Anisotropy of pump-probe absorption of the hydrated electron: A statistical model

A statistical model is presented to interpret the pump-probe absorption anisotropy of the hydrated electron. Its basic ingredient is the assumption of cascade-type deactivation processes between its excited p-states. The theory uses the correlation function formalism of nonlinear statistical mechanics. A number of technical problems are discussed: construction of a relaxation operator describing the cascade, solution of the corresponding Heisenberg equation of motion, calculation of various tensorial quantities, etc. According to this model, the presence of a cascade reduces the pump-probe dichroism, particularly if the p-state splitting is small. Very short cascade relaxation times of the order of 10 fs are required for that purpose. As the upper state of an s↔p transition is quasi-degenerate, the magic angle theories do not apply here. The similarity between the deactivation processes in excited electronic states of polyatomic molecules and of the hydrated electron is emphasized.