h o t o m u l t i p l i e r w i t h a 470 ohm load r e s i s t a n c e ^1 46 T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the DMSO p o s i t i v e i o n i n the presence of 0.2 M H^SO^ at 650 nm and 450 nm. The l o n g e r - l i v e d SO^ t r a n s i e n t i s r e a d i l y observed a t 450 nm 152 47 End of pulse spectrum of t r a n s i e n t s produced i n a DMSO s o l u t i o n 0.5 M i n A g + 1 5 3 48 Abso r p t i o n s p e c t r a of s o l v a t e d e l e c t r o n s i n DMSO-H^O mixtures; 0, 0.20, 0.28, 0.43, 0.72, 0.93 and 1.0 mole f r a c t i o n DMSO. Data p o i n t s were obtained at 50 nm i n t e r v a l s . The data f o r pure water have been m u l t i p l i e d by a f a c t o r of 0.3 r e l a t i v e to the others. 159 -• x v i i - Fi g u r e Page 49 P l o t of the values of Ge f o r the s o l v a t e d e l e c t r o n max absor p t i o n bands presented i n Figure 48 as a f u n c t i o n of the mole f r a c t i o n DMSO f o r the DMS0-H20 mixtures. The n o n - l i n e a r a x i s showing the change as a f u n c t i o n of s t a t i c d i e l e c t r i c constant of the bulk mixture i s shown on the top a b s c i s s a . O > a c t u a l observed absorbance peak h e i g h t s . 9 , c o r r e c t e d f o r decay during the pulse and response time of the det e c t o r 162 50 P l o t of the photon energy of the absor p t i o n band maximum f o r the s o l v a t e d e l e c t r o n i n the DMSO-H^O mixtures against the bulk s t a t i c d i e l e c t r i c constant of the mixtures (at 25°C). The n o n - l i n e a r a x i s showing the corresponding mole f r a c t i o n DMSO i s shown i n the upper a b s c i s s a 51 P l o t of the photon energy of the absor p t i o n band maximum f o r the s o l v a t e d e l e c t r o n i n the DMSO-l^O mixtures against the mole f r a c t i o n of water. The non- l i n e a r a x i s showing the corresponding bulk d i e l e c t r i c constants of the mixtures i s shown i n the upper a b s c i s s a 168 52 (a) Absorption s p e c t r a a t t r i b u t e d to the DMSO p o s i t i v e i o n produced i n DMSO-H^O mixtures. Curve 6 i s pure DMSO; 5, 0.93; 4, 0.72; 3, 0.43; 2, 0.28; and 1, 0.20 mole f r a c t i o n DMSO. (b) Peak absorbance (at 550 nm) of the bands shown i n (a) p l o t t e d against the f r a c t i o n of dose absorbed i n i t i a l l y by DMSO 170 - x v i i i - Figure Page 53 Abso r p t i o n spectrum of t r a n s i e n t s produced by the pulse r a d i o l y s i s of a DMSO-H20 g l a s s (39 mole % DMSO) at 77°K. The pulse width was 500 nsec, the dose per pulse being ^ 10 krad. The dotted curve i s that of the DMSO p o s i t i v e i o n i n pure DMSO normalized at 600 nm.. 175 54 T h e o r e t i c a l esr l i n e shapes f o r (a) a x i a l l y symmetric and (b) completely asymmetric g tensors. The upper curves r e f e r to absorption s p e c t r a of the para- magnetic species. The lower curves r e f e r to the exp e r i m e n t a l l y observed f i r s t d e r i v a t i v e s p e c t r a . . . . 185 55 T h e o r e t i c a l f i r s t d e r i v a t i v e e sr spectrum f o r a paramagnetic species w i t h S = 1/2, I = 1/2 and w i t h a x i a l l y asymmetric g and A tens o r s . The c e n t r a l dotted p o r t i o n i s the t h e o r e t i c a l spectrum i n the absence of the h y p e r f i n e i n t e r a c t i o n s 186 56 E l e c t r o n s p i n resonance spectrum obtained a f t e r the u l t r a v i o l e t p h o t o l y s i s of p o l y c r y s t a l l i n e DMSO at 77°K. The arrows correspond to the asymmetric g- f a c t o r s of the s u l f u r r a d i c a l CH^SO. The methyl r a d i c a l quartet i s i n d i c a t e d by the s t i c k p l o t 189 57 E l e c t r o n s p i n resonance s p e c t r a of Y - i r r a d i a t e d DMSO. The sample was i r r a d i a t e d i n the dark at 77°K to a t o t a l absorbed dose of 0.72 Mrad. (a) microwave power 0.44 mW; (b) microwave power 10 mW. g--,.,,, = 2.0036.. 191 DP r H - x i x - Fi g u r e Page 58 E l e c t r o n s p i n resonance s p e c t r a of Y - i r r a d i a t e d DMSO a f t e r b l e a c h i n g i r r a d i a t e d sample w i t h u l t r a v i o l e t l i g h t f o r f o r t y minutes (in spectrometer c a v i t y ) . Sample y - i r r a d i a t e d at 77°K i n the dark to a t o t a l absorbed dose of 0.72 Mrad. (b) microwave power 0.52 mW; (b) microwave power 10 mW 192 59 E l e c t r o n s p i n resonance spectrum of Y ~ i r r a d i a t e d p o l y c r y s t a l l i n e i c e at 77°K. Resonance p a t t e r n corresponds to that of the «0H r a d i c a l 194 60 Resonance p a t t e r n showing behaviour of »X w i t h (a) i n c r e a s i n g water composition (microwave power 0.42 mW) and (b) i n c r e a s i n g microwave power (pure DMSO). Numbers corresponding to s p e c t r a on l e f t r e f e r to mole f r a c t i o n DMSO. The arrows r e f e r to 8 D P P H * 1 9 8 61 E l e c t r o n s p i n resonance s p e c t r a of p o l y c r y s t a l l i n e Y - i r r a d i a t e d DMSO-water mixture (0.80 mole f r a c t i o n DMSO) at 77°K. (a) microwave power 0.42 mW; (b) a f t e r b l e a c h i n g w i t h u l t r a v i o l e t l i g h t f o r 20 minutes, microwave power 0.42 mW; (c) same as ( b ) , microwave power 10 mW 199 62 E l e c t r o n s p i n resonance s p e c t r a of Y ~ i r r a d i a t e d DMSO- water g l a s s (0.20 mole f r a c t i o n DMSO) at 77°K. The " s u l f u r p a t t e r n " and methyl r a d i c a l quartet are r e a d i l y observed, (a) microwave power 0.42 mW; (b) microwave power 10 mW 200 - XX - Figure Page 63 E l e c t r o n s p i n resonance s p e c t r a of y i r r a d i a t e d p o l y c r y s t a l l i n e DMSO-water mixture (0.01 mole f r a c t i o n DMSO) at 77 °K. (a) microwave power 0.42 mW; (b) microwave power 10 mW. The low f i e l d "hump" and doublet of the »0H r a d i c a l s are evident at 10 mW power (see Figure 59) 202 64 E l e c t r o n s p i n resonance spectrum obtained a f t e r p h o t o i o n i z a t i o n at 77°K of 0.01 M K^Fe(CN) 6 i n 0.20 mole f r a c t i o n DMSO-water g l a s s (compare to Fig u r e 62(a)) 207 65 E l e c t r o n s p i n resonance s p e c t r a obtained f o r the p h o t o i o n i z a t i o n of 0.01 M K^Fe(CN) 6 i n 8 M NaOH gl a s s at 77°K. (a) no DMSO added; (b) 1.0 M DMSO present i n g l a s s 208 66 E l e c t r o n s p i n resonance s p e c t r a obtained f o r y- i r r a d i a t e d (0.24 Mrad) of 8 M NaOH gl a s s at 77°K. (a) no DMSO added; (b) 1.0 M DMSO added; (c) a f t e r photobleaching (b) w i t h u l t r a v i o l e t l i g h t f o r 20 minutes 209 - x x i - ACKNOWLEDGMENTS The author would l i k e to s i n c e r e l y thank Dr. D.C. Walker f o r h i s encouragement and guidance during the course of t h i s study as w e l l as f o r i n t r o d u c i n g him to R a d i a t i o n Chemistry. I t i s the author's pleasure to acknowledge the cooperation of the N a t i o n a l Research C o u n c i l f o r use of the pulse r a d i a t i o n f a c i l i t i e s i n Ottawa. The author i s e s p e c i a l l y g r a t e f u l to Dr. Hugh A. G i l l i s , Dr. Norman V. Klass e n and Mr. George G. l e a t h e r f o r t h e i r i n v a l u a b l e a s s i s t a n c e i n performing the pulse r a d i o l y s i s experiments. S p e c i a l thanks are due to the N a t i o n a l Research C o u n c i l and the F.J. N i cholson Family f o r f i n a n c i a l support i n the form of post graduate s c h o l a r s h i p s . F i n a l l y , the author would l i k e to thank h i s w i f e C a r o l , who has shown great understanding and forbearance during the p r e p a r a t i o n and w r i t i n g of t h i s t h e s i s . - 1 - CHAPTER I INTRODUCTION R a d i a t i o n chemistry encompasses the study of the chemical e f f e c t s induced i n a system by high-energy r a d i a t i o n s such as those made a v a i l a b l e by r a d i o a c t i v e substances, p a r t i c l e a c c e l e r a t o r s and n u c l e a r r e a c t o r s . In c a r r y i n g out such a study an attempt i s made to i d e n t i f y the products formed, decide how they were formed and what t h e i r p r ecursors were w i t h the hope of e l u c i d a t i n g and understanding the chemical and p h y s i c a l processes i n v o l v e d . This d i s s e r t a t i o n i s concerned w i t h the e f f e c t of i o n i z i n g r a d i a t i o n on dimethyl s u l f o x i d e (DMSO) and b i n a r y mixtures of DMSO and water. P a r t i c u l a r emphasis i s p l a c e d on one of the e a r l y processes i n r a d i a t i o n chemistry, namely e l e c t r o n s t a b i l i z a t i o n , and how t h i s process i s r e l a t e d to the p h y s i c a l and chemical p r o p e r t i e s of the medium. In order to u n r a v e l the many c o m p l e x i t i e s i n v o l v e d i n studying r a d i a t i o n - c h e m i c a l phenomena, i t i s necessary to know how high-energy r a d i a t i o n i n t e r a c t s w i t h matter to produce the u l t i m a t e chemical e f f e c t . - 2 - A. INTERACTION OF HIGH-ENERGY RADIATION WITH MATTER In the study of the r a d i a t i o n chemistry of dimethyl s u l f o x i d e , two sources of high-energy r a d i a t i o n were used: (1) electromagnetic 60 r a d i a t i o n i n the form of Co y ~ r a y s and (2) short pulses of h i g h - energy e l e c t r o n s from a l i n e a r a c c e l e r a t o r . 1. Electromagnetic R a d i a t i o n There are f o u r p r i n c i p a l processes by which electromagnetic r a d i a t i o n may be absorbed by matter"'": ( i ) p h o t o e l e c t r i c , ( i i ) Compton, ( i i i ) p a i r - p r o d u c t i o n , and ( i v ) photonuclear r e a c t i o n s . Each of these processes depends p r i m a r i l y on the energy of the i n c i d e n t r a d i a t i o n but does depend upon the atomic number (Z) of the absorbing medium to some extent. Furthermore, the i n c i d e n t electromagnetic beam may be s c a t t e r e d by the e l e c t r o n s of the medium w i t h l i t t l e or no l o s s i n energy and i s termed coherent s c a t t e r i n g . This s c a t t e r i n g becomes important only f o r high-Z m a t e r i a l s and low photon energies (< 0.1 MeV). whenever electromagnetic r a d i a t i o n passes through matter, i t s i n t e n s i t y i s governed by the r e l a t i o n s h i p I = I Qe (1.1) where I i s the i n t e n s i t y of the r a d i a t i o n t r a n s m i t t e d through a t h i c k n e s s , x, of absorber and I i s the i n t e n s i t y of the i n c i d e n t o 3 r a d i a t i o n . The l i n e a r a b s o r p t i o n c o e f f i c i e n t , y , i s the sum of a l l 3. the p a r t i a l c o e f f i c i e n t s r e p r e s e n t i n g the v a r i o u s processes of a b s o r p t i o n mentioned above. - 3 - In the p h o t o e l e c t r i c e f f e c t , the e n t i r e energy of the photon, E^, i s t r a n s f e r r e d t o a s i n g l e atomic e l e c t r o n . This e l e c t r o n i s then e j e c t e d from the atom w i t h an energy, E £, equal to the d i f f e r e n c e between the photon energy and the b i n d i n g energy, B, of the e l e c t r o n i n the atom. E = E - B (1.2) e p Photoelectrons may be e j e c t e d from any of the K, L, M,... s h e l l s of an atom. However a f r e e e l e c t r o n cannot absorb a photon to become a photo- e l e c t r o n because a t h i r d body, i n t h i s case the nucleus, i s necessary i n order to conserve momentum. The p r o b a b i l i t y of p h o t o e l e c t r o n a b s o r p t i o n i n c r e a s e s w i t h the t i g h t n e s s of the b i n d i n g e l e c t r o n so that at photon energies g r e a t e r than the b i n d i n g energies of the K- and L- s h e l l s , e j e c t i o n from the outer s h e l l s i s n e g l i g i b l e . The vacancy created by the l o s s of an e l e c t r o n from the i n n e r s h e l l w i l l be f i l l e d by an e l e c t r o n from the outer s h e l l and the excess energy i s d i s s i p a t e d by the emission of low - energy Auger e l e c t r o n s or X-rays. P h o t o e l e c t r i c a b s o r p t i o n i s predominant at low photon energies (< 0.1 MeV). In m a t e r i a l s of h i g h atomic number the a b s o r p t i o n cross s e c t i o n , x , 3. i s given approximately by^ T * kZ 4/E 3 (1.3) a p where k i s a constant. At energies > 0.1 MeV, the photon i n t e r a c t s w i t h l o o s e l y bound or f r e e e l e c t r o n s r e s u l t i n g i n a r e f l e c t i o n of the photon w i t h reduced energy. - 4 - The e l e c t r o n i s e j e c t e d w i t h energy, E e , equal to the d i f f e r e n c e between the i n c i d e n t and s c a t t e r e d photon energy (the b i n d i n g energy i s n e g l e c t e d ) . E = E - E (1.4) e p Y The energy of the s c a t t e r e d photon i s given by"'" E E = \ (1.5) Y 1 + (E /m c ) (1 - cose) p o 2 where 0 i s the s c a t t e r i n g angle and m oc i s the r e s t mass energy of the e l e c t r o n (0.51 MeV). These r e c o i l e l e c t r o n s are c a l l e d Compton e l e c t r o n s and have t h e i r maximum energy when the s c a t t e r i n g angle i s 180°. In g e n e r a l , however, Compton e l e c t r o n s have a f a i r l y uniform d i s t r i b u t i o n i n energy, the average being about h a l f the i n c i d e n t photon energy. Compton i n t e r a c t i o n s are the predominant a b s o r p t i o n process f o r photon energies between 1 and 5 MeV i n h i g h atomic number m a t e r i a l s such as l e a d . In low-Z materials such as water and DMSO, Compton i n t e r a c t i o n s are dominant over a much wider range; that i s , from about 30 keV to 20 MeV (see Figure 1 ). The complete a b s o r p t i o n of a photon i n the v i c i n i t y of an atomic nucleus r e s u l t i n g i n the p r o d u c t i o n of two p a r t i c l e s , an e l e c t r o n and a p o s i t r o n , i s c a l l e d p a i r - p r o d u c t i o n . Because p a r t of the photon energy i s used to create the p o s i t r o n and e l e c t r o n , p a i r - p r o d u c t i o n cannot occur at photon energies l e s s than the sum of t h e i r r e s t mass energ i e s , namely 1.02 MeV. The remaining photon energy i s d i v i d e d between the k i n e t i c energies of the e l e c t r o n and p o s i t r o n s i n c e energy - 5 - t r a n s f e r to the nucleus and i t s subsequent r e c o i l i s considered n e g l i g i b l e . The p o s i t r o n , e i t h e r before or a f t e r l o s i n g i t s k i n e t i c energy, i s a n n i h i l a t e d by combining w i t h an e l e c t r o n w i t h the subsequent emission, i n opposite d i r e c t i o n s , of two 0.51 MeV Y - r a y s « P a i r p r o d u c t i o n i s of major importance only w i t h h i g h atomic number m a t e r i a l s and photon energies > 10 MeV. In the p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n processes, the photons e i t h e r e j e c t or create high-energy e l e c t r o n s . However, at energies above about 8 MeV f o r high-Z m a t e r i a l s and i n the r e g i o n of 10 to 20 MeV f o r low-Z m a t e r i a l s , the photons may have s u f f i c i e n t energy to e j e c t a proton or neutron from the nucleus of an atom. How- ever photonuclear cross s e c t i o n s are g e n e r a l l y s m a l l e r than Compton and p a i r - p r o d u c t i o n cross s e c t i o n s at the same energy so that the photonuclear process g e n e r a l l y makes a n e g l i g i b l e c o n t r i b u t i o n to the t o t a l energy a b s o r p t i o n . The three main processes then i n which high-energy photons may be absorbed by matter are the p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n processes. F i g u r e 1 shows the v a r i a t i o n of the t o t a l and p a r t i a l molecular 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 water as a f u n c t i o n of photon energy. The atomic a b s o r p t i o n c o e f f i c i e n t , y , i s r e l a t e d to the 3. 3. l i n e a r a b s o r p t i o n c o e f f i c i e n t by the relationship''' 2 -1 y = y A/pN cm atom (1.6) 3 3 3 O where p i s the d e n s i t y and A the atomic weight of the stopping m a t e r i a l , N being Avogadro's number. Since the p r o b a b i l i t y of a photon i PHOTON E N E R G Y ( M e V ) Figure 1. Atomic absorption coefficients for water. Curve A, total absorption coefficient (with coherent scattering); B, photoelectric absorption coeff ic ient ; C, Compton coefficient (with coherent scattering); D, Compton coefficient (without coherent scattering); E, pair-production coeff ic ient . -24 2' 1 barn = 10 cm . (Adapted from Figure 3.6, page 54, reference 1). - 7 - i n t e r a c t i n g w i t h an atom i s independent of i t s environment, the molecular a b s o r p t i o n c o e f f i c i e n t i s simply the sum of the atomic a b s o r p t i o n c o e f f i c i e n t s . T h u s the molecular absorption c o e f f i c i e n t f o r water i s : WR2O " 2 W H + Wo ( 1' 7 ) As i n d i c a t e d p r e v i o u s l y , the abs o r p t i o n c o e f f i c i e n t s f o r the va r i o u s processes mentioned above may be added to give the t o t a l a bsorption c o e f f i c i e n t , y = T + a + K (1.8) a a a a where T , a and K are the l i n e a r a b s o r p t i o n c o e f f i c i e n t s f o r the a a a c p h o t o e l e c t r i c , Compton and p a i r - p r o d u c t i o n processes r e s p e c t i v e l y . The sm a l l c o n t r i b u t i o n s from coherent s c a t t e r i n g and photonuclear r e a c t i o n s are neglected. Since the two Y ~ r a y s emitted by ̂ C o are 1.33 MeV and 1.17 MeV i n energy, the absor p t i o n i n water and other low-Z m a t e r i a l s (such as DMSO) w i l l be predominantly by the Compton process f o r the primary Y - r a y s o t h a t y - a . However Compton photons of a a lower energy w i l l have a much more important p h o t o e l e c t r i c c o n t r i b u t i o n . The Compton e l e c t r o n i c a b s o r p t i o n c o e f f i c i e n t , eo" a, i s r e l a t e d to the l i n e a r a b s o r p t i o n c o e f f i c i e n t i n the f o l l o w i n g manner, (a /o) = a (Z/A)N cm2gm 1 (1.9) a e a o where p i s the density,A i s the atomic weight (molecular weight) and Z i s - 8 - the atomic number (or sum of atomic numbers i f a compound) of the stopping medium. For r a d i a t i o n of a given energy the Compton e l e c t r o n energy c o e f f i c i e n t i s the same f o r a l l materials"'" so that (a /p) a (Z/A). Thus f o r r a d i a t i o n energies f o r which m a t e r i a l s absorb energy predominantly by the Compton process, energy a b s o r p t i o n i s p r o p o r t i o n a l to the e l e c t r o n d e n s i t y (the number of e l e c t r o n s per gram). Most of the y-ray energy i s t r a n s f e r r e d to the k i n e t i c energy of one or two high-energy e l e c t r o n s . Since these Compton e l e c t r o n s are r e s p o n s i b l e f o r the chemical e f f e c t s observed, i t i s necessary to know how high-energy e l e c t r o n s subsequently i n t e r a c t w i t h the chemical c o n s t i t u e n t s of matter. 2. High-Energy E l e c t r o n s High-energy e l e c t r o n s i n t e r a c t w i t h matter through i n e l a s t i c and e l a s t i c c o l l i s i o n s and by the emission of electromagnetic r a d i a t i o n c a l l e d bremsstrahlung emission. When a high-energy e l e c t r o n passes c l o s e to the nucleus of an atom, i t i s de c e l e r a t e d by the e l e c t r i c f i e l d and r a d i a t e s e l e c t r o - magnetic r a d i a t i o n (bremsstrahlung) w i t h a r a t e , -dE/dx, p r o p o r t i o n a l 2 2 2 to e Z /m where e and Z are the charge on the e l e c t r o n and nucleus r e s p e c t i v e l y and m i s the e l e c t r o n mass. As a r e s u l t , bremsstrahlung emission w i l l be grea t e s t f o r stopping m a t e r i a l s of high atomic number. Thus lead and tungsten are g e n e r a l l y used f o r e f f i c i e n t X-ray production from e l e c t r o n beams. The X - r a d i a t i o n emitted may then be p a r t i a l l y - 9 - absorbed i n " the medium by the processes described e a r l i e r . Bremsstrahlung emission i s unimportant below 100 keV but predominates above 100 MeV. For 10 MeV e l e c t r o n s bombarding tungsten % 50% of the energy i s d i s s i p a t e d by bremsstrahlung emission. At low energies where bremsstrahlung emission i s unimportant, e l e c t r o n d e c e l e r a t i o n i s predominantly through coulombic i n t e r a c t i o n w i t h the e l e c t r o n s of the stopping m a t e r i a l . The l i n e a r r a t e of energy l o s s , c a l l e d l i n e a r energy t r a n s f e r (LET) and expressed as -dE/dx, f o r an e l e c t r o n having r e l a t i v i s t i c v e l o c i t y v i s given by the 3 Bethe e x p r e s s i o n . -dE dx 2irNe Z m v E - (2>JlH52 - 1 + g 2 ) l n 2 + 1 - 3 2 [In o m v o 2 2 2 2I Z(1-3 Z) (1.10) Here, N i s the number of atoms per cubic centimeter, e i s the e l e c t r o n charge, Z i s the atomic number of the stopping m a t e r i a l , m i s the e l e c t r o n mass, o E i s the k i n e t i c energy of the e l e c t r o n , e r g s , v i s the v e l o c i t y of the e l e c t r o n , g equals v / c , where c i s the v e l o c i t y of l i g h t , I i s the average e x c i t a t i o n p o t e n t i a l of the stopping medium. - 10 - As these e l e c t r o n s are slowed down by these i n e l a s t i c c o l l i s i o n s , s u f f i c i e n t energy i s t r a n s f e r r e d to the medium or stopping m a t e r i a l to cause i o n i z a t i o n s and e x c i t a t i o n s . From expression (1.10), i t can be seen that the LET f o r t h i s process i n c r e a s e s w i t h the atomic number of the medium (Z) and decreases w i t h i n c r e a s i n g k i n e t i c energy of the e l e c t r o n . In water, the LET i s ̂  0.02 eV/A° f o r e l e c t r o n s having approximately 1 MeV of energy but increases to about 0.22 eV/A° as the k i n e t i c energy f a l l s to 10 keV."'' The r a t i o of energy l o s s per u n i t path length by bremsstrahlung emission to that by i n e l a s t i c c o l l i s i o n s i s given by"*" (- d E/ d x>brem EZ (-dE/dx) n i 2 (1.11) c o l l 1600 m c o where tie terms have already been def i n e d . For Compton e l e c t r o n s of up to 1.3 MeV from ^ C o y - r a y s t r a v e r s i n g DMSO, l e s s than 1% of the energy w i l l be emitted as bremmstrahlung r a d i a t i o n . However, i n the case of 35 MeV e l e c t r o n s from the l i n e a r a c c e l e r a t o r , approximately 25% of the energy i s l o s t as bremsstrahlung emission. In a d d i t i o n to bremsstrahlung r a d i a t i o n , there i s another form of r a d i a t i v e emission which occurs when high-energy e l e c t r o n s pass through matter, namely Cerenkov r a d i a t i o n . This occurs whenever a charged p a r t i c l e passes through any medium w i t h a v e l o c i t y greater than the phas v e l o c i t y of l i g h t i n the m a t e r i a l . As the e l e c t r o n s t r a v e r s e the d i e l e c t r i c medium, the molecules become temp o r a r i l y p o l a r i z e d . When - l i - the e l e c t r o n passes, they r e l a x and emit an electromagnetic wave which i s i n the v i s i b l e and u l t r a v i o l e t r e g i o n of the spectrum. Only those photons emitted at an angle 6 w i t h respect to the e l e c t r o n ' s t r a j e c t o r y w i l l c o n s t r u c t i v e l y i n t e r f e r e as shown i n the simple Huygens c o n s t r u c t i o n i n Figure 2. d i r e c t i o n of propagation d i r e c t i o n of propagation F i g u r e 2. Huygens c o n s t r u c t i o n of e l e c t r o n t r a j e c t o r y and r e s u l t i n g Cerenkov waveform. I f the e l e c t r o n t r a v e l s a d i s t a n c e to P^ i n the time the l i g h t wave t r a v e l s from P^ to X, then the wavelets from P^, and P^ w i l l 4 c o n s t r u c t i v e l y i n t e r f e r e . From Figure 2, the "Cerenkov r e l a t i o n " cos6 = 1/Bn (1.12) i s e s t a b l i s h e d where B i s the v e l o c i t y of the p a r t i c l e r e l a t i v e to the - 12 - v e l o c i t y of l i g h t i n vacuo, v/c, and n i s the r e f r a c t i v e index of the medium. Since cose < 1, Cerenkov emission w i l l occur only i f f3 > 1/n; that i s , i f the v e l o c i t y of the e l e c t r o n i s gre a t e r than the phase v e l o c i t y of l i g h t i n the medium. According to the Frank-Tamm theory"* the r a t e of energy l o s s per u n i t d i s t ance i n the form of 4 Cerenkov r a d i a t i o n i s given by c J gn > I 6 N where to = 2T T V. Since the l i g h t i n t e n s i t y of frequency co i s E = Nfito, where N i s the number of photons, i t can be shown that equation (1.13) 4 may be w r i t t e n as equation (1.14) f o r a l l frequencies where n > 1. d 2N 1 f A According to equation (1.14) the number of photons emitted per u n i t path l e n g t h , £., of the e l e c t r o n ' s t r a c k per u n i t wavelength i n t e r v a l f o l l o w s 2 a 1/X s p e c t r a l d i s t r i b u t i o n i n the v i s i b l e and u l t r a v i o l e t r e g i o n s . Although energy l o s s by Cerenkov emission i s n e g l i g i b l e compared to that by i n e l a s t i c c o l l i s i o n s and bremsstrahlung r a d i a t i o n , i t s i n t e n s i t y exceeds the bremsstrahlung by a very large f a c t o r i n the v i s i b l e r e g i o n . For t h i s reason, and the f a c t that the emission i s "instantaneous", Cerenkov emission i s often used to monitor the pulse shape of an e l e c t r o n a c c e l e r a t o r . This w i l l be discussed more f u l l y l a t e r . - 13 - 3. Range of E l e c t r o n s U n l i k e electromagnetic r a d i a t i o n , high-energy e l e c t r o n s have a f i n i t e range i n an absorbing medium. As e l e c t r o n s t r a v e r s e the medium they are c o n s t a n t l y d e f l e c t e d and slowed down by e l a s t i c and i n e l a s t i c c o l l i s i o n s w i t h the medium e l e c t r o n s or by i n t e r a c t i o n s w i t h the atomic n u c l e i (bremsstrahlung). Because of these d e f l e c t i o n s t h e i r t o t a l path l e n g t h w i l l exceed t h e i r depth of p e n e t r a t i o n . Therefore the path l e n g t h i s d e fined as the d i s t a n c e t r a v e l l e d by the impinging e l e c t r o n along i t s path before i t i s brought to r e s t whereas the range or p e n e t r a t i o n i s the d i s t a n c e t r a v e l l e d i n the d i r e c t i o n of o r i g i n a l momentum. In theory, the range may be obtained by numerical i n t e g r a t i o n of a s u i t a b l e stopping-power formula s i m i l a r to equation (1.10). I f one n e g l e c t s the v e l o c i t y dependence of LET then the range would be p r o p o r t i o n a l to the square of the e l e c t r o n energy. For high-energy e l e c t r o n s t h i s approximation i s reasonable s i n c e the range i s roughly p r o p o r t i o n a l to E m , m being only s l i g h t l y l e s s than 2 but decreasing as E decreases.** E m p i r i c a l formulae have been developed to r e l a t e the range and energy of e l e c t r o n s i n aluminum absorbers. For e l e c t r o n s of energy 0.01 to -2 1 2.5 MeV, the range i n mg cm i s given by Range = 412 1.265-0.0954 InE (1.15) where E i s the k i n e t i c energy of the e l e c t r o n s (MeV). For e l e c t r o n energies greater than 2.5 MeV, the range i s given by (1.16).''" Range = 530E - 106 (1.16) - 14 - These formulae can be a p p l i e d to other l i g h t elements s i n c e the range (mg cm ) v a r i e s only s l i g h t l y w i t h atomic number. Thus f o r a 1 MeV e l e c t r o n i n DMSO, the range, c a l c u l a t e d using (1.15), corresponds to -2 1.41 gm cm or to a th i c k n e s s of 1.28 cm. Using 35 MeV e l e c t r o n s , an approximate range, c a l c u l a t e d u s i n g (1.16), i s found to be 17 cm. This i s only a rough estimate s i n c e equation (1.16) i s good only up to about 20 MeV. 4. Track E n t i t i e s ; Spurs, Blobs, and Short Tracks In r a d i a t i o n chemistry the primary chemical events a r i s e from the i o n i z a t i o n s and e x c i t a t i o n s of the molecules produced by the secondary high-energy e l e c t r o n s or other charged p a r t i c l e s . The d i s t r i b u t i o n of these a c t i v e species i n the i r r a d i a t e d m a t e r i a l i s not homogeneous or random. Instead the a c t i v e species are produced only along the t r a c k of the i i c i d e n t p a r t i c l e and i t i s only a f t e r these species have d i f f u s e d throughout the r e a c t i o n volume that the system can be considered homogeneous. In a d d i t i o n , the a c t u a l y i e l d s of a c t i v e s pecies and t h e i r s p e c i f i c d i s t r i b u t i o n i n space depend upon the LET of the p a r t i c u l a r i n c i d e n t p a r t i c l e . Consequently, d i f f e r e n t o v e r a l l chemical changes can a r i s e from d i f f e r e n t i o n i z i n g r a d i a t i o n s simply because of the v a r i o u s s p a t i a l d i s t r i b u t i o n s of the primary species formed. As the primary e l e c t r o n s (or Compton e l e c t r o n s ) are slowed down by i n t e r a c t i o n s w i t h the medium, they produce a t r a i l of e x c i t e d and i o n i z e d species along t h e i r t r a c k s . The e l e c t r o n s which are e j e c t e d - 15 - as a consequence of the i o n i z a t i o n s may themselves be s u f f i c i e n t l y e n e r g e t i c to produce f u r t h e r i o n i z a t i o n s and e x c i t a t i o n s . I f the energy of these secondary e l e c t r o n s i s s m a l l (< 100 eV), t h e i r range i n a l i q u i d or s o l i d w i l l be s h o r t . Consequently any f u r t h e r i o n i z a t i o n s or e x c i t a t i o n s produced w i l l be l o c a l i z e d i n a small r e g i o n (perhaps o roughly approximated to a sphere) w i t h a mean diameter of ^20 A. These s m a l l c l u s t e r s of e x c i t e d and/or i o n i z e d species are c a l l e d spurs. -2 0 For a 1 MeV e l e c t r o n , whose LET i s ^ 10 eV/A, the spurs w i l l be separated, on the average, by s e v e r a l thousand Angstrom u n i t s . However there w i l l be a continuous d i s t r i b u t i o n of e j e c t e d e l e c t r o n energies and t h e r e f o r e a d i s t r i b u t i o n i n i n t e r - s p u r d i s t a n c e s . In some cases the secondary e l e c t r o n s may have s u f f i c i e n t energy to form branch t r a c k s of t h e i r own. These e l e c t r o n s are c a l l e d 6-rays. In other cases the spurs may overlap so that there i s e s s e n t i a l l y a continuous c y l i n d r i c a l r e g i o n of a c t i v a t e d s p e c i e s . Mozumder and Magee^ considered t h i s d i s t r i b u t i o n of species f o r the case of water i n the f o l l o w i n g way. Those e l e c t r o n s which have energies i n excess of 100 eV but i n s u f f i c i e n t energy to a l l o w them to escape the coulombic a t t r a c t i o n of t h e i r p o s i t i v e " h o l e " are c a l l e d super spurs or "blobs". I f the e l e c t r o n s are e n e r g e t i c enough to escape the "hole" but not e n e r g e t i c enough to prevent spur o v e r l a p , they form "short t r a c k s " . In the case of water, these energies are divided as f o l l o w s : spur, ^6-100 eV; b l o b , ^ 100-500 eV; short t r a c k , 'v, 500 eV-5 keV. Mozumder and Magee^ estimated the p a r t i t i o n of the primary energy of the e l e c t r o n i n t o these v a r i o u s track e n t i t i e s and the r e s u l t s of t h e i r c a l c u l a t i o n s are shown i n Figure 3. ^ C o y r a y s PRIMARY E L E C T R O N E N E R G Y (MeV) Figure 3. Schematic p l o t of percentage of energy s p l i t between spurs, blobs and short tracks f o r e l e c t r o n s i n water. (Adapted from Figure 5, page 211, reference 6). - 17 - i n water give r i s e to e l e c t r o n s having a mean energy of 440 keV. About 64% of t h i s energy i s deposited i n the form of i s o l a t e d spurs, about 25% i n the form of short t r a c k s , and about 11% as "blobs". From Figure 3, i t can be seen that as the primary e l e c t r o n energy i n c r e a s e s , the f r a c t i o n of energy deposited i n i s o l a t e d spurs i n c r e a s e s . Knowledge of t h i s approximate d i s t r i b u t i o n i s u s e f u l i n d e s c r i b i n g spur d i f f u s i o n processes s i n c e r e a c t i o n s i n i s o l a t e d spurs are expected to be d i f f e r e n t from those i n "bl o b s " and'short tracks"where i o n and r a d i c a l recombination r e a c t i o n s w i l l be more h i g h l y favoured. B. CHEMICAL CONSEQUENCES FOLLOWING ABSORPTION OF HIGH-ENERGY RADIATION The o v e r a l l process of producing chemical changes i n a medium w i t h i o n i z i n g r a d i a t i o n begins w i t h the bombardment and pro d u c t i o n of i o n i z e d and e x c i t e d species and terminates w i t h the reestablishment of chemical e q u i l i b r i u m . However there are s e v e r a l orders of magnitude i n time between these two stages. The chemical events observed before or a f t e r the attainment of chemical e q u i l i b r i u m ma}' not n e c e s s a r i l y a r i s e d i r e c t l y from the primary processes but r a t h e r from the intermediates so produced. For t h i s reason i t i s necessary to know the types of processes which may occur and t h e i r r e l a t i v e time s c a l e i n order to c o r r e c t l y i n t e r p r e t the chemical events observed. - 18 - 1. Time Scale of Events In r a d i a t i o n chemistry one u s u a l l y recognizes three stages f o l l o w i n g the abso r p t i o n of high-energy r a d i a t i o n and l a s t i n g up to the time of the production of the u l t i m a t e chemical e f f e c t . These are r e f e r r e d t o , i n order of i n c r e a s i n g time, as the p h y s i c a l stage, the physico-chemical stage and the chemical stage. In the p h y s i c a l stage, energy i s t r a n s f e r r e d to the system by the processes mentioned e a r l i e r . This i n v o l v e s the primary process and i t s d u r a t i o n i s of the order of 10 to 10 sec, the upper l i m i t being f i x e d by the Heisenberg U n c e r t a i n t y P r i n c i p l e (AE«At ^ "fl) f o r an e l e c t r o n d e p o s i t i n g 20 eV to 30 eV of energy. As the primary, secondary and higher-order e l e c t r o n s are slowed down by i n e l a s t i c c o l l i s i o n s w i t h the medium, they lo s e t h e i r excess energy by molecular e x c i t a t i o n s or i o n i z a t i o n s . When t h e i r energy has dropped below the lowest e l e c t r o n i c e x c i t a t i o n l e v e l ( u s u a l l y 1 to 10 eV), the e l e c t r o n s are c a l l e d " s u b e x c i t a t i o n e l e c t r o n s " and they subsequently lose t h e i r energy by e x c i t a t i o n of molecular v i b r a t i o n s and r o t a t i o n s . Since the -14 p e r i o d of a molecular v i b r a t i o n or r o t a t i o n i s of the order 10 to -12 10 sec, energy t r a n s f e r by these processes occurs over t h i s time range. These i n t e r a c t i o n s are the germane i n i t i a l events and thus the primary r e a c t i v e species are the p o s i t i v e i o n s , n e a r l y thermalized e l e c t r o n s , v i b r a t i o n a l l y and e l e c t r o n i c a l l y e x c i t e d molecules, a l l of which are the precursors of the observed chemical consequences of the r a d i a t i o n a b s o r p t i o n . The next stage i s c a l l e d the physico-chemical stage and l a s t s up to about 10 ± X sec. I t i s during t h i s p e r i o d that the unstable primary - 19 - species undergo secondary r e a c t i o n s , e i t h e r spontaneously or by c o l l i s i o n s w i t h adjacent ions or molecules. The p o s i t i v e ions may be i n v o l v e d i n charge n e u t r a l i z a t i o n , proton t r a n s f e r or they may decompose i n t o other fragments whereas the h i g h l y e x c i t e d molecules may decompose i n t o r a d i c a l species or los e energy by i n t e r n a l conversion. The e l e c t r o n reaches thermal e q u i l i b r i u m w i t h i t s environment, w i t h E % kT (0.025 eV) , at which time i t faces s e v e r a l d i f f e r e n t p o s s i b i l i t i e s . I t may be ( i ) recaptured by i t s parent i o n , ( i i ) captured by a solvent molecule to produce a negative i o n , ( i i i ) captured by a r a d i a t i o n - produced i o n or product other than i t s parent, ( i v ) captured by a scavenger molecule or i o n i n i t i a l l y present i n the system, or i t may (v) become trapped or s t a b i l i z e d among the molecules of the medium. The formation and p r o p e r t i e s of these t e m p o r a r i l y s t a b i l i z e d , or s o l v a t e d e l e c t r o n s w i l l be discussed l a t e r . U l t i m a t e l y the system a t t a i n s thermal e q u i l i b r i u m and the chemical stage begins. In the chemical stage the newly formed r e a c t i v e species which have escap- ed geminate recombination i n the spurs d i f f u s e out i n t o the bulk of the medium and e v e n t u a l l y they become homogeneously d i s t r i b u t e d . These primary or secondary species undergo thermal chemical r e a c t i o n s w i t h each other, w i t h scavengers or w i t h the medium i t s e l f . E x c i t e d molecules, formed e i t h e r i n the primary process or from i o n recombina- t i o n , can undergo v a r i o u s luminescent processes. These events continue u n t i l the system once again a t t a i n s chemical e q u i l i b r i u m . The above i s a r a t h e r general d e s c r i p t i o n of the types of chemical processes p o s s i b l e when a system i s bombarded by i o n i z i n g r a d i a t i o n . These processes and t h e i r time s c a l e s are i n d i c a t e d s c h e m a t i c a l l y i n Figure 4. The time s c a l e r e f e r s to the l i q u i d phase and would be Dielectric Relaxation Molecular Vibrations 16 < — 15 14 —> 13 Electronic Energy Deposit Molecules Jump < > 12 11 Electron Thermalization Spur Diffusion Processes Radiative Electronic Transitions 10 8 Ion Recombination Subexcitation Electron Electron Recapture o Internal Conversion of < > Electronic States (non - radiative) Figure 4. T h e o r e t i c a l time sc a l e f o r the i n i t i a l processes i n r a d i a t i o n chemistry. The numbers are the negative logarithm of time (pt = - l o g t ( s e c ) ) . The time s c a l e r e f e r s to the l i q u i d s t a t e . - 21 - d i f f e r e n t i n s e v e r a l respects when t h i s d e s c r i p t i o n i s a p p l i e d to the s o l i d or gas phase. 