Electron Capture in y-Irradiated Methylcyclohexane Glass mal activation systems. In any case, the ’conclusion that can be drawn is that helium is best represented by a transition probability model intermediate between the two extreme cases of SL and E X P distributions. References and Notes (1) This work was supported by the National Science Foundation and is abstracted from the Ph.D. Thesis of F. M. W., 1974. (2) D. C. Tardy and B. S. Rabinovitch, J. Chem. Phys., 48, 1282 (1968), (3) Y. N. Lin and B. S.Rabinovitch, J. Phys. Chem, 72, 1726 (1968). (4) S. C. Chan. J. T. Bryant, and B. S. Rabinovitch. J. Phys. Chem., 74.
867 (7) T. Fujimoto, F. M. Wang, and B. S. Rabinovitch, Can. J. Chem., 50, 19 (1970). (8) S. C. Chan, 6 .S. Rabinovitch, J. T. Bryant, L. D. Spicer, T. Fujimoto, Y. N. Lin, and S.P.Pavlou, J. Phys. Chem., 74, 3160 (1970). (9) F. W. Schneider and 6 .S. Rabinovitch, J. Amer. Chem. SOC., 84, 4215 (1962); part I . (IO) S. C. Chan, J. T. Bryant, L. D. Spicer, and B. S. Rabinovitch, J. Phys. Chem., 74, 2058 (1970). (11) B. S. Rabinovitch, Y. N. Lin, S. C. Chan, and K. W. Watkins, J. Phys. Chem., 71, 3715 (1967). (12) F. J. Fletcher, 6.S. Rabinovitch, K. W. Watkins, and D. J. Locker, J. Pbys. Chem., 7 0 , 2823 (1966). (13) B. S. Rabinovitch, D. C. Tardy, and Y. N. Lin, J. Phys. Chem., 71, 1549 (1967). (14) G. Kohimaier and B. S. Rabinovitch. J. Chem. Phys., 38, 1692 (1963) (15) B. S. Rabinovitch, H. F. Carroll, J. D. Rynbrandt, J. H. Georgakakos, B. A. Thrush, and R. Atkinson, J. Phys. Chem., 75, 3376 (1 971).
On Electron Capture in y-Irradiated Methylcyclohexane Glass T. B. Truong, A. Bernas,” Equipe de Recherches CNRS n098, Universite Paris V I , 91405 Orsay, France
and J. Roncin Laboratoire Associe de Physico-Chimie des Rayonnements, Universite Paris X1
(Received August 6, 1973)
Publication costs assisted by CNRS
Recent and present stimulated luminescence experiments have shown (besides trapped electrons et-) a solvent-derived negative species X- in y-irradiated methylcyclohexane (MCH) glass or in uv-excited TMPD-MCH system a t 77 K . In the former case, an ir et- bleaching is necessary for X - formation. The following conclusions are made. (1) The small decrease-A[X] due to X - formation has not been detected by epr but a comparison with the uv-excited TMPD-MCH case makes plausible that X is the methylcyclohexyl radical R-McH. (2) X - production efficiency examined as a function of et - bleaching wavelength displays a sharp maximum and its resonance character resembles that of a typical resonance electron attachment cross-section curve. (3) An INDO calculation of the lowest electronic energy states of the A and u structures of R*MCH and R’MCH- has shown that electron attachment to Re is not expected to occur in the gas phase and that the attachment observed in the solid phase necessitates a distortion from R . ( A ) to R.(o). The energy (-0.2 eV) corresponding to the maximum of X - formation efficiency curve may correlate with the activation energy of this r to u structural change. Implications on electron kinetic energy relaxation in y-irradiated glasses and phase effects on electron attachment processes are evoked.
Electron-ion pairs are produced in y-irradiated organic glasses and, in the absence of electron scavengers, most of the photoelectrons become matrix trapped. A subsequent optical excitation or “bleaching” of these electrons, under wavelength Ab, induces a neutralization luminescence1 and it has been noticed that the luminescence excitation spectrum closely parallels the trapped electrons (et - ) and anions absorption bands.2 Corroborating previous optical absorption observations,3 recent stimulated luminescence experiments4 have shown (besides et -) the existence of a solvent-derived negative species X - in y-irradiated 3-methylpentane (3MP) and methylcyclohexane (MCH) glasses or in uv-excited TMPD, 3MP, or TMPD-MCH systems a t 77 K (TMPD = tetramethyl-p-phenylenediamine). An identification of X is now being attempted and the
necessary conditions for X - formation and implications more precisely specified. y-Irradiated pure MCH will exemplify present experimental findings. Experimental Section The experimental conditions for y and uv irradiations and luminescence recordings have been described earlier.4 The y-ray doses range from 8 X 1019to 1.4 x 1021eV 8-1. Esr analysis are performed at 77 K with a JEOL Jesme 1 X spectrometer. A quartz iodine lamp with a silicium filter (A 11100 nm) was used for bleaching the irradiated sample when located in the epr spectrometer cavity. Results (a) As in the 3MP case,4 if the uv stimulation of the yirradiated MCH sample is performed directly after y-irraThe Journal of Pbysicai Chemistry. Vol. 78. No. 9. 7974
a6a
T. B. Truong, A . Bernas, and J. Roncin
!
ISL [arbitrary units)
‘1 \
I 300
I
3 50
I
-
1
100 hb(nm) L5O
Figure 1. Intensity of stimulated luminescence ( I ~ L ) in the uv region for y-irradiated MCH glasses (analyzing wavelength,,A, 445 nm: (a) y dose = 1.1 x lozoeV I , bleaching at Ab 1400 nm for 2 min; (b) y dose = 1.1 x loz%-ev 9-1, bleaching at Ab 1100 nm for 30 min; (c) y dose = 4 X l o z oeV g - I , bleaching at hb 1400 nm for 30 min. As shown in ref 10, et- is maximum for a dose of 1.5 X lozoe V g- and decreases at higher doses.
diation, the luminescence excitation spectrum or so-called stimulated spectrum attributable to X - is absent. However, following a 1400-nm bleaching (which lies in the etabsorption band) the X- stimulated spectrum appears very similar to the one previously recorded for 3MPa4The stimulated luminescence intensity is similarly found to increase with the 1400-nm bleaching time (Figure 1). On the other hand, when thermally released, the mobile electrons do not give rise to X-. The observation that only the optically excited trapped electrons can be captured has also been made for dissociative attachment on acetonitrile,5 methyl vinyl ether,6 and dimethyl ether7 following y-irradiations of mixed systems. Such a situation is, however, by no means general. Careful epr and optical spectroscopic studies have shown that a direct electron attachment under y-irradiations does occur on MTHF radicals.8 (b) After y-irradiation and before the trapped electron infrared bleaching, the epr spectrum consists of eight bands corresponding to the methylcyclohexyl radicalQ (R’MCH) and in the trapped electron singlet. After bleaching the samples under x b 21100 nm, the et,- signal disappears completely. The remaining R-MCH signal shows no decrease in intensity. However, the neutralization luminescence recorded later on the same prebleached sample does exhibit a luminescence for hb
