A classical phase r-centroid approach to molecular wave packet dynamics – illustrating the danger of using an incomplete set of initial states for thermal averaging

An inexpensive semiclassical method to simulate time-resolved pump–probe spectroscopy on molecular wave packets is applied to NaK molecules at high temperature. The method builds on the introduction of classical phase factors related to the r-centroids for vibronic transitions and assumes instantaneous laser–molecule interaction. All observed quantum mechanical features are reproduced – for short times where experimental data are available even quantitatively. Furthermore, it is shown that fully quantum dynamical molecular wave packet calculations on molecules at elevated temperatures, which do not include all rovibrational states, must be regarded with caution, as they easily might yield even qualitatively incorrect results.