Zero-Field Exciton Magnetic Resonance

Abstract

The zero-field magnetic resonance spectrum of triplet excitons in the solid ionic free radical (φ3AsCH3)+(TCNQ)2— has been observed in the 75°—140°K temperature range using marginal oscillator spectrometers. Zero-field transition frequencies of 61.4, 163.8, and 225.4 Mc/sec were observed at a temperature of 111°K. The transition frequencies decrease by roughly 0.1%—1% per 10° in the 75°—130°K range. The transition frequencies are in good agreement with the frequencies that can be calculated from the earlier high-field spectra of this compound obtained by Chesnut and Phillips. At low temperatures where exciton—exciton spin exchange is weak, the dominant temperature dependence of the zero-field transition frequencies is shown to be due to molecular torsional oscillations that are quite similar to those that are known to produce temperature-dependent transition frequencies in nuclear quadrupole resonance. In a sample contaminated with spin ½ free radicals, small temperature-independent frequency shifts are found and are attributed to second-order exciton—free radical spin exchange. An analysis of exciton linewidths in terms of an exciton diffusional motion leads to a site-to-site jump rate of the order of τ∼1012 sec—1.