The semiclassical regime of intramolecular vibrational dynamics

We report on a study of intramolecular dynamics after an ultrashort excitation. We show that a semiclassical regime of the vibrational dynamics exists on an intermediate time scale between the initial dephasing on the ultrashort time of the excitation pulse and the Heisenberg time scale associated with the mean level spacing. In this semiclassical regime, intramolecular dynamics presents time recurrences at the periods of the classical orbits. The amplitudes of these semiclassical time recurrences are evaluated for classically integrable and chaotic systems using Gutzwiller and the Berry-Tabor trace formulas. (C) 1999 American Institute of Physics. [S0021-9606(99)00812-0].