Nanosecond pulse radiolysis investigation of solute excited states formation in 3-methylpentane at low temperatures

Radlat. Phys. Chem. Vol. 32, No. 3, pp. 519-524, 1988 Int. J. Radlat. Appi. lnstrum. Part C

0146-5724/88 $3.00+0.00 Copyright © 1988PergamonPreu plc

Printed in Great Britain. All rights reserved

N A N O S E C O N D :PULSE RADIOLYSIS INVESTIGATION OF SOLUTE EXCITED STATES F O R M A T I O N IN 3-METHYLPENTANE AT L O W T E M P E R A T U R E S J. M^YEg, M. SZADKOWSKA-NIczE and J. K.ROH The Institute of Applied Radiation Chemistry, Technical University, 93-590 L6dY., Wrbblewsklego 15, Poland (Received 16 July 1987)

Abstract--The influence of temperature down to 90 K, on the formation of excited states in the nanosecond pulse radiolysis of aromatic solutes (9,10-diphenylanthracene, naphthalene) in 3-methylpentane was investigated. Lowering the temperature decreases the yield Of solute singlets and triplets generated during the pulse and extends the time range of excited state formation up to m~seconds. The mechanism of solute excited state formation in irradiated saturated hydrocarbons is discussed.

INTRODUCTION

tion. 9,10-Diphenylanthracene (DPA; Koch-Light, puriss) and anthracene (A; POCh, analytical grade) It is commonly accepted on) that at least part of the were used as received. singiet and triplet excited states of aromatic molecules All solutions were deaerated in the "Spectrosil A" observed by the pulse radiolysis of their solutions in sample cell either by bubbling with argon (helium) aliphatic hydrocarbons have ions a s their precursors. just before the irradiation or by the freeze-thaw However, excited states can be produced as a result technique using vacuum line. The home-made of other processes, i.e. energy transfer, photochemical excitation by (~'erenkov radiation, or direct excitation styrofoam-copper cryostat enabled experiments at temperatures down to 90 K. of the solute by subexcitation electrons. The excited states of the aromatic solute were A possible way of establishing the relative importproduced under the influence of 17 ns electron pulses ance of ion recombination and other processes is to examine the influence of temperature -on the yield of delivered from ELU-6 linear accelerator (U.S.S.R. solute excited states. There are only few data concern- made). The dose in the pulse was in the range 50ing the pulse radiolysis investigation of solute excited 60 Gy as measured by the KCNS dosimeter. Each state formation in aliphatic hydrocarbon systems sample during the experiment received no more than at low temperature. ¢2-1n) At room temperature the I0 electron pUlses. No influence of dose on the yield geminate ion neutralization is very rapid and pro- 0f excited states and ions was found in this dose range at low temperature. duces excited states that are observed as immediate The changes in absorbance following the electron products on the nanosecond time scale. ¢~) Under pulse were monitored using a Xenon lamp (150 W, these conditions the after-the-pulse growth of solute XBO Osram, 3 ms pulse) and a Bausch & Lomb excited states matches the solute-free ion decay. ¢~e) grating monochromator (u.v.-vis). Usually, the slits At low temperature due to the increase of solvent viscosity, one may expect to observe the solute of monochromator were set to give a bandpass of 2.4 and 4.8 rnn for the u.v. and vis range, respectively. excited state formation after the pulse, in the time range corresponding to the decay of geminate ions. Either the Hamamatsu R-928 or IP28 photomultiWe attempt to estimate the influence of temperature plier (input resistance 5(g]) in line with a Tektronix 7834 storage type oscilloscope were used as a (down to ca 90 K) on the formation of solute singiet detection system for both emission and absorption and triplet excited states in 3-methylpentane. Some measurements. The signal was recorded on an Iwatsu preliminary results concerning the temperature effects on solute singiet states formation were presented TS 8123 digital storage oscilloscope and processed by means of an Apple II microcomputer. previously:~ 1) To eliminate saturation and overloading effects on the phototube, the initial emission as well as analysing lights were always kept on the level, which EXPERIMENTAL allowed the tube to operate at current < 10 mA. The The solvent, 3-methylpentane (3MP; Fluka pure Cerenkov radiation of the same order as the initial grade) was chromatographed through activated silica solute emission was measured and its "tail" was gel and kept under argon. Naphthalene (Nph; POCh, compared with the solute delayed emission observed analytical grade) was purified by vacuum sublima- after t h e pulse. 519