2. Phase-Dependent Phenomena whenever a compound i s exposed to a source of i o n i z i n g r a d i a t i o n the primary processes are l a r g e l y independent of the phase of the medium. However the subsequent chemical processes w i l l depend markedly on the phase. For i n s t a n c e , there are no t r a c k e f f e c t s or s p u r - c o n t r o l l e d r e a c t i o n s of consequence i n the gas phase. The i o n - p a i r s formed i n the gas phase have a n e g l i g i b l e p r o b a b i l i t y of undergoing geminate recombination because the mean f r e e path of the e l e c t r o n i s too long. Consequently the p o s i t i v e i o n may experience s e v e r a l c o l l i s i o n s w i t h n e u t r a l molecules before n e u t r a l i z a t i o n occurs and thus has the o p p o r t u n i t y to decompose or undergo ion-molecule r e a c t i o n s . The e l e c t r o n , on the other hand, may a t t a c h i t s e l f to a n e u t r a l molecule and the r e s u l t i n g negative i o n may d i s p r o p o r t i o n a t e before undergoing n e u t r a l i z a t i o n w i t h a p o s i t i v e i o n . However, i n an i d e n t i c a l i o n i z a t i o n event i n the l i q u i d or s o l i d phase, the i o n i z e d and e x c i t e d molecules and molecular fragments formed by decomposition of e x c i t e d molecules are produced at h i g h l o c a l c o ncentrations i n the t r a c k s and spurs. Consequently the p r o b a b i l i t y of t h e i r r e a c t i o n w i t h each other i s i n c r e a s e d r e l a t i v e to t h e i r r e a c t i o n w i t h the medium or added scavengers. Furthermore, concomitant partners formed by the rupture of a given molecule w i l l be trapped w i t h i n the same s o l v e n t cage thereby i n c r e a s i n g the p r o b a b i l i t y of geminate recombination. Because of t h i s d i f f e r e n c e i n s p a t i a l d i s t r i b u t i o n of the r e a c t i v e i n t e r m e d i a t e s , the r a d i a t i o n chemistry of a gaseous system i s o f t e n d i f f e r e n t from i t s - 22 - liquid or solid phase counterpart, and solid phase were undertaken in be limited to the condensed phase. Since only studies on the liquid this work, further discussions w i l l 3. Studies on the Chemical Events in Condensed Phases Radiation chemical studies are not just concerned with the net chemical effect but with an understanding of the detailed mechanism leading to the change. The ionized and excited molecules i n i t i a l l y formed subsequently give rise to a series of ±>nic or radical intermediates which then produce the stable chemical products. By identifying these products and studying the effects on their yields caused by the addition of various radical, ionic or electron scavengers, i t i s possible, by inference, to speculate on the identity of the intermediates and thereby propose a reaction scheme. The lifetime of the transient species which have escaped the - 9 - 6 intra-spur reactions is often of the order 10 seconds to 10 seconds, and assuming the scavengers react at diffusion-controlled rates, 10"^ -1 -1 -1 -4 M sec , scavenger concentrations of the order 10 M to 10 M are required to compete effectively with their alternative decay processes. Those species which decay by intra-spur reactions and hence cannot be scavenged at these concentrations give rise to products which are referred to as "molecular products". By the method of pulse 7-9 radiolysis, i t is possible to observe the formation and decay of many of these transient intermediates. In this technique short pulses (typically 10 ^ to 10 ^ sec) of high energy electrons are used as the radiation source at intensities sufficiently high to produce "instantaneous" concentrations of transient species which may be detected - 23 - and ident i f ied by various fast physical methods, part icular ly by optical absorption spectroscopy. From the absorption spectra, i t is often possible to identify free radicals , molecular ions, solvated electrons or excited molecules. Sometimes this ident i f i ca t ion comes from comparison to other systems where the species have been well characterized but in many cases they have to be assigned on the basis of their chemical behaviour to various scavengers. In the l i q u i d phase these active species w i l l usually react with each other or with the solvent or scavengers in times of microseconds or less . As a result , they can only be observed, i f at a l l , by using very fast pulse radiolysis techniques. However, i f the medium is in the s o l i d state, and at s u f f i c i e n t l y low temperatures (usually 77°K or lower), the rate of reaction of these transient intermediates may be slowed down so that they can be observed over periods of minutes or even years. This approach is often referred to as the " i sola t ion technique". If these species are paramagnetic, such as radicals and trapped electrons, they can then be studied by electron spin resonance spectroscopy. They may also be observed by optical spectro- scopy i f the sample is s u f f i c i e n t l y transparent. The formation and decay of the various intermediates, i n particular the electron, depend to a great extent upon whether or not the medium is glassy (amorphous) or polycrystal l ine . Often studies on the sol id state give a valuable insight into the processes occurring in the l iquid state, especially since not a l l transients can be posit ively identif ied through their optical and chemical behaviour alone. The techniques of optical spectroscopy, electron spin resonance - 24 - (esr) spectroscopy and pulse r a d i o l y s i s w i l l be d e s c r i b e d more f u l l y i n the next chapter. However, i t must be s t r e s s e d that the t r a n s i e n t i ntermediates observed using these techniques are those a c t i v e species r e a c t i n g during the chemical stage only. The e a r l i e s t events which have been r e s o l v e d e x p e r i m e n t a l l y using pulse r a d i o l y s i s occur l a t e r than 10 picoseconds a f t e r energy d e p o s i t i o n ; consequently any e x t r a - p o l a t i o n to the events o c c u r r i n g from 10 to 10 ± X seconds can only be s p e c u l a t i v e . 4. Chemical Y i e l d s Any s u c c e s s f u l d e s c r i p t i o n of the e f f e c t of r a d i a t i o n on matter must i n v o l v e the y i e l d s of the r a d i o l y t i c products. In r a d i a t i o n chemistry chemical y i e l d s are expressed as G v a l u e s , the number of events of a s p e c i f i c k i n d induced i n a medium per 100 eV of energy absorbed. The u n i t energy, 100 eV, i s an e n t i r e l y a r b i t r a r y magnitude and, as such, G values have no i n t r i n s i c or s t o i c h i o m e t r i c s i g n i f i c a n c e . In general the y i e l d s are w r i t t e n as G(X) where X r e f e r s to the atoms, i o n s , r a d i c a l s , e x c i t e d species or molecules used up or produced by the energy d e p o s i t i o n . Often c o r r e l a t i o n s are drawn between the G value f o r i o n i z a t i o n i n the gas phase and the W v a l u e , which i s the mean energ r e q u i r e d i n i o n - p a i r formation. T y p i c a l W values f o r gases are i n the r e g i o n of 30 eV. Since i o n i z a t i o n p o t e n t i a l s are t y p i c a l l y ^ 10 eV i t f o l l o w s that only 30-40% of the energy i s d i s s i p a t e d i n i o n i z i n g processes. The r e l a t i o n s h i p may be expressed as f o l l o w s : G ( i o n - p a i r s ) = 100/W (1.17) - 25' - I f one assumes that the W value f o r the condensed phase i s the same as i t s gas phase counte r p a r t , then G ( i o n - p a i r s ) i n the condensed phases should be 3-4. Scavenging s t u d i e s at very high concentrations have i n d i c a t e d i o n i c y i e l d s not j u s t between 3 and 4 but even up to 5, to i n d i c a t e that W values may be somewhat sm a l l e r i n the condensed phase than i n the gas phase. Another parameter which i s o f t e n used to c h a r a c t e r i z e r a d i a t i o n e f f e c t s i s the i o n - p a i r y i e l d , M/N, where M i s the number of species X produced and N i s the number of i o n - p a i r s . This parameter was w i d e l y used i n the e a r l i e r years of r a d i a t i o n chemistry when i t was b e l i e v e d that v i r t u a l l y a l l induced chemical changes arose from i o n i c precursors. I t i s r e l a t e d to the G value by equation (1.18). G ( X , - f • 2f (1.18) However, expr e s s i o n (1.18) i s not u s e f u l when a p p l i e d to condensed systems s i n c e N and W cannot be measured d i r e c t l y . For t h i s reason, G values are used to express r a d i a t i o n chemical y i e l d s s i n c e they can be obtained d i r e c t l y and do not imply, as does the i o n - p a i r y i e l d , that the chemical a c t i o n i s c o n t r o l l e d by the number of ions formed. A l l that i s r e q u i r e d i s a knowledge of the number of species produced and the dose absorbed by the medium. The l a t t e r i s obtained by dosimetry and w i l l be d e s c r i b e d l a t e r . G e n e rally the G values of species produced by i o n i z i n g r a d i a t i o n range from C to 5. Y i e l d s greater than 5 u s u a l l y s i g n i f y chain r e a c t i o n s . However i t must be emphasized that the G values measured - 26 - represent average values. They are averaged over the range of LET involved and therefore small yields can arise, for instance, exclusively from the specific chemistry peculiar to short tracks or "blobs" and may not be at a l l representative of isolated spurs. C. STABILIZED ELECTRONS In many systems stabilized electrons are the principal chemically- reducing species produced by the interaction of ionizing radiation with matter. Although the existence of stabilized electrons i n solutions of a l k a l i metals in ammonia has been known for over f i f t y years, i t was only with the advent of pulse radiolysis that they were directly observed as intermediates i n the radiation chemistry of water.L® This was because the electrons have a high reduction potential and as such are extremely reactive and very short-lived. In the last decade stabilized electrons have been identified and studied in many other systems; indeed, no other species studied in radiation chemistry has lik e l y commanded as much attention."''"'' 1. Stabilization In gaseous media the thermalized electrons collide e l a s t i c a l l y or ine l a s t i c a l l y with unreactive molecules u n t i l they are eventually captured by reactive molecules or ions, undergoing electron attachment or dissociative reactions. However, in condensed media electrons may become confined to a cavity, either by the thermalized electron - 27 - p o l a r i z i n g the medium through r e p u l s i v e f o r c e s followed by r e o r i e n t a t i o n of the molecules to produce a b e t t e r t r a p , or by the e l e c t r o n " f a l l i n g i n t o " a p r e - e x i s t i n g , s u i t a b l y o r i e n t e d v o i d . The l i f e t i m e of these l o c a l i z e d or s t a b i l i z e d e l e c t r o n s w i l l depend upon the thermal motion of the molecules forming the c a v i t y w a l l s . In the l i q u i d phase the l i f e t i m e of these e l e c t r o n s , which w i l l be c a l l e d s o l v a t e d e l e c t r o n s and denoted by e g , i s o f t e n l e s s than 10 ^ sec due to t h e i r m o b i l i t y and h i g h r e a c t i v i t y towards the medium, other r a d i c a l s , p o s i t i v e i o n s , r a d i a t i o n products or scavengers. In the s o l i d phase, however, and at low enough temperatures, the c a v i t i e s may be "frozen i n " and the e l e c t r o n l i f e t i m e may be extended by s e v e r a l orders of magnitude. Such e l e c t r o n s are c a l l e d trapped e l e c t r o n s and are designated by e^ . The s i m i l a r i t y between the chemical and p h y s i c a l p r o p e r t i e s of e l e c t r o n s s o l v a t e d i n l i q u i d s and those trapped i n the s o l i d phase suggest that the two species are i d e n t i c a l except f o r m o b i l i t y . By studying these p r o p e r t i e s i n f o r m a t i o n regarding the s t a b i l i z a t i o n process can h o p e f u l l y be obtained, f o r , d e s p i t e the amount of work being c a r r i e d out on the s t a b i l i z e d e l e c t r o n , no s a t i s f a c t o r y theory regarding the nature of the t r a p p i n g s i t e s nor the mechanism of s o l v a t i o n has yet been proposed. 2. P r o p e r t i e s The s t a b i l i z e d e l e c t r o n i s regarded as the s i m p l e s t and most r e a c t i v e chemical e n t i t y . I t i s h i g h l y c h a r a c t e r i z e d by i t s i n t e n s e a b s o r p t i o n spectrum i n the v i s i b l e and near i n f r a - r e d r e g i o n , i t s paramagnetism and i t s h i g h m o b i l i t y . - 28 - 2.1 O p t i c a l Spectrum The most prominent feature of s t a b i l i z e d e l e c t r o n s i s t h e i r i n t e n s e a b s o r p t i o n s p e c t r a i n the v i s i b l e and near i n f r a r e d r e g i o n . The s p e c t r a are c h a r a c t e r i z e d by t h e i r broadness and l a c k of s t r u c t u r e , t h e i r asymmetry on the high-energy s i d e and by t h e i r i n t e n s i t y , w i t h 4 -1 -1 t y p i c a l molar e x t i n c t i o n c o e f f i c i e n t s > 10 M cm at the maximum. A l l c urrent t h e o r i e s regard the s t a b i l i z e d e l e c t r o n as being confined to a type of p o t e n t i a l w e l l , or c a v i t y , the depth of which depends upon the p o l a r i z a t i o n energy a c t i n g back on the e l e c t r o n . L i k e any l o c a l i z e d system, there w i l l be quantized energy l e v e l s a s s o c i a t e d w i t h each c a v i t y . The a b s o r p t i o n spectrum may then be a t t r i b u t e d to t r a n s i t i o n s between these l e v e l s and thus the t r a n s i t i o n energy may be taken as r e p r e s e n t a t i v e of the w e l l depth or the s o l v a t i o n energy of the e l e c t r o n . In nonpolar media, such as the hydrocarbons, s t a b i l i z a t i o n can only occur through short-range r e p u l s i o n s or induced e l e c t r o n i c p o l a r i z a t i o n of the surrounding molecules, so that the c a v i t y depth w i l l be r a t h e r shallow. On the other hand, w i t h p o l a r molecules such as water, a l c o h o l s and amines, o r i e n t a t i o n (atomic and d i p o l e ) p o l a r i z a t i o n can a l s o occur thereby i n c r e a s i n g s t a b i l i z a t i o n . Since the absorption s p e c t r a of e t i n v i t r e o u s , g l a s s y s o l i d s are v i r t u a l l y the same as those of t h e i r l i q u i d c ounterpart, t h i s suggests that these c a v i t i e s , w i t h the optimum d i p o l e arrangement, e x i s t e d before the a r r i v a l of the e l e c t r o n . This i s p a r t i c u l a r l y true of p o l a r media which owe t h e i r strong s o l v a t i o n to o r i e n t a t i o n p o l a r i z a t i o n . In the l i q u i d s t a t e t h i s d i p o l e r e l a x a t i o n may f o l l o w the i n i t i a l e l e c t r o n i c p o l a r i z a t i o n by the e l e c t r o n but may take s e v e r a l orders of magnitude longer at lower temperatures. However d i e l e c t r i c r e l a x a t i o n times are measured on a macroscopic s c a l e whereas r o t a t i o n a l - 29 - o s c i l l a t i o n s by the p o l a r molecules forming the c a v i t y w a l l s may be p o s s i b l e under the e l e c t r o n ' s f i e l d at a l l temperatures. Recently 12 Richards and Thomas observed a s h i f t i n the absor p t i o n s p e c t r a of trapped e l e c t r o n s i n glassy ethanol at 77°K towards the blue on a microsecond time s c a l e . S i m i l a r observations i n b i n a r y mixtures of 13 a l c o h o l s and water were made by Kevan. This blue s h i f t was a t t r i b u t e d to i n i t i a l t r a p p i n g of the thermal e l e c t r o n s i n l e s s than optimum traps f o l l o w e d by mic r o s c o p i c r e l a x a t i o n to produce deeper, more s t a b l e t r a p s . Thus the s t a b i l i z a t i o n of e l e c t r o n s i s probably f a c i l i t a t e d by the pre - e x i s t e n c e i n the medium of s u i t a b l y o r i e n t e d v o i d s , but r e o r g a n i z a t i o n of the c a v i t i e s occurs a f t e r e l e c t r o n capture. U n f o r t u n a t e l y the time s c a l e of current apparatus i s l i m i t e d to the 14 20 picosecond range, by which time e l e c t r o n s i n l i q u i d media at room temperature have already been s o l v a t e d . Extension of these s t u d i e s to low temperature l i q u i d systems i s a l s o p o s s i b l e . In p o l y c r y s t a l l i n e media, the s p e c t r a are s i m i l a r to the l i q u i d phase but the y i e l d of trapped e l e c t r o n s i s much lower. Because of the long-range order i n c r y s t a l s , there are l e s s s u i t a b l y o r i e n t e d v o i d s f o r i n i t i a l l y t r a p p i n g the e l e c t r o n compared to the gl a s s y s t a t e . As a r e s u l t , the e l e c t r o n s can only be trapped at defect s i t e s i n the c r y s t a l l a t t i c e . Besides the medium p o l a r i z a b i l i t y , the c a v i t y or v o i d r a d i u s i s a l s o a f a c t o r i n governing the s o l v a t i o n energy of the e l e c t r o n . A l l t h e o r i e s concerning the l o c a l i z e d e l e c t r o n p r e d i c t that the energy l e v e l s eparations should d i m i n i s h w i t h i n c r e a s i n g c a v i t y r a d i u s . That i s , the s e l f - i n d u c e d energy a c t i n g on the e l e c t r o n f a l l s o f f w i t h the - 30 - d i s t a n c e from the c a v i t y center. However, i t should be noted that the c a v i t y r a d i u s i s not to be confused w i t h the e f f e c t i v e i o n i c r a d i u s . The s t a b i l i z e d e l e c t r o n has a s m a l l mass and low momentum and i s there f o r e "smeared out" over a l a r g e volume i n accordance w i t h the u n c e r t a i n t y p r i n c i p l e . This i s represented i n Figure 5 where the e l e c t r o n , represented by the shaded p o r t i o n , i s spread over s e v e r a l molecules. The arrows represent the induced or permanent d i p o l e moments. The t i p s of the d i p o l e v e c t o r s represent the c a v i t y , or v o i d , i n the medium. Figure 5 . Schematic r e p r e s e n t a t i o n of e l e c t r o n l o c a l i z a t i o n (shaded region) produced by p o l a r i z a t i o n of the medium. The dotted area represents the v o i d or c a v i t y i n which the e l e c t r o n i s centered. As the c a v i t y r a d i u s i s decreased, the s e l f - e n e r g y i n c r e a s e s , and the s o l v a t i o n energy i s incr e a s e d . This has been demonstrated i n s t u d i e s on systems at low temperatures and high pressures where the s h i f t i n the e l e c t r o n a b s o r p t i o n band towards the blue has been a t t r i b u t e d to a decrease i n the c a v i t y radius caused by these e x t e r n a l e f f e c t s . The shape and p o s i t i o n of the absorption band w i l l depend upon the - 31 - manner i n which the energies of the ground and e x c i t e d s t a t e s vary w i t h the shape of the c a v i t y due to the va r i o u s arrangements of the molecules forming the c a v i t y w a l l s as w e l l as any d i s t r i b u t i o n i n c a v i t y s i z e s . This would suggest a reasonable e x p l a n a t i o n f o r the e x c e p t i o n a l width (% 1 eV) of the o p t i c a l s p e c t r a of s t a b i l i z e d e l e c t r o n s and why they show no s t r u c t u r e . Each e l e c t r o n w i l l be s t a b i l i z e d i n a p a r t i c u l a r environment and w i l l e x h i b i t a c h a r a c t e r i s t i c a b s o r p t i o n . The observed spectrum i s then simply the envelope of the i n d i v i d u a l s p e c t r a i n which the maximum represents the most probable c a v i t y or p o t e n t i a l w e l l . The asymmetric t a i l of the abso r p t i o n band on the high-energy s i d e of the a b s o r p t i o n maximum may be i n t e r p r e t e d , perhaps, i n terms of t r a n s i t i o n s of the s t a b i l i z e d e l e c t r o n s to higher e x c i t e d s t a t e s or an energy continuum (conduction band). In the d i s c u s s i o n so f a r i t has been assumed that the e x c i t e d s t a t e of the e l e c t r o n i s a bound s t a t e . I f one t r e a t s the c a v i t y as a s p h e r i c a l l y symmetric p o t e n t i a l w e l l and the e l e c t r o n i c wave f u n c t i o n s as hydrogenic i n nature, then the s e p a r a t i o n between successive quantized energy l e v e l s w i l l converge, the f i r s t t r a n s i t i o n , (2p I s ) , being three quarters of the w e l l depth. However the f i r s t e x c i t e d s t a t e may be very c l o s e t o , or perhaps ov e r l a p , w i t h e i t h e r the conduction band or an a u t o - i o n i z a t i o n s t a t e . This i s i n keeping w i t h the observations obtained when the e l e c t r o n s are photobleached i n low temperature g l a s s e s . By i l l u m i n a t i n g Y - i - r r a d i a t e d n-propanol"'""' and a l k a l i n e aqueous glasses"'"^ at 77°K a l t e r n a t e l y w i t h red (X > 640 nm) and blue ( X < 500 nm) l i g h t i t was p o s s i b l e to " p h o t o s h u t t l e " the trapped e l e c t r o n s between shallow and deep traps without causing any l o s s of - 32 - e l e c t r o n s . A l l that changed was the shape of the a b s o r p t i o n band. Furthermore, by using near-monochromatic l i g h t , i t was p o s s i b l e to show that p h o t o c o n d u c t i v i t y could be induced i n y _ : L r r a d i a t e d aqueous 17 18 a l k a l i n e g l a s s e s and methyltetrahydrofuran glasses by l i g h t which matches the a b s o r p t i o n spectrum. 2.2 E l e c t r o n s p i n resonance In c o n t r a s t to the o p t i c a l a b s o r p t i o n spectrum of the s t a b i l i z e d e l e c t r o n , e l e c t r o n s p i n resonance (esr) can be used to give a more s e n s i t i v e i n d i c a t i o n of the e l e c t r o n ' s immediate environment. The e s r s i g n a l c o n s i s t s of a s i n g l e , narrow l i n e w i t h a g - f a c t o r near t h a t of the f r e e e l e c t r o n , g = 2.0023. In l i q u i d s the l i n e width i s t y p i c a l l y l e s s than 0.1 gauss whereas i n s o l i d media the l i n e width i s much broader, g e n e r a l l y 5-15 gauss, due to the magnetic i n t e r a c t i o n of the e l e c t r o n s p i n w i t h the n u c l e a r magnetic moments of the n u c l e i surrounding i t . The narrowness of the l i n e i n the l i q u i d s t a t e i s due to the e x t e n s i v e time averaging of the n u c l e a r d i p o l a r i n t e r a c t i o n s . The absence of any r e s o l v a b l e h y p e r f i n e s p l i t t i n g i n d i c a t e s that the e l e c t r o n i s not s t r o n g l y l o c a l i z e d on any p a r t i c u l a r molecule but r a t h e r i n t e r a c t s weakly w i t h the molecules forming the c a v i t y . Furthermore, the s i g n a l s are e a s i l y power s a t u r a t e d at low temperatures. In nonpolar hydrocarbon m a t r i c e s , the esr s i g n a l becomes s a t u r a t e d at only 0.02 mw", whereas i n p o l a r media, s a t u r a t i o n becomes a p p r e c i a b l e l l h i f the microwave power s a t u r a t i o n exceeds 0.2 mW. One i n t e r e s t i n g f e a t u r e of esr s t u d i e s on s t a b i l i z e d e l e c t r o n s i s that the l i n e widths of trapped e l e c t r o n s i n p o l a r media are c o n s i d e r a b l y - 33 - broader than those i n nonpolar systems. The c a v i t y r a d i i f o r nonpolar media are l a r g e r , consequently there i s a diminished i n t e r a c t i o n of the e l e c t r o n w i t h the n u c l e i of the c a v i t y w a l l s as compared to those of p o l a r media. In the m a j o r i t y of s t u d i e s to date, trapped e l e c t r o n s have shown no r e s o l v a b l e h y p e r f i n e s t r u c t u r e suggesting that the e l e c t r o n i s not s t r o n g l y l o c a l i z e d . However a few cases of observable h y p e r f i n e s t r u c t u r e have been r e p o r t e d , i n p a r t i c u l a r that by Bennett, Milne 19 and Thomas i n which trapped e l e c t r o n s at 77°K were prepared by the c o - d e p o s i t i o n of sodium and water vapour on a r o t a t i n g c r y o s t a t . Upon s l o w l y warming the sample to 140°K, the s i n g l e e s r l i n e was s p i t i n t o seven e q u a l l y spaced l i n e s and was a t t r i b u t e d to the i n t e r a c t i o n of the e l e c t r o n w i t h s i x e q u i v a l e n t protons arranged o c t a h e d r a l l y around the e l e c t r o n . I t has been suggested that t h i s o b s ervation may be due to a phase t r a n s i t i o n of i c e from the amorphous to the cubic s t a t e which l i e leads to the p r e f e r r e d o r i e n t a t i o n of the s o l v a t i o n s h e l l . Other examples of h y p e r f i n e i n t e r a c t i o n were found i n the case of 20 21 deuterated p y r r o l i d i n e and c r y s t a l l i n e deuterated a c e t o n i t r i l e at 77°K. In the former case a seven l i n e spectrum was observed, i n d i c a t i n g i n t e r a c t i o n of the e l e c t r o n w i t h three or four e q u i v a l e n t n i t r o g e n atoms. In the l a t t e r case, a f i v e l i n e spectrum was observed and i t was suggested that the e l e c t r o n was trapped between the d i p o l e s and i n t e r a c t i n g w i t h the two n i t r o g e n n u c l e i . However t h i s observation has s i n c e been 22 23 a t t r i b u t e d to a dimer r a d i c a l anion of a c e t o n i t r i l e . ' To date, no other experiments have been reported regarding h y p e r f i n e s t r u c t u r e of s t a b i l i z e d e l e c t r o n s . - 34 - A more d e t a i l e d d e s c r i p t i o n of esr spectroscopy and i t s a p p l i c a t i o n to t h i s study on dimethyl s u l f o x i d e i s given i n Chapter V. 2.3 C o n d u c t i v i t y A t h i r d important property of s t a b i l i z e d e l e c t r o n s i s t h e i r h i g h c o n d u c t i v i t y . The s t a b i l i z e d e l e c t r o n i s as s o c i a t e d w i t h s e v e r a l molecules of i t s c a v i t y . Therefore, i f the c l a s s i c a l p i c t u r e of i o n i c m o b i l i t y , i n which the i o n moves c a r r y i n g i t s s o l v a t i o n sheath w i t h i t , holds f o r s t a b i l i z e d e l e c t r o n s , then the e l e c t r o n ' s m o b i l i t y should be comparable to other i o n i c s p e c i e s . However, i t s m o b i l i t y i s s i g n i f i c a n t l y g reater than most other negative i o n s , suggesting that, the e l e c t r o n moves from t r a p - t o - t r a p by quantum mechanical t u n n e l l i n g o r through voids formed i n the c a v i t y w a l l by the r o t a t i o n of one of the c a v i t y molecules. In the s o l i d s t a t e at low temperatures such r o t a t i o n i s i n h i b i t e d and the e l e c t r o n s can only be m o b i l i z e d by the absorption of l i g h t (photo- c o n d u c t i v i t y ) or by thermal r e l e a s e . Measurements of e l e c t r o n c o n d u c t i v i t i e s are h e l p f u l i n determining the extent of e l e c t r o n s o l v a t i o n . Those e l e c t r o n s which are s t r o n g l y s o l v a t e d , as i n p o l a r a l c o h o l s and water, e x h i b i t a much lower m o b i l i t y than those i n hydrocarbons where the p o l a r i z a t i o n f o r c e s are very weak. However, because of the high background conductance i n p o l a r and p a r t i a l l y i o n i c s o l v e n t s , experiments d e a l i n g w i t h r a d i a t i o n - i n d u c e d conductances are u s u a l l y r e s t r i c t e d to m a t e r i a l s of low d i e l e c t r i c constant, such as the sa t u r a t e d hydrocarbons and e t h e r s . Studies on e l e c t r o n c o n d u c t i v i t y i n DMSO were not undertaken. - 35 - 3. Models In order to account f o r the observed p h y s i c a l and chemical p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s i n v a r i o u s media, s e v e r a l t h e o r e t i c a l models have been proposed. Such models are e s s e n t i a l i f one i s to understand a l l f e a t u r e s of the experimental r e s u l t s , i n p a r t i c u l a r the v a r i a t i o n i n p r o p e r t i e s between d i f f e r e n t systems. 24 25 Probably the most w i d e l y accepted theory i s J o r t n e r ' s ' continuum model i n which the e l e c t r o n i s regarded as being s t a b i l i z e d by the induced p o l a r i z a t i o n f i e l d of the medium. The e l e c t r o n i n i t i a l l y p o l a r i z e s the medium by e l e c t r o n i c p o l a r i z a t i o n which i s f o l l o w e d , i n the l i q u i d phase, by o r i e n t a t i o n (atomic and d i p o l e ) p o l a r i z a t i o n i f the molecules of the medium have a permanent d i p o l e . In the case of low temperature g l a s s e s these traps are considered as b eing preformed w i t h the d i p o l e s s u i t a b l y o r i e n t e d before the a r r i v a l of the e l e c t r o n . In t h i s way one can e x p l a i n the s i m i l a r s p e c t r a l p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s i n the l i q u i d and glassy phases. Once s t a b i l i z e d , the e l e c t r o n i s regarded as being l a r g e l y confined to a s p h e r i c a l c a v i t y of r a d i u s R i n which the e l e c t r o s t a t i c p o t e n t i a l f u n c t i o n i s constant. Outside the c a v i t y the f u n c t i o n i s continuous. The s t a b i l i z a t i o n i s a t t r i b u t e d to long-range p o l a r i z a t i o n i n t e r a c t i o n s , the short-range a t t r a c t i o n s being o f f s e t by short-range r e p u l s i o n s . The s e l f - e n e r g y , E., of the e l e c t r o n may be represented by E i = T + V(r) (1.19) where T i s the inherent k i n e t i c energy (< kT) and V(r) i s the p o t e n t i a l energy a c t i n g on the e l e c t r o n produced by the p o l a r i z a t i o n . The Landau o r i e n t a t i o n p o t e n t i a l energy f u n c t i o n , P ( r ) , i s given as f o l l o w s : P ( r ) = - g e 2 / r f o r r > R (1.20) 2 = -ge /R f o r r < R where ft = (1/D - 1/D ). D and D are the o p t i c a l and s t a t i c d i -^ ' op s op s r e l e c t r i c constants and r i s the d i s t a n c e from the centre of the e l e c t r o n c a v i t y . The ground and f i r s t e x c i t e d s t a t e (presumed to be bound) of the e l e c t r o n are considered as being s i m i l a r to one-parameter hydrogenic- type Is and 2p wave f u n c t i o n s , which take the f o l l o w i n g form: , 3, .1/2 - y r * l s = ( y / 7 r ) 6 ,5, .1/2 Q -ar i|>2p = (a /TT) r cosy e (1.21) where y and a are the v a r i a t i o n a l parameters. The energy of the ground s t a t e can be represented by the e x p r e s s i o n w i s -7* *ls[- ^~2~ V 2 + P ( r ) ] * l s dx (1.22) 8TT m 2 where V i s the L a p l a c i a n operator, P ( r ) i s the p o t e n t i a l energy oper a t o r , h i s Planck's constant and m i s the e l e c t r o n mass. For a given value of R and 3, W ĝ i s then obtained as a f u n c t i o n of y Using the v a r i a t i o n procedure - 37 - the best value of y i s obtained which, when s u b s t i t u t e d i n t o (1.21) and (1.22) y i e l d s the wave f u n c t i o n and energy f o r the ground s t a t e of the s t a b i l i z e d e l e c t r o n . The f i r s t e x c i t e d s t a t e i s t r e a t e d i n a s i m i l a r manner. However, according to the Frank-Condon p r i n c i p l e , the value of R and the form of the p o t e n t i a l s e l e c t e d f o r the ground s t a t e must be the same f o r the 2p-type s t a t e s i n c e the molecules cannot r e o r i e n t themselves f a s t enough to f o l l o w the e x c i t a t i o n . The e l e c t r o n i c p o l a r i z a t i o n energy i s represented, approximately, by the expression S = - e 2 (1 - 1/D ) (1.24) x op where r ^ i s the mean radius of the charge d i s t r i b u t i o n i n the i s t a t e , 3 5 being ^ and — ^ f o r the ground and f i r s t e x c i t e d s t a t e r e s p e c t i v e l y . Thus the total energy of the ground and f i r s t e x c i t e d s t a t e of the e l e c t r o n may be represented as: E l s = W l s + S l s (1.25) E 0 = W0 + S_ 2p 2p 2p The energy f o r the (2p Is) t r a n s i t i o n i s then given by: ^ = E 2 p - E l s (1.26) which i s regarded to correspond to the e x c i t a t i o n energy at the a b s o r p t i o n maximum. - 38 - I t should be s t r e s s e d , however, that t h i s model i s s e m i - e m p i r i c a l because the c a v i t y radius i s introduced as an a d j u s t a b l e parameter and does not provide any r e a l p h y s i c a l i n f o r m a t i o n regarding the s h o r t - range s t r u c t u r a l m o d i f i c a t i o n s and i n t e r a c t i o n s . Furthermore, because D g i s f a i r l y l a r g e f o r most p o l a r media, and D i s % 1 t o 2 f o r a l l media, the terms ( 1 / D ^ - 1/Dg) and (1 - 1/D^) i n the energy expressions are f a i r l y constant so that the s p e c t r a l and thermodynamic p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s are o f t e n a t t r i b u t e d to changes i n the c a v i t y 26 r a d i u s . Assuming a constant value of 2 f o r DQp» Noda, Fueki and K u r i have drawn up a contour map of the o p t i c a l t r a n s i t i o n energy i n 0.1 eV steps as a f u n c t i o n of R and D g. From t h i s map i t i s p o s s i b l e to draw a c o r r e l a t i o n curve of the t r a n s i t i o n energy and c a v i t y r a d i u s f o r any s o l v e n t knowing i t s s t a t i c d i e l e c t r i c constant. The curve f o r DMSO i s shown i n Figure 6, t a k i n g D g = 48. Using Jortner's model, experimental data can be f i t t e d f o r any s o l v e n t (using e m p i r i c a l values of R) and has been done f o r s e v e r a l p o l a r media, i n p a r t i c u l a r ammonia, water and the a l c o h o l s , w i t h reasonable success. However, i n s p i t e of the apparent success of t h i s q u a l i t a t i v e approach, there e x i s t very s e r i o u s t h e o r e t i c a l arguments ag a i n s t i t s a p p l i c a t i o n , e s p e c i a l l y f o r s t r o n g l y s o l v a t e d e l e c t r o n s . The treatment i s r e a l l y an e l e c t r o n i c a d i a b a t i c approximation s i n c e i t assumes that the e x t r a e l e c t r o n i s much more l o o s e l y bound and t h e r e f o r e of lower mean v e l o c i t y than the valence or core e l e c t r o n s of the medium. In p o l a r media the b i n d i n g or s o l v a t i o n energy i s *v. 1 to 2 eV so t h a t the e l e c t r o n i s not a p p r e c i a b l y more weakly bound than the valence e l e c t r o n s . The e x t r a e l e c t r o n should be considered on an equal b a s i s w i t h the I I I I I I 0 1.0 2.0 3.0 4.0 CAVITY RADIUS (A°) Figure 6. C o r r e l a t i o n curve of the t r a n s i t i o n energy of the e l e c t r o n i n DMSO and i t s c a v i t y radius using the continuous d i e l e c t r i c model w i t h an a d i a b a t i c approximation. - AO - medium e l e c t r o n s by i n c l u d i n g the e l e c t r o n i c p o l a r i z a t i o n p o t e n t i a l i n the eigenvalue equation f o r determining the wavefunction parameters and energies of the e l e c t r o n i c s t a t e s . T r e a t i n g the t o t a l p o l a r i z a t i o n energy i n t h i s way thus c o n s t i t u t e s an independent p a r t i c l e or s e l f - c o n s i s t e n t f i e l d (Hartree-Fock) type approximation. When t h i s approximation i s a p p l i e d to the hydrated e l e c t r o n , the c a v i t y r a d i u s must be made v a n i s h i n g l y s m a l l (R ̂  0 ) f o r the t h e o r e t i c a l o p t i c a l 24 t r a n s i t i o n energy to agree reasonably w i t h the experimental r e s u l t s . However short-range overlap i n t e r a c t i o n s are not accounted f o r by t h i s continuum model and i s r e a l l y only a p p l i c a b l e provided that the c a v i t y r a d i u s i s taken to be equal or l a r g e r than a molecular or atomic r a d i u s . Moreover, the s h i f t i n the abso r p t i o n spectrum w i t h temperature and pressure cannot be adequately e x p l a i n e d assuming a near zero c a v i t y r a d i u s . For these reasons both the s e m i - e m p i r i c a l a d i a b a t i c and s e l f - c o n s i s t e n t f i e l d approximation to the continuous d i e l e c t r i c model are too l i m i t e d to provide a proper i n t e r p r e t a t i o n of the p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s i n p o l a r s o l v e n t s . Whereas the continuous d i e l e c t r i c model accounts f o r e l e c t r o n s t a b i l i z a t i o n through long-range p o l a r i z a t i o n f o r c e s , other models a t t r i b u t e these f o r c e s to short-range e l e c t r o n - s o l v e n t i n t e r a c t i o n s . 