Quantized semiclassical trajectory approach for evaluation of vibronic transitions in anharmonic molecules

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Abstract
Significant progress has been made in recent years in simulation of molecular spectra. It appears, however, that the available methods do not offer a reliable and practical way for simulation of the vibronic line shapes of large anharmonic molecules. As a step in the effort of attacking this problem we develop a semiclassical approach that generates quantized vibronic transitions between electronic surfaces of anharmonic molecules. The method evaluates vibronic transition probabilities by using the energy representation (Green’s functions) of the corresponding semiclassical propagators. The resulting expression generates a u t o m a t i c a l l y interference effects for nonquantized actions and gives significant intensity only for quantized vibronic transitions. The method is demonstrated in several test cases including the simulation of the line shape for the X→B transition in I2. The agreement between the quantum mechanical and classical results is excellent. The powerful physical insight provided by the method and its potential use in fundamental studies of line broadening in anharmonic molecules is outlined.