Protons colliding with ice: Bouncing, sticking, splashing

Results of classical trajectory calculations on the sticking of H+ to the basal plane (0 0 0 1) face of crystalline ice, at a surface temperature of 80 K are presented. The calculations were performed for incidence energies ranging from 0.1 to 4.0 eV, for normal and off-normal incidence. Significant reflection is predicted at low incidence energies and large incidence angles. This reflection is due to repulsive electrostatic interactions between the incident proton and the positively charged upward pointing H-atoms of the surface water molecules. The sticking probability converges to 1 for incidence energies ⩾0.8 eV, for all values of incidence angles considered here. The strong interaction with the neighbouring water molecules leads to a local rupture of the hydrogen bonding network, resulting in collision induced desorption of water (puffing), a process that occurs with significant probability even at the lowest incidence energy considered. The water desorption probability is found to be independent on the angle of incidence, but increases with the incidence energy. In nearly all trajectories in which water desorption occurs, a single water molecule is desorbed from the topmost monolayer. In nearly all sticking trajectories, the proton is implanted into the ice surface, with a penetration depth that increases with the incidence energy, and decreases with the angle of incidence for high incidence energies.