- Email: [email protected]
Dissociative electron attachment to methylhalides in 3-methylhexane glassy matrix
Znt. J. Radiat. Phys. Chem. 1976, Vol. 8, pp. 331-338. Pergamon
Press. Printed in Great Britain
DISSOCIATIVE ELECTRON ATTACHMENT TO METHYLHALIDES IN 3-METHYLHEXANE GLASSY MATRIX KATSUYOSHI HARADA, MASAHIRO IRIE* and HIROSHI YOSHIDA Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan (Received in revised form 20 June 197.5) Abstract-Dissociative electron attachment reaction to CH,I, CH&l and CH,F in a 3-methylhexane glassy matrix was studied by determining the yield of trapped electrons and that of methyl radicals immediately after y-irradiation at 77 K as a function of the scavenger concentration. The efficiency of conversion from the trapped electrons to the methyl radicals was also studied by photobleaching the trapped electrons. The results obtained are (1) the dissociative electron attachment occurs to CH,F, for which the gas phase data indicate that the reaction is endothermic by 1.2 eV, during either the y-irradiation or the photobleaching, and (2) CH,F is relatively less efficient in scavenging photo-liberated electrons than in scavenging the electrons during the y-irradiation, whereas CH,I and CHaCl are efficient scavengers for both the electrons. The dependence of the yields of the trapped electrons and the methyl radicals is discussed in terms of the electron-tunnelling mechanism and the epithermal electronscavenging mechanism. INTRODUCTION of electrons in y-irradiated glassy matrices have been studied for those solutes which undergo dissociative or non-dissociative electron Some of these studies attachment reactions. suggested that the behavior of electrons in the glassy matrices was different from that in gas phase. For example, the present authors reported that dissociative electron attachment occurs to solutes such as methylvinylether(r&) and dimethylether(ib) for which the reaction is thought to be endothermic from gas phase data, when trapped electrons are photobleached. This feature seems to indicate the specific nature and behavior of electrons in the glassy matrices, though it may be attributed partly to the solvation energy of ionic entities involved. Bonin et al. also reported an interesting observation that dissociative electron attachment occurred to acetonitrile upon photobleaching the trapped electrons in 2-methyltetrahydrofuran glassy matrix, while it did not occur during thermal bleachingf2). In order to obtain more information about dissociative electron attachment in glassy matrices, the study is extended here to the reaction for methyliodide, methylchloride and methylfluoride for which the heat of reaction is calculated to be Reactions
* Present address: The Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka 564, Japan.
0.6, 0.0 and - 1.2 eV, respectively”‘l, from the gas phase data. Methylhalides (iodide, chloride and bromide as well) are known to be good electron scavengers and are very often used in organic glassy matricesC4p5). However, in the present investigation trapped electrons were generated by y-irradiating a 3-methylhexane glassy matrix in the presence of a very small amount of methylhalides and then the trapped electrons were photobleached to result in the formation of methyl radicals through dissociative electron attachment to the solutes. The efficiency of the attachment during either y-irradiation or photobleaching is examined for these three scavengers. The methyl radicals, as well as the trapped electrons, were qualitatively measured by electron spin resonance (E.S.R.) method quite easily, which is the reason why methylhalides were used as electron scavengers in the present investigation. EXPERIMENTAL 3-Methylhexane was purified as mentioned previously’6’. CH,I and CH,Cl of analytical grade were used as received without further purification. CH,F was synthesized from CH,I and hydrogen fluoride”) and purified by passing through water. The concentration of methylhalides was determined by volumetric measurements under reduced pressures. Sample solutions were sealed in E.S.R. sample tubes of pure quartz under a vacuum of less than 10d6 Torr, irradiated with 6oCo y-rays at a dose rate of 1.5 Mrad h-l to a dose of 0.075 Mrad at 77 K in the dark and subjected to
332
KATSUYOSHIHARADA, MASAHIROIRIE and HIROSHI YOSHI~A
E.S.R. measurements at 77 K. Occasionally a ““Co source of 0.5 Mrad h-l was used but the decay of both the methyl radicals and the trapped electrons during the long irradiation time was properly corrected. Photobleaching of the trapped electrons was carried out with light of wavelength longer than 520 nm from the incandescent lamp of a slide projector. RESULTS Outline of observedphenomena When the concentration of the scavengers in a 3-methylhexane glassy matrix is low enough, the trapped electrons and the methyl radicals are simultaneously formed in the matrix by y-irradiation and observed from their E.S.R. spectra: a single-line spectrum of 0.37 mT width (peak-topeak width in derivative curve) due to the trapped electrons and a four-line spectrum with a separation of 2.3 mT due to the methyl radicals as shown in Fig. l(A) and (D). The initial yield of the trapped electrons and that of the methyl radicals are determined by extrapolating their decay curves to zero time (the time when the irradiation is stopped). Although the initial yields include some uncertainty due to the decay during irradiation, it has no serious effect on the analysis of the data in the following sections.
After observing the decay curves for an adequate period, the trapped electrons were photobleached completely. The conversion efficiency from the trapped electron to the methyl radical was detrrmined from the ratio of the increment of the methyl radical spectrum to the photobleached spectral intensity of the trapped electron [compare spectrum (C) with (B) and spectrum (E) with (D) in Fig. I]. The photobleaching is almost completed by the first 10 s. The change in the concentration of the trapped electrons and that of the methyl radicals during the course of the experimental procedures are illustrated in Fig. 2 representatively for CH,I as a scavenger. The conversion efficiency is dependent upon the scavenger concentration as expected. The dependence of the conversion efficiency was examined for CH,I upon either the wavelength of light or the survival fraction of trapped electrons to be bleached, but no systematic trend could be observed. The methyl radicals decrease in their spectral intensity in first-order kinetics, which contrasts to the decay of the methyl radicals formed through the dissociative electron attachment to methylvinyl This observation ether and dimethyl ether”‘. supports the interpretation that the methyl radicals
FIG. 1. E.S.R. spectra recorded from a 3-methylhexane glassy matrix containing 0.014 mole :c of CHJ irradiated to a dose of 0.075 Mrad at 77 K (A) immediatelv after the irradiation. (B) befbre and (C) after photobleaching of trapped electrons, and the giassy matrix containing 0.41 mole % of CH,F irradiated to the same dose (D) before and (E) after photobleaching the trapped electrons. The black arrows indicate the trapped electron spectra and the white arrows the methyl radical spectra.
Dissociative electron attachment to methylhalides in 3-methylhexane glassy matrix ,-f-irradiation
,- photobleach Isl(
1
333
between the trapped electron and the scavenger when the dissociative electron attachment is an endothermic reaction. Electron scavenging during y-irradiation
0
I
0
I
I LI
I
20
40
TIME
I
60
(min)
FIG. 2. Change in (0) the concentration of trapped electrons and (0) that of methyl radicals in a 3-methylhexane glassy matrix containing a small amount of CH,I irradiated to a dose of 0.25 Mrad (higher than the usual experimental condition) during the whole experimental procedure, determined by E.S.R. measurements at 77 K.
decay through geminate recombination with halide ionsn’). The trapped electron spectrum decays slowly (20 ‘A within the first 1 h) in the neat matrix, whereas it decays more quickly in the presence of a small amount of CHJ as shown in Fig. 2. It may be worthwhile to note, though qualitatively, that this enhancement of decay after y-irradiation is dependent upon the nature of the scavengers used. It is enhanced remarkably by CHJ and slightly by CH,CI present in small concentration. However, the addition of CH,F to the matrix caused no effect on the decay of the trapped electron up to a concentration as high as a few mole y0 as shown Fig. 3. This indicates that no reaction occurs 1.0
0.2 0
20
40 TIME
FIG.
