COMMUNICAT~ONS TO THE EDITOR selected value above for the atomization energy of SbP. With recourse to appropriate literature data,“’ the __ molecule SbP was further characterized by the folloxing thermodynamic parameters: AHf0298(SbP(g)) = 57.78 0.8 kcal molF1 (241.8 =t3.3 kJ mol-’) or AHf’o (SbP(g)) = 57.83 I-t 0.8 kcal mol-’ (242.0 =t 3.3 kJ mol-’) and S0298(SbP(g)) = 54.86 f 0.14 cal mol-l K-l (229.5 A 0.6 J mol-’ K-I) .18
*
Acknowledgment.
We gratefully acknowledge the
2341 financial support of The Robert A. Welch Foundation under Grant, A-387. (13) NOTEADDEDIN PROOF.It has come to our attention that an emission srJectrum of SbP had been obtained (K. K. Yee and W. E. Jones, J . i o l . Spectrosc., 33, 119 (1970)). The vibrational constants for Sb121P and Sb123P are 499.9 and 499.1 cm-’, respectively, as compared t o our estimate of 504.9 cm -1 for SbP. The free energy functions are affected but slightly by this revision and the derived thermodynamic data remain entirely unchanged. For the record, the revised free energy functions, -(GOT - H 0 o ) / T , are 60.265, 61.790, 63.091, 64.226, 65,223, and 66.138 cal mol-‘ K-I at 1000,1200, 1400,1600,1800, and 2000K, respectively.
C O M M U N I C A T I O N S T O THE E D I T O R
Wavelength Dependence of Photobleaching of Trapped Electrons in 3-Methylpentane Glass] Publication costs assisted by the U.S.Atomic Energy Commission
Sir: Early investigation2 of the quantum yields of photobleaching of trapped electrons produced by y irradiation of 3-methylpentane (331P) glass at 77’K indicated initial yields of near unity at 950 nm and ca. 0.4 at 1300-1600 nm. A yield “indistinguishable from zero” was reported for measurements using a glow-bar source, said to emit 97% of its radiation at wavelengths greater than 1700 nm, in conjunction with a I/win. germanium filter said to transmit beyond 1700 nm. If most of the intensity from the glow-bar-germanium filter combination was actually above the upper limit of the electron absorption spectrum (ea. 2200 nrn), rather than having the assumed “effective wavelength” of 1900 nm, the zero bleaching yield observed would be misleading. Since threshold e n e r g i e ~ ~for - ~ photobleaching of trapped electrons give some indication of the trapping energies, we have investigated bleaching from 1300 to 2150 nm in 3MP using a tungsten lamp source with a monochromator with a 100-nm bandwidth at halfheight. We find nonzero quantum yields out to 2150 nm, the values at 1800 and 2000 nm being ea. 0.7 and 0.2 of thc value at 1600 nm (Figure 1). The bleaching rate at 2150 nm was more t,han five times the minimum detectable rate. S o bleaching was detected with the monochromator set at 2300 nm, indicating that none of the bleaching at shorter. wavelengths was due to overtones or other impurity wavelengths. The evidence for zero photobleaching yield2 in the long wavelength portion of the trapped electron spectrum suggested transitions to bound excited states. However, the data of Figure 1 show that electrons are
removed from their traps throughout the absorption spectrum, indicating that the entire spectrum may be the photodetachment continuum for electrons bound in their traps by ea. 0.53 eV. The decrease in the quantum yield at long wavelengths may be due to participation of transitions to bound excited states, but alternatively to a high probability for an electron to become retrapped if it is ejected from its trap with little excess energy. If the trapped electron absorption spectrum is a continuum of direct transitions to unbound states, this can explain why it is so broad and does not change nppreciably during thermal decay6 or photobleaching. The spectrum may also include transitions to localized states which occur near the edges of conduction bands in amorphous s01ids.~ The bleaching experiments were done by observing, with repetitive scanning, the esr signal height from ?-irradiated samples of 3J1P in 2-mm i.d. Suprasil tubes while illuminating them in a Variaii 4531 ear cavity. A microwave power of 10 pW was used, partially saturating the electron singlet but allowing use of a resistive termination to prevent any dispersive contribution to the signal. Illuminations were started ea. 5 min after the end of irradiation and were continued until about one-third of the electrons were bleached. (1) This work is supported in part by the U. S. Atomic Energy Commission under Contract AT (11-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) D. W. Skelly and W. H. Hamill, J . Chem. Phys., 44, 2891 (1966). (3) P. J. Dyne and 0. A. Miller, Can. J . Chem., 43,2696 (1965). (4) A. Bernas and D. Grand, J . Chem. Soc. I),1667 (1970). (5) A. Habersbergerova, Lj.Josimovic, and J. Teply, Trans. Faraday SOC.,66,656 (1970). (6) (a) J. B. Gallivan and W. H. Hainill, J. Chem. f h y s . , 44, 1279 (1966) ; (b) R. A. Fass and J. E. Willard, unpublished results. (7) M. H. Cohen, J. ]Van-Cryst. Solids, 4, 391 (1970). The Journal o j Physical Chemistry, Vol. 76,No. 16, 1973
COMMUNICATIONS TO THE EDITOR
2342 LV
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Figure 1. Relative photobleaching quantum yields in 3 M P glass, and t h e absorption spectrum for trapped electrons in 3MP glass; y dose 2 x 1019 eV g-1. The relative quantum yield scale is set a t 0.4 a t 1300 nm t o be in approximate correspondence with the reported yield2 a t this wavelength.
No thermal decay (