27 N a t o r i and Watanabe proposed a s t r u c t u r a l model f o r the hydrated e l e c t r o n which i n v o l v e s a t r a p p i n g center c o n s i s t i n g of four water molecules w i t h one OH bond of each water molecule t e t r a h e d r a l l y o r i e n t e d around the e l e c t r o n . The t h e o r e t i c a l t r a n s i t i o n energy was f i t t e d to the experimental value by v a r y i n g the oxygen-oxygen d i s t a n c e and by s t r e t c h i n g the bond distances and angles. However, the c a l c u l a t e d value - 41 - was much l o v e r than the e x p e r i m e n t a l l y observed value (0.8 eV versus 1.7 eV). This was undoubtedly due to the f a c t the dominant lo n g - range p o l a r i z a t i o n i n t e r a c t i o n s were neglected. A second model, c a l l e d an o r i e n t e d d i p o l e model, has been suggested 28 by I g u c h i . In t h i s model the p o t e n t i a l f i e l d a r i s e s from the molecular d i p o l e s o r i e n t e d i n a s p h e r i c a l manner about the excess e l e c t r o n . The approximations used are e s s e n t i a l l y the same as f o r the e l e c t r o n i c a d i a b a t i c continuum model i n which the e l e c t r o n i c p o l a r i z a t i o n energy i s added as a c o r r e c t i o n term to the computed energy. The thermal dependence of the a b s o r p t i o n s p e c t r a are a t t r i b u t e d to the degree of o r i e n t a t i o n of the d i p o l e s as w e l l as by the thermal expansion of the l i q u i d r a t h e r than a simple change i n c a v i t y r a d i u s . The success of any of these treatments f o r e l e c t r o n s o l v a t i o n and trap p i n g are judged e s s e n t i a l l y i n terms of t h e i r a b i l i t y to produce "agreement" between theory and experiment. However, i n view of the crude approximations used, such as the n e g l e c t of e i t h e r the s h o r t - or long-range i n t e r a c t i o n s , none of these models can be considered as g i v i n g a true p i c t u r e of e l e c t r o n s t a b i l i z a t i o n . Furthermore, e i t h e r the c a v i t y r a d i u s (continuum model) or a s p e c i f i c d i p o l e o r i e n t a t i o n (atomic models) are v a r i e d u n t i l theory and experiment agree, i m p l y i n g these are the only f a c t o r s i n v o l v e d . Recently a semicontinuum model f o r the s t a b i l i z e d e l e c t r o n i n 29 30 31 ammonia, water and methanol was developed which i n c l u d e d both long- and short-range i n t e r a c t i o n s . In t h i s treatment the excess e l e c t r o n i n t e r a c t s w i t h the induced and permanent d i p o l e moments of the molecules i n the f i r s t s o l v a t i o n s h e l l by a short-range charge-dipole - 42 - a t t r a c t i v e p o t e n t i a l , s i m i l a r to that proposed by I g u c h i , whereas the s o l v e n t molecules beyond the f i r s t s o l v a t i o n sheath are t r e a t e d as a continuous d i e l e c t r i c medium w i t h which the e l e c t r o n i n t e r a c t s by a long-range p o l a r i z a t i o n p o t e n t i a l (continuum model). By performing v a r i a t i o n a l c a l c u l a t i o n s s i m i l a r to those described p r e v i o u s l y f o r v a r i o u s c a v i t y r a d i i , the authors were able to c o n s t r u c t c o n f i g u r a t i o n a l coordinate diagrams f o r the ground and e x c i t e d s t a t e s . In t h i s manner a minimum i n the t o t a l energy of the system could be e s t a b l i s h e d , which could not be obtained at a f i n i t e r a d i u s by only i n c l u d i n g long-range p o l a r i z a t i o n i n t e r a c t i o n s , and a unique c a v i t y r a d i u s could be p r e d i c t e d . In the case of ammonia, a s e l f - c o n s i s t e n t f i e l d treatment was used f o r short-range i n t e r a c t i o n s and a Landau-type p o t e n t i a l f o r long-range i n t e r a c t i o n s ( a d i a b a t i c approximation) whereas both s h o r t - and long-range i n t e r a c t i o n s were t r e a t e d s e l f - c o n s i s t e n t l y i n the case of the e l e c t r o n i n methanol and water. Using such a treatment, the observed o p t i c a l a b s o r p t i o n of both the s o l v a t e d e l e c t r o n i n ammonia and methanol and the hydrated e l e c t r o n i n water (using D g = 78) and i n p o l y c r y s t a l l i n e i c e at 77°K (using D g = 3) could be s a t i s f a c t o r i l y e x p l a i n e d . Moreover, the energy l e v e l s were moved toward the continuum so that the e x c i t e d s t a t e s are l e s s s t r o n g l y bound. This i s i n keeping w i t h the photobleaching and c o n d u c t i v i t y s t u d i e s on low temperature glasses mentioned e a r l i e r . Thermodynamic and s t r u c t u r a l data, such as the heat of s o l u t i o n and charge d i s t r i b u t i o n of the s t a b i l i z e d e l e c t r o n , were i n good agreement w i t h the experimental r e s u l t s . Although the semicontinuum model i s only approximate i n nature, i t does show that both s h o r t - and long-range i n t e r a c t i o n s must be - 43 - considered when d e s c r i b i n g the t r a p p i n g and s t a b i l i z i n g of e l e c t r o n s and when a n t i c i p a t i n g t h e i r inherent chemical and p h y s i c a l p r o p e r t i e s . Such c o n s i d e r a t i o n s are e s s e n t i a l i f one i s to understand a l l the fea t u r e s of the experimental r e s u l t s , e s p e c i a l l y the v a r i a t i o n i n p r o p e r t i e s between d i f f e r e n t s o l v e n t s . Studies of other systems, i n p a r t i c u l a r p o l a r a p r o t i c s o l v e n t s such as DMSO, should provide a ri g o r o u s t e s t t o these models and a b e t t e r i n s i g h t i n t o the s t r u c t u r a l , thermodynamic and o p t i c a l p r o p e r t i e s of the s t a b i l i z e d e l e c t r o n . 4 . Free Ion Y i e l d s In the preceding s e c t i o n i t was suggested t h a t the s o l v a t i o n energy of the e l e c t r o n depends to a great extent on the p o l a r i z a b i l i t y of the medium. S i m i l a r l y , the r a d i a t i o n y i e l d of s t a b i l i z e d e l e c t r o n s depends on the b u l k d i e l e c t r i c constant of the medium. The primary, secondary and hi g h e r - o r d e r e l e c t r o n s produced by the incident r a d i a t i o n are reduced to thermal energies by the processes mentioned e a r l i e r . Because of the random nature of the c o l l i s i o n s and s c a t t e r i n g , there w i l l be a d i s t r i b u t i o n of the t h e r m a l i z a t i o n d i s t a n c e s r e s u l t i n g i n a range of i n i t i a l s e p a r a t i o n d i s t a n c e s between the thermalized e l e c t r o n s and t h e i r parent p o s i t i v e i o n . Once thermalized, the e l e c t r o n s are doomed e i t h e r to geminate recombination w i t h t h e i r concomitant p a r t n e r or to d i f f u s i o n outwards i n t o the m i l i e u to undergo r e a c t i o n there. Those that escape are r e f e r r e d to as " f r e e i o n s " , those undergoing geminate combinations as "geminate i o n s " . Since the coulombic energy of a t t r a c t i o n , E , i s given as f o l l o w s , E a t t = ^ / D r (1.27) where q i s the e l e c t r o n charge, D i s the d i e l e c t r i c constant of the medium and r i s the s e p a r a t i o n d i s t a n c e , i t f o l l o w s that f o r a given i n i t i a l s e p a r a t i o n d i s t r i b u t i o n the f r e e i o n y i e l d w i l l i n c r e a s e w i t h d i e l e c t r i c constant. The d i s t a n c e y at which the thermal k i n e t i c energy, kT, of the e l e c t r o n equals that of the coulombic a t t r a c t i v e p o t e n t i a l i s given as f o l l o w s : y = q 2/DkT ( 1 < 2 8 ) The p r o b a b i l i t y of escaping geminate recombination, <j>(esc), i s given by equation ( 1 . 2 9 ) ' E a t t / k T .{.(esc) = e a c c (1.29) which, when combined w i t h (1.27) and (1.28), y i e l d s the c e l e b r a t e d Onsager r e l a t i o n s h i p (1.30). <J>(esc) = e~V^T (1.30) I f one were to make the crude approximations that ( i ) the d i s t r i b u t i o n of s e p a r a t i o n d i s t a n c e s and ( i i ) the i n i t i a l i o n i z a t i o n y i e l d were the same f o r a l l media, then the y i e l d of f r e e ions should vary e x p o n e n t i a l l y w i t h the d i e l e c t r i c constant of the medium. The dependence of the f r e e i o n y i e l d on the s t a t i c d i e l e c t r i c constant of the l i q u i d s i s shown i n Figure 7. The e m p i r i c a l curve appears to f o l l o w the e x p o n e n t i a l behavior pre- d i c t e d although there are some s i g n i f i c a n t exceptions such as l i q u i d ammonia. Apart from the approximations s t a t e d e a r l i e r there are other f a c t o r s which must be i n c l u d e d when c o n s i d e r i n g t h i s e l e c t r o s t a t i c model. I f e l e c t r o n s o l v a t i o n i s to be a t t r i b u t e d to the r e l a x a t i o n of the medium around the thermal e l e c t r o n , then the s o l v a t i o n time w i l l have to be comparable to the macroscopic d i e l e c t r i c r e l a x a t i o n time of the medium. However i f the s o l v a t i o n time i s i n the v i c i n i t y or s m a l l e r than the d i e l e c t r i c r e l a x a t i o n time, T,of the l i q u i d , then a time-averaged d i e l e c t r i c constant must be used. For nonpolar l i q u i d s , D = D , and the d i e l e c t r i c constant i s time independent. In p o l a r Op s' r r media the permanent molecular d i p o l e s r e q u i r e a c e r t a i n time to r o t a t e and l i n e up w i t h the e l e c t r i c f i e l d of the e l e c t r o n so th a t D i n equation (1.27) w i l l be l e s s than Dg. This i s supported by the f a c t that recent s t u d i e s on the pulse r a d i o l y s i s of water i n d i c a t e that the s o l v a t i o n time of the hydrated e l e c t r o n i s s h o r t e r than i t s b u l k 14 35 d i e l e c t r i c r e l a x a t i o n time. Therefore, as suggested by Mozumder, the d i e l e c t r i c constant should be t r e a t e d as a time-dependent v a r i a b l e , g i ven as f o l l o w s , D D(t) = 22 ( 1.31) 1 - (1 - D /D ) (1 - e" t / < ! )) op s where 6 = (D /D )x and t i s the time a f t e r the s t a r t of the p o l a r i z a t i o n op s r of the medium by the e l e c t r o n . U n f o r t u n a t e l y the time r e s o l u t i o n of present o p t i c a l d e t e c t i o n equipment i s l i m i t e d to 20 picoseconds, by which 3.0 c o <u L. H — o 2 0 1.0 - FORMAMIDE . — D 2 ° 1 — 1 PROPYLENE S T CARBONATE NITRILES ^ / 61 ooro ALCOHOLS o ' . • AMMONIA HYDROCARBONS r . i . I 1 . 1 fl— 1 _ _ , •c- O N 20 40 60 80 100 Figure 7. P l o t of G (free ion) as a f u n c t i o n of the s t a t i c d i e l e c t r i c constant of the medium, references 32, 33', 34, 46, 101. Data taken from - 47 - time the e l e c t r o n s are already s o l v a t e d , so that a p p l i c a t i o n of t h i s r e l a t i o n s h i p i s not yet p o s s i b l e . Another parameter which has not r e c e i v e d the a t t e n t i o n i t deserves i s the i n i t i a l d i s t r i b u t i o n of t h e r m a l i z a t i o n d i s t a n c e s . The low energy e l e c t r o n s l o s e t h e i r excess energy mainly by e l e c t r o n i c a l l y e x c i t i n g the molecules they encounter. As t h e i r k i n e t i c energy drops below the lowest e x c i t e d l e v e l , f u r t h e r energy degradation proceeds by e x c i t i n g i n t r a m o l e c u l a r and i n t e r m o l e c u l a r v i b r a t i o n s and molecular r o t a t i o n s . The d i s t a n c e t r a v e l l e d by the e l e c t r o n d u r i n g . t h i s second p a r t of i t s t h e r m a l i z a t i o n , the s o - c a l l e d s u b e x c i t a t i o n range, makes the g r e a t e s t c o n t r i b u t i o n to the t o t a l e l e c t r o n range. According to L a s s e t t r e and 36 Silverman, the Born approximation, upon which the Bethe stopping power formula given i n equation (1.10) i s based, i s only good above about 120 eV so that t h e o r e t i c a l t h e r m a l i z a t i o n d i s t a n c e s cannot be c a l c u l a t e d u s i n g t h i s formula. U n f o r t u n a t e l y no s u c c e s s f u l treatment has been proposed to account f o r t h e r m a l i z a t i o n d i s t a n c e s f o r e l e c t r o n s having energies below 100 eV. I t would appear that e l e c t r o n s have s h o r t e r t h e r m a l i z a t i o n paths i n media i n which the molecules have s e v e r a l i n t e r n a l degrees of freedom. However, the i o n i c m o b i l i t y of the e l e c t r o n b e f o r e , during and a f t e r the medium begins to r e l a x w i l l a l s o c o n t r i b u t e to the t h e r m a l i z a t i o n l e n g t h . Although the f r e e i o n y i e l d i s dependent on s e v e r a l v a r i a b l e s , the s t a t i c d i e l e c t r i c constant i s the only parameter w i t h which y i e l d s i n v a r i o u s media have been compared. From the e m p i r i c a l r e l a t i o n s h i p shown i n Figure 7, i t appears that t h i s parameter does indeed p l a y the dominant r o l e i n most media; but t h i s r e l a t i o n s h i p needs to be much more widely t e s t e d , p a r t i c u l a r l y i n media such as the a p r o t i c p o l a r s o l v e n t s . - 48 - D. BINARY MIXTURES The chemical y i e l d s of solvated electrons and t h e i r o p t i c a l absorption spectra i n binary solvent systems are of i n t e r e s t because they o f f e r a te s t of the t h e o r e t i c a l models described p r e v i o u s l y . I f the continuum or semi-continuum models are ap p l i c a b l e , then the electrons should be delocalized, thereby sampling the average environment of the mixture and thus the o p t i c a l properties and y i e l d s should be determined by the macroscopic properties of the medium. On the other hand, i f the electrons are strongly l o c a l i z e d through short-range i n t e r a c t i o n s , then the y i e l d s and o p t i c a l c h a r a c t e r i s t i c s should be governed by the s o l v a t i o n due to a small number of solvent molecules, perhaps 4 to 6. Thus an examination of e l e c t r o n y i e l d s , absorption band maxima and the width-at- h a l f - h e i g h t , AW, of the absorption bands as a function of binary solvent composition should provide a b e t t e r i n s i g h t i n t o the nature of e l e c t r o n s o l v a t i o n . Perhaps f o r these reasons several i n v e s t i g a t i o n s have been undertaken very recently on binary mixtures of widely d i f f e r e n t p o l a r i t y . 37 A r a i and Sauer have studied binary mixtures of water and alcohols at various concentrations. In a l l the mixtures examined the e l e c t r o n absorption had only one peak and the absorption maxima and half-width were intermediate to those of the pure components. I f the absorption spectra were a mere superposition of the bands a r i s i n g from the i n d i v i d u a l components, a much broader band (enhanced AW) with well-defined shoulders at intermediate concentrations would be expected. Furthermore, i n the case of the ethanol-water mixture, G(e ) increased l i n e a r l y with the - 49 - d i e l e c t r i c constant of the mixture. These r e s u l t s were taken as suggesting that the s o l v a t i o n of the e l e c t r o n depends on the aggregate p r o p e r t i e s of the mixture, such as the macroscopic d i e l e c t r i c constant, i n which i t i n t e r a c t s w i t h a l a r g e number of molecules. S i m i l a r c o n c l u - 38 s i o n s were a r r i v e d at by Dorfman et a l . from t h e i r i n v e s t i g a t i o n s on b i n a r y mixtures of water w i t h ammonia and ethylenediamine. Whereas the X of the pure components i n the w a t e r - a l c o h o l b i n a r y mixtures are max tr r J very c l o s e (see Table 1 ) , so that any changes would be s m a l l , those of water (720 nm), ethylenediamine (1350 nm) and ammonia (1550 nm) are s u f f i c i e n t l y w i d e l y separated so that any broadening e f f e c t s would be r e a d i l y observed. Bi n a r y mixtures of p r o t i c p o l a r media w i t h nonpolar hydrocarbons or s l i g h t l y p o l a r ethers have a l s o been s t u d i e d but the r e s u l t s are 39 d i f f e r e n t from the mixtures mentioned above. Kemp et a l . pulse i r r a d i a t e d b i n a r y mixtures of methanol i n t e t r a h y d r o f u r a n (67 mole % methanol) and cyclohexane (4% methanol). In both mixtures the l i f e t i m e of the s o l v a t e d e l e c t r o n was unchanged w i t h respect to t h a t i n pure methanol. Although the abso r p t i o n maximum was s h i f t e d s l i g h t l y (% 80 nm), the band was not broadened. The maximum absorbances, expressed as G(e )e , were 13,900 (pure methanol), 14,000 (33% s insx t e t r a h y d r o f u r a n , 67% methanol) and 3500 (96% cyclohexane-4% methanol) i o n s (100 eV) Hi "'"cm L. I f one assumes that the e x t i n c t i o n c o e f f i c i e n t s of e g are not d r a s t i c a l l y changed i n these mixtures from that i n methanol, then the l a t t e r value suggests that aggregates of methanol molecules must e x i s t i n the mixture and that these aggregates are capable of tr a p p i n g and s o l v a t i n g e l e c t r o n s formed i n the hydrocarbon component. This c o n c l u s i o n a r i s e s because the y i e l d of s o l v a t e d e l e c t r o n s - 50 - was approximately s i x times g r e a t e r than that expected simply from the mole f r a c t i o n of methanol i n the mixture. Once trapped i n the methanol c l u s t e r , the e l e c t r o n decays by the same processes of e l e c t r o n decay i n pure methanol, hence i t s l i f e t i m e i s unchanged. A s i m i l a r r e s u l t was observed f o r s c l v a t e d e l e c t r o n s i n mixtures of 3-methylhexane i n AO e t h a n o l and methanol. Analogous i n v e s t i g a t i o n s i n ethanol/n-hexane mixtures over a c o n c e n t r a t i o n range of 2-100 mole % eth a n o l have a l s o 41 been reported. No s h i f t i n the p o s i t i o n of the ab s o r p t i o n spectrum maximum of the s o l v a t e d e l e c t r o n from that i n ethanol was observed. T h i s , together w i t h the f a c t t h a t the e l e c t r o n had a constant h a l f - l i f e , was i n t e r p r e t e d to mean that the e l e c t r o n was trapped i n b a s i c a l l y the same type of p o t e n t i a l w e l l , r e g a r d l e s s of the n-hexane c o n c e n t r a t i o n . The e l e c t r o n y i e l d s i n the ethanol/n-hexane mixtures were observed to be g r e a t e r than those f r e e i o n y i e l d s p r e d i c t e d (using Figure 7) f o r pure l i q u i d s which have d i e l e c t r i c constants equal to the b u l k d i e l e c t r i c constants of the mixtures. However t h i s comparison was 42 c r i t i c i z e d by Freeman on two p o i n t s . F i r s t l y , the b u l k d i e l e c t r i c constant of the ethanol/n-hexane mixture i s lower than the average m i c r o s c o p i c d i e l e c t r i c constant. Secondly, some of the e l e c t r o n s and ions generated by the i o n i z a t i o n of the hydrocarbon are probably scavenged by c l u s t e r s of eth a n o l molecules r e s u l t i n g i n an i n c r e a s e i n the average d i e l e c t r i c constant i n the v i c i n i t y of the geminate i o n p a i r s , thereby decreasing the p r o b a b i l i t y of geminate recombination. 43 Recently Dorfman et a l . , u s i n g f a s t i n f r a r e d o p t i c a l techniques, were able to i n v e s t i g a t e the o p t i c a l p r o p e r t i e s of tetrahydrofuran-water mixtures over the complete range of compositions. They observed that - 51 - both the peak p o s i t i o n and half-width, although intermediate between the two pure components, were dominated by the water despite the f a c t that the value of the macroscopic d i e l e c t r i c constant of the mixtures was dominated by tetrahydrofuran. A s i m i l a r observation was observed f o r 44 water-1,4 dioxane mixtures. In the l a t t e r study there was a gradual s h i f t of the e l e c t r o n band with increasing water content u n t i l at 34 mole % water the peak absorption corresponded to that of the hydrated e l e c t r o n . A possible explanation i s that these s l i g h t l y p o l ar organic molecules are able to disrupt the water aggregates somewhat through weak dipole i n t e r - actions thereby causing a v a r i e t y of traps of d i f f e r e n t p o l a r i t y to be formed at low water concentrations. At higher water concentrations, there i s presumably a predominance of s u i t a b l e "pure water" traps i n which the electrons are more stable and therefore the o p t i c a l spectrum i s governed by the water aggregates. When ethylenediamine was added to tetrahydrofuran i n various concentrations, both the peak p o s i t i o n and half-width showed an almost l i n e a r dependence upon composition analogous to the mixtures of p r o t i c 43 40 polar media. Magnusson et a l . i n v e s t i g a t e d mixtures of 3-methyl hexane and diethylether and observed an almost l i n e a r dependence of the e l e c t r o n y i e l d on the ether concentration. The spectrum showed no s h i f t with concentration, being the same as that of the pure ether. However, the detection apparatus was too slow to observe the pure hydrocarbon spectrum so the:,question of whether or not there was a s h i f t at low ether concentrations i s uncertain. Because of the l i n e a r i t y of y i e l d with ether concentration, the authors suggest that the e l e c t r o n s t a b i l i z a t i o n i s by an electron-ether molecule complex with the e l e c t r o n being bound by a charge-dipole i n t e r a c t i o n . - 52 - Recently a study on b i n a r y mixtures of a h i g h l y p o l a r a p r o t i c 45 s o l v e n t , formamide (D = 109 at 20°C), w i t h water was re p o r t e d i n s which only the hydrated e l e c t r o n band was observed, i t s p o s i t i o n and shape being u n a l t e r e d by changes i n composition. Since no a b s o r p t i o n band that could be a t t r i b u t e d to the s o l v a t e d e l e c t r o n i n formamide was observed, i t was suggested that the h i g h f r e e i o n y i e l d (G(free ion) = 46 3.3) r e p o r t e d e a r l i e r was due to r e a c t i v e r a d i c a l anions produced by e l e c t r o n r e a c t i o n w i t h the s o l v e n t molecules. The diminished i n t e n s i t y of the a b s o r p t i o n band of the hydrated e l e c t r o n i n these mixtures was r a t i o n a l i z e d on the b a s i s t h a t water aggregates were competing w i t h the formamide molecules f o r the thermalized e l e c t r o n . K i n e t i c data showed tha t the hydrated e l e c t r o n formed then decayed by r e a c t i o n w i t h the formamide m i l i e u . Thus, from the s t u d i e s to date on b i n a r y m i x t u r e s , i t appears that the macroscopic d i e l e c t r i c constant i s not the major f a c t o r i n determining the e l e c t r o n y i e l d s and s o l v a t i o n p r o p e r t i e s ; but r a t h e r I t i s the b i n a r y s o l v e n t s t r u c t u r e that c o n t r o l s these p r o p e r t i e s . Mixtures i n which the pure components do not i n t e r a c t s t r o n g l y , such as the p r o t i c p o l a r s o l v e n t s and nonpolar media, the e l e c t r o n y i e l d and o p t i c a l p r o p e r t i e s are determined by the a b i l i t y of the p o l a r aggregates to scavenge and s t a b i l i z e the e l e c t r o n s . On the other hand, components which may i n t e r a c t s t r o n g l y so t h a t the medium becomes a homogeneous mixture, the e l e c t r o n y i e l d and o p t i c a l c h a r a c t e r i s t i c s appear to depend upon the macroscopic p r o p e r t i e s . I t should be noted that the b i n a r y mixtures mentioned above were 47 f o r s t u d i e s on the l i q u i d s t a t e . Binary mixtures of g l a s s y a l c o h o l s - 53 - and water-alcohols have been i n v e s t i g a t e d . They showed s i m i l a r c h a r a c t e r - i s t i c s to t h e i r l i q u i d c o u n t e r p a r t s ; that i s , e l e c t r o n a b s o r p t i o n maxima vary c o n t i n u o u s l y w i t h change i n composition, s h i f t i n g towards the s h o r t e r wavelengths w i t h i n c r e a s i n g c o n c e n t r a t i o n of the more p o l a r component. Support f o r the formation of p o l a r aggregates i n nonpolar media comes from the ob s e r v a t i o n that y - i r r a d i a t e d g l a s s y mixtures of 47 n-propanol w i t h 3-methylpentane y i e l d two absorption bands a t t r i b u t a b l e to trapped e l e c t r o n s , one c h a r a c t e r i s t i c of each pure component. S i m i l a r l y , y - i r r a d i a t e d g l a s s y mixtures of methyltetrahydrofuran and eth a n o l show two abso r p t i o n peaks, one a t t r i b u t a b l e to m e t h y l t e t r a - 49 hydrofuran and the other to et h a n o l . E. SCOPE OF STUDY The d i e l e c t r i c constant has been used e x t e n s i v e l y i n attempts at exp l a n a t i o n s of v a r i o u s types of s o l v e n t e f f e c t s . In the f i e l d of r a d i a t i o n chemistry both the y i e l d of f r e e ions and the s t a b i l i z a t i o n of e l e c t r o n s are considered as being dominated by the d i e l e c t r i c constant. However t h i s parameter i s a macroscopic property of the medium and flie e l e c t r o s t a t i c s i t u a t i o n i n the v i c i n i t y of any p a r t i c u l a r s o l v e n t molecule or " c l u s t e r " may be q u i t e d i f f e r e n t from t h i s average value. In general there i s a d i r e c t correspondence between the s o l v a t i o n energy of s t a b i l i z e d e l e c t r o n s , as c h a r a c t e r i z e d by t h e i r o p t i c a l a b s o r p t i o n band maxima, and the s t a t i c d i e l e c t r i c constant of the medium. Dorfman showed that f o r a homologous s e r i e s of a l i p h a t i c - 54 - a l c o h o l s , the energy corresponding to the ab s o r p t i o n maximum inc r e a s e s smoothly w i t h an in c r e a s e i n d i e l e c t r i c c o n s t a n t . A s i m i l a r c o r r e l a t i o n was found by Ekstrom and W i l l a r d f o r a s e r i e s of organic gl a s s e s a t 47 77°K, ranging from 3-methylpentane (D g = 2.0) to g l y c e r o l (D g = 42.5). However, n e i t h e r water nor ammonia obey t h i s e m p i r i c a l r e l a t i o n s h i p (see Table I) suggesting that other f a c t o r s may be i n v o l v e d . One such property i s the " s o l v a t i n g power" of the medium. For the systems s t u d i e d to date, those media w i t h h i g h d i e l e c t r i c constants are a l s o p r o t i c s o l v e n t s and hence r e a d i l y s o l v a t e i o n i c species by hydrogen bonding. These s o l v e n t s , such as water and the a l c o h o l s , are c h a r a c t e r i z e d by high d i e l e c t r i c c o n s t a n t s , e x c e l l e n t i o n i c s o l v a t i n g a b i l i t y , h i g h f r e e i o n y i e l d s and abs o r p t i o n maxima i n the v i s i b l e r e g i o n of the spectrum. On the other hand, a p r o t i c nonpolar hydrocarbons and s l i g h t l y p o l a r ethers are c h a r a c t e r i z e d by low s t a t i c d i e l e c t r i c c o n s t a n t s , poor i o n i c s o l v a t i n g a b i l i t y , low fr e e i o n y i e l d s and abs o r p t i o n maxima i n the i n f r a r e d r e g i o n of the spectrum. As a r e s u l t , s t u d i e s on these media are not i n d i c a t i v e of whether or not the medium's s o l v a t i n g power i s a determining f a c t o r i n e l e c t r o n s t a b i l i z a t i o n . The d i e l e c t r i c constant i n f l u e n c e s the s t a b i l i t y of an i o n , but the i n f l u e n c e i s only important when the d i e l e c t r i c constant i s s m a l l . In media of d i e l e c t r i c constant l e s s than 10 or so the e f f e c t of the d i e l e c t r i c constant should be at l e a s t comparable to that of i t s s p e c i f i c s o l v a t i n g power. However, i n media of d i e l e c t r i c constants g r e a t e r than 30 or so the e f f e c t of the d i e l e c t r i c constant i s of 51 52 minor importance compared w i t h the s p e c i f i c s o l v a t i n g a c t i o n . ' For t h i s reason DMSO seemed p a r t i c u l a r l y s u i t e d to such a study. DMSO i s a - 55 - p o l a r a p r o t i c s o l v e n t which i s w i d e l y used i n organic and i n o r g a n i c chemistry because of i t s i n a b i l i t y to s o l v a t e negative i o n s ; indeed bases appear e x c e p t i o n a l l y strong i n t h i s medium. However DMSO has a f a i r l y h i g h s t a t i c d i e l e c t r i c constant (D g = 48 at 20°C) and l a r g e d i p o l e moment (4.3 D ) . ^ Therefore, i f the s t a t i c d i e l e c t r i c constant of the continuum i s the important c r i t e r i u m i n determining e l e c t r o n s t a b i l i t y , then one would expect the s o l v a t i o n energy to be intermediate between that of the p o l a r a l c o h o l s and water; that i s , the e l e c t r o n a b s o r p t i o n band maximum should be i n the v i s i b l e r e g i o n of the spectrum. On the other hand, i f the s o l v a t i n g power of the medium i s the dominant f a c t o r , as given by i t s s p e c i f i c i n t e r a c t i o n s , then the s t a b i l i z e d e l e c t r o n i n DMSO should resemble the a p r o t i c hydrocarbons. In a d d i t i o n , comparison of the f r e e i o n y i e l d w i t h the e m p i r i c a l c o r r e l a t i o n shown i n Figure 7 should s i m i l a r l y i n d i c a t e whether or not the s t a t i c d i e l e c t r i c has a commanding i n f l u e n c e on the y i e l d i n DMSO. Mixtures of DMSO and water were a l s o i n v e s t i g a t e d i n order to determine whether or not the f r e e i o n y i e l d and o p t i c a l p r o p e r t i e s of the s t a b i l i z e d e l e c t r o n s are dependent upon the b u l k or l o c a l p r o p e r t i e s of the d i e l e c t r i c medium. DMSO and water are completely m i s c i b l e i n a l l p r o p o r t i o n s , consequently aggregate e f f e c t s which hinder s t u d i e s on other p r o t i c - a p r o t i c mixtures should be absent i n t h i s system. In a d d i t i o n , s t u d i e s were made on trapped e l e c t r o n s and other species i n the s o l i d s t a t e at 77°K on y - i r r a d i a t e d p o l y c r y s t a l l i n e DMSO as w e l l as p o l y c r y s t a l l i n e and gl a s s y s o l i d s of DMSO-water mixtures. Such studies were of i n t e r e s t i n t h e i r own r i g h t but a l s o f o r c o r r e l a t i o n w i t h the l i q u i d phase work. TABLE I. Optical data for electrons stabilized in liquid media at room temperature. Medium A (nm) max E x (eV) max G(e;> f 3 , -1 -1. e (M cm ) max AW1/2(eV) s water 720 1.72 2.7 0.65 18,500 0.92 78 glycerol 528 2.35 — — — 1.5 43 ethylene glycol 580 2.16 1.2 0.68 14,000 1.35 39 methanol 630 1.97 1.1 0.78 17,000 1.29 33 ethanol 700 1.77 1.0 0.87 15,000 1.55 25 n-propanol 740 1.67 1.0 0.59 13,000 21 isopropanol 820 1.51 1.0 0.67 14,000 1.22 19 n-butanol 680 1.82 — — — 1.47 18 ethylenediamine 1350 0.92 — — 20,000 0.88 — ammonia 1550 0.80 0.45 0.77 49,000 0.46 22C tetrahydrofuran 2100+50 0.59 — — 14,000d — 7.4 d ime thoxye th ane 1900+150 0.65 — — — — 7.2 diethylether 2050+150 0.60 0.19 — 7,500d — 4.3 diethylamine 1900+80 0.65 — — 10,000 — 3.6 dioxane >1100 <1.1 0.04,0.10 — — — 2.2 TABLE I (continued) Medium X (nm) E, (eV) G(e~) f a e (M" 1cm _ 1) AW. ,.(eV) D b max X s max 1/2 s max n-hexane >1500 <0.80 0 . 1 0 — > 1 0 , 0 0 0 ~ 1 . 9 methylcyclohexane >1500 <0.80 — — > 1 0 , 0 0 0 - - 2 . 0 Data taken from references l i e , 2 9 , 3 7 , 3 8 , 4 3 , 4 4 , 1 0 2 , 103 O s c i l l a t o r strength of the sol v a t e d e l e c t r o n band. b S t a t i c d i e l e c t r i c constant at 20°C. C - 3 3 ° C . ^ Uncertainty may be as much as 5 0 % . - 58 - CHAPTER I I EXPERIMENTAL A. 6°Co Y-RAMOLYSIS 1. M a t e r i a l s DMSO (Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade) was s t i r r e d i n a cl o s e d v e s s e l over calcium hydride f o r at l e a s t two days i n a n i t r o g e n dry box. I t was then d i s t i l l e d under reduced pressure (> 1 t o r r at 48-50°C). Only the middle 50% was c o l l e c t e d and s t o r e d over a l a y e r of Linde 4A molecular s i e v e s and sealed i n an atmosphere of pure n i t r o g e n . P r i o r to use, a p o r t i o n of i t was r e d i s t i l l e d and the samples immediately prepared. A n a l y s i s by gas chromatography showed that the water content was l e s s than 0.001% by volume. The only other i m p u r i t y detected was a t r a c e (> 0.005% by volume) of dimethyl s u l f i d e . T r i p l y d i s t i l l e d water was prepared by f i r s t l y r e f l u x i n g an a c i d i c dichromate s o l u t i o n made wi t h s i n g l y d i s t i l l e d water. The d i s t i l l a t e was y - i r r a d i a t e d w i t h a dose of ̂  0.5 Mrad to remove any t r a c e i m p u r i t i e s . I t was then placed under continuous r e f l u x d i s t i l l a t i o n from a l k a l i n e permanganate u n t i l r e q u i r e d . N i t r o u s oxide, obtained from Matheson, was p u r i f i e d by " t r a p - t o - t r a p " d i s t i l l a t i o n i n a high vacuum system to remove any t r a c e s of n i t r o g e n - 59 - and oxygen. I t was subsequently s t o r e d i n a f i v e l i t r e f l a s k on the vacuum l i n e u n t i l r e q u i r e d . The argon and helium used f o r degassing the l i q u i d samples and as a c a r r i e r gas f o r the gas chromatographs were obtained from Canada L i q u i d A i r . A l l other chemicals used i n t h i s study were a n a l y t i c a l grade or b e t t e r and were not p u r i f i e d f u r t h e r . A l l glassware used i n the experiments was s c r u p u l o u s l y cleaned by washing i n permanganic a c i d f o l l o w e d by r i n s i n g w i t h d i s t i l l e d water and a s o l u t i o n of concentrated hydrogen peroxide and n i t r i c a c i d to remove any r e s i d u a l MnC^ on the g l a s s s u r f a c e . F i n a l l y the glassware was thoroughly r i n s e d w i t h s i n g l y d i s t i l l e d and then t r i p l y d i s t i l l e d water before d r y i n g i n the oven at 250°C. In the case of the i r r a d i a t i o n sample c e l l s , the excess grease from the stopcocks and stoppers was f i r s t removed w i t h hexane and the c e l l was then r i n s e d w i t h hexane and water before p u t t i n g them i n the a c i d bath. G e n e r a l l y , however, the sample c e l l s were annealed i n the g l a s s b l o w e r s 1 oven p r i o r to washing i n the a c i d because some of the r a d i a t i o n products could not be removed u s i n g the above procedure. 2. R a d i a t i o n Source The- r a d i a t i o n source was a 4000 c u r i e ^ C o Atomic Energy of Canada Gammacell 220. 