60
(mid
3. Isothermal decay of trapped electrons at 77 K after y-irradiation of (0) a neat 3-methylhexane glassy matrix, (0) matrix containing 0.025 mole % of CHJ, and (0) matrix containing 041 mole % of CH,F.
Figure 4 shows, representatively for CHJ, the dependence of the yields immediately after yirradiation for the trapped electrons and the methyl radicals upon the concentration of the added scavenger. Electrons are scavenged only partially at a scavenger concentration of less than 0.05 mole % and the rest of electrons are trapped in the matrix. The yield of the trapped electrons in the neat matrix is estimated to 0.53 (for 100 eV energy absorbed), which is comparable with the value previously reported by UP) and is smaller than the values determined by several groups”“. The primary concern here is, however, not in the absolute value of the yield but in its value relative to the methyl radical yield.
0
’
0
i
‘%
I
’ 03 0.3 0.2 CONCENTRATION OF CH,I (mole%)
I I
0.4
FIG. 4. Dependence of (0) the yield of trapped electrons and (0) that of methyl radicals immediately after y-irradiation of a 3-methylhexane glassy matrix at 77 K upon the concentration of CHJ added to the matrix as an electron scavenger.
With the increasing concentration of CHJ, the trapped electron yield rapidly decreases and the methyl radical yield increases somewhat slowly from zero to a constant value. The constancy of the methyl radical yield at a scavenger concentration higher than O-1 mole % results from the complete scavenging of the electrons. It should be noted that the ultimate yield of the methyl radicals, G,(CHa), is higher than the trapped electron yield in the neat matrix, GJet-), which indicates that the electron scavenging reaction of CHJ during yirradiation competes not only with electron trapping but also with prompt charge recombination in the matrix. The scavenger concentration at which the trapped electron yield becomes one-half of the yield in the absence of the scavenger can give a
334
KATSUYOSHI HARADA, MASAHIRO IRIE and HIROSHI
measure of efficiency of the scavenging reaction. The efficiency is also given by the scavenger concentration at which the methyl radical yield is one-half the plateau value at high scavenger concentrations. For CH31 as scavenger, the efficiency is given by a scavenger concentration of 0.007 mole “/: (from a decrease in the trapped electron yield) and 0.01, mole “/, (from an increase in the methyl radical yield) from Fig. 4. Again, the difference between these two values indicates that a fraction of the electrons which undergo the prompt charge recombination in the absence of the scavenger is scavenged by CH,I. For CH,Cl, the results obtained are similar to those for CHJ and are essentially the same as those reported previously(lO’, except that CH,Cl is less efficient in scavenging electrons. This is indicated by the scavenger concentration, 0.035 and O.OSj mol %, at which the trapped electron yield decreases to one-half and the methyl radical yield increases to one-half, respectively. Although dissociative electron attachment to CH,F is thought to be endothermic, it is shown to occur by observing the formation of methyl radicals and the decrease in the trapped electron yield [see Fig. l(D) and (E)]. The yield of the methyl radicals is low because of the low scavenging efficiency of CH,F. However, the formation of methyl radicals is not due to the direct effect of radiation on CH$F but due to the dissociative electron attachment, because the trapped electrons are eliminated by adding a few mole “/, of CH,F to the matrix. The scavenger concentration of 0.24 mole o/o lowers the trapped electron yield to onehalf of its value in the neat matrix. The plateau of the methyl radical yield could not be determined, because the direct effect becomes apparent at high scavenger concentrations. However, the extrapolation to 100 mole % CH$F of the linear increase in the methyl radical yield at low scavenger concentrations gives a yield as high as 50. This also indicates that the methyl radicals are formed essentially due to the dissociative electron attachment when the concentration of CH,F is not very high. Shiotani et al. also noted the occurrence of dissociative electron attachment to fluorinated acetic acids in glassy matrices where the reaction was thought to be endothermic”l). Dissociatiz;e electron attachment induced byphotobleaching of trapped electrons When the trapped electrons are photobleached, the methyl radical spectrum increases in its intensity in the presence of methylhalides in the matrix [see
YOSHIDA
Fig. l(C) and (E)]. This indicates that a fraction of the electrons photoliberated from their trap attach dissociatively with the scavengers. The conversion efficiency from the trapped electron to the methyl radical is dependent upon the scavenger concentration; it increases with increasing concentration and then reaches a plateau value as shown in Fig. 5. The microwave power saturation factor should be known for the trapped electron spectra in order to determine the absolute values of the conversion efficiency (the methyl radical spectra are not saturated at the microwave power level used), and can be estimated by presuming that the Dlateau value corresponds to an eficiency of unity. 0
0
0.L
0.8
1 ,? -1
C 0.08 0.04 CONCENTRATION OF SOLUTES (moie’io)
FIG. 5. Efficiency of conversion from trapped electrons to methyl radicals when the former is photobleached in a y-irradiated 3-methylhexane glassy matrix containing (c:) CH,I, (JJ) CH,CI or (0) CH,F as electron scavengers as a function of the scavenger concentration. The abscissa for CH,F is different from those for the other two scavengers. The scavenging of photoliberated electrons occurs more readily for CH,I than for CH:,CI, because the conversion efficiency reaches unity at lower concentrations of the scavengers for the former than for the latter. The conversion efficiency for CHBF is so low that the plateau of the efficiency concentration curve could not be obtained. The dependence of the conversion efficiency on the scavenger concentration indicates the competition between the scavenger reaction and the charge recombination reaction and gives the reactivity of the photoliberated electrons towards the scavengers. The scavenger concentration of 0.003, 0.006 and 0.4 mole “’ ,
Dissociative
electron attachment
to methylhalides
in 3-methylhexane
glassy matrix
335
TABLE I. EFFICIENCY 0~ ELECTRON SCAVENGING BY METHYLHALIDES IN 3-METHYLHEXANE GLASSY MATRIXDURINGy-IRRADIATIONORDURING PHOTOBLEACHING OF TRAPPED ELECTRONS AT 77 K
Method
CH3Cl
CH3I (0.6eV)
_
G(et)/Go(et)=1/2
From
-
G(CH3)/GUKH3)=1/2
From
conversion
of
0.2
0.03
(0.035) 0.1
(0.24) -__-
(0.017)
(0.055)
efficiency
l/2 in photobleaching
(-1.2&J)
(0.007) 0.4
1.0
Fran
CH3F
lO.OeV)
1
z 2 (%0.003)
0.02
(0.006)
(0.4)
Numbers in parentheses indicate the scavenger concentrations in mole % unit which give the observation indicated in the first column. The heat of reaction for dissociative electron attachment calculated from the gas phase data is also indicated for the scavengers used. three scavengers is shown in Table I, where the scavenger efficiencies during y-irradiation are also shown for comparison. The latter efficiencies are
obtained alternatively from the reciprocal of the scavenger concentration giving the trapped electron yield of one-half of that in the neat matrix, G(et-)/G,(et-) = 4, or from the reciprocal concentration giving the methyl radical yield one-half of that for the scavenger concentrations which are sufficiently high, G(CH,)/G,(CH,) = 3. All the efficiencies shown in Table I are normalized, so that the efficiency derived from G(et-)/G,(et-) = 4 is unity for CHJ. In any method of determining the efficiency, it decreases in the order of CHJ, CH,Cl and CH,F as expected from the decreasing heat of reaction for the dissociative electron attachment to them. Although the electron attachment to CH,F is thought to be endothermic, it certainly occurs, though less efficiently, in the 3-methylhexane glassy matrix during either y-irradiation or photobleaching of the trapped electrons. The E.S.R. spectrum in Fig. l(E) shows apparently the decay of the trapped electrons and the formation of the methyl radicals during the photobleaching of the trapped electrons in the y-irradiated matrix containing CH,F. It is noticed that CH,F shows a reaction different from that of CHJ and of CH,Cl in scavenging electrons; the former scavenger is relatively inefficient in scavenging the photoliberated electrons, whereas the latter scavengers are efficient both during y-irradiation and during photobleaching. DISCUSSION to CHJ and CH,Cl It has been commonly assumed that electron scavenging and trapping in glassy matrices involve Dissociative
electron attachment
