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Nanosecond pulse radiolysis studies. 1968

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NANOSECOND PULSE RADIOLYSIS STUDIES by GERALDINE ANNE KENNEY L i c e n t i a t e o f the Royal I n s t i t u t e of Chemistry, London 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE 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 A p r i l , 1968 i In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t 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 f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of Chemistry The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, B.C., Canada A p r i l 16, 1968 i i A b stract Nanosecond pulse r a d i o l y s i s s t u d i e s on the behaviour of e aq at high concentrations as a p r e l i m i n a r y t o the i n v e s t i g a t i o n of e aq* have shown that contrary to normal c l a s s i c a l homogeneous k i n e t i c s the e l e c t r o n decays i n i t i a l l y i n a f i r s t order manner, moving i n t o second order decay w i t h i n about 100 nanoseconds a f t e r the e l e c t r o n pulse. Further i n v e s t i g a t i o n s have shown that f o r a comparable time a f t e r the pulse the d i s t r i b u t i o n o f the absorbing species i s not homogeneous thus rendering any c l a s s i c a l k i n e t i c i n t e r p r e t a t i o n i n v a l i d . Q u a l i t a t i v e c a l c u l a t i o n s on the duration of the inhomogeneity were performed and the experimental r e s u l t s are i n reasonable agreement with t h e i r p r e d i c t i o n s , The f i r s t order decay that i s observed i s considered to be more than a random sequence of r e a c t i o n s and two p o s s i b l e models are t e n t a t i v e l y proposed to account f o r these events. Comparisons are made between t h i s work and others i n which inhomogeneity undoubtedly accounts f o r the unusually f a s t b i m o l e c u l a r rate constants f o r the primary decay e aq + e aq *• + 20H aq The deuterated e l e c t r o n was i n v e s t i g a t e d w i t h s i m i l a r conclusions. The rate constants evaluated from t h i s work were: i " aq k = 8.80 ± .80 x 10 6 s e c " 1 k 2 = 5.88 ± 1.20 x 1 0 1 0 M _ 1 s e c " 1 e d k = 8.14 ± .21 x 10 6 s e c " 1 . ,.11 .,-1 -1 k 2 = ^10 M sec i i i and the b i m o l e c u l a r rate constant, determined i n a homogeneous environ- ment, i s i n good agreement w i t h the accepted l i t e r a t u r e values f o r the probable r e a c t i o n s i n v o l v i n g e aq i n our system. iv Table of Contents Page I. I n t e r a c t i o n s o f I o n i s i n g R a d i a t i o n and Matter . 1 I n t r o d u c t i o n 1 Electromagnetic Rca-.diation ( i ) P h o t o e l e c t r i c e f f e c t 2 ( i i ) Compton e f f e c t 3 ( i i i ) P a i r production 3 Summary 4 P a r t i c l e Radiation ( i ) E l a s t i c c o l l i s i o n s 7 ( i i ) I n e l a s t i c c o l l i s i o n s 7 ( i i i ) Bremsstrahlung 7 E l e c t r o n Range 7' Cerenkov 10 Dose and Y i e l d 1 1 I r r a d i a t i o n of L i q u i d Water ( i ) (a) P h y s i c a l 12 (b) Energy t r a n s f e r e f f e c t s 14 ( i i ) Physicochemical 15 ( i i i ) Chemical 17 The Hydrated E l e c t r o n ( i ) Background 18' ( i i ) A p h y s i c a l model 20 ( i i i ) The fa t e of the hydrated e l e c t r o n 27 The Problem 29 V Page I I . The Technique of Pulse R a d i o l y s i s and K i n e t i c Spectroscopy The Radiation C e l l 32 The E l e c t r o n A c c e l e r a t o r 36 So l u t i o n s and Flow Techniques 37 The Detection System - O p t i c a l . 41 ( i ) The l a s e r 41 ( i i ) F i l t e r s and lenses 44 The Detection System - E l e c t r o n i c 45 ( i ) The p h o t o m u l t i p l i e r 45 ( i i ) O s c i l l o s c o p e and camera 48 The Grounding System 49 I I I . The Computation of Data and the Results The input data 52 Computation of Data 52 V a r i a t i o n o f o p t i c a l density with pathlength 56 The formation and decay of e aq a f t e r a 3 Nsec e l e c t r o n pulse . . 58 The formation and decay of e aq a f t e r a 50 Nsec e l e c t r o n pulse. . 63 The e f f e c t s of m u l t i p l e p u l s i n g 63 The e f f e c t of H 65 The e f f e c t o f an a l c o h o l 65 The e f f e c t of oxygen 66 The pinhole experiments 68 The deuterated e l e c t r o n 73 Int e r f e r e n c e phenomena 7 ? v i Page IV. Discussion and I n t e r p r e t a t i o n of the Results ( i ) The behaviour of e aq at high dose rates 83 ( i i ) A model r e l a t i n g to the d i s t r i b u t i o n o f the spurs i n space and time 86 ( i i i ) C a l c u l a t i o n s and the r e s u l t s 87 ( i v ) The b i m o l e c u l a r decay 91 (v) F i r s t order decay 92 Epilogue 95 References 97 v i i L i s t of Tables Page Table I. Rate Constants of some r a d i c a l r e a c t i o n i n r a d i o l y s e d water. . \- 28 Table I I . Bimolecular ra t e constants f o r the decay of e aq + e aq 30 Table I I I . Experimentally determined ra t e constants f o r e aq + e aq 61 Table IV. Data from the pinhole experiments 6p Table V. Data f o r the deuterated e l e c t r o n 76 Table VI. C a l c u l a t i o n s from the spur overlap model 88 v i i i L i s t o f I l l u s t r a t i o n s Page Diagram 1. Mass absorption c o e f f i c i e n t s f o r energy t r a n s f e r processes i n water 5 Diagram 2. Range of a monoenergetic beam of e l e c t r o n s i n an absorbing m a t e r i a l 9 Diagram 3. Sequence of events f o l l o w i n g the impact of a primary p a r t i c l e 13 Diagram 4. Absorption spectrum of the hydrated e l e c t r o n . . . . 23 Diagram 5. Energy models f o r the hydrated e l e c t r o n 26 Diagram 6. The a c c e l e r a t o r l a b o r a t o r y 31 Diagram 7. The p l e x i c e l l and components (photograph) 34 Diagram 8. The e l e c t r o n i c d e t e c t i o n system (photograph). . . . 40 Diagram 9. Showing the apparatus i n experimental p o s i t i o n s (photograph) 43 DiagranulO. Faraday cup; t y p i c a l pulsed waveforms from the two e l e c t r o n tubes 38 Diagram 11. The flow system (photograph) 46 Diagram 12. The grounding system 50 Diagram 13. O p t i c a l density as a f u n c t i o n of path length. . . . 57 Diagram 14. F i r s t and second order decay of e aq 59 Diagram 15a. F i r s t and Second order decay of e d 75 Diagram 15b. Some o s c i l l o s c o p e t r a c e s of e aq + s o l u t e s 62 Diagram 16. Second order decay of e aq 60 Diagram 17. The e f f e c t s of m u l t i p l e p u l s i n g on rate constants and experimental h a l f l i f e of e aq 64 Diagram 18. The e f f e c t s of s o l u t e s on the experimental h a l f l i f e o f the hydrated e l e c t r o n 67 Diagram 19. O p t i c a l d e n s i t i e s and k i n e t i c data from the pnhole experiments 71 i x L i s t of I l l u s t r a t i o n s ( c o nt). Page Diagram 20. O s c i l l o s c o p e t r a c e s of some i n t e r f e r e n c e s i g n a l s . . 78 Diagram 21. P i c t o r i a l r e p r e s e n t a t i o n of the r e s u l t s of the spur overlap c a l c u l a t i o n s 89 The as s i s t a n c e of Mr. D.A. Head i s acknowledged i n Diagrams 1, 2 and 20. X Acknowledgment I would l i k e to thank Dr. D.C. Walker f o r h i s valued advice, patience and encouragement during the course of t h i s research and i n the pr e p a r a t i o n of t h i s t h e s i s . 1 I n t e r a c t i o n s of I o n i s i n g R a d i a t i o n w i t h Matter The absorption o f high energy r a d i a t i o n s of e i t h e r p a r t i c l e or electromagnetic o r i g i n induces i o n i s a t i o n , e x c i t a t i o n and a s e r i e s of physicochemical processes a l l o f which occur i n the absorbing target m a t e r i a l . R a d i c a l s , ions and e x c i t e d species produced as a consequence of these primary e f f e c t s then i n t e r a c t with t h e i r molecular environment and give r i s e to the s t a b l e chemical products. Although termed " i o n i s i n g " these r a d i a t i o n s are not n e c e s s a r i l y r e s t r i c t e d to i o n i s i n g behaviour as w i l l become c l e a r during t h i s i n t r o d u c t o r y s e c t i o n . Some o f the aspects of the p h y s i c a l consequences of the i n t e r a c t i o n s o f r a d i a t i o n and matter w i l l be discussed as a b a s i s f o r the chemical r e a c t i o n s that u l t i m a t e l y occur. The energy of a ray or p a r t i c l e can be wholly or i n pa r t t r a n s f e r r e d to the medium through which the r a d i a t i o n i s passing. The mechanisms by which energy i s t r a n s f e r r e d t o , and then- d i s s i p a t e d i n the medium are to. some extent c h a r a c t e r i s t i c of the i n c i d e n t r a d i a t i o n , and to some extent dependent on the m a t e r i a l i t s e l f . For electromagnetic r a d i a t i o n s which w i l l now be discussed there are three main processes, each of which may dominate under s p e c i f i e d circumstances. Electromagnetic r a d i a t i o n As a photon impinges on the surface o f a target m a t e r i a l there may be a t r a n s f e r of energy r e s u l t i n g i n a change o f d i r e c t i o n and energy of the i n c i d e n t photon; a l t e r n a t i v e l y there may be simply a red u c t i o n i n the i n t e n s i t y o f the tr a n s m i t t e d r a d i a t i o n i n accordance with A l = -uIAx 2 where x i s the thickness of the absorbing m a t e r i a l , A l the l o s s of i n t e n s i t y and u the l i n e a r absorption c o e f f i c i e n t . For a non-monoenergetic beam of photons i n c i d e n t on the m a t e r i a l T T Hi i x T -u?x T -u^x I = I e H 1 + I e M Z + I ;e K J where (I.., u . . ) c h a r a c t e r i s e s a photon o f energy E... The three important i n t e r a c t i o n s to consider are ( i ) the p h o t o e l e c t r i c e f f e c t ( i i ) the Compton e f f e c t ( i i i ) p a i r production and of these only one under any given set of co n d i t i o n s w i l l c o n t r i b u t e s i g n i f i c a n t l y to the energy exchange processes. ( i ) The p h o t o e l e c t r i c e f f e c t describes the absorption of photons of low energy, 1 KeV < E.. < 500 KeV, by m a t e r i a l s o f high atomic number, Z, which r e s u l t s i n the simultaneous angular e j e c t i o n of a photoelectron whose energy i s given by E = E. - d> pe J In metals $ i s the work f u n c t i o n , f o r other media <j> i s g e n e r a l l y thought of as the bi n d i n g energy but i n e i t h e r case may be equal to or more than the i o n i s a t i o n p o t e n t i a l of the medium. The photoelectrons tend to be e j e c t e d at angles approaching 90° as the energy of the photon decreases. The sharp d i s c o n t i n u i t i e s observed on a g r a p h i c a l p l o t o f photon energies versus atomic absorption c o e f f i c i e n t s r e l a t e d to the bindi n g energy o f the e l e c t r o n s i n the d i f f e r e n t s h e l l s . Vacancies i n the inner s h e l l s a r i s i n g from a p h o t o e l e c t r i c process w i l l be f i l l e d by outer s h e l l e l e c t r o n s ; energy i s conserved w i t h the emission o f X-rays or low-energy 1 auger e l e c t r o n s . Coherent s c a t t e r i n g of the i n c i d e n t photon by the atomic e l e c t r o n s increases as the e l e c t r o n d e n s i t y o f the m a t e r i a l increases and i s g e n e r a l l y most pronounced at low photon energies. This s c a t t e r i n g occurs i n the same energy regions as the p h o t o e l e c t r i c e f f e c t but i s by comparison a weak i n t e r a c t i o n . ( i i ) The Compton e f f e c t describes p a r t i a l l y e l a s t i c c o l l i s i o n o f a photon with an e l e c t r o n during which the photon i s s c a t t e r e d w i t h a diminished energy E and the e l e c t r o n r e c o i l s with an increase i n energy E = E. - E r J As the s c a t t e r i n g i s angular the absolute value o f the r e c o i l energy E^ i s a f u n c t i o n o f the angular r e l a t i o n s h i p between the i n c i d e n t and s c a t t e r e d photons, and the ac c e l e r a t e d e l e c t r o n . E = Eo (1 + E o / m Q c 2 ) ( l - Cos-6-) Er has values o f zero (Er = Eo) and the maximum energy i s given by l e t t i n g 8 ->- 180° Er = { Eo } m a x i n or /c 1 + 0.25/Eo The Compton e f f e c t dominates r a d i a t i o n i n t e r a c t i o n s between i n c i d e n t photons of s e v e r a l MeV and m a t e r i a l s of high e l e c t r o n d e n s i t y , and photons of lower i n c i d e n t energies (20 KeV < E.. < 2MeV) f o r m a t e r i a l s of lower e l e c t r o n d e n s i t y . In a medium such as water Compton e f f e c t s have been observed over the range 30 KeV < E.. < 20 MeV. ( i i i ) p a i r production r e f e r s to the appearance o f a p o s i t r o n and e l e c t r o n at the disappearance of the i n c i d e n t photon ( i n the v i c i n i t y o f the atomic nucleus) whose energy i s converted i n t o the k i n e t i c energy 4 and r e s t mass of these two p a r t i c l e s . Photons of energy l e s s than 1.02 MeV cannot p a r t i c i p a t e i n t h i s process as the r e l a t i o n s h i p 2 Ep + E = E. - 2mc * e j must be s a t i s f i e d . Following the formation o f the p a i r the p o s i t r o n combines with an e l e c t r o n and two 0.51 MeV Y _ r a y s a r e simultaneously emitted. These are r e f e r r e d to as a n n i h i l a t i o n r a d i a t i o n . Summary The atomic cross s e c t i o n s (or absorption c o e f f i c i e n t s ) f o r a l l these processes i n c r e a s e w i t h i n c r e a s i n g e l e c t r o n d e n s i t y o f the m a t e r i a l i n v o l v e d . In a given medium and at low photon energies the most important e f f e c t i s the p h o t o e l e c t r i c phenomenon; at medium photon energies the Compton e f f e c t dominates any other processes and at high photon energies p a i r production i s the p r e v a i l i n g i n i t i a l energy t r a n s f e r mechanism. The mass absorption c o e f f i c i e n t s f o r these various processes i n water have been p l o t t e d against i n c i d e n t photon energies i n diagram I. The mass absorption c o e f f i c i e n t i s defined as — . where p i s the -3 de n s i t y i n grams cm , and f i s a c o r r e c t i o n f a c t o r f o r s c a t t e r i n g , fluorescence losses and bremsstrahlung emission. The tr u e mass absorption c o e f f i c i e n t i s e s s e n t i a l l y the sum of a l l the i n d i v i d u a l c o e f f i c i e n t s f o r the e f f e c t s discussed above: (see legend on diagram) ±E = X + — + — + i i p p p p p energy c o n t r i b u t i o n s from s c a t t e r e d photons are o f t e n neglected to a f i r s t approximation. At very high i n c i d e n t photon energies photonuclear r e a c t i o n s may occur o f the general form (y,n) or (y,p)• there i s a high 5 l 1 r E n e r g y (Mev) Diagram 1. Mass various absorption c o e f f i c i e n t s processes i n water. f o r the 6 t h r e s h o l d f o r these r e a c t i o n s and one can j u s t i f i a b l y assume that i n most cases they o f f e r no s i g n i f i c a n t c o n t r i b u t i o n to the t o t a l energy absorbed. P a r t i c l e r a d i a t i o n s As a high energy charged p a r t i c l e t r a v e l s through a medium energy may be t r a n s f e r r e d v i a d i r e c t c o l l i s i o n s and e x c i t a t i o n s w i t h the predominant appearance of i o n i s e d and e x c i t e d atoms and the production of r a d i a t i o n . At lower energies the p a r t i c l e s w i l l be in v o l v e d i n e l a s t i c s c a t t e r i n g and i n e l a s t i c c o l l i s i o n s to a more s i g n i f i c a n t degree. The i n t e r a c t i o n s of heavy p a r t i c l e s such as alpha p a r t i c l e s and those of e l e c t r o n s o f the same energy i n any absorbing medium can be only q u a l i t a t i v e l y compared because the e l e c t r o n s w i l l have a greater v e l o c i t y and as such cause less s p e c i f i c i o n i s a t i o n . With a beam o f high energy e l e c t r o n s as an i n c i d e n t r a d i a t i o n source the energy t r a n s f e r processes that must be considered are: ( i ) e l a s t i c s c a t t e r i n g , d e f l e c t i o n ( i i ) i n e l a s t i c s c a t t e r i n g and e x c i t a t i o n ( i i i ) emission o f bremsstrahlung r a d i a t i o n . There w i l l be a s t a t i s t i c a l variance i n the number of c o l l i s i o n s between the i n c i d e n t p a r t i c l e and the e l e c t r o n s of the medium and consequently i n the energy t r a n s f e r r e d during these events. This i s i n d i c a t e d by the ranges o f the p a r t i c l e s i n a given absorber, and i n the case of an e l e c t r o n (which can lose a s u b s t a n t i a l amount of i t s energy i n one c o l l i s i o n ) the spread i s even more pronounced. 