Adsorption and photochemistry of dinitrogen tetroxide on low temperature ice layers

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.