Abstract

A theory for ultrafast pump-probe spectroscopy of large polyatomic molecules and molecular aggregates in condensed phases is developed. A multimode Brownian oscillator model is used to account for high frequency molecular vibrations, local intermolecular modes as well as collective solvent motions. A semiclassical picture is provided using the density matrix in Liouville space. The pump field creates a doorway state which propagates for a specified time interval, and the spectrum is calculated by finding its overlap with a window state, prepared by the probe pulse. The doorway and the window states are wavepackets in phase space. For high frequency modes and long pulses they are expanded in the vibronic eigenstates, whereas for low frequency modes and with impulsive pulses the Wigner (phase space) representation is more adequate. Conditions for the observation of quantum beats, spectral diffusion and solvation dynamics (dynamical Stokes shift) are specified.

© 1990 Optical Society of America