thermalized electrons and that the yield of the trapped electron and that of scavenger products (the methyl radical in the present investigation) are determined by the competition between trapping, scavenging and charge recombination reactions: (1)
e- + trap (Tt
(2) e-+scavenger (3)
e- + cation A
et-, n’ > scavenger product, charge recombination,
where o’s are the cross-section for these reactions. The observed scavenging efficiencies shown in Table 1 are first examined on the basis of this simple competition scheme. The dependence of the trapped electron yield upon the scavenger concentration is derived directly from the scheme as (4)
G,(et-) = 08 cs G(et-) 1 + Ut CTf U, cc ’
-
where cs, CTand cc are the scavenger concentration, the effective concentration of physical traps in the matrix and the formal concentration of the cation, respectively. The concentration of the cation, as well as that of physical traps, is presumed to be constant, because every ionization event is regarded as an isolated event and a single cation exists for an electron to recombine with. The reciprocal of the scavenger concentration giving the trapped electron yield of G(et-)/G,(et-) = + during y-irradiation is u&o, cc + ut CT). Reaction (1) is effectively absent during photobleaching, because all electrons are liberated from their traps until they react with either the scavenger or the cation. The efficiency of conversion from the trapped electron to the methyl radical is given by q, c&u, cc + a, cs) and, therefore, the reciprocal concentration of the scavenger giving the conversion efficiency of one-half is
KATSUYOSHI HARADA, MASAHIRO IRIE and HIROSHI YOSHIDA
336
Q/O, cc. These considerations lead to the expectation that the scavenger efficiency shown in Table 1 is higher for the photoliberated electron during photobleaching than for the thermal electron during y-irradiation. This is really the case for CH,I and CH,Cl as scavengers, for which the electron attachment is an exothermic reaction. However, the competition scheme is too simple to interpret qualitatively the observed features of electron scavenging by these scavengers. According to the scheme, the scavenging efficiency determined from the methyl radical yield, G(CH,)/G,(CH,) = a is predicted to be equal to the scavenging efficiency determined from the trapped electron yield, G(e,--)/G,(et-) = _2. However, the former efficiency was found to be smaller than the latter for both CHJ and CH,Cl as shown in Table I. Furthermore, the dependence of the trapped electron yield upon the concentration of CHJ (see Fig. 4) does not follow equation (4), but is better expressed by an exponential relationship, G(et-9 G&--I as shown
= exp (- ;um),
in Fig. 6.
7
20
r
I
0
0.05
CONCENTRATION
0.10
OF CHJI
(mole%)
FIG. 6. Dependence of the initial yield of trapped electrons and that of methyl radicals in a 3-methylhexane glassy matrix containing CHJ upon the concentration of the scavenger. (A) The reciprocal plot of the trapped electron yield, (B) the exponential plot of the trapped electron yield and (C) the exponential plot (see the text) of the methyl radical yield. The alotted points in this figure were taken from ;he points &pled from the curves in Fig. 3. This kind of relationship has been observed for some electron scavenging reactions in glassy matricesu2p In), and was attributed recently to longrange quantum tunnelling of electrons from their
trap to scavenger n~olecules’lL-“‘l. In the context ot tunnelling mechanism, all the secondary electrons are trapped once after escaping the prompt charge recombination, and a fraction of them tunnel to the scavenger. The yield of surviving electrons (the yield of trapped electrons at the time of measurement) depends on the distribution of the trap scavenger distance and the potential barrier to be tunnelled. Among the present results, the enhancement of the electron decay due to the added scavenger (see Fig. 3) also seems to indicate that the scavenging of electrons by CH,I is brought about by the tunnelling mechanism’13’. In the present case, however, the Franck-Condon effect is one of the important features to be considered for dissociative electron attachment by the tunnelling mechanism as pointed out previouslyu9). Even though the dissociative attachment to CHJ (and probably to CH,CI also’21’) is exothermic, it is not expected to have a positive molecular electron affinity and, therefore, any low-energy vacant molecular orbitals. For the tunnelling mechanism to be operative in the electron scavenging reaction, the level of the lowest vacant molecular orbital (the acceptor level) should be resonant (or nearly resonant) to the level of the trapped electron. This condition does not seem to be fulfilled for the dissociative electron attachment to methylhalides. As a matter of fact, the electron tunnelling in non-dissociative electron attachment was observed to occur to scavengers having a positive electron affinity (or an electron athnity larger than the bound energy of the trapped electron) but not to scavengers having a negative electron afinity in an alkaline ice matrix’““‘. Although some of the present results seem to indicate a tunnelling mechanism in the electron scavenger reaction of CH,I on the basis of the criteria so far used to judge the electron transport mechanism, further studies are needed especially on the Franck--Condon effect before reaching a conclusion. We are not insisting on excluding the tunnclling mechanism but it may be worth while to interpret otherwise the exponential dependence of the trapped electron yield upon the scavenger concentration. In order to explain the exponential dependence of hydrated electrons upon the scavenger concentration in aqueous solutions, Hunt et cd. proposed a model where secondary electrons can react with the scavenger only in theit epithermal energy range and they become solvated when they have been thermatired without being scavenged’““. If an electron has a probability P (~I
Dissociative electron attachment to methylhalides in 3-methylhexane glassy matrix reacting with a scavenger molecule on each encounter in the epithermal range and if it undergoes N effective collisions during its thermalization process, the probability it will not have reacted with a scavenger molecule is (1 - cP)~, where c is the fractional molecular concentration of the scavenger. This probability is readily approximated by exp (-cPN) for N%l and cP
(6)
-
Wet->
G(CHd
= [l -(a* cs +
UC
CC)IN,
where Gu(CH,) gives the yield of all electrons involved. This leads readily to N
(7)
1 0,csN1. (-~l-a,cc
G(w) _ l-(e,cs+e,cc!) l--U,@ [ G,(et-) -exp
On the other hand, the probability of not reacting with scavenger molecule is given by (1 -u~cs)~, and therefore
(8)
1
_
-G(C&) = exp (- uS cs N). GJCH,)
Exactly speaking, cs is the fractional concentration of the scavenger molecule, cc the ‘effective fractional concentration’ of the cation (a constant value for a low dose), and a, and uc are the probabilities of the reactions (2) and (3) on each encounter. The observed dependence of the methyl radical yield upon the CHJ concentration shown in Fig. 4 is replotted in Fig. 6, which shows the straight line as expected from equation (8). The slope of the line is found to be smaller than that for the exponential dependence of the trapped electron yield, consistent also with the expectation that a, N/(1 - uc cc) > u,N. The efficiencies of the electron scavenging reaction are calculated from equations (7) and (8): the efficiency determined from G(et-)/G,(et-) = 4 is given by (a, N/in 2)/(1- uc CC) and that from G(CH,)/ G,(CH,) = ) is u,N/ln2. The former efficiency is expected to be larger than the latter, and this is really the case as shown in Table I for CHJ and CH,Cl The conversion of the photoliberated electrons to the methyl radicals can be explained also in the
337
framework of the model and it is expected to occur with the efficiency (g)
-AD-M = (I-
a, Cfj)N or exp (- u, cs N’), G(w) where A[CH,] is the increase in the methyl radical concentration in G units during photobleaching, and N’ is the number of effective collisions that a photoliberated electron undergoes before charge recombination. From equation (9), the efficiency of the electron scavenging reaction during photobleaching is calculated to be u, N’/ln 2. The observed efficiency during photobleaching is larger than the efficiency during y-irradiation, a, Nlln 2, as shown in Table I and indicates that N’> N. If a small excess energy attainable by photobleaching the trapped electrons suffices for the scavenging reaction, N’ should be larger than N and the ratio N’IN gives the average number of detrap-retrap cycles before a photoliberated electron recombines with the cation in the absence of a scavenger. Thus, the epithermal electron-scavenging model can also interpret all the present results observed for CHSI and CH,Cl as electron scavengers in the matrix.
