Dissociative excitation of water by metastable rare gas atoms: Rg(3P/sub 0,2/)+H2O→Rg+OH(A 2Σ+)+H (Rg = Ar,Kr)

The title reactions were studied over the relative collision energy range 0.3--1.8 eV in crossed molecular beams. Vibrational and rotational state distributions of the nascent OH(A 2Σ+) product were determined by analysis of fluorescence from the OH(A 2Σ+--X 2Pi) bands. The rotational distributions could be represented by simple Boltzmann distributions. With Ar*(3P/sub 0,2/) excitation, both vibrational and rotational distributions were found to have no significant dependence on the collision energy and compare well with results previously obtained at near-thermal energies. With Kr(3P/sub 0,2/) excitation, however, the state distributions were found to be strongly collision-energy dependent, the rotational temperature T/sub r/ (v = 0) increasing from 850 to 1750 K and the vibrational population ratio N/sub v//sub //sub = /1/N/sub v//sub //sub = /0 from ≤0.09 to 0.14 as the collision energy was increased from 0.35 to 0.65 eV. Time-of-flight (TOF) energy selection was used to measure the integral cross sections for the formation of the OH(A). The collision energy dependence of the cross section for the reaction Ar(3P/sub 0,2/) +H2O→Ar+OH(A 2Σ+)+H was found to be negative, whereas that for the reaction Kr(3P/sub 0,2/)+H2O→Kr +OH(A 2Σ+)+H exhibited a positive dependence. To understand the above experimental findings, possible mechanisms for the formation of OH(A 2Σ+) from Rg(3P/sub 0,2/)+H2O are considered that are consistent with the hypothesis that the reaction is governed by spin conservation