Subpicosecond pump-probe absorption of the hydrated electron: Nonlinear response theory and computer simulation

A theory is presented to study the subpicosecond transient absorption of the hydrated electron. Applicable in equilibrium conditions, this theory is a statistical theory using a correlation function approach to the nonlinear optical process involved. Four-time correlation functions of the electric moment and of the incident electric fields are employed extensively. An analytical expression for the transient signal is obtained; both coherent and incoherent electric fields are considered. The parameters of this theory are evaluated by means of a mixed quantum-classical molecular-dynamics simulation; water molecules are assumed to be rigid. The hydrated electron is described in terms of floating Gaussian orbitals; the solvent polarizability is accounted for in a self-consistent way. Theoretical time- and frequency-resolved spectra are compared with experiment. Nonmonotonic behavior of spectral transients is attributed to a competition between thermic and nonthermic subbands of the signal. Most of the spectral evolution is achieved in 20-30 fs by the fast inertial response of the solvent.