Dissociative electron attachment to CHsF The gas phase data indicate that the heat of reaction for the dissociative electron attachment to CH,F is as high as l-2 eV and suggest that it is endothermic even in the glassy matrix, so that the attachment is not brought about by thermal electrons. However, the attachment reaction really occurred and the formation of the methyl radicals was observed during either y-irradiation or photobleaching. The most significant feature in electron scavenging by CH,F is that the scavenging efficiency during photobleaching, a, N’/ln 2, is smaller than that during y-irradiation, u, N/in 2. This is just the reverse to what was observed for CHJ and CH,Cl as shown in Table I, and can be interpreted by considering that only epithermal electrons undergo dissociative electron attachment to CHsF for which the attachment reaction is endothermic. In the gas phase, a transient molecular anion of scavengers must be a precursor of radicals formed through dissociative electron attachment, and the dissociative attachment has a positive energy threshold’23). The threshold for CH3F is higher than 1.2 eV (the heat of reaction for the dissociative electron attachment to CH,F) in neglecting a small energy of electronic polarization due to charged entities involved. The secondary electrons necessarily transmit the epithermal energy range during the thermalization process and have a chance of colliding with CH,F molecules to be effectively
KATSUYOSHIHARADA, MASAHIRO IRIE and HIROSHIYOSHIDA
338
scavenged. On the other hand, the threshold is so high that only a fraction of the photoliberated electrons can attain to it and can be scavenged by CH,F. The highest attainable energy is estimated to be 1.6eV [the highest energy limit of the bleaching light (2.4 eV) minus the bound energy of the trapped electron (about 0.8 eV)] and is thought to be higher than the threshold for the dissociative electron attachment to CH3F. Therefore, the effective number of collisions during photobleaching, N’, can be on average smaller than that during y-irradiation, N, for CHIF as scavenger. The value of N’ is thus dependent upon either the threshold energy for the attachment reaction or the initial energy of the photoliberated electrons. As a matter of fact, the conversion from the trapped electrons into the methyl radicals was found qualitatively to be dependent upon the wavelength of the bleaching light for CHJF; the conversion efficiency for blue light (35(rSOO nm) is about twice as high as that for 1.R. light (wavelength longer than 900 nm) for a CHBF concentration of 0.1-0.2 mole %. The electron photoliberated with the shorter wavelength has the greater initial excess energy and the higher probability of colliding effectively with CH,F molecules and is scavenged more efficiently. In this respect, it may be worth while noting that the scavenging efficiency of SF, for photoliberated electrons was found to be higher for the shorter wavelength, which was attributed to the longer range the electron travelled before being retrappedtz4). Although it could not be examined whether the exponential dependence of the trapped electron yield upon the scavenger concentration holds for CH,F or not, the decay of the trapped electrons was not found to be increased in its presence. This observation and a large negative heat of reaction for electron attachment imply that the epithermal electron-scavenging rather than the electron tunnelling is the mechanism operative in the dissociative electron attachment to CHBF in a glassy matrix.
REFERENCES 1. (a) H.
M. IRIE, K. HAYASHI, S. OKAMURA and YOSHIDA, J. phys. Chem. 1971, 75, 476: (b) H. YOSHIDA, M. IRIE, 0. SHIMADA and K. HAYASHI, J. phys. Chem. 1972, 76, 3747.
2. M. A. BONIN, J. LIN, K. TSUJI and F. WILLIAMS, Adv. Chem. Ser. 1968, 82, 269. 3. Calculated from the data of CH,-X dissociation energies and atomic electron affinities of X in The - Chemistr) of the Carbow Hakagen Bond, edited by S. PATAI, John Wiley. New York, 1973, pp. 9, 687.
4. (a) J. P. GUAKINO, M. C. RONAYNT: and W. H. HAMILL, Radiat. Res. 1962, 17, 379; (b) E. I’. BERTINand W. H. HAMILL. J. Am. them. Sot. 1964. 86, 1301; (c) D. W. SKELLY. R. G. HAYES and W. H. HAMILL.,J. them. Phys. 1965, 43, 2795. 5. (a) R. F. CLARIDGE and J. E. WILLARD. J. Am. them. Sot. 1965, 87, 4992: (b) M. SHIKOM and J. E. WILLARD, J. phys. Chem. 1968, 72, 1702. 6. M. IRIE, K. HAYASHI, S. OKAMURA and H. YOSHIDA, Int. J. Radiat. Phys. Chem. 1969, I, 297. 7. F. SWARTS, Bull. Sot. Chim. Beg. 1937, 46, 10. 8. C. R. ROY and J. E. WILLARD, J. phys. Chem. 1972. 76, 1406.
9. L. KF.VAN, Advances in Radiation Chemistry, Vol. 4. edited by M. BURTON and J. L. MAGEE, Wiley, New York, 1974, p. 182. 10. K. OKA, H. YAMAZAKI and S. SHIDA, Bull. &CIII. Sot. Japatz 1973, 46, 2347. 11. M. SHIOTANI, D. IIMURA, S. MURABAYASHI and J. SOHMA, Int. J. Radiat. Phys. Chenl. 1975, 7, 267. 12. (a) P. J. DYNF. and A. 0. MILLER, Can. J. Chew?. 1965, 43, 2696; (b) A. NAMIKI. .I. them. P~J~s. 1975. 62, 990.
13. (a) L. T. BUOAENKOand 0. S. PO~OLOTSKAYA.Klrirfr ~ys. Eneug. 1967, 1, 480; (b) H. H. STFEN, 0. KAALHUS and M. KOKOSHANA, J. phys. Chem.
1971.75, 1971. 14. J. R. MILLEI~,J. them. Phyx. 1972, 56, 5173. 15. A. I. MIKHAILOV. Dokl. Akad. Xuuk SSSR. 1971. 197, 136. 16. K. I. ZAMARAEV, R. F. KHAIRUTDI~XO~, A. I. MIKHAILOV and V. E. GOLDANSKI, Dokl. Akad. Nauk SSSR, 1971, 199. 640. 17. J. KROH and Cz. STRADOWSKI, Irrt. J. Radiat. Phys. Chem. 1973, 5, 243. 18. K. 1. ZAMARAEV and R. F. KHAIRUTDINOV, Cllef?l. Phys. 1974, 4, 181. 19. M. TACHIYA and A. MOZUMDER, Chem. Phys. Letr. 1974, 28, 87. 20. J. R. MILLER, J. phys. Chem. 3975, 79, 1070. 21. W. E. WENTWORTH, R. GEORGE and H. Kt.ITH. J. them. Phys. 1969, 51, 1791. 22. (a) R. K. WOLFF, M. J. BRONSKILLand J. W. HUNT, J. them. Phys. 1970, 53, 4211; (b) J. E. ALDRICH, M. J. BRONSKILL. R. K. WOLFF and J. W. HUE;T.
J. them. Phys. 1972, 56, 5173. 23 (a) M. W. HICKAM and D. BERG, J. c/rem. Phys. 1958, 29, 517; (b) L. G. CHRISTOPHOROU,R. N. COMPTON, G.
S. HURST and P. W.
J. them. Phys. 1966, 45, 536. 24. K. OKA, H. YAMAZAKI and Y. them. Sor. Jaoan 1975. 48.1107.
REINHARDT,
HATANO, Bull.