520

J. MAYERet al.

Am~ and with the same optical geometry. The ratio of the rate constants for energy transfer from benzene [ t I to D P A and benzene excited singlet deactivation was 0.3 300 found to be 990 tool -] dm 3. The fluorescence lifetimes and fluorescence quantum yields of D P A in both solvents are very simi!ar(ls~ and do not depend on .% O.Z ~_. 200 the temperature in 3MP: °s) The G(DPA*) found in such a way for 10 -3 mol dm -3 D P A solution in 3MP, was treated as a standard for D P A - 3 M P solutions at different temperatures and another solute concentra0.1 1O0 tions. The correction due to (~erenkov emission from the solvent was also necessary. The yield of benzene singlets was assumed to be 1.55Y* The G(DPA*) gradually decreases with decreasing temperature as it 10-6 10-5 10-4 10-3 was reported for microsecond pulses. C6)It is apparent [oPAl that the observed temperature effects are dependent Fig. 1. (a) Influence of DPA concentration on the yield on DPA concentration. Due to the poor solubility of DPA singlets formed during the pulse at different temperatures; [] 295 K; O 90K. Co) Dependence of(AG) -~ of D P A in 3MP at low temperature the solute concentration was limited to 10 -3 tool dm -3. vs [DPA] -°'7 (@). Assuming that the quantum yield of DPA fluorescence does not depend significantly on temperature Such delayed fluorescence leads to an apparent in the range 90-298 K (~s']6) one may conclude that decrease in the observed absorption signal in the spectral region where both fluorescence and triplet the observed temperature effects are due to the temperature-affected radiation-induced processes, i.e. bands appear. The computer corrected absorption signal was obtained from the difference between the ion recombination. When the solution of aromatic solute with a short analysing "light on" and "light off" signals. fluorescence lifetime (7.88 ns for D P A in 3 M P (]5']6) More details concerning the accelerator and detection equipment have been published elsewhere. (]3) is pulse irradiated the delayed emisston due to ion recombination should appear after the pulse. The influence of temperature on the intensity of the RESULTS AND DISCUSSION delayed emission is shown in Fig. 3. There is an increase of intensity as the temperature decreases Singlet excited states formation to ca 130-150 K with a maximum at ca 130 K. The The influence of temperature on the yield of D P A delayed emission in this temperature range was still fluorescence, G(DPA*) measured at 430nm during observable even a few tenths of a microsecond after the pulse is shown in Figs 1 and 2. The G(DPA~') the pulse. were calculated using DPA-benzene solutions as The influence of temperature on the relative Nph a standard. The G-value at room temperature was fluorescence measured at 325 nm is shown in Fig. 4. obtained by comparing the integrated time emissions at a ~m=~for D P A (10 -3 tool dm -3) in 3MP with the integrated emission of the solute in benzene at the [ O p A ] - ° ' T x l O -3

1

2

3

=.

o

0.5

100

I

lOO

I

I

ZOO

300

T(K)

Fig. 2. Temperature profiles of relative DPA fluorescence intensities for different DPA concentrations: & 10-6tool dm-3; • 10-4mol dm-3; • 10-~mol dm -3

200

300

T(K)

Fig. 3. Influence of temperature on the intensity of delayed emission for 10-3tool dm -3 DPA in 3MP measured at 300 ns after the pulse. Inset: after-the-pulse emission of DPA singlets taken at different temperatures, 500ns/div: (1) 295K, gain x l; (2) 130K, gain x 2; (3) 93K, gain x 2.8.