3. Dosimetry In order to c a l c u l a t e G values i t i s necessary to know the absorbed r a d i a t i o n dose. The u n i t of absorbed dose i s c a l l e d the rad and i s equal - 60 - 13 -1 to 100 ergs per gram or 6.24 x 10 eV gm Often one r e f e r s to the absorbed dose r a t e of the source which i s the absorbed dose per u n i t time. Since the primary energy a b s o r p t i o n by the Compton process depends only on the e l e c t r o n d e n s i t y of the medium, the energy absorbed by a sample can be determined from a standard simply by comparing the e l e c t r o n d e n s i t i e s of the two samples as given by the r e l a t i o n s h i p (2.1) (Z/A) . R . - R „. x s a m P l e (2.1) sample s t d . (Z/A) , s t d . where R i s ftie absorbed dose r a t e , Z the atomic number and A the gram atomic weight of the medium considered. The chemical standard, or dosimeter, most commonly used, and the one used i n t h i s study, i s the F r i c k e dosimeter which measures the o x i d a t i o n of f e r r o u s ions to f e r r i c i o n s . The F r i c k e s o l u t i o n was prepared by d i s s o l v i n g 0.4 grams of F e ( N H ^ ) 2 ( S 0 4 ) 2 * 6 H 2 0 , 0.060 grams NaCl and 22 ml concentrated (95-98%) H 2S0^ i n s u f f i c i e n t t r i p l y d i s t i l l e d water to make 1 l i t r e of s o l u t i o n . The s o l u t i o n was then 0.001 M w i t h respect to f e r r o u s ammonium s u l f a t e , 0.001 M w i t h respect to sodium c h l o r i d e and 0.4 M w i t h respect to s u l f u r i c a c i d . A l l the reagents used were a n a l y t i c a l grade or b e t t e r . Since the r a d i a t i o n f i e l d i n the Gammacell 220 chamber i s not uniform, i r r a d i a t i o n s of the F r i c k e s o l u t i o n s were done i n the same c e l l s and p l a c e d i n the same p o s i t i o n i n the c a v i t y as the DMSO samples. When water i s i r r a d i a t e d , the f o l l o w i n g process occurs: - 61 - In the presence of 0.8 N a c i d , a i r (oxygen) and f e r r o u s ions e~ + H + >• H- (2) aq H - + 0 2 *- H C y (3) H0 2- + F e 2 + + H + *- F e + 3 + H 2 0 2 (4) H 2 0 2 + F e + 2 + H + *• F e 3 + + «0H + H 20 (5) •OH + F e 2 + + H + F e 3 + + H 20 (6) so that G ( E e 3 + ) = 3[G(e ) + G(H•)] + G(-OH) + 2G(H.0 o) (2.2) aq 11 which equals 15.5. This value has been determined on an absolute b a s i s 53 by measuring by c a l o r i m e t r y the energy absorbed by the F r i c k e s o l u t i o n . Any f a c t o r which a l t e r s the molecular or r a d i c a l y i e l d i n water 3+ w i l l a l t e r the y i e l d of Fe as seen by equation (2.2). The F r i c k e dosimeter i s very s e n s i t i v e to organic i m p u r i t i e s . O x i d a t i o n of the organic i m p u r i t y by 'OH r a d i c a l s and subsequent r e a c t i o n of the organic r a d i c a l w i t h oxygen v i a r e a c t i o n s (7) and (8) produces an org a n i c p e r o x i d e , R0_•. - 62 - *0H + RH R- + H 20 (7) R* + 0 2 R0 2- (8) The o r g a n i c peroxide then r e a c t s w i t h the ferrous i o n i n an analogous manner to H0 2', thus i n c r e a s i n g the y i e l d of the f e r r i c i o n . 3+ As a r e s u l t , G(Fe ) > 15.5 i n the presence of some o x i d i z a b l e organic i m p u r i t i e s . To suppress r e a c t i o n (7) the c h l o r i d e i o n was added to the dosimeter s o l u t i o n . In i t s presence, the h y d r o x y l r a d i c a l i s r e a d i l y reduced and the c h l o r i n e atom then o x i d i z e s the f e r r o u s i o n so 3+ t h a t the o v e r a l l y i e l d of G(Fe ) i s the same. •OH + C l " *~ OH + C l - (9) C l - + F e 2 + F e 3 + + C l (10) The c o n c e n t r a t i o n of f e r r i c ions produced was measured s p e c t r o - p h o t o m e t r i c a l l y on a Cary IA spectrophotometer using an u n i r r a d i a t e d F r i c k e sample as the blank and reading the absorbance at 30A nm. The 3+ l i n e a r i t y of the absorbance of Fe versus the i r r a d i a t i o n time i s shown i n Figure 8 . The i r r a d i a t i o n time i s that of the automatic timer on the Gammacell. The p o s i t i v e i n t e r c e p t i s due to the f a c t that the m i c r o - s w i t c h which a c t i v a t e s the automatic timer i s engaged only when the Gammacell drawer reaches i t s f u l l y lowered p o s i t i o n . Consequently, the samples were exposed to the r a d i a t i o n f i e l d f o r a short p e r i o d d u r i n g the r a i s i n g and lowering of the drawer which was not IRRADIATION TIME (minutes) F i g u r e 8 . F r i c k e d o s i m e t e r r e s u l t s o b t a i n e d f r o m t h e r a d i o l y s i s o f t h e s o l u t i o n s i n the i r r a d i a t i o n c e l l u s e d i n t h i s s t u d y . - 64 - accounted f o r by the timer. This s m a l l c o r r e c t i o n f a c t o r was taken i n t o account i n subsequent dose c a l c u l a t i o n s and i s e q u i v a l e n t to a 5 sec exposure w i t h the timer o p e r a t i n g . From the slope of the graph, the absorbed dose r a t e , R, of the 53 F r i c k e s o l u t i o n was c a l c u l a t e d from the f o l l o w i n g r e l a t i o n s h i p 0.965 x 10 9 x (AO.D./At) . -1 ,„ R_ . , = TT. rads mm (2.3) F r i c k e „ 3-K £304 X X P X ^ ' 3+ where e_„. i s the molar e x t i n c t i o n c o e f f i c i e n t . o f Fe at 304 nm (2174 304 M ''"cm L ) , £ i s the path l e n g t h of the c e l l (1 cm) , p i s the d e n s i t y of 3+ the F r i c k e s o l u t i o n (1.024 + 0.001 between 15 and 25°C) and G(Fe ) = 15.5. A l l i r r a d i a t i o n s on the dosimetry s o l u t i o n s and subsequent DMSO samples were conducted at 23 + 2°C. The dose r a t e corresponding to Figure 8 was found to be 5500 rads minute ± . Since ( Z / A ) f o r the F r i c k e s o l u t i o n i s 0.553 and ( Z / A ) f o r DMSO i s 0.538, the dose r a t e f o r DMSO i s 0.971 that of the F r i c k e s o l u t i o n f o r DMSO i n the same c e l l and i n the same p o s i t i o n i n the Gammacell chamber. A f u r t h e r c o r r e c t i o n must be made f o r the r a d i o a c t i v e decay of the ^ C o . The a c t i v i t y , A^, a f t e r a p e r i o d of decay, t , i s r e l a t e d to 53 the o r i g i n a l a c t i v i t y A q by the e x p r e s s i o n A ^ = A e " X t (2.4) t o where X i s the decay constant (X = 0.693/T^^2^ T i / 2 i s t* i e h a l f - l i f e of ^ C o , 5.27 years. As a r e s u l t , the dose r a t e , R^, at day t a f t e r the dosimetry was performed, R , was obtained from r e l a t i o n s h i p (2.5). - 65 - R = R E " ( 0 - 6 9 3 x t / 1 9 2 5 days) t o 54 A computer program was w r i t t e n to give the dose r a t e and the absorbed dose for a given i r r a d i a t i o n time f o r a p a r t i c u l a r l i q u i d on any given day. 4. Sample P r e p a r a t i o n Two d i f f e r e n t types of pyrex g l a s s sample c e l l s were used, depending upon whether l i q u i d or gaseous products were to be analyzed. Schematic diagrams of the sample c e l l s are shown i n F i g u r e 9 . For gas a n a l y s i s , the DMSO was d i s t i l l e d d i r e c t l y i n t o the pre- weighed sample c e l l (Figure 9 ( a ) ) through the B 7 socket using a mo d i f i e d P e r k i n t r i a n g l e . A f t e r about 2 0 - 2 5 ml of DMSO had been c o l l e c t e d , the c e l l was q u i c k l y removed and stoppered. The B 7 cone and socket j o i n t were greased s p a r i n g l y w i t h high vacuum Apiezon N grease and h e l d together w i t h two s t a i n l e s s s t e e l s p r i n g s . The four-way stopcock was s i m i l a r l y greased w i t h Apiezon N and was h e l d i n pla c e w i t h an aluminum stopcock r e t a i n e r . The stopcock r e t a i n e r and sp r i n g s were r e q u i r e d because the c e l l was p r e s s u r i z e d during the a n a l y s i s procedure. The c e l l was then reweighed on a beam balance to the nearest 0 . 0 1 gram and the sample weight determined. This was necessary i n order to c a l c u l a t e the t o t a l absorbed dose and hence G va l u e s . F o l l o w i n g t h i s weighing and g r e a s i n g , the sample was deoxygenated by f l u s h i n g w i t h argon f o r 3 0 minutes a f t e r which the c e l l stopcock was turned about 4 5 ° , thereby s e a l i n g the c e l l under an argon atmosphere. The stopcock was only turned 4 5 ° so that the sample (a) Irradiation cell for gas products (b) Irradiation cell for liquid products Figure 9. Pyrex i r r a d i a t i o n c e l l s used f o r deoxygenation of the l i q u i d samples. - 67 - would remain above the f r i t t e d glass disk. If i t was turned 90°, the sample tended to flow through the sintered disk and f i l l the opposite sidearm of the c e l l . Since the dosimetry had been done with the solution above the disk, the actual absorbed dose would be different. After deoxygenation, the sample was irradiated immediately or else attached to the vacuum line for degassing and addition of nitrous oxide prior to irradiation. DMSO samples containing solid and liquid scavengers were prepared by weighing a given quantity of the scavenger into a volumetric flask and immediately made up to the required volume with freshly d i s t i l l e d DMSO. The sample was then added to the pre-veighed c e l l through the B7 cone, after which the c e l l was greased, reweighed and degassed. For nitrous oxide studies, the deoxygenated DMSO samples were attached to the vacuum line illustrated in Figure 10 and degassed. The S13 b a l l joints of the c e l l were greased and then connected to the two S13 sockets of the vacuum line. The b a l l and sockets were held together by metal clips in order to produce a hard vacuum. Each of the connections was isolated from the vacuum line by small stopcocks, and S^. A third stopcock, S^, further separated the c e l l and these external connections from the main vacuum manifold. With the four-way stopcock turned i n i t i a l l y at 45°, the vacuum line up to c e l l bore was pumped to a hard vacuum, typically 10 ̂  torr, using a three-stage mercury diffusion pump backed by a rotary o i l pump. Then, with stopcock S 2 closed and S^ part i a l l y closed, the four-way stopcock of the c e l l was gradually rotated un t i l the argon inside the c e l l started to bubble out through S^. It was necessary to turn the stopcock slowly because opening the sample to the vacuum line too quickly resulted in excessive to mercury diffusion pump x- 5 litre n i t rous oxide storage flask U cell y A mercury manometer ure 10. Schematic diagram of vacuum line used for degassing the liquid samples and adding nitrous oxide to the samples. - 69 - b u b b l i n g and l o s s of sample by s p l a s h i n g i n t o the connectors. Once the bu b b l i n g had subsided, was sl o w l y opened and tr a c e s of gas remaining on that s i d e of the s i n t e r e d d i s k were removed. A f t e r pumping down to a good vacuum, stopcocks S^, and were c l o s e d . N i t r o u s o x i d e , which had been p r e v i o u s l y degassed and trapped out i n bulb T using l i q u i d n i t r o g e n , was s l o w l y vapourized i n t o the evacuated l i n e between stopcocks S^, and From the i n i t i a l n i t r o u s oxide pressure, as read from the mercury manometer, and from the p r e v i o u s l y determined volume of the l i n e between the stopcocks S^, and S^, the i n i t i a l number of moles of n i t r o u s oxide could be obtained from the i d e a l gas law, n = PV/RT. Then was s l o w l y opened and the n i t r o u s oxide allowed to bubble u n t i l e q u i l i b r i u m was e s t a b l i s h e d , u s u a l l y about 30 minutes. was then opened, the four-way stopcock turned 45° and the f i n a l pressure read. Knowing the volume of the DMSO sample i n the c e l l , the volume of the u n f i l l e d c e l l and the volume between the bore and stopcocks S^ and S^, one could c a l c u l a t e the amount of n i t r o u s oxide d i s s o l v e d i n the sample. The v a r i a t i o n of n i t r o u s oxide c o n c e n t r a t i o n w i t h the e q u i l i b r i u m p a r t i a l pressure i s shown i n Figure 11 f o r two s i m i l a r c e l l s . One c e l l (A) had a medium p o r o s i t y s i n t e r e d d i s k whereas the second c e l l (B) had a f i n e p o r o s i t y d i s k . I t can be seen that n e i t h e r c e l l gives a "zero" i n t e r c e p t although both have the same sl o p e . The "non-zero" i n t e r c e p t i s b e l i e v e d to be an e m p i r i c a l a r t i f a c t of the system, probably a r i s i n g because of an " e f f e c t i v e " back-pressure due to the presence of the s i n t e r e d d i s k . From the i n v e r s e of the s l o p e , the s o l u b i l i t y f a c t o r -4 -1 f o r n i t r o u s oxide i n DMSO was found to be 1.10 + 0.05 x 10 M t o r r at 23°C. [ N 2 0 ] (M) Figure 11. Plot showing the re l a t i o n s h i p of the p a r t i a l pressure of nitrous oxide to i t s s o l u b i l i t y i n DMSO at 23°C for the two bubbler c e l l s containing a medium ( c e l l A) and f i n e ( c e l l B) porosity sintered disk. - 71 - The sample c e l l used.for l i q u i d product a n a l y s i s i s shown i n Fi g u r e 9(b). This c e l l c o n s i s t e d of a three-necked, 50 ml f l a s k w i t h BIO sockets. A s i n t e r e d g l a s s bubbler was attached to the centre neck by means of a BIO cone. The 25 ml sample was p i p e t t e d i n t o the f l a s k through a s i d e arm which was then sealed w i t h a rubber septum. The sample was deoxygenated by f l u s h i n g w i t h helium and then se a l e d under a s m a l l excess pressure of helium. 5. Product A n a l y s i s F o l l o w i n g i r r a d i a t i o n the sample c e l l f o r gaseous a n a l y s i s was attached to the e x t e r n a l loop of the gas chromatograph v i a S13 sockets. A schematic diagram of the experimental setup i s shown i n Fi g u r e 12. Attached to the e x t e r n a l loop were the sample c e l l , an 18 x 1/8 i n c h Porapak Q "pre-column" and an " o n - l i n e " sample loop. The Porapak Q "pre-column" was used to prevent DMSO vapours from e n t e r i n g the gas chromatograph system. A f t e r each experiment, the "pre-column" was back-flushed f o r about 30 minutes to remove the c o l l e c t e d vapour. The " o n - l i n e " sample loop contained a four-way stopcock so that the loop could be bypassed. I n i t i a l l y , the four-way stopcock of the sample c e l l was turned so tha t the e x t e r i o r loop of the gas chromatograph and i t s attachments, i n c l u d i n g the ou t s i d e bore of the sample c e l l , could be f l u s h e d of a i r . Once the a i r had e l u t e d , the stopcock was r o t a t e d by 90° and the gases were f l u s h e d i n t o the chromatograph. The chromatograph used was a V a r i a n Aerograph S e r i e s 1700 c o n t a i n i n g dual 20 f t . x 1/4 i n c h s t a i n l e s s s t e e l 13X molecular s i e v e columns. D e t e c t i o n was made using WX thermal c o n d u c t i v i t y d e t e c t o r s maintained atl25°C at a f i l a m e n t current of - 72 - Figure 12. Schematic diagram of apparatus used f o r f l u s h i n g the v o l a t i l e gaseous products i n t o the gas chromatograph. - 73 - 100 ma and the signal registered on a Westronics, variable speed chart recorder. Using a column temperature of 55°C and an argon flow rate of 30 ml a minute. H_, 0 o, N„ and CH. were eluted within 30 2 2 2 4 minutes after the start of flushing. Ethane took much longer, consequently i t was measured by temperature programming. After the other gas products were detected the column was heated to 130°C at a rate of 20°C per minute. With a flow rate of 30 ml a minute, i t was found that over 95% of these vol a t i l e gases were extracted in the f i r s t 3 minutes so that t a i l i n g of the peaks was very small. However, the bubbling was continued throughout the analysis (except for ethane) to avoid a pressure change and resulting base line d r i f t on terminating the bubbling. At the start of the temperature programming the four-way stopcock of the sample c e l l was rotated by 45°, the exterior loop of the gas chromatograph bypassed, and the Porapak Q "pre-column" back- flushed. This allowed another sample to be prepared while C^H^ was being eluted (̂  45 minutes). The "on-line" sample loop was used for monitoring the variation of the detector sensitivity from day to day. During the elution of the sample gases the sample loop was in the bypass position. After CH^ had been detected the four-way stopcock of the sample loop was rotated by 90° and the standard gas sample, which was N 2 > was flushed into the column. After the ̂  standard had been detected, the chromato- graph was temperature programmed for ethane as previously described. The linearity of the detector response and sensitivity towards the various gases were established by injecting known quantities of sample gases using a second sample loop in place of the irradiation c e l l . This - 74 - secondary loop was f i l l e d w i t h the r e q u i r e d amount of gas on the vacuum l i n e , i t s amount being measured u s i n g a McLeod gauge. The response of the d e t e c t o r was l i n e a r f o r a l l the gases over the range s t u d i e d . Under the c o n d i t i o n s used, the s e n s i t i v i t y or response 2 f a c t o r s (cm /ymole gas) f o r H„, N„, CH. and C„H, were 59.5, 4.85, 15.2, I £. 4 Z D and 12.7, r e s p e c t i v e l y . The peak areas were measured by manual t r i a n g u l a t i o n . A t y p i c a l chromatogram i s shown i n F i g u r e 13. The l i q u i d products were analyzed by f i r s t l y i n s e r t i n g the needle of a l i q u i d s y r i n g e through the rubber septum of the sample c e l l t o e x t r a c t a known volume of i r r a d i a t e d l i q u i d (25 y£) and then i n j e c t i n g t h i s sample i n t o a V a r i a n Aerograph A-90-P2 gas chromatograph. This chromatograph used a Porapak Q column maintained at 215°C. D e t e c t i o n was made usin g WX thermal c o n d u c t i v i t y d e t e c t o r s at 215°C coupled to a Leeds and Northrup Speedomax chart r e c o r d e r . The f i l a m e n t c u r r e n t was maintained at 170 ma and the helium c a r r i e r gas had a flow r a t e 2 of 50 ml per minute. The s e n s i t i v i t y f a c t o r s (cm /ymole) f o r water and dimethyl s u l f i d e were 17 and 8 r e s p e c t i v e l y . A r e p r e s e n t a t i v e chromatogram i s shown i n F i g u r e 14. Products e l u t i n g at r e t e n t i o n times g r e a t e r than that f o r DMSO were not observed. No thermal decomposition of DMSO occurred d e s p i t e the h i g h column and d e t e c t o r temperatures used. argon flow rate 30 mls/min column temperature 55°C detector temperature 125°C '2 filament current 100 ma I I 1 I . I 1 1—//J 1 -L— 0 20 40 60 " 1 0 0 1 2 0 TIME (minutes) Figure 13. T y p i c a l chromatograph obtained f o r 20 ml DMSO sample c o n t a i n i n g 0.05 M n i t r o u s oxide and 4 r e c e i v i n g an absorbed dose of 8 x 10 rads. helium flow rate 50 mis/min column temperature 215 °C detector temperature 215 T£ filament current 170 ma to z O o_ LU Q : DC O h- U LU h- LU Q DMSO CH 3 SSCH 3 Figure 14. I 1 o 5 10 TIME (minutes) 15 Typical chromatograph obtained after injection of 25 U£ of irradiated DMSO sample, dose was 6 Mrad. Total absorbed - 77 - B. PULSE RADIOLYSIS 1. O u t l i n e of the Technique Pulse r a d i o l y s i s s t u d i e s of DMSO and DMSO-l^O mixtures were performed during a s e r i e s of f i e l d t r i p s to the Phys i c s D i v i s i o n of the N a t i o n a l Research C o u n c i l i n Ottawa. As mentioned i n the I n t r o d u c t i o n , p u l s e r a d i o l y s i s enables one to detect and observe the formation and decay of the r e a c t i v e intermediate s p e c i e s . The s t u d i e s reported are a l l concerned w i t h a b s o r p t i o n s p e c t r o s c o p i c measurements. These were done s p e c t r o p h o t o m e t r i c a l l y w i t h a l i m i t of ̂  10 nsec on the time r e s o l u t i o n . A schematic p l a n of the layout of the apparatus which was used i s shown i n Figure 15. The e l e c t r o n beam was p a r t i a l l y absorbed i n a s m a l l i r r a d i a t i o n c e l l through which the l i g h t beam passed i n a d i r e c t i o n at r i g h t angles to the e l e c t r o n beam. The t r a n s m i t t e d l i g h t was then d i r e c t e d , by means of a s e r i e s of lenses and m i r r o r s , out of the i r r a d i a t i o n area through a s m a l l aperature i n the concrete w a l l where i t was s p l i t i n t o two beams by a p a r t i a l l y r e f l e c t i n g m i r r o r . A f t e r s p l i t t i n g the beam, the r e s u l t a n t l i g h t beams were passed through s u i t a b l e f i l t e r s and focussed onto the entrance s l i t s of the monochromators, the outputs of which were observed by photodiodes or p h o t o m u l t i p l i e r s coupled to a dual-beam o s c i l l o s c o p e . The o s c i l l o s c o p e t r a c e s corresponding to v a r i o u s o s c i l l o s c o p e sweep speeds and monochromator wavelengths were obtained f o r a permanent record by photographing the o s c i l l o s c o p e screen. In t h i s way absor p t i o n s p e c t r a could be constructed and decay r a t e s at s e l e c t e d wavelengths could be assessed. - 78 - xenon arc lamp secondary emission monitor lens light shutter pyrex f i l t e r 35 MeV linear accelerator irradiation V c e l l lens mirro mirror concrete shielding par t i a l l y reflecting mirror Corning glass f i l t e r Bauch and Lomb monochromato . 0 ty photodetector lens concrete shielding mirror Corning glass f i l t e r Bauch and Lomb monochromator photodetector Tektronix 556 dual-beam oscilloscope Figure 15. Lay-out of the pulse radiolysis equipment at the National Research Council radiation laboratory in Ottawa, Ontario - 79 - 2. Radiation Source The source of high-energy electrons was a 35 MeV e l e c t r o n microwave l i n e a r a c c e l e r a t o r ( l i n a c ) operated by the National Research Council i n Ottawa. E l e c t r o n pulse widths of 10 nsec and 40 nsec were used which deposited about 900 rads and 2200 rads per pulse r e s p e c t i v e l y . Pulse-to-pulse v a r i a t i o n s were monitored with a secondary emission monitor (SEM) behind the i r r a d i a t i o n c e l l as shown i n Figure 15. The monitor consisted of a s e r i e s of t h i n aluminum f o i l s which c o l l e c t the secondary electrons emitted by the impinging primary e l e c t r o n s . The t o t a l charge c o l l e c t e d was measured by a charge i n t e g r a t o r , the reading being p r o p o r t i o n a l to the beam current or the number of high- energy electrons t r a v e r s i n g the i r r a d i a t i o n c e l l . The monitor was c a l i b r a t e d d a i l y using an aqueous potassium thiocyanate dosimeter s o l u t i o n saturated with nitrous oxide. For a 10 nsec pulse, the peak beam current was ^ 1 Amp. 3. I r r a d i a t i o n C e l l and O p t i c a l Detection System The i r r a d i a t i o n c e l l used i s shown i n Figure 16. The c e l l was placed i n a r i g i d holder so that the l i g h t beam passed through the o p t i c a l l y f l a t end windows. The windows were made of high p u r i t y s i l i c a so that they were r e s i s t e n t to r a d i a t i o n c o l o r a t i o n . The o p t i c a l path length was 1 cm. The white l i g h t source was a 900-watt xenon arc lamp. In order to prevent photochemical changes i n the s o l u t i o n by continuous i l l u m i n a t i o n from the high i n t e n s i t y lamp, a pyrex f i l t e r was placed between the c e l l and l i g h t source to cut out the u l t r a v i o l e t l i g h t . E f f e c t s of t h i s kind were further reduced by using Figure 16. Schematic diagram of i r r a d i a t i o n c e l l used f o r deoxygenation of l i q u i d samples used i n the pulse r a d i o l y s i s study. The spectroscopic c e l l was f i l l e d by t i p p i n g the c e l l h o r i z o n t a l l y a f t e r f l u s h i n g with high p u r i t y argon. - 81 - a remotely operated s h u t t e r between the lamp and i r r a d i a t i o n c e l l which was kept close d u n t i l j u s t before the e l e c t r o n p u l s e . Although the lamp was u s u a l l y used i n the continuous mode, a few experiments used the lamp under pulsed o p e r a t i o n i n which the current was in c r e a s e d -3 b r i e f l y 10 sec) to produce a hi g h e r l i g h t i n t e n s i t y . The t r a n s m i t t e d l i g h t beam was s p l i t using e i t h e r p a r t i a l l y a luminized m i r r o r s or m i r r o r s which t r a n s m i t t e d only s e l e c t e d wave- l e n g t h s , r e f l e c t i n g a l l others. The d e s i r e d wavelengths were i s o l a t e d u s i n g e i t h e r narrow band pass i n t e r f e r e n c e f i l t e r s or Bauch and Lomb g r a t i n g monochromators. App r o p r i a t e Corning colour g l a s s f i l t e r s placed before the monochromator entrance s p l i t were used to e l i m i n a t e second- and h i g h e r - o r d e r d i f f r a c t e d l i g h t from the g r a t i n g s . For the wave- le n g t h range below 450 nm a P h i l l i p s XP1003 p h o t o m u l t i p l i e r was used. Above 450 nm two d i f f e r e n t photodiodes were used, a S i photodiode (HP 5082-4207) f o r the range 450-750 nm and a Ge photodiode ( P h i l c o - Ford L 4521) f o r the range 750-1500 nm. In most cases e i t h e r a 93 ohm or a 50 ohm load r e s i s t o r was used i n the photodetector anode c i r c u i t . The v o l t a g e s i g n a l s from the p h o t o m u l t i p l i e r or photodiode were a m p l i f i e d and d i s p l a y e d on a dual-beam o s c i l l o s c o p e . ( T e k t r o n i x 556) and photographed usi n g high speed f i l m ( P o l a r o i d type 410). The h o r i z o n t a l sweep of the o s c i l l o s c o p e trace was normally t r i g g e r e d o f f the e l e c t r o n pulse although i t was o c c a s i o n a l l y i n i t i a t e d by an e l e c t r o n pick-up placed c l o s e to the SEM chamber. Because of the dual-beam o s c i l l o s c o p e and s p l i t beam arrangement, i t was p o s s i b l e to f o l l o w c o n c u r r e n t l y the absor p t i o n and decay of two d i f f e r e n t t r a n s i e n t s . This was a l s o u s e f u l when measuring t h e i r - 82 - a b s o r p t i o n s p e c t r a . By s p l i t t i n g the l i g h t beam and usin g one beam as a v a r i a b l e wavelength and the other at a f i x e d (reference) wavelength, the s p e c t r a could be p r o p e r l y normalized to compensate d i r e c t l y f o r v a r i a t i o n s i n the pulse to pulse amplitude or any s l i g h t wandering of the e l e c t r o n beam. 4. O s c i l l o s c o p e Measurements The photodetectors used produce an anode current which i s p r o p o r t i o n a l to the l i g h t i n t e n s i t y s t r i k i n g the photocathode. This current creates a v o l t a g e drop across the anode load and consequently the v o l t a g e measured by the o s c i l l o s c o p e i s d i r e c t l y p r o p o r t i o n a l to the l i g h t i n t e n s i t y . Changes i n vo l t a g e are then p r o p o r t i o n a l to changes i n l i g h t i n t e n s i t y . F i g ure 17 shows a h y p o t h e t i c a l o s c i l l o s c o p e t r a c e i n which I i s the 100% l i g h t t r a n s m i s s i o n before the p u l s e , I the l i g h t t r a n s m i t t e d at the end of the pul s e (AI the l i g h t absorbed). The absorbance,D,of a s o l u t i o n is given by: D " l o 8 i o V J t (2.6) - - l o g 1 0 (1 - A I / I 0 ) In the experiments reported the absorbances measured were g e n e r a l l y very s m a l l such t h a t I >> A l so th a t A l / I , the f r a c t i o n a b s o r p t i o n , was determined from the o s c i l l o s c o p e t r a c e . This was normally achieved by u s i n g the d i f f e r e n t i a l comparator to o f f s e t the d.c. l e v e l so that I appeared on the lower p a r t of the screen and A* f i l l e d the screen. 0 % t i g h t l i l CD < f— _i O > 1 0 0 % light Jt transient build up t ransient decay electron pulse CO % ABSORPTION (%A) = Al/T X 100 TIME Figure 17. Hypothetical oscilloscope trace showing the b u i l d up and decay of transient absorbing species. The time p r o f i l e of the electron pulse i s shown as the dotted curve. - 84 - When A I / I Q i s very s m a l l then i t i s simply p r o p o r t i o n a l to D. I n the o s c i l l o s c o p e t r a c e s shown, %A r e f e r s to the % a b s o r p t i o n or A l / I Q x 100. 5. Cerenkov Emission . As mentioned p r e v i o u s l y , Cerenkov r a d i a t i o n i s emitted whenever high-energy electrons pass through matter w i t h a v e l o c i t y g r e a t e r than the phase v e l o c i t y of l i g h t i n the medium. Although the emission i n t e r f e r r e d w i t h the absorbance measurements, i t was u s e f u l i n determining the time response of the d e t e c t i o n system. Since the emission c o i n c i d e s w i t h the time p r o f i l e of the e l e c t r o n pulse and assuming the e l e c t r o n p u l s e i s square, then the r i s e t i m e of the d e t e c t i o n system may be observed from the o s c i l l o s c o p e t r a c e . The r i s e t i m e i s d e f i n e d as the time i n t e r v a l between the 10% and 90% amplitude p o i n t s f o r a step v o l t a g e change. For the T e t r o n i x 556 dual-beam o s c i l l o s c o p e the inherent r i s e t i m e of the a m p l i f i e r i s 9 nsec, consequently the t o t a l r i s e t i m e of the o p t i c a l d e t e c t i o n apparatus was g r e a t e r than t h i s . The Cerenkov l i g h t used f o r measuring the speed of the d e t e c t i o n systems was generated i n a p i e c e of S u p r a s i l fused s i l i c a backed by a m i r r o r . The m i r r o r was r o t a t e d so that the emitted l i g h t was p i c k e d up by the o p t i c a l system. F i g u r e 18 shows t y p i c a l o s c i l l o s c o p e traces obtained f o r the Ge and S i photodiodes. I t can be seen that the response times were a l l < 20 nsec. However, because the pulse i s not r e a l l y square, the a c t u a l response times are undoubtedly s h o r t e r , probably about 15 nsec. Larger load r e s i s t o r s increased the s i g n a l - t o - n o i s e r a t i o but lengthened the response time of the d e t e c t i o n system. - 85 - 20mv 20 nsec 20 nsec (a) \ = 1275 nm Ge photodiode 93 ohm load r \ 1 v 1 \ J 3« te (b) X= 1275 nm 6e photodiode 50 ohm load (c) X= 500 nm Si photodiode 50 ohm load 20 nsec Figure 18. T y p i c a l o s c i l l o s c o p e traces•showing the response time of the detection apparatus to Cerenkov l i g h t generated using a 40 nsec wide e l e c t r o n pulse. - 86 - This can be seen i n Figures 18(a) and 18(b) f o r the Ge d e t e c t o r u s i n g a 93 ohm and 50 ohm load r e s i s t o r i n which the former load r e s i s t o r d i s p l a y s a more pronounced t a i l . G e n e r a l l y the 50 ohm or 93 ohm l o a d r e s i s t o r s were used i n a l l experiments, e s p e c i a l l y when stud y i n g the s h o r t - l i v e d t r a n s i e n t s , such as the s o l v a t e d e l e c t r o n , but l a r g e r load r e s i s t o r s were sometimes used when studying l o n g e r - l i v e d t r a n s i e n t s when a b e t t e r s i g n a l - t o - n o i s e r a t i o was d e s i r e d . At wavelengths g r e a t e r than 1500 nm i t was shown that the r i s e t i m e of the Ge photodiode system inc r e a s e s n o t i c e a b l y , t a k i n g s e v e r a l hundred nanoseconds to respond completely to the new l i g h t l e v e l . This i n c r e a s e i n response time was a t t r i b u t e d to the longer time r e q u i r e d f o r the e l e c t r o n s to d i f f u s e to the reversed-biased p - j u n c t i o n . At wavelengths above 1500 nm the absorption c o e f f i c i e n t of Ge decreases r a p i d l y thereby causing the e l e c t r o n s to be formed deeper i n the n - j u n c t i o n of the photodiode. The e f f e c t of Cerenkov emission on the a b s o r p t i o n at 500 nm of the t r a n s i e n t s produced i n DMSO usin g a 40 nsec pulse i s shown i n Figure 19. The pure Cerenkov emission was obtained by i r r a d i a t i n g the s o l u t i o n w i t h the a n a l y z i n g l i g h t source o f f ( F i g . 1 9 ( a ) ) . The a c t u a l a b s o r p t i o n that would have been observed i f there was no Cerenkov emission ( F i g . 19(c)) was obtained by adding the emission ( F i g . 19(a)) to the observed abs o r p t i o n ( F i g . 19(b)). As shown, the e f f e c t of Cerenkov emission on the a b s o r p t i o n maximum i s q u i t e s m a l l . However, when the 10 nsec pulses were used, the c o n t r i b u t i o n was not n e g l i g i b l e and had to be c o r r e c t e d f o r when c a l c u l a t i n g the end of pulse a b s o r p t i o n . - 87 - 0.95% \ (a) / / (b) A t m I • *• (O j • * • S • • • • • • =» 6 20 nsec Figure 19. E f f e c t of Cerenkov emission on the a b s o r p t i o n of the t r a n s i e n t s absorbing at 500 nm i n pure DMSO. (a) pure Cerenkov ( a n a l y z i n g l i g h t o f f ) ; (b) observed a b s o r p t i o n ; (c) a b s o r p t i o n that would have been observed had there been no Cerenkov emission. The detector was a S i photodiode w i t h a 93 ohm load r e s i s t o r . The pulse w i d t h was 40 nsec. - 88 - 6. Dosimetry The dose per pulse absorbed by the samples was measured usi n g an aqueous potassium thiocyanate chemical d o s i m e t e r . A s the absorbed dose depended very c r i t i c a l l y on the p o s i t i o n of the c e l l on the o p t i c a l bench, the dosimetry was c a r r i e d out i n the same c e l l i n the same p o s i t i o n w i t h the same o p t i c a l alignment as the other samples. In the r a d i o l y s i s of aqueous s o l u t i o n s of potassium thiocyanate the h y d r o x y l r a d i c a l r e a c t s r a p i d l y w i t h the thiocyanate i o n to produce a t r a n s i e n t absorbing r a d i c a l s p e c i e s , (CNS)^ , which has an ab s o r p t i o n maximum at 480 nm. Reactions (11) and (12) i n d i c a t e the p o s t u l a t e d mechanism.^ •OR + CNS~ *~ CNS- + 0R~ (11) CNS- + CNS (CNS) 2 (12) Because the molar e x t i n c t i o n c o e f f i c i e n t of (CNS)^ i s l a r g e 3 - 1 - 1 ^ e500 = 7.1 x 10 M cm ) and the abso r p t i o n r e l a t i v e l y long l i v e d , i t i s p a r t i c u l a r l y u s e f u l as a dosimeter s o l u t i o n . In n e u t r a l water, G(-OH) = 2.9 so that f o r a c o n c e n t r a t i o n of CNS s u f f i c i e n t to -3 suppress a l l r e a c t i o n s of 'OH except (11), such as 5 x 10 M, one w i l l have G(-OH) = G((CNS) 2") = 2.9. In N 20 sa t u r a t e d s o l u t i o n s one a l s o has r e a c t i o n (13) H O e + N_0 N_ + 0~ — — * - 0H~ + -OH (13) aq Z Z so that the o v e r a l l processes lead to r e l a t i o n s h i p (2.7) - 89 - G((CNS)_ ) = GC-OH) + G(e ) L aq (2.7) = 5.7 where the y i e l d of hydrated e l e c t r o n s scavenged i s taken as 2.8. T y p i c a l o s c i l l o s c o p e t r a c e s of the formation and decay of (CNS) 2 -3 at 500 nm i n a n i t r o u s oxide s a t u r a t e d s o l u t i o n of 5 x 10 M thiocyanate i n water using 40 nsec pulses of 35 MeV e l e c t r o n s are shown i n F i g u r e 20. From the absorbance a t the end of the p u l s e the dose (rads pulse ^) absorbed by the dosimeter s o l u t i o n , f o r a given SEM r e a d i n g , was obtained using equation 2.8 (compare to 2.3), 9 0.965 x 10 x (absorbance) r r. n , „ 500 nm , . -1 R,rHC-, - = 7 7 — ^ : rads p u l s e (CNS) 2 ( G e ) 5 ( ) 0 n m x £ x p (2.8) where Z i s the c e l l path l e n g t h (1 cm) and p the d e n s i t y of the -3 dosimeter s o l u t i o n (1.0 gm cm ). The dose per pulse of any other system, f o r the same SEM r e a d i n g , was obtained by m u l t i p l y i n g by the correspond- i n g e l e c t r o n d e n s i t y r a t i o . For other SEM readings i t was assumed that the dose was p r o p o r t i o n a l to SEM. For DMSO, the absorbed doses f o r 10 nsec and 40 nsec pulses were u s u a l l y about 900 rads pulse ^ and 2200 rads pulse ^ r e s p e c t i v e l y . From equation (2.8) i t can be seen t h a t the absorbance of a given t r a n s i e n t i s r e l a t e d to the product of i t s G value and e x t i n c t i o n c o e f f i c i e n t ; consequently once the absorbed dose of any sample i s known, the absorbance can be expressed i n terms of Ge. - 90 - 4.8% A (a) 100 nsec 5.6 % A Figure 20. (b) 500 nsec Typical oscilloscope traces for the formation and decay of (CNS>2 at 500 nm obtained by using a 40 nsec pulse of 35 MeV electrons on a nitrous oxide saturated solution of -3 5 x 10 M thiocyanate in water. (a) Si photodiode, 93 ohm load resistor; (b) photomultiplier, 470 ohm load resistor. - 91 - 7. M a t e r i a l s and P u r i f i c a t i o n DMSO (Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade) was used without f u r t h e r p u r i f i c a t i o n . Gas chromatography showed i t to cont a i n l e s s than 0.01% by volume water and dimethyl s u l f i d e . 58 Previous pulse r a d i o l y s i s s t u d i e s on DMSO showed that there was no d i f f e r e n c e i n the o p t i c a l p r o p e r t i e s or decay r a t e s of the t r a n s i e n t s between those samples subjected to f r a c t i o n a l d i s t i l l a t i o n or f r a c t i o n a l c r y s t a l l i z a t i o n , f o l l o w e d by d r y i n g w i t h molecular s i e v e s and deaerated on a vacuum l i n e and those used d i r e c t l y from the manufacturer ( F i s h e r S c i e n t i f i c ) and bubbled w i t h h i g h p u r i t y argon. The deuterated DMSO (99.5%) was obtained from Matheson, Coleman and B e l l and d r i e d over Linde 4A molecular s i e v e s . A n a l y s i s by gas chromatography showed i t t o have no det e c t a b l e i m p u r i t y . The water was t r i p l y d i s t i l l e d by the methods des c r i b e d e a r l i e r . Potassium bromide, s i l v e r n i t r a t e and concentrated s u l f u r i c a c i d were a l l a n a l y t i c a l grade or b e t t e r . The b l u e - v i o l e t f l u o r e s c e n t anthracene was obtained from Eastman Organic Chemicals. A l l glassware used was cleaned by the procedure described p r e v i o u s l y . A f t e r each s e r i e s of experiments the i r r a d i a t i o n c e l l s were r i n s e d w e l l w i t h s i n g l y - f o l l o w e d by t r i p l y - d i s t i l l e d water and d r i e d i n the oven u n t i l r e q u i r e d . At the end of the day, a l l the c e l l s were placed i n the annealing oven and l e f t o vernight. The i r r a d i a t i o n c e l l s contained about 5 ml of sample which was added by p i p e t t e through the BIO cone shown i n Figure 16. The samples were then deoxygenated by bubbling w i t h h i g h p u r i t y argon f o r at l e a s t 30 minutes a f t e r which the stopcocks were closed and the c e l l t i p p e d h o r i z o n t a l l y to f i l l the sidearm c a r r y i n g the s p e c t r o s c o p i c c e l l . - 92 - C. ELECTRON SPIN RESONANCE 1. M a t e r i a l s and P u r i f i c a t i o n Matheson, Coleman and B e l l , s p e c t r o s c o p i c grade DMSO was p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n as described p r e v i o u s l y . The water was t r i p l y d i s t i l l e d by the method des c r i b e d e a r l i e r . A l l other chemicals used were a n a l y t i c a l grade or b e t t e r . The argon used f o r deoxygenation was obtained from Canada L i q u i d A i r . A l l glassware used i n the experiments was cleaned as o u t l i n e d p r e v i o u s l y . 