Press. Printed in Great Britain
DISSOCIATIVE ELECTRON ATTACHMENT TO METHYLHALIDES IN 3-METHYLHEXANE GLASSY MATRIX KATSUYOSHI HARADA, MASAHIRO IRIE* and HIROSHI YOSHIDA Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan (Received in revised form 20 June 197.5) Abstract-Dissociative electron attachment reaction to CH,I, CH&l and CH,F in a 3-methylhexane glassy matrix was studied by determining the yield of trapped electrons and that of methyl radicals immediately after y-irradiation at 77 K as a function of the scavenger concentration. The efficiency of conversion from the trapped electrons to the methyl radicals was also studied by photobleaching the trapped electrons. The results obtained are (1) the dissociative electron attachment occurs to CH,F, for which the gas phase data indicate that the reaction is endothermic by 1.2 eV, during either the y-irradiation or the photobleaching, and (2) CH,F is relatively less efficient in scavenging photo-liberated electrons than in scavenging the electrons during the y-irradiation, whereas CH,I and CHaCl are efficient scavengers for both the electrons. The dependence of the yields of the trapped electrons and the methyl radicals is discussed in terms of the electron-tunnelling mechanism and the epithermal electronscavenging mechanism. INTRODUCTION of electrons in y-irradiated glassy matrices have been studied for those solutes which undergo dissociative or non-dissociative electron Some of these studies attachment reactions. suggested that the behavior of electrons in the glassy matrices was different from that in gas phase. For example, the present authors reported that dissociative electron attachment occurs to solutes such as methylvinylether(r&) and dimethylether(ib) for which the reaction is thought to be endothermic from gas phase data, when trapped electrons are photobleached. This feature seems to indicate the specific nature and behavior of electrons in the glassy matrices, though it may be attributed partly to the solvation energy of ionic entities involved. Bonin et al. also reported an interesting observation that dissociative electron attachment occurred to acetonitrile upon photobleaching the trapped electrons in 2-methyltetrahydrofuran glassy matrix, while it did not occur during thermal bleachingf2). In order to obtain more information about dissociative electron attachment in glassy matrices, the study is extended here to the reaction for methyliodide, methylchloride and methylfluoride for which the heat of reaction is calculated to be Reactions
* Present address: The Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka 564, Japan.
0.6, 0.0 and - 1.2 eV, respectively”‘l, from the gas phase data. Methylhalides (iodide, chloride and bromide as well) are known to be good electron scavengers and are very often used in organic glassy matricesC4p5). However, in the present investigation trapped electrons were generated by y-irradiating a 3-methylhexane glassy matrix in the presence of a very small amount of methylhalides and then the trapped electrons were photobleached to result in the formation of methyl radicals through dissociative electron attachment to the solutes. The efficiency of the attachment during either y-irradiation or photobleaching is examined for these three scavengers. The methyl radicals, as well as the trapped electrons, were qualitatively measured by electron spin resonance (E.S.R.) method quite easily, which is the reason why methylhalides were used as electron scavengers in the present investigation. EXPERIMENTAL 3-Methylhexane was purified as mentioned previously’6’. CH,I and CH,Cl of analytical grade were used as received without further purification. CH,F was synthesized from CH,I and hydrogen fluoride”) and purified by passing through water. The concentration of methylhalides was determined by volumetric measurements under reduced pressures. Sample solutions were sealed in E.S.R. sample tubes of pure quartz under a vacuum of less than 10d6 Torr, irradiated with 6oCo y-rays at a dose rate of 1.5 Mrad h-l to a dose of 0.075 Mrad at 77 K in the dark and subjected to
332
KATSUYOSHIHARADA, MASAHIROIRIE and HIROSHI YOSHI~A
E.S.R. measurements at 77 K. Occasionally a ““Co source of 0.5 Mrad h-l was used but the decay of both the methyl radicals and the trapped electrons during the long irradiation time was properly corrected. Photobleaching of the trapped electrons was carried out with light of wavelength longer than 520 nm from the incandescent lamp of a slide projector. RESULTS Outline of observedphenomena When the concentration of the scavengers in a 3-methylhexane glassy matrix is low enough, the trapped electrons and the methyl radicals are simultaneously formed in the matrix by y-irradiation and observed from their E.S.R. spectra: a single-line spectrum of 0.37 mT width (peak-topeak width in derivative curve) due to the trapped electrons and a four-line spectrum with a separation of 2.3 mT due to the methyl radicals as shown in Fig. l(A) and (D). The initial yield of the trapped electrons and that of the methyl radicals are determined by extrapolating their decay curves to zero time (the time when the irradiation is stopped). Although the initial yields include some uncertainty due to the decay during irradiation, it has no serious effect on the analysis of the data in the following sections.
After observing the decay curves for an adequate period, the trapped electrons were photobleached completely. The conversion efficiency from the trapped electron to the methyl radical was detrrmined from the ratio of the increment of the methyl radical spectrum to the photobleached spectral intensity of the trapped electron [compare spectrum (C) with (B) and spectrum (E) with (D) in Fig. I]. The photobleaching is almost completed by the first 10 s. The change in the concentration of the trapped electrons and that of the methyl radicals during the course of the experimental procedures are illustrated in Fig. 2 representatively for CH,I as a scavenger. The conversion efficiency is dependent upon the scavenger concentration as expected. The dependence of the conversion efficiency was examined for CH,I upon either the wavelength of light or the survival fraction of trapped electrons to be bleached, but no systematic trend could be observed. The methyl radicals decrease in their spectral intensity in first-order kinetics, which contrasts to the decay of the methyl radicals formed through the dissociative electron attachment to methylvinyl This observation ether and dimethyl ether”‘. supports the interpretation that the methyl radicals
FIG. 1. E.S.R. spectra recorded from a 3-methylhexane glassy matrix containing 0.014 mole :c of CHJ irradiated to a dose of 0.075 Mrad at 77 K (A) immediatelv after the irradiation. (B) befbre and (C) after photobleaching of trapped electrons, and the giassy matrix containing 0.41 mole % of CH,F irradiated to the same dose (D) before and (E) after photobleaching the trapped electrons. The black arrows indicate the trapped electron spectra and the white arrows the methyl radical spectra.
Dissociative electron attachment to methylhalides in 3-methylhexane glassy matrix ,-f-irradiation
,- photobleach Isl(
1
333
between the trapped electron and the scavenger when the dissociative electron attachment is an endothermic reaction. Electron scavenging during y-irradiation
0
I
0
I
I LI
I
20
40
TIME
I
60
(min)
FIG. 2. Change in (0) the concentration of trapped electrons and (0) that of methyl radicals in a 3-methylhexane glassy matrix containing a small amount of CH,I irradiated to a dose of 0.25 Mrad (higher than the usual experimental condition) during the whole experimental procedure, determined by E.S.R. measurements at 77 K.
decay through geminate recombination with halide ionsn’). The trapped electron spectrum decays slowly (20 ‘A within the first 1 h) in the neat matrix, whereas it decays more quickly in the presence of a small amount of CHJ as shown in Fig. 2. It may be worthwhile to note, though qualitatively, that this enhancement of decay after y-irradiation is dependent upon the nature of the scavengers used. It is enhanced remarkably by CHJ and slightly by CH,CI present in small concentration. However, the addition of CH,F to the matrix caused no effect on the decay of the trapped electron up to a concentration as high as a few mole y0 as shown Fig. 3. This indicates that no reaction occurs 1.0
0.2 0
20
40 TIME
FIG.
60
(mid
3. Isothermal decay of trapped electrons at 77 K after y-irradiation of (0) a neat 3-methylhexane glassy matrix, (0) matrix containing 0.025 mole % of CHJ, and (0) matrix containing 041 mole % of CH,F.
