VIBRATIONAL/TRANSLATIONAL RELAXATION RATE MEASUREMENTS BY THE PHOTOACOUSTIC PHASE SHIFT METHOD

The photoacoustic and infrared fluorescence signal following $CO_{2}$ laser excitation was measured for $CO_{2}, N_{2}O$, and $SF_{6}$. The amplitude and phase of the resulting signal was measured as a function of modulation frequency and the V=V and V=T relaxation rates were determined by least squares fitting. The branching ratio for collisional deexcitation from (0,0,1) to the (0,m,0) states is determined for $N_{2}O$ and $CO_{2}$. The effect of thermal diffusion on the photoacoustic signal is observed at low pressure and included in the data analysis. Radiation trapping of the infrared fluorescence signal is also observed