The perturbation of coherent wave-packet dynamics by atom/molecule collisions: the NaI + Ar system

The results of recent femtosecond pump/probe experiments on the NaI molecule interacting with Ar atoms under different pressure conditions (Knopp et al., J. Phys. Chem., in press) are interpreted using classical and quantum mechanical calculations. It is shown how the periodic quasibound NaI wave-packet dynamics initiated by ultrashort pulse excitation is modified through the collision process and how the pump/probe fluorescence signal changes accordingly. Furthermore it is demonstrated how the energy transfer in the collision processes yields internally cooled NaI molecules which are more stable with respect to electronic predissociation.