Relaxation of C2D2(ν4, ν5) by vibration–rotation, translation energy transfer

Abstract

A pulsed laser–photoacoustic technique has been used to measure effective collisional probabilities for relaxation of the two lowest coupled vibrational modes of C₂D₂ by several collision partners. At 300 K these probabilities are: C₂D₂, (8.2 ± 1)× 10^–4; He, (10.5 ± 1)× 10^–4; Ne, (5.2 ± 1)× 10^–4; Ar, (5.9 ± 1)× 10^–4; Kr, (8.1 ± 1)× 10^–4; Xe, (8.6 ± 1.5)× 10^–4; H₂, (20 ± 4)× 10^–4; D₂, (23 ± 3)× 10^–4; N₂, (7.8 ± 1.7)× 10^–4; CO, (6.9 ± 0.6)× 10^–4. The experimental results for C₂D₂+ noble-gas mixtures are compared with: (a) laser-induced vibrational fluorescence measurements on the relaxation of C₂H₂ and (b) estimates of probabilities based on a simple first-order perturbation treatment which includes the effect of molecular rotation. The latter comparison suggests that rotation facilitates relaxation of the bending vibrations in C₂D₂(and C₂H₂).