PUMP-PROBE SPECTROSCOPY OF H-BONDS AS THE LEVEL-CROSSING PROBLEM

Saturation of the H-bond absorption spectra by a strong laser field is described as a level-crossing problem with diffusional motion along t he reaction coordinate. The stationary populations of the ground and e xcited vibronic states were found, assuming that the light induced tra nsitions between them occur only at the point of exact resonance and t he nonradiative decay of the upper state is uniform. The stationary di stributions in both states are also obtained and shown to be Gaussian at slow vibrational relaxation, but essentially distorted when relaxat ion is faster than diffusion. The absorption spectra of the weak probe light in the frame of two-level and three-level models are calculated , which show either loss or gain in different spectral regions. The ex istence of bleaching and superabsorption branches in corresponding tra nsmission spectra are confirmed. (C) 1998 American Institute of Physic s.