The Branching Ratio between Reaction and Relaxation in the Removal of H₂O from its /04>⁻ Vibrational State in Collisions with H Atoms

The removal of H2O molecules from their \04](-) vibrational state in collisions with H atoms can occur both by reaction, producing OH(v=0)+H-2, and by nonreactive relaxation. We report an experimental measurement of the fraction (f(reac)) that occurs by reaction. The value of f(reac) is determined by comparing the yields of OH from three experiments in which the same concentration of H2O(\04](-)) is prepared by overtone absorption of pulsed laser radiation and OH(v=0) is produced: (i) solely by the H+H2O(\04](-)) reaction; (ii) solely by the photodissociation of H2O(\04](-)) at 266 nm; and (iii) both by the photodissociation of H2O(\04](-)) and by the subsequent reaction of a fraction of the remaining H2O(\04](-)) with H atoms. Analysis of these experiments shows that f(reac)=(0.34+/-0.11). The experimental results are compared with the results of two kinds of scattering calculations performed on a potential energy surface developed recently, specifically with this problem in mind. Using the vibrational coupled-channel infinite-order-sudden (VCC-IOS) method, rate coefficients have been calculated for individual vibrationally inelastic processes and then summed to find the rate coefficient (k(relax)(H)) for total nonreactive relaxation from the \04](-) state. The quasiclassical trajectory (QCT) method has been used to calculate the rate coefficient (k(reac)) for reaction between H atoms and H2O(\04](-)). Both the calculated rate coefficient (i.e., k(relax)(H)+k(reac)) for total loss from H2O(\04](-)) and the calculated branching ratio, f(reac)=k(reac)/(k(relax)(H)+k(reac))=0.38, are in quite good agreement with the experimental values. (C) 2001 American Institute of Physics.