Photochemistry in the charge transfer and neutral excited states of HCl in Xe and Kr matrices.

Abstract

HCl-doped Xe and Kr films are irradiated with wavelength dispersed synchrotron radiation in the wavelength range from 200 to 130 nm. The growth of H, Cl, Xe2H+, XeH2, HXeCl, Kr2H+, and HKrCl as well as the decomposition of HCl are recorded by a combination of UV, VIS, and IR spectroscopy. A turnover in the formation of Xe2H+ and Kr2H+ by a predominant two-step reaction on neutral surfaces at low energies to a one-step formation on ionic surfaces is determined at 172 and 155 nm in Xe and Kr, respectively. A potential energy diagram for neutral and ionic states is derived that is consistent with a DIIS calculation, with new UV fluorescence bands from Xe+HCl- centers, with the turnover energies and with a deconvolution of the absorption spectra in neutral and ionic contributions. The cage exit of charged as well as of neutral H, the latter via a harpoon reaction, is discussed for the ionic surfaces. The self-limitation of HCl decomposition on the neutral surfaces due to absorption by H and Cl fragments is treated quantatively. Dissociation efficiencies phi(e), together with absolute absorption cross sections sigma(H) and sigma(Cl) of the fragments, are derived. sigma(H) and sigma(Cl) are of the order of 10(-16) cm(2) compared to 10(-18) cm(2) for sigma(HCl). Dissociation is accompanied by many excitation cycles of the fragments, which leads to light-induced migration of H and recombination. phi(e) therefore represents a product of the cage exit probability phi that was treated theoretically and the survival probability concerning geminate and nongeminate recombination.