7 ( i ) e l a s t i c s c a t t e r i n g arises from the d e f l e c t i o n o f the i n c i d e n t e l e c t r o n without l o s s of energy by the coulombic f i e l d s about the n u c l e i of the medium; the process i s most important f o r low energy e l e c t r o n s and mat e r i a l s of high e l e c t r o n d e n s i t y . Any large angle d e f l e c t i o n s w i l l be from n u c l e a r s c a t t e r i n g . ( i i ) i n e l a s t i c s c a t t e r i n g occurs when the i n c i d e n t e l e c t r o n ' i n t e r a c t s w i t h these coulombic f i e l d s of the medium and a large f r a c t i o n of the energy o f the i n c i d e n t e l e c t r o n may be t r a n s f e r r e d i n a s i n g l e c o l l i s i o n . Energy imparted to the e l e c t r o n o f the medium experiencing a c o l l i s i o n i s often s u f f i c i e n t to cause t h i s e l e c t r o n to take part i n secondary c o l l i s i o n s i t s e l f . These primary and secondary events both lead to i o n i s a t i o n and e x c i t a t i o n of the atoms i n the absorbing m a t e r i a l and are the dominant process f o r the energy loss of mfedl-um energy e l e c t r o n s <1 MeV below which bremsstrahlung emission i s not an important f a c t o r . ( i i i ) Bremsstrahlung r a d i a t i o n i s emitted to conserve energy and momentum r e l a t i o n s h i p s when a high speed e l e c t r o n i s deaccelerated i n the environment o f a nucleus of the atoms i n the absorbing m a t e r i a l and i s predominant mode of energy l o s s f o r el e c t r o n s >10 MeV. The r a t e -dE z^Z^ of l o s s of energy —3— i s p r o p o r t i o n a l to — — where z, Z are the dx m 2 charges on the p a r t i c l e and nucleus r e s p e c t i v e l y , and m i s the mass o f the p a r t i c l e . The range of the e l e c t r o n The track o f a charged p a r t i c l e passing through a medium w i l l not n e c e s s a r i l y be a s t r a i g h t one, and f o r e l e c t r o n s i n p a r t i c u l a r the 8 true range i s much greater than that determined ex p e r i m e n t a l l y , owing to the appreciable and d i v e r s i f i e d s c a t t e r i n g already discussed. However the average range i s s t i l l dependent on the r a t e of loss of energy of the i n c i d e n t e l e c t r o n -- r e f e r r e d to as the stopping power o f the m a t e r i a l with respect to that p a r t i c l e . In B e t h e l s formula below t h i s l o s s per u n i t length i s r e l a t e d to both the c h a r a c t e r i s t i c s o f the medium and the i n c i d e n t p a r t i c l e . J F 9 4 m v 2E >—tr- 9 - ( G £ ) = N . Z [ i n _ ° _ ( 2 / _ g 2 . ! + g ^ ) l n 2 + mQv 21 ( 1 - 3 ) c o l l e c t i v e + 1 - g 2 + 1_(1 - y/l - g 2 ) 2 ] ergs cm - 1 8 v = v e l o c i t y of i n c i d e n t p a r t i c l e , cm sec g = c i s the v e l o c i t y of l i g h t , cm sec ^. I = mean e x c i t a t i o n p o t e n t i a l of the atoms, ergs. N = number of atoms per c.c. o f stopping m a t e r i a l . e = charge on e l e c t r o n , esu. m = r e s t mass o f the e l e c t r o n , grams. o • Z = atomic number of the stopping m a t e r i a l . dE The stopping power i s - (-5—) ,, . . , which d i v i d e d by the r r 6 r Mx ; c o l l e c t i v e 1 d e n s i t y of the m a t e r i a l p gives the mass stopping power. The lower the i n c i d e n t energy of the p a r t i c l e the more r e a d i l y w i l l i t reach thermal energies. In diagram 2 the number o f i n c i d e n t monoenergetic e l e c t r o n s that penetrate to a c e r t a i n p o s i t i o n i n the absorbing m a t e r i a l has been p l o t t e d as a f u n c t i o n o f dista n c e . The t a i l o f the curve represents the d i s t r i b u t i o n o f ranges and the mean range R i s shown with the 6 • m ext r a p o l a t e d range R e x> the l a t t e r value being the one used i n e x p e r i - mental work. ( I t i s only p o s s i b l e to determine a maximum range f o r T r 1 r 01 0-2 0-3 0-4 Distance Diagram 2. Range o f a beam o f monoenergetic e l e c t r o n s in an absorber 10 non-monoenergetic beams by means of a pe n e t r a t i o n study.) As the e l e c t r o n spends a time i n v e r s e l y p r o p o r t i o n a l to i t s v e l o c i t y w i t h i n an i n t e r a c t i o n d i s t ance o f the atomic e l e c t r o n s any changes i n i t s momentum and energy t r a n s f e r must be accomplished w i t h i n t h i s p e r i o d . However i n r e l a t d v i s t i c regions the k i n e t i c energy of the p a r t i c l e exceeds i t s rest-mass energy (6 -»• 1) and s i g n i f i c a n t 2 v a r i a t i o n s i n the In (1-3 ) term lead to an increase i n the s p e c i f i c i o n i s a t i o n (-dE/dx) . . p a r a l l e l i n g the increase i n v: the i n t e r -v ^ c o l l e c t i v e r 6 ' p r e t a t i o n of t h i s i s based on the apparent s h o r t e r time o f c o l l i s i o n (thereby i n c r e a s i n g the impact parameters) due to the Lorentz c o n t r a c t i o n . -dE At n o n - r e l a t i v i s t i c v e l o c i t i e s , (—j—) I T -̂ decreases as v inc r e a s e s . ' d x ^ c o l l e c t i v e Cerenkov r a d i a t i o n -13 The instantaneous ( w i t h i n 10 seconds) emission of l i g h t at wavelengths i n the I.R., v i s i b l e and U.V. during the i r r a d i a t i o n of various media with f a s t charged p a r t i c l e s i s c l a s s i f i e d as Cerenkov r a d i a t i o n and has s p e c i a l s i g n i f i c a n c e i n any study of the absorption and emission e f f e c t s i n i r r a d i a t e d l i q u i d s . The i n t e n s i t y and duration of the luminescence i s such that i t can mask weak or very short l i v e d emission or absorption and t h e r e f o r e appropfnte experimental precautions must be taken. The d u r a t i o n o f the emission i s determined by the v e l o c i t y o f the charged p a r t i c l e and the length o f i t s t r a c k before i t slows down to below-threshold c o n d i t i o n s . The emission arises from r e l a t i v i t i c c o n s i d e r a t i o n s . I f the v e l o c i t y v of a f a s t charged p a r t i c l e moving i n a medium of r e f r a c t i v e index n exceeds the phase v e l o c i t y o f l i g h t (—) i n the same medium, then an electromagnetic shock wave i s produced at an angle to the tr a c k o f the p a r t i c l e as a r e s u l t o f the 11 p o l a r i s a t i o n and subsequent r e l a x a t i o n of the molecules i n the immediate v i c i n i t y o f the t r a c k . The threshold c o n d i t i o n i s t h e r e f o r e ng>>l where g i s the r e l a t i v i s t i c v e l o c i t y \ The angle 6 at which the l i g h t i s emitted i s r e l a t e d to the tr a c k d i r e c t i o n Cos 0 = and f o r water i f n = 1.332, 6 w i l l be 0.751 corresponding to 265 KeV el e c t r o n s f o r the t h r e s h o l d p a r t i c l e energy. Dose and y i e l d The y i e l d o f a species formed or destroyed i n the chemical processes induced by r a d i a t i o n i s expressed i n terms of the number of molecules per u n i t o f absorbed energy. g _ number of molecules o f x formed (x) 100 eV absorbed energy E v a l u a t i o n o f the absorbed energy i s th e r e f o r e c r i t i c a l and a range of techniques are a v a i l a b l e to measure t h i s energy. In dosimetry the common u n i t of absorbed dose i s the rad. One rad i s equivalent to 13 the d e p o s i t i o n o f 100 ergs or 6.24 x 10 eV per gram of m a t e r i a l . I r r a d i a t i o n o f l i q u i d water The r a d i a t i o n chemistry of l i q u i d water i s a remarkably complex subject whose t h e o r e t i c a l tenets based on p h y s i c a l and chemical evidence are not f u l l y understood even at the present moment. To emphasise the changing environment i n the v i c i n i t y o f the i n c i d e n t charged p a r t i c l e as i t t r a v e l s through the l i q u i d medium t h i s s e c t i o n 12 w i l l be presented i n three c h r o n o l o g i c a l stages f o l l o w i n g the impact of the primary p a r t i c l e . -18 -16 ( i ) (a) p h y s i c a l 10 to 10 seconds (b) energy t r a n s f e r e f f e c t s -13 -7 ( i i ) physicochemical 10 to 10 seconds _7 ( i i i ) chemical >10 seconds In view of the exponential increase i n the number of papers on the r a d i a t i o n e f f e c t s i n b i o l o g i c a l l y important molecules and i n v i t r o systems where water plays a major r o l e , i t would be j u s t i f i e d to complete the sequence with 7 (i v ) b i o c h e m i c a l , b i o l o g i c a l >10 seconds,even i f d i s c u s s i o n as such i s beyond the scope of t h i s t h e s i s . The sequence i s i l l u s t r a t e d i n diagram 3. ( i ) a. Stage I - p h y s i c a l The absorption of r a d i a t i o n i s a very f a s t process and does not produce a simultaneous response that can be experimentally observed other than Cerenkov r a d i a t i o n . The primary e l e c t r o n (or Compton e l e c t r o n i n the case of i n c i d e n t X or yvrays) f o l l o w s a l i n e a r t r a c k at high energies but i s d e f l e c t e d as i t slows down; secondary i o n i s a t i o n s and 2 e x c i t a t i o n s are produced along t h i s t r a c k g i v i n g r i s e t o both low (£10 eV) 2 and high (>10 eV) secondary e l e c t r o n s . The l a t t e r , i f s u f f i c i e n t l y e n e r g e t i c w i l l branch o f f to form another s h o r t e r t r a c k r e f e r r e d to as a § ray, along which the remaining energy w i l l be deposited. The low energy secondary e l e c t r o n s s u f f e r f u r t h e r d e f l e c t i o n s causing i o n i s a t i o n and e x c i t a t i o n i n a defined area of the medium. The energy of the primary p a r t i c l e i s thus deposited along these 13 Impact of Primary P a r t i c l e 0 sees 10 sees 10 sees 10 sees 10 sees 10 sees i n " 7 10 sees P h y s i c a l Stage e high energy e sub e x c i t e d Physicochemical Stage e thermal e s o l v a t e d e r e a c t s m s chemical stage Diagram 3: Sequence of events a f t e r the impact of the i n c i d e n t p a r t i c l e (41) 14 tracks as a number of events or spWs; about 60% o f the t o t a l energy i s t r a n s f e r r e d to the medium i n t h i s way. A spur i s considered to be an average d e p o s i t i o n of 100 eV and w i l l be a c l u s t e r o f e l e c t r o n i c a l l y e x c i t e d s p e c i e s , p o s i t i v e ions and e l e c t r o n s , a l l o f which now enter the physicochemical stage. ( i ) b. Energy t r a n s f e r e f f e c t s The distance between the spurs i s r e l a t e d to the nature of the primary p a r t i c l e and i s c a l c u l a b l e from a knowledge of the rate o f energy t r a n s f e r to the medium. The l i n e a r energy t r a n s f e r or L.E.T. fo r m a l l y expresses t h i s as a rat e of energy l o s s of the i n c i d e n t p a r t i c l e i n eV o per A, and puts the energy loss f o r d i f f e r e n t p a r t i c l e s on a q u a n t i t a t i v e b a s i s . The L.E.T. increases along the tr a c k as the p a r t i c l e slows down implying that the spurs become c l o s e r together towards the end o f a t r a c k - - and w i t h high L.E.T. t r a c k s the spurs w i l l n e c e s s a r i l y overlap. Since the chemical products of r a d i o l y s i s are governed by the behaviour o f the species formed i n the spur, and the s p a t i a l d i s t r i b u t i o n of the spurs by the L.E.T. c h a r a c t e r i s t i c s of the p a r t i c l e , L.E.T. e f f e c t s are extremely important. 60 ° For a t y p i c a l Co y-ray the L.E.T. i s 0.02 eV per A which i s small and the spurs w i l l be r e l a t i v e l y i s o l a t e d . Low energy e l e c t r o n s have L.E.T. values increased by an order o f magnitude while a heavy o alpha p a r t i c l e has a high L.E.T. of ̂  10 eV per A. The model of spurs, t r a c k s and y-rays has been elaborated (6,7) i n an attempt to give a more r e a l i s t i c p i c t u r e o f the a r b i t r a r i l y c l a s s i f i e d secondary e l e c t r o n s i n the l i g h t of these d i f f e r e n t L.E.T. e f f e c t s . 15 An energetic secondary e l e c t r o n or d - e l e c t r o n of energy gr e a t e r than 5000 eV w i l l form i t s own branch t r a c k and because of the low L.E.T. the spurs w i l l be w e l l separated, of the order 10^ A apart. As the§- e l e c t r o n loses energy the L.E.T. increases and at energies below 5000 eV the spurs begin to overlap g i v i n g r i s e to a short t r a c k . F i n a l l y as the ^ - e l e c t r o n approaches energies of 500 to 100 eV i t becomes d i f f i c u l t f o r the e l e c t r o n or products of i t s i o n i s a t i o n power to move f a r from t h e i r o r i g i n , thus g i v i n g r i s e to a densely packed area of i o n i s a t i o n and e x c i t a t i o n w i t h a v i s u a l i s e d pear-shape geometry, often r e f e r r e d to as a blob. The secondary e l e c t r o n s w i t h energies below 100 eV w i l l form c l u s t e r s of i o n i s a t i o n s i n i s o l a t e d spurs. As the molecular products, formed i n i n t r a s p u r r e a c t i o n s or on the recombination of i o n p a i r s w i t h i n the spur, d i f f e r i n y i e l d from those a t t r i b u t e d to say r a d i c a l r e a c t i o n s between species d i f f u s i n g out of the spurs, i t i s not s u p r i s i n g that v a r i a t i o n s i n the i n t e n s i t y and o r i g i n o f the i o n i s i n g r a d i a t i o n lead to d i f f e r e n t y i e l d s . ( i i ) Stage I I - physicochemical The c l u s t e r of i o n i s e d and e x c i t e d species designated as a spur may lose energy by c o l l i s i o n and d i s s o c i a t i o n i n t o r a d i c a l s . Solvent -12 quenching of e x c i t e d species can occur w i t h i n 10 seconds and metastable s t a t e s are thought t o r e t u r n to the ground s t a t e by n o n - r a d i a t i v e processes. As about 30 eV i s expended on average per i o n - p a i r produced, a t y p i c a l spur w i l l c ontain three i o n - p a i r s or s i x r a d i c a l s . Energetic e l e c t r o n s have already been discussed but a sub-excited e l e c t r o n (<10 eV) can become thermalised and w i t h i n 10 1 1 seconds (the d i e l e c t r i c r e l a x a t i o n 16 time of water) s o l v a t e d . The p o s i t i o n of the thermalised e l e c t r o n w i t h respect to the p o s i t i v e parent i o n must be c a r e f u l l y considered -- i f the thermal energy of the e l e c t r o n exceeds the a t t r a c t i v e p o t e n t i a l energy r e s u l t i n g from the coulombic f i e l d of the concomittant p o s i t i v e i o n , the e l e c t r o n w i l l not be recaptured. The net separation of the i o n p a i r due to random walk i s c r i t i c a l and f o r aqueous s o l u t i o n s two opposing t h e o r i e s r e l a t i n g to the y i e l d s of molecular products are based on d i f f e r e n t evaluations of t h i s parameter. Obviously media of high d i e l e c t r i c constants w i l l permit the e l e c t r o n to t r a v e l f u r t h e r before t h e r m a l i s a t i o n . The Samuel-Magee model (8) assumes that recapture does occur i n the coulombic f i e l d , while the Lea-Platzman model permits the e l e c t r o n to escape before t h e r m a l i s a t i o n . Current s p e c t r o s c o p i c (10) and chemical evidence (11) tend to favour the l a t t e r model while m o d i f i c a t i o n s to the former accounts f o r some d i s c r e p a n c i e s , but there i s s t i l l a l a rge d i f f e r e n c e i n the estimates o f the mean free path of the escaping e l e c t r o n . The d i f f u s i o n o f r a d i c a l s begins i n t h i s stage and the spur i s t h e r e f o r e i n c r e a s i n g i n s i z e which r a t h e r i n v a l i d a t e s the accepted f i r s t approximation of regarding the spur as having s p h e r i c a l geometry. Unfortunately i t i s a l s o during t h i s p e r i o d that one requires good values f o r spur s i z e and overlap i n mechanistic c a l c u l a t i o n s . I f the species escapes from the spur i t must e v e n t u a l l y react with another r a d i c a l , s o l u t e or solvent molecules, perhaps at d i f f u s i o n c o n t r o l l e d r a t e s . A t h e o r e t i c a l treatment of such a o n e - r a d i c a l one s o l u t e problem has been given by Kupperman (12) and extended to more complex systems. The mechanisms occuring during the physiochemical stage are 17 H 20 v H 20 +.F , p o s s i b l y H 20* H 20 + + H 20 • H 30 + + OH H 20* »• H + OH e" thermal »• e aquated ( i i i ) Stage I I I - chemical Fluorescence from e l e c t r o n i c a l l y e x c i t e d s t a t e s may be observed and, as low energy e l e c t r o n s i n the £-ray area w i l l give r i s e to dis a l l o w e d e x c i t a t i o n s , phosphorescence on a delayed time s c a l e . Unimolecular r e a c t i o n s i n v o l v i n g a breakdown o f the o r i g i n a l molecule, rearrangements (H atom migration) b i m o l e c u l a r ion-molecule r e a c t i o n s , d i s s o c i a t i v e and n o n - d i s s o c i a t i v e charge t r a n s f e r (depending on the s t a b i l i t y o f the ions formed) are a l l p o s t u l a t e d to account f o r the r a d i o l y t i c products. The i r r a d i a t i o n o f l i q u i d water at pH 7 with a beam of high energy e l e c t r o n s gives r i s e t o the f o l l o w i n g products; the G values are i n b rackets. H 20 >-H ( 0 . 4 2 ) , H O 2 ( 0 . 7 1 ) , H(0.6), OH (2.2) eaq(2.3) and (H.OH, H 20*)? The f o l l o w i n g i n t r a spur mechanisms i n the previous stage lead to the product formation observed: f" aq + e" aq >• H 2 + 20H~aq H + H > H 2 OH + OH • H 2 0 2 H + OH »• H 20 H + e aq > H 2 + OH" I f the species d i f f u s e from the spur they can a l s o react w i t h solvent or s o l u t e molecules S 18 H,e" aq, OH + S • products. These are the main r e a c t i o n s i n the i r r a d i a t i o n of l i q u i d water although some others r e l e v a n t to the behaviour o f e x c i t e d water, i t s e l f a c o n t r o v e r s i a l specieS(13,14), have not been i n c l u d e d . The hydrated e l e c t r o n ( i ) In 1962 the p u b l i c a t i o n of a paper d e s c r i b i n g an e l e c t r o n pulse-induced broad absorption i n the v i s i b l e r e gion o f the spectrum which was p o s i t i v e l y a t t r i b u t e d to the hydrated e l e c t r o n , climaxed s e v e r a l years o f experiments and p r e d i c t i o n s i n d i f f e r e n t l a b o r a t o r i e s and opened up an e n t i r e l y new concept o f primary r e a c t i o n mechanisms. (In r e t r o s p e c t the now c l a s s i c paper o f Hart and Boag (15,16) not only c h a r a c t e r i s e d t h i s curious specieSbut a l s o w i t h the technique o f pulse r a d i o l y s i s and so l u t e scavenging provided the f i r s t r e a l b a s i s f o r the determination of absolute r a t e constants). Perhaps the success o f the fr e e r a d i c a l hypothesis o f Weiss (17) i n accounting f o r the i r r a d i a t i o n processes i n l i q u i d water was i n part the reason f o r the prolonged lack of i n t e r e s t i n the secondary e l e c t r o n s . However, i n 1953 Platzman (18) i n a t h e o r e t i c a l paper had questioned the r e a c t i o n e" u . + H_0 • H + OH" thermal 2 on the grounds that there was a s i g n i f i c a n t time delay between the r e a c t i o n time and the time r e q u i r e d to u t i l i s e the hy d r a t i o n energy which made the r e a c t i o n e n e r g e t i c a l l y f e a s i b l e . He continued " . . . f o r t h i s reason the e l e c t r o n becomes hydrated.... I mean (here) that the e l e c t r o n p o l a r i s e s the d i e l e c t r i c and i s bound i n a s t a b l e quantum s t a t e to i t there i s time f o r the h y d r a t i o n to take p l a c e , which must as 19 Dr. Onsager s a i d be a minimum of the r e l a x a t i o n time 10 seconds." The i m p l i c a t i o n s of Platzman's remarks were not f u l l y r e a l i s e d f o r many years although S t e i n (19) i n 1952 had suggested that the hydrated e l e c t r o n might be present i n an i r r a d i a t e d methylene blue system. By the l a t e f i f t i e s s u f f i c i e n t work had been reported to e s t a b l i s h ' the f a c t that there had to be two d i f f e r e n t reducing species i n i r r a d i a t e d watery: there was no other way of i n t e r p r e t i n g the anomalous k i n e t i c s i n the v a r i e t y of aqueous systems i n v e s t i g a t e d . These in c l u d e d the formic a c i d and f e r r o u s - c u p r i c (Hart (20)) the c h l o r o a c e t i c a c i d (Hayon"' § Weiss (21)) the peroxide (Barr $ A l l e n (22)) and the methanol- c u p r i c sulphate systems (Baxendale £ Hughes (23)). When the dominant reducing species i n i r r a d i a t e d n e u t r a l s o l u t i o n s was shown to have u n i t negative charge (Czapski § Schwarz (24) l a t e r confirmed by C o l l i n s o n et a l (25) and Dainton £ Watts (26)) the r o l e of the e l e c t r o n i n these systems was no longer s p e c u l a t i v e . The t r a n s i e n t absorption reported by Keene (27) i n h i s pulse r a d i o l y s i s experiments i n aqueous s o l u t i o n had been t e n t a t i v e l y a t t r i b u t e d to a hydrated e l e c t r o n (Matheson (28)) i n view of the f a c t that the absorption d i d not appear i n s o l u t i o n s c o n t a i n i n g e l e c t r o n scavenging s o l u t e s . The hydrated e l e c t r o n i d e n t i f i e d by Hart § Boag i n t h e i r s e n s i t i v e combination of f l a s h spectroscopy and pulse r a d