Vibrational relaxation of D2O by collisions with Ar atoms

Abstract

Vibrational relaxation of D2O(v2, v13) molecules by collisions with Ar atoms was studied at 298 K (v2 denotes the bending vibrational mode and v13 denotes the collisionally coupled v1 and v3 stretching modes). The vibrationally excited D2O molecules were generated in two ways: exothermic abstraction reactions of OD radicals with different RD reactants and unimolecular decomposition of chemically activated compounds. The D2O(v2, v13) distributions were observed by infrared emission from a fast-flow reactor as a function of Ar pressure and reaction time. State-specific relaxation rate constants were obtained by comparison of the time evolution of the experimental vibrational distributions with numerical kinetic calculations of the vibrational populations. The relaxation mechanism was based on the relaxation model of H2O studied earlier, with the addition of a few channels specific for D2O. The average rate constants of the loss of population from (01), (02), (03), and (04) stretching states are (1.2 ± 0.2) × 10−14, (2.8 ± 0.6) × 10−14, (5.3 ± 0.8) × 10−14, and (11.3 ± 2.2) × 10−14 cm3 molecule−1 s−1, respectively. A rate constant of (3.9 ± 0.9) × 10−14 cm3 molecule−1 s−1 was assigned to the relaxation of the first level (v2 = 1, v13 = 0), which is in agreement with two other measurements. The rate constants of higher bending states increase linearly with the v2 quantum number and decrease slightly as v13 increases from v13 = 0 to v13 = 1–3. In general, the rate constants for the relaxation of D2O(v2,v13) are 3–5 times smaller than those for H2O(v2, v13) reported in an earlier study, despite the smaller energy defects for D2O. Our results for D2O and H2O relaxation by collisions with Ar are compared with existing experimental studies and theoretical models.