Collisional effects in the multiphoton dissociation of CF2CFCl

Abstract

We report results of a combined experimental and theoretical study of the effects of collisions with an inert buffer gas, on the CO2 laser induced MPD of CF2CFCl to form CF2 and CFCl. Rates of formation of the primary product CF2 have been determined, in real time using the laser excited fluorescence technique, at four IR laser intensities (Imax = 35, 47, 73, 220 MW/cm2) and a range of argon buffer gas pressures (0⩽PAr⩽500 Torr). The experimental data clearly show the effects of collisional hole filling at low pressures and V–T collisional deactivation at higher (≳100 Torr) pressures. We present a generally applicable theoretical model for collisional effects in MPD, in which two parameters (1/τ, ΔE) specify the collisional deactivation, two parameters (s, <ω≳) specify the density of states of the absorber, two parameters (A∞,Eact) specify the microscopic reaction rates and one parameter (δ) specifies the radiative pumping rates. In applying this model to CF2CFCl five of these seven parameters are readily estimated from independent kinetic or spectroscopic data. Treating only the remaining two parameters ΔE and δ as adjustable, an excellent theoretical fit of the high pressure experimental data is obtained. In this fit a value (ΔE=2.6 kcal/mole) is found for the mean energy transferred per collision to the buffer gas molecules. This value lies within the range expected for ΔE on the basis of chemical activation studies. The overall closeness of fit confirms the basic soundness of the ’’single quantum exchange’’ model for T2 relaxation, from which expressions are derived for the microscopic radiative pumping rates. Using only parameters determined in the MPD fit, we predict as a function of laser intensity and buffer gas pressure the energy absorbed from the light field. This prediction can be tested in an experiment that measures absorbed energy by the acousto-optic technique.