Energy transfer in highly vibrationally excited acetylene: Relaxation for vibrational energies from 6500 to 13 000 cm−1

Abstract

Vibrational overtone excitation of acetylene molecules to energies between 6500 and 13 000 cm−1 followed by interrogation of the excited states during collisional relaxation determines both the mechanism and rates of energy transfer. A pulsed visible or near-infrared laser excites a single rotational state of C2H2 in the region of the first (2νCH), second (3νCH), or third (4νCH) overtone of the C–H stretching vibration, and an ultraviolet laser probes the excited molecules by laser-induced fluorescence after a variable delay. The self-relaxation rate constant of about 9×10−10 cm3 molecules−1 s−1 is almost twice the Lennard-Jones collision rate constant and is nearly invariant with vibrational level. The energy-transfer rate constants from these population transfer measurements agree with those extracted from pressure-broadening data in both their size and insensitivity to vibrational state. Relaxation by the rare-gas atoms He, Ar, and Xe is nearly half as efficient as self-relaxation, suggesting that the internal structure of the collision partner is not particularly important in determining the relaxation rate. The invariance with vibrational level and the efficiency of rare-gas quenching indicate that rotational energy transfer is the most important relaxation pathway.