i o l y s i s i s now known to be a chemical e n t i t y i n i t s own r i g h t with d i f f u s i o n and i n t e r - a c t i o n p r o p e r t i e s . In the s i x years s i n c e i t s discovery the r a t e constants f o r some s i x hundred of i t s r e a c t i o n s with i n o r g a n i c , organic and b i o - chemical systems have been compiled (29); most of these are d i f f u s i o n 20 c o n t r o l l e d r a t e s and are among the f a s t e s t r e a c t i o n s known. P a r a l l e l to the i n v e s t i g a t i o n s of e l e c t r o n - s o l u t e i n t e r a c t i o n s were experiments designed to produce the hydrated e l e c t r o n i n d i f f e r e n t ways f o r i t had been immediately recognised that many r e a c t i o n s p r e v i o u s l y a t t r i b u t e d to the hydrogen atom, at a s u i t a b l e pH, may have been i n t i t a t e d by the hydrated e l e c t r o n , i t s conjugate base. At the present time i t i s g e n e r a l l y accepted that e~ aq i s the precursor to hydrogen whenever water i s reduced; as a most powerful reducing s p e c i e , e" aq may be generated r a d i o l y t i c a l l y , photochemically, e l e c t r o l y t i c a l l y , by chemical r e d u c t i o n , from H atoms, by photo-induced e l e c t r o n emission from metals and from s t a b l e s o l v a t e d e l e c t r o n s i n other media (5). The deuterated e l e c t r o n has a l s o been prepared and i t s r e a c t i o n s s t u d i e d (30). ( i i ) A p h y s i c a l model The p r e c i s e sequence of events t h a t lead to the formation of a solvent sheaf about the e l e c t r o n and the nature o f the s t a b l e quantum s t a t e i n which the e l e c t r o n i s then trapped, continues to invoke much d i s c u s s i o n as the experimental evidence i s not unambiguous. Two d i f f e r e n t t h e o r i e s on the immediate f a t e of the thermalised e l e c t r o n s w i l l be b r i e f l y discussed. As an e l e c t r o n approaches thermal energies i t i s s t i l l moving through the water r e l a t i v e l y q u i c k l y and although i t a f f e c t s p o l a r i s a t i o n of the water molecules near the t r a c k i t i s not long enough i n the region to be trapped by t h i s p o l a r i s a t i o n ; i f there already e x i s t s a region of a c c i d e n t a l p o l a r i s a t i o n due to the random thermal motions of the molecules themselves then the water dip o l e s w i l l be p o l a r i s e d t o a more s i g n i f i c a n t degree i n the f i e l d of the excess 21 e l e c t r o n . In l i q u i d water both e l e c t r o n i c and o r i e n t a t i o n a l p o l a r i s a t i o n are important, the f i n i t e time a s s o c i a t e d w i t h the l a t t e r due to molecular r o t a t i o n . The d i e l e c t r i c r e l a x a t i o n time of water i s 10 seconds. S c h i l l e r (31) regards the d i e l e c t r i c r e l a x a t i o n time i n the t r a p p i n g procedure as the d e c i s i v e paramter. He assumes the t r a c k and spur model and considers the time-dependent d i e l e c t r i c p r o p e r t i e s of the medium with a non-conservative e l e c t r i c f i e l d ; the p r o b a b i l i t y of trapping the e l e c t r o n increases as the r e l a x a t i o n time (T ) i n c r e a s e s . I n v e s t i g a t i o n s on e l e c t r o n capture i n media of d i f f e r e n t x but s i m i l a r s t a t i c and o p t i c a l d i e l e c t r i c constants ( l i q u i d water, supercooled water and i c e ; v a r i a t i o n of 10^ i n T ) have supported h i s ideas (32). The other model df e l e c t r o n capture i s based on a time-independent d i e l e c t r i c constant and a s t a t i c e l e c t r i c f i e l d . Here freeman and Fayadh (33) use the c a v i t i e s present i n the l i q u i d s t r u c t u r e as the i n i t i a l t r a p p i n g centres, and suggest that the l i m i t i n g f a c t o r i n the m o b i l i t y of the e l e c t r o n i s the p h y s i c a l m i g r a t i o n of these c a v i t i e s . Experimental evidence i s given. At the c o n c l u s i o n , on e i t h e r model, the e l e c t r o n i s trapped i n a p o t e n t i a l w e l l w i t h p a r t i a l o r i e n t a t i o n o f a second and t h i r d l a y e r of water molecules i n the outer s o l v a t i o n sheaf, although thermal a g i t a t i o n w i l l cause these arrangements to be time-dependent. The number of water molecules i n the i n n e r solvent sheaf could be from four to s i x , i t i s not known, and there may be a range of b i n d i n g energies f o r the e l e c t r o n i n i t s t r a p , or traps of d i f f e r e n t depth. To what extent t h i s general p i c t u r e can be r e s p o n s i b l e f o r the broad absorption spectrum o f the hydrated e l e c t r o n , and e l e c t r o n s s o l v a t e d i n other media, i s open to s p e c u l a t i o n . 22 The absorption spectrum of the hydrated e l e c t r o n (see diagram ;o ° 4) i s evident at 5000A with a maximum at 7200A and shows no convergence o l i m i t at 5400A. Trapped e l e c t r o n s i n i c e have the same ;\ m ax' a n d f e a t u r e l e s s absorption spectrum although they are i n an ordered matrix but i t i s i n t e r e s t i n g to note an increase i n i n t e n s i t y i n the near u l t r a v i o l e t region has been reported (34). Yet i n another study (35) the appearance of a d i s t i n c t shoulder i n the lower wavelength region O o ^5500A to 6000A was observed f o r the absorption s p e c t r a o f some frozen aqueous s o l u t i o n s and c r y s t a l l i n e i c e . A narrowing of the absorption band was al s o seen down to 200°K by these workers; such behaviour has not been observed by others. I t may w e l l be that the mode of i n t r o d u c t i o n of the e l e c t r o n i n t o a s o l u t i o n or a frozen matrix may account i n par t f o r these d i s c r e p a n c i e s . The assymmetry of the spectrum o f the high energy side of the spectrum of e" aq has al s o been observed f o r ammonlated e l e c t r o n s , but n e i t h e r s p e c t r a shows, any discernible f i n e s t r u c t u r e . Dorfman has found a c o r r e l a t i o n between the s t a t i c d i e l e c t r i c constants and the values of A.max f o r e l e c t r o n s s o l v a t e d i n a s e r i e s of alc o h o l s (36). The e x t i n c t i o n c o e f f i c i e n t f o r e~ aq i n the region of A max i s 4 - 1 - 1 =10 l i t r e mole cm and thus the t r a n s i t i o n corresponding to the energy of'Xmax must be allowed. J o r t n e r (37) pub l i s h e d v a r i a t i o n a l c a l c u - l a t i o n s based on a quantum mechanical model which a t t r i b u t e s the Xmax to a (2p -*r Is) t r a n s i t i o n i n the p o t e n t i a l w e l l . He t r e a t e d the solvent as a continuous d i e l e c t r i c medium i n which an excess e l e c t r o n has been caught i n a s e l f - i n d u c e d p o l a r i s a t i o n t r a p . The e l e c t r o n i s incompletely confined to a c a v i t y of uniform f i e l d and experiences a decrease i n the E X T I N C T I O N C O E F F I C I E N T ( X 1 0 ~ 4 V 24 coulombic f i e l d as i t wanders outside the inner s o l v e n t sheaf i n t o the bulk medium. The parameters that define the trap are the s t a t i c and o p t i c a l d i e l e c t r i c constants. With hydrogenic wave funct i o n s he a n t i c i p a t e s the f i r s t e x c i t e d s t a t e to be a 2p s t a t e , and the (2p <- Is) t r a n s i t i o n to occur at an energy E(hv) = 1.35 eV, the o s c i l l a t o r s trength being f i = 1.1. The c a v i t y radius under these conditions'equates to zero. Experimentally Amax equates to 1.75 eV, f i = 0.8 and the radius of i n f l u e n c e o f the o hydrated e l e c t r o n to be 2.5 to 3.OA. The most recent determination i s o 2.9A (38). On t h i s hydrogen=type model treatment the f i r s t t r a n s i t i o n may correspond to three quarters of the w e l l depth (diagram 5 ( a ) ) . Higher t r a n s i t i o n s (3p -«- Is) w i l l be very weak i f they occur at a l l as f o r a one e l e c t r o n system the sum of the o s c i l l a t o r strengths has to be u n i t y ; the broad absorption band c e r t a i n l y shows no f i n e s t r u c t u r e but i t may be masking a very weak band. J o r t n e r ' s model encounters more complications i n e x p l a i n i n g the nature of the hydrated e l e c t r o n than the ammoniated e l e c t r o n (37). The temperature dependence of max f o r e~ aq was p r e d i c t e d to be -3.3 - 3 ' - 3 x 10 - dE_ max - -2.2 x 10 (eV per degree) and recent r e s u l t s (38) dT _ 3 6 Q report a s h i f t of -2.9-10 eV per degree i n Co y r a d i o l y s e d aqueous systems. The i n i t i a l o p t i c a l d e n s i t y o f Xmax at 10°C had decreased by 70% at 96°C. I t appeared that the radius of the hydrated e l e c t r o n was i n c r e a s i n g . However pulse r a d i o l y s e d systems i n d i c a t e d no such behaviour i n Xmax over the same temperature range. Chemical evidence of photo bleaching and p h o t o c o n d u c t i v i t y (39) ambiguously r e l a t e to the proposed 25 2p upper bound s t a t e of s o l v a t e d e l e c t r o n s and the appearance o f temperature dependent X max f o r other s o l v a t e d e l e c t r o n s outside the framework set by e" aq and e" ::ammn. s t r o n g l y suggests the need f o r a modifie d or a l t e r n a t i v e model. I t i s not intended to discuss these models i n d e t a i l but to emphasise the importance of the e x c i t e d s t a t e i n the a l t e r n a t i v e t h e o r i e s i n accounting f o r the t r a n s i t i o n s a s s o c i a t e d w i t h the observed A.max. The broadness, asymmetry and lack o f f i n e s t r u c t u r e must a l s o be accounted f o r . The energy of A max has been "likened? ' to an i o n i s a t i o n p o t e n t i a l (I ), to r e l e a s e of the e l e c t r o n from i t s t r a p . The hyd r a t i o n energy of the e" aq was c a l c u l a t e d to be 1.72 eV (40). The r e l a t i o n s h i p between any p o l a r i s a t i o n energy of the s o l v e n t , I and the hyd r a t i o n energy AH i s shown i n diagram (5b). V a r i a t i o n i n the s i z e of t r a p s , and thermal motions of the solvent sheaf i t s e l f may give r i s e to the broadness of the spectrum (5d). Even i f the traps had e s s e n t i a l l y the same immediate environment i n the ground s t a t e there could s t i l l be a wide v a r i a t i o n i n t h i s environment at e x c i t e d l e v e l s . The l a t t e r may i n f a c t be a continuum analogous to the conduction band of a semi conductor i n t o which the el e c t r o n s are e x c i t e d by wavelengths l e s s than X.max (5c). The p o s s i b i l i t y of a symmetrical charge t r a n s f e r absorption i n combination with an i o n i s a t i o n continuuii has a l s o been proposed i n a d i s c u s s i o n of these models i n (41). I f the hydrated e l e c t r o n were e x c i t e d i . e . photolysed at s u i t a b l e energies, i n t o t h i s u n s p e c i f i e d upper s t a t e and i t s behaviour followed through s u i t a b l e means one could d i s t i n g u i s h between the models through the changing co-ordinates of Amax; an attempt to e x c i t e the e l e c t r o n from t h i s s t a t e i n t o an even higher one would a l s o give v a l u a b l e 26 Diagram 5. Model r e l a t i n g E ^ ^ ^ to the Structure o'f the Hydrated E l e c t r o n 27 in f o r m a t i o n . The work described i n t h i s t h e s i s proved to be a necessary p r e l i m i n a r y b a s i s f o r experiments designed to study the nature o f the e x c i t e d s t a t e of the hydrated e l e c t r o n , the formation and decay of which may be f o l l o w e d s p e c t r o s c o p i c a l l y at wavelengths approaching Xmax. ( i i i ) Fate o f the hydrated e l e c t r o n The processes that c o n t r i b u t e to the l o s s o f the hydrated e l e c t r o n vary i n e f f i c i e n c y w i t h the dose r a t e o f the i o n i s i n g r a d i a t i o n , the pH of the medium, other species that may be present as i m p u r i t i e s o r scavengers and the temperature o f the s o l u t i o n s . H 20 • e" aq, H, OH, H , H ^ , H 30 +, 0H~ The f o l l o w i n g r e a c t i o n s may occur during and a f t e r the i r r a d i a t i o n ; only OH and H 2 do not react w i t h e aq. M-1 -1 pH Rate (29)M sec (1) e aq + e aq >- H 2 + 20H"aq 10.5 4.5-10 9 (2) e aq + OH • OH" aq 10.5 3.0-10 1 0 (3) e aq + H > H~ aq 10.5 2.5-10 1 0 (4) e aq + H 30 + • H + H 2 ° 4 - 5 2.32'10 1 0 (5) e aq + H 20 > H + OH" aq 8.4 1.6-10 1 (6) e aq + R 0 ^OH + OH" aq 7 1.23'10 1 0 With nanomolar concentrations o f e aq from low dose rates (50 rads per pulse) the decay i s a f i r s t order process; as the dose increases the [e aq] moves to the micromolar region and the decay now resembles second order k i n e t i c s . At higher doses the hydroxyl r a d i c a l s , hydroxonium ions and hydrated e l e c t r o n s are a l l i n s u f f i c i e n t concentra- t i o n to give c l a s s i c a l second order k i n e t i c s and the ra t e s are d i f f u s i o n c o n t r o l l e d . When [e aq] i s i n m i l l i m o l a r concentrations the t e c h n i c a l 28 s i t u a t i o n becomes more complex as the h a l f l i f e i s considerably reduced and the i n t e r p r e t a t i o n o f the k i n e t i c s i n a c l a s s i c a l sense appears ambiguous. The dominant decay mechanism i s (1) e aq + e" aq • H 2 + 20H - aq but r a d i c a l - r a d i c a l and r a d i c a l - i o n r e a c t i o n s account f o r n e a r l y h a l f of the t o t a l number of processes that must be considered before a r a t e constant f o r the primary decay can be evaluated. Fortunately not a l l of these r e a c t i o n s are k i n e t i c a l l y s i g n i f i c a n t under any given set of experimental conditions and with the j u d i c i o u s use of pH and scavenging solu t e s many of the r e a c t i o n s can be at l e a s t c o n t r o l l e d i f not el i m i n a t e d . The r a d i c a l r e a c t i o n s are l i s t e d i n Table I below Table I Reaction pH k" M 1 s e c 1 H + H • H 2.0 1.0-10 1 0 H + OH • H 20 3.0 1.2-10 1 0 OH + OH y H 20 2 7.0 4-10 9 H + H 2 0 2 • H 20 + OH 2.1 9-10 7 H 3 0 + + OH" —^2H 20 7.0 1.43-10 1 1 Under high dose r a t e c o n d i t i o n s the species i n n e u t r a l water that could appreciably i n t e r f e r e with the "pure" biomolecular decay are the hydroxyl r a d i c a l and the hydrogen atom. In the presence of a c a l c u l a t e d amount of methanol (or higher alcohol) the hydroxy r a d i c a l s w i l l be removed with the hydrogen atoms; i n both instances the r e l a t i v e l y i n e r t CH20H r a d i c a l i s produced. (7) H + CH30H • CH20H + H 2 K y = 1.7-10 6 M _ 1 s e c ' 1 (8) OH + CH OH • CHo0H + H o0 K D = 5.1-10 8 M" 1sec" 1 o Z I o In a d d i t i o n common i m p u r i t i e s such as d i s s o l v e d 0 2 or C0 2 i n 29 the l i q u i d w i l l have appreciable e f f e c t s on the i n i t i a l r a t e of decay of the hydrated e l e c t r o n and must be removed as much as p o s s i b l e . (9) OH + C0 3 • C0~ pH 11, K g = 3.0"-108 M _ 1 s e c _ 1 (10) e aq + 0 2 t 0~ pH 7, K = 2.10 1 0 M _ 1 s e c _ 1 The r e a c t i o n w i t h C0 3 i s p a r t i c u l a r l y undesirable as the CO^ o i o n absorbs at 6000A. Peroxides and other r a d i o l y t i c products are dea l t w i t h according to the p r e v a i l i n g experimental c o n d i t i o n s . In many instances however the e f f e c t i v e concentration of competing species does not decrease as r a p i d l y as the concentration of e aq (30). The Problem The predominant decay mechanism of the hydrated e l e c t r o n i n n e u t r a l water under high dose ra t e c o n d i t i o n s i s (1) e aq + e aq , • > ( e ) 2 aq >- H^ + 20H aq and would be described i n a c l a s s i c a l sense as a bi m o l e c u l a r decay. 2-(The t r a n s i e n t (e)„ , specieShas been i d e n t i f i e d i n s e v e r a l other 2 s o l v systems, l i q u i d , g l a s s y and s o l i d matrices (34,42,43,44) g e n e r a l l y through the absorption or e.s.r. spectra.) The rate constant f o r t h i s r e a c t i o n i s d i f f u s i o n c o n t r o l l e d but the l i t e r a t u r e contains an abundance of data that f i t s n e i t h e r a f i r s t order nor a second order k i n e t i c treatment. There would appear to be a tr a n s i t i o n from one mode of decay to another w i t h i n a very short time and thus the data not unexpectedly shows v a r i a t i o n s according to the r a d i a t i o n source, the duration and i n t e n s i t y of pulsed r a d i a t i o n and the speed with which the decay can then be followed. The d i f f e r e n t values f o r k'̂  were a l l w i t h i n an order of magnitude (see Table II:below) u n t i l the s t a r t l i n g l y high value o f 30 3.2 1 0 1 1 M 1 sec 1 was reported by K l e i n and Warner (45) i n 1966; t h i s i s the f a s t e s t r e a c t i o n i n the l i q u i d phase ever p u b l i s h e d , and they suggest that the model used to e x p l a i n the r a d i a t i o n chemistry of water at low dose rates i s inadequate at very high dose rates such as they had employed. Other values f o r and r e c e n t l y c a l c u l a t e d are given below. Table I I Reference Author K M 1 sec 1 Technique Dose Rate pH 45 Klein.Warner 3.2.10 1 1 p.r. ^ 1 0 2 5 e V A _ 1 >8 s e c ~ l 9 60 19 -1 38 G o t t s c h a l l . H a r t 6.3 ± 1.10 Co y ^10 eV/l >8 -1 sec ,9 46 Matheson.Rabani 5.5 ± 0.75.10 p.r. high 7-14 47 Dorfman.Taub <7.0.10 9 p.r. 'high 12 9 24 -1 30 H a r t . F i e l d e n _ 6.0.10 p.r. ^10 eV/£_J pD13.4 for(ed+ed) sec In order to examine the' e aq e aq mechanism i t i s necessary to have an accurate p r o f i l e of the ground s t a t e behaviour o f the hydrated e l e c t r o n , that i s the decay of t h i s species at high concentrations. Experiments were th e r e f o r e designed to f o l l o w the formation and decay 2 of the hydrated e l e c t r o n at r a d i a t i o n i n t e n s i t i e s a f a c t o r o f 10 above those employed by K l e i n and Warner. 