Ionization of Nitric Acid on Crystalline Ice: The Role of Defects and Collective Proton Movement

Ionization of nitric acid (HNO₃) on a model ice surface is studied using ab initio molecular dynamics at temperatures of 200 and 40 K with a surface slab model that consists of the ideal ice basal plane with locally optimized and annealed defects. Pico- and subpicosecond ionization of nitric acid can be achieved in the defect sites. Key features of the rapid ionization are (a) the efficient solvation of the polyatomic nitrate anion, by stealing hydrogen bonds from the weakened hydrogen bonds at defect sites, (b) formation of contact ion pairs to stable “presolvated” molecular species that are present at the defects, (c) rapid formation of the “solvent-separated” ion pair, which is facilitated by collective proton migration that is typical to ice, and (d) the facile formation of Eigen ions on the ice basal plane.