Temperature dependence as a probe of intramolecular relaxation in the infrared multiphoton excitation of CF2Cl2

Abstract

Systematic variations have been seen as a function of temperature and irradiation frequency in the multiphoton dissociative product yields for CF₂Cl₂. Thermal shifts in the small signal absorption spectrum do not account for the observed effects, which are attributed instead to an interference between initial thermal vibrational excitation and laser pumping through the anharmonic bottle neck. A simple stochastic classical model for the excitation of an active oscillator coupled to an intramolecular heat bath is developed to explain our results. Temperature dependence in this model comes from the dependence of the dephasing rate in the driven system on the initial energy of the heat bath. Comparison of the model with experiment yields good agreement with characteristic times for our derived dephasing parameter of 0.1 nsec at 300 °K and 2 psec at 500 °K. It is predicted that under certain conditions of irradiation frequency and pressure, enhanced relaxation will lead to increased primary isotope separation efficiency at elevated temperatures.