The Effect of Initial Rotational Energy on the Adsorption of CO to the (0001) Face of Crystalline Ice Ih at Hyperthermal Energies

Abstract

The influence of initial rotational energy on the adsorption of CO to the (0001) crystalline ice Ih surface has been predicted using molecular dynamics (MD) simulations. The simulations were carried out at a surface temperature (Ts) of 150 K for J = 0−50 and incidence energies (Ei) of 24.1, 48.2, and 96.5 kJ mol-1, at normal incidence. Predictions were made for both the trapping probabilities (Pads) and the adsorption lifetimes, τ. At low incidence energies (viz. 24.1 and 48.2 kJ mol-1), there is an approximately exponential decrease in the trapping probability with increased initial rotation; a similar trend is obtained for the adsorption lifetime. At these incidence energies, rotationally mediated adsorption, trapping promoted by energy transfer from motion toward the surface to rotation, acts in tandem with energy transferred to surface vibrations to enhance the adsorption of CO. At higher incidence energies (96.5 kJ mol-1), the trapping probability decreases much less with increasing initial rotational-energy of the CO molecule than at lower Ei. A surprising finding of this work is that the trapping of CO to the ice surface is much more hindered by putting rotational energy into the molecule than it is by putting an equivalent amount of energy into motion toward the surface, even though it is the latter mode from which energy has to flow for trapping to occur. To our knowledge, this is the first system for which this phenomenon has been reported.