J. Phys. Chem. 1985, 89, 3583-3586
3583
Structure of Spurs in y-I rradiated Alcohol Matrices Determined by Electron Spin-Echo Method Tsuneki Ichikawa,* Shin-ichi Wakasugi, and Hiroshi Yoshida Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan (Received: February 21, 1985)
Paramagnetic relaxation rates of radiation-generated tetracyanoethylene anion radicals (TCNE-) and chemically prepared TCNE- in the glassy matrices of ethanol, 1-propanol and 1-butanol, and radiation-generated CHJCHOH radicals in neat ethanol matrices have been measured by an electron spin-echo method for elucidating the detailed structure of spurs generated in the alcohol matrices y-irradiated at 77 K. The local concentrationsof hydroxylalkyl radicals generated in pair with TCNEor CH3CHOH radicals in isolated spurs were determined by analyzing the rates of excitation transfer from the magnetically excited TCNE- or CH3CHOH radicals to the hydroxyalkyl radicals. Comparison of the local concentrations of CH3CHOH radicals in ethanol matrices with and without solute tetracyanoethylene revealed that the average number of ion pairs in a spur is close to unity. The distribution function of the distance r between TCNE- and the paired hydroxyalkyl radical, #(r), was derived from relaxation kinetics of TCNE-, and was found to be expressed by # ( r ) = 3 e ~ p ( - r ~ / r ~ ~ ) / ( 2 p ~ / ~ r ~ ) . The average distances between TCNE- and the paired hydroxylalkyl radicals were determined from the local concentrations of the hydroxyalkyl radicals, and were about 5 nm for all the alcohol matrices.
Introduction Ionizing radiations passing through condensed substances deposit energy inhomogeneously in the form of ionization and excitation of molecules. The local concentrations of ionized and excited molecules are therefore much higher than the bulk concentrations. The extent of the inhomogeneity of energy deposition depends on the rate of energy losses of ionizing radiations per unit path length (linear energy transfer, LET).' Low LET radiations such as y-rays and high energy 8-rays cause the formation of isolated spurs containing a few pairs of ionized molecules. High LET radiations such as a-rays cause the formation of overlapped spurs in which dozens of ion pairs are included. Reactive free radicals formed in pair by the ionization or excitation of molecules further react with each other or with diamagnetic molecules and finally convert to stable radiationchemical products. The local concentration of radicals affects the yield of the stable products. For example, the molecular products by initial radical-radical reactions can be obtained with high efficiency if the radicals are formed in close vicinity of other radicals. To determine the extent of the inhomogeneity is therefore important for elucidating radiation-chemical reactions. In rigid matrices paramagnetic ions and radicals are immobilized in the tracks of ionizing radiations, so that the local concentration of the paramagnetic species is possible to be measured after stopping the irradiation. One of the most direct methods for determining the local concentration of paramagnetic species is to measure their paramagnetic relaxation rates by means of the electron spin-echo (ESE) methodeZ The relaxation rates depend on the extent of electron spinspin interactions which are inversely proportional to the cube of the distance between interacting electron spins. The relaxation rates are therefore highly sensitive to the local concentration or the spatial distribution of the paramagnetic species.) Tsvetkov et al. have applied the ESE method for investigating the spatial inhomogeneities of C H 2 0 H radicals in solid methanol4 and 0- anion in aqueous alkaline glasses5 irradiated with ionizing radiations of different LET. It has been shown that the local concentration of these paramagnetic species is higher for the high (1) Mozumder, A. In "Advances in Radiation Chemistry"; Burton, M., Magee, J., Eds.; Wiley-Interscience: New York, 1969; Vol. 1, p 1. (2) Salikhov, K. M.; Tsvetkov, Yu. D. In "Time Domain Electron Spin Resonance"; Kevan, L., Schwartz, R. N., Eds.;Wiley-Interscience: New York, 1979; Chapter 7. (3) Bowman, M. K.; Norris, J. R. J. Phys. Chem. 1982,86, 3385. (4) Raitsimring, A. M.; Tsvetkov, Yu. D.; Moralev, V. M. Int. J . Radiat. Phys. Chem. 1973,5, 249, (5) Raitsimring, A. M.; Samilova, P. I.; Moralev, V. M.; Tsvetkov, Yu. D. Inst. Chem. Kinet. Combust., Novosibirsk 1976, I , 4.
