LOW-TEMPERATURE REFLECTION ABSORPTION IR STUDY OF THIN-FILMS OF NITRIC-ACID HYDRATES AND AMMONIUM-NITRATE ADSORBED ON GOLD FOIL/

Grazing angle reflection/absorption FTIR spectroscopy has been employe d to characterise thin layers of nitric acid hydrates and ammonium nit rate 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 observ ed IR band intensities suggests that the nitrate and oxonium ions alig n preferentially with their C-3 axes parallel to each other and perpen dicular to the underlying substrate. A similar orientation was observe d for the nitrate ions of thin ammonium nitrate films.