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

Vibrational relaxation of H2O(v2,v13) molecules by collisions with Ar was studied at 298 K (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) × 10−14 cm3 molecule−1 s−1, respectively. [ABSTRACT FROM AUTHOR]