Experiment and theory for CO2 laser-induced CF2HCl decomposition rate dependence on pressure and intensity

Abstract

A laser‐excited fluorescence method has been used to determine the rates at which CF₂HCl molecules dissociate into CF₂+HCl fragments during CO₂ laser pulses of uniform fluence and known intensity. A study has been made of the dependence of the rate on CO₂ laser intensity and pressure of Ar buffer gas from the collision‐free regime to atmospheric pressure. The effect of increasing Ar pressure is initially to increase the CF₂HCl dissociation rate; above a moderate pressure (∼50 Torr), the rate is independent of Ar pressure up to atmospheric pressure. The data has been compared to a model, which adequately reproduces all the experimental data. The model treats the effect of collision between CF₂HCl and the argon buffer gas in terms of rotational equilibration or ’’hole filling’’ in the discrete energy level region of CF₂HCl. The discrete energy levels are interfaced to a quasicontinuum of vibrational–rotational states in a self‐consistent manner which incorporates a background of nonpumpable CF₂HCl states as a finite heat bath interacting with the pump mode. The model is used to calculate the rate of formation of product CF₂ molecules as a function of argon pressure and CO₂ laser intensity. The quasicontinuum for CF₂HCl is predicted to begin about four quanta above the ground state. The absorption cross section in the quasicontinuum is shown to decrease from 10⁻¹⁸ to 10⁻²⁰ cm² at V=15. The energy distribution in CF₂HCl is predicted to be decidedly nonthermal both below and beyond threshold.