Phase and Energy Relaxation of Vibrational Motion and Its Manifestation in Femtosecond Pump−Probe Experiments on I2 in Rare Gas Environments

Abstract

We present a classical description of femtosecond time-resolved experiments on collision-induced vibrational relaxation processes. For I2 embedded in Ar and He gases, a hot distribution of molecular vibrational states is prepared by a pump laser pulse. Collisions of I2 with gas atoms lead to dephasing and energy relaxation in the vibrational degree-of-freedom. In employing energy-phase coordinates, we are able to separate these two effects. Decay rates as obtained from the Fourier analysis of pump−probe signals are compared to those derived from the coordinate autocorrelation-function. It is shown that the vibrational energy relaxation influences the decay of the oscillatory structure in transient signals, leading to a nontrivial density dependence of the dephasing rate as observed in the experiments of Liu et al. [J. Phys. Chem. 1996, 100, 18650].