31 a c c e l e r a t o r c o n t r o l console • p l e x i c e l l l a s e r l e a d screens flow system lead box p h o t o m u l t i p l i e r l e n s , f i l t e r , i r i s j u n c t i o n box power supply to H o s c i l l o s c o p e copper buss bar, ground stake Diagram 6. The a c c e l e r a t o r Laboratory 32 The Technique of Pulse R a d i o l y s i s , arid K i n e t i c Spectroscopy The formation and decay of the hydrated e l e c t r o n produced during the pulse r a d i o l y s i s of aqueous media was followed by a k i n e t i c spectrophotometric technique. A Helium-Neon l a s e r was used as a monitor- ing l i g h t source and t r a n s i e n t o p t i c a l absorptions produced during and a f t e r the pulse were detected by a p h o t o m u l t i p l i e r , d i s p l a y e d on an o s c i l l o s c o p e and photographed. A schematic diagram of the a c c e l e r a t o r l a b o r a t o r y i n which these experiments were c a r r i e d out i s shown i n diagram 6; d e t a i l s of the equipment are l i s t e d i n the f o l l o w i n g s e c t i o n s and i n diagrams 7, 8 and 9. The I r r a d i a t i o n C e l l In designing the i r r a d i a t i o n c e l l s e v e r a l important f a c t o r s had to be considered: ( i ) the window of the c e l l should be of s u i t a b l e m a t e r i a l and thickness to allow the maximum number of e l e c t r o n s i n the pulsed beam to penetrate the s o l u t i o n . ( i i ) I f too deep a volume of s o l u t i o n i s i r r a d i a t e d the e l e c t r o n s w i l l penetrate to only a c e r t a i n depth and thus space charges may appear. ( i i i ) I t was necessary to remove the i r r a d i a t e d s o l u t i o n and r e p l e n i s h the c e l l q u i c k l y between the pu l s e s . ( i v ) A v a r i a b l e path length was d e s i r a b l e i n view of the nature of the assumptions i n r e l a t i n g charges i n o p t i c a l d e n s i t y to charges i n the concentration of the absorbing specie. (v) The widthof the i r r a d i a t i o n c e l l should be comparable to 33 the width of the l a s e r beam i n order to detect simultaneously events i n c a l l areas of the c e l l . The f i n a l design of the i r r a d i a t i o n c e l l i n i t s supporting framework i s shown i n diagram 7. The support was constructed from 25 mm t h i c k p o l i s h e d p l e x i g l a s s and thus the i r r a d i a t i o n c e l l has been c a l l e d the p l e x i c e l l . The p l e x i c e l l i s 11.2 mm i n length and stands 10.4 mm high. An upper s e c t i o n of the p l e x i g l a s s block was removed, the area o f the r e s u l t i n g space being approximately that of the e l e c t r o n tube window i n the a c c e l e r a t o r . A hole was d r i l l e d through the remaining p l e x i g l a s s e i t h e r s i d e of the space, i n t o which a long t h i n g l a s s tube could be i n s e r t e d . A recess f o r an end window and a port were al s o machined at each end of the block as can be seen i n the diagram. The t h i n transparent 100 mm long glass tube of 1.5 mm i n t e r n a l diameter and 0.2 mm t h i c k w a l l s acted as the i r r a d i a t i o n c e l l proper along which the l a s e r beam was d i r e c t e d . ( A f t e r s e v e r a l pulses the glass d i s c o l o u r e d due to the presence o f trapped e l e c t r o n s , and so the tubes were r e g u l a r l y replaced. To ensure u n i f o r m i t y i n thickness and t r a n s - parency i n a l l the experiments a supply o f KIMAX (U.S.A.) c a p i l l a r y tubes was purchased and the f i n a l s p e c i f i c a t i o n s of the p l e x i c e l l m odified to f i t these tubes.) The glass tube was r e t a i n e d f i r m l y i n p o s i t i o n by a set of aluminium p l a t e s at both ends of the p l e x i c e l l ; these a l s o supported the windows and t h e i r rubber backing d i s c s ensured the c e l l to be water t i g h t when a gentle pressure was a p p l i e d to the p l a t e screws. The windows of the c e l l were p o l i s h e d 15 mm diameter pyrex d i s c s . The glass tube reached to w i t h i n 5 mm of e i t h e r window; a smaller distance between them l e d to the trapping of gas bubbles o f t e n introduced i n the 34 Diagram 7. The P l e x i c e l l and Components 35 flow of the sample s o l u t i o n s and otherwise e a s i l y removed. F i t t e d p l e x i - g l a s s ports of 6.7 mm diameter t a p e r i n g to 2.5 mm diameter j u s t above the ends o f the gl a s s tube c o n t r o l l e d the flow of s o l u t i o n through the p l e x i c e l l . A block of aluminium 9.6 mm t h i c k and 4.5 mm by 4.1 mm high was machined to f i t i n t o the empty s e c t i o n i n the p l e x i c e l l . The 20 mm by 1 mm wide s l i t cut along the centre o f the block was a l i g n e d w i t h the gla s s tube, and two small doors of 0.8 mm aluminium s l i d along a recess ( i n the block) f a c i n g the tube. The aluminium block had a two-fold purpose. F i r s t l y i t represented the v a r i a b l e path length f o r the irradiations as i t was i n s e r t e d on the f r o n t s i d e of the p l e x i c e l l f a c i n g the a c c e l e r a t o r . When the aluminium doors were f u l l y closed the t o t a l e l e c t r o n beam was absorbed by them. Secondly the s l i t i n the block served to c o l l i m a t e the e l e c t r o n beam. Previous studies using the whole o f the e l e c t r o n beam to i r r a d i a t e the s o l u t i o n s i n the p l e x i c e l l had shown some very curious v a r i a t i o n s i n the l i g h t i n t e n s i t y that completely masked the absorption s i g n a l under i n v e s t i g a t i o n . The probable o r i g i n o f these w i l l be discussed b r i e f l y elsewhere, but t h e i r e l i m i n a t i o n was only p o s s i b l e by r e s t r i c t i n g the e l e c t r o n beam to an area comparable to the i r r a d i a t i o n tube. For a while the r e s t r i c t o r was a c i r c u l a r p l a t e o f 0.8 mm t h i c k aluminium containing a s l i t of the re q u i r e d path length t h a t could be attached to the f r o n t face of the a c c e l e r a t o r . However t h i s s l i t d i d not define the path length with s u f f i c i e n t accuracy due to the undetermined divergence o f the e l e c t r o n s between the s l i t and the gl a s s tube; the concentrations of e aq and observed ra t e constants c a l c u l a t e d from these e a r l y s t u d i e s were to g r e a t l y i n f l u e n c e the design of l a t e r experiments. 36 Three holes d r i l l e d i n t o the lower p a r t of the p l e x i c e l l permitted screw attachment of the c e l l d i r e c t l y to the f r o n t o f the a c c e l e r a t o r , o r to a brass support (or the o p t i c a l bench) on which the p l e x i c e l l could be r o t a t e d i n any d i r e c t i o n . In a l l the experiments discussed here the p l e x i c e l l was attached to the Febetron as t h i s markedly reduced the high noise l e v e l on the s i g n a l s recorded; the d i r e c t grounding of the c e l l when i n contact with the machine and the reduced a i r space between the window of the e l e c t r o n tube and the i r r a d i a t i o n c e l l probably both c o n t r i b u t e d to the improved s i g n a l to noise r a t i o . The E l e c t r o n A c c e l e r a t o r In these experiments the source o f high energy i o n i s i n g r a d i a - t i o n was a pulsed e l e c t r o n a c c e l e r a t o r manufactured by F i e l d Emission Corporation (Oregon). The Febetron which operates on the f i e l d emission p r i n c i p l e produced s i n g l e intense pulses o f nanosecond duration o f 0.5 MeV e l e c t r o n s . In the i n i t i a l stages o f t h i s work the Febetron model 701-2660 p u l s e r and 5235 e l e c t r o n tube were employed to produce a 50 nanosecond pulse o f 0.52 MeV e l e c t r o n s . The peak beam current observed at the window of t h i s e l e c t r o n tube was 1000 amperes. The energy o f the el e c t r o n s i n the pulse can be v a r i e d according to the D.C. charging v o l t a g e ; the charging c i r c u i t of the Febetron operates on the Marx Surge C i r c u i t p r i n c i p l e . The impedance o f the e l e c t r o n tube was f a i r l y constant thus the beam current v a r i e d w i t h the charging v o l t a g e . The beam current and pulse shape (with a h a l f width o f ̂ 20 nanoseconds) were repr o d u c i b l e to w i t h i n a few percent (a = ±3%) under i d e n t i c a l charging c o n d i t i o n s . 37 At peak performance the Febetron gave a 50 nanosecond pulse of 0.52 MeV 19 e l e c t r o n s amounting to a t o t a l d e p o s i t i o n of energy of 'vlO eV. The m a j o r i t y of experiments were c a r r i e d out with the 3 nano- second pulse which was obtained by a t t a c h i n g the model 2770 pulse shortner to the Model 2660 p u l s e r case and r e p l a c i n g the e l e c t r o n tube with a s m a l l e r model, no 5510. At peak performance t h i s assembly gave a 3 nanosecond pulse of 0.5 MeV e l e c t r o n s and a beam current of ^1000 amperes. The 19 t o t a l energy deposited was ^10 eV but the dose r a t e was increased to 26 1 1 26 1 5 x 10 eV gram sec compared to 10 eV gram * sec with the long pulse. The r e p r o d u c i b i l i t y was w i t h i n 5%, but with both e l e c t r o n tubes the g r e a t e r the number of pulses i n excess of the mean " l i f e t i m e " of the tube, the worse the r e p r o d u c i b i l i t y o f t h e i r quoted c h a r a c t e r i s t i c s . T y p i c a l pulse shapes are shown i n f i g u r e 10a. The e l e c t r o n beam current was measured with an apertured Faraday Cup shown i n f i g u r e 10b. The cup contains a T § M Research Products Model GR-1-05 current viewing r e s i s t o r (C.V.R.) with an impedance o f 0.0507 ohms. The voltage pulse across the C.V.R. was fed i n t o the v e r t i c a l d i s p l a y of a Tektronix model 454 o s c i l l o s c o p e v i a doubly s h i e l d e d RG58 c o a x i a l cable coupled to a 50 ohm terminator at the input to the o s c i l l o s c o p e . S o l u t i o n s and Flow Techniques Laboratory d i s t i l l e d water was r e d i s t i l l e d from a c i d i f i e d potassium permangate s o l u t i o n and kept i n a r e s e v o i r f l a s k ( b u i l t i n t o the flow system) under an atmosphere of Helium gas. S o l u t i o n s prepared containing other s o l u t e s were always made up from t h i s supply of doubly d i s t i l l e d water. 38 50 n s e c s / d i v ( h o r i z ) t y p i c a l pulsed wave forms observed from the e l e c t r o n i r r a d i a t i o n tube. J U ' I V - I M (a) Diagram 10. 10 n s e c s / d i v ( h o r i z ) (b) 39 The 0.26 M i s o p r o p y l a l c o h o l s o l u t i o n was always f r e s h l y prepared from analar B.D.H. reagent without f u r t h e r p u r i f i c a t i o n . A supply o f 0.0025 M and 0.1 M H^SO^ s o l u t i o n s were al s o made up from analar B.D.H. a c i d , and a f t e r degassing kept under an i n e r t atmosphere of helium i n a second r e s e v o i r f l a s k i n the flow system. These r e s e v o i r s were 2 l i t r e three necked f l a s k s w i t h i n d i v i d u a l flow l i n e s to the i r r a d i a t i o n c e l l that could be sealed when not i n use. Each f l a s k was connected to a common threeway stopcock which c o n t r o l l e d the flow o f helium gas through the f l a s k s ; an arrangement of traps and secondary stopcocks made improbable any a c c i d e n t a l f i l l i n g o f the gas l i n e w i t h s o l u t i o n . Helium gas was bubbled v i g o r o u s l y through the s o l u t i o n s v i a a f r i t t e r e d g l a s s oval to ensure e f f i c i e n t degassing; the oxygen content o f the s o l u t i o n s was ̂ 1 ppm, i . e . that contained i n the gas i t s e l f . The degassing took place f o r s e v e r a l hours when the r e s e v o i r s were completely r e f i l l e d w i t h f r e s h s o l u t i o n s , otherwise f o r 30 minutes before the experiments began. (When i s o p r o p y l a l c o h o l s o l u t i o n s were used degassing was gentle and only f o r 10 minute periods due to the high vapour pressure o f the alco h o l . ) During t h i s time the helium escaped to the atmosphere through an open stopcock i n the f l a s k ; the s o l u t i o n i n the p l e x i c e l l was replaced with a f r e s h volume o f l i q u i d by c l o s i n g t h i s stopcock, b r i e f l y p r e s s u r i s i n g the f l a s k and then opening the appropriate flow l i n e to the c e l l . The surplus l i q u i d was c o l l e c t e d through another l i n e on the e x i t port o f the c e l l that l e d i n t o a residue f l a s k . The temperature o f a l l these s o l u t i o n s was 19° ±1°C. Unless s t a t e d otherwise i n the t e x t the s o l u t i o n was rep l e n i s h e d i n the i r r a d i a t i o n c e l l a f t e r each pulse. A p r c t u n e i r e p r e s e n t a t i o n o f the. f^low system is-shown i n diagram 11. Diagram 8. The E l e c t r o n i c Detection System 41 The Detection System - O p t i c a l The l o c a t i o n s o f the l a s e r and p h o t o m u l t i p l i e r w i t h respect to the p l e x i c e l l and the a c c e l e r a t o r are shown i n diagram 6. (i ) The l a s e r The analysing l i g h t source was a Spectra physics model 130C D.C. e x c i t e d Helium-Neon l a s e r , that emitted a continuous p a r a l l e l beam o at 6328A. The beam width was 1.4 mm and the maximum output power 1 m watt. The i n t e n s i t y o f the beam could be v a r i e d ; the l a s e r was operated under co n d i t i o n s o f minimum "no i s e " which proved to be j u s t below the maximum output power. The reason f o r t h i s i s given below. Small i r r e g u l a r i t i e s on the i n s i d e o f the plasma tube bore and the e x i t aperture tend to d i f f r a c t some of the l i g h t away from the p r i n c i p a l a x i s , and although the l o s s i s so small as to not n o t i c e a b l y a f f e c t the power output, the i n t e n s i t y d i s t r i b u t i o n of the beam v a r i e s with distance from the aperture. In t h i s case the i r r a d i a t i o n c e l l was 90 cm from the aperture and the i n t e n s i t y spread was s t i l l contained w i t h i n 2 mm, the diameter o f the glass i r r a d i a t i o n tube; there was no appreciable l o s s o f i n t e n s i t y as the beam passed through the s o l u t i o n but the d i f f r a c t i o n was enhanced to a small extent. At the entrance to the p h o t o m u l t i p l i e r box some 3 metres away the main beam was s t i l l 2 mm i n diameter but the d i f f r a c t e d l i g h t was i r r e g u l a r l y s c a t t e r e d about i t . An adjustable non r e f l e c t i n g i r i s was mounted on the o p t i c a l bench immediately before the focussing lens and i n t h i s way the s c a t t e r e d l i g h t was e l i m i n a t e d . An i n e v i t a b l e degree o f i n s t a b i l i t y i n the l a s e r and 60 Hz f l u c t u a t i o n s i n the i n t e n s i t y from the main power l i n e s were detected on 42 the p h o t o m u l t i p l i e r and added to the " n o i s e " on the absorption and decay t r a c e of the hydrated e l e c t r o n . I t was p o s s i b l e as w e l l to p i c k up extremely f a s t and r e p r o d u c i b l e s i g n a l s of %200 M Hz frequency and higher i n the l a s e r beam. This was a t t r i b u t e d to one o f the a d d i t i o n a l resonance frequencies i n the o s c i l l a t i o n as t h i s l a s e r does not operate on a "single-mode" technique. The resonant frequencies are spaced f = c_ , 2c, 3c .. . where c = speed of l i g h t and L i s the o p t i c a l c a v i t y 2L 2L 2L length. In t h i s system the o p t i c a l c a v i t y length i s fv30 cm) which could correspond to the high frequency observed. In a d d i t i o n these frequencies are perturbed by small amounts according to how f a r the resonance i s from the centre of the Neon resonance and any s l i g h t changes i n the c a v i t y length (due p r i n c i p a l l y to thermal e f f e c t s ) . These p e r t u r - b a t i o n frequencies can a l s o be detected i n the I K Hz to 100 K Hz range, w i l l be present simultaneously and are not i n phase. As a r e s u l t the main s i g n a l from the l a s e r on a wide-band o s c i l l o s c o p e has superimposed noise l i k e t races which only disappear when these c a v i t y resonances become symmetrical about the Neon resonance l i n e and, through an apparent self-phase l o c k i n g , go to zero. The extent to which t h i s r i p p l e on the l a s e r s i g n a l i n t e r f e r e d w i t h the t r a n s i e n t absorption s i g n a l s i s considered i n the r e s u l t s and d i s c u s s i o n . . In the experiment designed to f o l l o w the formation o f absorbing species i n d i f f e r e n t areas i n the i r r a d i a t i o n c e l l , the l a s e r beam passed along the whole tube as usual and the changes i n i n t e n s i t y across the diameter o f the l a s e r beam i t s e l f were monitored. This was done by a l i g n - in g a t h i n brass r e s t r i c t i o n p l a t e c o n t a i n i n g four pinholes of 0.031, (9.020, 0.0225 and 0.0135 (thousandths of an inch) bore p a r a l l e l to the Diagram 9. Showing the apparatus i n experimental p o s i t i o n s 44 l a s e r beam, i n a p o s i t i o n on the o p t i c a l bench a f t e r the p l e x i c e l l and before the i r i s . The r e s t r i c t i o n p l a t e was mounted on a stand which had a f i n e l a t e r a l adjustment; thus the pinhole of choice could be moved across the width of the l a s e r beam and e f f e c t i v e l y "scan" the i r r a d i a t i o n c e l l . ( i i ) F i l t e r s and Lenses o A 6328A (Baird-Atomic Inc. i n t e r f e r e n c e type B l l with <10% transmission outside the narrow band pass) f i l t e r that t r a n s m i t t e d about 60% o f the i n c i d e n t l a s e r beam was incorporated i n t o the o p t i c a l arrange- ments as shown i n diagram 6. The reasons were twofold. The high i n t e n s i t y Cerenkov emission occurs at a l l wavelengths and although the p l e x i c e l l had been blackened on i t s outer surfaces to prevent the r e f l e c t i o n or transm i s s i o n o f the Cerenkov l i g h t the c e l l windows were open and q u i t e transparent. I t was not expected however to observe a s i g n i f i c a n t i n t e n s i t y at 90° to the e l e c t r o n beam. The second reason f o r using a red f i l t e r concerns the r e l a t i v e s e n s i t i v i t y of the p h o t o m u l t i p l i e r to d i f f e r e n t wavelengths. The p h o t o m u l t i p l i e r i n use had a higher r e l a t i v e s e n s i t i v i t y o to s h o r t e r wavelengths with a maximum response at ̂ 3500A. A small f l u c t u a t i o n i n i n t e n s i t y at a low wavelength would correspond to a large change i n i n t e n s i t y i n the red and might t h e r e f o r e f a l s i f y the height and shape o f the s i g n a l recorded. The n e u t r a l density f i l t e r s used p e r i o d i c a l l y to t e s t the l i n e a r i t y of the system were 1% and 10%. These were Baird-Atomic i n t e r - ference f i l t e r s . Complete absorption o f the l a s e r beam gave a s i g n a l o f only ^25 mv from the p h o t o m u l t i p l i e r operating at 550 v o l t s . The d e t a i l s o f the e l e c t r o n i c d e t e c t i o n system are given l a t e r together w i t h the reasons 4 5 f o r t h i s small s i g n a l . U l t i m a t e l y the best s i g n a l to noise r a t i o (set by the l i m i t s of the maximum p e r m i s s i b l e gain i n the p h o t o m u l t i p l i e r and the noise observed at higher operating voltages used to amplify the small s i g n a l ) was obtained by focusing the l a s e r beam onto the photocathode. A 8.5 mm f o c a l length p o l i s h e d quartz lens was mounted on a support capable of f i n e l a t e r a l and v e r t i c a l adjustment and o r i e n t a t e d to give the best p o s s i b l e s i g n a l from the l a s e r beam. Normally readjustment was unnecessary as the geometry of a l l the equipment was r e p r o d u c i b l e even a f t e r any displacement or m o d i f i c a t i o n s to the system. The Detection System - E l e c t