Formation of H₂⁺ by Ultra-Low-Energy Collisions of Protons with Water Ice Surfaces

The molecular ion of dihydrogen (H 2 + ) is produced by 1 eV collisions of protons (H + ) on amorphous water ice surfaces. The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D + on amorphous H 2 O and D 2 O ices yield D 2 + on the former, subsequent to isotope exchange on the H 2 O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency as the alkyl chain length increases. There is no corresponding reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a result of stabilization of the other reaction product, a hydroxyl radical (OH • ), on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids.