Low-temperature photochemistry of submicrometer nitric acid and ammonium nitrate layers

Reflection-absorption infrared spectroscopy has been employed in order to investigate the low-temperature photochemistry (90-140 K) of thin films of nitric acid and ammonium nitrate grown in vacuo. Photolysis of amorphous nitric acid hydrate, the crystalline dihydrate (NAD) and trihydrate (NAT) at lambda > 230 nm resulted in the formation of molecular nitric acid due to rapid protonation of the excited nitrate ion. Secondary photolysis of HONO2 produced NO2 and NO. If a neat film of molecular, anhydrous nitric acid was irradiated, nitrate and nitronium ions were observed. In contrast, ammonium nitrate photolysis at 140 K did not result in a proton transfer to produce NH3 and HONO2 but in the formation of the peroxynitrite ion (ONOO-) as a precursor for NO2-. Molecular dinitrogen tetraoxide and nitrous oxide were also detected in the film. Mechanistic details and possible implications for the chemistry of the polar atmosphere are discussed.