Mechanisms and rate constants for the vibrational relaxation of HD(v=4,5, and 6) in collisions with HD, ⁴He, and D₂

The relaxation of highly vibrationally excited HD with several collision partners is examined using direct overtone excitation and time-resolved photoacoustic detection of the subsequent change in translational energy of the gas. Analysis of such data for HD(v=4, 5 and 6) self-relaxation yields vibration-translation (V-T) and vibration-vibration (V-V) rate constants for collisions between HD(v=1 to 6) and HD(v=0). The V-T rate constant becomes larger than the V-V rate constant above v=4. Similarly, relaxation data for the HD(v=4 and 5)-⁴He systems is analyzed and the HD-⁴He V-T rate constants at v=4 and v=5 are presented. The differences in the quantum number scaling of the V-T rates for the HD-HD and HD-⁴He systems are discussed. Finally, the time-resolved photoacoustic data for HD(v=4) relaxing in collisions with D₂ shows the effect of the rapid, near-resonant V-V energy transfer between HD in the v=1 through v=4 levels and D₂ in v=0.