Molecular dynamics simulation of the optical absorption spectrum of the hydrated electron

Abstract

The optical absorption spectrum of the hydrated electron was computed at 300 K using the Feynman path integral formulation of quantum statistical mechanics in conjunction with molecular dynamics simulations. In addition, the potential energy of the hydrated electron was studied at 700 K as a function of the liquid density between 0.02 and 1.0 g/cm3 and correlated with the maximum of the absorption spectrum. The procedure to calculate the optical spectrum makes use of the solution of the Schrödinger equation for an ensemble of model potentials which span the region allowed by the fluctuations of the potential well which traps the electron in thermodynamic equilibrium. The results indicate that the absorption band is due to a strongly allowed 1s→2p transition and the breadth of the spectrum is a consequence of the fluctuations in the trap dimensions. Moreover, these calculations imply that the long tail of the absorption spectrum in the UV arises from electrons which are in deep traps. Such traps occur with low but finite probability in the ensemble of solvent configurations which are accessible to the system in thermal equilibrium. Calculations of the average potential energy of the electron as a function of the density reproduced well the measured shift in the absorption spectrum of the excess electron in water.