Scattering of High-Incident-Energy Kr and Xe from Ice: Evidence that a Major Channel Involves Penetration into the Bulk

Abstract

The scattering of high kinetic energy (1–6.5 eV) xenon and krypton atoms from ice was experimentally measured and theoretically modeled. The ice efficiently accommodates translational energy, but the extent to which the energy is quenched suggests the mechanism for the highest energies and near-normal incidence angles involves more than interaction with just the molecules at the ice surface. Simulations show that for these conditions the xenon penetrates into the selvedge. This penetration into the solid manifests experimentally in that the postcollision translational energy is essentially independent of the incident translational energy. This observation is in stark contrast with what is usually the situation for scattering from a surface. Finally, postexposure desorption measurements showed that some of the incident gas atoms were trapped in the bulk of the ice at temperatures well above where the absorption is thermodynamically stable. The above evidence leads to the conclusion that under conditions of near-normal angles of incidence and high collision energy, much, if not most, of the rare gas penetrates below the ice surface and into the selvedge. A fraction is stably embedded in the near surface region of the ice, whereas the remaining rare gas escapes with two distinct velocity distributions; the first is a result of a thermal process and the second, faster distribution, is the result of ejection from the solid.

Molecular interactions with ice: Molecular embedding, adsorption, detection, and release

“…However, it also appears that the uptake rate is also less than for Kr, which has almost the same mass as CF 4 . The previous experiments 10 with Xe and Kr showed that the rate of embedding decreased with exposure. The rates of CF 4 embedding vs. exposure curves were linear for all of the energies and exposures used, but the amount of CF 4 embedded had not reached a value as large as for the longest exposures of the rare gases.…”

“…For both Xe and Kr, the rate of implantation was directly related to the translational energy, and for a given energy, Xe embedded at a higher rate than the lighter Kr. 10,24 The amount embedded appeared to asymptotically approach a final value that was dependent on energy and mass.…”

“…[4][5][6][7] Energetic collisions of small molecules with ice surfaces are of particular relevance for the capture and matrix preconcentration of trace gases; 8,9 for gases with insufficient momentum are unable to penetrate into the ice, whereas heavier, or more energetic, species will be trapped in the near surface region of the solid. 10 Energetic collisions on ice surfaces are also highly relevant for the evolution of the composition of icy bodies in space. [11][12][13] In particular, collisions with impact energy of several electron volts are representative of the encounters between the surfaces of comets and the ambient molecules and atoms in interplanetary space as a comet orbits a star.…”

“…10,24 On both amorphous solid water (ASW) and crystalline ice (CI), the energetic incident Xe atoms sputtered water molecules from the surface, and a well-defined, water isotope-dependent, threshold energy was observed for the sputtering that was on the order of the sublimation energy. The scattering experiments and chemical dynamics simulations 10,24 showed that the ice surfaces very efficiently accommodate the incident kinetic energy of the Xe or Kr atoms, and the energy of the scattered atoms is weakly correlated to their incident energy. These findings suggested that the scattering was not the result of simple binary collisions.…”

“…In the course of these experiments, we discovered that a small, but significant, amount of the inert gases could be absorbed at surface temperatures well above where they could stably adsorb. 3,10 Ice was grown on single crystal metal surfaces, either Rh(111) or Au(111), in ultra-high vacuum (UHV) and were then exposed to beams of energetic Xe or Kr atoms. After the exposure, temperature programmed desorption (TPD) measurements were taken of the ice, and the signal from D 2 O and either Xe or Kr was monitored as the ice temperature was ramped upwards.…”