Radioluminescence of methylcyclohexane in the glassy state

lnt. J. Radiat. Phys, Chem. 1976, Vol. 8, pp~ 247--256. Pergamon Press. Printed in Great Britain

RADIOLUMINESCENCE OF METHYLCYCLOHEXANE IN THE GLASSY STATE J. KROH, R. Lr~CzY~SKI and J. MA~[ER InsOtute of Radiation Chemistry, Technical University, Politechnika, L6dz, Poland (Received 8 May 1974; in revised form 21 October 1974)

Abstract--The radioluminescence of pure, frozen methylcyclohexane (MCH) has been investigated. y-Irradiation of MCH at 77 K produces at least three negative species--trapped electrons, et-, and two types of anions, which are formed in the reactions of electrons with radicals, R-, and with stable radiolysis products, P-. It is shown that the radiothermoluminef~enae (RTL) emission is due to the recombination of et- (RTL peak at 90 K) and R-, P- (RTL peak at 95 K) with cations. The intensity-dose dependence of the RTL peaks hasbeen interptet~l using a simple k|netic model.

INTRODUCTION BOTH main ionic entities primarily formed in the irradiated frozen alkanes, i.e. electrons and positive holes, are fairly stable in the glassy matrices at 77 K. The electrons are stabifized competitively either in the matrix traps o r b y attachment to the impurity molecules. The chemical traps may also be formed as a result of radiation destruction o f the matrix itself. In such a case, both radicals and stable radiolysis products (14) often act as additional trapping sites. The recombination o f negative and positive species becomes a source o f deferred luminescence~u~, which can be observed at the temperature of irradiation (isothermal luminescence, ITL) and/or during the warming-up o f the sample (radiothermoluminescence, RTL). Comparatively little attention has been paid to the radioluminescence of pure frozen hydrocarbons (U-s~. The work reported here was undertaken initially as a study of the I T L and R T L o f glassy methylcyclohe.xane (MCH), in order to elucidate certain phenomena accompanying electron-cation recombination and electron decay.

EXPERIMENTAL Koch-Light Lab. Ltd. pure grade MCH was passed through a column (ca. 2 m long) of freshly activated silica gel and stored under nitrogen. The purity of the MCH was tested by absorption measurements in the U.V. (see curve A in Fig. 6). The samples of MCH, in most cases 0-5 cms in volume, were prepared in a thin-waU brass vessel using the technique as described by Btfl]ot et al. ~loa). Deaeration was carried out in the same vessel by the freeze-pump--thaw technique, In order to obtain a glassy structure, the samples had to be cooled very rapidiy~X°~); this was achieved by placing the container directly into liquid nitrogen. The luminescence measurements were performed using a modified cryostat as desoribed by Builot et al. (~e'~. For fight intensity measurements a visible sensitive photomultiplier (M12-FS35, of East German make) was used. The signal was amplified and recorded simultaneously with the temperature of the sample. The frozen samples were irradiated with WCo y-rays at liquid nitrogen temperature. The dose rate was varied from 1.4 × ]0~. to ] .7 × lOt. eV dm --as -x. During the photoblenching experiments the irradiated samples were kept under the surface of the liquid nitrogen. The pathiength of the bleaching light in the MCH sample was ca. 0"2 cm. A Bausch & Lomb high intensity monochromator, cat. 33-86-25 with gratings: cat. 33-86-02 (blaze 247

248

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~I~IOtoH, R. L~zcz,~srd and J, MAY,~a

pass), was ~ t'or p r ~ i o n 0 ~ ~ ~ in~t~ ~ ~ near-l,g, r a n ~ . Appropriate filters reduced the amount of ligiM~ tl~ ~ l i i t w t ~ shorter than the monochromator setting. As a light source, the standard Bausch & Lomb, cat. 33-86-39-01, 45 W tungsten (silica giass-iodine)lamp was used. Since in the bleaching efficiency mc~orements a constant intensity of the photon flux should b e ~ th~ ~ i a e t m ' s for ~ W ~ were calculated, taking into account the radiant fiux~-wgvelen~h ~ curves of the light source. Special attention

has been ~

to t h ¢ ~

~

d t ~ ~ ~ .

As ~

sourou, a ~00 W

tungsten and Q-400 Original I-Ianau (for U.V. range) lamps were used in the photobleaching experiments. In both ~ , the distane¢ hatsmen ~ ¢ ~ampand sample was the same, c a . 10 cm. No collimating lenses were used. RESULTS

The dose depemlotee o f R T L peaks ~

~

RTL~i~-.His~wnin~i~~ofFig.

