Reaction of Metastable Ar*(3P2) and Kr*(3P2) Atoms with Water Vapor: Excitation Functions for Electronic Quenching Collisions

Abstract

In this paper, we examine the functional nature of the state-specific cross section for electronic quenching σQ(E) in collisions of metastable noble gas atoms [Ar*(3P2) and Kr*(3P2)] with ground-state water molecules confined to a scattering-gas cell. The relative kinetic energy range studied is E = 0.0463−0.772 eV for Ar*(3P2) and 0.0432−0.692 eV for Kr*(3P2). These beam-gas-luminescence experiments incorporate a novel state-specific monitor system employing Xe gas. Photon emission from a state-specific Xe state produced in quenching collisions with the respective metastable noble gas (Ng*) atoms is a direct measure of the total disappearance of the metastable state under scrutiny in the reaction with water vapor. We use the classical orbiting and absorbing sphere models with empirically derived parameters [London dispersion Cdisp, induction Cind (spherically averaged or aligned dipole), and absorbing sphere radius ras] to predict energy-dependent cross sections for the total quenching reactions. The underlying premise of the models is that although the quenching process itself is envisioned to be a short-range two-electron exchange interaction, it is the long-range attractive forces between the respective collision partners that are critical in determining whether such short-range processes ultimately occur. Values of σQ determined from the models are in very good agreement with our experiment. This work supersedes our previous study for Ar*(3P2,0) [Novicki, S.; Krenos, J. J. Chem. Phys. 1988, 89, 7031].