2. R a d i a t i o n Source The i r r a d i a t i o n source used was the ^ C o Gammacell 220. The approximate dose r a t e was 4000 rads min 1 as estimated by F r i c k e dosimetry i n the other experimental apparatus. T o t a l absorbed doses 5 5 used i n the ESR study ranged from 1.2 x 10 to 9.6 x 10 rads. 3. Sample P r e p a r a t i o n The samples f o r i r r a d i a t i o n were prepared by dropping t i n y s p h e r i c a l drops of the l i q u i d from the c a p i l l a r y t i p of a p i p e t t e i n t o a dewar of l i q u i d n i t r o g e n . W i t h i n a few seconds the l i q u i d drops f r o z e i n t o n e a r l y s p h e r i c a l b a l l s and dropped to the bottom of the dewar. Each b a l l was approximately 2-3 mm i n diameter. Pure DMSO and DMSO, formed p o l y c r y s t a l l i n e b a l l s although s e v e r a l b i n a r y mixtures of DMSO and water of v a r y i n g composition formed completely transparent g l a s s y b a l l s . The s o l i d b a l l s were i r r a d i a t e d i n a s m a l l pyrex dewar c o n t a i n i n g l i q u i d n i t r o g e n . A f t e r the i r r a d i a t i o n the b a l l s were t r a n s f e r r e d at l i q u i d n i t r o g e n temperature to a quartz dewar i n the spectrometer c a v i t y . 4. E l e c t r o n Spin Resonance Spectra A l l e l e c t r o n s p i n resonance s p e c t r a were taken u s i n g a V a r i a n A s s o c i a t e s E-3 spectrometer which operated a t 9.1 GHz (X-band) and w i t h a 100 kHz f i e l d modulation. The magnetic f i e l d s t r e n g t h and f i e l d scan l i n e a r i t y were c a l i b r a t e d w i t h a proton-probe gaussmeter and the microwave frequency was checked w i t h a Hewlett-Packard model 5255 A d i g i t a l frequency counter. However, the microwave power l e v e l s were read d i r e c t l y from the power c o n t r o l d i a l of the spectrometer and were not c a l i b r a t e d . The s p e c t r o s c o p i c s p l i t t i n g f a c t o r s , or g-values, were determined u s i n g a f i n e l y powdered sample of DPPH ( d i p h e n y l p i c r y l h y d r a z y l ) sealed i n a t h i n quartz tube and placed i n the quartz dewar along w i t h the sample b a l l s . Taking g n p p H = 2.0036, the unknown g-values, g , were c a l c u l a t e d u s i n g equation (2.9) gv = 2.0036 (1 - |5. ) (2.9) X M X where AH i s the d i f f e r e n c e i n magnetic f i e l d between the centre of the unknown resonance and th a t of the DPPH sample and H^ i s the magnetic 59 f i e l d of the unknown resonance. - 94 - 5. P h o t o l y s i s Apparatus An u n f i l t e r e d low pressure mercury lamp (Hanovia #687A45) w i t h a Vycor envelope was used f o r the u l t r a v i o l e t p h o t o l y s i s experiments. G e n e r a l l y the photobleaching and p h o t o l y s i s experiments were c a r r i e d out by removing the quartz dewar from the c a v i t y and p l a c i n g the lamp envelope d i r e c t l y a gainst the p o r t i o n of the dewar c o n t a i n i n g the sample b a l l s . Surrounding the dewar and lamp w i t h aluminum f o i l enhanced the p h o t o l y s i n g l i g h t i n t e n s i t y . Changes i n the e s r s p e c t r a were f o l l o w e d by p h o t o l y s i n g the sample i n the c a v i t y . The c a v i t y g r i d cut down the l i g h t i n t e n s i t y so th a t the changes were not so r a p i d . A 100 watt tungsten lamp was used i n some p h o t o l y s i s experiments. These experiments were done outside the c a v i t y but, because of the heat given o f f , the lamp was not placed too clos e to the quartz dewar. - 95 - CHAPTER I I I STUDIES ON LIQUID DMSO A. 6°Co Y-RADIOLYSIS 1. Gaseous Products The gaseous products obtained under the Y - r a d i o l y s i s of pure DMSO were hydrogen, methane and ethane. Hydrogen and ethane were formed w i t h y i e l d s of G(H_) = 0.20 + 0.01 and G(C„H,) = 0.49 + 0.03, JL — / 0 — which were independent of dose up to at l e a s t 1.2 x 10^ rads as shown i n Figure 21. Methane formation was not l i n e a r w i t h dose and the data suggest that some n o n - v o l a t i l e r a d i a t i o n product was being b u i l t up which scavenges the precursor of par t of the methane. This can be seen i n Fi g u r e 22 where the v o l a t i l e gases were removed a f t e r each i r r a d i a t i o n and the t o t a l accumulated y i e l d i s p l o t t e d as a f u n c t i o n of dose. In the case of methane, three d i f f e r e n t s e r i e s were c a r r i e d out; one i n which a l l the doses were the same, another i n which each dose was d i f f e r e n t and a t h i r d i n which a s m a l l dose was fo l l o w e d by a l a r g e dose and then followed by the same sm a l l dose. As can be seen i n Fi g u r e 22 a l l the p o i n t s f a l l on a smooth curve. From the i n s e t of Figure 22 i t i s c l e a r that the methane y i e l d i s only l i n e a r up to about 1.8 x 10^ rads. Consequently, a l l scavenging s t u d i e s w i t h 3.2 GCX) 2.8 - 2.4 2.0 0.8 0.4 ^^PQ ° Q - O C r O - O X ) O- 4 0 8 0 DOSE ( r a d x I O " 4 ) TT • \0 ON 120 Figure 21. R a d i a t i o n y i e l d s as a funct i o n of absorbed dose: • , C,^ and O > H 2* The methane curve was obtained by t a k i n g the slope at various portions of the curve shown i n Figure 22. 40r- 9 0 - U) jL) O E 6 0 - ^ Q _ J L J > l/) < o o LLJ 3 0 < U ( J < 4 0 80 120 A C C U M U L A T E D DOSE (rad x 1 0 " 4 ) 160 F i g u r e 2 2 . A c c u m u l a t e d gas y i e l d s as a f u n c t i o n o f a c c u m u l a t e d dose:#,A >A > A> CH^ a t v a r i o u s d o s e s ; • - 98 - vari o u s added s o l u t e s were c a r r i e d out w i t h doses l e s s than 1.8 x 10^ rads so that the r e a l e f f e c t of these scavengers on the methane y i e l d could be observed. From the slope at v a r i o u s p o r t i o n s of the curve the change i n G(CH^) w i t h absorbed dose was obtained and t h i s i s shown i n Figure 21. I t can be seen that G(CH^) f a l l s from 3.3+0.1 to 2.1 + 0.1 over the dose range s t u d i e d (up to 1.3 x 10^ r a d s ) . The only other r e p o r t on the r a d i o l y s i s of DMSO i n which the gaseous y i e l d s have been measured i s by Koulkes-Pujo and B e r t h o u ^ i n which they r e p o r t that the hydrogen and methane y i e l d s were both independent of dose. The y i e l d s , G ( H 2 ) = 0.19 + 0.006 and G ( C H 4 ) = 3.4 + 0.3, agree reasonably w e l l w i t h those observed i n t h i s study provided t h e i r t o t a l absorbed dose was l e s s than 1.8 x 10~* rads. Since the dose range was not s t a t e d , i t i s not p o s s i b l e to a s c e r t a i n whether or not the dose independence of the methane y i e l d i s i n c o n f l i c t w i t h the r e s u l t s obtained i n t h i s work. No mention was made of ethane being observed. 2. L i q u i d Products The only l i q u i d products i d e n t i f i e d i n the Y - r a d i o l y s i s of pure DMSO were dimethyl s u l f i d e and dimethyl d i s u l f i d e . Other s t u d i e s on the r a d i o l y s i s of DMSO have quoted dimethyl s u l f i d e , m e t h y l mercaptan^ 2 and dimethyl s u l f o n e ^ as l i q u i d products but no y i e l d measurements were given. I n j e c t i o n of methyl mercaptan as a standard showed that i t d i d not belong to any of the unresolved peaks shown i n Figure 14. In t h i s study, only dimethyl s u l f i d e was obtained as a measurable product; dimethyl d i s u l f i d e w a s observed only i n t r a c e q u a n t i t i e s . As shown i n Figure 23, the y i e l d of ( C H ^ ^ S was dependent on dose as - 99 - 00 X o E UJ > LO Q Q LU < ID U u < 3 6 9 A C C U M U L A T E D D O S E ( rads x 10 ) Figure 23. Accumulated dimethyl s u l f i d e y i e l d as a f u n c t i o n of accumulated dose. The e x t r a p o l a t e d p o r t i o n of the curve corresponds to G(DMS) = 1.2 + 0.2. - 100 - i t i n c r e a s e d w i t h accumulated dose. A p o s s i b l e e x p l a n a t i o n i s that some product i s being b u i l t up which y i e l d s dimethyl s u l f i d e upon r a d i o l y s i s . The e x t r a p o l a t e d y i e l d of dimethyl s u l f i d e was found to be G(DMS) =1.2+0.2. 3. Scavenger Studies The gaseous and l i q u i d products observed are the chemical consequence of the i n t e r a c t i o n of high-energy r a d i a t i o n w i t h DMSO. In order to t e s t the nature of t h e i r p r e c u r s o r s , and i n p a r t i c u l a r the primary reducing s p e c i e s , v a r i o u s e l e c t r o n and r a d i c a l scavengers were added and the e f f e c t of these scavengers on the chemical y i e l d s observed. Probably the most wi d e l y used scavanger f o r s o l v a t e d e l e c t r o n s i n l i q u i d s i s n i t r o u s oxide which y i e l d s molecular n i t r o g e n upon r e d u c t i o n . The e f f e c t on the gas y i e l d s due to the a d d i t i o n of various c o n c e n t r a t i o n s of n i t r o u s oxide to DMSO i s shown i n Fi g u r e 24. The methane y i e l d was found to decrease by about 20% (from 3.3 to 2.6) w h i l e hydrogen and ethane were una f f e c t e d . As the n i t r o u s oxide c o n c e n t r a t i o n was inc r e a s e d up to 0.09 M, G(N^) s t e a d i l y i n c r e a s e d w i t h no simple p l a t e a u as shown i n Figure 25. At [^0] = 0.054 M, the n i t r o g e n y i e l d was independent of dose up to at l e a s t 7.5 x 10^ rads (Figure 26) suggesting that whatever the n o n - v o l a t i l e product i s which i n h i b i t s the methane y i e l d , i t does not a f f e c t the n i t r o g e n y i e l d . From the slope of the l i n e i n Figure 26, G(N 2) = 1.6 + 0.1 at t h i s c o n c e n t r a t i o n . These r e s u l t s suggest that p a r t of the methane y i e l d a r i s e s from I I li I I I I I I 1 I—I 0.02 0.04 0 . 0 6 0 - 0 8 0.10 [ N 2 O ] M Figure 24. R a d i a t i o n y i e l d s of CH^ (A), (•) and H 2 (O) as a f u n c t i o n of the N 20 c o n c e n t r a t i o n . In the case of CH^, a l l doses were < 1.8 x 10^ rads. Figure 25. Radiation y i e l d of n i t r o g e n as a f u n c t i o n of N.O c o n c e n t r a t i o n i n DMSO. ACCUMULATED D O S E ( r a d . x l O ^ ) Figure 26. Accumulated nitrogen y i e l d as a f u n c t i o n of accumulated dose fo r a 0.054 M s o l u t i o n of N . O i n DMSO. - 104 - some st r o n g reducing p r e c u r s o r . To t e s t the nature of t h i s chemical species w i t h which the n i t r o u s oxide was r e a c t i n g , v a r i o u s second scavengers were added to 0.05 M N^O s o l u t i o n s to compete w i t h i t . Data on the e f f e c t of these scavengers i s shown i n Table I I . Methanol and i s o p r o p a n o l are known to r e a c t r a p i d l y w i t h hydrogen atoms but very s l o w l y w i t h s o l v a t e d e l e c t r o n s . On the other hand, acetone, s i l v e r n i t r a t e , chloroform, i o d i n e , carbon t e t r a c h l o r i d e and a c i d are known to r e a c t very r a p i d l y w i t h s o l v a t e d e l e c t r o n s i n other p o l a r s o l v e n t s . Water was added because DMSO i s very hygroscopic and there could have been a t r a c e of water i n the samples d e s p i t e the s t r i n g e n t precautions taken to e l i m i n a t e i t . As can be seen from Table I I the added water had no a p p r e c i a b l e e f f e c t on any of the gas y i e l d s , so that e i t h e r a l l samples contained enough a d v e n t i t i o u s water f o r i t s e f f e c t to be f u l l y f e l t or water cannot compete w i t h the precursors of the gaseous products at t h i s c o n c e n t r a t i o n (a. 0.5 % by volume). With the assumption that n i t r o u s oxide and the second scavenger (S) compete f o r the p r e c u r s o r (P) of n i t r o g e n according to r e a c t i o n s (14) and (15) N 20 + P N + products (14) k S + P s products (15) then k ^ o K P ] G(N 2) = G(P) [ Q [ N 2 0 ] [ P ] + k j S H P ] ] (3.1) - 105 - TABLE I I . R a d i a t i o n y i e l d s f r o m i r r a d i a t e d DMSO c o n t a i n i n g 0 . 0 5 M N 2 0 p l u s t h e s e c o n d s c a v e n g e r i n d i c a t e d i n c o l u m n 1 a t c o n c e n t r a - t i o n g i v e n i n c o l u m n 2 . S e c o n d s c a v e n g e r C o n c e n t r a t i o n (M) G ( N 2 ) G ( C H 4 ) G ( H 2 ) G ( C 2 H W a t e r 0 . 2 2 1 . 7 3 . 1 0 . 2 1 0 . 4 6 M e t h a n o l 0 . 0 9 8 1 . 7 3 . 0 0 . 2 3 0 . 4 2 0 . 1 9 6 1 . 7 3 . 0 0 . 2 3 0 . 4 3 (a ) 0 . 1 9 6 - 3 . 6 0 . 2 4 0 . 4 6 I s o p r o p a n o l 0 . 1 0 3 1 . 8 2 . 8 0 . 2 6 0 . 4 5 (a ) 0 . 1 0 3 - 3 . 1 0 . 2 5 0 . 4 6 A c e t o n e 0 . 1 0 7 1 . 7 2 . 8 0 . 2 3 0 . 4 5 0 . 2 6 4 1 . 8 2 . 6 0 . 2 3 0 . 4 4 A g N 0 3 0 . 0 0 9 4 7 1 . 4 2 . 6 0 . 2 1 0 . 4 0 0 . 0 2 4 9 1 . 2 2 . 4 0 . 2 1 0 . 4 2 0 . 0 5 1 6 0 . 9 5 2 . 3 0 . 2 0 0 . 4 1 0 . 1 0 2 0 . 6 5 2 . 1 0 . 1 9 0 . 4 1 0 . 1 5 8 0 . 5 7 2 . 1 0 . 2 0 0 . 4 1 C H C 1 3 0 . 0 1 2 4 1 . 2 2 . 3 0 . 2 0 0 . 3 6 0 . 0 6 9 6 0 . 6 8 2 . 2 0 . 2 1 0 . 4 1 0 . 0 9 9 0 0 . 6 3 2 . 2 0 . 1 9 0 . 3 9 0 . 1 9 7 0 . 4 3 2 . 1 0 . 2 1 0 . 4 0 0 . 3 0 4 0 . 3 8 1 . 9 0 . 2 0 0 . 3 7 I o d i n e 0 . 0 0 6 5 2 1 . 5 0 . 3 2 0 . 2 2 0 . 4 2 0 . 0 2 0 8 1 . 1 0 . 2 6 0 . 2 1 0 . 2 9 0 . 0 3 0 0 0 . 9 9 0 . 2 5 0 . 2 2 0 . 2 8 0 . 0 9 4 1 0 . 5 9 0 . 2 0 0 . 2 0 0 . 2 5 0 . 1 2 6 0 . 4 5 0 . 2 1 0 . 1 9 0 . 2 6 - 106 - TABLE I I (continued) Second scavenger Concentration (M) G(N 2) G(CH 4) G(H 2) G(C 2H 6) c c i 4 0.0179 1.11 2.2 0.21 0.45 0.0490 0.74 1.8 0.21 0.38 0.0685 0.68 1.7 0.21 0.37 0.0740 0.65 1.7 0.20 0.36 0.103 0.45 1.5 0.20 0.36 Anhydrous 0.00918 1.4 2.4 0.20 0.41 s u l f u r i c a c i d 0.0195 1.2 2.5 0.20 0.45 0.0295 1.1 2.7 0.21 0.45 0.0377 1.0 2.8 0.20 0.47 0.0559 0.80 2.7 0.19 0.47 0.0895 0.68 3.0 0.20 0.50 0.114 0.63 3.0 0.20 0.53 0.137 0.55 3.2 0.20 0.50 (b) 0.0718 0.78 2.8 0.23 (a) No N 20 added (b) 0.5 M methanol added - 107 - where k g i s the r a t e constant of P w i t h S, ^ i s the r a t e constant f o r the r e a c t i o n of P w i t h H^O and G(P) i s the y i e l d of the pr e c u r s o r . Rearranging (3.1), the f o l l o w i n g r e l a t i o n s h i p was obtained and a p p l i e d to the competitions. G T J p " GW [ I + ^ I S I / ^ O I ^ O ] ] (3.2) The data from Table I I are p l o t t e d i n Figures 27 and 28 and appear to f o l l o w the simple competition r e l a t i o n s h i p . From the slopes of the p l o t s the values f o r k g / k j j n were obtained and are given i n Table I I I and compared w i t h the pu b l i s h e d values of these r a t i o s f o r the corresponding r e a c t i o n s i n water i n which the precursor i s the hydrated e l e c t r o n , e aq Although the i n t e r c e p t of these p l o t s a l l give G(P) = 1.6+0.2 t h i s i s not the tru e p r e c u r s o r y i e l d . In f a c t G(N2) does not show a p l a t e a u value at high n i t r o u s oxide concentrations which would correspond to complete scavenging of the precursor ; thus the y i e l d of G(P) depends on the n i t r o u s oxide concentrations used above 0.03 M (see F i g u r e 25). This i s evident from the H + competition shown i n Figure 28 i n which two d i f f e r e n t but high values (0.05 M and 0.07 M N 20) of n i t r o u s oxide were used. Although both l i n e s have the same slope two d i f f e r e n t i n t e r c e p t s were obtained. For t h i s reason the n i t r o u s oxide c o n c e n t r a t i o n was h e l d constant at 0.05 M w h i l e the co n c e n t r a t i o n of the second scavenger was v a r i e d . F i g u r e s 29 and 30 show the e f f e c t of i o d i n e and hydrogen ions on the gaseous products. A c i d appears to have no e f f e c t on the gas - 108 - Figure 27. P l o t of 1/G(N ) as a f u n c t i o n of [Scavenger]/[NO]. The N O c o n c e n t r a t i o n was 0.05 M and the second scavenger concentra- t i o n was v a r i e d . The data was taken from Table 2. # , C C l ^ ; A , I _ ; • , CHC1-; O , A g + and • , acetone. 2.0 concentrations: O , 0.05 M 1^0; A , 0.04 M N 20 and • , 0.07 M N 20. • corresponds to 0.5 M methanol added to the a c i d s o l u t i o n . - 110 - TABLE I I I . R a t i o of k /lc obtained f o r the precursor of N- i n DMSO w i t h v a r i o u s second scavengers (S) added to a 0.05 M s o l u t i o n of N 20. Column headed water r e f e r s to the p u b l i s h e d r a t e constant r a t i o f o r the hydrated e l e c t r o n . S k S / k N 2 0 ( D M S 0 ) k S / k N 2 0 < w a t e r ) a CC1. 4 0.85 + 0.09 5.4 h 0.60 + 0.06 9.1 CHC1 3 0.45 + 0.05 3.6 A g + 0.40 + 0.04 5.7 H + 0.25 + 0.03 3.9 (CH 3) 2CO <0.01 1.1 Data taken from the c o m p i l a t i o n of r a t e constants by M. Anbar and P. Neta, I n t e r . J . Appl. Rad. I s o t . 18, 493 (1967).  3.6 - G C X ) o JO—O-O-O O O- -O- 0.04 0 0 8 0-12 [ H 2 S Q 4 ] M to I 0.20 Figure 30. Rad i a t i o n y i e l d s of CH^ (A), (•) and H 2 (O). as a f u n c t i o n of H + c o n c e n t r a t i o n . A corresponds to the y i e l d of CH, when 0.05 M N~0 was added to the corresponding a c i d s o l u t i o n of 5 DMSO. In the case of CH., a l l doses were < 1.8 x 10 rads. 4 - 113 - y i e l d s , i n c o n t r a s t to n i t r o u s o x i d e , whereas i o d i n e reduces the y i e l d of ethane and methane without a f f e c t i n g the hydrogen y i e l d . 4. D i s c u s s i o n Although n i t r o u s oxide has been w i d e l y used as a s p e c i f i c scavenger f o r s o l v a t e d e l e c t r o n s i n many l i q u i d s , other s t r o n g l y reducing species formed i n the r a d i o l y s i s may gi v e r i s e to the n i t r o g e n observed. I n the case of DMSO, s o l v a t e d e l e c t r o n s , hydrogen atoms, f r e e r a d i c a l s (R) or s o l v e n t r a d i c a l anions could y i e l d n i t r o g e n v i a r e a c t i o n s (16), (17), (18), or (19) i f t h e i r a l t e r n a t e f a t e s were comparatively slow. e + N_0 *- N. + 0 (16) s 2 2 s H- + N 20 *- N 2 + -OH (17) R* + N 20 N 2 + R0» (18) DMSO~ + N 20 *~ N 2 + DMS02~ " (19) In t h i s r e a c t i o n scheme the so l v e n t r a d i c a l anion, DMSO , i s to be regarded as an anion formed e i t h e r by e l e c t r o n attachment of s o l v a t e d or f r e e e l e c t r o n s or i s a decomposition product formed from i t . I n order to e x p l a i n the observed r e s u l t s and to d i f f e r e n t i a t e between the p o s s i b l e p r e c u r s o r s of the n i t r o g e n , strong i n f e r e n c e s must be drawn from the competition s t u d i e s w i t h the other scavengers. From Table I I i t can be seen that the a d d i t i o n of methanol and - 1 1 4 - i s o p r o p a n o l d i d not a l t e r the y i e l d of n i t r o g e n . Since these a l c o h o l s are g e n e r a l l y much b e t t e r hydrogen atom scavengers than i s n i t r o u s o x i d e , t h i s suggests that r e a c t i o n (17) does not c o n t r i b u t e to the n i t r o g e n y i e l d . Furthermore, the s l i g h t i n c r e a s e i n hydrogen y i e l d , which can be p a r t i a l l y a t t r i b u t e d to the y i e l d obtained from the d i r e c t a c t i o n of the r a d i a t i o n on the a l c o h o l s , i m p l i e s t h a t the hydrogen atoms are produced i n low y i e l d . In a s i m i l a r manner, r e a c t i o n (18) can a l s o be disregarded. From Table I I I i t can be seen that i o d i n e competes on an almost equal b a s i s w i t h n i t r o u s a i d e f o r the precursor of n i t r o g e n d e s p i t e the f a c t t h a t i o d i n e i s a much b e t t e r r a d i c a l scavenger than n i t r o u s oxide. I f r e a c t i o n (18) were the pre c u r s o r of molecular n i t r o g e n one would expect a grea t e r r e d u c t i o n i n the y i e l d when i o d i n e was added than i s observed. This i n f e r e n c e i s f u r t h e r supported i n Fi g u r e 29 where i t can be seen that i o d i n e v i r t u a l l y e l i m i n a t e s methane, even at 0.006 M. This suggests that methane a r i s e s from methyl r a d i c a l s which a b s t r a c t a hydrogen atom from DMSO. In t h i s regard, i t should be mentioned that methyl r a d i c a l s were observed i n y - i r r a d i a t e d p o l y c r y s t a l l i n e DMSO at 77°K (to be discussed l a t e r ) . From Figure 29 i t can a l s o be seen that i o d i n e decreases the ethane y i e l d s l i g h t l y i m p l y i n g t h a t p a r t of i t i s s i m i l a r l y formed by r a d i c a l r e a c t i o n s . On the other hand, the hydrogen y i e l d i s unaffected by i o d i n e i n d i c a t i n g i t i s formed i n a "molecular process", such as molecular detachment of hydrogen or an ion-molecule or "hot" atom r e a c t i o n o c c u r r i n g w i t h i n a few c o l l i s i o n s . S i m i l a r l y about h a l f the ethane y i e l d and about 10% of the methane y i e l d may be due to "molecular processes". - 115 - The i n f e r e n c e then i s that the reducing precursor of n i t r o g e n i s a s o l v a t e d e l e c t r o n or r a d i c a l anion. The reducing p r o p e r t i e s and chemical behavior of these two species may be very s i m i l a r ; indeed perhaps only p u l s e r a d i o l y s i s a b s o r p t i o n spectroscopy or es r can r e a l l y d i f f e r e n t i a t e between them. As mentioned e a r l i e r the absor p t i o n s p e c t r a of s o l v a t e d e l e c t r o n s are c h a r a c t e r i z e d by t h e i r broadness and i n t e n s i t y i n the v i s i b l e and near i n f r a r e d r e g i o n of the spectrum. By c o n t r a s t , r a d i c a l anions would probably have weak, narrow bands i n the v i s i b l e . T his s i m i l a r i t y i n behavior was demonstrated i n a 46 recent study on the r a d i o l y s i s of l i q u i d formamide i n which the p a t t e r n of r e a c t i v i t y of the reducing species conformed remarkably w e l l to that of s o l v a t e d e l e c t r o n s i n other media. However l a t e r p u l s e r a d i o l y s i s s t u d i e s showed that formamide does not form s o l v a t e d e l e c t r o n s w i t h l i f e t i m e s > 10 ± L seconds but ra t h e r r e a c t s r a p i d l y w i t h the thermalized e l e c t r o n s , presumably to give r a d i c a l anions 45 as t h e i r decomposition products. These r a d i c a l anions thus represent the " f r e e i o n s " which r e a c t w i t h the e l e c t r o n scavengers. As i n the case of formamide the chemical evidence f o r the presence or absence of s o l v a t e d e l e c t r o n s i n DMSO i s not c o n c l u s i v e . The pulse r a d i o l y s i s study which w i l l be described l a t e r showed that s o l v a t e d e l e c t r o n s are formed as the primary reducing species i n DMSO. However, tha t study a l s o showed that the e l e c t r o n s appear to re a c t w i t h the sol v e n t q u i t e r a p i d l y so that r a d i c a l anions are probably a l s o i n v o l v e d i n the r e d u c t i o n of U^O and other scavengers at low concentrations i n these steady s t a t e experiments. From the data given i n Tables I I and I I I , i t i s evident t h a t n i t r o u s - 116 - oxide i s a b e t t e r scavenger of reducing species than the other scavengers. The r e l a t i v e r a t e constants obtained from the slopes of F i g u r e s 27 and 28 and compared w i t h the r e l a t i v e r a t e s of these scavengers w i t h hydrated e l e c t r o n s show them to be markedly d i f f e r e n t . Furthermore acetone does not compete e f f e c t i v e l y w i t h n i t r o u s oxide although C C l ^ , I 2 > CHC13» A g + and H + do. T h i s i s at v a r i a n c e w i t h the r e s u l t s obtained i n the pulse r a d i o l y s i s of DMSO c o n t a i n i n g 0.19 M 58 acetone i n which the s o l v a t e d e l e c t r o n was completely e l i m i n a t e d . A. p o s s i b l e e x p l a n a t i o n i s that the acetone r a d i c a l anion, (CH^COCH^) , undergoes a charge t r a n s f e r w i t h n i t r o u s oxide according to the f o l l o w i n g sequence: e + N„0 >• N_ + 0 (16) s 2 2 s e + CH.COCH, (CH-COCH.) (20) s 3 3 3 3 (CH 3C0CH 3) + N 20 N 2 + °s + C H 3 C 0 C H 3 ( 2 1> Moreover the negative ions of C C l ^ , 1^ and CHCl^ may a l s o undergo charge t r a n s f e r w i t h the n i t r o u s oxide so that the a c t u a l r a t e constant, k^ Q, could be a c t u a l l y s m a l l e r than t h a t observed. The curvature of G(N 2) at [N 20] > 0.03 M i s d i f f i c u l t to i n t e r p r e t without knowing the a l t e r n a t e f a t e s of the n i t r o g e n precursors ( s o l v a t e d e l e c t r o n s and other reducing s p e c i e s , i f any) and t h e i r a bsolute r a t e constants. A p o s s i b l e i n t e r p r e t a t i o n i s that the n i t r o u s oxide i s scavenging e l e c t r o n s i n s i d e the spur. However t h i s i s - 117 - disregarded because pulse r a d i o l y s i s of a s o l u t i o n of 0.07 M n i t r o u s 5 8 oxide i n DMSO only scavenged about 80% of the s o l v a t e d e l e c t r o n s . I f one equates the n i t r o g e n y i e l d at the h i g h n i t r o u s oxide concentra- t i o n s w i t h the t o t a l f r e e i o n y i e l d , then G(free ion) = 1.8 +0.2 (see Figure 25). However, using anthracene as a scavenger i t was shown by pulse r a d i o l y s i s t h a t the y i e l d of s o l v a t e d e l e c t r o n s i s only 1.3 (to be discussed s h o r t l y ) . Perhaps the higher y i e l d i n the case of n i t r o u s oxide i s due to scavenging of other h i g h l y reducing s p e c i e s i n a d d i t i o n to s o l v a t e d e l e c t r o n s . However, another p o s s i b i l i t y e x i s t s , namely that each e gives r i s e to more than one N„ molecule. Studies s 2 on other systems using n i t r o u s oxide have given higher n i t r o g e n y i e l d s than the t o t a l f r e e i o n y i e l d and i t has been proposed that 0 may re a c t f u r t h e r to produce more n i t r o g e n according to r e a c t i o n (22). e + N„0 N 0 + 0 (16) s 2 2 s 0 + N O *~ N + products (22) E a r l i e r i t was suggested t h a t most of the methane a r i s e s from methyl r a d i c a l s which a b s t r a c t a hydrogen atom from DMSO because the methane was v i r t u a l l y e l i m i n a t e d by i o d i n e , an e x c e l l e n t r a d i c a l scavenger. However, the f a c t that n i t r o u s oxide can decrease the methane y i e l d by 20% suggests t h a t i t can a l s o i n t e r f e r e w i t h a minor r e a c t i o n l e a d i n g to CH^ formation. Because no decrease i n the methane y i e l d was observed i n the presence of methanol and i s o p r o p a n o l , which are b e t t e r r a d i c a l scavengers than n i t r o u s o x i d e , the i n t e r f e r e n c e i s - 118 - probably i o n i c i n nature. This i s s t i l l i n keeping w i t h the i o d i n e r e s u l t s s i n c e 1^ i s a l s o a good e l e c t r o n scavenger as seen from the r e l a t i v e r a t e constants i n Table I I I . I t i s suggested that p a r t of the methane y i e l d a r i s e s from the decomposition of the DMSO r a d i c a l anion by the r e a c t i o n sequence shown below. e + DMSO >- DMSO (23) s DMSO *~ CH 3- + products (24) CR3- + DMSO CH 4 + products (25) This i s supported by the f a c t t h a t e l e c t r o n s generated i n DMSO-water glasses at 77°K by u l t r a v i o l e t p h o t o l y s i s of K 4Fe(CN)^ undergo r e a c t i o n w i t h the DMSO molecules t o produce methyl r a d i c a l s (see l a t e r ) . N i t r o u s o x i d e , and other e l e c t r o n scavengers (except H ), c o u l d i n t e r f e r e w i t h the methane formation e i t h e r by scavenging the s o l v a t e d e l e c t r o n before i t r e a c t s w i t h the s o l v e n t molecules to produce the anion or by charge t r a n s f e r w i t h the s o l v e n t anion. Furthermore, s t r o n g l y reducing metals (M) such as sodium and potassium i n excess DMSO decompose the s o l v e n t y i e l d i n g a mixture of methane, dimethyl s u l f i d e and the s a l t s of methane s u l f e n a t e and m e t h y l s u l f i n y l carbanion 63 according to the r e a c t i o n sequence below. CR 3SOCH 3 + 2M *- CH3SO~M+ + CH 3~M + (26) CH 3SOCH 3 + CH 3"M + *- CH 3SOCH 2~M + + CH^ (27) - 119 - CH 3SOCH 3 + 2M *- CH 3SCH 3 + (28) CH 3SOCH 3 + M 20 *• CH 3SOCH 2~M + + MOH (29) Reactions (26) and (28) were found t o be about equal w i t h sodium whereas w i t h potassium r e a c t i o n (26) g r e a t l y predominated. Reactions (26) and (27) may be considered somewhat analogous to r e a c t i o n s (23) to (25). I t i s i n t e r e s t i n g to note that dimethyl s u l f i d e i s a major decomposition product i n a l k a l i metal s o l u t i o n s of DMSO. The l a r g e y i e l d of dimethyl s u l f i d e , G(DMS) = 1.2 + 0.2, observed i n the r a d i o l y s i s of DMSO may a r i s e d i r e c t l y from the r e d u c t i o n of the so l v e n t according to r e a c t i o n (30). CH.SOCH- + e~ *~ CH.SCH_ + products (30) 3 3 s 3 3 The a d d i t i o n of 0.5 M AgN0 3 which would r e a d i l y scavenge a l l the s o l v a t e d e l e c t r o n s caused no n o t i c e a b l e change i n the dimethyl s u l f i d e y i e l d , which argues a g a i n s t r e a c t i o n (30). However, i t i s p o s s i b l e that the s i l v e r atoms produced undergo charge t r a n s f e r w i t h the s o l v e n t i n an analogous manner to r e a c t i o n (28). + Ag ^ - ^ A g ^ (31) CH 3S0CH 3 + A g 2 + >• Ag 20 + CH 3SCH 3 (32) This i s supported by the f a c t that when DMSO was allowed to remain i n - 120 - contact w i t h a f r e s h l y prepared s i l v e r m i r r o r f o r s e v e r a l days, a strong odour of dimethyl s u l f i d e was observed. On the other hand i t i s p o s s i b l e that DMS does not a r i s e from e^ or i s a molecular product whose precursor cannot be scavenged even at 0.5 M Ag +. The a d d i t i o n of hydrogen ions at concentrations up to 0.2 M H + d i d not a f f e c t the gaseous y i e l d s from pure i r r a d i a t e d DMSO (see Figure 30) i n marked c o n t r a s t to the presence of n i t r o u s oxide. This e f f e c t of H + i s i n agreement w i t h the r e s u l t s of P.ujo and Berthou.^^ The strong e l e c t r o n - d o n a t i n g power of the s u l f o x i d e group makes DMSO a powerful Lewis base which i s r e a d i l y protonated by hydrogen ions i n accordance w i t h r e a c t i o n (33). H + + CH 3SOCH 3 *- (CH 3S0HCH 3) + (33) The experimental observations can be ex p l a i n e d i f the r e a c t i o n of the e l e c t r o n w i t h the p o s i t i v e i o n (CH 3S0HCH 3) + has the same f a t e as the DMSO r a d i c a l anion. e~ + (CR* 3S0HCH 3) + (CH^OHCH^ »-CH3' + products (34) The p o s i t i v e i o n would compete w i t h n i t r o u s oxide f o r the e l e c t r o n and perhaps other reducing species produced i n the r a d i o l y s i s . At the same time i t would counteract the s l i g h t e f f e c t n i t r o u s oxide has on the methane y i e l d . This e f f e c t i s shown i n Figure 30. The e l e c t r o n s which are not scavenged and undergo geminate recombination would - 121 - produce a h i g h l y e x c i t e d DMSO molecule which would probably d i s s o c i a t e according to r e a c t i o n (35). e~ + DMSO+ *~ DMSO* *~ CHy + products (35) Whereas n i t r o u s oxide at 0.07 M d i d not seem to be i n t e r f e r i n g w i t h geminate recombination, s u l f u r i c a c i d at 0.2 M may w e l l be doing so, as the p u l s e r a d i o l y s i s data i n d i c a t e s . Reactions (34) and (35) could t h e r e f o r e e x p l a i n why h i g h a c i d concentrations had no e f f e c t on the methane y i e l d . The r e a c t i o n of the s o l v a t e d e l e c t r o n w i t h the hydrogen i o n , according to r e a c t i o n (36), e~ + H + *~ H- (36) s or w i t h the protonated DMSO molecule to produce a hydrogen atom does not appear to occur i n view of the competition between H +, ^ 0 and methanol (see Table I I ) . I f r e a c t i o n (34) produced a hydrogen atom or i f r e a c t i o n (36) was i n competition w i t h r e a c t i o n (16), then the hydrogen y i e l d would be i n c r e a s e d by the presence of methanol i n the a c i d i c s o l u t i o n of DMSO. No such i n c r e a s e was observed. However, i t i s p o s s i b l e that hydrogen atoms are produced but t h a t they r e a c t r a p i d l y w i t h DMSO to give non-gaseous products. - 122 - B. PULSE RADIOLYSIS 1. A b s o r p t i o n Spectra i n DMSO and DMSO^ Previous i n v e s t i g a t i o n s on the pulse r a d i o l y s i s of pure DMSO at room t e m p e r a t u r e " * ^ ' h a v e shown that the a b s o r p t i o n s p e c t r a belonging to the t r a n s i e n t s f a l l i n t o three main c a t e g o r i e s . From t h e i r d i s t i n c t decay c h a r a c t e r i s t i c s and behavior towards s p e c i f i c scavengers they were a t t r i b u t e d to d i f f e r e n t s p e c i e s . The three p r i n c i p a l branches, w i t h t h e i r c h a r a c t e r i s t i c f e a t u r e s , are given as f o l l o w s : ( i ) a broad, s t r u c t u r e l e s s a b s o r p t i o n s t r e t c h i n g from the v i s i b l e i n t o the near i n f r a r e d w i t h a X > 1500 nm and having a h a l f - l i f e of ̂  15 nsec max which was a t t r i b u t e d to the s o l v a t e d e l e c t r o n ; ( i i ) a f a i r l y broad band centered at % 600 nm whose h a l f - l i f e was <\> 1 usee and a t t r i b u t e d to an o x i d i z i n g s p e c i e s ; and ( i i i ) a b s o r p t i o n bands p r o g r e s s i n g from 400 nm i n t o the near u l t r a v i o l e t r e g i o n belonging to at l e a s t two components possessing h a l f - l i v e s > 1 msec and ̂  12 usee. However f u r t h e r s t u d i e s on these t r a n s i e n t s , i n p a r t i c u l a r that of the o x i d i z i n g species and the s o l v a t e d e l e c t r o n , were warranted i n view of the f a c t that the f r e e i o n y i e l d had not been determined nor the i d e n t i t y of the o x i d i z i n g s p e c i e s . Unless s t a t e d otherwise, the a b s o r p t i o n s p e c t r a of these and other t r a n s i e n t s were obtained using e l e c t r o n pulse widths of 40 nsec d e p o s i t i n g an average dose of 2.2 krad per p u l s e . The t o t a l absorbed dose which any one sample r e c e i v e d was always l e s s than 10"* rads. The a b s o r p t i o n s p e c t r a of the o x i d i z i n g species (X = 550 nm) and the s o l v a t e d e l e c t r o n (X > 1500 nm) obtained i n pure (CH o)_S0 max 5 1 and (CD~)_S0 are given i n Figure 31. The t o t a l a b s o r p t i o n was 3 0 0 5 0 0 7 0 0 9 0 0 1100 1300 1500 WAVELENGTH (nm) Figure 31. Transient spectra observed i n (CI^^SO and (CD-j^SO. The c i r c l e s refer to the so l v a t e d e l e c t r o n band c o r r e c t e d f o r the detector response time and the t r i a n g l e s r e f e r to the DMSO p o s i t i v e i o n , or o x i d i z i n g species. O and A are f o r ( C l ^ ^ S O ; • and A are f o r (003)280. Almost a l l data p o i n t s are the mean of at l e a s t two measurements. The x m a x f o r the & spectrum was e s t a b l i s h e d to be at 550 nm from a previous set of experiments. - 124 - obtained from the observed absorption peak heights at the end of the pulse. The bands attributed to the solvated electron in the deuterated and undeuterated DMSO had almost completely decayed within 100 nsec; consequently the maximum intensity of the bands centered at 550 nm were deduced by extrapolation to the end of the pulse from times > 100 nsec. This approximation is reasonable since the 550 nm band decays in a first-order manner with a h a l f - l i f e of 2.3 + 0.2 jisec (see Figure 35). Typical oscilloscope traces of the electron and oxidizing species are shown i n Figures 32 and 33. In order to compare the relative intensity of the bands, i t was necessary to correct for the decay of the electron during the pulse. By using 10 nsec pulses i t was determined that the electron decays by first-order kinetics with a h a l f - l i f e of 15 + 2 nsec (see Figure 34). Therefore with a 40 nsec pulse, which was used in the determination of the absorption spectra, considerable decay of the electron would have occurred during the pulse. Furthermore, the time response of the detection system (oscilloscope and photodiodes) i s comparable to the h a l f - l i f e of the electron so that the i n i t i a l absorbance observed i s not the true end of pulse absorbance. Using reasonable values for the time constants of the photodiodes and oscilloscope, the electron pulse width and the h a l f - l i f e of the electron, suitable correction factors were computed and applied to the observed absorbance.