1. For. a low dose

(ca. 1"3 x 10m eVdm -s) only one peak at 90 K, Pw, has been observed. When the dose

became higher (1-2 x 10u eV dm q ) a second peak at 95 K, P~s, appears. It is worth mentioning that unpurified MCIggi~es,both ltamks at low doses and the intensity of the emitted light is higher than for the pure compound. '

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F1G. 1. The dopendznce of RTL l ~ k intuaRy vs time of irradiation, t, for

MCH, Dose rate 1.7xl@SeV~4s-!; O,l'¢ak e,,, x - - ~

e,~.

The curves are ~ t e d according t o ~ 0 1 ) a ~ C l 3 ~ . Inset: RTL spectra of y-irradiated MCH, - ' - -, d ~ I-3 x 10w eY din- ; , (Jose 1"2 x 10n eV dm -s.

The dose de~ndenee of both RTL ~ of y - i ~ gla~y MCH is shown in Fig. 1. The curves ~ there are caloalated according to equations (11) and (13) (see later). The curve for peak Pm has a maximum at ca. 7 x 10n eV dm -s. Similar dose dependence curves have been found in the eases of trappedelectron concentrations in nonpo!~ ~ i a ( ! ,s'11"n). Shirom ~ ~ i : u , have o b t a i ~ the maximum on an electron ~ ~ n ~ curve for ~ H at ca. 10x 10meVdm -8, Keeping the MCH samples aP~r irradiation over a long period in darkness at 77 K, it has been

Radioluminescence of methylcyct0hexatie in the glassy state

249

found that peak Pse decreases. For example,, after ca. 1300 h of storage peak P,o decreases by about 50 per cent. In orde~ to eliminate any effects due to phase changes, which might occur during that time, iu~rradiated samples were kept over the same period and were then irradiated and used as blanks. According to Lin et al. (xm, the decay of trapped electrons, • t- seems ~o be ~negligible at 77 K. This is in good agreement with the above experiment. The int~sity of peak Pgs does not depend on storage time. Photobleaching efficiency curve In order to identify the negative species responsible for RTL peaks the photobleaching efficiencycurve has been examined. This determination is based on the wellknown fact that the intensity of RTL emission decreases under the influence of light of suitable wavelength. Photobleaching is repeated at different wavelengths with the same intensity of incident beam. Asseming tha t ~ amount of emitted RTL light, ~IRTL, is proportional to the et- conoentration, the following equation has been derived(s),

(1)

E~ = In ~]~/RTL° = K~, l~l° eai it,

where ]~IRTL° and ]~IRTLt denote the amount of light emitted by unbleached and bleached (with X~) samples, respectively; K~l is the probability coefficient of electron disappearance upon absorption of photons at A,; la ° is the constant incident light intensity at Ai; ,aj is the extinction coefficient of the negative species absorption spectrum at Al; I is the optical path length of thesample~ and t is the bleaching time. Since for every series of expeMments the values l~l° 1, t are constant, Ea should vary with bleaching wavelength as eat and KAy. In particular, when K~j has a constant value above a certain threshold of energy required for the detrapping or detachment of the electron, E~ varies with bleaching Ai like ,~, i.e. in the same manner as the a b s o ~ o n spectra of the electrons and anions responsible for RTL. The plot ofE~ vs wavelength of bleaching light gives a "bleaching efficiency curve". The above method is similar to that developed by Deroul~le (xs). Huang and Kevan (14) have found that in the case of 3-methylhexane (3MH) glass the relative optical bleaching rate constant ofej, at 77 K, which is related to Kao varies with the wavelength of the bleaching light. Assuming that there are no differences between the bleaching behaviour of • t- in 3MH and MCH glasses, E~ should be a function of K~, and ~ . It is worth noting that in the range 400-1200 nm the absorption spectrum of et- in MCH glass (12) is represented by a smooth curve slightly rising towards the I.R. It seems to us that in such a case the Ea should be mainly influenced by K~i. The photobleaching efficiencycurve for peak P00 is plotted in Fig. 2. This curve has a maximum at 800 rim and probably another peak appears above 1200 nm. The peak in Fig. 2 is similar to photocurrent and photobleaching rate constant peaks found by Huang and Kevan (x*) in the case of 3MH glass. The rate of decay of Pie under the influence of bleaching light at 800 nm is shown in Fig. (3a). In contrast the RTL peak Pee does not decrease after illumination of the sample by monochromatic light in the range 400-1200 nm. The photobleaching profile, using light at 800 nm for peak P~, is shown in Fig. 3(b). It is worth noting that there is a small maximum at the beginning

