Ionization and hydrolysis of dinitrogen pentoxide in low-temperature solids

Solid layers of interest for the chemistry of polar stratospheric clouds were investigated. Mixtures of covalent N₂O₅ and water in various molar ratios are deposited from vapor phase on a cold 12 K substrate. By repeatedly recording Fourier transform infrared spectra of the samples during gradual warm up to 200 K over a period of several hours the hydrolysis process can be followed. At each concentration the process is found to proceed in several distinct steps with sharp temperature thresholds. In samples containing only small amounts of water the covalent N₂O₅ is first around 110 K converted to an ionic nitronium nitrate, and only in a subsequent step NO₂⁺ NO₃^- reacts to nitric acid.