Modelling of relaxation measurements on highly vibrationally excited HD using direct overtone pumping and photoacoustic detection

The relaxation of highly vibrationally excited HD is modelled. In particular, we discuss measurement of vibration-to-vibration and vibration-to-translation rates from the upper vibrational levels of HD (2 ⩽ υ ⩽ 6) by detection of the change in translational energy of the gas following direct overtone excitation with a pulsed laser. Relaxation rates for HD have been estimated using a recently developed energy corrected sudden approximation scaling theory. The modelled curves of translational energy as a function of time are uniquely characteristic of the mechanism of relaxation and reflect its variation with increasing vibrational level. We have examined the use of photoacoustic detection to make these rate measurements and find that this is feasible over a reasonable range of temperature and pressure. This conclusion is based on consideration of the known absorption coefficients, the scaled rates, and background noise measurements made in our laboratory.