0022-3654/85/2089-3583$01.50/0
LET radiation because of overlapping of spurs in a track. However, the local concentration of paramagnetic species in an isolated spur has not been determined. We have recently studied the spatial distribution of paramagnetic species trapped in y-irradiated 2-methyltetrahydrofuran matrices by means of the ESE method! Assuming that a spur contains a single ion pair, distribution of the intrapair distance has been determined from the longitudinal relaxation rates of the paramagnetic species. However, the number of ion pairs in the spur has not been experimentally determined. Up to now, as far as the authors know, no experimental attempt has been made for determining the number of ion pairs in an isolated spur in condensed media. In the present work the longitudinal relaxation rates of paramagnetic species in y-irradiated glassy matrices of alcohols are measured by the ESE method to deduce the number of ion pairs in a spur and the distribution of the intrapair distances. The longitudinal relaxation rates are measured for CH3CHOH radicals in y-irradiated matrices of neat ethanol, tetracyanoethylene anion radicals (TCNE-) in y-irradiated matrices of ethanol, 1-propanol and 1-butanol containing 0.1 mol/dm3 of T C N E as an electron scavenger, and the lithium salt of TCNE (TCNE-/Li+) in the alcohol matrices. Hydroxyalkyl radicals and localized electrons are generated as paramagnetic products by the y-irradiation of neat alcohol matrices at low temperature.' The hydroxyalkyl radicals are mainly generated through the reaction of the molecular cations of alcohols with intact alcohol molecules RCHzOH RCHZOH'
--
+ RCHzOH
RCH20H+
-+
RCHOH
+ e+ RCH20H2+
(1)
Excess electrons, the counterpart of the cations, are localized in the vicinity of the hydroxyalkyl radicals. In the presence of TCNE, the localized electrons react with adjacent TCNE8 to convert to TCNEe-
+ TCNE
-
TCNE-
(2)
In a neat ethanol matrix, the localized electrons react with adjacent ethanol molecules by the illumination of visible light to convert to CH,CHOH radicalsg (6) Ichikawa, T.; Yoshida, H. J . Phy. Chem. 1984, 88, 3199. (7) Pshezhetskii, S. Ya.; Kotov, A. G.; Milinchuk, V. K.; Roginskii, V. A,; Tupikov, V. I. In "EPR of Free Radicals in Radiation Chemistry"; Wiley: New York, 1974; Chapter 5. (8) Beitz, J. V.;Miller, J. R. J. Chem. Phys. 1979, 71, 4579 and reference therein.
0 1985 American Chemical Society
3584
e-
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
+ 2CH3CH20H
-
CH3CHOH
+ CH3CHO- + H2
Ichikawa et al.
(3)
The initial spatial correlation between a molecular cation and a localized electron is therefore maintained between a hydroxyalkyl radical and TCNE- or between a pair of CH3CHOH radicals.
Experimental Section Analytical grade ethanol 1-propanol and 1-butanol were dried over magnesium amalgam in vacuo, vacuum distilled, and then degassed by a freezepumpthaw procedure. Tetracyanoethylene (TCNE) was purified by sublimation in vacuo. Lithium salt of tetracyanoethylene anion radical (TCNE-/Li+) was prepared by the reduction of T C N E with LiI.Io Three kinds of samples were prepared as follows: (1) neat ethanol, (2) the alcohol solutions of 0.1 mol/dm3 TCNE, and (3) the alcohol solutions of TCNE-/Li+. These samples were sealed under vacuum into high purity quartz tubes and then plunged into liquid nitrogen to get glasslike samples. The samples 1 and 2 were irradiated at 77 K with %o y-rays at a dose rate of 0.45 Mrad/h. After the y-irradiation, the samples 1 were illuminated with visible light for converting all the localized electrons to CH3CHOH radicals. One of the samples 1 was then transferred into a continuous flow cryostat mounted in a ESR cavity. The temperature of the sample was then raised to 100 K by decreasing the flow rate of cold nitrogen gas while monitoring the ESR intensity of the CH3CHOH radicals. The CH3CHOH radicals are mobile at 100 K, so that the concentration of the radicals was gradually decreased during the thermal annealing. The concentrations of the paramagnetic species were determined by comparing