Semiclassical and Quantum Vibrational Intensities

The semiclassical theory of infrared vibrational intensities is extended and applied to multidimensional systems. The validity and utility of the semiclassical approach is assessed by comparison with perturbative and exact quantum results for one nonresonant and two resonant model Hamiltonian systems. Approximate mean trajectories are found to provide intensities accurate to within ~20% and frequencies accurate to within ~ 1% for almost all transitions studied. More accurate semiclassical results can be obtained by using exact mean trajectories and, for intensities in the nonresonant system, trajectories determined by an improved correspondence rule. The advantages and limitations of the semiclassical method are summarized.