PUMP-PROBE SPECTROSCOPY OF THE HYDRATED ELECTRON - A QUANTUM MOLECULAR-DYNAMICS SIMULATION

Quantum nonadiabatic molecular dynamics simulations are used to directly compute the transient absorption spectroscopy following photoexcitation of equilibrium hydrated electrons. The calculated spectral transients are found to be in excellent agreement with ultrafast traces measured in recent transient spectral hole-burning experiments [Barbara and co-workers, J. Chem. Phys. 98, 5996 (1993); J. Phys. Chem. 98, 3450 (1994)], indicating that the computer model correctly captures the underlying physics. The model transients are dissected into ground state bleach, excited state absorption, and stimulated emission spectral components, each of which is examined individually and analyzed in terms of the microscopic solvent response following photoexcitation. Although there is no distinct spectral hole, bleaching dynamics are found to play an important role in the overall transient spectroscopy. The excited state absorption spectrum undergoes a complex evolution due to solvation dynamics which alters both the frequencies and the oscillator strengths of the relevant quantum transitions. Calculated excited state emission from the electron is characterized by an enormous dynamic Stokes shift as well as an overall spectral narrowing in time. In combination, these three components allow the assignment of features of the measured ultrafast spectroscopic transients in terms of specific details of the microscopic solvent response.