Nanosecond pulse radiolysis investigation

521

temperature on the Nph excimer emission at 415 nm for 7 x 10-3mol dm -3 Nph in 3MP is shown in Fig. 4. The maximum seen on Nph excimer emission curve coincides with the minimum seen on Nph fluorescence curve explaining the strange shapes of the Nph fluorescenee intensity vs temperature dependences. Similar results were found by us for a pyrene-3MP system. (:~) The lifetime of Nph fluorescence-is relatively long at low temperature °7) (Fig, 5) and the contribution of after-the-pulse ion recombination can be seen as a 0 / AL..,~" I, I I gradual growth o f emission during the first 100 ns 100 140 180 220 260 300 after the pulse in the case of more concentrated Nph T(KI solution (7.5 x 10 -3 mol dm-3). The Nph fluorescence Fig. 4. (a) Influence of temperature on the relative integrated decay hehaviour at low temperature is similar to that flcorescence intensities lr/l~s x for Nph in 3MP measured at 325nm: C) 10-4mol dm -3 Nph; • 7.5 x 10-3mol dm -3 observed for a pyrene-3MP system, em Nph. (b) Influence of temperature on Nph excimer emission Triplet excited state formation intensity at 415nm; • [Nph] ffi 7.5 x 10-3 tool dm -3. In order to observe at room temperature the solute triplets formed in the nanosecond time range as a The fluorescence intensities were corrected for result of ion recombination the appropriate scavenger variation of quantum efficiency of Nph fluorescence must be used: Anthracene, with high fluore'~:ance and with temperature, t~7) The observed effects are Nph intersystem crossing rates,( ~ as well as having a high concentration dependent but one can find some extinction coefficient, e for triplet absorption, To - Ti common features, Initially, the fluorescence intensity at 425nm (~ ffi6.4 x 104mol - t dm 3 cm-lC22)), seems decreases as the temperature decreases to 180-200 K to he the most suitable although its poor solubility reaching a minimum. Then, at lower temperatures in 3MP at low temperature excludes additional there is certain increase of fluorescence yield. In the experiments in such a temperature range. The pulse radiolysis of A in 3MP (5 x 10 -3 tool temperature range 120-130 K a maximum may he seen. At the lowest temperature ( ~ 9 0 K) the remain- dm -3) shows transient absorption with a sharp ing emissions amount to about 80 and 35% of the maximum at 425 nm due to A triplets and broad value corresponding to room temperature for 10 -4 absorption band in the red part of the spectrum and 7 . 5 x 10 -3 tool dm -3 Nph in 3MP, respectively. corresponding to A ions. (23) The time profiles of F o r a more concentrated solution of Nph it was detected absorption signals at 425 and 600 um are possible to observe excimer emission (in the range shown in :Fig. 6: As seen in Fig. 6a, after the initial 400-430 um(~s-20)a t l o w temperature. The influence of absor)tion formed during the pulse there is a slower m m i

i;

mmm

m

mm,,,, - -

mmm .,m %.,mmmm

--,mmmmmmmmm

mmu

,mnmmmmm

,am mm

(a)

(b)

Fig. 5. Influence of Nph concentration on the solute fluorescence decay curve: (a) 145K, 10-s tool dm -3 Nph in 3MP, 20 mV/div, 100 ns/div; (b) 143 K, 7.5 x 10-3 tool dm -3 Nph in 3MP, 50 mV/div, I00 ns/div.

(a)

(b)

m

_J

l •

[

Fig. 6, Osciltog:ope traces of absorptiom o f A tripletsat 425nm (a) and A ions at 600 nm (b) in 3MP (concentration of A, 5 x 10-3 mol dm-3): (a) 10.6%/div, 100ns/div; Co)1.04%/div, 100ns/div.