the intensities of their ESR spectra with the ESR intensity of the ethanol solution of TCNE-/Li+ of a known concentration. The ESR spectra were recorded at 77 K with a Varian E- 109 X-band spectrometer. ESE measurements were carried out with a home-built X-band spectrometer." The longitudinal relaxation rates were determined from the dependence of the two-pulse ESE intensity at a fixed T (time interval between a 9 0' and a 180' pulse, 500 ns) on the repetition period, t, of the two-pulse echo series. The longitudinal relaxation rates of TCNE- and TCNE-/Li+ were measured at 77 K, whereas those of CH3CHOH radicals were measured at 4.2 K because the relaxation rates were too fast at 77 K. Results and Discussion Radiation-Chemical Yield of Paramagnetic Species. The concentrations of TCNE- and hydroxyalkyl radicals in y-irradiated ethanol, 1-propanol, and 1-butanol were measured as a function of radiation dose. The G values (number of products per 100 eV energy absorbed) of TCNE- and the hydroxyalkyl radicals were 2.6 f 0.2 and 3.0 f 0.2, respectively, for all the alcohol matrices within whole dose range examined. The G values of the hydroxyalkyl radicals and TCNE- close to each other indicate the pairwise formation of a hydroxyalkyl radical and a localized electron and the conversion of the latter into TCNE-. The yield of the hydroxyalkyl radicals is 15% higher than that of TCNE-, which indicates that 7.5% of the hydroxyalkyl radicals are paired with the hydroxyalkyl radicals. These neutral radical-radical pairs are considered to be formed by the dissociation of excited molecules into hydroxyalkyl radicals (RCH,OH)* RCHOH + H H
+ RCH20H
-
-+
RCHOH
+ H2
(4)
or the reaction of excess electrons with solvent molecules e-
+ 2RCH20H
-
RCHOH
+ RCHO- + H2
,
-2
(9) Shida, T.; Imamura, M. J. P h p . Chem. 1974, 78, 232. (10) Webster, 0.W.; Mahler, W.; Benson, R. E. J . Am. Chem. SOC.1962, 84, 3678. (1 1) Ichikawa, T.;Yoshida, H. Bull. Fac. Eng.,Hokkaido Wniv. 1984, 121,
-1
0 In(t/ms)
1
2
Examples of observed recovery kinetics of longitudinal magnetization for 2.0 mmol/dm3 TCNE- in ethanol matrices at 77 K monitored by the two-pulse spin-echoes with pulse repetition period t varied. The anion radicals, TCNE-, were generated by (0)the y-irradiation of ethanol matrices at 77 K containing 0.1 mol/dm3 TCNE as an electron scavenger, or ( 0 )the chemical reduction of TCNE with LiI. The observed relaxation kinetics are expressed by the functional form of V ( t ) / V , = 1 - e~p(-btO,~). Figure 1.
51 4
0
1 2 3 4 ITC NE-] Im mMm3 Figure 2. Dependence of the rate constant, b, of the longitudinal relaxation for TCNE- in ethanol matrices at 77 K on the concentration of (0) y-rays-generated and ( 0 )chemically prepared TCNE-. The yield of CH3CHOH radicals in neat ethanol matrices after photobleaching of localized electrons is equal to the sum of the yields of the localized electrons and the CH3CHOH radicals in the matrices before the photobleaching, which confirms the validity of eq 3. The two-pulse ESE envelopes for TCNE-/Li+ in alcohol matrices measured as a function of T, the time between the first and second pulses, do not show any Li nuclear modulation, which indicates no detectable magnetic interactions between TCNE- and Li'. In the following sections the magnetic property of TCNEitself is taken to be the same for TCNE-/Li+ and y-irradiated samples. Number of Ion Pairs in a Spur. Shown in Figure 1 is the dependence of ESE intensity on the pulse repetition period t for TCNE-/Li+ in a ethanol matrix and TCNE- in a y-irradiated ethanol matrix. The dependence shows the recovery kinetics of magnetization to a thermal equilibrium value after the saturation pulse. The recovery kinetics follow the equation
v(t)= vm{l- e ~ p ( - b t ~ . ~ ) ]
(6)
for all the concentrations of TCNE-/Li+ and TCNE- examined. As shown in Figure 2, the rate constant, b, of the longitudinal relaxation increases linearly with the concentration of TCNE-/Li+ or TCNE-. For TCNE-/Li+ the rate constant is expressed by the relation b = 0.97
(5)
taking place during y-irradiation.
41.
