Nature of Excess Electrons in Polar Fluids: Anion-Solvated Electron Equilibrium and Polarized Hole-Burning in Liquid Acetonitrile

Unlike most polar liquids, excess electrons in liquid CH3CN take on two distinct forms - solvated electrons (e(solv)(-)) and solvated molecular anions - that are in equilibrium with each other. We find that excitation of e(solv)(-), leads to a short-lived excited state that has no effect on the equilibrium but that excitation of the molecular anion instantaneously leads to the production of new e(solv)(-). We also find that e(solv)(-) produced by excitation of dimer anions relocalize to places far enough from their original location to alter their recombination dynamics. Finally, we show using polarized transient hole burning that e(solv)(-) in liquid CH3CN have an inhomogeneously broadened spectrum, demonstrating that these electrons almost certainly reside in a cavity. Because there is no polarized hole-burning for e(solv)(-) in water or methanol, these results have important implications for the nature of excess electrons in all polar liquids.