Decoherence of molecular vibrational wave packets: Oscillatory decay of purity

We study vibrational wave packet dynamics on realistic surfaces of diatomic molecules and focus on the loss of coherence induced by rovibrational coupling at finite temperature T. The expected revivals of the wave packet depend sensitively on the coherence between the vibrational states, and their suppression indicates decoherence. In order to investigate this loss of vibrational coherence more quantitatively, we focus on purity decay and observe, apart from an overall decay, a remarkable oscillatory behavior of purity on the vibrational time scale. We present fully quantum-mechanical calculations of purity decay and the signal measured in a typical femtosecond pump-probe experiment.