-1 '
+ O.O55[TCNE-]
(7) where b is in m ~ ~and 3 .[TCNE-] ~ is the concentration of TCNEin mmol/dm3. In the TCNE-/Li+ samples TCNE- anion radicals excited by microwave pulses can transfer their excess energy to diamagnetic matrix molecules or to unexcited TCNE-. The former and the latter relaxation processes can be designated as lattice relaxation and excitation transfer, respectively. The first and the second term in the right-hand side of eq 7 are therefore the lattice relaxation rate and the rate of excitation transfer to TCNE- which
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3585
Structure of Spurs in Alcohols
0
1
2 3 In( t Ims)
4
J 0
Figure 3. Longitudinal relaxation kinetics for (0)2.2 mmol/dm3, (A) 4.5 mmol/dm3, ( 0 )8.9 mmol/dm3, and (A) 17.9 mmol/dm3 of CH2CHOH radicals at 4.2 K in y-irradiated and then photobleached neat ethanol matrices. The y-irradiation and photobleaching were carried out at 77 K. The relaxation kinetics are expressed by the functional form
of
v(t)/v,= 1 - exp(-br0.68).
are homogeneously distributed around the excited TCNE-. The rate constant for TCNE- in the y-irradiated ethanol matrices is expressed by
b = 1.97 + 0.76[TCNE-],
(8)
where [TCNE-1, is the concentration of TCNE- in mmol/dm3. The first term in the right-hand side of eq 8 is the relaxation rate of TCNE- in a completely isolated spur. The second term in the right-hand side of eq 8 is the sum of the rates of excitation transfer to TCNE- and CH3CHOH radicals in spurs which are homogeneously distributed around the spur containing the magnetically excited TCNE-. Equation 8 is then rewritten as
b = 1.97
+ O.O55[TCNE-] + 0.61[CH3CHOH]
(9)
where [CH3CHOH] is the concentration of CH3CHOH radicals in mmol/dm3. Equation 9 indicates that the rate of excitation transfer to TCNE- is much slower than that to CH3CHOH radicals. Assuming that an isolated spur contains TCNE- and CH3CHOH radicals of the number n, the magnetically excited TCNEin a completely isolated spur can transfer its e x w s energy to the lattice, and CH3CHOHradicals of the number n and TCNE- of the number (n - 1) in the same spur. Since the rate of excitation transfer to TCNE- is negligibly slower than that to the CH3CHOH radicals, the relaxation rate of TCNE- in a completely isolated spur is given by the sum of the lattice relaxation rate and the relaxation rate to the CH,CHOH radicals of the number n. The total relaxation rate of TCNE- in y-irradiated ethanol matrices is then given by
b = 0.97
+ O.OSS[TCNE-] + 0.61(n[Co] + [CH3CHOH]j
(10) where [C,] is the effective local concentration of a CH3CHOH . radical in a spur defined as the concentration of homogeneously distributed CH3CHOHradicals necessary for giving the same rate of excitation transfer to the CHSCHOH radical in the spur. Comparison of eq 9 and 10 gives the effective local concentration as [C,] = 1.64/n mmol/dm3
(11)
Only CH3CHOH radicals exist in ethanol matrices that are y-irradiated and then photoilluminated. As shown in Figure 3, the relaxation kinetics at 4.2 K are expressed by V ( t ) = V,(1 - exp(-bt0,6s))
(12) for all the concentrations of the CH3CHOH radicals examined. As shown in Figure 4, the rate constant depends linearly on the concentration of the CH3CHOH radicals as b = 0.115 - 0.019[CH3CHOH] (13) where b is the rate constant in ms-’.6s. The first term in the right-hand side of eq 13 is the relaxation rate of a magnetically excited CH,CHOH radical in a completely isolated spur, and is the sum of the lattice relaxation rate and the rate of excitation transfer to the CH3CHOH radicals in the same spur as that for
5 10 15 20 [ C H M ]ImmoW3
Figure 4. Dependence of the rate constant, b, of the longitudinal relaxation at 4.2 K on the concentration of CH3CHOHradicals in neat ethanol matrices ( 0 )with and (0)without thermal annealing at 100 K.
