Electron spin resonance studies of localized excess electron states in frozen solutions of alkali metals in hexamethylphosphoramide

Electron spin resonance spectra of frozen solutions of sodium, potassium, rubidium, and cesium metals in HMPA are reported. Spectra at low microwave power levels (≲1 mW) showed the existence of several, discrete, localized excess electron states with unpaired electron spin density lying between ∼36 and ∼80% of the free atom value. In potassium, rubidium, and cesium solutions, a localized state with spin density ≲1% of the free atom value was also identified. Magnetic parameters (hyperfine coupling constant, electronic g factor) for both the high and low atomic character states in frozen metal-HMPA solutions showed similarities with results for fluid metal-amine solutions. The relevance of our observations to current models for metal-amine solutions is discussed. The experimental distribution function for localized excess electron states provides strong support for a multistate model for fluid metal-amine solutions. The high atomic character states are described in terms of intermediate impurity states in the host (HMPA) matrix, while the low atomic character state approximates closely to a true Wannier-Mott impurity ground state