Vibrational relaxation of H2O by H2, HCl, and H2O at 295 K

Abstract

A laser induced fluorescence method has been used to measure rate constants for vibrational relaxation of the equilibrated ν1 and ν3 stretching level reservoir, the 2ν2 bending overtone level, and the ν2 bending level of H2O by H2 and HCl at 295 K. The rate constants for relaxation by H2 were found to be (8.0±1.1), (29±4), and (13±2)×10−13 cm3 molecule−1 s−1, respectively. For relaxation by HCl the rate constants were (8.4±1.2), (47±7), and (24±4)×10−12 cm3 molecule−1 s−1, respectively. Relaxation by intermolecular V→V transfer was experimentally determined to account for <8% of the rate constant for relaxation of the H2O stretching levels by HCl and was estimated to contribute <16% of the rate constant for relaxation of the bending overtone level. For both collision partners, the H2O stretching levels were relaxed predominantly to the 2ν2 level and the 2ν2 level was relaxed predominantly to ν2. Following direct laser excitation, the rate constant for relaxation of the equilibrated ν1+ν2 and ν3+ν2 combination vibrational levels of H2O by H2O was measured to be (1.6±0.2)×10−10 cm3 molecule−1 s−1. Direct relaxation to the ν1 and ν3 fundamental levels was experimentally determined to account for <40% of the relaxation rate constant.