Figure 4 shows, representatively for CHJ, the dependence of the yields immediately after yirradiation for the trapped electrons and the methyl radicals upon the concentration of the added scavenger. Electrons are scavenged only partially at a scavenger concentration of less than 0.05 mole % and the rest of electrons are trapped in the matrix. The yield of the trapped electrons in the neat matrix is estimated to 0.53 (for 100 eV energy absorbed), which is comparable with the value previously reported by UP) and is smaller than the values determined by several groups”“. The primary concern here is, however, not in the absolute value of the yield but in its value relative to the methyl radical yield.
0
’
0
i
‘%
I
’ 03 0.3 0.2 CONCENTRATION OF CH,I (mole%)
I I
0.4
FIG. 4. Dependence of (0) the yield of trapped electrons and (0) that of methyl radicals immediately after y-irradiation of a 3-methylhexane glassy matrix at 77 K upon the concentration of CHJ added to the matrix as an electron scavenger.
With the increasing concentration of CHJ, the trapped electron yield rapidly decreases and the methyl radical yield increases somewhat slowly from zero to a constant value. The constancy of the methyl radical yield at a scavenger concentration higher than O-1 mole % results from the complete scavenging of the electrons. It should be noted that the ultimate yield of the methyl radicals, G,(CHa), is higher than the trapped electron yield in the neat matrix, GJet-), which indicates that the electron scavenging reaction of CHJ during yirradiation competes not only with electron trapping but also with prompt charge recombination in the matrix. The scavenger concentration at which the trapped electron yield becomes one-half of the yield in the absence of the scavenger can give a
334
KATSUYOSHI HARADA, MASAHIRO IRIE and HIROSHI
measure of efficiency of the scavenging reaction. The efficiency is also given by the scavenger concentration at which the methyl radical yield is one-half the plateau value at high scavenger concentrations. For CH31 as scavenger, the efficiency is given by a scavenger concentration of 0.007 mole “/: (from a decrease in the trapped electron yield) and 0.01, mole “/, (from an increase in the methyl radical yield) from Fig. 4. Again, the difference between these two values indicates that a fraction of the electrons which undergo the prompt charge recombination in the absence of the scavenger is scavenged by CH,I. For CH,Cl, the results obtained are similar to those for CHJ and are essentially the same as those reported previously(lO’, except that CH,Cl is less efficient in scavenging electrons. This is indicated by the scavenger concentration, 0.035 and O.OSj mol %, at which the trapped electron yield decreases to one-half and the methyl radical yield increases to one-half, respectively. Although dissociative electron attachment to CH,F is thought to be endothermic, it is shown to occur by observing the formation of methyl radicals and the decrease in the trapped electron yield [see Fig. l(D) and (E)]. The yield of the methyl radicals is low because of the low scavenging efficiency of CH,F. However, the formation of methyl radicals is not due to the direct effect of radiation on CH$F but due to the dissociative electron attachment, because the trapped electrons are eliminated by adding a few mole “/, of CH,F to the matrix. The scavenger concentration of 0.24 mole o/o lowers the trapped electron yield to onehalf of its value in the neat matrix. The plateau of the methyl radical yield could not be determined, because the direct effect becomes apparent at high scavenger concentrations. However, the extrapolation to 100 mole % CH$F of the linear increase in the methyl radical yield at low scavenger concentrations gives a yield as high as 50. This also indicates that the methyl radicals are formed essentially due to the dissociative electron attachment when the concentration of CH,F is not very high. Shiotani et al. also noted the occurrence of dissociative electron attachment to fluorinated acetic acids in glassy matrices where the reaction was thought to be endothermic”l). Dissociatiz;e electron attachment induced byphotobleaching of trapped electrons When the trapped electrons are photobleached, the methyl radical spectrum increases in its intensity in the presence of methylhalides in the matrix [see
YOSHIDA
Fig. l(C) and (E)]. This indicates that a fraction of the electrons photoliberated from their trap attach dissociatively with the scavengers. The conversion efficiency from the trapped electron to the methyl radical is dependent upon the scavenger concentration; it increases with increasing concentration and then reaches a plateau value as shown in Fig. 5. The microwave power saturation factor should be known for the trapped electron spectra in order to determine the absolute values of the conversion efficiency (the methyl radical spectra are not saturated at the microwave power level used), and can be estimated by presuming that the Dlateau value corresponds to an eficiency of unity. 0
0
0.L
0.8
1 ,? -1
C 0.08 0.04 CONCENTRATION OF SOLUTES (moie’io)
FIG. 5. Efficiency of conversion from trapped electrons to methyl radicals when the former is photobleached in a y-irradiated 3-methylhexane glassy matrix containing (c:) CH,I, (JJ) CH,CI or (0) CH,F as electron scavengers as a function of the scavenger concentration. The abscissa for CH,F is different from those for the other two scavengers. The scavenging of photoliberated electrons occurs more readily for CH,I than for CH:,CI, because the conversion efficiency reaches unity at lower concentrations of the scavengers for the former than for the latter. The conversion efficiency for CHBF is so low that the plateau of the efficiency concentration curve could not be obtained. The dependence of the conversion efficiency on the scavenger concentration indicates the competition between the scavenger reaction and the charge recombination reaction and gives the reactivity of the photoliberated electrons towards the scavengers. The scavenger concentration of 0.003, 0.006 and 0.4 mole “’ ,
Dissociative
electron attachment
to methylhalides
in 3-methylhexane
glassy matrix
335
TABLE I. EFFICIENCY 0~ ELECTRON SCAVENGING BY METHYLHALIDES IN 3-METHYLHEXANE GLASSY MATRIXDURINGy-IRRADIATIONORDURING PHOTOBLEACHING OF TRAPPED ELECTRONS AT 77 K
Method
CH3Cl
CH3I (0.6eV)
_
G(et)/Go(et)=1/2
From
-
G(CH3)/GUKH3)=1/2
From
conversion
of
0.2
0.03
(0.035) 0.1
(0.24) -__-
(0.017)
(0.055)
efficiency
l/2 in photobleaching
(-1.2&J)
(0.007) 0.4
1.0
Fran
CH3F
lO.OeV)
1
z 2 (%0.003)
0.02
(0.006)
(0.4)
Numbers in parentheses indicate the scavenger concentrations in mole % unit which give the observation indicated in the first column. The heat of reaction for dissociative electron attachment calculated from the gas phase data is also indicated for the scavengers used. three scavengers is shown in Table I, where the scavenger efficiencies during y-irradiation are also shown for comparison. The latter efficiencies are
obtained alternatively from the reciprocal of the scavenger concentration giving the trapped electron yield of one-half of that in the neat matrix, G(et-)/G,(et-) = 4, or from the reciprocal concentration giving the methyl radical yield one-half of that for the scavenger concentrations which are sufficiently high, G(CH,)/G,(CH,) = 3. All the efficiencies shown in Table I are normalized, so that the efficiency derived from G(et-)/G,(et-) = 4 is unity for CHJ. In any method of determining the efficiency, it decreases in the order of CHJ, CH,Cl and CH,F as expected from the decreasing heat of reaction for the dissociative electron attachment to them. Although the electron attachment to CH,F is thought to be endothermic, it certainly occurs, though less efficiently, in the 3-methylhexane glassy matrix during either y-irradiation or photobleaching of the trapped electrons. The E.S.R. spectrum in Fig. l(E) shows apparently the decay of the trapped electrons and the formation of the methyl radicals during the photobleaching of the trapped electrons in the y-irradiated matrix containing CH,F. It is noticed that CH,F shows a reaction different from that of CHJ and of CH,Cl in scavenging electrons; the former scavenger is relatively inefficient in scavenging the photoliberated electrons, whereas the latter scavengers are efficient both during y-irradiation and during photobleaching. DISCUSSION to CHJ and CH,Cl It has been commonly assumed that electron scavenging and trapping in glassy matrices involve Dissociative
electron attachment
thermalized electrons and that the yield of the trapped electron and that of scavenger products (the methyl radical in the present investigation) are determined by the competition between trapping, scavenging and charge recombination reactions: (1)
