Vibrational relaxation of acetylene and acetylene–rare-gas mixtures

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.