^ These calculations show that under the experimental conditions, the maximum electron absorbance observed was only 43% of the absorbance which would have been observed had there been no decay. Consequently the observed absorbances for the electron band were multiplied by 2.3 relative to the 550 nm band data shown i n Figure 31. - 125. - 0.82%A 20 nsec (a) \=1300 nm 10 nsec pulse ( b ) \ = 1275 nm U0 nsec pulse 20 nsec Figure 32. T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the s o l v a t e d e l e c t r o n i n DMSO. Both tr a c e s were obtained u s i n g a Ge photodiode w i t h a 50 ohm load r e s i s t a n c e , (a) p u l s e width 10 nsec; (b) pulse w i d t h 40 nsec. - 126 V 1 A J > e ( a ) 100 nsec 1.0% A (b) F i g u r e 33. T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the DMSO p o s i t i v e i o n or o x i d i z i n g species at 550 nm. The f a s t i n i t i a l decay i n (a) i s due to the s o l v a t e d e l e c t r o n . Both tr a c e s were obtained u s i n g a pulse width of 40 nsec. (a ) S i photodiode w i t h 93 ohm load r e s i s t o r ; (b) photo- m u l t i p l i e r w i t h 470 ohm load r e s i s t o r . - 127 - At X > 1500 nm the Ge photodiode response i s too slow to make measurements on the e l e c t r o n band f o r the reasons mentioned e a r l i e r . However, by observing the steady s t a t e a b s o r p t i o n during long 58 i r r a d i a t i o n pulses (> 1 usee) u s i n g slow InAs ( r i s e - t i m e 2 usee) and InSb ( r i s e - t i m e 100 n s e c ) ^ d e t e c t o r s , i t appears t h a t the maximum e x t i n c t i o n c o e f f i c i e n t f o r the e l e c t r o n band occurs between 1600 and 1800 nm and tha t Ge i s < 25% l a r g e r than Ge at 1500 nm. max There are s e v e r a l reasons f o r a s s i g n i n g the abs o r p t i o n band w i t h X > 1500 nm to the s o l v a t e d e l e c t r o n i n DMSO. F i r s t l y , i t i s max 4 -1 -1 extremely broad and in t e n s e (e > 10 M cm ), a c h a r a c t e r i s t i c J max fe a t u r e of s o l v a t e d e l e c t r o n bands. Secondly, the band i s e f f i c i e n t l y removed or reduced by known e l e c t r o n scavengers, such as O2, ̂ 0 , anthracene, + + Ag , acetone, C C l ^ and H . Moreover, a d d i t i o n of known p o s i t i v e i o n scavengers such as Br and water had no a f f e c t on the spectrum. There i s a l s o evidence, which w i l l be discussed more f u l l y l a t e r , f o r the assignment of the band centered at 550 nm to the DMSO p o s i t i v e i o n . In the presence of Br the 550 nm band was completely e l i m i n a t e d and re p l a c e d by the B ^ abs o r p t i o n . A d d i t i o n of e l e c t r o n scavengers 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 to j u s t e l i m i n a t e the e l e c t r o n band d i d not a f f e c t the 550 nm absor p t i o n or i t s decay r a t e , whereas a d d i t i o n + + of H and Ag i n s u f f i c i e n t c o n c e n t r a t i o n to i n t e r f e r e w i t h geminate recombination (> 0.1 M) inc r e a s e d i t s absorbance very s i g n i f i c a n t l y (by 90%). The evidence f o r the assignment of these bands to the s o l v a t e d e l e c t r o n and the primary o x i d i z i n g - s p e c i e s i s f u r t h e r s u b s t a n t i a t e d by the f a c t that both bands show an eq u i v a l e n t i n c r e a s e upon i s o t o p i c - 128 - s u b s t i t u t i o n , suggesting that the species r e s p o n s i b l e have a common o r i g i n . As shown i n Figure 31, Ge i s % 30% l a r g e r f o r the deuterated DMSO than i t i s f o r the undeuterated. Since i t i s u n l i k e l y both bands would have e x a c t l y the same change i n e x t i n c t i o n c o e f f i c i e n t s i n going from the deuterated m a t e r i a l to the u n s u b s t i t u t e d , i t i m p l i e s that the change a r i s e s from a G value e f f e c t . This suggests then that both primary s p e c i e s have an equal p r o b a b i l i t y of s u r v i v i n g geminate spur r e a c t i o n s . A s i m i l a r e f f e c t occurs i n water i n which the f r a c t i o n of i o n s which escape geminate recombination and become s o l v a t e d i s 67 68 s u b s t a n t i a l l y l a r g e r i n the deuterated m a t e r i a l . ' B^O and IL^O have i d e n t i c a l b u l k d i e l e c t r i c constants so that the i n c r e a s e i n y i e l d cannot be a simple d i e l e c t r i c constant e f f e c t . What i t does suggest i s that there i s a wider range of i n i t i a l s e p a r a t i o n d i s t a n c e s between the thermalized e l e c t r o n and i t s parent p o s i t i v e i o n i n D^O than i n 1^0. I t i s known that D^O has a slower d i e l e c t r i c r e l a x a t i o n time than rl^O; consequently the thermal e l e c t r o n s w i l l have to t r a v e l f u r t h e r before they become s o l v a t e d by o r i e n t a t i o n a l p o l a r i z a - t i o n and hence w i l l have a higher p r o b a b i l i t y of escaping geminate recombination. On the other hand, i t i s p o s s i b l e that the m a t e r i a l c o n t a i n i n g the h e a v i e r isotope i s l e s s e f f i c i e n t i n i t s i n e l a s t i c s c a t t e r i n g of the " s u b e x c i t a t i o n e l e c t r o n " s o t h a t , on the average, the e l e c t r o n gets f u r t h e r away from i t s concomitant p a r t n e r before i t becomes th e r m a l i z e d . The decay r a t e s of both the e l e c t r o n and p o s i t i v e i o n were the same w i t h i n experimental e r r o r i n the deuterated DMSO as they were i n the protonated m a t e r i a l . In both cases the decay of the e l e c t r o n being - 129 - n e a r l y 100 times f a s t e r . Both species decay by f i r s t - o r d e r k i n e t i c s , as shown i n Figures 34 and 35; the pseudo f i r s t - o r d e r r a t e constants f o r the e l e c t r o n and p o s i t i v e i o n are (4.8 +0.1) x lO^sec L and (3.0 + 0.1) x 10~*sec L r e s p e c t i v e l y . This suggests both t r a n s i e n t s r e a c t w i t h the s o l v e n t or e l s e w i t h some r e s i d u a l i m p u r i t y . However a d d i t i o n of % 0.1 M dimethyl s u l f i d e and water, which were the only d e t e c t a b l e i m p u r i t i e s , showed no observable i n c r e a s e i n decay r a t e s . I t i s thought that the e l e c t r o n decays by r e a c t i n g w i t h the s o l v e n t medium f o r reasons which have already been mentioned. Although the DMSO r a d i c a l anion has been observed to have an ab s o r p t i o n maximum at 350 nm i n i r r a d i a t e d aqueous a l k a l i n e s o l u t i o n s (pH >10) c o n t a i n i n g 69 0.7 M DMSO, i t was not observed i n the present study. I f i t i s formed, i t i s p o s s i b l e t h a t i t i s very unstable and decays f a s t e r than the response time of the d e t e c t i o n apparatus. I t i s a l s o p o s s i b l e that i t s e x t i n c t i o n c o e f f i c i e n t i s too s m a l l to be observed. Under the experimental c o n d i t i o n s used i n t h i s study the l i m i t of d e t e c t a b i l i t y p r o v i d i n g , of course, that the mean l i f e time of the species i s long compared to the pulse l e n g t h and response time of the d e t e c t i o n apparatus. For a G value of 1.3 t h i s imposes an upper l i m i t of 750 M ^cm x on the e x t i n c t i o n c o e f f i c i e n t of the so l v e n t anion. The DMSO p o s i t i v e i o n can presumably r e a d i l y undergo ion-molecule r e a c t i o n s w i t h the s o l v e n t or spontaneous unimolecular decomposition. I t should 64 be noted that Koulkes-Pujo et a l . found that the o x i d i z i n g species decayed by a second-order r e a c t i o n which had a f i r s t h a l f - l i f e of 0.36 usee. This was probably due to r e a c t i o n w i t h a negative i o n of an ab s o r p t i o n i s given by G e ^ 1000 - 130 - - 2 . 0 0 - - 2 . 2 0 - - 2 . 4 0 - - 2 . 6 0 - - 2 . 8 0 TIME (nsec) ure 34. F i r s t - o r d e r decay p l o t of the so l v a t e d e l e c t r o n i n DMSO taken at 1300 nm. The pulse width was 10 nsec, g i v i n g an absorbed dose of 900 rads per pu l s e . The decay was measured usi n g a Ge photodiode w i t h a 50 ohm load r e s i s t o r . -1.90 - 131 - - 2 . 1 0 d c o - 2 . 3 0 - 2 . 5 0 - 2 - 7 0 - 2 . 9 0 2.0 4.0 TIME (jisec) F i g u r e 35. F i r s t - o r d e r p l o t of the decay of the DMSO p o s i t i v e i o n . The decay was measured at 550 nm using the p h o t o m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r . The pulse width was 40 nsec, g i v i n g an absorbed dose of 2200 rads per p u l s e . - 132 - produced from the e l e c t r o n decay. T h e i r dose r a t e was n e a r l y 60 times th a t used i n t h i s study (3 x 10"^ rads sec X compared to 5 x 1 0 ^ rads sec X ) so tha t the "instantaneous" c o n c e n t r a t i o n of t r a n s i e n t i o n i c s p e c i e s would be much higher i n t h e i r study. 2. Free Ion Y i e l d s Anthracene has been w i d e l y used as a scavenger of s o l v a t e d e l e c t r o n s and r a d i c a l anions because of i t s h i g h r e a c t i v i t y and the i n t e n s e 4 -1 -1 abs o r p t i o n (e = 10 M cm ) of i t s r a d i c a l anion i n the v i s i b l e max re g i o n of the spectrum (* m a x ^ 720 nm). In t h i s study anthracene was added to DMSO at v a r i o u s concentrations up to i t s s o l u b i l i t y l i m i t , 0.02 M. The s o l u t i o n s were prepared j u s t p r i o r to i r r a d i a t i o n s i n c e i t was observed that s o l u t i o n s c o n t a i n i n g anthracene "aged" on standing i n the f l u o r e s c e n t room l i g h t i n g f o r a day or more, probably producing the anthracene photodimer, which caused a long l i v e d t r a n s i e n t to be formed i n the r e g i o n 450-800 nm upon i r r a d i a t i o n . In t a k i n g the ab s o r p t i o n spectrum of the r a d i c a l anion (A ) and e v a l u a t i n g i t s absorbance, i t was necessary to make c o r r e c t i o n s f o r the DMSO p o s i t i v e i o n and the s o l v a t e d e l e c t r o n , both of which absorb to some extent i n the same region. The abs o r p t i o n s p e c t r a of these l a t t e r two t r a n s i e n t s had already been determined and consequently the c o r r e c t i o n s were r e l a t i v e l y easy to apply because A does not absorb at 550 nm (the DMS0 + peak) nor at 1275 nm (where e absorbs s t r o n g l y ) . s The c o n t r i b u t i o n s of these two species at each wavelength were simply s u b s t r a c t e d from the t o t a l observed end of pulse absorbance. The split-beam o p t i c a l method was used to normalize the three s p e c t r a . - 133 - The spectrum of A obtained in this manner for 0.02 M anthracene in DMSO i s shown in Figure 36, where i t can be seen that X = 750 nm. & ' max Figures 37 and 38 show the oscilloscope traces at 750 nm for 0.001, 0.005, 0.01 and 0.02 M anthracene in DMSO. For a l l traces shown, the contribution from the positive ion decreases only sl i g h t l y over the time period shown since i t s h a l f - l i f e i s > 2 ysec. At the lowest concentration, the contribution made by the short lived solvated electron can readily be observed. From these traces i t can be deduced that the absorbance due to A consists of two components, one of which builds up during the pulse and is larger at the higher concentrations and the other which builds in after the pulse with a rate of growth which increases with the anthracene concentration. Furthermore, i t is clear from Figure 37(b) that the slow component builds in at a much slower rate than the electron decays, which strongly suggests that the anthracene is scavenging both the solvated electron and the reducing species resulting from the electron decay, either the DMSO radical anion or i t s decomposition product. Thus the savenging reactions may be written as e + A *~ A s e + S s S + A (37) (38) (39) CO I o \— X O u> 500 600 700 800 WAVELENGTH (nm) 900 Figure 36. End of pulse absorption spectrum of the anthracene r a d i c a l anion obtained from a DMSO s o l u t i o n 0.02 M i n anthracene a f t e r absorbances due to the e l e c t r o n and o x i d i z i n g species had been subtracted. - 135 100 nsec 0.97% A IT 200 nsec 2.1% A (a) X = 750 nm no anthracene added (b) X = 750 nm 0.001 M anthracene (O \ = 750 nm 0.005 M anthracene Figure 37. 100 nsec T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the e l e c t r o n and b u i l d up of the anthracene r a d i c a l anion at 750 nm. (a) no anthracene added; (b) 0.001 M anthracene i n DMSO; (c) 0.005 M anthracene i n DMSO. - 136 - Figure 3 8 . Typical oscilloscope traces showing the build up and decay of the anthracene radical anion at 750 nm. (a) 0.01 M anthracene in DMSO; (b) 0.02 M anthracene in DMSO. - 137 - where S i s the s o l v e n t medium and S i s the r a d i c a l anion or i t s decomposition product. Figure 39 shows how the absorbance observed immediately at the end of pulse (curve 2) and the maximum absorbance observed a f t e r the pulse (curve 1) of A~ at 750 nm vary w i t h the anthracene c o n c e n t r a t i o n . The immediate absorbances were c o r r e c t e d f o r the p o s i t i v e i o n and e l e c t r o n c o n t r i b u t i o n s at 750 nm by measuring t h e i r r e s p e c t i v e absorbances at 550 nm and 1275 nm and a p p l y i n g the appropriate c o r r e c t i o n f a c t o r according to £759^550 a n <^ e75c/ e1275" e l e c t r o n absorbance was monitored at 1275 nm u s i n g the s p l i t - b e a m o p t i c a l method. The maximum absorbances were obtained by e x t r a p o l a t i n g the decaying p o r t i o n of the absorbance back to the end of the p u l s e . The decay of the p o s i t i v e i o n , obtained at 550 nm and u s i n g the c o r r e c t i o n procedure described above, was added to the observed decay at 750 nm so t h a t the true A b u i l d up and decay could be obtained. The e f f e c t of the anthracene on the y i e l d s of the p o s i t i v e i o n (curve 3) and the s o l v a t e d e l e c t r o n s (curve 4) are a l s o shown i n Figure 39. The s l i g h t i n c r e a s e i n the p o s i t i v e i o n y i e l d may be due to Ihe scavenging of e l e c t r o n s or other reducing species which were otherwise doomed to recombination w i t h the c a t i o n . N e v e r t h e l e s s , the f a c t t h a t the y i e l d of the p o s i t i v e i o n was not decreased nor was i t s decay r a t e a p p r e c i a b l y i n c r e a s e d , suggests that p a r t of the absorbance a t t r i b u t e d to A cannot be due to the presence of anthracene c a t i o n s , A +, as has been suggested i n some other s y s t e m s . ^ The anthracene c a t i o n i s b e l i e v e d to have a very s i m i l a r e x t i n c t i o n c o e f f i c i e n t and a b s o r p t i o n spectrum as i t s anion counterpart and i s formed through 15 Figure 39. Graph shewing the scavenging of solvated electrons i n pure DMSO by anthracene and the formation of anthracene radical anions. 9, absorbance at 750 nm due to A~ immediately at the end of the pulse; O > maximum in the absorbance at 750 nm due to A - after the pulse; A , absorbance due to the positive ions at 550 nm; B , absorbance due to solvated electrons at 1275 nm (not corrected for decay during the pulse nor for the detector response time). / . " ' - 139 - charge t r a n s f e r w i t h the primary o x i d i z i n g species i n some media. 34 64 In the previous s t u d i e s on DMSO, ' anthracene was used at a con c e n t r a t i o n of 0.005 M. As can be seen from Figure 39, anthracene at 0.005 M reduced the observable y i e l d of e g to about 50% of i t s y i e l d i n the pure system although i t then scavenged the decay product of most of these unscavenged e l e c t r o n s . By assuming the e x t i n c t i o n 4 - 1 - 1 c o e f f i c i e n t of A at X (750 nm)to be 1.0 x 10 M cm , as used max ' 34 64 p r e v i o u s l y , ' i t can be c a l c u l a t e d that the y i e l d immediately at the end of the pulse i s G(A ) = 0.6. This then i n c r e a s e d to G(A ) = 1.1 + 0.1 due to the slower component of the scavenging process. 34 Hayon's val u e of 1.62 i s much l a r g e r than t h i s but i s probably i n e r r o r because i t was not c o r r e c t e d f o r the s i g n i f i c a n t absorbance by the p o s i t i v e ions at the wavelength used (720 nm) to determine G(A ). Indeed a d d i t i o n of the two absorbances at t h i s wavelength gives an " e f f e c t i v e " G of 1.4 + 0.1, assuming e 7 2(/ e55o = °**8 f o r t h e D M S 0 64 p o s i t i v e i o n . On the other hand Koulkes-Pujo et a l . obtained a much lower y i e l d , G(A ) = 0.64, which i s more d i f f i c u l t to r e c o n c i l e w i t h the data obtained i n t h i s study. T h e i r y i e l d was obtained by i r r a d - i a t i n g a s o l u t i o n of 0.1 M i n Br (to scavenge the o x i d i z i n g species) and 0.005 M i n anthracene. When a DMSO s o l u t i o n which contained 0.1 M Br and 0.02 M anthracene was i r r a d i a t e d , the anthracene anion y i e l d was reduced by about 40%, to G(A ) = 0.8, from what i t was without the a d d i t i o n of Br , des p i t e the f a c t that the e l e c t r o n was completely e l i m i n a t e d . However the chemistry must be r a t h e r d i f f e r e n t i n t h i s mixture because a white p r e c i p i t a t e was produced at the end of the i r r a d i a t i o n . For the reasons mentioned e a r l i e r , however, t h i s decrease i n absorbance at 750 nm i n the presence of Br i s thought not - 140 - to a r i s e from the presence of A + c o n t r i b u t i n g to the 750 nm abs o r p t i o n . Thus the y i e l d of f r e e i ons i n DMSO, as obtained from G(A ) at 4 0.02 M, i s c a l c u l a t e d to be 1.3 + 0.15, assuming e (A ) = 1.0 x 10 — max M "̂cm At 0.02 M anthracene, the s o l v a t e d e l e c t r o n y i e l d was reduced by about 90%, as shown i n Figure 39, the remaining 10% being scavenged as the s o l v e n t anions or t h e i r decomposition product. However, even at t h i s c o n c e n t r a t i o n the t o t a l y i e l d of A i s not e n t i r e l y independent of the anthracene c o n c e n t r a t i o n , as evidenced by the lack of a good scavenging p l a t e a u . Perhaps at 0.02 M some i n t r a - spur scavenging occurs. From G(e ) = 1.3 one c a l c u l a t e s that e, C r t^ f o r the s o l v a t e d s 1500 nm e l e c t r o n band i n DMSO i s about 14,000 M "*"cm \ I f Ge f o r e i n max s DMSO i s ^25% g r e a t e r than Ge, c r v r. , then e i s presumably of the . 1500 nm max r J order of 17,000 M^cm" 1. In view of the d i f f e r e n c e i n the f r e e i o n y i e l d of reducing species obtained from the n i t r o u s oxide steady s t a t e experiments and anthracene p u l s e r a d i o l y s i s experiments, i t was imperative t h a t the y i e l d of the p o s i t i v e i o n (or primary o x i d i z i n g s p ecies) be determined. Br ions have been used i n aqueous systems as a scavenger f o r o x i d i z i n g s p e c i e s such as the 'OH r a d i c a l s and p o s s i b l y a l s o ^ 0 * . I t i s very convenient because the t r a n s i e n t product, the B ^ i o n , has a strong 72—76 o p t i c a l a b s o r p t i o n around 360 nm. Consequently KBr was s e l e c t e d as a p o s i t i v e i o n scavenger i n DMSO and used at v a r i o u s concentrations ranging from 0.001 to 0.1 M. At [Br ]> 0.01 M the a b s o r p t i o n band centered at 550 nm was completely e l i m i n a t e d i n d i c a t i n g complete scavenging of the f r e e p o s i t i v e - 141 - ions and a new abs o r p t i o n band centered at 375 nm was produced. On account of i t s s i m i l a r i t y to the Br,, spectrum i n water the new band was a t t r i b u t e d to t h i s s p e c i e s . The spectrum a t t r i b u t e d to the B r 2 i o n at two concentrations of Br i s shown i n Figure 40. The i n s e t of Fig u r e 40 shows the absorbance of B r 2 at 375 nm as a f u n c t i o n of the con c e n t r a t i o n of Br presented on a semi-log p l o t . Even at the highest c o n c e n t r a t i o n s t u d i e d , [Br ] = 0.1 M, the e l e c t r o n a b s o r p t i o n band was un a l t e r e d which suggests t h a t the Br i o n d i d not i n t e r f e r e w i t h geminate recombination. The co n c e n t r a t i o n independent value of Ge,,-,,. = 15 x 1 0 3 ions (100 eV^hf^cnT1 f o r [Br~] > 0.01 M was J/D nm 64 i d e n t i c a l t o that r e p o r t e d by Koulkes-Pujo e t a l . I f one assumes that the e f o r Br_ i n DMSO i s the same as the r e c e n t l y r e p o r t e d max 2 value f o r B r 2 i n water,^namely 1.2 x 10^M "'"cm \ then one c a l c u l a t e s G ( B r 2 ) = 1.3+0.1. This value should be compared to t h a t obtained by Kemp et a l . ^ ^ i n which the p o s i t i v e ions were scavenged us i n g 0.01 M TMPD (N,N,N*,N'-tetramethyl-p-phenylenediamine). G(DMS0+) = 1.7 was c a l c u l a t e d from the y i e l d of TMPD+ which i s somewhat l a r g e r than the value obtained here. S e v e r a l i n t e r e s t i n g f e a t u r e s arose from the study of the KBr s o l u t i o n s i n DMSO. From the o s c i l l o s c o p e traces i n F i g u r e 41 i t can be seen that at [Br ]= 0.01 or 0.001 M, the absor p t i o n at 365 nm d i d not grow i n du r i n g the 40 nsec p u l s e . Instead a s h o r t - l i v e d t r a n s i e n t a b s o r p t i o n was observed whose spectrum i s centered at 450-500 nm (see Figure 40). This t r a n s i e n t could not be confused w i t h the s o l v e n t c a t i o n because i t s a b s o r p t i o n was almost zero at 650 nm i n a 0.001 M KBr WAVELENGTH (nm) Figure 40. Absorption s p e c t r a f o r KBr s o l u t i o n s i n DMSO. S o l i d curve r e f e r s to Br2 s p e c t r a ; the dotted curve r e f e r s to (he t r a n s i e n t precursor of B ^ .A, B ^ from 0.1 M KBr s o l u t i o n ; # , B ^ from 0.01 M KBr s o l u t i o n ; H , t r a n s i e n t precursor of Br^ at 0.01 M KBr. The i n s e t i s a p l o t of Ge at 375 nm f o r Br " against l o g ([Br~]/M). 143" - 17% A r —̂ 9» «£ 1.4% A J & e 2.0% A (a) X= 365 nm 0.1 M KBr (b) X = 365 nm 0.01 M KBr (c) \ = 365 nm 0.001 M KBr 1 ^.sec F i g u r e 4 1 . T y p i c a l o s c i l l o s c o p e t r a c e s showing the b u i l d up and decay of B r 2 at 365 nm i n pure DMSO. De t e c t i o n made usin g the photo- m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r . (a) 0 . 1 M KBr; (b) 0 . 0 1 M KBr; (c) 0 . 0 0 1 M KBr. - 144 - s o l u t i o n . At t h i s c o n c e n t r a t i o n , the t r a n s i e n t decayed w i t h a h a l f - l i f e of ̂ 0.8 usee, which was approximately the same r a t e at which the B r 2 a b s o r p t i o n at 365 nm grew i n . This b u i l d up, shown i n F i g u r e 4 1 ( c ) , was observed to f o l l o w f i r s t - o r d e r k i n e t i c s as shown i n Figure 42. In t h i s p l o t D^ i s the absorbance of Br^ at 365 nm at time t a f t e r the pulse and D i s the maximum absorbance of Br„ a t t a i n e d a f t e r the CO 2 b u i l d up. Since the b u i l d up and decay of Br^ at 0.001 M KBr were not w e l l separated i n time, D m was obtained by e x t r a p o l a t i n g the decay curve back to the end of the pulse and t a k i n g the value of the absorbance on the e x t r a p o l a t i o n corresponding to time t . The pseudo f i r s t - o r d e r r a t e constant obtained from t h i s p l o t , k = (8.7 +0.3) 5 —1 8 x 10 sec , y i e l d s a second-order r a t e constant of (8.7 +0.3) x 10 M ^sec L f o r <he r e a c t i o n between the t r a n s i e n t and Br to form the Br2 i o n . I t i s proposed t h a t the decay of DMS0+ and the formation of B r 2 goes v i a a two step process,as shown below, w i t h the second step being r a t e determining. DMS0+ + B r " [DMS0 + Br~] — — * - B r 2 ~ + DMSO (40) The t r a n s i e n t i n t e r m e d i a t e centered at 450-500 nm i s a t t r i b u t e d to the charge t r a n s f e r complex [DMS0 +....Br ]. The t r a n s i e n t complex i s observed at the end of the 40 nsec pulse even i n 0.001 M KBr, as shown i n Fi g u r e 43, which suggests that the h a l f - l i f e of the DMSO p o s i t i v e i o n i s < 1 0 - 1 1 seconds i n 0.1 M KBr. Scavenging on t h i s time s c a l e should a f f e c t geminate recombination and 145 - -1.90 0.4 0 . 8 1.2 T IME tyisec) F i g u r e 42. P l o t showing f i r s t - o r d e r b u i l d up of Br^ (Figure 41(c)) f o r DMSO s o l u t i o n 0.001 M i n KBr. - 146 - 1.4% A 1.0% A 1.9% A ^ ^ ^ ^ ^ ^ 1 yisec (a) X = 400 nm 0.1 M KBr \ \ \ ii *> s> £ (b) X = 450 nm 0.01 M KBr (c) X = 450 nm 0.001 M KBr Figure 43. T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of Br^ and i t s t r a n s i e n t p r e c u r s o r . The f a s t i n i t i a l decay i n (b) and (c) i s a t t r i b u t e d to the t r a n s i e n t complex. D e t e c t i o n was made using the p h o t o m u l t i p l i e r w i t h a 470 ohm load r e s i s t o r . - 147 - thereby increase the y i e l d of e g ; however no such i n c r e a s e was observed. This i m p l i e s that the e l e c t r o n can r e a c t w i t h the complex during an i n t r a - s p u r decay process i n the same manner as i t does w i t h the p o s i t i v e i o n according to r e a c t i o n (41), [DMSO+ Br~] + e~ DMSO* + Br~ (41) * where DMSO i s an e x c i t e d DMSO molecule. From the second-order r a t e — 8 constant f o r the r e a c t i o n between t h i s complex and Br , 8.7 x 10 M ''"sec \ the h a l f - l i f e of t h i s complex i s evaluated to be ̂  8 nsec i n 0.1 M KBr. This i s too long t o a f f e c t the recombination w i t h the spur e l e c t r o n given i n equation (41); consequently the y i e l d of e g i s expected to be unchanged, as observed. However, the f a c t that the y i e l d of TSr^ has diminished by about 30% at 0.001 M i m p l i e s that the charge t r a n s f e r complex has an a l t e r n a t i v e f a t e , perhaps d i s s o c i a - t i o n back i n t o the Br i o n and the p o s i t i v e i o n . There i s a tr a c e of some t r a n s i e n t species i n the wavelength r e g i o n where the p o s i t i v e i o n absorbs which i s too l o n g - l i v e d to be e i t h e r the e l e c t r o n or the t r a n s i e n t complex. This species could be the p o s i t i v e i o n i t s e l f or e l s e i t must be the a t t r i b u t e d to yet another t r a n s i e n t s p e c i e s . So f a r i t has been assumed th a t the o x i d i z i n g species observed to have an ab s o r p t i o n band centered at 550 nm i s the DMSO p o s i t i v e i o n . However, the r e s u l t s are compatible w i t h t h i s t r a n s i e n t being some other o x i d i z i n g s p e c i e s , perhaps e i t h e r a f r e e r a d i c a l or other c a t i o n produced by the r a p i d decomposition of DMS0+. In any event i t seems c l e a r that G ( o x i d i z i n g species) = 1.3, i n which case c f o r ° max the 550 nm band i s 3500 M~1cm~1. - 148 - Thus, from the pulse r a d i o l y s i s s t u d i e s , i t t r a n s p i r e s t h a t the y i e l d of primary reducing species ( e g ) and o x i d i z i n g species (probably DMSO+) are i d e n t i c a l , being 1.3 +0.1. This y i e l d can hence be equated to the f r e e i o n y i e l d i n DMSO. 3. Geminate Ion Scavenging At h i g h concentrations of I^SO^,which i s an e f f i c i e n t e l e c t r o n scavenger»the e l e c t r o n band was completely e l i m i n a t e d but the band centered at 550 nm was found to be inc r e a s e d markedly. This i s a t t r i b u t e d to the scavenging of e l e c t r o n s which were otherwise doomed to geminate recombination w i t h t h e i r concomitant p a r t n e r ; consequently the p o s i t i v e i o n y i e l d i s i n c r e a s e d . The inc r e a s e i n absorbance f o r a DMSO s o l u t i o n c o n t a i n i n g 0.2 M H^SO^ i s shown i n F i g u r e 44 and i s 64 s i m i l a r to th a t reported p r e v i o u s l y using 0.1 M I^SO^. In a d d i t i o n , a l o n g - l i v e d t r a n s i e n t having an absor p t i o n centered at 450 nm w i t h 3 -L -1 -1 Ge ^ 10 mols. (100 eV) M cm at i t s X was a l s o observed. This max t r a n s i e n t i s thought to be the s u l f a t e r a d i c a l anion, SO^ , which i s known to be formed i n the p u l s e r a d i o l y s i s of aqueous s u l f u r i c a c i d 78—80 systems. In the aqueous system i t i s thought to a r i s e from 'OH + HSO. >- H„0 + SO. (42) 4 2 4 In DMSO i t may a r i s e from the methyl r a d i c a l s according to r e a c t i o n (43) s i n c e the methyl r a d i c a l y i e l d i s h i g h (G(CH 3*) > 3). 'CH_ + HSO. *- CH. + SO. (43) 3 4 4 4 149 - 1 0 r 4 0 0 5 0 0 6 0 0 7 0 0 WAVELENGTH (nm) F i g u r e 44. End of pulse a b s o r p t i o n spectrum of the DMSO p o s i t i v e i o n [0] i n 0.2 M H„S0.. The dotted l i n e r e f e r s to the absorbance of the 2 4 p o s i t i v e i o n i n pure DMSO. X r e f e r s to the l o n g - l i v e d SO^ inter m e d i a t e produced i n the a c i d s o l u t i o n . - 150 - When c o r r e c t i o n s f o r the decay of t h i s s u l f a t e r a d i c a l anion were a p p l i e d , i t was noted t h a t the DMSO p o s i t i v e i o n decays by a f i r s t - order process w i t h almost the same h a l f - l i f e (2.5 usee) and r a t e constant (2.7 x 10~*sec X ) as i t d i d i n pure DMSO (see F i g u r e 45). T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the p o s i t i v e i o n and the l o n g - l i v e d SO^ r a d i c a l anion are presented i n F i g u r e 46. The a b s o r p t i o n spectrum obtained f o r a DMSO s o l u t i o n c o n t a i n i n g + 81—83 0.5 M Ag i s shown i n Figure 47. By analogy w i t h aqueous systems the a b s o r p t i o n maximum at X < 330 nm i s a t t r i b u t e d to the s i l v e r atom formed by the r e a c t i o n e~ + A g + »- Ag° (44) However at X > 350 nm i t appears that there i s a b s o r p t i o n from other species i n a d d i t i o n to DMS0+, p o s s i b l y one centered at 400-500 nm. In c o n t r a s t to the H + system discussed above, the absorbances i n the range 320-700 nm were a l l much longer l i v e d w i t h f i r s t h a l f - l i v e s > 100 usee. The decay k i n e t i c s were n e i t h e r f i r s t - o r d e r nor second- order. Although a v a r i e t y of s i l v e r i o n adducts, such as A g 2 + and Ag^ +, are known to be formed i n the pulse r a d i o l y s i s of aqueous s i l v e r s o l u t i o n s , they are not known to absorb at X > 400 nm. I t i s p o s s i b l e that the peak centered at 400-500 nm i s due to a complex of the DMS0 + i o n and a s i l v e r atom or other s i l v e r aggregates. Because the absorbances were not w e l l separated i n time, i t i s not p o s s i b l e to s t a t e whether the o x i d i z i n g species r e s p o n s i b l e f o r the 550 nm band i n pure DMSO e x i s t e d as such i n these s i l v e r s o l u t i o n s . - 1 . 8 0 - 151 - - 2 . 0 0 - 2 . 2 0 6 o o o - 2 . 4 0 - 2 . 6 0 - 2 . 8 0 _L 1 JL 2.0 8 .0 4.0 6.0 T IME (^sec) Figure 45. F i r s t - o r d e r decay p l o t of the DMSO p o s i t i v e i o n i n the presence of 0.2 M Ĥ SÔ .. Decay measured at 550 nm usin g the photo- m u l t i p l i e r w i t h a 470 ohm load r e s i s t a n c e . - 152 - 1 t % A ***** **** i > «s X = 650 nm sec X=450 nm 2^sec Figure 46. T y p i c a l o s c i l l o s c o p e t r a c e s showing the decay of the DMSO p o s i t i v e i o n i n the presence of 0.2 M ^SO^ at 650 nm and 450 nm. The longer- l i v e d . S O ^ t r a n s i e n t i s r e a d i l y observed at 450 nm.  - 1 5 4 - 4. D i e l e c t r i c Constant and E l e c t r o n S t a b i l i z a t i o n The y i e l d of f r e e ions i n DMSO, G(free ion) = 1.3 + 0.1, appears to be c o n s i s t e n t w i t h i t s r e l a t i v e l y h i g h d i e l e c t r i c constant and the e m p i r i c a l r e l a t i o n s h i p shown i n Figure 7. This would suggest that the s t a t i c d i e l e c t r i c constant i s the major f a c t o r determining the f r e e i o n y i e l d i n DMSO. On the other hand, the a b s o r p t i o n maximum of the so l v a t e d e l e c t r o n i s at X > 1500 nm, corresponding to an o p t i c a l t r a n s i t i o n energy < 0.8 eV. In t h i s respect DMSO behaves l i k e a sa t u r a t e d hydrocarbon towards e l e c t r o n s o l v a t i o n and suggests t h a t the s o l v a t i n g power of the medium plays the dominant r o l e i n e l e c t r o n s t a b i l i z a t i o n . The nature of these weak i o n - d i p o l e f o r c e s s o l v a t i n g the e l e c t r o n may be due to the i n a b i l i t y of the DMSO molecules to a l i g n t h e i r d i p o l e s f o r maximum i n t e r a c t i o n w i t h the e l e c t r o n or due to a l a r g e c a v i t y r a d i u s . In p o l a r p r o t i c media, the p o s i t i v e charge i s l o c a l i z e d on the hydrogen atom and i s able to f i t more c l o s e l y about the negative centre whereas i n DMSO the p o s i t i v e charge due to the S -> 0 d i p o l e i s s h i e l d e d by the two methyl groups. Furthermore, the bulky methyl groups w i l l prevent an optimum o r i e n t a t i o n of the d i p o l e s around the e l e c t r o n , as w e l l as producing a l a r g e v o i d , so that the p o l a r i z a t i o n p o t e n t i a l would be consider a b l y l e s s than i f there was no s t e r i c hinderance. I t i s a l s o i n t e r e s t i n g to note that o the c o r r e l a t i o n curve i n F i g u r e 6 gives a c a v i t y r a d i u s > 3.3 A f o r the e l e c t r o n i n DMSO based on hv < 0.8 eV. max The data suggest, t h e r e f o r e , that the e l e c t r o s t a t i c i n t e r a c t i o n s are not the same i n s o l v e n t s of e q u i v a l e n t d i e l e c t r i c constant and that e l e c t r o n s t a b i l i z a t i o n occurs through s p e c i f i c i n t e r a c t i o n s as - 155 - governed by the m i c r o s c o p i c p r o p e r t i e s of the medium. The e l e c t r o n s are s t i l l to be regarded as being s t a b i l i z e d by i o n - d i p o l e i n t e r a c t i o n s , but the extent of t h i s i n t e r a c t i o n i s r e l a t e d to the i n t r i n s i c p r o p e r t i e s of the s o l v e n t , such as the molecule shape, s i z e , type of f u n c t i o n a l groups, d i p o l e moment and so f o r t h , and not s o l e l y to the macroscopic d i e l e c t r i c constant of the continuum. - 156 - CHAPTER IV PULSE RADIOLYSIS OF DMSO-WATER BINARY MIXTURES In the previous chapter i t was shown that the e l e c t r o n i n DMSO i s very weakly trapped as i n d i c a t e d by i t s a b s o r p t i o n band i n the near i n f r a r e d w i t h a X > 1500 nm. On the other hand, p o l a r p r o t i c max s o l v e n t s , such as water and the a l c o h o l s , are c h a r a c t e r i z e d by e l e c t r o n a b s o r p t i o n bands i n the v i s i b l e r e g i o n . This i s i n keeping w i t h t h e i r a b i l i t y to s o l v a t e negative ions through strong i o n - d i p o l e i n t e r a c t i o n s and hydrogen bonding. The e l e c t r o n i s regarded as being s t a b i l i z e d through i n t e r a c t i o n w i t h s e v e r a l molecules so that s t u d i e s on the a b s o r p t i o n s p e c t r a , and y i e l d s , of e l e c t r o n s s o l v a t e d i n b i n a r y mixtures are of i n t e r e s t , p a r t i c u l a r l y when the extent and manner of s o l v a t i o n d i f f e r markedly i n the i n d i v i d u a l components. As mentioned i n the I n t r o d u c t i o n , s t u d i e s have been attempted on b i n a r y mixtures of a l c o h o l s or water w i t h a p r o t i c 40 41 39 43 44 hydrocarbons ' or et h e r s . ' ' However these were not i d e a l combinations s i n c e they formed Very inhomogeneous mixtures on the microscopic s c a l e . This r e s u l t e d i n aggregates of the p o l a r h y d r o x y l i c s o l v e n t a c t i n g as scavenging centers f o r e l e c t r o n s which would otherwise have been l o s t through geminate recombination i n the a p r o t i c component. Consequently, both the y i e l d and o p t i c a l p r o p e r t i e s of the so l v a t e d e l e c t r o n s were dominated by the p o l a r component. - 157 - DMSO would appear to be a model hydrocarbon-like s o l v e n t f o r s t u d y i n g such b i n a r y mixtures. U n l i k e the a p r o t i c s o l v e n t s mentioned above i t has a high f r e e i o n y i e l d , comparable to that of the h y d r o x y l i c s o l v e n t s . As a r e s u l t , the scavenging of geminate ions by p o l a r aggregates should not be a dominant f a c t o r . Furthermore, DMSO i s completely m i s c i b l e w i t h water i n a l l p r o p o r t i o n s ; i n f a c t DMSO-water mixtures show more i n t e r m o l e c u l a r s t r u c t u r e than i s present i n * i • -A 8 Z » ~ 9 2 e i t h e r of the pure l i q u i d s . 