Laser Induced Infrared Fluorescence: Thermal Heating, Mass Diffusion, and Collisional Relaxation in SF6

Experimental results of the study of 10-μ and 16-μ laser induced fluorescence in SF6 and SF6-rare gas mixtures are presented. The fluorescence at both wavelengths is found to exhibit a fast and a slow decay process. The major features of the experimental fluorescence decay curves are well described by a simple model with only two energy levels. The rate equations for this model system include terms which couple collisional relaxation with heat and mass transport. The fast fluorescence decay process at each wavelength is found to be characteristic of binary collisional deactivation. The rate of decay of the slow process, however, is controlled by transport of both heat and mass to the wall of the fluorescence cell with heat transport being the dominant effect. The experimental results and the analysis based on a two level model system are consistent with a two step vibrational deactivation mechanism. The first step is a rapid equilibration of the vibrational modes. The second step is a somewhat slower equilibration of vibrational and translational degrees of freedom.