e- + trap (Tt
(2) e-+scavenger (3)
e- + cation A
et-, n’ > scavenger product, charge recombination,
where o’s are the cross-section for these reactions. The observed scavenging efficiencies shown in Table 1 are first examined on the basis of this simple competition scheme. The dependence of the trapped electron yield upon the scavenger concentration is derived directly from the scheme as (4)
G,(et-) = 08 cs G(et-) 1 + Ut CTf U, cc ’
-
where cs, CTand cc are the scavenger concentration, the effective concentration of physical traps in the matrix and the formal concentration of the cation, respectively. The concentration of the cation, as well as that of physical traps, is presumed to be constant, because every ionization event is regarded as an isolated event and a single cation exists for an electron to recombine with. The reciprocal of the scavenger concentration giving the trapped electron yield of G(et-)/G,(et-) = + during y-irradiation is u&o, cc + ut CT). Reaction (1) is effectively absent during photobleaching, because all electrons are liberated from their traps until they react with either the scavenger or the cation. The efficiency of conversion from the trapped electron to the methyl radical is given by q, c&u, cc + a, cs) and, therefore, the reciprocal concentration of the scavenger giving the conversion efficiency of one-half is
KATSUYOSHI HARADA, MASAHIRO IRIE and HIROSHI YOSHIDA
336
Q/O, cc. These considerations lead to the expectation that the scavenger efficiency shown in Table 1 is higher for the photoliberated electron during photobleaching than for the thermal electron during y-irradiation. This is really the case for CH,I and CH,Cl as scavengers, for which the electron attachment is an exothermic reaction. However, the competition scheme is too simple to interpret qualitatively the observed features of electron scavenging by these scavengers. According to the scheme, the scavenging efficiency determined from the methyl radical yield, G(CH,)/G,(CH,) = a is predicted to be equal to the scavenging efficiency determined from the trapped electron yield, G(e,--)/G,(et-) = _2. However, the former efficiency was found to be smaller than the latter for both CHJ and CH,Cl as shown in Table I. Furthermore, the dependence of the trapped electron yield upon the concentration of CHJ (see Fig. 4) does not follow equation (4), but is better expressed by an exponential relationship, G(et-9 G&--I as shown
= exp (- ;um),
in Fig. 6.
7
20
r
I
0
0.05
CONCENTRATION
0.10
OF CHJI
(mole%)
FIG. 6. Dependence of the initial yield of trapped electrons and that of methyl radicals in a 3-methylhexane glassy matrix containing CHJ upon the concentration of the scavenger. (A) The reciprocal plot of the trapped electron yield, (B) the exponential plot of the trapped electron yield and (C) the exponential plot (see the text) of the methyl radical yield. The alotted points in this figure were taken from ;he points &pled from the curves in Fig. 3. This kind of relationship has been observed for some electron scavenging reactions in glassy matricesu2p In), and was attributed recently to longrange quantum tunnelling of electrons from their
trap to scavenger n~olecules’lL-“‘l. In the context ot tunnelling mechanism, all the secondary electrons are trapped once after escaping the prompt charge recombination, and a fraction of them tunnel to the scavenger. The yield of surviving electrons (the yield of trapped electrons at the time of measurement) depends on the distribution of the trap scavenger distance and the potential barrier to be tunnelled. Among the present results, the enhancement of the electron decay due to the added scavenger (see Fig. 3) also seems to indicate that the scavenging of electrons by CH,I is brought about by the tunnelling mechanism’13’. In the present case, however, the Franck-Condon effect is one of the important features to be considered for dissociative electron attachment by the tunnelling mechanism as pointed out previouslyu9). Even though the dissociative attachment to CHJ (and probably to CH,CI also’21’) is exothermic, it is not expected to have a positive molecular electron affinity and, therefore, any low-energy vacant molecular orbitals. For the tunnelling mechanism to be operative in the electron scavenging reaction, the level of the lowest vacant molecular orbital (the acceptor level) should be resonant (or nearly resonant) to the level of the trapped electron. This condition does not seem to be fulfilled for the dissociative electron attachment to methylhalides. As a matter of fact, the electron tunnelling in non-dissociative electron attachment was observed to occur to scavengers having a positive electron affinity (or an electron athnity larger than the bound energy of the trapped electron) but not to scavengers having a negative electron afinity in an alkaline ice matrix’““‘. Although some of the present results seem to indicate a tunnelling mechanism in the electron scavenger reaction of CH,I on the basis of the criteria so far used to judge the electron transport mechanism, further studies are needed especially on the Franck--Condon effect before reaching a conclusion. We are not insisting on excluding the tunnclling mechanism but it may be worth while to interpret otherwise the exponential dependence of the trapped electron yield upon the scavenger concentration. In order to explain the exponential dependence of hydrated electrons upon the scavenger concentration in aqueous solutions, Hunt et cd. proposed a model where secondary electrons can react with the scavenger only in theit epithermal energy range and they become solvated when they have been thermatired without being scavenged’““. If an electron has a probability P (~I
Dissociative electron attachment to methylhalides in 3-methylhexane glassy matrix reacting with a scavenger molecule on each encounter in the epithermal range and if it undergoes N effective collisions during its thermalization process, the probability it will not have reacted with a scavenger molecule is (1 - cP)~, where c is the fractional molecular concentration of the scavenger. This probability is readily approximated by exp (-cPN) for N%l and cP
(6)
-
Wet->
G(CHd
= [l -(a* cs +
UC
CC)IN,
where Gu(CH,) gives the yield of all electrons involved. This leads readily to N
(7)
1 0,csN1. (-~l-a,cc
G(w) _ l-(e,cs+e,cc!) l--U,@ [ G,(et-) -exp
On the other hand, the probability of not reacting with scavenger molecule is given by (1 -u~cs)~, and therefore
(8)
1
_
-G(C&) = exp (- uS cs N). GJCH,)
Exactly speaking, cs is the fractional concentration of the scavenger molecule, cc the ‘effective fractional concentration’ of the cation (a constant value for a low dose), and a, and uc are the probabilities of the reactions (2) and (3) on each encounter. The observed dependence of the methyl radical yield upon the CHJ concentration shown in Fig. 4 is replotted in Fig. 6, which shows the straight line as expected from equation (8). The slope of the line is found to be smaller than that for the exponential dependence of the trapped electron yield, consistent also with the expectation that a, N/(1 - uc cc) > u,N. The efficiencies of the electron scavenging reaction are calculated from equations (7) and (8): the efficiency determined from G(et-)/G,(et-) = 4 is given by (a, N/in 2)/(1- uc CC) and that from G(CH,)/ G,(CH,) = ) is u,N/ln2. The former efficiency is expected to be larger than the latter, and this is really the case as shown in Table I for CHJ and CH,Cl The conversion of the photoliberated electrons to the methyl radicals can be explained also in the
337
framework of the model and it is expected to occur with the efficiency (g)
-AD-M = (I-
a, Cfj)N or exp (- u, cs N’), G(w) where A[CH,] is the increase in the methyl radical concentration in G units during photobleaching, and N’ is the number of effective collisions that a photoliberated electron undergoes before charge recombination. From equation (9), the efficiency of the electron scavenging reaction during photobleaching is calculated to be u, N’/ln 2. The observed efficiency during photobleaching is larger than the efficiency during y-irradiation, a, Nlln 2, as shown in Table I and indicates that N’> N. If a small excess energy attainable by photobleaching the trapped electrons suffices for the scavenging reaction, N’ should be larger than N and the ratio N’IN gives the average number of detrap-retrap cycles before a photoliberated electron recombines with the cation in the absence of a scavenger. Thus, the epithermal electron-scavenging model can also interpret all the present results observed for CHSI and CH,Cl as electron scavengers in the matrix.