r o n i c ( i ) The p h o t o m u l t i p l i e r A R.C.A. V i c t o r 1P28 p h o t o m u l t i p l i e r with a S5 s p e c t r a l response and a 100 ohm load r e s i s t a n c e was used to f o l l o w the v a r i a t i o n s i n the i n t e n s i t y of the t r a n s m i t t e d l a s e r beam. The load r e s i s t a n c e and r e s i s t o r s i n the dynode chain were f i t t e d i n t o a compact metal base and the whole assembly was housed i n a copper box 19 cm high by 12 cm by 17 cm that had been l i n e d w i t h %" t h i c k lead (to p r o t e c t the p h o t o m u l t i p l i e r from X- r a d i a t i o n ) . The l i d of the box contained e i g h t small holes that provided a minimum amount of v e n t i l a t i o n but because of the a c t u a l p o s i t i o n of the phototube no d i f f u s e l i g h t could be p i c k e d up and a m p l i f i e d with the s i g n a l . The ambient temperature i n the box rose appreciably a f t e r prolonged use of the p h o t o m u l t i p l i e r and t h e r e f o r e the duration o f experiments was kept w i t h i n a reasonable time and the p h o t o m u l t i p l i e r p e r i o d i c a l l y checked f o r any marked increase i n dark current. A m o d i f i c a t i o n to the housing to provide a constant temperature c o o l i n g system has been designed and w i l l be i n use 4 6 Diagram 11. The Flow System 47 f o r the next experiments i n t h i s p r o j e c t . Under the present conditions the temperature i n s i d e the copper box was 21° ±1°C. A pi n h o l e of 1.5 mm diameter was d r i l l e d through the f r o n t face of the s h i e l d i n g box thus a l l o w i n g l i g h t focused through the pi n h o l e to f a l l on to the photocathode. Between experiments the photocathode was pro t e c t e d from the l a s e r beam by covering the p i n h o l e . The 1P28 has f a s t time r e s o l u t i o n c h a r a c t e r i s t i c s the anode pulse -8 -9 r i s e time of 10 seconds and time spread o f 2 x 10 seconds at the voltages employed. The minimum length o f RG71/B/U c o a x i a l cable (used to match impedance with the load r e s i s t a n c e ) necessary to transmit the s i g n a l to the o s c i l l o s c o p e was IB"; the cable was doubly s h i e l d e d and fed the s i g n a l through a 93 ohm Tektronix terminator i n t o the v e r t i c a l a m p l i f i e r input o f the o s c i l l o s c o p e . The maximum current t h a t the dynode chain could s u s t a i n was 10 m amps but to avoid s a t u r a t i o n e f f e c t s and problems o f non l i n e a r i t y i n the response o f the p h o t o m u l t i p l i e r tube a working voltage of 550 v o l t s from a power supply (5.5 mA D.C. across the chain) was normally used and the s i g n a l s attenuated on the o s c i l l o s c o p e . For very small v a r i a t i o n s i n i n t e n s i t y o f the l a s e r beam and the "pinhole experiments" the voltage was r a i s e d to a maximum o f 750 V. At very low voltages i . e . low current along the dynode r e s i s t o r chain, f l u c t u a t i o n s i n the dynode voltages conr t r i b u t e d towards the n o n - l i n e a r behaviour o f the tube; at voltages above 750 V the l e v e l of the noise on the s i g n a l , which had c o n t r i b u t i o n s from dark currents i n the tube, a m p l i f i e d l a s e r r i p p l e , and feedback e f f e c t s was unacceptable. In a d d i t i o n during the experiments there were adverse changes i n the dark current and noise pulses due to the intense i o n i s i n g 48 r a d i a t i o n and f i e l d s generated by the e l e c t r o n pulse. However these e f f e c t s were reduced to as low a l i m i t as p o s s i b l e by the lead screening i n the copper box, the r a d i a t i o n s h i e l d s and the double s h i e l d i n g on a l l the t r a n s m i s s i o n l i n e s , although t h i s l e v e l was by no means completely s a t i s f a c t o r y . The p h o t o m u l t i p l i e r was powered by a Fluke model 412B high voltage D.C. power supply whose output was s t a b i l i s e d to w i t h i n ±0.005% per hour or ±0.02% per day's operation. The power supply was connected to the p h o t o m u l t i p l i e r w i t h c o a x i a l doubly s h i e l d e d transmission cable. ( i i ) O s c i l l o s c o p e and camera The Tektronix model 454 wide band path o s c i l l o s c o p e was used to d i s p l a y the v a r i a t i o n s i n the i n t e n s i t y of the l a s e r beam ( i . e . the change i n anode current across the load r e s i s t a n c e i n the photomultipler) during and a f t e r the e l e c t r o n pulse from the a c c e l e r a t o r as a time dependent voltage s i g n a l . The r i s e time o f the o s c i l l o s c o p e under these conditions was 2.6 nanoseconds. The input power l i n e from the mains 110 v o l t s supply was f i l t e r e d through a radiofrequency l i n e f i l t e r attached to the back of the o s c i l l o - scope. As already mentioned the s i g n a l from the p h o t o m u l t i p l i e r was terminated with 93 ohms at the ircput on channel 1 o f the o s c i l l o s c o p e . The t r a c e on the screen was t r i g g e d by an A.C. p o s i t i v e f l u c t u a t i o n i n the s i g n a l and a l l low frequency s i g n a l s were r e j e c t e d . In each experiment the 100% l i g h t l e v e l was recorded immediately a f t e r the pulse by manually chopping the l a s e r beam (at approximately 1 K Hz) and recording the r i s e of the s i g n a l and subsequent f a l l to 100% transmission again. 49 A Tektronix Type C-40 (pola r o i d ) camera s u p p l i e d with attachments f o r the o s c i l l o s c o p e was used to record the traces on p o l a r o i d polascope type 410 f i l m which i s p a r t i c u l a r l y s u i t a b l e f o r extra-high speed (ra t e d as 10,000 ASA equivalent) photography and low l e v e l l i g h t sources such as the t r a c e i n t e n s i t i e s a v a i l a b l e with the f a s t e s t sweeprate on the o s c i l l o - scope . The Grounding System I t was imperative to have a good high frequency grounding system with a l l the equipment grounded e f f e c t i v e l y to one p o i n t as otherwise the intense magnetic and e l e c t r i c f i e l d s produced by the a c c e l e r a t o r during the e l e c t r o n pulse completely i n t e r f e r e d with any measurements. The power supply, o s c i l l o s c o p e and p h o t o m u l t i p l i e r were a l l grounded to one another through the outer s h i e l d i n g on the doubly s h i e l d e d c o a x i a l cable. However i t also appeared necessary to ground the noise f i l t e r on the o s c i l l o s c o p e s e p a r a t e l y , to take a ground lead from the p h o t o m u l t i p l i e r box to the "common" ground p o i n t and to remove a l l the equipment from the mains earth by f l o a t i n g the plugs i n a common j u n c t i o n box. I t i s probably more in f o r m a t i v e to i l l u s t r a t e the complete grounding scheme i n a diagram (see diagram 12). The ground stake was attached to a long copper bar to which i n d i v i d u a l grounding tapes were b o l t e d . During the experiments i t was discovered that many of the long t h i c k grounding tapes and unshielded cables were merely a c t i n g as attennae, so a l l grounding tapes were kept to a minimal s i z e and no cable remained unshielded. The l a s e r was als o attached to a f l o a t i n g plug and grounded then A' a c c e l e r a t o r B c o n t r o l console C p l e x i c e l l D l a s e r E lead screens H pho.tomul.tiplier L o s c i l l o s c o p e K power supply to H M copper buss bar, . ground stake Diagram -12... The Grounding System 51 to the copper bar approximately three fee t from the grounding stake. A piece of %" lead was f o l d e d around the s i d e o f the l a s e r near to the a c c e l e r a t o r to prevent any adverse e f f e c t s on the l a s e r output and r i p p l e . There was a t h r e e - s i d e d lead box immediately behind the p l e x i c e l l which i n the absence of the c e l l or other attachment on the f r o n t o f the a c c e l e r a - t o r acted as a sink f o r the r a d i a t i o n produced. As the alignment of the l a s e r , p l e x i c e l l , focusing lens and p h o t o m u l t i p l i e r was c r i t i c a l , any v i b r a t i o n o f these pieces of equipment markedly a f f e c t e d the s i g n a l , and o f t e n heavy " n o i s e " on the s i g n a l could be t r a c k e d back to imperfect alignment. The other f a c t o r s to consider were the occurrence of ground loops from doubly grounding some of the e l e c t r o n i c equipment and the i n e v i t a b l e pickup from the mains of high frequency s i g n a l s from machines i n nearby l a b o r a t o r i e s . 52 The Computation o f Data and the Results The Input Data This c o n s i s t e d o f the recorded absorption s i g n a l as a f u n c t i o n of time during the decay o f the hydrated e l e c t r o n . The measurements were made on the photographs o f the o s c i l l o s c o p e traces using a Micro- s c a l e w i t h 0.1 mm d i v i s i o n s , and reasonable estimate was p o s s i b l e f o r the next decimal p l a c e . A t y p i c a l i n i t i a l absorption s i g n a l would be 14.8 ± .05 mm ( f o r a 2.5 mm path length) decreasing to ̂ 10 mm a f t e r 150 nanoseconds, w i t h a maximum p o s s i b l e absorption corresponding to 30 mm. Measurements were made every 5, 10 or 20 nanoseconds the time i n t e r v a l depending on the sweeprate of the o s c i l l o s c o p e , and sometimes estimates of the tru e height of the s i g n a l had t o be made due to the noise l e v e l . Because of t h i s u n c e r t a i n t y a f t e r 100 or 200 nanoseconds, i t was often d i f f i c u l t to determine anything but an approximate h a l f l i f e f o r e aq under the experimental conditions and t h e r e f o r e a comparison has been made between the d i f f e r e n t r e s u l t s that r e l a t e s the % decrease i n the absorption s i g n a l at a given time a f t e r the pulse. In those traces where the noise and l a s e r i n t e r f e r e n c e (see s e c t i o n I I ) were more than an annoying width on the s i g n a l the e aq decay was not analysed. Attempts were made t o enlarge the p o l a r o i d photographs i n t o both negatives and p r i n t s f o r e a s i e r working but the i n e v i t a b l e d i s t o r t i o n s i n the s c a l e and on the trac e although small were s u f f i c i e n t to outweigh any v i s u a l advantages. Computation o f Data The a n a l y s i s o f the r e s u l t s c o l l e c t e d i n the long pulse (50 Nsec) experiments i n d i c a t e d that the data could not be simply matched to 53 e i t h e r a f i r s t or second order k i n e t i c treatment. In a d d i t i o n the duration o f the pulse meant that the hydrated e l e c t r o n s were r e a c t i n g before the e l e c t r o n pulse (and thus the formation of the hydrated e l e c t r o n s ) was f i n i s h e d . With the short (3 Nsec) e l e c t r o n pulse the l a t t e r problem was i n s i g n i f i c a n t but the k i n e t i c s were not s i m p l i f i e d . The r e s u l t s from these experiments were t h e r e f o r e considered from three independent approaches, covering the d i f f e r e n t mechanisms f e a s i b l e i n t h i s system. ( i ) Two s i g n i f i c a n t simultaneous processes o c c u r r i n g immediately a f t e r the pu l s e . ( i i ) One s i g n i f i c a n t process only o c c u r r i n g a f t e r the pulse. ( i i i ) Two s i g n i f i c a n t processes o c c u r r i n g c o n s e c u t i v e l y a f t e r the pulse. Let x be the absorbing specie ( i e . the hydrated e l e c t r o n ) and y any other specie subsequently formed i n the system i n s i g n i f i c a n t c oncentrations; k^ i s a f i r s t order r a t e constant and a second order r a t e constant. ( i ) I f s e v e r a l processes are o c c u r r i n g simultaneously an expression can be derived r e l a t i n g the change i n concentration o f x to these r e a c t i o n rates and the concentrations of any other species i n v o l v e d . From an i n i t i a l study of the data i t was c l e a r that i f there were two simultaneous processes i n t h i s system they would be f i r s t and second order type r e a c t i o n s and so the f o l l o w i n g equation describes t h i s case: -d[x] = k [x] + k [ x ] 2 (or k xy) dt -1_ . d [x] = k + k ? [x] which on rearrangement shows that a [x] ~dt~ 1 p l o t of 1 d[x] against [x] should give a s t r a i g h t l i n e of slope - k„ [x] dt 2 54 and i n t e r c e p t k^. A f u r t h e r rearrangement gives mathematically i d e n t i c a l p l o t but an i n t e r e s t i n g check on the way the data f i t s the treatment. -dfeix = k + k [ X] dt 1 1 In the event of only one process occuring the appropriate k w i l l become i n s i g n i f i c a n t and the term w i l l equate to zero. The para- meters evaluated from these p l o t s were then f i t t e d to a t h i r d degree polynomial. The assumption that o p t i c a l d e n s i t y could be l i n e a r l y r e l a t e d to the concentration of the absorbing specie O.D. = c. el e e x t i n c t i o n c o e f f i c i e n t % path length i n cms., was i n v e s t i g a t e d experimentally and the r e s u l t s w i l l be discussed l a t e r . The o p t i c a l , d ensity was considered as the v a r i a b l e , and the two expressions below used t o separate the two simultaneous processes -1 d O.D. = k + k 2 O.D. O.D. dt e l -dJto O.D. = k + k 2 O.D. dt el up t o 200 Nseconds a f t e r : t h e pulse. ( i i ) I f there were only one s i g n i f i c a n t process o c c u r r i n g then a d e t a i l e d g r a p h i c a l a n a l y s i s of the changes i n o p t i c a l d ensity ( i n the f i r s t 100 Nseconds or so a f t e r the pulse) should i n d i c a t e the r e a c t i o n order. F i r s t order process: Jinx = k T + constant A p l o t o f ln(0.D.) against t should give a s t r a i g h t l i n e o f slope k . el ~" Second order process: 1_ = k 2 x + constant x  k A p l o t of 1_ against x should give a s t r a i g h t l i n e of slope 2. The O.D. eT 55 k„ value was c a l c u l a t e d from 1_ - 1_ = _2_ O . D . O . D . eT T O ( i i i ) I f the mode of decay of the hydrated e l e c t r o n changes a f t e r a c e r t a i n time and, f o r example, both f i r s t and second order processes are i n v o l v e d i n sequence, then the behaviour of the specie s t i l l may be s p e c i f i c a l l y c l a s s i f i e d before and a f t e r the t r a n s i t i o n p e r i o d . Here one must al s o i n c l u d e the p o s s i b i l i t y that the two mechanisms are of the same r e a c t i o n order but occur with s i g n i f i c a n t l y d i f f e r e n t rate constants. Any changes i n the slopes observed from the treatment i n ( i i ) should occur at approximately the same time a f t e r the pulse and i n the same d i r e c t i o n . The u n d e r l y i n g assumption i n these approaches i s that c l a s s i c a l k i n e t i c theory can be used to describe the behaviour o f a system whose p r i n c i p a l a c t i v e specie has been consumed by the products of i t s own i n i t i a l r e a c t i o n s i n about 200 nanoseconds, and can be scavenged by a d d i t i v e s of s u i t a b l e concentrations i n only a few nanoseconds. The data was fed i n t o a computer; the l e a s t - s q u a r e s - f i t programme c o n t a i n i n g the treatments f o r ( i ) and ( i i ) was a l s o designed to provide i n f o r m a t i o n f o r ( i i i ) by s y s t e m a t i c a l l y e l i m i n a t i n g the data p o i n t s up to a given time a f t e r the pulse and r e c o r d i n g the v a r i a t i o n s i n the ra t e constants and the q u a l i t y of the f i t . * A l l the long pulse data had been analysed assuming ( i i ) and i t was from these graphs that the f i r s t apparent k^ and values were c a l c u l a t e d ; these were;.-a good i n d i c a t i o n of the order o f magnitude t o expect on the short pulse data some of which was a l s o analysed i n t h i s * The author g r a t e f u l l y acknowledges the a s s i s t a n c e of Mr. S.C. Wallace i n p r e p a r i n g the programme. 56 way as a precaution against e r r o r s i n the input data or computer programme. The r e s u l t s o f each of the s e r i e s of experiments w i l l now be given i n r e l a t i o n t o the three treatments. V a r i a t i o n s o f o p t i c a l d ensity w i t h path length Path lengths of 1.0 mm, 1.5 mm, 2.7 mm, 3.5 mm, 5.0 mm and 5.5 mm were used. When i t became c l e a r that the l a r g e r path lengths were g i v i n g r i s e to o p t i c a l d e n s i t i e s (O.D.) that deviated from the a n t i c i p a t e d values ( d e r i v a t i v e s from Beer's law) then attempts were made to look at O.D. at even s h o r t e r path lengths. However the t e c h n i c a l problems a s s o c i a t e d w i t h the d e t e c t i o n and a m p l i f i c a t i o n of the s i g n a l prevented such experiments. Two p o s s i b l e reasons f o r t h i s d e v i a t i o n are (a) the e f f e c t i v e path length of absorbing species was greater than the width of the s l i t s due t o an undetermined divergence of the e l e c t r o n beam, a more s i g n i f i c a n t e r r o r at longer path lengths; and (b) the r a d i a l i d i s t r i b u t i o n of the e l e c t r o n beam i t s e l f i s such that i f the centre point of the s l i t s d i d not c o i n c i d e with that o f the e l e c t r o n beam then there may be d i f f e r e n t numbers of absorbing species along the path l e n g t h , a r i s i n g from a s p a t i a l v a r i a t i o n i n the energy deposited i n the medium. The data i s summarised i n diagram 13. The v a r i a t i o n s i n O.D. observed at the same path length may be i n p a r t due t o (b) above and a l s o t o the f a c t that r e c e n t l y f l u c t u a t i o n s i n the output of the e l e c t r o n a c c e l e r a t o r have been observed i n t h i s l a b o r a t o r y . The energy of the e l e c t r o n beam v a r i e d w i t h the a c t u a l changing c o n d i t i o n s of the a c c e l e r a t o r and the time i n t e r v a l between pulses to a more s i g n i f i c a n t degree than the manufacturers claimed. The pulse to pulse r e p r o d u c i b i l i t y 57 .0 .5 1.5 2.5 3.5 45 5.5 mm Diagram 13.