522

J. MAYERet

ai.

due to D P A triplets becomes indistinguishable even at longer time scale (Fig. 7b). At 105 K one obtains the initial spectrum with DPA ions absorption superI K2,:;' ' ...... . . . . 0.02 ~ _ o . imposed over the absorption of unscavenged electrons e.etrapped in 3MP. In the microsecond time range, o.o, the absorption in the range 500-650 nm starts to decrease probably due to the electron absorption I I I I I -- 1-~ *1" • "I" -- -r - i1- ,,~ - 1- •~l-r -~ decay, whereas at 430-440 nm no significant changes of absorption may be seen. For A < 430 nm one may find some slight increase of the absorption probably due to D P A ion formation but nevertheless all o.o,15 I-t ....... 'M I "0 absorption changes for ,~ ~<430 nm are in the range I= of experimental error. Both transient absorption bands at 440 and 600 nm decay in a similar way at 90K, the lowest temperature in our experiments. 0.02 ~ For the case of Nph at room temperature in the intermediate absorption spectrum (Fig. 8a) one can find the characteristic maximum at 415nm which I I I I I i I I I i I t I I corresponds to Nph triplets. (21) As for A, two modes 400 500 600 700 of solute triplet state production can be observed, one ), (nrn) which is completed during the pulse and another one Fig. 7. DPA intermediateabsorption speotrumin the range which generates triplets over times greater than tens 405-700nm at different temperatures. DPA concentration of nanoseconds. The latter one corresponds to triplets in 3MP 10-3mol dm -3. (a) 298K: O end-of-pulse spectrum; • 8#s after the pulse. Insets: oscilloscope traces of 0.03 absorptions at 440 nm [(1) 3.7%/div, I/zs/div] and 500 nm [(2) 1.15%/div, 1/As/div], (b) 105K: O end-of-pulse spec0.02 D trum; • 16/is after the pulse. Insets: oscilloscope traces of absorptions at 440 nm [(1) 2.1%/div, 2/~s/div] and 600 nm [(2) 2.9%/div, 2/~s/div]. For comparison the end-of-pulse oo, spectrum of irradiated 3MP solvent at 90 K is shown (r-l).

ca)

]..i,,7,7-: ; - :

,-/+t.

increase of absorption over ca 400 n s which may be attributed to a triplet formation by ion recombination (Fig. 610) as was observed earlier, (n'24) The kinetics of these reactions are very complex, neither first nor second order as one may expect for geminate ion recombination. Both processes during the first 400 ns can be approximated by first-order kinetics giving rate constants equal to 2.9 x l0 s and 3.2 x l06 s- 1 for the ion decay and triplet formation, respectively. Pulse radiolysis of l0 -3 tool dm -3 D P A solution in 3MP at room temperature gives intermediate absorption, as shown in Fig. 7a. The end-of-pulse broad absorption band centred at 430-450nm overlaps with a short-lived absorption due to D P A ions. (25) The longer-lived band with, Am~x~ 440 nm, which can be seen at 8/~s, corresponds to D P A triplets. (26) All detected D P A triplets were found to be formed immediately during the pulse. At room temperature the triplet absorption was constant over a period of few microseconds after the pulse [Fig. 7a, inset (1)] without traces of after-the-pulse growth corresponding to ion recombination as it was observed for anthracene. The broad band with )-m~ ~ 600-625 nm can be assigned to the absorption of DPA ions. (25) When the temperature of the solution decreases the contribution of ion absorption to the end-of-pulse spectrum increases whereas the maximum at 440 nm

I

•

F.:;iiiii'!'

i i)771 ;
t |

,

P ........

0 . 0 4 ~ ~ < ~ . .

.4

002

°%-°-..o_~

hbl

(c)

350

.

......

0.02

0

czl

I_

•

s-

~..:

. . .

IF ..... I

I

r, ....

