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D. P. LIN AND LARRYKEVAN
Optical Bleaching Effects on the Paramagnetic Relaxation of Trapped Electrons in Methyltetrahydrofuran at 77°K D. P. Lin and Larry Kevan* Department o j Chemistry, Wayne State University, Detroit, Michigan &S,Oi?
(Received October $6, 1971)
Publication costs assisted by the U.S. Atomic Energy Commksion
The effect of optical bleaching on the characteristic relaxation time, TIT,)'/^, of trapped electrons (et-) in methyltetrahydrofuran (MTHF) glass at 77'K has been measured for different doses corresponding to different initial spatial distributions. At low radiation doses (T1T2)'/2increases as et- are bleached. This confirms the spur model of electron trapping in this matrix and indicates that the spurs contain more than one eton the average. At high radiation doses TIT^)^/^ remains constant as et- are bleached. This is interpreted in terms of spin-spin relaxation of et- with MTHF as well as with other et-. A quantitative treatment of TITz data us. total spin concentration confirms this latter interpretation,
Introduction The spatial distribution of trapped species produced by r-irradiation is of interest in radiation chemistry because radiation energy is deposited inhomogeneously in condensed systems. For paramagnetic species the study of spin-spin interactions provides a method to examine local spatial distribution of the spins. Previous work has shown how changes in T,, the spin-spin relaxation time, measured by microwave saturation can indicate changes in the spatial distribution of the trapped spins. Trapped electrons in and organic g l a s ~ e s ~are - ~ distributed inhomogeneously in spurs with a spur size that increases with decreasing matrix polarity.* The effect of thermal decay on the spatial distribution of radicals in poly(methy1methacrylate) has also been studied by paramagnetic relaxation methods.7 Optical bleaching of electrons trapped in glassy matrices is expected to remove electrons homogeneously throughout the system. This should have a predictable effect on the relaxation time of the electrons as measured by T Zdepending on the initial spatial distribution. I n this work we test this conjecture on electrons trapped in methyltetrahydrofuran (MTHF) a t 77°K and confirm the spur model of electron trapping in this matrix. It is also shown that the spin-spin relaxation of trapped electrons in MTHF involves interaction with the MTHF radicals as well as with other trapped electrons.
Actual doses to the samples were corrected for the 6oCo decay rate. Epr measurements were made with a Varian 4502 X-band spectrometer. The magnetic field modulation frequency was 400 Hz and the modulation amplitude waB 0.6 G. Saturation curves were determined by measuring the epr derivative signal intensity, V (height X width2), vs. the microwave magnetic field, H I . Slow passage conditions as defined by Hl/(w,Hm) >> TIT^)'" were approximated. H , is the magnetic field modulation amplitude in gauss and wm is the modulation angular frequency in radians sec-'. Adiabatic passage conditions as defined by l/rH1
