Low-temperature reflection/absorption IR study of thin films of nitric acid hydrates and ammonium nitrate adsorbed on gold foil

Abstract

Grazing angle reflection/absorption FTIR spectroscopy has been employed to characterise thin layers of nitric acid hydrates and ammonium nitrate in a high-vacuum system. Ordered films of frozen nitric acid di- and tri-hydrate were produced by slow effusive vapour deposition onto a polycrystalline gold foil at 80 K followed by annealing to 190 K. On the basis of the metal surface selection rule, analysis of the observed IR band intensities suggests that the nitrate and oxonium ions align preferentially with their C₃ axes parallel to each other and perpendicular to the underlying substrate. A similar orientation was observed for the nitrate ions of thin ammonium nitrate films.