the excited CH3CHOH radical. Since the initial anionic species in the neat ethanol matrices have been completely converted to CH3CHOH radicals, an isolated spur contains 2n CH3CHOH radicals. The magnetically excited CH3CHOH radical can transfer its excess energy to the 2n-1 radicals in the same spur. The lattice relaxation rate of the CH3CHOH radical is obtained by measuring the relaxation rate of the sample thermally annealed at 100 K. The thermal annealing of the sample causes two effects, a decrease of the CH3CHOH radicals by radical recombination reactions and a change of the spatial distribution of the CH3CHOH radicals from an initial inhomogeneous one to a homogeneous one. Shown in Figure 4 with solid circles are the relaxation rates of the CH3CHOH radicals in a successively annealed sample. Since the thermal annealing causes the loss of paired radicals, the relaxation rate for the annealed sample is lower than that for the unannealed samples even if the radical concentrations are the same for both of the samples. The relaxation rate of the annealed sample extrapolated a t zero radical concentration, 0.08, is that of a CH3CHOH radical without a paired one, so that it is equal to the lattice relaxation rate. The second term in the right-hand side of eq 13 is the rate of excitation transfer to the CH3CHOH radicals in spurs other than that for the excited CH3CHOH radical. When the measured lattice relaxation rate is used, the total relaxation rate of the CH3CHOH radicals in the y-irradiated ethanol matrices is expressed by b = 0.08 0.019((2n - l)[Co] [CH3CHOH]) (14)
+
+
where the definition of [C,] is similar to that for the TCNE- case. The effective local concentration [C,] is deduced by comparing eq 13 and 14 as [C,] = 1.8/(2n- 1) (15) The average number of ion pairs in a spur is calculated to be n = 1.1 by substituting eq 15 into eq 1 1. The number of ion pairs determined in the present study is considerably different from a theoretical value obtained by Kowari and Sato.lz They calculated the degradation spectra of secondary electrons in y-irradiated water and found that only 27% of ion pairs are produced in the spurs consisting of one ion pair and the other ion pairs are produced in the condensed spurs consisting of many ion pairs. This discrepancy may arise from recombination reaction of ion pairs taking place during y-irradiation at 77 K. The mobile electrons ejected from matrix molecules either recombine with cationic species or are localized in trapping sites. The theoretical G value of ionization in alcohols is about 4,13 whereas the observed G value for ion pair formation is 2.6. This indicates that 35% of ion pairs recombine during y-irradiation at 77 K. The recombination probability increases with the number of cationic species in a spur, so that condensed spurs consisting of many ion pairs are probably converted to isolated spurs consisting of one ion pair during irradiation at 77 K. The distribution of the separation distance of ion pairs has been determined by analyzing the decay of ionic species by charge ~
~
(12) Kowari, K.;Sato, S. Bull. Chem. S O ~Jpn. . 1981, 54, 2878. (13) Okazaki, K.;Yamabe, M.; Sato, S . Bull. Chem. SOC.Jpn. 1977, 50, 1409.
3586 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
Ichikawa et al. TABLE I: Distribution of Intrapair Distance of TCNE--Hydroxyalkyl Radical Pairs in y-Irradiated Alcohol Matrices at 77 K CJmmol. matrix polarity" paired radical dm--l r,,/nmc ?/nmd ethanol 24.3 CHiCHOH 1.64 6.5 5.0 1-propanol 20.1 CH;CH2CHOH 1.24 7.3 5.6 1-butanol 17.1 CH3CH2CH21.65 6.6 5.0 CHOH
0
2 3 [TCNE-I/ mmobdK3
1
4
Figure 5. Concentration dependence of the rate constant, 6, of the longitudinal relaxations at 77 K for y-rays-generated TCNE- in (0) 1-propanol and (A)1-butanol matrices, and chemically prepared TCNEin ( 0 )1-propanol and (A) 1-butanol matrices. The relaxation kinetics are expressed by the functional form of V(r)/V, = 1 - exp(-bto5).