- O p t i c a l density as a f u n c t i o n of path .length 58 has now been improved by changing a l l the n i t r o g e n gas i n the high pressure chambers to oxygen. The formation and decay of e aq a f t e r a 5 Nsec e l e c t r o n pulse This s e c t i o n r e f e r s to the data c o l l e c t e d on the decay of e aq a f t e r a s i n g l e e l e c t r o n pulse at d i f f e r e n t path lengths. Some t y p i c a l p l o t s are given i n diagrams 14, 15,16 which show the f i r s t and second order treatments and the apparent t r a n s i t i o n from one to the other at about 80 ± 10 nanoseconds i n each case. Unimolecular and b i m o l e c u l a r decay processes might have been o c c u r r i n g simultaneously with one mode dominating the other at d i f f e r e n t times a f t e r the p u l s e , but none o f the data f i t t e d treatment ( i ) . In some cases the t r a n s i t i o n p e r i o d i n the f i r s t order p l o t s was then followed by a long l i n e a r p o r t i o n which might i t s e l f be i n d i c a t i v e of another f i r s t order decay o c c u r r i n g at a d i f f e r e n t r a t e . The slope of t h i s l i n e was l i n e a r to about 150 nanoseconds and gave a r a t e constant slower by a f a c t o r of 3. The ra t e of decay of e aq was f i r s t t a b u l a t e d from those experiments i n which i t had been p o s s i b l e to c o l l e c t data every 10 nano- seconds and i n most cases every 5 nanoseconds. The average value f o r the f i r s t order ra t e constant as c a l c u l a t e d from a p l o t of l o g (O.D.) 6 —1 against time was 8.80 ± .8 x 10 sec . With data taken i n other experiments only every 20 nanoseconds there were not as many p o i n t s on the 'graph up to the 80 nanosecond t r a n s i t i o n p e r i o d and i t was apparent that t h i s average k^ value was s l i g h t l y lower, v i z . 5.7 ± .8 x 10^ sec The p l o t s of 1_ against time were l i n e a r a f t e r about 80 O.D. nanoseconds i n a l l the experiments, and i r r e s p e c t i v e of the time i n t e r v a l s i n which the data was c o l l e c t e d the values f o r k„ were i n reasonable J_ O.D. 8l 7 6 0 / P ft / / Log fQD el 4.1 3.9 3.7' —•V . 0 d% 9 ' 0 3<K^ 2 1 path length @ 1 mm © 25 mm © 1.5 mm y O ^ 0 ft' 3.5 .0 o 0 o 1 mm © 2.7 mm © 1.5 mm 0 50 100 150 200 250 Time (nsccs) Diagram 14. Some f i r s t and second order p l o t s f o r the decay of the hydrated e l e c t r o n 60 0 20 60 Time (nsecs)180 220 260 Diagram 16. Second order p l o t s f o r the decay o f e" 61 agreement w i t h an average value of 5.88 ± 1.2 x 1 0 ^ M ^ sec ^. I n d i v i d u a l experimental r e s u l t s are compared i n Table ( I I I ) Table I I I .. , 1f,-6 -1 , i n-10 ..-1 -1 path k^ 10 sec k 2 10 M sec length cm Data every 5, 10 nanoseconds 0.1 7.17 ± .15 4.49 + .07 0.15 8.38 ± .02 6.45 ± .23 0.15 8.26 ± .13 6.11 ± .11 0.35 9.56 ± .54 6.00 ± .42 0.55 8.19 ± .07 i n s u f f i c i e n t data 0.55 8.19 ± .09 i n s u f f i c i e n t data Data every 20 nanoseconds or longer 0.1 6.72 ± .14 4.25 ± .23 0.1 5.73 ± .42 4.14 ± .09 0.1 5.18 ± .29 5.23 ± .26 0.27 4.92 ± .02 8.20 ± .33 0.27 6.06 ± .00 6.60 ± .11 0.27 i n s u f f i c i e n t 7.49 ± .29 data 0.55 5.30 ± .45 i n s u f f i c i e n t data The evidence s t r o n g l y suggests that f o r the f i r s t 80 nanoseconds at l e a s t the e aq decays i n a 1st order process; t h i s i s a p u z z l i n g concept i n view of the g e n e r a l l y accepted b i m o l e c u l a r nature of the decay of e aq i n i r r a d i a t e d n e u t r a l water. A f t e r t h i s p e r i o d there appears to be a t r a n s i t i o n t o 2nd order behaviour which holds over s e v e r a l h a l f l i v e s . The apparent l i n e a r i t y of some f i r s t order p l o t s a f t e r 80 and up to about 150 nanoseconds with an average k^ value of 2.62 ± 1 x 62 e Q q , 1 pulse. 20mV/div. I'Vl rrti i 100 nsocs/div. , 2 pulses. .50mV/div. •v, ft 50nsecs/div. aq 10 mV/div. 50 nsecs/div. e + R-OH aq 20mV/div. ... .•• p 50 nsecs/div. Diagram 15 (b). O s c i l l o s c o p e traces showing' the e aq decay i n the presence of d i f f e r e n t s o l u t e s . 63 10 sec may or may not be a r e a l e f f e c t : t h i s might simply be the beginning of a very long slow curve which would only be n o t i c e d i f data up t o s e v e r a l hundred nanoseconds could be taken at small time i n t e r v a l s . The l e v e l of noise (that masked the absorption s i g n a l ) on the t r a c e s was too great to permit accurate measurements up t o t h i s stage. Formation and decay o f e aq a f t e r a 50 Nsec e l e c t r o n pulse These experiments were c a r r i e d out i n a d i f f e r e n t l a b o r a t o r y but w i t h e s s e n t i a l l y the same equipment. The/important d i f f e r e n c e s are the length of the p u l s e , during which time the e aq i s formed and begins to decay and may p o s s i b l y reach a steady s t a t e c o n c e n t r a t i o n , and the path length of the c e l l which was always 7 mm. The decay o f e aq was shown to f o l l o w f i r s t order k i n e t i c s over about 100 nanoseconds, and a h a l f l i f e of about 130 nanoseconds. The r a t e constants are given as k = 1.91 ± .07 x 10 7 s e c " 1 • 1 1 ..-1 -1 k = ^10 M sec 2 The e f f e c t s of m u l t i p l e p u l s i n g The decay of e aq was bi m o l e c u l a r a f t e r s e v e r a l pulses and the rates i n c r e a s e d s i g n i f i c a n t l y . Although some f i r s t order character was s t i l l evident i n the data a f t e r three or four pulses the increase i n r ates was such as to i n d i c a t e pseudo f i r s t order behaviour i n the presence of r e l a t i v e l y high concentrations o f r a d i o l y t i c products. The average h a l f l i f e of [e aq] was reduced by about a f a c t o r o f 2 a f t e r 64 16 14 12 K 8 6 4 2 J i i t i i 1 - 1 n-6 -1 x 10 sec , i n-10 ..-1 -1 x 10 M sec 1 2 3 4 Pulses 8 9 10 (a) 1 2 3 4 Pulses 8 .9 10 ( b) Diagram 17. (a) -The e f f e c t s of m u l t i p l e p u l s i n g on (a) r a t e constants and k^, and (b) time taken f o r the i n i t i a l absorption s i g n a l to to reduce by a f a c t o r of 2 65 10 p u l s e s . A t y p i c a l p l o t from a m u l t i p l e - p u l s e experiment i s shown i n diagram 17 where the r e l a t i v e changes i n and k^, and the h a l f l i f e of e aq are i n c l u d e d as- a f u n c t i o n of the number of pulses f o r two d i f f e r e n t path lengths, 1.0 mm and 2.6 mm. The E f f e c t o f H + The a d d i t i o n of a c i d to the aqueous system markedly reduced the l i f e t i m e o f the hydrated e l e c t r o n . With a path length of 3.6 mm and 5 x 10 ^ M H + from s u l p h u r i c a c i d the absorption s i g n a l had disappeared i n about 40 nanoseconds (whereas the n e u t r a l water c o n t r o l i n d i c a t e d only a 17.5% decrease during the same time i n t e r v a l ) . From the p o i n t s a v a i l a b l e f o r the e aq decay i n a c i d s o l u t i o n the r e a c t i o n was unambiguously f i r s t order; t h i s i s explained as the concentration of H + exceeds that of e aq (as c a l c u l a t e d from the i n i t i a l 2 o p t i c a l density) by 10 and thus pseudo f i r s t order k i n e t i c s might be a n t i c i p a t e d . e aq + H + >• H + OH aq 7 -1 The ra t e constant was c a l c u l a t e d to be 7.4 ± .5 x 10 sec . The c o n t r o l sample data showed a f i r s t order decay (k = 9.46 ± .16 x 10^ sec ^) up t o about 80 nanoseconds and then a second order process appeared to take over. The probable reasons f o r t h i s e f f e c t commonly seen i n n e u t r a l water w i l l be discussed i n part IV. The e f f e c t of an a l c o h o l Isopropyl a l c o h o l reacts r e a d i l y w i t h OH and H r a d i c a l s and 66 t h e r e f o r e should increase the l i f e t i m e of the hydrated e l e c t r o n . Whereas i n the c o n t r o l sample the i n i t i a l absorption s i g n a l had decreased by 42% 100 Nsecs a f t e r the e l e c t r o n p u l s e , the s i g n a l from the a l c o h o l i c s o l u t i o n had only decreased by 20%. Again data from the c o n t r o l sample f i t t e d a f i r s t order treatment up t o about 75 nano- seconds. The data from the al c o h o l experiments gave good second order p l o t s over the complete time p e r i o d examined (140 nanoseconds) and with one exception the f i r s t order treatments could be ignored. The reasons for. the exception are not known -- the p o i n t s were not s c a t t e r e d . In treatment ( i ) although the f i t s were not s a t i s f a c t o r y the evaluated r a t e constants were o f the c o r r e c t order o f magnitude. The k^ c a l c u l a t e d from treatment ( i i ) was 3.8 ± .1 x l O 1 ^ M sec 1 . The e f f e c t of oxygen The v a r i a t i o n i n the rate of decay o f e aq i n the presence o f d i f f e r e n t concentrations of d i s s o l v e d oxygen was followed during three s e r i e s o f experiments; the f i r s t s e r i e s used l i q u i d water c o n t a i n i n g "atmospheric" oxygen (1/5 atmos), the second employed oxygen saturated s o l u t i o n s (1 atmos) and the t h i r d a c o n t r o l s o l u t i o n degassed i n the usual manner. A 3.6 mm path length was used. The data from the decay of e aq i n the c o n t r o l s o l u t i o n i n i t i a l l y f i t t e d a 1° order treatment to about 90 nanoseconds and the decay i n the oxygen satura t e d s o l u t i o n s f o l l o w e d the same p a t t e r n . I t i s suggested that pseudo f i r s t order k i n e t i c s are given i n the l a t t e r case because of the much higher concentration of oxygen i n r e l a t i o n to the hydrated e l e c t r o n . The atmospheric oxygen samples supported a second order decay f o r the hydrated 67 control Oxygen Alcohol Acid Diagram 18. The time taken f o r the i n i t i a l absorption s i g n a l of e~ aq to decrease by a f a c t o r of 2 i n the presence of d i f f e r e n t s o l u t e s . 68 e l e c t r o n which was to be expected although the curves on the f i r s t order p l o t s had long l i n e a r p o r t i o n s to which the computer f i t t e d a r a t e constant. Data up to 120 nanoseconds used f o r the second order p l o t s . i "I k^ sec Con t r o l sample 8.75 ± .47 x 10 6 7 atomospheric C>2 (1.03 * ; .04 x 10 ) 7 saturated 0 2 1.28 ± .01 x 10 The e f f e c t s of these s o l u t e s are compared i n diagram 18. The p i n h o l e experiments In these experiments a study was made of the d i f f e r e n t i n i t i a l o p t i c a l d e n s i t i e s and r a t e constants observed f o r the e aq decay at va r y i n g p o s i t i o n s across the width of the r a d i a t i o n tube. I n t u i t i v e l y one would expect a higher concentration of r e a c t i n g species i n the sid e of the c e l l through which the e l e c t r o n beam f i r s t penetrates as i t i s here that the m a j o r i t y of secondary i o n i s a t i o n s w i l l occur. The pinh o l e r e s t r i c t o r p l a t e has already been described and t r i a l experiments i n d i c a t e d that p i n h o l e no. 2 (0.0225 thouO was the most s a t i s f a c t o r y compromise between an adequate i n t e n s i t y f o r the emerging l a s e r beam and the r a t i o o f the diameter of the pi n h o l e to that of the beam. The p o s i t i o n s were recorded as near, centre and f a r (see diagram 19). Great care was taken to scan only the main p o r t i o n and avoid the d i f f r a c t e d and s c a t t e r e d periphery of the l a s e r beam. Attempts to reproduce these p o s i t i o n s on d i f f e r e n t occasions by v i s u a l s e t t i n g s (with the a i d of the f i n e l a t e r a l adjustment on the base of the p l a t e ) were reasonably s u c c e s s f u l as i n d i c a t e d by the good agreement between the s e r i e s of experiments. k 2 M sec 8.43 ± .25 x 10 1.12 ± .25 x 10 1.45 * .35 x 10 69 The data i s tab u l a t e d below. In the near p o s i t i o n about 86% of the l i g h t was i n i t i a l l y absorbed, about 66% i n centre and only about 43% at the f a r s i d e . These are average percentages over a l l e x p e r i - ments and do not vary more than ±5%. The&I<_Q i s the decrease i n the absorption s i g n a l a f t e r 50 nanoseconds expressed as a percentage of the i n i t i a l a bsorption s i g n a l ; although the l a t t e r v a r i e s c o n s i d e r a b l y across the tube ( i n d i c a t i n g a s u b s t a n t i a l c oncentration gradient) t h i s value does not. Despite the conc e n t r a t i o n gradient i t appears as though the r a t e of l o s s of the absorbing species i s not very d i f f e r e n t across the tube which i s perhaps a q u a l i t a t i v e statement of f i r s t order behaviour. Table IV -1 -1 -1 P o s i t i o n I n i t i a l O.D. % I , , A V AI_. k,sec k„M' sec O.D. at 90 5 0 1 2 TNsecs. 7 10 NEAR .89 ± .15 .34 86% 21% 1.01 ± .04 x 10 7.02 ±.03 x 10 CENTRE .47 ± .06 .23 66% 28% 8.73 ± 1.3 x 10 6 8.03 ± 1.7 x 1 0 1 0 pAR .25 ± .04 .16 43% 23% poor f i t 1.36 ± .12 x 1 0 1 1 WHOLE .95 ,-• .32 85% 25% 6.10 6 7.10 1 0 BEAM A d e t a i l e d examination of f i r s t and second order p l o t s (with p o i n t s every 5 nanoseconds) of a l l the data provided no obvious conclusions concerning the k i n e t i c behaviour of e aq i n these s p e c i f i c areas. In the near p o s i t i o n s the f i r s t order p l o t s were c o n s i s t e n t l y good f o r about 40 nanoseconds a f t e r the pulse and then they t a i l e d o f f ; 70 at approximately the same time the second order p l o t s became l i n e a r and continued so f o r the remainder of the absorption s i g n a l f o l l o w e d , which was normally 100 nanoseconds. In no instance was the change i n slope a dramatic one. (In t a k i n g data from o s c i l l o s c o p e traces every f i v e nanoseconds one i s l i m i t e d by the d u r a t i o n and noise l e v e l on the t r a c e : on photographs covering s e v e r a l hundred nanoseconds r e s o l u t i o n of l e s s than 10 nanoseconds becomes very u n c e r t a i n ) . This appears to be a r e a l e f f e c t , not an a r t e f a c t due to say some l i m i t a t i o n of the de t e c t i o n system or widely s c a t t e r e d p o i n t s . The rates given i n the t a b l e are averaged over a l l experiments, and have an absolute value w i t h i n framework of the pinhole experiments but are i n the f i r s t instance f o r comparison only w i t h the f u l l beam experiments. The agreement f o r the unimolecular (now r e f e r e d to as k^) and bi m o l e c u l a r (now r e f e r r e d to as k^) r a t e constants were good, and both k^ and k^ represent very f a s t r e a c t i o n s . Treatment ( i ) d i d not give any u s e f u l r e s u l t s . In the centre p o s i t i o n the graphs were almost as w e l l matched as those of the near p o s i t i o n s but i t was more d i f f i c u l t t o judge the slopes. I t d i d not appear that two mechanisms were o c c u r r i n g simultaneously as treatment ( i ) gave very poor r e s u l t s . The only c l e a r f a c t was that the data could f i t both f i r s t and second order treatments q u i t e adequately, and t h i s i n i t s e l f was confusing. A s u b t l e change i n slope appeared i n some of second order p l o t s i n d i c a t i n g l i n e a r i t y above ^35 nanoseconds ( a f t e r the pulse) when the i n i t i a l absorption s i g n a l had decreased by some 15%, but to what extent i f any t h i s i s a change i n slope i s due to a change i n mechanism i s open to s p e c u l a t i o n . The f i r s t order p l o t s 71 AOD = (OD - OD t = 9 ( )) far centre near © e o Diagram 19. The f i r s t order decay of e aq i n the pinhole experiments. T " i I i 1 1 1 i 1 1 1 r J 1 1 1 1 i t t i i t 0 10 20 Time (nsecs) 50 6.0 70 72 were l i n e a r as f a r as they were fol l o w e d , to 65 nanoseconds; the reasons f o r t h i s seemingly short data-period have already been given. The and k^ evaluated from these slopes (k^ a f t e r 35 nanoseconds) are again high but although the average values are s l i g h t l y lower than the k^, k^ (near), i n d i v i d u a l values are i n the same range, and w i t h i n experimental e r r o r can be considered constant. F i n a l l y the data from the f a r p o s i t i o n was compared to that discussed above and t h i s time g r a p h i c a l a n a l y s i s i n d i c a t e d r a t h e r poor f i r s t order and reasonable second order f i t s . Treatment ( i ) was u n i n f o r - mative Iher.e.-'too:. However another curious feature appeared i n the absorp- t i o n and decay s i g n a l s of most of these experiments. The i n i t i a l 25 ± 5 nanoseconds were c h a r a c t e r i s e d by a p l a t e a u a f t e r which the decay of the absorbing specie was b i m o l e c u l a r . In view of the r e l a t i v e l y weak absorption observed i n the f a r p o s i t i o n and thus the poor s i g n a l to noise r a t i o i t i s not at present p o s s i b l e to decide unequivocally whether the p l a t e a u was i n f a c t an extremely slow decay whose slope could not be determined at the l e v e l s of s e n s i t i v i t y employed, or whether i t repre- sented a time " e q u i l i b r i u m " i n that f a r region of the r a d i a t i o n c e l l . Again i t i s f e l t that t h i s i s a genuine r e s u l t and not a t e c h n i c a l a r t e f a c t as i t was reproducible on d i f f e r e n t occasions when the components of the d e t e c t i o n and monitoring system were i n d i v i d u a l l y checked. The second order r a t e constant f o r the disappearance of the absorbing specie was determined to be 1.36 ± .12 x 10^^ which i s s i g n i f i c a n t l y f a s t e r than the other k^ values. In summary the pinhole experiments lead t o three conclusions. F i r s t that there i s a marked change i n o p t i c a l d e n s i t y across the depth 73 of the i r r a d i a t e d volume of l i q u i d which i n terms of the concentration -4 of the hydrated e l e c t r o n means about a 70% decrease from 2.9 x 10 M at the near edge to 8 x 10 ^ M at the f a r edge. Secondly the k i n e t i c s are complex even i n the r e s t r i c t e d regions of the i r r a d i a t i o n l i q u i d that were s t u d i e d ; only i n the f a r region do the k i n e t i c s appear to be second order, while a preference f o r f i r s t order behaviour i s i n d i c a t e d e l s e - 7 -1 where. T h i r d l y the ra t e constants are h i g h , k^ being of the order 10 sec and k^ v a r y i n g from 0.70 t o 1.36 !x ''lOsecM"} sec 1 The v a r i a t i o n s i n k^ w i l l be averaged i n a f u l l beam study and thus i l l u s t r a t e the hazards o f making d e t a i l e d k i n e t i c i n t e r p r e t a t i o n s on systems w i t h an inhomogeneous d i s t r i b u t i o n s of r e a c t i n g s p ecies. The deuterated e l e c t r o n ( i ) The formation and decay of the deuterated e l e c t r o n were a l s o s t u d i e d w i t h the f o l l o w i n g m o d i f i c a t i o n s t o the system. A 15 mwatt sp e c t r a physics model 124 Helium Neon l a s e r replaced the lm watt l a s e r and the beam emerging from the window of the p l e x i c e l l was d i r e c t e d on to the photocathode wires i n the p h o t o m u l t i p l i e r tube now out o f i t s copper housing (and t h e r e f o r e f u l l y exposed to the beam). The aperture of the i r i s some 10 cms away from the p h o t o m u l t i p l i e r was adjusted to remove a l l the s c a t t e r e d l i g h t around the main beam, and a l l the experiments were c a r r i e d out i n a darkened room. The lens was omitted from the alignment. The very high frequency s i g n a l s observed from the 1 m watt l a s e r were not unexpectedly present ( i n view of t h e i r probable o r i g i n ) i n t h i s more powerful l a s e r , but o p e r a t i n g the p h o t o m u l t i p l i e r at 500 v o l t s gave a S:N r a t i o of 150:1 under steady s t a t e c o n d i t i o n s and about 25:1 during and 74 immediately a f t e r the e l e c t r o n