A low-temperature reflection-absorption infrared spectroscopic study of ultrathin films of dinitrogen tetroxide and dinitrogen pentoxide on gold foil

An ultrahigh-vacuum system optically coupled to a reflection-absorption infrared d probe at a grazing angle of incidence has been employed to characterize thin layers of N2O4 and N2O5. An ordered film of D-2h-N2O4 molecules was produced by slow effusive deposition of NO2 on a polycrystalline gold foil at 80 K. Analysis of the resulting infrared bands suggests that the molecules align with their central N-N bond axes parallel to each other and perpendicular to the underlying substrate, consistent with the known structure of monoclinic beta-N2O4. This orientation effect was not observed upon adsorption at 120 K. Condensation of N2O5 on the substrate at 80, 100, and 120 K resulted in a multilayer film of amorphous covalent, crystalline covalent, and ionic N2O5, respectively. As observed for N2O4 at 80 K, covalent N2O5 molecules in the crystalline phase order their molecular planes perpendicular to the surface. This previously unreported crystalline covalent phase is metastable with the respect to the ionic form, nitronium nitrate.