Rare gas collisions with molten metal surfaces

Atom-surface scattering in the classical multiphonon regime

Many experiments that utilize beams of incident atoms colliding with surfaces as a probe of surface properties are carried out at large energies, high temperatures and with large mass atoms. Under these conditions the scattering process does not exhibit quantum mechanical properties such as diffraction or single-phonon excitation, but rather can be treated with classical physics. This is a review of work carried out by the authors over a span of several years to develop theoretical frameworks using classical physics for describing the scattering interactions of atom with surfaces.

Scattering of CO and N2 molecules by a graphite surface

Measurements of angular distributions for the scattering of well-defined incident beams of CO and N(2) molecules from a graphite surface are presented. The measurements were carried out over a range of graphite surface temperatures from 150 to 400 K and a range of incident translational energies from 275 to over 600 meV. The behavior of the widths, positions and relative intensities of the angular distributions for both CO and N(2) were found to be quite similar. The experimental measurements are discussed in comparison with calculations using a classical mechanical model that describes single collisions with a surface. Based on the behavior of the angular distributions as functions of temperature and incident translational energy, and the agreement between measured data and calculations of the single-collision model, it is concluded that the scattering process is predominantly a single collision with a collective surface for which the effective mass is significantly larger than that of a single carbon atom. This conclusion is consistent with that of earlier experiments for molecular beams of O(2) molecules and Xe atoms scattering from graphite. Further calculations are carried out with the theoretical molecular scattering model in order to predict translational and rotational energy transfers to and from the molecule during scattering events under similar initial conditions.

Determination of the surface corrugation amplitude from classical atom scattering

The energy landscape of an atomic or molecular projectile interacting with a surface is often described in terms of a corrugation function that gives the classical turning point as a function of position vector parallel to the surface. It is shown here that the relative height variation of the corrugation function for scattering of atoms under classical conditions can be determined by a measurement of the maximum intensity in energy-resolved scattering spectra as a function of surface temperature. This is demonstrated by developing a semiclassical quantum theory of atomic scattering from corrugated surfaces and then extending the theory to the classical limit of large incident energies and high surface temperatures. Comparisons of calculations with available data for Ar atom scattering determine the corrugation amplitude for a molten In surface to be about 29% of the mean interparticle spacing in the bulk liquid.

Scattering of Xe from graphite

Recently a series of experimental measurements for the scattering of Xe atoms from graphite has been reported for both energy-resolved spectra and angular distributions. This system is of fundamental interest because the projectile Xe atoms are considerably more massive than the carbon atoms making up the graphite surface. These measurements were initially analyzed using the hard cubes model and molecular dynamics simulations, and both treatments indicated that the scattering process was a single collision in which the incoming Xe atom interacted strongly with a large number of carbon atoms in the outermost graphite layer. In this work we analyze the data using a single scattering theory that has been shown to explain a number of other experiments on molecular beam scattering from surfaces. These calculations confirm that the scattering process is a single collision with an effective surface mass that is substantially larger than that of the basic graphite ring.

A stochastic, local mode study of neon-liquid surface collision dynamics

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.

He/Ar-atom scattering from molecular monolayers: C60/Pt(111) and graphene/Pt(111)

Supersonic He and Ar atomic beam scattering from C(60) and graphene monolayers adsorbed on a Pt(111) surface are demonstrated in order to obtain detailed insight into a gas-molecule collision that has not been studied in detail so far. The effective masses and phonon spectral densities of the monolayers seen by different projectiles are discussed based on classical models such as the hard cube model and the recently developed smooth surface model. Large effective masses are deduced for both the monolayers, suggesting collective effects of surface atoms in the single collision event. The effective Debye temperature of graphene was found to be similar to that reported in highly oriented pyrolytic graphite (HOPG), indicating that the graphene is decoupled well from the Pt substrate. A much smaller Debye-Waller factor was found for the C(60) layer, probably reflecting the strong C(60)-Pt(111) interaction.

Calculations of trapping and desorption in heavy atom collisions with surfaces

Calculations are carried out for the scattering of heavy rare gas atoms with surfaces using a recently developed classical theory that can track particles trapped in the physisorption potential well and follow them until ultimate desorption. Comparisons are made with recent experimental data for xenon scattering from molten gallium and indium, systems for which the rare gas is heavier than the surface atoms. The good agreement with the data obtained for both time-of-flight energy-resolved spectra and for total scattered angular distributions yields an estimate of the physisorption well depths for the two systems.

Direct scattering, trapping, and desorption in atom-surface collisions

Maxwell is credited as the first to invoke the assumption that an impinging gas beam scatters from a surface with a direct contribution exhibiting little change in state and a trapping-desorption fraction that desorbs in equilibrium [J. C. Maxwell, Phil. Trans. R. Soc. London 170, 231 (1879)]. Here a classical mechanical scattering theory is developed to describe direct scattering, trapping, and subsequent desorption of the incident beam. This theory allows a rigorous test of the Maxwell assumption and determines the conditions under which it is valid. The theory also gives quantitative explanations of important new experimental measurements [K. D. Gibson, N. Isa, and S. J. Sibener, J. Chem. Phys. 119, 13 083 (2003)] for direct and trapping-desorption scattering of Ar atoms by a self-assembled layer of 1-decanethiol on Au(111).