pulse. The p h o t o m u l t i p l i e r was not saturated by the l i g h t i n t e n s i t y ( although the concept of s h i n i n g such a l a s e r at a p h o t o m u l t i p l i e r i s not to be too r e a d i l y accepted) 'under the working c o n d i t i o n s . The apparent improvement i n the S:N r a t i o can be a t t r i b u t e d to the lower working voltage on the p h o t o m u l t i p l i e r and the increase i n a v a i l a b l e l i g h t i n t e n s i t y which permitted a higher attenuation to be used i n re c o r d i n g the s i g n a l s . ( i i ) U nfortunately the l i m i t e d supply o f D̂ O immediately a v a i l a b l e was i n s u f f i c i e n t f o r degassing to take place i n the manner p r e v i o u s l y described. A 50 ml sample o f D^O (Merck.S§D.) i n a sealed, s t e r i l i s e d polythene c o n t a i n e r was v i g o r o u s l y degassed with helium s u p p l i e d through a f r i t t e r e d glass oval and escaping v i a a temporary opening. A f t e r t h i r t y minutes of shaking and degassing the g a s - l i n e on the container was q u i c k l y exchanged f o r a short f i t t e d n o z z l e . The l a t t e r s l i p p e d i n t o the f i l l i n g p a r t of the p l e x i c e l l and thus the 0^0 was introduced by p r e s s u r i s i n g the container. When the c e l l was f u l l and there were no gas bubbles present the nozzle was removed and both p a r t s sealed. The oxygen content of the D2O was c e r t a i n l y l e s s than atmospheric but greater than that estimated f o r the l i q u i d water experiments. The e f f e c t of oxygen on the decay o f e aq has been s t u d i e d and from t h i s i t i s f e l t that the probable amount of oxygen present would not g r e a t l y e f f e c t the observed ra t e constants f o r the decay of e^, but nevertheless more accurate determinations can be made. The pinhole experiment was repeated w i t h D 0, but here the presence of oxygen and r a d i a t i o n products owing 2 to the n e c e s s i t y of r e p e t i t i v e p u l s i n g i n many instances permits only a  76 q u a l i t a t i v e statement o f the r e s u l t s . ( i i i ) The decay o f e^ was undoubtedly f i r s t order f o r the i n i t i a l 90 nanoseconds a f t e r the pulse u s i n g the f u l l beam but the d e v i a t i o n from the l i n e a r p l o t was not very pronounced even at 120 nano- seconds; second order treatment gave no s a t i s f a c t o r y f i t s although am1 approximate estimate of the b i m o l e c u l a r rate constant was 1 0 1 1 M 1 sec 1 as the f i t improved. The decay of e^ a f t e r 2 consecutive pulses essen- t i a l l y f o l l o wed second order k i n e t i c s . In n e i t h e r case d i d treatment ( i ) i n d i c a t e two simultaneously o c c u r r i n g processes. The pinhole experiments again showed a marked concentration gradient across the depth o f the i r r a d i a t e d volume but no attempt w i l l be made to d i f f e r e n t i a t e between the processes o c c u r r i n g i n the d i f f e r e n t areas f o r reasons given above. The A I^Q values v a r i e d l i t t l e i n these areas, but c u r i o u s l y decreased a f t e r two pulses . This may be an a r t e f a c t . A few comparisons are made below. A path length o f 0.25 mm was used. Table V Region O.D. A l 5 0 ^1 s e c ^ ^2 M ^ s e c _ 1 F u l l beam -i-67 ±.16 30% 8.14 ± .21 x 10 6 ( 1 0 U ) * 1 pulse F u l l beam 6 7 ± > 1 6 20% (10 7) 9.88 ± .50 x 1 0 1 0 2 pulses NEAR I t = 0 14% (106') (3.10 1 0) CENTRE 1.8 ± .1 13% (10 6) (4.10 1 0) FAR .77 ± .06 11% poor f i t (6.10 1 0) *the data i n brackets are only i n c l u d e d t o show the r e l a t i v e rates that were i n d i c a t e d and imply nothing more. 77 In summary the decay of the deuterated e l e c t r o n appears t o f o l l o w f i r s t order k i n e t i c s f o r about 90 nanoseconds a f t e r the pulse and then tends towards second order behaviour as was observed f o r the hydrated e l e c t r o n . Interference phenomena The d e t e c t i o n of appreciable f l u c t u a t i o n s i n high i n t e n s i t y a f t e r the e l e c t r o n pulse induced absorption, f o r some while prevented any accurate measurements on the r e a l absorption and decay s i g n a l o f the hydrated e l e c t r o n . At t h i s stage the p l e x i c e l l only was being used with no e l e c t r o n beam r e s t r i c t o r block or grounding p l a t e . The f l u c t u a t i o n s sometimes amounted to 70% of t h e . i n i t i a l a b s o r p tion, were qu i t e reproducible and l a s t e d s e v e r a l microseconds. The p o s s i b i l i t y that e aq* was being photolysed i n s i g n i f i c a n t q u a n t i t i e s by the high i n t e n s i t y o f the l a s e r beam and on r e t u r n i n g t o the ground s t a t e proceeded to absorb once more could not be r u l e d out. However i n experiments under i d e n t i c a l c o n d i t i o n s but w i t h a c i d i f i e d water (0.1 M H +) the e aq absorption completely disap- peared as expected but the i n t e n s i t y f l u c t u a t i o n s remained as strong and c h a r a c t e r i s t i c as i n the n e u t r a l water experiments. The t r a c e s are reproduced i n diagram (20) i n c l u d i n g one showing l a s e r i n t e r f e r e n c e . Other p o s s i b l e absorbing species were a l s o r u l e d out. Because of the i n t e r f e r i n g but reproducible nature of these f l u c t u a t i o n s a systematic study was made to t r y and d i s c o v e r t h e i r o r i g i n and to e l i m i n a t e them ( i f not completely) at l e a s t from the f i r s t 500 nanoseconds on the t r a c e s . For convenience these s i g n a l s w i l l now be c a l l e d "waves" as t h i s best describes t h e i r i r r e g u l a r u n d u l a t i n g No restrictor plate top-- H 2 0 bottom: acid Long pulse 200 nsec/div. Beam restrictor and A l block in place Short pulse - h-LO 10mV/div. • ***** T P 100nsec/div. Typ ica l high frequency laser s ignal , 20mV/div. A) u u r 10 nsec/div. Diagram 20. O s c i l l o s c o p e t r a c e s o f the I n t e r f e r e n c e S i g n a l s 79 appearance. There were three p o s s i b l e general causes f o r these waves ( i ) e l e c t r i c a l ; r i n g i n g i n the p h o t o m u l t i p l i e r or r e f l e c t i o n s i n the grounding system, space charge e f f e c t s , mismatch of tran s m i s s i o n l i n e s . ( i i ) o p t i c a l ; malfunction of the l a s e r , sudden l o c a l i s e d changes i n r e f r a c t i v e index i n the l i q u i d , s c a t t e r i n g of l i g h t . ( i i i ) pressure waves; across the i r r a d i a t i o n c e l l a f t e r the impact of the e l e c t r o n p u l s e , shock wave through the a i r space between the e l e c t r o n tube window and the c e l l . The r e s u l t s o f the study w i l l be summarised b r i e f l y ; u l t i m a t e l y i t was only p o s s i b l e to delay the onset and reduce the magnitude of the waves, the p r e c i s e nature o f t h e i r o r i g i n remaining u n c e r t a i n . In as much as they were not the r e s u l t o f chemical a c t i v i t i e s of e aq and the absorption and decay s i g n a l s were f i n a l l y r e s o l v e d the problem remained a t e c h n i c a l one i n the context of t h i s research. I t would be extremely i n t e r e s t i n g however t o pursue t h e i r o r i g i n as a primary o b j e c t i v e . ( i ) e l e c t r i c a l : i n the absence o f the p l e x i c e l l but otherwise i d e n t i c a l c o n d i t i o n s the waves could not be observed. Several gf'ound loops were d e l i b e r a t e l y i n c o r p o r a t e d to see i f the frequency or appearance of the waves could be a l t e r e d i n the presence of the p l e x i c e l l , but with negative r e s u l t s . The p h o t o m u l t i p l i e r was checked f o r s a t u r a t i o n e f f e c t s and overshoot; the waves d i d not reduce or disappear as the p h o t o m u l t i p l i e r o p e r a t i n g voltage was increased and t h e r e f o r e space charges as w e l l were r u l e d out. I t appeared that t h i s approach would not lead to the o r i g i n o f the waves. 8 0 ( i i ) o p t i c a l : i f the l a s e r were switched o f f (but remained i n the c i r c u i t ) and the acceleratJ>Ja f i r e d under normal experimental c o n d i t i o n s the waves disappeared, t h e r e f o r e no emission was c o n t r i b u t i n g to t h e i r shape and the v a l i d response of the p h o t o m u l t i p l i e r was confirmed. I f the l a s e r was switched on but the p l e x i c e l l emptied of a l l s o l u t i o n the waves again were not observed. On s u b s t i t u t i n g a 2 mm diameter glass :iod w i t h p o l i s h e d ends f o r the glass i r r a d i a t i o n tube i n the c e l l an absor- p t i o n s i g n a l and permanent change i n o p t i c a l d e n s i t y occurred a f t e r 1 ̂ .second (due to the presence of trapped e l e c t r o n s ) but the height and shape of the s i g n a l i n d i c a t e d that there were no waves superimposed during the i n i t i a l 500 nanoseconds. S c a t t e r i n g of the l a s e r beam.by d u s t - p a r t i c l e s i n s o l u t i o n or m i c r o s p l i n t e r s of glass that may be present i n the i r r a d i a t i o n tube a f t e r s e v e r a l e l e c t r o n pulses was u n l i k e l y to cause such enormous f l u c t u a t i o n s i n the i n t e n s i t y ; the waves remained unaffected by changing r a d i a t i o n tubes and u s i n g f r e s h s o l u t i o n s . The waves t h e r e f o r e were a f u n c t i o n of the l i q u i d and i t seemed probable that they arose from the sudden l o c a l i s e d changes i n r e f r a c t i v e index t h a t occurred immediately a f t e r the pulse along the length of the i r r a d i a t i o n tube. The r a d i a l d i s t r i b u t i o n of the e l e c t r o n beam i t s e l f l e d t o a f a r higher concentration of e l e c t r o n s i n the centre of the i r r a d i a t i o n tube i n comparison t o e i t h e r end of the tube. However t h i s could not be the complete explanation as the waves continued to appear q u i t e s t r o n g l y f o r tens of microseconds a f t e r the pulse and any i.nhomogeneity would be smoothed out w i t h i n a microsecond. 81 ( i i i ) pressure waves: the impact o f the e l e c t r o n beam i s powerful -- i n the 2 MeV Febetron 30 nanosecond e l e c t r o n pulse o f 1.8 MeV e l e c t r o n s h a t t e r s any glass windows on the c e l l s and s t a i n l e s s steerl must be used (48.) . I t would not be s u r p r i s i n g t h e r e f o r e t o have a pressure wave r e f l e c t i n g back and f o r t h across the diameter of the i r r a d i a - t i o n c e l l . This pressure wave could a r i s e from the energy t r a n s f e r between the glass i r r a d i t i o n tube and the l i q u i d a f t e r the impact of the e l e c t r o n beam, or could be t r a n s m i t t e d from a shock wave moving behind the e l e c t r o n beam (as i t emerges from the electron-tube window towards the glass tube) across the e n t i r e p l e x i c e l l . Another p o s s i b i l i t y f o r an i n t e r n a l pressure wave has been described as ' m i c r o c a v i t a t i o n ' (49); i t i s suggested that there i s a sudden increase i n volume when the e l e c t r o n s f i n a l l y become s o l v a t e d (10 1 1 seconds) and t h i s amounts to pocket " e x p l o s i o n s " i n the l i q u i d . A pressure wave w i l l give r i s e to changes i n r e f r a c t i v e index and thus i t i s d i f f i c u l t to separate the two ideas experimentally. That there was a s i g n i f i c a n t impact on the r a d i a t i o n tube was shown by f i l l i n g the p l e x i c e l l w i t h water c o n t a i n i n g a very small q u a n t i t y of o i l . The l a t t e r s e t t l e d to the bottom o f the tube and d i d not i n t e r f e r e w i t h the l a s e r beams. On p u l s i n g t h i s s o l u t i o n the same s o r t of wave s i g n a l s were recorded w i t h the e aq absorption; but p r o j e c t i n g the t r a n s m i t t e d l a s e r beam on t o a screen one could even see the changes i n i n t e n s i t y as the o i l d r o p l e t s moved i n t o the path of the beam. The same e f f e c t cOuld be produced by ge n t l y tapping the tube i n the p l e x i c e l l to mix the l i q u i d s . Other l i q u i d s w i t h d i f f e r e n t v i s c o s i t i e s and non H-bonded 82 solvents were examined i n the p l e x i c e l l and l a t e r r a d i a t i o n tubes of up t o 1.5 cm i n diameter i n i n d i v i d u a l supporting frameworks used to contain these l i q u i d s and i n v e s t i g a t e the p r o p e r t i e s of the waves ait'some distance from the formation of the hydrated e l e c t r o n . In these areas the waves were very weak and s t r u c t u r e l e s s s i g n a l s as one might a n t i c i p a t e from a d i s s i p a t e d pressure wave, but i n the l a r g e r tubes the problem of space charges could no longer be ignored. In solvents of high v i s c o s i t y the amplitude and frequency of the waves were considerably reduced. ( i v ) summary: on the assumption that these waves were phenomenon a r i s i n g from sudden ( i n t e r n a l or e x t e r n a l ) pressure or concentration changes i n the l i q u i d i n the c e l l attempts were made to reduce the a i r space between the p l e x i c e l l and the electron-tube window and to c o l l i m a t e the e l e c t r o n beam. The most s a t i s f a c t o r y t e c h n i c a l arrangements have been described i n the experimental s e c t i o n although the beginning of a much weaker wave s i g n a l could s t i l l be observed a f t e r about 600 nanoseconds. 83 D i s c u s s i o n and I n t e r p r e t a t i o n of the Results ( i ) The behaviour of e aq at high dose rates The aim of t h i s research was to provide a f i r m b a s i s f o r the studycof e aq* and t h e r e f o r e i n v e s t i g a t e d the decay of e aq produced i n high concentrations by an e l e c t r o n p u l s e , the i n t e n s i t y of which gave 2 dose rates ^10 above those used i n previous i n v e s t i g a t i o n s . The disappearance of e aq over a p e r i o d of about 70 to 80 nanoseconds a f t e r the 3 nanosecond e l e c t r o n pulse has been shown to f o l l o w a f i r s t order decay; there i s subsequently a slow t r a n s i t i o n towards b i m o l e c u l a r behaviour and t h i s appears to be f i r m l y e s t a b l i s h e d - i n about 100 to 110 nanoseconds a f t e r the pulse. I t i s worth n o t i n g that t h i s t r a n s i t i o n i s . contrary to normal c l a s s i c a l homogeneous k i n e t i c s where a t r a n s i t i o n from l r \ d -»- Isd. order would be expected as the concentration of the r e a c t i v e species decreases. The second-order decay can be followed f o r s e v e r a l hundred nanoseconds a f t e r the pulse although'by t h i s stage the r e a c t i o n s are not e x c l u s i v e l y due to the combination o f two hydrated e l e c t r o n s but g e n e r a l l y to r e a c t i o n s w i t h the products of r a d i o l y s i s . The pinhole experiments however demonstrated very c l e a r l y that the system was not homogeneous during the f i r s t 90 nanoseconds and that even i n l o c a l i s e d regions of the i r r a d i a t e d volume the k i n e t i c s are n e i t h e r f i r s t nor second order i n the c l a s s i c a l sense but are more complex. The p a t t e r n of behaviour observed f o r the deuterated e l e c t r o n followed the same time sequence and again the pinhole experiments demon- s t r a t e d that the d i s t r i b u t i o n of absorbing species was inhomogeneous f o r 84 a s i g n i f i c a n t i n t e r v a l a f t e r the pulse. The f i r s t and second order r a t e constants evaluated at appropriate times a f t e r the pulse are t a b u l a t e d below together w i t h some second order ra t e constants published f o r these r e a c t i o n s . Table VII, .' e" aq y ? k' = 8.80 ± .8 x 10 6 s e c " 1 * e" aq + e" aq y H 2 + 20H" k 2 = 5.88 ± 1.2 x 1 0 1 0 M _ 1 sec" 1*' 10 -1 -1** = 1.22 ± .1 x 10 M sec = 3.2 x 1 0 U M"1 s e c " 1 (46) ed y ? k" = 8.14 ± .21 x 10 6 s e c " 1 * i d + ed y D„ + 20D" k, = ^lO11 N f s e c " 1 * 2 1.20 ± .1 x 1 0 1 0 M 1 s e c " 1 (30) * t h i s work **average value from s e v e r a l l a b o r a t o r i e s : see t a b l e I I The b i m o l e c u l a r r a t e constant determined .in t h i s work i s a f a c t o r o f ^5 higher than that p r e v i o u s l y reported and a f a c t o r of ^5 lower than K l e i n § Warner's r e s u l t s ; a f i g u r e approaching t h e i r value was estimated f o r the decay i n the f a r regions of the i r r a d i a t i o n tube where the d i s t r i b u t i o n was c e r t a i n l y inhomogeneous and (as l a t e r c a l c u l a - t i o n s show) would remain so f o r s e v e r a l hundred nanoseconds. These authors <have made the p r i o r assumption of homogeneous k i n e t i c s i n t h e i r system - 8 on the b a s i s of the mean l i f e t i m e o f a spur being ^10 seconds (2). I t would appear that the changes i n o p t i c a l d e n s i t y they i n i t i a l l y recorded were averages of q u i t e v a r i e d changes across t h e i r r a d i a t i o n c e l l due to the inhomogeneous d i s t r i b u t i o n of e aq. 85 In c o n t r a s t , the behaviour of e aq i n the presence of scavenging species d i d not show any unpredictable trends. The presence of e aq i n the system was confirmed by p u l s i n g an a c i d i f i e d aqueous s o l u t i o n i n which no absorption at 6328 A was detected. Under these c o n d i t i o n s the hydrated e l e c t r o n i s converted to a hydrogen atom too q u i c k l y f o r e aq to be observed, w h i l e at a s l i g h t l y higher pH the absorption could be observed very b r i e f l y before t h i s conversion was complete. In i s o p r o p y l a l c o h o l s o l u t i o n s the experimental h a l f l i f e (time taken f o r the i n i t i a l a bsorption s i g n a l to f a l l t o h a l f i t s value) increased as the a l c o h o l removed H and OH r a d i c a l s before they could react w i t h e aq and deplete i t s c oncentration. The decay was b i m o l e c u l a r although e a r l y stages were ambiguous. The e f f e c t o f d i s s o l v e d oxygen i n the system was to decrease the h a l f - l i f e of e aq but even inthe oxygen saturat e d s o l u t i o n s , the e aq would have t o move from the spur i n t o the bulk medium before r e a c t i n g w i t h the oxygen, as the s o l u t e concentration i s s t i l l only miHi>molar. The fundamental problem that has been r a i s e d by the events of the f i r s t 100 nanoseconds or so a f t e r the e l e c t r o n pulse i s one of inhomogeneity, and one that i n v a l i d a t e s the i n t e r p r e t a t i o n of the data from the c l a s s i c a l k i n e t i c viewpoint. In order to c a l c u l a t e the time lapse between the end of the e l e c t r o n pulse and the overlap of the spurs, that i s homogeneous d i s t r i b u t i o n o f the hydrated e l e c t r o n , the f o l l o w i n g c a l c u l a t i o n s were made. 86 ( i i ) A model r e l a t i n g to the