450

• t)

;'12; ,.,

, • ,

550

X(nm) Fig. 8. Nph intermediate absorption spectrum in the range 350-550 nm at different temperatures. Nph concentration in 3MP = 7 . 5 x 10-3 tool dm -3. (a) 298 K: O end-of-pulse spectrum. Insets: oscilloscope traces of absorptions at 415 nm [(1) 10%/div, 100 ns/div] and 440 nm [(2) 0.9%/div, 100ns/div]. (b) 103K: O end.of-pulse spectrum; • 90#s after the pulse. Insets: oscilloscope traces of absorptions at 415 nm [(1) 7.7%/div, I0/~s/div] and 440 nm [(2) 6.55%/div, 10/~s/div]. (c) 95K: O end-of-pulse spectrum; • 4.5#s after the pulse. Insets: oscilloscope traces of absorptions at 415 nm [(1) 4.7%/div, 500 ns/div] and 440 nm [(2) 4.3%/div, 500 ns/div].

Nanosecond pulse radiolysis investigation Ln ~t "~"~'

.~ I

e- + S--.S-

10 I

13 'x.\.\

15 I

3MP + + S--,S + + 3MP

-~

"~'o~m~ ,\,\~,\,~

12

523

(4)

e- + S+--*S *

(5)

3MP + + S---,S*

(6)

3MP* + S---,S* + 3MP. _5

(3)

e- + 3MP+--)3MP *

S- + S+---,S + S*

11

(2)

(7) (8)

lo 9

8 ?

"aN Lt I t I 6

7

8

9

I 10

, I 11

T -I x t 0 3

Fig. 9. (a) Temperature dependence of first-order rate constants for: Nph triplet formation at 415 nm (O) and Nph ion decay at 550 nm (I-7). (b) The viscosity dependence of first-order rate constants for Nph triplet formation (0) and Nph ion decay (m), The viscosities of 3MP at different temperatures were taken from Ref. 34,

formed as a result of ion recombination and/or from the Nph singlets because the lifetime of Nph fluorescence equals to c a I00 ns. ~2~) At the lower temperature, i.e. 103 K (Fig. 8b) it was possible to observe the formation of triplets at 415 nm as a result of geminate ion recombination. The decay of Nph ions was observed at 440 nm. (27,2s) At the lowest temperature, 95 K (Fig. 8c) in the end-of-pulse absorption spectrum only the traces of Nph triplet absorption at 415-420 nm can he seen. Probably these Nph triplets are formed as a result of intersystem crossing from Nph singlets. In the temperature range 93-130 K it was possible to follow the growth of Nph triplets at 415 nm and the decay of Nph ion absorption at 550 nm. Assuming that both reactions can he treated as complex first-order processes (29) one may approximate t h e initial part of the kinetic curves (first half-life) using first-order equation. The influence of the temperature on the first-order rate constants is presented in Fig. 9. It can be seen that the Arrhenius relation is quite well fulfilled. The activation energy for both reactions, i.e. triplet formation and ionic recombination is about 16.8 kJ tool -l. The mechanism if solute excited state formation in irradiated hydrocarbon solution involves the scavenging of electrons and positive ions by the aromatic acceptor, followed by ion recombination. The overall reaction scheme to be considered is the following: 3MPH-~3MP + + eR.P.C. 32/3---N

(1)

