Solvation of excess electrons in supercritical ammonia

Abstract

Molecular dynamics simulations have been performed to study equilibrium and dynamical aspects of solvation of excess electrons in supercritical ammonia along the T=450 K isotherm. The interval of supercritical densities investigated spans from typically dense liquid down to dilute vapor ambients. Equilibrium aspects of solvation were analyzed using combined path integral-molecular dynamics techniques. The transition from localized to quasifree states, described in terms of the isomorphic electron-polymer spatial extent, was observed at approximately one fourth of the triple point density, a value somewhat higher than that recently reported for supercritical water [D. Laria and M. Skaf, J. Phys. Chem. A 106, 8066 (2002)]. The density of electronic eigenstates shows typically one s- and three p-like bound states that gradually lose their symmetry characteristics as the density lowers. The computed ground state absorption spectrum exhibits redshifts in the absorption bands as the density decreases; these shifts are much larger than those reported by pulse radiolysis experiments. By performing adiabatic dynamics, we also investigate mechanisms for solvent relaxation at high and intermediate supercritical densities following a vertical excitation of the electron.