Electron Spin-Lattice Relaxation of Toluene and p-Xylene Anions

Tata Institute of Fundamental Research, Howl Bhabha Road, Bombay-400 005

Electron spin-lattice relaxation times (T_1e) of toluene and p-xylene anions have been determined by the pulsed saturation recovery technique. Toluene anion exhibits short and nearly temperature independent T_1e (1.8±0.4 µs) in the temperature range studied (188 to 205 K). This is in agreement with the fact that toluene anion has a close lying excited orbital state at about 500 cm^-1 and the temperature behaviour follows closely that observed earlier for T_1e of benzene anion (orbitally degenerate system) in CW saturation studies. p-Xylene anion exhibits larger T_1e's (9.5 µs at 205 K) and is a linear function of (ŋ/ T) with zero intercept. p-Xylene anion has been estimated to have its first electronic excited state at 1 500 cm^-1. It is proposed that Orbach type mechanisms arising from a closely lying excited electronic level are domi­nant for spin-lattice relaxation in toluene anion. In p-xylene anion, however, spin rotation interaction is believed to play the dominant role with Orbach type processes being insignificant since 1 500 cm-1 >> > kT.