The solute excited state S* can be either a triplet or a singlet, 3MP + and S + denote the solvent and solute cations, respectively; e- and S- denote electrons and solute anions, respectively. A l l aromatic scavengers used in our experiments react very efficiently with electrons as well as with solvent cations. In order to explain the influence of temperature on solute excited state formation in 3MP one should take into account that the lowering of temperature decreases the mobilities of electrons (~32) and positive ions (33) as well as corresponding rate constants of reactions (2) and (3). Moreover, the rate of ion recombination reactions (5-7), slows down with decreasing temperature ~29)and hence the yield of these reactions during the pulse diminishes. This effect is accompanied by the lowering of fluorescence intensity during the pulse (for DPA and N p h ) w i t h decreasing temperature (Figs 1, 2 and 4). Similarly the initial yield of solute triplets decreases with lowering of temperature (Figs 7b and 8b,c). At the lowest temperatures the traces of solute triplets can be found. In the case of Nph the intersystem crossing from singtets seems to be responsible for triplet formation during the pulse at the 93 K. The decrease of solute fluorescence yield formed during the pulse with decreasing temperature is accompanied by an increase of the delayed emission intensity for DPA (Fig. 3) and gradual growth of fluorescence after the pulse for Nph in the temperature range 135-145 K (Fig. 5). The time scale for geminate ion recombination at low temperature gradually changes from nanoseconds to microseconds and hence the solute triplet generation one can observe in microsecond/millisecond time range instead during the pulse (Fig. 9). The apparent activation energy for both processes, i.e. solute triplet formation and ion decay is considerably lower than the activation energy of viscosity, ~/ changes in 3MP in the examined temperature range.<34) It is known, that diffusion-controlled processes in liquids proceed at a rate proportional to the fluidity, i.e. to ~/- 1. The influence of viscosity on the first-order rate constants is shown in Fig. 9. The straight line was obtained with a slope of -0.5. Similar "anomalous" viscosity dependence of the recombination time of ions in several hydrocarbons at low temperatures has been observed by other authors. ~3'z9"35-37)Thus it seems that in viscous hydro-

524

J. MAYERet al.

carbons the anomalous viscosity behaviour is a general feature of reactions in which diffusion mechanism is involved. Assuming that the observed temperature effects are due only to ionic reactions one may conclude that for DPA fluorescence yields the AG ffi G ( D P A ~ ) ~ G(DPAf)90 should be proportional to the yield of geminate ions. If so, the (AG) -1 ought to be proportional to [DPA] -°7, as seen in Fig. 1. This is the concentration dependence one would expect for geminate ion recombination.°8) At the lowest temperature some scavenger singlets are still formed during the pulse. The residual emission might arise from direct absorption of {~erenkov radiation by the solute. ~39)A certain contribution of energy transfer (8) from the excited states of 3MP to the solute molecules might be taken into account too although this problem needs some further investigations. No significant yield of initial solute triplets was observed at the lowest temperature (Figs 7b and 8c). The lack of an early peak due to solute triplet state is in agreement with picosecond observations.(4°'4t) Acknowledgements--We are grateful for the co-operation of R. Lubis, K. Hodyr and K. Paryjczak of the accelerator group. This research was supported by CPBP 01.19

Program. REFERENCES 1. H. T. Choi, F~ Hirayama and S. Lipeky, J. Phys. Chem. 1984, 88, 4246 and references cited therin. 2. E.J. Land and A. J. Swallow, Trans. Faraday Soc. 1968, 64, 1221. 3. J. Fuller, N. Petelski, D. Ruppel and M. Tomlinson, J. Phys. Chem. 1970, 74, 3006. 4. J. H. Baxendale and P. Wardman, Int, J. Radiat. Phys. Chem. 1971, 3, 377. 5. J. T. Richards and J. K. Thomas, J. Chem. Phys. 1970, 55, 218. 6. J. Kroh, J. Mayer, J. L. Gebiekl and J. Grodkowski, Int. J. Radiat. Phys. Chem. 1976, 8, 433. 7. T. Tanaka, T. MiyaT~ki and Z. Kuri, Int. J. Radiat. Phys. Chem. 1976, 8, 645. 8. G. A. Salmon and K. Selby, Faraday Discuss. Chem. Soc. 1977, 63, 189. 9. J. Kroh and J. Mayer, Nukleonika 1979, 24, 901. I0. T. Miyazaki, N. Kato and K. Fueki, Radiat. Phys. Chem. 1983, 21, 489. 11. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Proc. 6th "Tihany" Symp. on Radiat. Chem. Balatonsz~lak, 1986.