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~ ~ ~, E~ vs A,, of bleachin2fightfor peak Proresults ltitve ~ ~ fbr speet~ di~butiOn of blmcht~ I ~ t . I

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t. $ photobleaching wavelenltthA=[IO0~m., dose 2.5x10meVdm "4, t, time of illumination at 77 K. of that curve. Peak P,~ can be removed under the influence of the nonfdtered light of a tungsten lamp (I00 W). For the sample irradiated with a dose of 2.5 × 10w eVdm -s the time required to remove ca. 80 Per cent of peak Pc6 is ca. 1800 s. Peak Pge may be weakened by a factor of 150 as a result of 300 s illumination with the Q-400 lamp.

The identification of RTL peak P~ In order to identify RTL peak Poe the following experiments have been carried out. The degassed sample of MCH has been 7,-irradiated at 77 K. After melting, the pre-irradiated liquid has been used for the pztparation of a RTL sample. The influence of pre-irradiation dose on the intensity of the RTL peaks is shown in Fig. 4. It is clearly seen that such a treatx~eat causes an ~ of peak Pgt and a decrease of peak Pu- Pre-irradiation also affec~ RTL-dose d e p e n d s . As seen in Fig. 5, the maximum on the dose dependence curve for peak Pm is shifted towards the low dose range whereas the slope of peak P~-dose dependence is increased compared with the untreated samples.

Radioluminescence of methylcyclohexane in the glassy state

251

The U.V. spectra o f irradiated M C H samples after melting are presented in Fig. 6. The U.V. absorption band o f irradiated M C H shifts towards the visible range with increasing pre-irradiation dose. 2.0

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FiG. 4. Dependence of RTL peak intensity vs pre-irradJation dose. The RTL samples have been prepared using freshly melted MCH after pre-irradiation at 77 K. (D--peakPse, A--peakPt~. The dose of RTL measurements 8.4x 10JneV dm -a. 2.o~_

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Flo. 5. Influence of pre-irradiation treatment on the dependence of RTL peak intensity vs time of irradiation, ~. Dose rate 1.7 × lO~t eV dm "4 s -1. A and • : blank experiments for peaks Pie and Pte, respectively. Pro-irradiation dose: 5 × 10u eV din-*; © and • for peaks P~e and P,e respectively. Another series of experiments was designed to prove the formation o f anions o f radicals in M C H glass. Such species might be responsible for peak Pn. The samples of frozen M C H after ~,-irradiation at 77 K have been illuminated with the full light intensity o f the tungsten lamp (100W) in order to photobleach the ionic species present in the samples. Under such conditions, the radicals formed in glassy M C H should remain fully or at least partly intact. Then the samples were 7,-irradiated again and the R T L spectra recorded. The results o f these experiments are shown in Fig. 7. Peak Pgo decreases whereas peak P95 increases with the pre-irradiation dose.

252

J. K~oa, R. l . a ~ c z v t ~ u and J. MAYER

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FIG. 6. U l t r a - v i o l e t ~ j ~ f ~ , liquid MCH aftex i~tdiation at 77 K. Optical path l e l ~ : - T ~ [ U - ~ : I ~ : ~ A--pu~:I~lCX-I, no irradiation, B--dose 5 x 10ta eV dm -a, C--dose 3 x 10m eV dm -s.

0

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FIG. 7. Dependence of RTL peak i n ~ vs pre.4xradiation time, t, at 77 K. Dose rate 1.7 x 10It eV dm -s s -1. The RTL ~ l~tv~l~l~ recorded for samples ~ with a n ~ d o , of 8.4~ ]0 m eV ~ = 4 after complete photobleaching of pre-irradiat~ sample with light from a tungsten lamp. © J p e a k Pro, x --peak P05.

Isothermal luminescence of M C H The dose d e ~ o f I T L is shown in Fig. 8. As a measure of I T L light the integrated area under the I T L curve between 180 and 900 s after the end of irradiation has been taken. In the range o f the doses used in our experiments no saturation level has been achieved. It :is worth ~ ~ t h a t , compared t o pure M C H , unpurified M C H gives a more intense ~ L , The experimental I T L d e c a y carves for different doses have. been represented using Bagdasar' yrm's equation lo/1I -- 1 + at cL1~ (Fig~ 9), where I 0 and/i denote the intensities o f 1TL determined: at the b e S i n ~ o f measurement a n d after time t, respectively, and denotes an experimental constant. As c a n b e seen the vatu¢ of ct is dose dependent. T h e intensity o f ~ defreases after prior bleaching o f the y-irradiated M C H sample with I.R. light.