"The values given are the dielectric constant of the alcohols at 293 K. bThe definition of the effective local concentration, C,, is given in the text. CThedistribution function for the intrapair distance is given by b(r) = 3 ( e ~ p ( - @ / r ~ ~ ) / ( 2 ~ ~ / dThe ~ $ ) ) .average distance between TCNE- and the paired hydroxyalkyl radicals. unit. The average distance, f , between TCNE- and the paired CH3CHOH radicals is obtained from eq 16 as
recombination during thermal annealing of irradiated s01ids'~J~ T = 0.764ro (21) or after the irradiation of liquids with pulsed radiation s~urces.~~J~ In these analyses the number of ion pairs in a spur has been Substituting [Co] = 1.64 mmol/dm3 into eq 20, the value of ro tentatively assumed to be one. The present study indicates that and p are determined as 6.5 and 5.0 nm, respectively. the assumption of one ion pair is valid as long as the early stage The same procedure was used for deriving the distribution of the recombination reaction is ignored. function of the distance between TCNE- and a paired hydroxyDistribution of Intrapair Distance. The relaxation kinetics of alkyl radical in y-irradiated l-propanol and l-butanol matrices. the excited TCNE- by excitation transfer to the paired CH3CHThe relaxation kinetics for radiation-generated and chemically OH radicals are given by6 prepared TCNE- in these matrices follow eq 6. This implies that the distribution function of the distance between TCNE- and the V ( t ) = Vm[1 - l m d r l f f 2 a r 2 4 ( rexp(-A&(l ) paired CH3CH2CHOHor CH3CH2CH2CHOHradical is also 0 0 expressed by eq 16. Figure 5 shows the rate constant, b, of the 3 cosz sin 0 do] (16) longitudinal relaxation as a function of TCNE- concentration. The rate constants are expressed by a relation similar to eq 7 and 9. The effective local concentrations of CH3CH2CHOHradicals where 4(r) is the distribution function of the distance between and CH3CH2CH2CHOHradicals were derived from these rate TCNE- and the paired CH3CHOH radical, and 0 is the angle constants and are summarized in Table I together with the values between an external magnetic field and the vector joining the of ro and p . TCNE- and the CH3CHOH radical. Since the exponent of t for It can be seen from Table I that the average separation distance, the relaxation kinetics is 0.5 even at the lowest TCNE- concenp , is not so much dependent on the polarity of the matrix molecules. tration, the relaxation kinetics by the excitation transfer alone This su8gests that the average distance between localized electrons must also be expressed by eq 6. The distribution function giving and counterpart cations (precursors of TCNE- and hydroxyalkyl an exponent of t close to 0.5 is given by6 radicals, respectively) is not so much dependent on matrix polarity. 4(r) = 3 ( e ~ p ( - P / r , 6 ) / ( 2 * ~ / ~ r , ~ ) ) (17) Sawai, Shinozaki, and Meshitsuka'* studied the electron scavenging efficiencies of solute molecules in several alcohol matrices Substituting eq 17 into eq 16, one can obtain at 77 K. They found that the scavenging efficiencies decrease with increasing matrix polarity. On the basis of the assumption v(t) N V,[I - e ~ p ( - r ~ ~ ( A t ) ' / ~ ) ] (18) that mobile electrons are captured by solute molecules before The value of ro can be obtained from the effective local concenconverting to localized electrons, they concluded that the average tration, [C,],of the CH3CHOH radicals as follows. The relaxation distance between localized electrons and counterpart cations inkinetics by the excitation transfer to the CH3CHOH radicals with creases with decreasing matrix polarity. However, recent studies a homogeneous spatial distribution are given by6 on electron scavenging reactions in rigid matrices indicate that electrons are captured by solute molecules through electronV(t) = VJ 1 - e ~ p ( - l 6 . r r ~ / ~ C ( A t ) ' / ~ / 3 ~ / ~(19) )] tunneling reactions after being localized into trapping sites.* The where C is the number density of the CH3CHOHradicals. Since increase of the scavenging efficiency does not imply an increase the relaxation kinetics of the paired CH3CHOH radical are the of the average separation distance but only implies an increase same as those of the homogeneously distributed radicals of the of the tunneling probability. The tunneling rate of electrons toward concentration [C,],by comparing eq 18 and 19, one can express scavenging molecules increases with decreasing matrix polarity ro with [Co] as because the trapping sites for localized electrons are shallower in less polar mat rice^.'^ ro = (3s/z/16r3/z[Co])1/3 (20) Registry No. TCNE, 670-54-2; TCNE-/Li+, 34500-72-6; TCNE-, 34512-48-6; CH-lCHOH, 2348-46-1; CH~CHZCHOH,5723-77-3; where [Co] is the effective local concentration in number density CH,CH2CH2CHOH,21 576-64-7; ethanol, 64-17-5; 1-propanol,7 1-23-8; 1-butanol, 7 1-36-3. (14) Ichikawa, T.; Yoshida, H.; Hayashi, K. Bull. Chem. Sor. Jpn. 1973, 46, 8 12. (15) Ichikawa, T.;Yoshida, H.; Hayashi, K. Bull. Chem. SOC.Jpn. 1975, 48, 2685.
(16) Leone, J. A.; Hamill, W. H. J . Chem. Phys. 1969, 50, 1787. (17) Sawai, T.; Hamill, W. H. J . Chem. Phys. 1972, 56, 5524.
(18) Sawai, T.; Shinozaki, Y.; Meshitsuka, G. Bull. Chem. Sor. Jpn. 1972, 45, 984.
(19) Kevan, L. In "Advances in Radiation Chemistry"; Burton, M., Magee, J. L., Eds.; Wiley-Interscience: New York, 1973; Vol. 4, p 181.
