Molecular Beam Studies of Carbon Monoxide Interactions with Water Ice

Abstract

The dynamics of CO interactions with water ice surfaces are studied using molecular beam techniques. The experimental method allows for detailed investigations of interaction between molecules and ice surfaces under single collision conditions, and collision dynamics and energy transfer are characterized. An ice surface is prepared by deposition of water vapor on a cold substrate and the surface is maintained in a vacuum chamber with a partial water vapor pressure up to 10-4 mbar, which allows for experiments with ice surfaces in the temperature range 100–190 K. A molecular beam is directed towards the ice surface, and the molecular flux from the surface is detected by mass spectrometry. Angular-resolved intensity and time-of-flight distributions are measured, and the effects of surface temperature, incident translational energy and incident angle are investigated. The surface collisions are highly inelastic with large energy loss observed for the directly scattered flux, similar to the results for the previously studied Ar-ice and HCl-ice systems. The data for the energy loss as a function of scattering angle show that energy transfer is substantial both parallel and perpendicular to the surface plane. The trapping of CO on the surface is found to be very effective under typical thermal conditions. The molecules accommodate to the temperature of the ice, but rapidly leave the surface by desorption because of the low binding energy to the surface.