1. Solvated E l e c t r o n s In DMSO-water b i n a r y mixtures two e a s i l y r e s o l v a b l e bands were observed f o r wavelengths > 400 nm. As i n pure DMSO, the l o n g e r - l i v e d band ( Tn/2 ^ 2 usee) was a t t r i b u t e d to the DMSO p o s i t i v e i o n and was centered at 550 nm f o r a l l mixtures. The other band was assigned t o the s o l v a t e d e l e c t r o n and was very s h o r t - l i v e d ( t - , ^ < 25 nsec) . The X of the e l e c t r o n band s h i f t e d from that i n pure water (X = max max 720 nm) to tha t i n pure DMSO (X > 1500 nm) w i t h i n c r e a s i n g DMSO r max co n c e n t r a t i o n . In each mixture only a s i n g l e e l e c t r o n band was observed. The l a c k of "shoulders" or broadening i n the bands suggest that these s p e c t r a are not the r e s u l t of a simple combination or overlap of the e and e „ W O r t absorbances. To prove t h i s a s e r i e s of aq DMSO r r e p r e s e n t a t i v e s p e c t r a at v a r i o u s concentrations were cons t r u c t e d based on simple band overlap but the r e s u l t a n t s p e c t r a d i d not agree at a l l w i t h the observed s p e c t r a . Furthermore, simple combination of the two s p e c t r a would r e q u i r e f r e e " i c e b e r g s " of DMSO and water molecules i n the m i l i e u which i s u n l i k e l y i n view of the strong i n t e r - molecular i n t e r a c t i o n s e x h i b i t e d by the two components. - 158 - Figure 48 shows the s p e c t r a f o r the s o l v a t e d e l e c t r o n i n mixtures c o n s i s t i n g of 0, 0.20, 0.28, 0.43, 0.72, 0.93 and 1.0 mole f r a c t i o n DMSO. The s p e c t r a were a l l taken using 40 nsec p u l s e s . They have not been c o r r e c t e d f o r decay during the p u l s e nor f o r the response time of the d e t e c t i o n apparatus; but the c o n t r i b u t i o n s from the DMSO p o s i t i v e ions were deduced by e x t r a p o l a t i o n to the end of the pulse from times > 100 nsec when the e l e c t r o n band had f u l l y decayed, and these have been su b t r a c t e d . The a b s o r p t i o n band of the 4 - l - I -1 hydrated e l e c t r o n , w i t h Ge = 4.3 x 10 mols. (100 eV) M. cm at - 93 \nax nm), corresponds c l o s e l y to the p u b l i s h e d spectrum of e a q ' The s o l v a t e d e l e c t r o n decay f o l l o w e d good f i r s t - o r d e r k i n e t i c s i n a l l b i n a r y mixtures. In pure water the k i n e t i c s were a mixture of f i r s t - and second-order and were not analyzed f u r t h e r although the mean l i f e t i m e was > 10 ̂  sec. Decay p l o t s f o r e g were obtained u s i n g 10 nsec pulses and fol l o w e d at X > 1000 nm where the c o n t r i b u t i o n of the p o s i t i v e i o n was n e g l i g i b l e f o r a l l mixtures. As i n pure DMSO, the s o l v a t e d e l e c t r o n i s thought to decay by r e a c t i n g w i t h DMSO. The pseudo f i r s t - o r d e r r a t e constants thus obtained are expressed as h a l f - l i v e s (column 6) i n Table IV. This t a b l e a l s o records the observed X , max the w i d t h - a t - h a l f - h e i g h t i n energy u n i t s , AW, and the observed end of p u l s e absorbance, Ge (obs) of the so l v a t e d e l e c t r o n band f o r each max of the mixtures. A second-order r a t e constant was c a l c u l a t e d f o r each mixture by combining the observed f i r s t - o r d e r r a t e constant w i t h the b u lk c o n c e n t r a t i o n of DMSO. These are given i n column 7 of Table IV and are observed to be f a i r l y constant a t 4.5 x 10^ M "'"sec "*". This i s somewhat higher than that f o r pure DMSO and f o r the d i l u t e 30 LU <J < g 10 to CD < — /A H 2 ° / A x0 .3 - / / 0 . 2 0 \ \ / / \V — / • *• \ 1 ; . \ / / 0.28 \ \ # • *. \ / •• •• • \ DMSO ^ - / : • \ / / • \\  • •  .  # . • • • • \ 0.93 // / 0.43 • - . . . . . 0 . 7 2 1 , 1 . 1 . 1 . 1 4 0 0 6 0 0 1400 Figure 48. 8 0 0 1000 1200 WAVELENGTH (nm) Absorption spectra of solvated electrons in DMS0-H20 mixtures; 0, 0.20, 0.28, 0.43, 0.72, 0.93 and 1.0 mole fraction DMSO. Data points were obtained at 50 nm intervals. The data for pure water have been multiplied by a factor of 0.3 relative to the others. TABLE IV. Summary of data obtained from studies on pulse irradiated DMSO-water mixtures at room temperature. Mole Fraction DMSO D 3 s X max AW ^Gmax(obs) k 2 ( c a l c ) b Ge (corr)° max (nm) (eV) (x 10"3) (nsec) ,M-1 -1, (M sec ) (x 10~ 3) 0 78 720 0.88 43.7 >103 . — 43.7 0.017 78 — — — 340 2.9 x 10 6 — 0.20 75 750 0.76 11.0 22 4.5 x 10 6 20.5 0.28 73 800 0.83 7.22 19 4.4 x 10 6 14.8 0.43 67 900 0.97 4.24 15 4.5 x 10 6 10.2 0.72 57 1000 0.86 3.85 12 4.6 x 10 6 11.2 0.93 48 1350 >0.65 5.85 — — — 1.00 46 >1500 __ >7.44 14.5 3.4 x 10 6 >18.9 ON o Data taken from references (84,85) for mixture temperature of 25°C. Calculated from the observed pseudo-first order rate constant using bulk concentration of DMSO. Corrected for decay during the pulse and response time of the detection system. - 161 - s o l u t i o n , 0 . 0 1 7 mole f r a c t i o n DMSO ( 0 . 7 M i n DMSO). The r a t e constant i n t h i s l a t t e r mixture, 2 . 9 x 1 0 ^ M ^"sec \ i s higher than the pub l i s h e d values f o r the r e a c t i o n of DMSO w i t h hydrated e l e c t r o n s a t concen t r a t i o n s up to and i n c l u d i n g 0 . 7 M DMSO i n water (k = 1 . 6 x i n 6 . , - l - 1 . 6 9 , 9 4 10 M sec ). The observed v a l u e s , G E (obs), were c o r r e c t e d f o r decay d u r i n g max the p u l s e and response time of the o p t i c a l d e t e c t i o n system u s i n g the same computational procedures mentioned p r e v i o u s l y . The values of Ge (obs) and Ge ( c o r r ) f o r e are p l o t t e d as a f u n c t i o n of the max max s r mole f r a c t i o n DMSO i n Figure 49 and both are seen t o pass through a minimum, the c o r r e c t e d one being more pronounced. This minimum may a r i s e from a change i n G, e or a combination of both. I t i s b e l i e v e d t h a t the minimum a r i s e s from a G va l u e e f f e c t f o r two reasons. F i r s t l y , the value of e was estimated i n the previous chapter to be max % 1 7 , 0 0 0 M "'"cm ̂  which i s s i m i l a r to e = 1 8 , 5 0 0 M "'"cm "*" f o r max 95 e at 720 nm. Consequently the pure components have very s i m i l a r c o e f f i c i e n t s so th a t i t seems u n l i k e l y that the mixture w i l l show the s o r t of minimum given i n Figure 4 9 . This i s f u r t h e r supported by the f a c t t h a t AW i s f a i r l y constant f o r a l l m ixtures. The o s c i l l a t o r s t r e n g t h of an abs o r p t i o n band, f > i s r e l a t e d to the i n t e g r a t e d molar e x t i n c t i o n c o e f f i c i e n t e by the r e l a t i o n s h i p ^ f = 4 . 3 2 x 1 0 ~ 9 F / e d ai ( 4 . 1 ) mn / ^ l where the i n t e g r a t i o n extends over the e n t i r e band r e l a t e d to the t r a n s i t i o n from the s t a t e n-«-m and F i s a c o r r e c t i o n f a c t o r near u n i t y - 162 - 7 8 7 5 6 8 61 5 2 4 6 I f— 1 i 1 1 1 • i i I i 1 i — I — . — I O 0.2 0.4 0.6 0.8 1.0 M O L E FRACTION DMSO F i g u r e 49. P l o t of the values of Ge f o r the s o l v a t e d e l e c t r o n a b s o r p t i o n max r bands presented i n Fi g u r e 48 as a f u n c t i o n of the mole f r a c t i o n DMSO f o r the DMSO-l^O mixtures. The n o n - l i n e a r a x i s showing the change as a f u n c t i o n of s t a t i c d i e l e c t r i c constant of the bulk mixture i s shown on the top a b s c i s s a . O » a c t u a l observed absorbance peak h e i g h t s . • , c o r r e c t e d f o r decay d u r i n g the puls e and response time of the d e t e c t o r . - 163 - r e l a t e d to the r e f r a c t i v e index of the medium which contains the absorbing s p e c i e s . For a s i n g l e e l e c t r o n Ef. = 1 (4.2) x where Ihe summation extends over a l l absorptions i n v o l v i n g the e l e c t r o n . For most s o l v a t e d e l e c t r o n bands the o s c i l l a t o r s t r e n g t h i s ^ 0.7 + 0.2 which i m p l i e s t h a t t r a n s i t i o n s other than (2p «- I s ) must be very weak i f they occur at a l l . Assuming a Gaussian shaped abs o r p t i o n band f o r the e l e c t r o n , to ' e d a) - e AW (4.3) co, / max so t h a t (4.1) becomes f - (constant) e AW (4.4) mn max By analogy w i t h other s o l v a t e d e l e c t r o n bands the o s c i l l a t o r s t r e n g t h i s not expected to change a p p r e c i a b l y f o r the m i x t u r e s , AW i s observed to be constant and hence the e should remain reasonably max constant. U n f o r t u n a t e l y s u i t a b l e e l e c t r o n scavengers, such as anthracene, are not s o l u b l e enough i n these b i n a r y mixtures to give an e l e c t r o n y i e l d measurable by pulse r a d i o l y s i s so that t h i s i n f e r e n c e could not be v e r i f i e d . I t should be noted that the observed minimum may a l s o a r i s e from an experimental a r t i f a c t . One of the parameters used i n the procedure f o r c o r r e c t i n g the observed absorbance i s the e l e c t r o n h a l f - l i f e . Since t h i s minimum a l s o c o i n c i d e s approximately w i t h the f a s t e s t - 164 - decaying e l e c t r o n s (see Table IV) i t i s p o s s i b l e that the measured decay r a t e may have over-estimated the h a l f - l i f e s l i g h t l y , producing a more pronounced minimum. However, the second order r a t e constants are a l l reasonably constant i n t h i s r e g i o n which would suggest that t h i s cannot be the reason f o r the minimum. I t i s suggested t h a t t h i s minimum i n the s o l v a t e d e l e c t r o n y i e l d , i f i t i s a r e a l e f f e c t , a r i s e s not from a decrease i n the f r e e i o n y i e l d but r a t h e r from an enhanced r e a c t i o n of the thermal e l e c t r o n s p r i o r to s o l v a t i o n . There are s e v e r a l reasons f o r t h i s c o n c l u s i o n . The b u l k d i e l e c t r i c constant f o r the mixtures changes m o n o t o n i c a l l y w i t h composition, from D = 78 (water) to D = 4 6 (DMSO), e x h i b i t i n g s s no minimum. Consequently one would not expect the f r e e i o n y i e l d to pass through a minimum. Furthermore, i f the minimum arose from a f r e e i o n y i e l d e f f e c t , then the y i e l d of DMSO p o s i t i v e ions should have shown a s i m i l a r e f f e c t i n these mixtures. In f a c t they d i d n o t , as w i l l be shown l a t e r , so tha t i f t h i s minimum i s to be a t t r i b u t e d t o a change i n the free i o n y i e l d i t must a r i s e s o l e l y from the c o n t r i b u t i o n of water to the t o t a l f r e e i o n y i e l d of e g i n the mixtures. However, i t i s unreasonable to suppose that water could a f f e c t the y i e l d so markedly at > 0.8 mole f r a c t i o n DMSO where l e s s than 10% of the primary i o n i z a t i o n events i n v o l v e water molecules. I t i s observed that the l i f e t i m e of the s o l v a t e d e l e c t r o n i n these mixtures passes through a s i m i l a r minimum to the Ge values and t h i s suggests that e g react f a s t e r w i t h DMSO i n the mixtures than i n pure DMSO. I f the p r e c u r s o r e l e c t r o n s of e g are a l s o more r e a c t i v e i n these m i x t u r e s , then t h i s would account f o r the minimum i n the - 165 - 9 4 s o l v a t e d e l e c t r o n y i e l d . Recently Koulkes-Pujo e t a l . repo r t e d the e f f e c t of d i l u t e s o l u t i o n s of DMSO on the hydrated e l e c t r o n up to 3 . 5 M i n DMSO ( 0 . 0 7 7 mole f r a c t i o n DMSO) and observed an i n c r e a s i n g value of k/G w i t h i n c r e a s i n g DMSO co n c e n t r a t i o n . In t h i s e x p r e s s i o n k i s the r a t e constant f o r the r e a c t i o n of e + DMSO ( 1 . 6 x 1 0 ^ aq M ''sec *) and G i s the apparent y i e l d of e ^ . The r a t e constant i s t r e a t e d as a constant i n t h i s study so tha t the i n c r e a s e i n k/G corresponds to a decrease i n G. This behavior was con t r a r y to what the authors observed w i t h other e l e c t r o n scavengers, such as N^O, H + and CH^Cl, i n which the G value i n c r e a s e d due to spur p e n e t r a t i o n . This phenomena i n DMSO was a t t r i b u t e d to the unhydrated e l e c t r o n s being scavenged before they could become s o l v a t e d , thereby causing a decrease i n G. These r e s u l t s are e n t i r e l y c o n s i s t e n t w i t h the observations i n t h i s study. A decreased s o l v a t e d e l e c t r o n y i e l d (G) and/or an in c r e a s e d decay r a t e (k) as the c o n c e n t r a t i o n of DMSO i s i n c r e a s e d i n t h i s mole f r a c t i o n r e g i o n would give an in c r e a s e d k/G. I t i s i n t e r e s t i n g that t h i s r e g i o n of minimum absorbance due to e occurs where these DMSO-water mixtures show maximum i n t e r m o l e c u l a r s s t r u c t u r e as s i g n i f i e d by a l a r g e v i s c o s i t y i n c r e a s e , negative heat 8 4 - 9 2 of mixing and con s i d e r a b l e volume c o n t r a c t i o n . Furthermore, measurements on the s p i n - l a t t i c e (T^) and transverse (T^) r e l a x a t i o n times i n d i c a t e a minimum i n molecular m o b i l i t y around 0 . 3 5 mole 89 f r a c t i o n DMSO. Perhaps t h i s i n c r e a s e d molecular s t r u c t u r e provides f o r a lower a c t i v a t i o n energy f o r the r e a c t i o n of the e l e c t r o n , both q u a s i - f r e e and s o l v a t e d , w i t h DMSO. The l i f e t i m e s of these q u a s i - f r e e e l e c t r o n s may be f u r t h e r reduced by the increased molecular - 166 - r e l a x a t i o n time. Those e l e c t r o n s which are i n i t i a l l y trapped i n l e s s than optimum voids may react w i t h the medium before they become f u l l y s o l v a t e d . I n Figure 50, the values of the photon energy ( i n cm x ) at the ab s o r p t i o n band maxima of the m i x t u r e s , , are p l o t t e d against max t h e i r r e s p e c t i v e b u l k d i e l e c t r i c constant. The d i e l e c t r i c constant 8 data f o r the DMSO-water b i n a r y mixtures was obtained from the l i t e r a t u r e . I t i s i n t e r e s t i n g , but perhaps f o r t u i t o u s , that t h i s p l o t i s approximately l i n e a r , w i t h i n experimental e r r o r , out to 0.9 mole f r a c t i o n DMSO. The abs o r p t i o n band w i d t h - a t - h a l f - h e i g h t , AW, i s f a i r l y constant d e s p i t e the widely d i f f e r e n t s o l v a t i o n energies provided by DMSO and water s e p a r a t e l y . This behavior i s c o n s i s t e n t w i t h the continuum or semi- continuum model i n which the e l e c t r o n can sample the environment of the mixture . I f the e l e c t r o n was trapped by s o l v a t i o n w i t h only a s m a l l number of molecules then the a b s o r p t i o n bands could be expected to be much broader. The s h i f t i n a b s o r p t i o n from the near i n f r a r e d (pure DMSO) to the v i s i b l e (pure water) w i t h i n c r e a s i n g water content may be due t o e i t h e r a decrease i n the c a v i t y radius or to an in c r e a s e i n t e l e c t r o n - d i p o l e i n t e r a c t i o n or a combination of both. Figure 51 shows a p l o t of E a g a i n s t the mole f r a c t i o n of water i n the mixture. From A max t h i s c o r r e l a t i o n curve and Fig u r e 50 i t can be seen that n e i t h e r DMSO nor water dominate the o p t i c a l p r o p e r t i e s of the a b s o r p t i o n band and suggests that e l e c t r o n s o l v a t i o n occurs by a combination of strong (water) and weak (DMSO) i o n - d i p o l e i n t e r a c t i o n s , the extent of which i s governed l a r g e l y by the c a v i t y r a d i u s through the composition of the mixtures. - 167 - M O L E FRACTION DMSO 1.0 0.87 0.61 0.34 0.0 j 1 1 1 1 F i g u r e 50. P l o t of the photon energy of the absorption band maximum f o r the s o l v a t e d e l e c t r o n i n the DMSO-I^O mixtures against the bulk s t a t i c d i e l e c t r i c constant of the mixtures (at 25°C). The n o n - l i n e a r a x i s showing the corresponding mole f r a c t i o n DMSO i s shown i n the upper a b s c i s s a . - 168 - 4 6 h- 5 2 61 — f - 6 8 7 5 7 8 — l E u b X o E M O L E FRACTION H 2 0 Figure 51. P l o t of the photon energy of the absor p t i o n band maximum f o r the s o l v a t e d e l e c t r o n i n the DMSO-H^O mixtures against the mole f r a c t i o n of water. The n o n - l i n e a r a x i s showing the corresponding bulk d i e l e c t r i c constants of the mixtures i s shown i n the upper a b s c i s s a . - 169 - Thus the data are compatible w i t h that of aqueous b i n a r y . 3 8 . , . . . 3 8 . , . , 3 7 , 4 7 , 4 8 , , . , mixtures of ammonia, ethylenediamme and a l c o h o l s i n which the s t a b i l i z e d e l e c t r o n s have abso r p t i o n band maxima and h a l f - w i d t h s i n t e r m e d i a t e between those of the pure l i q u i d s . I n these mixtures the e l e c t r o n s see a p o l a r i z e d medium i n which the i n t e r a c t i o n energy i s dependent upon the p r o p e r t i e s of the continuum. However the results in DMSO- water mixtures are a t va r i a n c e w i t h mixtures of other h y d r o x y l i c media v A v 4 0 , 4 1 , . » , , . , „ . 3 9 , 4 3 , 4 4 and a p r o t i c non-polar hydrocarbons or s l i g h t l y p o l a r e t h e r s . The reason f o r t h i s i s that the thermal e l e c t r o n s do not see a d i e l e c t r i c continuum but r a t h e r a random d i s t r i b u t i o n of s t r o n g l y and weakly p o l a r i z a b l e aggregates\ consequently the o p t i c a l p r o p e r t i e s of the s t a b i l i z e d e l e c t r o n s are dominated by the p o l a r component. I t would appear then that f o r media which are completely m i s c i b l e i n a l l p r o p o r t i o n s the y i e l d and o p t i c a l p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s depend upon the mean bu l k p r o p e r t i e s of the m i x t u r e , r e g a r d l e s s of the s o l v a t i o n power e i t h e r component has f o r the e l e c t r o n s . 2. DMSO P o s i t i v e Ions The a b s o r p t i o n band centered at 550 nm, and a t t r i b u t e d to the DMSO p o s i t i v e i o n , d i d not change i n p o s i t i o n or shape but d i d i n c r e a s e i n magnitude as the DMSO c o n c e n t r a t i o n was in c r e a s e d . Figure 52(a) shows t h i s absorbance change over the composition range 0.20 to 1.00 mole f r a c t i o n DMSO. These s p e c t r a were obtained a f t e r the a b s o r p t i o n due to the s o l v a t e d e l e c t r o n had decayed ( l i f e t i m e < 100 nsec) and were c o r r e c t e d to zero time by e x t r a p o l a t i o n back to the end of the pulse(the h a l f - - 170 - Fract ion of Dose absorbed by DMSO F i g u r e 52. (a) Ab s o r p t i o n s p e c t r a a t t r i b u t e d to the DMSO p o s i t i v e i o n produced i n DMS0-H20 mixtures. Curve 6 i s pure DMSO; 5, 0.93; 4, 0.72; 3, 0.43; 2, 0.28; and 1, 0.20 mole f r a c t i o n DMSO. (b) Peak absorbance (at 550 nm) of the bands shown i n (a) p l o t t e d a g a i n s t the f r a c t i o n of dose absorbed i n i t i a l l y by DMSO. - 171 - l i f e f o r the p o s i t i v e i o n <v 2 usee f o r a l l m i x t u r e s ) . The change i n absorbance of the band at 550 nm as a f u n c t i o n of the dose i n i t i a l l y absorbed by the DMSO i n the mixture (as given by i t s e l e c t r o n f r a c t i o n and approximately equal t o i t s volume %) i s shown i n F i g u r e 52(b). I t can be seen to be reasonably l i n e a r over the range s t u d i e d . The data given i n Figure 52, when coupled w i t h the f a c t t h a t the l i f e t i m e of DMS0+ i s u n a l t e r e d , s t r o n g l y i n d i c a t e s that the c a t i o n i s u n a f f e c t e d by the water content, even at 0.80 mole f r a c t i o n water. This implies t h a t the DMSO p o s i t i v e i o n does not undergo proton or charge t r a n s f e r w i t h the water molecules nor i s there any exchange of primary o x i d i z i n g s pecies between the two components. I t should be 97 98 noted that v a r i o u s proton acceptors, such as ammonia and e t h a n o l , have been used i n nonpolar a p r o t i c media as scavengers of p o s i t i v e i o n s ; but the above data suggest t h a t r e a c t i o n (45) does not occur i n DMSO-water b i n a r y mixtures. ( C H 3 ) 2 S 0 + + H 20 CH 3SOCH 2 + H 3 0 + (45) However there i s an important d i f f e r e n c e between nonpolar a p r o t i c media, such as cyclohexane, and DMSO. In DMSO c a t i o n s are s t r o n g l y s o l v a t e d whereas they are extremely unstable i n the nonpolar a p r o t i c s o l v e n t s and hence w i l l r e a d i l y undergo proton t r a n s f e r . Since both DMSO and water r e a d i l y s o l v a t e c a t i o n s , there i s no " d r i v i n g f o r c e " f o r the i o n i z e d species to undergo charge or proton t r a n s f e r . In view of the f a i r l y l a r g e d i f f e r e n c e i n i o n i z a t i o n p o t e n t i a l 99 100 of DMSO (8.85 eV) and water (12.6 eV), i t i s s u r p r i s i n g that the - 172 - y i e l d of DMSO p o s i t i v e ions i s independent of the water content. One would have expected charge or i n t e r m o l e c u l a r energy t r a n s f e r to occur from the water t o the DMSO molecules. The i n i t i a l d e p o s i t i o n of energy by the high-energy e l e c t r o n s r e s u l t s i n h i g h l y e x c i t e d molecules, e i t h e r through d i r e c t e l e c t r o n i c e x c i t a t i o n or e x c i t a t i o n produced by i o n n e u t r a l i z a t i o n (geminate recombination). Since these s o l u t i o n s are completely homogeneous, e x c i t a t i o n t r a n s f e r between the e x c i t e d water molecules and neighbouring DMSO molecules could have r e s u l t e d i n the l a t t e r being i o n i z e d . N e v e r t h e l e s s , the l i n e a r i t y of Figure 52(b) suggests that the y i e l d of DMSO p o s i t i v e i ons i n the mixture i s simply p r o p o r t i o n a l to i t s e l e c t r o n f r a c t i o n . I t would a l s o appear that the y i e l d of f r e e ions (those e l e c t r o n s which escape geminate recombination) i s independent of the bulk d i e l e c t r i c constant of the medium. D i l u t i o n of DMSO by water (up to 50% by volume) causes the b u l k d i e l e c t r i c constant to change from 46 to 75 (at 25°C), yet the f r e e i o n y i e l d of DMSO, as given by the y i e l d of DMSO p o s i t i v e i o n s , i s not ap p r e c i a b l y changed. Perhaps the t h e r m a l i z a t i o n path of the low energy e l e c t r o n i s shortened by the " t i g h t e r " DMSO-water s t r u c t u r e but the inc r e a s e d coulombic a t t r a c t i o n energy i s compensated f o r by the higher d i e l e c t r i c constant. I t should be mentioned t h a t i n a mixture c o n s i s t i n g of 0.72 mole f r a c t i o n DMSO, the a b s o r p t i o n at 550 nm was inc r e a s e d approximately 90% by the presence of 0.5 M A g + and the e l e c t r o n band completely e l i m i n a t e d . As i n the case of pure DMSO, t h i s i n c r e a s e i s a t t r i b u t e d to scavenging of thoseelectrons which were otherwise doomed to geminate recombination w i t h t h e i r concomitant p a r t n e r . - 173 - 3. Tran s i e n t Intermediates at 77°K The pulse r a d i o l y s i s of a DMSO-water gl a s s at 77°K was undertaken f o r two reasons. F i r s t l y , e l e c t r o n s p i n resonance s t u d i e s on y - i r r a d i a t e d aqueous DMSO glasses d i d not i n d i c a t e the presence of trapped e l e c t r o n s (to be discussed i n the next c h a p t e r ) . However t h e i r n a t u r a l l i f e t i m e may have been very short ( l e s s than a few minutes) so tha t they would have decayed completely before e s r measurements could be taken. I t was hoped that the p u l s e r a d i o l y s i s of a gl a s s y mixture would show whether or not trapped e l e c t r o n s are formed and i f so, to measure t h e i r decay r a t e . Secondly, a pur p l e - c o l o u r e d intermediate was observed upon y - i r r a d i a t i o n of the g l a s s e s . I t was suspected t h a t t h i s t r a n s i e n t was the DMSO p o s i t i v e i o n but only by r e c o r d i n g i t s o p t i c a l spectrum could t h i s s u p p o s i t i o n be v e r i f i e d . An aqueous g l a s s c o n t a i n i n g 0.39 mole f r a c t i o n DMSO was prepared by r a p i d l y p l u nging the o p t i c a l c e l l c o n t a i n i n g the mixture i n t o a dewar w i t h o p t i c a l windows f i l l e d w i t h l i q u i d n i t r o g e n . The mixture had been p r e v i o u s l y deoxygenated by bubbling w i t h high p u r i t y argon. At the end of the experiment both the DMSO-water g l a s s and the c e l l were cracked. I t i s not known whether the c e l l cracked upon c o o l i n g down, warming up or during the experiment. However the " s p l i n t e r crack" would probably prevent contamination by a i r (oxygen) s i n c e the c e l l was immersed i n l i q u i d n i t r o g e n . The o p t i c a l path l e n g t h was 2 mm. The r e s t of the o p t i c a l d e t e c t i o n system was the same as described p r e v i o u s l y . An e l e c t r o n pulse width of 500 nsec was used which gave 4 an approximate dose of 10 rads per p u l s e . Dosimetry was performed using an aqueous KCNS s o l u t i o n as p r e v i o u s l y described except the s o l u t i o n - 174 - was s a t u r a t e d w i t h a i r r a t h e r than n i t r o u s oxide. As a r e s u l t , GCCNS)^ was taken as 2.9. The dosimetry was only approximate because the dewar d i d not c o n t a i n an appropriate medium (such as methanol) to take account of the s c a t t e r i n g of the high energy e l e c t r o n beam by the l i q u i d n i t r o g e n . The reason f o r t h i s was that the c e l l was cracked and contamination of the dosimeter s o l u t i o n would have r e s u l t e d . The a c t u a l dose absorbed by the g l a s s y sample was probably higher f o r a g i v e n SEM reading than that c a l c u l a t e d from the dosimeter r e s u l t s because the c e l l i n l i q u i d n i t r o g e n would be i n e l e c t r o n i c e q u i l i b r i u m ( e l e c t r o n s s c a t t e r e d out of the c e l l compensated by those s c a t t e r e d i n t o the c e l l i n dense surroundings). Consequently the absorbance readings, when expressed as Ge, are probably an upper l i m i t . The gl a s s y sample was photobleached between pulses and the t o t a l absorbed dose was < 3.5 x 10"* rads. The spectrum observed at the end of the 500 nsec pulse i s shown i n F i gure 53. Only two d i s t i n c t bands were observed, one centered at 550-600 nm and the other at X < 400 nm. The absor p t i o n band of max r the DMSO p o s i t i v e i o n i n l i q u i d DMSO was normalized to the spectrum of the g l a s s y sample at 600 nm and i s shown as the dotted l i n e i n Figure 53. I t can be seen that the observed spectrum devia t e s from the p o s i t i v e i o n band at X > 700 nm suggesting that another i n t e r m e d i a t e i s absorbing i n t h i s r e g i o n . This i s probably due to the t r a n s i e n t a b s o r p t i o n by the s i l i c a windows ( S u p r a c i l ) of the c e l l due to e l e c t r o n bombardment at 77°K. I t i s not b e l i e v e d to be due to the trapped e l e c t r o n f o r two reasons. F i r s t l y , the trapped e l e c t r o n should have an absorption maximum at ̂  800 nm on the b a s i s of the s p e c t r a given i n Figure 48 f o r 4 0 r - 3 0 0 5 0 0 7 0 0 9 0 0 1100 WAVELENGTH (nm) 1300 1500 Figure 53. Absorption spectrum of t r a n s i e n t s produced by the pulse r a d i o l y s i s of a DMSO-l^O g l a s s (39 mole % DMSO) at 77°K. The pulse width was 500 nsec, the dose per pulse being ^ 10 krad. The dotted curve that of the DMSO p o s i t i v e i o n i n pure DMSO normalized at 600 nm. - 176 - the l i q u i d mixtures (assuming no s p e c t r a l s h i f t ) whereas the observed ab s o r p t i o n i n the r e g i o n 600-1100 nm showed a very broad continuous t a i l w i t h no d i s c o n t i n u i t y or the presence of any "shoulders". Secondly,the a b s o r p t i o n maximum at 500-600 nm was c a l c u l a t e d to be Ge < 6000 mols. (100 eV) '"cm ^ which seems to be too s m a l l f o r a s t a b l e trapped e l e c t r o n . The end of p u l s e o s c i l l o s c o p e t r a c e s showed no change i n absorbance w i t h time suggesting t h a t any trapped or s o l v a t e d e l e c t r o n s had reacted by the end of the pul s e and hence th a t t h e i r l i f e t i m e was l e s s than 10 ^sec. This i s c o n s i s t e n t w i t h the e s r s t u d i e s on y- i r r a d i a t e d g l a s s y DMSO-water mixtures at 77°K i n which no trapped e l e c t r o n s i g n a l c o u l d be detected. Even when the e l e c t r o n s were generated i n the g l a s s a t 77°K i n the spectrometer c a v i t y by the u l t r a v i o l e t p h o t o l y s i s of K^Fe(CN)g, the e l e c t r o n s were unstable and decayed immediately, producing the same e s r spectrum as the Y - i r r a d i a t e d g l a s s . This w i l l be discussed more f u l l y i n the next chapter. I t i s worth n o t i n g t h a t i n many other p o l a r and nonpolar low temperature g l a s s e s e l e c t r o n s can be trapped and s t a b i l i z e d i n d e f i n i t e l y . I t i s suggested then that the a c t i v a t i o n energy i s very low f o r the r e a c t i o n of the e l e c t r o n w i t h a DMSO molecule forming a p a r t of i t s s o l v e n t cage so t h a t r e a c t i o n r e a d i l y proceeds even at 77°K. - 177 - CHAPTER V ELECTRON SPIN RESONANCE STUDIES ON DMSO AND DMSO-H20 MATRICES AT 77°K A. INTRODUCTION * 1. B a s i c P r i n c i p l e s of ESR Use of the technique of e l e c t r o n s p i n resonance (esr) i s r e s t r i c t e d to those molecules or atomic species c o n t a i n i n g unpaired e l e c t r o n s . Because of i t s charge and i n t r i n s i c angular momentum, or s p i n , the e l e c t r o n has a magnetic moment a s s o c i a t e d w i t h i t . According to quantum theory, a s i n g l e e l e c t r o n can s p i n i n e i t h e r of two d i r e c t i o n s as given by the quantum numbers M g = +1/2 (a spin) or M g = -1/2 (£ s p i n ) . In the absence of any e x t e r n a l magnetic f i e l d the e l e c t r o n has no preference f o r an a or g s p i n s i n c e they are of equal energy. When an e x t e r n a l magnetic f i e l d i s a p p l i e d to the paramagnetic system, t h i s degeneracy i s removed. The e l e c t r o n w i l l have lower energy i f i t s s p i n magnetic moment i s a l i g n e d so as t o be i n the d i r e c t i o n of the a p p l i e d f i e l d r a t h e r than a g a i n s t i t , which i s the only other allowed o r i e n t a t i o n . The energy s e p a r a t i o n between the two s p i n s t a t e s i s p r o p o r t i o n a l to the product of a constant, 8 , (the e l e c t r o n i c Bohr magneton) and the str e n g t h of the Prepared from references 59 and 104. - 178 - e x t e r n a l magnetic f i e l d , H. The p r o p o r t i o n a l i t y constant, g, i s r e f e r r e d to as the s p e c t r o s c o p i c s p l i t t i n g f a c t o r , or g - f a c t o r , and represents the r a t e of divergence of the magnetic energy l e v e l s w i t h the magnetic f i e l d . I f the paramagnetic species i s i r r a d i a t e d w i t h electromagnetic r a d i a t i o n possessing energy, hv, equal to the se p a r a t i o n between the two energy l e v e l s , the unpaired e l e c t r o n w i l l absorb energy and " f l i p over" to the s t a t e of higher energy i n which i t s s p i n magnetic moment i s a l i g n e d a n t i - p a r a l l e l t o the magnetic f i e l d . This resonance c o n d i t i o n i s described by equation (5.1). hv = ge3H (5.1) For a f r e e e l e c t r o n g = 2.0023 (the d e v i a t i o n from the i n t e g r a l number being a r e l a t i v i s t i c c o r r e c t i o n f o r the o r b i t a l v e l o c i t y of the e l e c t r o n ) . The term f r e e e l e c t r o n or f r e e s p i n r e f e r s to an unpaired e l e c t r o n having s p i n angular momentum but no o r b i t a l angular momentum (such as that possessed by an e l e c t r o n i n an s o r b i t a l ) . Although t h i s c o n d i t i o n i s r a r e l y r e a l i z e d f o r organic f r e e r a d i c a l s , the o r b i t a l angular momentum a s s o c i a t e d w i t h a p or d o r b i t a l e l e c t r o n i s u s u a l l y qienched so that the g - f a c t o r s are near that of f r e e e l e c t r o n s . In p r i n c i p l e e l e c t r o n s p i n resonance absorptions may be produced by v a r y i n g the magnetic f i e l d (H), the r a d i a t i o n frequency (v) or a combination of both. In p r a c t i c e , however, the frequency i s u s u a l l y f i x e d and the f i e l d s l o w l y v a r i e d s i n c e the k l y s t r o n o s c i l l a t o r which generates the microwave r a d i a t i o n i s tunable only over a very narrow range. For the X-band spectrometer used i n t h i s study the microwave frequency was about 9.1 GHz so that the r e q u i r e d magnetic f i e l d f o r the resonance - 179 - c o n d i t i o n of a f r e e e l e c t r o n occurred around 3200 gauss. Thus s t u d i e s i n e s r depend upon the absorption of microwave energy between non-degenerate s p i n s t a t e s of a paramagnetic s p e c i e s . However the probabilities of upward ( s t i m u l a t e d absorption) and downward ( s t i m u l a t e d emission) t r a n s i t i o n s are e q u a l ; consequently the c o n d i t i o n of:• resonance i s dependent upon there being a d i f f e r e n c e i n p o p u l a t i o n between these two s t a t e s . The r a t i o of paramagnetic species w i t h t h e i r s p i n s a l i g n e d i n the d i r e c t i o n of the a p p l i e d magnetic f i e l d (N ) to those a l i g n e d a g a i n s t i t (N +) at temperature T i s given by the Boltzman d i s t r i b u t i o n . N+/N~ = exp(-gBH/kT) (5.2) Since ggH << kT f o r temperatures above a few degrees a b s o l u t e , the e x p o n e n t i a l f a c t o r i s c l o s e to u n i t y so that the excess p o p u l a t i o n i n the ground s t a t e i s very s m a l l (̂  0.07% f o r g = 2, T = 300°K and H = 3000 gauss). I f the r a d i a t i o n f i e l d a t resonance i s a p p r e c i a b l y i n c r e a s e d , the upper and lower s p i n s t a t e s w i l l become e q u a l l y populated so t h a t there w i l l be no net energy a b s o r p t i o n and t h e r e f o r e no resonance s i g n a l . The s p i n system i s then s a i d to be power s a t u r a t e d . This s a t u r a t i o n i s counterbalanced by r e l a x a t i o n processes, as c h a r a c t e r i z e d by the s p i n - l a t t i c e r e l a x a t i o n time (T^) and s p i n - s p i n r e l a x a t i o n time (T ^ ) , which tend to r e s t o r e thermal e q u i l i b r i u m . In the former process the s p i n system i n t e r a c t s w i t h the medium or l a t t i c e by donating i t s excess energy to the v i b r a t i o n a l and r o t a t i o n a l modes of the surrounding molecules. Rapid d i s s i p a t i o n of t h i s excess - 180 - s p i n energy (short T^) i s e s s e n t i a l i f the p o p u l a t i o n d i f f e r e n c e of the s p i n s t a t e s i s to be maintained. Although t h i s p o p u l a t i o n d i f f e r e n c e i s g r e a t e s t at low temperatures, t h e r e f o r e a l l o w i n g a s t r o n g e r a b s o r p t i o n , the s p i n - l a t t i c e r e l a x a t i o n process i s l e s s e f f i c i e n t . Consequently f r e e r a d i c a l s are o f t e n e a s i l y power s a t u r a t e d at low temperatures. S p i n - s p i n r e l a x a t i o n i n v o l v e s magnetic i n t e r a c t i o n s between the unpaired e l e c t r o n s and surrounding magnetic d i p o l e s , such as other unpaired e l e c t r o n s or magnetic n u c l e i n a t i v e to the l a t t i c e . These i n t e r a c t i o n s are not energy d i s s i p a t i n g and t h e r e f o r e do not c o n t r i b u t e d i r e c t l y i n r e t u r n i n g the s p i n system to e q u i l i b r i u m . However the s p i n - l a t t i c e t r a n s i t i o n s described above may be enhanced i f the s p i n - s p i n process b r i n g s the excess energy to a p o s i t i o n f o r a p r o p i t i o u s t r a n s i t i o n to the l a t t i c e . These s p i n - s p i n t r a n s i t i o n s are important i n another sense i n that they cause broadening of the a b s o r p t i o n l i n e s . The t o t a l magnetic f i e l d experienced by a p a r t i c u l a r s p i n system w i l l i n c l u d e c o n t r i b u t i o n s from i t s neighbours as w e l l as the a p p l i e d magnetic f i e l d ; consequently the resonance t r a n s i t i o n s w i l l occur over a range of frequencies corresponding to the v a r i a t i o n s i n l o c a l f i e l d . In l i q u i d media molecules undergo r a p i d random motion so that these induced d i p o l a r f i e l d s are subjected to extensive time averaging. As a r e s u l t the l i n e s are narrower than those i n the corresponding s o l i d s t a t e where the paramagnetic species are prevented from r o t a t i o n a l or t r a n s l a t i o n a l motion. A s p e c i a l case of s p i n - s p i n i n t e r a c t i o n may occur between the - 181 - unpaired e l e c t r o n and nuc l e a r s p