d i s t r i b u t i o n of the spurs i n space and time Assume a simple p h y s i c a l model o f the i r r a d i a t e d volume of l i q u i d immediately a f t e r the p u l s e , with s p l ^ s randomly d i s t r i b u t e d i n small l o c a l i s e d areas, but a marked degree o f rinhomo.igeneity along the cross s e c t i o n of the volume through which the l a s e r beam i s passing. U t i l i s i n g the data from the pinhole experiments (see Table IV) the o p t i c a l d e n s i t y recorded at t = 0 nanoseconds i s taken as p r o p o r t i o n a l to the concentration of the absorbing specie i n a p a r t i c u l a r region of the i r r a d i a t e d volume and from t h i s concentration the energy deposited i n that region can be estimated. From the l a t t e r , the number of spurs i s c a l c u l a b l e and th e r e f o r e the mean volume space occupied by a spur. o The i n i t i a l radius* of a spur a f t e r the pulse i s b e l i e v e d to be^20 A (12) and i f one considers t h i s spur t o be c e n t r a l l y l ocated i n the mean volume space by regarding the two volumes as c o n c e n t r i c spheres then i t i s p o s s i b l e t o determine the distance between the surface o f the spur and the l i m i t s o f the volume i n which the spur must be s t a t i s t i c a l l y present. The time taken f o r the spur t o d i f f u s e to the l i m i t s o f the mean volume space i s then c a l c u l a t e d . As a l l the spheres r e p r e s e n t i n g the mean volume space are i n three dimensional contact w i t h neighbouring volumes the time taken f o r t h i s d i f f u s i o n i s the time lapse before spur overlap creates a homogeneous d i s t r i b u t i o n of hydrated e l e c t r o n s and r a d i c a l s . The approxi- mations necessary are that the spur i s s p h e r i c a l and d i f f u s e s w i t h s p h e r i c a l geometry and th a t the volume of l i q u i d can be represented as an area o f close packed spheres i n each o f which there i s u n i t p r o b a b i l i t y o f f i n d i n g a spur. There i s approximately one t h i r d f r e e volume space not accounted 87 f o r by l a t t e r assumption and so t h i s was i n c o r p o r a t e d i n t o another set of c a l c u l a t i o n s to give an upper l i m i t to the r e s u l t s ; the u l t i m a t e d i f f e r e n c e i n the time necessary f o r overlap was not very l a r g e , ^10%. ( i i i ) C a l c u l a t i o n s and the r e s u l t s The f o l l o w i n g constants were used: G- = 2.6 e aq 1 spur =, 100 eV e = 1.2 x 10 4 £, Path length ( f o r t h i s experiment) 2.6 mm No = 6.025 x 1 0 2 3 . _ , n-5 2 -1 „ = 4.7 x 10 cm sec D— aq e n The concentration of e aq i n a s p e c i f i c region i s 0D_ ei _ I f a d e p o s i t i o n o f 100 eV gives r i s e t o 2.6 molecules o f e aq, then the t o t a l energy deposited per mL that area i s E. E(eVmL _ 1) = 100 eV . [e aq] . No % aq 10 3 The numbers of spurs, C^ i s given by C = E_ spurs mL 1 S 100 The volume occupied by 1 spur i s 3 1 cm = t o a sphere of radius r . c~s S r = (.3 ) 1 / 3 x 10 8 A S 4 . H.C s A spur has radius r Q at time t Q ; the r a d i a l d i f f e r e n c e between the spur and volume of l i q u i d i t occupies i n t h i s region at t Q i s (f- s - r ). 88 The time t taken f o r a spur to d i f f u s e that distance 1/2 ( r - r ) = ( D t ) 1 / Z s o The r e s u l t s of the c a l c u l a t i o n s p e r t a i n i n g to three d i f f e r e n t areas i n the i r r a d i a t e d l i q u i d u sing data from the pinhole experiments are t a b u l a t e d below. The upper l i m i t t * on the time i s that c a l c u l a t e d w i t h the a d d i t i o n o f (:I ) to the mean volume space a v a i l a b l e . S i m i l a r 3C s c a l c u l a t i o n s u s i n g the o p t i c a l d e n s i t i e s recorded f o r the whole o f the area o f the i r r a d i a t e d l i q u i d l e d to overlap times ranging from 30 to Region [e aq]M NEAR 2.85 x 10~ 4 CENTRE 1.50 x 10~ 4 FAR 8.0 x 10" 5 90 nanoseconds. However i n these instances one i s working at an average s i t u a t i o n and the time taken f o r the a c t u a l d i s t r i b u t i o n o f the spurs to be t r u l y homogeneous may be q u i t e d i f f e r e n t . The r e s u l t s o f the c a l c u l a - t i o n s are shown p i c t o r i a l l y i n diagram 21. These c a l c u l a t i o n s show that about 50% of the energy deposited i n spurs (not a l l the energy i s t r a n s f e r r e d to the medium v i a spurs) may be found w i t h i n a very l o c a l i s e d region where the e l e c t r o n pulse f i r s t p e netrates, some 30% at a greater distance away and about 1.5 mm away from the dominant s i t e o f i o n i s a t i o n and e x c i t a t i o n about 15%. These are only general f i g u r e s but the l a s t one may e x p l a i n the appearance o f \ a pl a t e a u on the absorption and decay t r a c e s taken i n the f a r region of the tube. The spurs here are so i s o l a t e d that the e aq cannot react Table VI C ml" 1 s o r A s t Nsecs t* Nsecs 6.57 x 1 0 1 6 153 39 43 3.46 x 1 0 1 6 190 62 76 1.84 x 1 0 1 6 558 503 538 89 0 0 0 o 0 o o o O Q o normalised d i s t r i b u t i o n of spurs (immediately a f t e r the e l e c t r o n pulse) i n the l a s e r beam. D i f f u s i o n o f spurs i n t o bulk volume i n 40 nanoseconds s t i l l leaves regions of inhomogeneity at f a r s i d e of r a d i a t i o n c e l l . near • centre • far Diagram 21. P i c t o r a l Representation of the spur overlap c a l c u l a t i o n s . 9(X> except (assuming the b i m o l e c u l a r decay w i t h i t s e l f ) w i t h another e aq or r a d i c a l i n the same spur. As the spur i s d i f f u s i n g i n time the p r o b a b i l i t y of i n t r a spur r e a c t i o n s t e a d i l y decreases, although the random d i s t r i b u t i o n o f spurs may give r i s e to a greater extent of r e a c t i o n than i s p r e d i c t e d by assuming r e g u l a r i n t e r s p u r d i s t a n c e s . I t was s t a t e d i n part I I I that d e t a i l e d g r a p h i c a l a n a l y s i s showed c o n s i s t e n t l y good f i r s t order p l o t s f o r the decay of e aq i n the near region up to 40 nanoseconds and a f t e r that time the second order p l o t s became l i n e a r w i t h i n the margin of experimental e r r o r . Even f o r t u i t o u s r e s u l t s are encouraging as the time c a l c u l a t e d f o r the spurs to overlap i n t h i s r e gion was 39 to 43 Nsecs. The data had been s c r u t i n i s e d and t a b u l a t e d before these c a l c u l a t i o n s were performed and i t was s t a t e d that the f i r s t order decay f o r the centre p o s i t i o n was l i n e a r up t o 65 nanoseconds the length o f the data p e r i o d . In the l i g h t o f these c a l c u l a t i o n s the spurs were i n the process of overlapping at t h i s time; thus i t would not be p o s s i b l e to see a d e f i n i t e change in::slope as i n s u f f i c i e n t data was a v a i l a b l e . The " s u b t l e change" i n the slope around 35 nanoseconds i s s t i l l open to s p e c u l a t i o n . S i m i l a r l y i t i s not s u r p r i s i n g that the graphs from the f a r region were s l i g h t l y d i f f e r e n t i n r e l a t i o n t o these ot h e r s , and the 11 -1 -1 very f a s t b i m o l e c u l a r r a t e constant (>10 M sec ) must be considered to be an u n r e a l i s t i c e v a l u a t i o n because i n c o n v e r t i n g o p t i c a l d e n s i t y t o concentration homogeneity has been assumed. In conclusion these c a l c u l a t i o n s ( i n s p i t e of the reasonable arguments against the approximations but the lack o f e q u a l l y reasonable 91 a l t e r n a t i v e s ) appear to be q u a l i t a t i v e l y s u c c e s s f u l i n e x p l a i n i n g the inhomogeneity observed i n regions o f the i r r a d i a t e d volume and i n p r o v i d i n g an acceptable b a s i s f o r the change i n slope that appears i n a l l the f i r s t order p l o t s f o r the decay of the hydrated e l e c t r o n , ( i v ) The b i m o l e c u l a r decay A rate constant of (5.88 i l ^ l O 1 ^ M '''sec 1 has been determined f o r a second order decay a f t e r the i n i t i a l 50 nanoseconds . There w i l l be s i g n i f i c a n t amounts of other r a d i c a l s and ions at t h i s stage and so the decay w i l l not be s o l e l y due to e~ aq + e aq ^ >- H 2 + 20H" ̂  = 9.10 9 M _ 1 s e c " 1 but w i l l a l s o i n c l u d e — 2̂ 10 -1 -1 e»aq + OH — > OH aq K = 3.10 M sec — + ^ 1 0 - 1 - 1 e aq + H^O * — • H + OH k 3 = 2.32.10 M sec and c o n t r i b u t i o n s from OH w i l l decrease r a p i d l y due t o OH + OH — »• H 20 2 k 4 = 4 x 10 9 M _ 1 s e c " 1 As the G-values f o r a l l these other species are approximately the same one can assume that t h e i r i n i t i a l concentrations are the same and the expression f o r the loss o f e aq -d(e~ aq) = k [e a q ] 2 + k [e aq] [OH] + k [e aq] [H 0 +] dt * s can be s i m p l i f i e d t o = (k]_ + k 2 + k 3 ) [e~ a q ] 2 By adding the values f o r the rat e constants one has a composite rate constant f o r the bi m o l e c u l a r decay o f G^.IO 1^ M 1 sec 1 and the averaging experimentally determined value of S.88.10 1^ M 1 sec 1 i s i n good agreement. Alhtough not as much in f o r m a t i o n i s a v a i l a b l e f o r e^, the same explanation can be o f f e r e d to e x p l a i n the bimolec u l a r r a t e constant to 'vlO 1 1 M _ 1 s e c - 1 as i n a l l of i t s r e a c t i o n s s t u d i e d (except f o r e^ + D 20) e^ has r a t e constants very s i m i l a r to "e aq. 92 (v) F i r s t order decay Although the k i n e t i c s i t u a t i o n i s f a r from i d e a l , there must . be more than a casual sequence of events that gives r i s e to the c o n s i s - t e n t l y observed f i r s t order decay. And i f there were the events would be t a k i n g place i n the spurs themselves which immediately t r a n s f e r s the s i t u a t i o n i n t o an environment about which l i t t l e i s known and much i s discussed. (Even the second order process might be the o v e r a l l p i c t u r e of two f i r s t order processes though against a l l p ublished work t h i s i s d o u b t f u l ) . Therefore two p o s s i b l e mechanisms that could give r i s e to f i r s t order decay i n the c l a s s i c a l sense w i l l be o u t l i n e d b r i e f l y . The time c a l c u l a t e d f o r overlap i s an average parameter because i n the r e a l case there may at any time be a s i g n i f i c a n t number of spurs overlapping due to the p a r t i t i o n of the energy of the primary i n c i d e n t e l e c t r o n i n t o b l o b s , short t r a c k s and spurs. For a 0.5 MeV e l e c t r o n about 64% of the primary energy f i n d s i t s e l f i n i s o l a t e d spurs but there are s u b s t a n t i a l c o n t r i b u t i o n s from blobs (12%) and short t r a c k s (24%) . The chemical e f f e c t s that take place i n these areas occur i n i s o l a t i o n although, f o r example, at the end of a c y l i n d r i c a l t r a c k there may be q u i t e a high l o c a l c o ncentration of r a d i c a l species. These species are s t i l l t e m p o r a r i l y i s o l a t e d from the other events o c c u r r i n g i n the medium, and may r e a c t w i t h each other w i t h i n the spur. The r a d i c a l species thus d i s t r i b u t e d i n the l i q u i d a f t e r the e l e c t r o n pulse may d i f f u s e together to form - 'encounter-paiis', a t r a n s i e n t intermediate whose occurrence depends on the p r o x i m i t y of 93 the species and the encounter time. The r a t e o f disappearance o f t h i s species depends on two f a c t o r s ; these are ( i ) the frequency of p a i r encounter w i t h i n the spur and ( i i ) the number of p a i r s , t h e r e f o r e the number o f spurs i n the path o f the l a s e r beam. This p r o b a b i l i t y of an event t a k i n g place would give r i s e to an o v e r a l l f i r s t order. Suppose f o r instance that there were a c e r t a i n p r o b a b i l i t y o f two hydrated e l e c t r o n s forming such a t r a n s i e n t intermediate which could e i t h e r r e l a x i n t o the f r e e species again or react q u i c k l y to give the f a m i l i a r r a d i o l y t i c products. In the l a t t e r case the concentration o f the hydrated e l e c t r o n would be decreasing at a r a t e p r o p o r t i o n a l to the k l 2- ra t e o f formation o f the intermediate. Perhaps e aq + e aq v N (e) aq k - l 2 i s e s t a b l i s h e d and the p r o b a b i l i t y o f f u r t h e r r e a c t i o n i n the spur i s zero order; ( e ) 2 " a q ^ >Hg + 20H~ aq or, i f the encounter-pair r e l a x e s to give two hydrated electrons, 2e~ aq — • H 2 + 20H~ aq. But as the number of spurs i s r e f l e c t e d i n the o p t i c a l d e n s i t y measurements, the r a t e o f l o s s of e aq as measured experimentally by AOD w i l l give r i s e dt to a f i r s t order decay. The d i m i n i s h i n g concentration of e aq and the r a p i d l y expanding spur (due to the d i f f u s i o n o f species w i t h i n the spur) l i m i t the time up to which these i n t r a s p u r encounters have any s i g n i f i c a n c e . When the spurs have overlapped then the dominant decay i s through i n t e r s p u r mechanisms. — 2-I f an e q u i l i b r i u m between e aq and the ( e ) 2 d i d e x i s t then the k term may be la r g e enough to observe a tr u e e q u i l i b r i u m experimentally when the number of encounter p a i r s (or spurs) i s r e l a t i v e l y s m a l l , and 94 t h i s i s p o s s i b l y another reason behind the pl a t e a u observed i n the f a r re g i o n i n the pin h o l e experiments. There are many co n s i d e r a t i o n s to an approach of t h i s k i n d the Thermodynamics of the r e a c t i o n sequence and the magnitude of the for c e f i e l d s i n the l i q u i d (which can produce appreciable changes i n the s t r u c t u r e i n the l i q u i d ) being two of b a s i c importance. Whereas above the formation of an encounter-pair was proposed, an a l t e r n a t i v e i s the r a p i d formation i n the spur of a c o r r e l a t e d i o n - p a i r , e aq + H^O* » H^0+ e aq H 30 +.e aq f HO + H which then breakes up to give a hydrogen atom and water. I f the only reason f o r the decay of the species i n the spur i s through such a mechanism (analogous to e l e c t r o n - h o l e p a i r s ) then the separation and coulombic a t t r a c t i o n s between the ions i s of c r i t i c a l importance. The i o n i n d i f f u s i n g away from i t s o r i g i n w i l l see a time dependent coulombic f i e l d due to the s p a t i a l d i s t r i b u t i o n of the other ions i n the spur, and i f t h i s energy i s comparable to thermal energies i t i s u n l i k e l y such i o n p a i r combinations could occur. [ S i m i l a r menomer and dimer species that are e s s e n t i a l l y i o n - p a i r s have been p o s t u l a t e d f o r e l e c t r o n s i n ammonia (50)]. The c o n t r o v e r s i e s a s s o c i a t e d w i t h the recapture of el e c t r o n s are fundamental problems i n t h i s f i e l d of study (10,11). Work published on the recombination of trapped e l e c t r o n s i n glasses and fr o z e n matrices has a l s o shown f i r s t order decay behaviour and suggestions have been made that the disappearance of the e l e c t r o n i s through geminate recombination w i t h the p o s i t i v e ions (14) . According 95 to the d i s t r i b u t i o n of the ions both f i r s t and second order decays could be observed and the recombination times were markedly temperature dependent. S i m i l a r work (42,43) showed a temperature dependent f i r s t order going i n t o o v e r a l l second order decay i n glasses as the temperature rose and the e l e c t r o n s became f r e e to d i f f u s e . This was taken as evidence f o r d i f f e r e n t k i n d of t r a p s . Recently (51) the e l e c t r o n - h o l e p a i r has again been discussed i n terms of an e x c i t o n v i z [(H^O) +''''(H^O) ] , two of which u l t i m a t e l y give r i s e to molecular products of hydrogen, water and hydrogen peroxide. Although the proposal i n the main r e l a t e s to experimental r e s u l t s i n i c e , Weiss regards the s i t u a t i o n i n pulse r a d i o l y s e d water i n the nanosecond r e g i o n as analogous, ( v i ) Epilogue As a b a s i s f o r study on the nature of e* aq t h i s work provided some unexpected but not s u r p r i s i n g conclusions r e l a t i n g to the inhomo- geneous d i s t r i b u t i o n of the species whose k i n e t i c behaviour was under i n v e s t i g a t i o n . I f the i n i t i a l decay mechanism i s a r e a l f i r s t order process the p r e c i s e sequence of events i s open to s p e c u l a t i o n although an e q u i l i b r i u m or charge c o r r e l a t i o n model might be considered. The bimolecu l a r r a t e constant determined was a f a c t o r of 5 lower than K l e i n 2 and Warner's result d e s p i t e the 10 increase i n the dose r a t e . The inhomo- geneity o f the system adds some u n c e r t a i n t y to even t h i s determination and undoubtedly accounts f o r t h e i r high v a l u e . I t i s apparent that i n studying r e a c t i o n s o c c u r r i n g i n tens of nanoseconds one cannot j u s t i f i a b l y convert o p t i c a l d e n s i t i e s i n t o concentrations on which the bimolecular r a t e constants depend. Because of the existence of l o c a l i s e d areas of 96 very high concentrations of r e a c t i v e species the Beer-Lambert r e l a t i o n s h i p which a p p l i e s to homogeneous and continuous systems breaks down. Never- t h e l e s s the i n i t i a l mode of behaviour and h a l f l i f e of e aq i n our system i n the absence of any p h o t o l y s i n g l i g h t has been e s t a b l i s h e d and the next experiments w i l l be to look at the behaviour of e* aq by f l a s h i n g p h o t o l y s i n g the hydrated e l e c t r o n . 97 References 1. I.V. V e r e s h c h i n s k i i § A.K. Pikaev, I n t r o d u c t i o n t o Radiation Chemistry, E n g l i s h t r a n s l a t i o n from I s r e a l Program f o r S c i e n t i f i c t r a n s l a t i o n s , Jerusalem (1964). 2. J.W.T. Spinks § R.J. Woods, An I n t r o d u c t i o n to Radiation Chemistry, John Wiley $ Sons, N.Y. (1964). 3. A.E. Moelwyn-Hughes, P h y s i c a l Chemistry, Pergamon Press, Oxford (1961) 4. R.F. Gould ( E d i t o r ) Solvated E l e c t r o n , Am. Chem. Soc. P u b l i c a t i o n , Washington (1965) . 5. D.C. Walker, i n press. 6. A Mozumder § J.L. Magee, Rad. Res., 28_, 203-214, (1964). 7. A. Mozumder $ J.L. Magee, Rad. Res., 28_, 215-231 (1964). 8. A.H. Samuel $ J.L. Magee, J . Chem. Phys., 21_, 1080 (1955). 9. R.L. Platzman, P h y s i c a l § Chemical Aspects o f B a s i c Mechanisms i n Radiobiology, Pub. no. 305, 22 (U.S./NRC) (1953). 10. L.H. Gray, J . Chim. Phys. 48, 172. (1951). 11. J.L. Magee. Proc. I n f . 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Academic Press 1961. 98 r e f . 40 J.H. Baxendale 98 r e f . 42 G.A. Salmon

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