Investigation of the cw Saturation Technique for Measurement of Electron Spin-Lattice Relaxation: Application to the Benzene Anion Radical

A detailed consideration of the cw saturation technique for measuring spin‐lattice relaxation times (T₁) in liquid solutions is given. An improved method for more accurate measurement of cavity Q is described. The saturation technique was used to measure the T₁ of the benzene anion radical over the temperature range 150–250°K. The measurements were made at five different concentrations (0.01), 0.03, 0.1, 0.3, and 1.0M). For benzene concentrations $\text{≦0.1 M}$ , the linewidths and hence T₂ are independent of concentration. However, the linewidth displays a minimum and T₂ a maximum at 173°K for all benzene concentrations. Our measurements of T₁ show that although there is no discernable dependence upon benzene concentration, a maximum in T₁ is observed at approximately 188°K. Below 188°K, T₁ decreases to the solvent freezing point. In addition, analysis of the line shape of the benzene anion radical spectrum yields strong evidence that the benzene anion radical prepared from $\mathrm{Na}–\mathrm{K}$ alloy in 2:1 tetrahydrofuran‐1,2‐dimethoxyethane is ion paired.