Knowledge Bank

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 $HD(v=4,5,and6)$ self-relaxation yields vibration-translation (V-T) and vibration-vibration (V-V) rate constants for collisions between $HD(v=1to6)$ and $HD(v=0)$. The V-T rates become larger than the V-V rates above $v=4$. Similarly, relaxation data for the $HD(v=4and5)-4$He 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 $HD-\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}He$ 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 $V=0$.