Vibrational relaxation of high levels of H2O by collisions with Ar as studied by infrared chemiluminescence.

Abstract

Vibrational relaxation of H2O(v2,v13) molecules by collisions with Ar was studied at 298K (v2 denotes the bending vibrational mode and v13 denotes the sum of the symmetric, v1, and asymmetric, v3, vibrational modes). The H2O molecules from 14 different exothermic reactions of H-atom abstraction by OH radicals were observed by infrared emission from a fast flow reactor as a function of Ar pressure and reaction time. Numerical kinetic calculations were used to obtain rate constants for stretch-to-bend energy conversion, (v2,v13) → (v2 + 2,v13 - 1), and pure bend relaxation, (v2,v13) → (v2 - 1,v13). Rate constants for states up to v13 = 4 were based on the average values from all reactions. The rate constant for the (2,0) → (1,0) bending relaxation is in agreement with the published values from laser-induced fluorescent experiments; the rate constants for higher levels increase with v2. Our average rate constant for the (0,1) → (2,0) stretch-to-bend conversion is somewhat smaller but falls within the uncertainty limit of the published value. The average rate constants for the stretch-to-bend process for (01), (02), (03), and (04) stretching states are (4.3 ± 0.8) × 10-14, (7.7 ± 1.1) × 10-14, (14.3 ± 4.2) × 10-14, and (20.6 ± 6.2) × 10-14cm3molecule-1s-1, respectively.