Dissociative excitation of water by metastable rare gas atoms: Rg(3P0,2)+H2O→Rg+OH(A 2Σ+)+H (Rg=Ar,Kr)

Abstract

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 2Π) bands. The rotational distributions could be represented by simple Boltzmann distributions. With Ar*(3P0,2) 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(3P0,2) excitation, however, the state distributions were found to be strongly collision-energy dependent, the rotational temperature Tr (v=0) increasing from 850 to 1750 K and the vibrational population ratio Nv=1/Nv=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(3P0,2) +H2O→Ar+OH(A 2Σ+)+H was found to be negative, whereas that for the reaction Kr(3P0,2)+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(3P0,2)+H2O are considered that are consistent with the hypothesis that the reaction is governed by spin conservation. It is found that the present results clearly display the characteristic features of reaction dynamics that involve triplet excited potential surfaces. The formation of OH(A) is well interpreted as dissociation of an excited intermediate H2O*(d̃ 3A1) state produced competitively in the decay of triplet Rydberg states.