Intramolecular and solvent dynamics in femtosecond pump-probe spectroscopy

A density-matrix theory is applied to the calculation of femtosecond pump-probe experiments on solvated polyatomic molecules. Specific calculations are performed for a two-mode system, in which an optically active solute mode is represented by a pair of harmonic oscillators in the state representation, and the solute is modeled as an overdamped harmonic mode, using the Wigner phase space representation. No restrictions are placed on the relative time scales of the solute vibrational period, the solvent relaxation time scale and the pump pulse duration. The calculations demonstrate the physical phenomena observed in recent experiments on solvated dyes, both in the hole-burning limit, where the pump pulse is long compared to the relevant solute and solvent time scales, and the impulsive limit, for which the pump is short compared to the solute vibrational period.