Radioluminescence of m e t h y l c y c l o h e x u e in the glassy state

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FIo. 8. The dependence o f I T L intensity a t 77 K vs time of irradiation, t. Dose rate 1.7 × 101' eV d m 4 s -1.

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DISCUSSION

According to our interpretation, the above results indicate that three types of negative species are involved in the formation of excited states which are responsible for the emission of peaks P,0 and P w The above conclusions seem to be justified in view of the following experimental facts. (i) The dose dependence of peak P,0 is similar to that found by Shirom and Willard m) for e c in glassy MCH at 77 K. Moreover, peak Pg0 can be easily p h o t o b l ~ with I.R. fight. If the Huang and Kevan (u) energy level diagram for trapped electrons in 3MH glass is correct, then the maximum of the photobleaching efficiency curve (Fig. 2) at800 nm (1.55 eV) should be due to a transition from ground level to the state which undergoes auto-ionization to the conduction band. The corresponding values given by Kevan's group for 3MH and M T H F are 1.24eV (14) and 2.38 eV ¢1s), respectively. The threshold energy of the maximum at 800 nm should correspond to the bottom Of the conduction band and is about 1.38 eV (900 nm, Fig. 2). The corresponding threshold values for 3MH and M T H F are 14)eV (14) and 1.6_+0'2 eV (16), respectively. Thus our results fit into the sequence o f energy values incaeasing with increasing matrix polarity, since the static dielectric constants of 3MH, MCH and MTHF are, respectively, 1.93, 2,03 and 4.6 (17).

254

J. K l t o H , R . [ n ~ c z v ~ s x J

a n d J. MAY~R

(ii) The dose dependence for peak P ~ has a shape similar to that found by Ekstrom

et al. ~1~,in the case of the anions of radicals in 3-methylpentane glass. Peak P05 can be easily photobleached by U,V. fight and its intensity increases with the dose applied in both "pre-irradiation" experi~nts (Figs~4 and 7). The formation of anions of radicals in irradiated nonpolar glasses has been suggested~b~ Truong and Bernas ~ . The nature of a stable radiolysis product which could scavenge eleetrons at 77 K in frozen MCH is speculative. Since no evidence exists for the formation o f vinylene-type negative ions ~ls~ the most likely species responsible for the peak P95 emission are anions of conjugated olefins. The formation of such olefins~19,20~and corresponding anions as~ has been proved experimentally. Brocklehurst and Robinson c21~ found that in the presence of conjugated olefins very strong visible emission in nonpolar glasses is produced. The U.V. spectra of irradiated liquid MCH seem to support the above interpretation (Fig. 6). If the above conclusions are correct our observed dose dependences (Fig. 1) for both RTL peaks may be e x e r t e d in the f0110wingway. Equations (2)-(9) represent the competitive reactions which probably take place in y-irradiated, frozen MCH: (2)

MCH

(3)

e-

(4)

MCH +

(5)

e - + M C H + ,,

aoZ

M C H + + e -,

kT > et-, > (MCH+)t, ~,' > MCH*,

(6)

e-+R

k. • R - ,

(7)

e-+P

k. • p-,

(8)

e~- + (MCH+)t

k > MCH*

(peak Ps0),

(9)

R - , P - +(MCH+)t

, MCH*

(peak P~).

MCH+,e - and (MCH+)t, et - denote dry and ~apped charges, respectively; R and P denote radical and unknown stable radiolysis products; R - and P - denote the products of electron attachment to R and P; Go and I show the primary ionization yield and the dose rate, respectively; kT, k r, etc. denote the reaction rate constants. The rate of trapped electron formation is given by =

(10)

~

cdk

"

kT+kr~l~,[R,p]-k[( MU

)0.

