An electron spin-lattice relaxation mechanism involving tunneling modes for trapped radicals in glassy matrixes. Theoretical development and application to trapped electrons in .gamma.-irradiated ethanol glasses

Abstract

A new electron spin-lattice relaxation mechanism for molecular radicals in glassy matrices is developed theoretically and tested experimentally. The mechanism depends on modulation of the electron nuclear dipolar interaction between a trapped radical and nearby magnetic nuclei by the motion of tunneling nuclei or groups of nuclei in the disordered glass. In glassy systems it appears that modulation by tunneling modes is much more effective than modulation by lattice phonos for electron spin-lattice relaxation in low and intermediate temperature ranges, typically to approximately 100 K. The quantitative mechanism predicts: (a) that the spin-lattice relaxation rate T/sub 1//sup -1/ is linearly proportional to temperature, (b) that T/sub 1//sup -1/ is dependent on glass preparation to the extent that this affects the number and distribution of tunneling groups, (c) that T/sub 1//sup -1/ is sensitive to the isotopic composition of the glass, (d) that T/sub 1//sup -1/ for a given radical is larger by several orders of magnitude in a glassy environment than in a crystalline one, and (e) that T/sub 1//sup -1/ varies as ..omega../sup -2/ where ..omega.. is the EPR frequency. Predictions (a) to (d) have been tested and supported by T/sub 1/ measurements vs. temperature on trapped electrons inmore » C/sub 2/H/sub 5/OH, C/sub 2/D/sub 5/OH, and C/sub 2/H/sub 5/OD glasses. The measurements were made with a pulsed EPR spectrometer by the saturation recovery technique.« less