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

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Abstract
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.