i n s w i t h i n the same atom or molecule. This i n t e r a c t i o n r e s u l t s , not i n l i n e broadening, but r a t h e r i n r e s o l v e d h y p e r f i n e s t r u c t u r e . J u s t l i k e the e l e c t r o n s p i n , the n u c l e a r s p i n i s quantized having 2 1 + 1 s t a t e s of equal energy f o r a nucleus of s p i n I . When a magnetic f i e l d i s a p p l i e d , these degenerate s t a t e s are s p l i t and the n u c l e a r magnetic moment forms the 2 1 + 1 allowed o r i e n t a t i o n s w i t h respect to the f i e l d d i r e c t i o n . Under the c o n d i t i o n s of an e s r experiment (H <\» 3000 gauss, v ^ 9.0 GHz) a l l 2 1 + 1 n u c l e a r moment o r i e n t a t i o n s may be considered as e q u a l l y probable s i n c e the d i f f e r e n c e i n p o p u l a t i o n of the n u c l e a r s u b - l e v e l s i s s e v e r a l orders of magnitude l e s s than the corresponding e l e c t r o n s p i n s t a t e s . This r e s u l t s i n each of the e l e c t r o n s p i n s t a t e s being f u r t h e r s p l i t i n t o 2 1 + 1 s u b - l e v e l s of equal s e p a r a t i o n . Since the n u c l e a r s p i n s are unaffe c t e d by the o s c i l l a t i n g microwave f i e l d which causes the e l e c t r o n i c t r a n s i t i o n s (the resonance frequency of a proton, f o r example, i n a f i e l d of 3000 gauss i s about 14 MHz), the s e l e c t i o n r u l e s f o r the e s r t r a n s i t i o n s are AM^ = + 1 and AM^ = 0. The e s r spectrum w i l l t h e r e f o r e c o n s i s t of 21 + 1 l i n e s . Often there are groups of n u c l e i i n c h e m i c a l l y and m a g n e t i c a l l y e q u i v a l e n t p o s i t i o n s , i n which case they act together to give a s p l i t t i n g c h a r a c t e r i z e d by t h e i r t o t a l s p i n nj_l£' Here n^ i s the number of e q u i v a l e n t n u c l e i w i t h n u c l e a r s p i n I . . The combined i n t e r a c t i o n produces 2n.I. + 1 l i n e s of equal s e p a r a t i o n , the i n t e n s i t i e s of which can be i d e n t i f i e d w i t h the c o e f f i c i e n t s of the app r o p r i a t e multinominal expansion. The r e s u l t i n g l i n e s e p a r a t i o n i s c a l l e d the h y p e r f i n e s p l i t t i n g . A much more complicated s i t u a t i o n a r i s e s when the e l e c t r o n i n t e r a c t s w i t h - 182 - more than one non-equivalent magnetic n u c l e i . In s o l i d s and h i g h l y v i s c o u s media, i n which the paramagnetic centres are not f r e e to tumble, the hy p e r f i n e s p l i t t i n g s w i l l depend upon the r e l a t i v e o r i e n t a t i o n of the l a t t i c e and a p p l i e d magnetic f i e l d . The magnetic d i p o l e - d i p o l e i n t e r a c t i o n s between the unpaired e l e c t r o n and n u c l e a r moments have a d i r e c t i o n a l c h a r a c t e r a s s o c i a t e d w i t h them so that the e f f e c t i v e magnetic f i e l d f e l t by the e l e c t r o n w i l l be a n i s o t r o p i c . This i s accounted f o r by expressing the s p l i t t i n g s i n terms of a h y p e r f i n e tensor A. I f the paramagnetic species i s r a p i d l y r e o r i e n t i n g , the a n i s o t r o p i c p a r t w i l l average to zero and the s p l i t t i n g s w i l l a r i s e s o l e l y from the i s o t r o p i c c o u p l i n g (Fermi contact i n t e r a c t i o n ) . The g - f a c t o r may s i m i l a r l y show a n i s o t r o p i c behaviour. I f the unpaired e l e c t r o n possesses o r b i t a l angular momentum i t w i l l ccuple w i t h the s p i n angular momentum. Because of the o r i e n t a t i o n a l dependence of t h i s s p i n - o r b i t c o u p l i n g , i t w i l l be e a s i e r to make the e l e c t r o n reverse i t s s p i n when the magnetic f i e l d i s a p p l i e d i n c e r t a i n d i r e c t i o n s as compared to o t h e r s . This a n i s o t r o p y r e s u l t s i n the paramagnetic system r e s o n a t i n g a t d i f f e r e n t magnetic f i e l d s . For t h i s reason the g - f a c t o r i s o f t e n expressed as a tensor i n which the p r i n c i p a l values are g , g and g . In most cases the o f f - d i a g o n a l xx yy zz elements are s m a l l and may be negl e c t e d so t h a t , to a good approximation, g = g > g = g and g = g • Here x, y, and z r e f e r to the °xx °x °yy °y °zz z l a b o r a t o r y f i x e d axes and are r e l a t e d to the c r y s t a l or molecular f i x e d axes by a s u i t a b l e coordinate t r a n s f o r m a t i o n m a t r i x (not n e c e s s a r i l y the same f o r the g and A t e n s o r s ) . In a l i q u i d or s l i g h t l y - 183 - v i s c o u s s o l i d the observed g-values may be considered i s o t r o p i c s i n c e the molecules w i l l be tumbling r a p i d l y and randomly; hence g = 1/3(g + 8 y y + g z z ) ' 2. Amorphous and P o l y c r y s t a l l i n e Media In amorphous or p o l y c r y s t a l l i n e s o l i d s the paramagnetic species are o r i e n t e d randomly i n the m a t r i x . I f the medium temperature i s s u f f i c i e n t l y low, r o t a t i o n a l and t r a n s l a t i o n a l motions w i l l be hindered so t h a t the time averaging p r o p e r t i e s of f l u i d systems are l o s t . Thus the observed resonance w i l l be a sum of the i n d i v i d u a l resonances of the randomly o r i e n t e d paramagnetic centres. In t h i s case the spectrum w i l l be governed by the a n i s o t r o p y of the g - f a c t o r and h y p e r f i n e s p l i t t i n g as w e l l as any broadening due to d i p o l e - d i p o l e i n t e r a c t i o n s . T his r e s u l t s i n a l e s s d e t a i l e d p i c t u r e of the s t r u c t u r e and e l e c t r o n i c d i s t r i b u t i o n of the r a d i c a l than i f i t was present i n a s i n g l e c r y s t a l o r s o l u t i o n . However, s i n c e the primary o b j e c t i v e of t h i s and other s t u d i e s i n r a d i a t i o n chemistry i s simply to i d e n t i f y the paramagnetic species i n v o l v e d i n the chemical processes, s t u d i e s on the amorphous or p o l y c r y s t a l l i n e s t a t e are u s u a l l y performed s i n c e the experimental procedure and mathematical a n a l y s i s are l e s s complicated. Spectral l i n e shapes have been computed f o r paramagnetic species randomly trapped i n s o l i d m a t r i c e s . Two of the most commonly o c c u r r i n g l i n e shapes are shown i n F i g u r e 54 f o r r a d i c a l s e x h i b i t i n g - 184 - no h y p e r f i n e s t r u c t u r e . F i g u r e 54(a) shows the resonance a b s o r p t i o n f o r a radical possessing an a x i a l l y symmetric g-tensor where g ^ = g ^ = g and g = g whereas Figure 54(b) r e f e r s to a completely X zz u asymmetric g-tensor where g^ < g^ < g-j. The a p p l i e d magnetic f i e l d i s taken as being a l i g n e d along the z a x i s . An example of a para- magnetic system having an a x i a l l y symmetric g and A tensor w i t h the same a x i s of symmetry f o r a s i n g l e nucleus w i t h n u c l e a r s p i n I = 1/2 i s shown i n Fi g u r e 55. In t h i s spectrum the a n i s o t r o p i c h y p e r f i n e s p l i t t i n g constants A ( | and A^ are >> 8H(g^ - g^). The c e n t r a l d o t t e d p o r t i o n i s the t h e o r e t i c a l spectrum i n the absence of the h y p e r f i n e c o u p l i n g . These three cases represent the optimum c o n d i t i o n s to be expected when d e a l i n g w i t h p o l y c r y s t a l l i n e and amorphous s o l i d s . I n some matr i c e s the s p e c i f i c f e a t u r e s corresponding to the tensor components of A and g are l o s t due to excessive l i n e broadening. This i s e s p e c i a l l y t r u e f o r r a d i c a l s trapped i n hydrocarbon matrices at low temperatures where the d i p o l a r broadening due to the protons i s o f t e n g r e a t e r than BH(g - g ) and (A - A The broadening r e s u l t s i n it j_ li j. l i n e s approximately Gaussian i n shape w i t h the peak-to-peak s e p a r a t i o n corresponding f a i r l y c l o s e l y to the i s o t r o p i c component of the hy p e r f i n e s p l i t t i n g constant. G e n e r a l l y the l i n e widths f o r the a l i p h a t i c r a d i c a l s at 77°K are about 10-15 gauss but can be narrower i f the r a d i c a l i s f r e e to r o t a t e or tumble w i t h i n the t r a p p i n g s i t e and thereby average out the d i p o l a r i n t e r a c t i o n s . Often the broadening can be reduced s i g n i f i c a n t l y by using deuterated molecules. Since the n u c l e a r moment of deuterium i s s m a l l e r than that of hydrogen, the h y p e r f i n e - 185 - Fi g u r e 54. T h e o r e t i c a l esr l i n e shapes f o r (a) a x i a l l y symmetric and (b) completely asymmetric g tensors. The upper curves r e f e r to a b s o r p t i o n s p e c t r a of the paramagnetic s p e c i e s . The lower curves r e f e r to the e x p e r i m e n t a l l y observed f i r s t d e r i v a t i v e s p e c t r a . (Adapted from Figures 9.3 and 9.4, pages 324-325, reference 104). Figure 5 5 . T h e o r e t i c a l f i r s t d e r i v a t i v e esr spectrum f o r a paramagnetic species w i t h S = 1 / 2 , I = 1/2 and w i t h a x i a l l y asymmetric g and A tensors. The c e n t r a l dotted p o r t i o n i s the t h e o r e t i c a l spectrum i n the absence of the hyperfine i n t e r a c t i o n s . (Adapted from Figure 9 . 7 , page 3 2 7 , reference 1 0 4 ) . - 187 - i n t e r a c t i o n s and l i n e widths w i l l be reduced compared to the u n s u b s t i t u t e d medium. In the preceding d i s c u s s i o n i t has been assumed that the resonance a b s o r p t i o n i s due to a s i n g l e paramagnetic species randomly o r i e n t e d i n the m a t r i x . G e n e r a l l y t h i s i s not the case. Often the spectrum i s f u r t h e r complicated by the appearance of other r a d i c a l s r e s o n a t i n g i n the same r e g i o n so tha t i d e n t i f i c a t i o n i s not always p o s s i b l e . This i s p a r t i c u l a r l y t r u e i f the medium has been exposed to i o n i z i n g r a d i a t i o n , such as X- or y-ra y s , which are not s e l e c t i v e i n the. types of r a d i c a l s they produce. In many systems s p e c t r a l a n a l y s i s may be s i m p l i f i e d by using a Hg lamp as the r a d i a t i o n source. Since u l t r a v i o l e t p h o t o l y s i s i s more s e l e c t i v e i n c l e a v i n g molecular bonds, only c e r t a i n r a d i c a l s w i l l be produced. B. RESULTS AND DISCUSSION 1. Pure DMSO 1.1 U l t r a v i o l e t I r r a d i a t e d The r a d i c a l s produced when p o l y c r y s t a l l i n e DMSO i s photolyzed at 77°K with u l t r a v i o l e t l i g h t e x h i b i t a broad, asymmetric spectrum c o n s i s t i n g of seven r e s o l v a b l e l i n e s centered near g = 2.004 as shown i n F i g u r e 56. Since the Vycor envelope i s opaque below 220 nm, the emitted l i g h t from the low-pressure mercury resonance a r c i s c h i e f l y 253.7 nm corresponding to an energy of = 110 k c a l mole \ DMSO absorbs s t r o n g l y below 260 nm suggesting that the f i r s t 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 i s comparable to the energy of t h i s mercury l i n e . I f d i s s o c i a t i o n occurs by i n t e r n a l conversion from the lowest e l e c t r o n i c a l l y - 188 - e x c i t e d s t a t e , then the p r o b a b i l i t y of r a d i c a l formation w i l l favour the weakest bond. In DMSO the C-S bond has a mean d i s s o c i a t i o n energy D o f ̂  50 kcal/mole 1 as compared to D(S-O) and D(C-H) which are 86 and 90-100 k c a l mole * r e s p e c t i v e l y . Since the C-S bond i s ap p r e c i a b l y weaker than the r e s t one would expect CH^SO and 'CH^ r a d i c a l s to predominate i n the primary p h o t o l y t i c process. The spectrum shown i n Figure 56 tends to confirm t h i s s u p p o s i t i o n . The q u a r t e t , centered a t g • 2.003 and having a h y p e r f i n e s p l i t t i n g of 22 + 1 gauss (as measured from the d e r i v a t i v e maxima),is t h a t expected f o r an unpaired e l e c t r o n i n t e r a c t i n g w i t h three e q u i v a l e n t a protons. The l i n e w i d t h , AH, as measured between the peaks of the derivative'maximum and minimum, i s approximately 6 to 7 gauss i n d i c a t i n g t h a t the methyl r a d i c a l i s tumbling f r e e l y i n the DMSO mat r i x a t 77°K. The i n t e n s i t y d i s t r i b u t i o n of the resonance l i n e s of the q u a r t e t , as measured by the peak-to-peak amplitudes, agrees w i t h the t h e o r e t i c a l r a t i o of 1:3:3:1. The asymmetric three l i n e spectrum a s c r i b e d to the CH^SO r a d i c a l s i s i n d i c a t i v e of the resonance p a t t e r n observed f o r other s u l f u r r a d i c a l s i n u l t r a v i o l e t and y - i r r a d i a t e d t h i o l s , s u l f i d e s , d i s u l f i d e s and s u l f o n e s where the anis o t r o p y i n the g - f a c t o r i s a t t r i b u t e d to the strong s p i n - o r b i t c o u p l i n g of the unpaired e l e c t r o n l o c a l i z e d on the s u l f u r a t o m s . S i n c e the s u l f u r resonance shows seven p o i n t s of i n f l e c t i o n , the three p r i n c i p l e g - f a c t o r s w i l l correspond to the maximum, zero and minimum p o i n t s i n the d e r i v a t i v e curve (see Figure 5 4 ( b ) ) . The g- f a c t o r s obtained i n t h i s manner are as f o l l o w s : g^ = 1.988, g 9 = 2.004 and g_ = 2.017. No r e s i d u a l h y p e r f i n e s p l i t t i n g was g 3 - 2.017 g 2 = 2.004 1.988 c v. L 10 gauss H L •CH3 J e 56. Electron spin resonance spectrum obtained after the ultraviolet photolysis of polycrystalline DMSO at 77°K. The arrows correspond to the asymmetric g-factors of the sulfur radical CH^SO. The methyl radical quartet is indicated by the stick plot. - 190 - observed suggesting that the unpaired e l e c t r o n i s l o c a l i z e d mainly on the s u l f u r atom and i s not i n t e r a c t i n g w i t h the B protons of the methyl group. I t i s p o s s i b l e , however, that the s p l i t t i n g i s too s m a l l to be r e s o l v e d due t o the d i p o l a r broadening. 1.2 Y ~ I r r a d i a t e d Figure 57 shows the e s r spectrum obtained when the p o l y c r y s t a l l i n e DMSO b a l l s were Y _ i r r a d i a t e d i n the dark at 77°K. U n l i k e the photolyzed sample (Figure 56), the resonance s p e c t r a of the r a d i c a l s produced were p o o r l y r e s o l v e d . The two d e r i v a t i v e peaks on the h i g h and low f i e l d p o r t i o n of the spectrum correspond to the resonance p o s i t i o n s of the outer l i n e s of the methyl r a d i c a l q u a r t e t . However the asymmetric s u l f u r p a t t e r n observed i n the photolyzed sample cannot be p o s i t i v e l y assigned because of the o v e r l a p p i n g resonance of other paramagnetic s p e c i e s . When the microwave power was increased above 1.0 mW, one or more d i f f e r e n t paramagnetic species present i n the Y _ i r r a d i a t e d sample and centered near g = 2.006 began to s a t u r a t e . This i s evident from F i g u r e 57(b) f o r which the microwave power i s 10 mW. Superimposed upon the resonance p a t t e r n shown i n F i g u r e 57 i s a broad asymmetric s i n g l e t centered at g = 2.007 w i t h a l i n e width of <\» 20 gauss. This s i n g l e t i s not due to trapped e l e c t r o n s because the abs o r p t i o n i s not s a t u r a t e d at hi g h power (160 mW) and i t s g - f a c t o r i s g r e a t e r than the f r e e s p i n v a l u e . I t i s worth n o t i n g that the r a d i c a l s produced i n the samples by y - i r r a d i a t i o n and u l t r a v i o l e t p h o t o l y s i s were s t a b l e and showed no sign s of decay when the i r r a d i a t e d samples were kept i n the dark i n l i q u i d n i t r o g e n . Even a f t e r s e v e r a l days the resonance p a t t e r n s and - 191 - (b) F i gure 57. E l e c t r o n s p i n resonance s p e c t r a of y - i ^ r a d i a t e d DMSO. The sample was i r r a d i a t e d i n the dark at 77"K to a t o t a l absorbed dose of 0.72 Mrad. (a) microwave power 0.44 mW; (b) microwave power 10 mW. g D p p H = 2.0036. - 192 - Figure 58. E l e c t r o n s p i n resonance s p e c t r a of y _ i r r a d i a t e d DMSO a f t e r b l e a c h i n g i r r a d i a t e d sample w i t h u l t r a v i o l e t l i g h t f o r f o r t y minutes ( i n spectrometer c a v i t y ) . Sample y i r r a d i a t e d at 77°K i n the dark to a t o t a l absorbed dose of 0.72 Mrad. (a) microwave power 0.52 mw; (b) microwave power 10 mW. - 193 - i n t e n s i t i e s were unchanged. When the y i r r a d i a t e d sample was photcbleached w i t h u l t r a v i o l e t l i g h t , the spectrum shown i n Figure 58 was observed and i s comparable to that found f o r d i r e c t u l t r a v i o l e t p h o t o l y s i s and given i n Figure 56. The s p e c t r a l change d i d not a r i s e from simple p h o t o l y s i s of the DMSO molecules s i n c e the methyl r a d i c a l c o n c e n t r a t i o n was unchanged (as measured by the outer peaks of the q u a r t e t ) . Furthermore, the sample was photobleached i n the esr c a v i t y f o r only 40 minutes which, on the ga i n s e t t i n g used, would not have caused any observable change i n the spectrum due to the d i r e c t p h o t o l y s i s of DMSO. On the other hand, the broad s i n g l e t was reduced i n i n t e n s i t y and the s a t u r a t i o n e f f e c t s at high microwave power (> 1.0 mW) were l e s s pronounced suggesting that the observed resonance change was due to the p a r t i a l b l e a c h i n g of these two paramagnetic s p e c i e s . 2. DMSO-Water M a t r i c e s 2.1 y - I r r a d i a t e d P o l y c r y s t a l l i n e Ĥ O In order to analyze the esr s p e c t r a of b i n a r y m i x t u r e s , i t i s e s s e n t i a l to have a d e t a i l e d knowledge of the type and s t a b i l i t y of the resonance p a t t e r n induced i n each of the pure components. The resonance spectrum of Y - i r * " a d i a t e d p o l y c r y s t a l l i n e water has been e x t e n s i v e l y s t u d i e d and the l i n e assignments are reasonably w e l l u n d e r s t o o d . T h e spectrum observed i n the present study i s shown i n Figure 59 and agrees w i t h t h a t observed by other authors. The "water resonance" i s c h a r a c t e r i z e d by a d i s t i n c t doublet s p l i t by about 40 gauss ( l i n e w i d t h ^ 12 gauss) and centered around g = 2.008. The broad "hump" to 20 gauss Figure 59. Electron spin resonance spectrum of y-irradiated polycrystalline ice at 77°K. Resonance pattern corresponds to that of the "OH radical. - 195 - the low f i e l d s i d e of the doublet had p r e v i o u s l y been a t t r i b u t e d to such paramagnetic species as R^o"*", Ĥ O or RG^*. However subsequent i n v e s t i g a t i o n s have shown that the e n t i r e esr spectrum may be assigned 115 to an a n i s o t r o p i c *0H r a d i c a l . Only the h y d r o x y l r a d i c a l i s observed by e s r i n y - i r r a d i a t e d i c e at 77 PC. Other paramagnetic species produced i n the r a d i o l y s i s are e i t h e r too unstable and immediately r e a c t w i t h the m a t r i x (H* atoms) or t h e i r y i e l d i s too low to be observed ( e t > H02*)» 2.2 y - I r r a d i a t e d DMSO-H^O Mix t u r e s E l e c t r o n s p i n resonance s t u d i e s were made on a s e r i e s of ten mixtures ranging from 0.01 to 0.89 mole f r a c t i o n DMSO. Table V gives the mixtures s t u d i e d and the type of matrix (amorphous or p o l y c r y s t a l l i n e ) formed i n each system when "shock cooled " i n l i q u i d n i t r o g e n by the " b a l l technique". Only two mixtures corresponding to 0.20 and 0.39 mole f r a c t i o n DMSO formed g l a s s y (amorphous) b a l l s . The sample b a l l s were y - i r r a d i a t e d i n the dark to a t o t a l absorbed dose of 0.96 Mrad. A l l p h o t o l y s i s or photobleaching experiments were performed w i t h the dewar i n the spectrometer c a v i t y . When using the u l t r a v i o l e t l i g h t source (Hg lamp), the b l e a c h i n g time was always l e s s than 20 minutes; consequently the observed changes i n the spectrum were not due to the p h o t o l y t i c decomposition of the DMSO molecules. Two d i s t i n c t resonance p a t t e r n s were observed f o r the composition range s t u d i e d and were not a simple combination of the pure components. At high DMSO concentrations (0.89, 0.80, 0.69 and 0.67 mole f r a c t i o n DMSO) the y - i r r a d i a t e d m atrices produced resonance s p e c t r a s i m i l a r to - 196 - TABLE V. Summary of data obtained from. s t u d i e s on Y - l r r a d i a t e d DMSO-water matrices at 77°K. Mole F r a c t i o n DMSO Volume f r a c t i o n DMSO M a t r i x at 77°K Sample co l o u r a f t e r y - i r r a d i a t i o n R a d i c a l s 0 observed 1.0 1.0 p o l y c r y s t a l l i n e y e l l o w t i n g e .X, 'SX.-CH^ (DMS0) + 0.89 0.97 p o l y c r y s t a l l i n e y e l l o w t i n g e .X,'SX,«CH3, (DMS0) + 0.80 0.94 p o l y c r y s t a l l i n e y e l l o w t i n g e •X,«SX,-CH3, (DMS0) + 0.69 0.90 p o l y c r y s t a l l i n e p urple t i n g e •X, »SX, *CH 3 > (DMS0) + 0.67 0.89 p o l y c r y s t a l l i n e p u r p l e t i n g e •X,-SX,'CH3, (DMS0) + 0.39 0.71 amorphous pu r p l e •SX,'CH 3,(DMS0) + 0.20 0.50 amorphous p u r p l e •SX,'CH 3,(DMS0) + 0.11 0.33 p o l y c r y s t a l l i n e p urple t i n g e •SX,*CH 3,(DMS0) + 0.06 0.20 p o l y c r y s t a l l i n e p urple t i n g e •SX,'CH 3,(DMS0) + 0.03 0.10 p o l y c r y s t a l l i n e c o l o u r l e s s •SX,*CH3,(DMS0)+,'0H 0.01 0.04 p o l y c r y s t a l l i n e c o l o u r l e s s •SX,'CH3,(DMS0)+,-OH Volume f r a c t i o n approximately equal to f r a c t i o n dose absorbed i n i t i a l l y by DMSO i n mixture. P u r p l e centre turned y e l l o w upon b l e a c h i n g w i t h u l t r a v i o l e t and v i s i b l e l i g h t . •X = paramagnetic species centered at g = 2.006, r e a d i l y power sa t u r a t e d above 1.0 mW and photobleached w i t h u l t r a v i o l e t l i g h t . •SX= asymmetric s u l f u r r a d i c a l w i t h g^ = 1.988, = 2.004 and g 3 = 2.017. - 197 - those observed f o r pure DMSO. The f i r s t two mixtures had a yellow tinge a f t e r Y ~ i r r a d i a t i o n whereas the l a t t e r two had a purple tinge. The paramagnetic species responsible f o r the satur a t i o n e f f e c t s at low microwave power (hereafter c a l l e d *X f o r brevity) were s t i l l present although t h e i r c o n t r i b u t i o n to the o v e r a l l spectrum diminished as the water content increased. This i s indic a t e d i n Figure 60 i n which the region of the spectrum where *X resonates shows the same behaviour upon power sa t u r a t i o n (pure DMSO) as with addition of water (DMSO-water mixtures). The remaining part of the esr spectra of these four mixtures are compatible with Figure 57 (pure DMSO). When the i r r a d i a t e d samples were photobleached f o r 20 minutes with u l t r a v i o l e t l i g h t , the broad asymmetric s i n g l e t and *X disappeared completely and the purple colour was replaced by a yellow tinge. The bleaching was accompanied by a s l i g h t increase 20%) i n the methyl r a d i c a l y i e l d . These features are shown i n Figure 61 f o r 0.80 mole f r a c t i o n DMSO and are representative of the other three mixtures. The methyl r a d i c a l quartet and the asymmetric " s u l f u r p a t t e r n " are c l e a r l y evident a f t e r photobleaching (Figures 61(b) and ( c ) ) . The remaining y - i r r a d i a t e d mixtures (0.39, 0.20, 0.11, 0.06, 0.03 and 0.01 mole f r a c t i o n DMSO) gave well-resolved esr spectra showing the broad asymmetric s i n g l e t , the c h a r a c t e r i s t i c " s u l f u r p a t t e r n " and the methyl r a d i c a l quartet. However there was no evidence of "X as ind i c a t e d by the power satu r a t i o n behaviour of the spectra. T y p i c a l esr spectra of these mixtures are given i n Figure 62 f o r 0.20 mole f r a c t i o n DMSO. Traces of the hydroxyl r a d i c a l were observed i n the le a s t concentrated DMSO mixtures (0.03 and 0.01 mole f r a c t i o n DMSO), (a) Figure 60. Resonance p a t t e r n showing behaviour of *X w i t h (a) i n c r e a s i n g water composition (microwave power 0.42 mW) and (b) i n c r e a s i n g microwave power (pure DMSO). Numbers corresponding to s p e c t r a on l e f t r e f e r to mole f r a c t i o n DMSO. The arrows r e f e r to g . Figure 61. E l e c t r o n resonance s p e c t r a of p o l y c r y s t a l l i n e y - i r r a d i a t e d DMSO- water mixture (0.80 mole f r a c t i o n DMSO) at 77°K. (a) microwave power 0.42 mW; (b) a f t e r b l e a c h i n g w i t h u l t r a v i o l e t l i g h t f o r 20 minutes, microwave power 0.42 mW; (c) same as ( b ) , microwave power - 200 - Figure 62. E l e c t r o n s p i n resonance s p e c t r a of y - i r r a d i a t e d DMSO-water gl a s s (0.20 mole f r a c t i o n DMSO) at 77°K. The " s u l f u r p a t t e r n " and methyl r a d i c a l quartet are r e a d i l y observed, (a) microwave power 0.42 mW; (b) microwave power 10 mW. - 201 - e s p e c i a l l y at high power, and the asymmetric s i n g l e t was not as pronounced (see Figure 63). The i r r a d i a t e d sample b a l l s of these l a t t e r two mixtures were c o l o u r l e s s whereas the other f o u r samples were s l i g h t l y p u r p l e . The pu r p l e c o l o u r and broad asymmetric s i n g l e t were r e a d i l y photobleached w i t h u l t r a v i o l e t and v i s i b l e l i g h t and the r e s u l t i n g s p e c t r a were i d e n t i c a l to that shown i n Figure 61(b). The methyl r a d i c a l c o n c e n t r a t i o n showed a s i m i l a r i n c r e a s e upon b l e a c h i n g and the b a l l s had a y e l l o w t i n g e . I t i s i n t e r e s t i n g that the h y d r o x y l r a d i c a l s were only observed a t the h i g h e s t water concentrations (0.97 and 0.99 mole f r a c t i o n water) d e s p i t e the f a c t that t h e i r y i e l d i n Y - i r r a d i a t e d i c e a t 77°K i s f a i r l y h i g h (G(-OH) = 0.8).^"^ E l e c t r o n s p i n resonance s t u d i e s on the l i q u i d s t a t e have shown that *0H r a d i c a l s r e act a t a d i f f u s i o n - c o n t r o l l e d r a t e w i t h DMSO to produce methyl r a d i c a l s by cleavage of 116—118 the C-S bond. The mechanism proposed i s given by equation (46). 0 0 - 0 II I II "OH + CH-SCH. [CH--S-CHJ CH.S0H + 'CH- (46) J J J | J j J OH I t i s suggested here that 'OH r a d i c a l s formed i n the r a d i o l y s i s of the DMSO-water mixtures r e a c t w i t h DMSO even at 77°K. This i n f e r e n c e i s supported by the o b s e r v a t i o n that h y d r o x y l r a d i c a l s generated by the photodecomposition of hydrogen peroxide reacted w i t h DMSO to give an e s r spectrum composed of a methyl r a d i c a l q u a r t e t and a broad, u n i d e n t i f i e d s i n g l e t (AH ^ 40 gauss) centered near g = 2.008. The gl a s s y sample was prepared from equal p a r t s (by volume) DMSO and 15% hydrogen peroxide and photolysed at 77°K w i t h u l t r a v i o l e t - 202 - Figure 63. E l e c t r o n s p i n resonance s p e c t r a of y - i r r a d i a t e d p o l y c r y s t a l l i n e DMSO-water mixture (0.01 mole f r a c t i o n DMSO) at 77°K. (a) micro- wave power 0.42 mW; (b) microwave power 10 mW. The low f i e l d "hump" and doublet of the 'OH r a d i c a l s are evident at 10 mW power (see Figure 59). - 203 - light. Another possibility i s that energy and charge transfer from the water to the DMSO molecules might be occurring in these mixtures at 77°K. Although these processes in the liquid state were discounted on the basis of the pulse radiolysis data discussed earlier, these transfer processes may be quite efficient in the solid state at low temperature. The esr spectra indicate there are at least four distinct paramagnetic species (in addition to the *0H radical) produced by the y-irradiation of DMSO-water mixtures and trapped in the matrices at 77°K. The methyl and asymmetric sulfur radicals are readily identified by their characteristic spectra and, as w i l l be shown in the next section, are formed primarily by the reaction of electrons with DMSO. The identities of the other two paramagnetic entities are less certain . As mentioned already *X i s only present in the polycrystalline media of high DMSO composition and i t s contribution to the resonance pattern decreases as the water content increases (see Figure 60). The radical i s easily power saturated, photobleached with v i s i b l e or ultraviolet light and i s centered at g = 2.006. Since a l l known trapped electrons have isotropic g-values less than the free spin 22 value (g = 2.0023), i t i s unlikely that *X i s a trapped electron. However this cannot be taken as distinct evidence for the absence of trapped electrons. Theoretical studies indicate that electrons may be bound in the f i e l d of a stationary dipole moment provided the 119 strength of the dipole moment is greater than 1.6 D. DMSO has a f a i r l y large dipole moment (4.3 D) so that i t i s theoretically possible for electrons to be trapped in crystalline DMSO at low temperatures - 204 - where the dipoles are not free to rotate. The c r y s t a l s tructure of DMSO has been reported and the data i n d i c a t e a strong coupling between a p a i r of DMSO molecules i n which t h e i r dipoles are aligned i n 120 opposite d i r e c t i o n s . Perhaps electrons are i n i t i a l l y trapped i n these strong d i p o l a r f i e l d s but immediately form DMSO r a d i c a l anions due to the presence of vacant low-lying d o r b i t a l s present on the s u l f u r atoms. Thus the DMSO r a d i c a l anion or i t s decomposition product could be responsible f o r the resonance c h a r a c t e r i s t i c s of *X. I f the s t a b i l i t y of these r a d i c a l anions i s governed by the dipo l a r i n t e r a c t i o n s associated with the DMSO molecules, then the addit i o n of water may lower the r a d i c a l s a c t i v a t i o n energy f o r reaction by breaking up these strong d i p o l a r f i e l d s . This could explain why *X decreases as the water concentration increases. The broad asymmetric s i n g l e t observed i n pure DMSO and i n a l l binary mixtures i s a t t r i b u t e d to DMSO p o s i t i v e ions f o r two reasons. F i r s t l y , the esr spectrum obtained when electrons were generated i n a DMSO-water glass (0.20 mole f r a c t i o n DMSO) by the photoionization of K.Fe(CN),(see below) i s comparable to that of the corresponding y - i r r a d i a t e d glass (Figure 62) except f o r the absence of the broad asymmetric s i n g l e t and purple colour. This absence suggests that the s i n g l e t i s due to an oxidized paramagnetic species produced by the i o n i z i n g r a d i a t i o n . Secondly, the purple centre and s i n g l e t show the same behaviour towards photobleaching i n f e r r i n g they correspond to the same species. It was shown i n the previous chapter that the absorption spectrum of a purple centre i s due to the DMSO p o s i t i v e ions. The absence of a d i s t i n c t purple tinge at the highest DMSO compositions (1.0, 0.89 and 0.80 mole f r a c t i o n DMSO) may be a y i e l d e f f e c t . I f the r e a c t i v i t y of the thermalized electrons i s enhanced by the presence of water, as suggested by studies on the l i q u i d mixtures, then the - 205 - f r a c t i o n of DMSO p o s i t i v e ions escaping recombination i n the p o l y - c r y s t a l l i n e s t a t e i s expected to inc r e a s e w i t h i n c r e a s i n g water composition. The increase i n methyl r a d i c a l s upon photobleaching the y - i r r a d i a t e d samples could be a t t r i b u t e d to the photodecomposition of the DMSO p o s i t i v e i o n according to equation (47). (CH 3SOCH 3) + + hv *- CH 3* + CH 3S0 + (47) U n f o r t u n a t e l y the e s r s p e c t r a corresponding to the p o s i t i v e i o n s and methyl r a d i c a l s are not w e l l - r e s o l v e d so t h a t i t i s i m p o s s i b l e to see i f the decay and b u i l d up of the r e s p e c t i v e r a d i c a l s are comparable. 2.3 P h o t o i o n i z a t i o n of K^Fe(CN)^ i n Aqueous Glasses E l e c t r o n s may be generated i n a system by exposing the sample to i o n i z i n g r a d i a t i o n but they can a l s o be formed by p h o t o l y s i s of s e v e r a l s o l u t e s at wavelengths w i t h i n t h e i r c h a r g e - t r a n s f e r - t o - s o l v e n t bands. The l a t t e r process i s p a r t i c u l a r l y u s e f u l when studying the r e a c t i o n s of e l e c t r o n s i n s o l i d m atrices a t low temperatures by esr s i n c e resonance i n t e r f e r e n c e by other r a d i c a l s produced by i o n i z i n g r a d i a t i o n may be e l i m i n a t e d . Ferrocyanide ions are o f t e n used as a reducing s o l u t e s i n c e they are e a s i l y p h o t o i o n i z e d by u l t r a v i o l e t l i g h t a t 254 nm according 121 t o equation (48). F e ( C N ) 4 4 _ + hv F e ( C N ) 6 3 ~ + e~ (48) In t h i s study, e l e c t r o n s were generated at 77°K by p h o t o l y s i n g w i t h a low pressure mercury resonance lamp g l a s s y samples c o n t a i n i n g 0.01 M - 206 - potassium ferrocyanide in the spectrometer cavity. Two different glassy systems were investigated. One consisted of an aqueous DMSO glass (0.20 mole fraction DMSO) and the other an aqueous alkaline glass (8 M sodium hydroxide) to which DMSO was added as a scavenger (1.0 M DMSO). Electrons stabilized in aqueous alkaline glasses at 77°K have been shown previously to exhibit an intense blue colour ( X m a x = 580 nm) and narrow esr singlet (AH ^ 16 gauss) centered at g = 2.0006."'"''""' Figure 65(a) shows the esr singlet obtained in the present study by the ultraviolet photolysis of the ferrocyanide ion in an aqueous 8 M sodium hydroxide glass at 77°K. The sample balls after photolysis were bright blue in colour. However trapped electrons were not observed i n either the DMSO-water glass (Figure 64) or the alkaline glass containing 1.0 M DMSO (Figure 65(b)) upon photolysis, as indicated by the total absence of an esr singlet or colour centre i n these samples. The esr spectrum of the DMSO-water glass i s identical to the y-irradiated sample (Figure 62) except for the absence of the broad singlet attributed to DMS0+. The resonance pattern of the alkaline glass (Figure 65(b)) i s similar to Figures 62 and 64 except that the "sulfur pattern" appears to be more prominant. Similar spectra were obtained when an alkaline 8 M sodium hydroxide glass containing 1.0 M DMSO was Y - i r r a d i a t e d at 77°K to an absorbed dose of 0.2 Mrad (Figure 66(b)). In this case not a l l the electrons were scavenged, as indicated by the blue tinge of the irradiated sample. Subsequent photobleaching with ultraviolet light eliminated the blue colour and caused a slight (^10-20%) increase in the methyl radical resonance (Figure 66(c)). It was not possible to discern i f the "sulfur pattern" changed in intensity with bleaching because of the Figure 64. Electron spin resonance spectrum obtained after photoionization at 77°K of 0.01 M K^Fe(CN) in 0.20 mole fraction DMSO-water glass (compare to Figure 62(a)). - 208 - (a) H > Figure 65. Ele c t r o n spin resonance spectra obtained for the photoionization of 0.01 M K 4Fe(CN) 6 i n 8 M NaOH glass at 77°K. (a) no DMSO added; (b) 1.0 M DMSO present i n glass. I I 10 gauss F i g u r e 66. E l e c t r o n s p i n resonance s p e c t r a obtained f o r y - i r r a d i a t i o n (0.24 Mrad) of 8 M NaOH g l a s s at 77°K. (a) no DMSO added; (b) 1.0 M DMSO added; (c) a f t e r photobleaching (b) w i t h u l t r a v i o l e t l i g h t f o r 20 minutes. - 210 - decay of the u n d e r l y i n g e l e c t r o n s i n g l e t . The e s r data shown i n Figures 64, 65 and 66 thus s t r o n g l y suggest th a t e l e c t r o n s react w i t h DMSO i n gl a s s y s o l i d s at 77°K, and that the r e a c t i o n produces methyl and s u l f u r r a d i c a l s . I t i s not known i f the u n i d e n t i f i e d s u l f u r r a d i c a l i s produced i n the same d i s s o c i a t i o n process as the methyl r a d i c a l s or by another competing process. Although the asymmetric " s u l f u r p a t t e r n " and g- f a c t o r s observed i n the Y - i r r a d i a t e d DMSO and DMSO-water matrices are i d e n t i c a l to those obtained i n the photolysed DMSO sample (Figure 56), t h i s s i m i l a r i t y does not n e c e s s a r i l y imply the s u l f u r resonance i s due to CH^SO. Other s u l f u r r a d i c a l s may be r e s p o n s i b l e f o r the observed resonance. For example, when p o l y c r y s t a l l i n e b a l l s of dimethyl s u l f i d e at 77°K were s i m i l a r l y photolysed by u l t r a v i o l e t l i g h t the CH_S» r a d i c a l s so produced gave almost i d e n t i c a l " s u l f u r patterns'.'. - 211 - REFERENCES 1. J.W.T. Spinks and R.J. 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