Adsorption and photochemistry of dinitrogen tetroxide on low temperature ice layers

Abstract

Monolayer coverages of N₂O₄ have been prepared via the adsorption of gaseous NO₂ at ca. 100 K on 5–10 monolayers (ML) of condensed H₂O in a high vacuum system. Exposure to continuous near-UV irradiation resulted in the photodesorption of NO₂ from the surface. The interaction with the ice was purely physical in character and there was no reaction between NO₂ and the water surface. Grazing incidence FTIR spectroscopy identified adsorbed N₂O₄ as the sole parent species which, in addition, had a preferred orientation on the surface: the N—N bond was oriented perpendicular to the surface plane. The temporal profiles of both the NO₂ photodesorption rate and of the surface-bound N₂O₄ concentration exhibited simple monotonic decays and were explained within a simple kinetic scheme for the photochemistry of adsorbed N₂O₄. The wavelength dependence of the NO₂ yield was comparable to the gas-phase absorption cross-section for N₂O₄. The experiments show that the photochemistry of N₂O₄ physisorbed on low-temperature ice is not significantly different to that of its gas-phase counterpart.