Vibrational relaxation of acetylene and acetylene–rare-gas mixtures

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Abstract
Vibrational relaxation of acetylene (C2H2) and acetylene–rare‐gas mixtures has been investigated using laser‐induced fluorescence. Time‐dependent fluorescence signals from the vibrational modes ν2 and ν5 have been recorded, following excitation of ν3 and (ν2+ν4+ν5) at 3300 cm−1 with pulses of a tunable optical parametric oscillator. The activation rate of ν2 and ν5 were estimated to be ≳880 ms−1 Torr−1 and ≳330 ms−1 Torr−1, respectively. The following V‐T/R relaxation rates have been obtained from the decay of the ν5 fluorescence signal: (12.7±1.1) ms−1 Torr−1 for C2H2 as collision partner, (0.46±0.05) ms−1 Torr−1 for Ne, (0.66±0.06) ms−1 Torr−1 for Ar, (0.67±0.04) ms−1 Torr−1 for Kr, and (0.57±0.04) ms−1 Torr−1 for Xe. Surprisingly small V–V deactivation rates of the CC stretching vibration ν2 have been measured. These are (21.3±1.0) ms−1 Torr−1 for C2H2 collisions, (0.63±0.06) ms−1 Torr−1 for Ne, (0.78±0.08) ms−1 Torr−1 for Ar, (0.93±0.09) ms−1 Torr−1 for Kr, and (0.77±0.09) ms−1 Torr−1 for Xe. Acetylene relaxation mechanisms compatible with these rates are considered.

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