12. J. K. Thomas, K. Johnson, T. Klippert and R. Lowers, J. Chem. Phys. 1968, 48, 1608. 13. S. Karolezak, K. Hodyr, R. Lubis and J. Kroh, J. Radioanal. Nucl. Chem. 1986, 101, 177. 14. P. Skarstad, R. Ma and S. Lipsky, Mol. Cryst. 1968, 4,3. 15. J. V. Morris, M. A. Mahaney and J. R. Huber, d. Phys. Chem. 1976, 80, 969. 16. J. R. Huber, M. A. Mahaney and N. W. Mantulin, J. Photochem. 1973/74, 2, 67. 17. J. Mayer, M. Szadkowska-Niezeand J. Kroh, J. Photochem. 1987, 38, 385 and references cited therin. 18. M. AI-Jarrah, B. Brecklehurst and M. Evans, J. Chem. Soc. Faraday Trans H 1976, 72, 1921. 19. J. B. Aladekono and J. B. Birks, Proc. R. Soc. A 1965, 284, 551. 20. J. Mayer, M. Szadkowska-Nicze and I. Zuchowicz, Radiat. Phys. Chem. 1983, 21, 401. 21. J. B. Birks, in Photophysics of Aromatic Molecules, Chaps 4 and 6. Wiley-Interseience, New York, 1970. 22. R. Bensasson and E. J. Land, Trans. Faraday Soc. 1971, 67, 1904. 23. W. Christodouleas and W. H. Hamill, J. Am. Chem. Soc. 1964, 86, 5413. 24. J. H. Baxendale and P. Wardman, Trans. Faraday Soc. 1971, 67, 2997. 25. W: H. Hamill, in B__ad3_'~!Ions (Edited by E. T. Kaiser and L. Kevan) Chap. 9, Wiley, New York, 1968, 26. T. Medinl~r and F. Wilkinson, Trans. Faraday Soc. 1965, 61, 620. 27. M. R. Ronayane, J. P. Guarino and W. H. Hamill, J. Am. Chem. $oc. 1962, 84, 4230. 28. B. Brocklehurst and R. D. Russell, Trans. Faraday Soc. 1969, 65, 2159. 29. N. Kato, T. Miyazaki, K. Fueki, S. Miyata and Y. Kawai, J. Phys. Chem. 1984, 88, 1445 and references cited therein. 30. G. F. Novikov and B. S. Yakovlev, Int. J. Radiat. Phys. Chem. 1976, 8, 517. 31. W. F. Sehmidt, Can. J. Chem. 1977, 55, 2197. 32. T. G. Ryan and G. R. Freeman, J. Chem. Phys. 1978, 48, 5144. 33. S. S. S. Huang and G. R. Freeman, J. Chem. Phys. 1979, 70, 1538. 34. G. A. von Salis and H. Labhart, J. Phys. Chem. 1968, 72, 752. 35. J. A. Leone and W. H. Hamill, J. Chem. Phys. 1968,49, 5294. 36. T. Sawai and W. H. Hamill, J. Chem. Phys. 1972, 56, 5524. 37. J. Cygler, Z. Czerwik, J. L. Gebieki and J. Mayer, Radiochem. RadioanaL Lett. 1979, 41, 287. 38. H. T. Choi, J. A. Haghmd and S. Lipsky, J. Phys. Chem. 1983, 87, 1583. 39. Y. Katsumura, Y. Tabata and S. Tagawa, Radiat. Phys. Chem. 1984, 24, 489 and references cited therein. 40. G. Beck and J. K. Thomas, J. Phys. Chem. 1972, 78, 3856. 41. Ch. D. Jonah and M. C. Sauer Jr, Chem. Phys. Lett. 1982, 90, 402.