Dissociative electron attachment to CHsF The gas phase data indicate that the heat of reaction for the dissociative electron attachment to CH,F is as high as l-2 eV and suggest that it is endothermic even in the glassy matrix, so that the attachment is not brought about by thermal electrons. However, the attachment reaction really occurred and the formation of the methyl radicals was observed during either y-irradiation or photobleaching. The most significant feature in electron scavenging by CH,F is that the scavenging efficiency during photobleaching, a, N’/ln 2, is smaller than that during y-irradiation, u, N/in 2. This is just the reverse to what was observed for CHJ and CH,Cl as shown in Table I, and can be interpreted by considering that only epithermal electrons undergo dissociative electron attachment to CHsF for which the attachment reaction is endothermic. In the gas phase, a transient molecular anion of scavengers must be a precursor of radicals formed through dissociative electron attachment, and the dissociative attachment has a positive energy threshold’23). The threshold for CH3F is higher than 1.2 eV (the heat of reaction for the dissociative electron attachment to CH,F) in neglecting a small energy of electronic polarization due to charged entities involved. The secondary electrons necessarily transmit the epithermal energy range during the thermalization process and have a chance of colliding with CH,F molecules to be effectively
KATSUYOSHIHARADA, MASAHIRO IRIE and HIROSHIYOSHIDA
338
scavenged. On the other hand, the threshold is so high that only a fraction of the photoliberated electrons can attain to it and can be scavenged by CH,F. The highest attainable energy is estimated to be 1.6eV [the highest energy limit of the bleaching light (2.4 eV) minus the bound energy of the trapped electron (about 0.8 eV)] and is thought to be higher than the threshold for the dissociative electron attachment to CH3F. Therefore, the effective number of collisions during photobleaching, N’, can be on average smaller than that during y-irradiation, N, for CHIF as scavenger. The value of N’ is thus dependent upon either the threshold energy for the attachment reaction or the initial energy of the photoliberated electrons. As a matter of fact, the conversion from the trapped electrons into the methyl radicals was found qualitatively to be dependent upon the wavelength of the bleaching light for CHJF; the conversion efficiency for blue light (35(rSOO nm) is about twice as high as that for 1.R. light (wavelength longer than 900 nm) for a CHBF concentration of 0.1-0.2 mole %. The electron photoliberated with the shorter wavelength has the greater initial excess energy and the higher probability of colliding effectively with CH,F molecules and is scavenged more efficiently. In this respect, it may be worth while noting that the scavenging efficiency of SF, for photoliberated electrons was found to be higher for the shorter wavelength, which was attributed to the longer range the electron travelled before being retrappedtz4). Although it could not be examined whether the exponential dependence of the trapped electron yield upon the scavenger concentration holds for CH,F or not, the decay of the trapped electrons was not found to be increased in its presence. This observation and a large negative heat of reaction for electron attachment imply that the epithermal electron-scavenging rather than the electron tunnelling is the mechanism operative in the dissociative electron attachment to CHBF in a glassy matrix.
REFERENCES 1. (a) H.
M. IRIE, K. HAYASHI, S. OKAMURA and YOSHIDA, J. phys. Chem. 1971, 75, 476: (b) H. YOSHIDA, M. IRIE, 0. SHIMADA and K. HAYASHI, J. phys. Chem. 1972, 76, 3747.
2. M. A. BONIN, J. LIN, K. TSUJI and F. WILLIAMS, Adv. Chem. Ser. 1968, 82, 269. 3. Calculated from the data of CH,-X dissociation energies and atomic electron affinities of X in The - Chemistr) of the Carbow Hakagen Bond, edited by S. PATAI, John Wiley. New York, 1973, pp. 9, 687.
4. (a) J. P. GUAKINO, M. C. RONAYNT: and W. H. HAMILL, Radiat. Res. 1962, 17, 379; (b) E. I’. BERTINand W. H. HAMILL. J. Am. them. Sot. 1964. 86, 1301; (c) D. W. SKELLY. R. G. HAYES and W. H. HAMILL.,J. them. Phys. 1965, 43, 2795. 5. (a) R. F. CLARIDGE and J. E. WILLARD. J. Am. them. Sot. 1965, 87, 4992: (b) M. SHIKOM and J. E. WILLARD, J. phys. Chem. 1968, 72, 1702. 6. M. IRIE, K. HAYASHI, S. OKAMURA and H. YOSHIDA, Int. J. Radiat. Phys. Chem. 1969, I, 297. 7. F. SWARTS, Bull. Sot. Chim. Beg. 1937, 46, 10. 8. C. R. ROY and J. E. WILLARD, J. phys. Chem. 1972. 76, 1406.
9. L. KF.VAN, Advances in Radiation Chemistry, Vol. 4. edited by M. BURTON and J. L. MAGEE, Wiley, New York, 1974, p. 182. 10. K. OKA, H. YAMAZAKI and S. SHIDA, Bull. &CIII. Sot. Japatz 1973, 46, 2347. 11. M. SHIOTANI, D. IIMURA, S. MURABAYASHI and J. SOHMA, Int. J. Radiat. Phys. Chenl. 1975, 7, 267. 12. (a) P. J. DYNF. and A. 0. MILLER, Can. J. Chew?. 1965, 43, 2696; (b) A. NAMIKI. .I. them. P~J~s. 1975. 62, 990.
13. (a) L. T. BUOAENKOand 0. S. PO~OLOTSKAYA.Klrirfr ~ys. Eneug. 1967, 1, 480; (b) H. H. STFEN, 0. KAALHUS and M. KOKOSHANA, J. phys. Chem.
1971.75, 1971. 14. J. R. MILLEI~,J. them. Phyx. 1972, 56, 5173. 15. A. I. MIKHAILOV. Dokl. Akad. Xuuk SSSR. 1971. 197, 136. 16. K. I. ZAMARAEV, R. F. KHAIRUTDI~XO~, A. I. MIKHAILOV and V. E. GOLDANSKI, Dokl. Akad. Nauk SSSR, 1971, 199. 640. 17. J. KROH and Cz. STRADOWSKI, Irrt. J. Radiat. Phys. Chem. 1973, 5, 243. 18. K. 1. ZAMARAEV and R. F. KHAIRUTDINOV, Cllef?l. Phys. 1974, 4, 181. 19. M. TACHIYA and A. MOZUMDER, Chem. Phys. Letr. 1974, 28, 87. 20. J. R. MILLER, J. phys. Chem. 3975, 79, 1070. 21. W. E. WENTWORTH, R. GEORGE and H. Kt.ITH. J. them. Phys. 1969, 51, 1791. 22. (a) R. K. WOLFF, M. J. BRONSKILLand J. W. HUNT, J. them. Phys. 1970, 53, 4211; (b) J. E. ALDRICH, M. J. BRONSKILL. R. K. WOLFF and J. W. HUE;T.
J. them. Phys. 1972, 56, 5173. 23 (a) M. W. HICKAM and D. BERG, J. c/rem. Phys. 1958, 29, 517; (b) L. G. CHRISTOPHOROU,R. N. COMPTON, G.
S. HURST and P. W.
J. them. Phys. 1966, 45, 536. 24. K. OKA, H. YAMAZAKI and Y. them. Sor. Jaoan 1975. 48.1107.
REINHARDT,
HATANO, Bull.