We have assumed that: ( i ) r e a s o n (5) is a first-order process with respect to the conceatration of dry e ~ n - - c a t i o n ~ ( i i ) ~ electrons are reactive towards It. and P b u t not towards ~ positive holes, (MCH+)t. This simplification seems to be • • (~) justified since in the presence o f hole ~ v e n g e r s the concentrauon of e t _ increases , most probably due to decrease of the ~ of reaction (5). The dry MCH + are replaced by trapped cations of tmsaturatedszavenger. The mine effect is observed when a few per cent of alcohol isadded ~ the 3 ~ matrix ~a~. In addition to these, in polar media, dry electrons do not react with ~ cations eu~. (iii) The trapped electrons d ~ l ~ durins irradiation by fi~t-order reaction (8) with positive ions (MCH+)t most probably by tunnelling ~.2e~. The reaction of etwith R and P is probably not important since dering the prolonged storage of the sample in the dark peak P~ remains constant, w~reas peak Pg0 decreases. Truong

Radioluminescence of methylcyclohexane in the glassy state

255

et al. ~z~have found that only opticaJgy (not thermally) released electrons are capable of producing the anions o f radicals. The observation that only optically excited trapped electrons can be captured h a s also been made for dissociative attachment on acetonitrile cs~ and methyl vinyl ether~m~following y-irradiation of mixed systems. Taking into account the above discussion the increase of P~s after illumination with light at 800 nm (Fig. 3b) is probably due to the reaction of photodetrapped electrons with R and P. The decreases from the maximum seen in Fig. 3(b) may be connected with certain overlapping of the Pm and Pss peaks. Assuming that the concentration of R and P should increase linearly with time, [R,P] = GR,plt (t = time of irradiation) and [(MCH+)t]--Ice-] the solution of equation (10) is given by

(11)

[et-] = e-~ [,41n(l +-~) + kat(l + ~ - k a b ) ] ,

where

a=~

GRipk s'

b = (GoI) -1 1+

,

,4 = a(1-kab+kSaSbS). The rate of growth of the anions is given by d[R-, P-]

(12)

dt

GoIksGR,pIt =kT+kr+ksGR,plt'

and thus (13)

[R-, P-]

.[ 1 +(kJk,r) " [1 ' (kJk'r)GR'plt] O°Jit--(kJkT)--'GR,plm~ ~" l+(kr/kT) J J"

We assume the yields of ionization to be Go -- 4 and Ga.p~4 because Ga in glassy MCH is ca. 3.3 ~2s', whereas the yield of conjngated olefins in 3MP glass was found to be ca. 0.5~z°L No data are available for kT/k,, kr/k T and k. The curves plotted in Fig. 1 lbr both RTL peaks are calculated assuming kT/k, --=104 , kr/kT -- 1 and k = 5 x 10.4 s-1. The experimental points fit the theoretical curves quite satisfactorily, which supports our model at least as a semiquantitative approximation. It is very difficult to evaluate to what extent the chosen numerical values have an experimental significance. The values of kJ(kT+k~) given by Oka et al. cm) in ~tbe case of 3MH glass, in the presence of certain scavengers, vary in the range 10-2 x 102. Our values of ks/k T and kr/kT fit very well in the above range. The reason for a constant value of k in process (8) is somewhat obscure: according to the tunnelling,hypothes/s, the rate constant k should be time or separation dependentC~2eL Probablyin our case the average separation e c - ( M C H + ) t remains at an approximately constan~ level. The equations (11) and (13) also explain the observed shift of maximum on the dose dependence curve of Pgo for pre-irradiated samples. The ITL of y-irradiated MCH glass at 77 K results from the recombination of trapped electrons with cations. The ITL decay can be expressed by the Bagdasar' yah equation o, 15) (Fig. 9), which seems to be generally observed when electron tunnell/ng occursCaXL The observed 1TL dose dependence (Fig. 8) is very difficult to explain since the intensity of ITL at high doses achieves almost saturation, despite.the low concentra-

256

J. Kltol~, R. I a ~ ; z c z ~ s ~ and J. M~Ymt

tion o f e t - (the intensity o f ,peak Pie is Iow)~ I t is p r o b a b l e .that at high doses a new type o f excited state is formed c o m p a r e d to the low dose range. Such excited states m a y b e generated as a result o f the reaction o f electrons with certain positive ions which are produeed by a charge transfer process. These might be cations o f either conje4gated olefmstxs~ or radicals. The highlumineseenee q u a n t u m yield and new spectral distribution ought to be characterigic for theseexcited states. For example, in the presence of various aromatic scavengers the spectral distribution of I T L in ?-irradiated ~nonpolar glasses at 77 K is characteristic for the excited state of added s c a v e n ~ moleccles t-rob). It suggests that in this case the electrons react with positive ions of the scavenger. In addition to this, Brocldehurst and Robinson (21) have found in the case of nonpolar Oasses that the spectral distribution o f the emitted R T L light is dose dependent. It is worth noting that Kieffer et al.
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