A COMPARISON OF EXACT QUANTUM-MECHANICAL AND VARIOUS SEMICLASSICAL TREATMENTS FOR THE VIBRONIC ABSORPTION-SPECTRUM - THE CASE OF FAST VIBRATIONAL-RELAXATION

We have extended our study of the vibronic absorption spectrum in cond ensed matter [S. A. Egorov, E. Rabani, and B. J. Berne, J. Chem. Phys. 108, 1407 (1998)] to the case when the electronic dephasing rate is s low compared to the vibrational relaxation rate in both electronic sta tes. We find that under such circumstances, unlike the case of fast el ectronic dephasing, treating all nuclear degrees of freedom classicall y provides better agreement with the exact quantum treatment than the mixed quantum-classical approximation. These results are consistent wi th the conclusions reached by Bader and Berne in their study of mixed quantum-classical treatments of vibrational relaxation processes. (C) 1998 American Institute of Physics. [S0021-9606(98)51439-0].