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Neutralization probabilities are presented for hyperthermal energy Na+ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated., Comment: Accepted for publication in Physical Review Letters. (10 pages, 5 figures)

Detailed KMC-MD (kinetic Monte Carlo-molecular dynamics) simulations of hyperthermal energy (10-100 eV) copper homoepitaxy have revealed a re-entrant layer-by-layer growth mode at low temperatures (50K) and reasonable fluxes (1 ML/s). This growth mode is the result of atoms with hyperthermal kinetic energies becoming inserted into islands when the impact site is near a step edge. The yield for atomic insertion as calculated with molecular dynamics near (111) step edges reaches a maxima near 18 eV. KMC-MD simulations of growing films and a minima in the RMS roughness as a function of energy near 25 eV. We find that the RMS roughness saturates just beyond 0.5 ML of coverage in films grown with energies greater than 25 eV due to the onset of adatom-vacancy formation near 20 eV. Adatom-vacancy pairs increase the island nuclei density and the step edge density which increases the number of sites available to insert atoms. Smoothest growth in this regime is achieved by maximizing island and step edge densities, which consequently reverses the traditional roles of temperature and flux: low temperatures and high fluxes produce the smoothest surfaces in these films. Dramatic increases in island densities are found to persist at room temperature,where island densities increase an order of magnitude from 20 to 150 eV., Comment: 9 pages, 2 tables, 7 figures

We present a new method for simulating crystal growth by energetic beam deposition. The method combines a Kinetic Monte-Carlo simulation for the thermal surface diffusion with a small scale molecular dynamics simulation of every single deposition event. We have implemented the method using the effective medium theory as a model potential for the atomic interactions, and present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to 35 eV. The method is capable of following the growth of several monolayers at realistic growth rates of 1 monolayer per second, correctly accounting for both energy-induced atomic mobility and thermal surface diffusion. We find that the energy influences island and step densities and can induce layer-by-layer growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag), which correlates with where the net impact-induced downward interlayer transport is at a maximum. A high step density is needed for energy induced layer-by-layer growth, hence the effect dies away at increased temperatures, where thermal surface diffusion reduces the step density. As part of the development of the method, we present molecular dynamics simulations of single atom-surface collisions on flat parts of the surface and near straight steps, we identify microscopic mechanisms by which the energy influences the growth, and we discuss the nature of the energy-induced atomic mobility.

A theory based on the thermodynamic Gibbs-Thomson relation is presented which provides the framework for understanding the time evolution of isolated nanoscale features (i.e., islands and pits) on surfaces. Two limiting cases are predicted, in which either diffusion or interface transfer is the limiting process. These cases correspond to similar regimes considered in previous works addressing the Ostwald ripening of ensembles of features. A third possible limiting case is noted for the special geometry of "stacked" islands. In these limiting cases, isolated features are predicted to decay in size with a power law scaling in time: A is proportional to (t0-t)^n, where A is the area of the feature, t0 is the time at which the feature disappears, and n=2/3 or 1. The constant of proportionality is related to parameters describing both the kinetic and equilibrium properties of the surface. A continuous time Monte Carlo simulation is used to test the application of this theory to generic surfaces with atomic scale features. A new method is described to obtain macroscopic kinetic parameters describing interfaces in such simulations. Simulation and analytic theory are compared directly, using measurements of the simulation to determine the constants of the analytic theory. Agreement between the two is very good over a range of surface parameters, suggesting that the analytic theory properly captures the necessary physics. It is anticipated that the simulation will be useful in modeling complex surface geometries often seen in experiments on physical surfaces, for which application of the analytic model is not straightforward., Comment: RevTeX (with .bbl file), 25 pages, 7 figures from 9 Postscript files embedded using epsf. Submitted to Phys. Rev. B A few minor changes made on 9/24/96

We use the Newns-Anderson Hamiltonian to describe many-body electronic processes that occur when hyperthermal alkali atoms scatter off metallic surfaces. Following Brako and Newns, we expand the electronic many-body wavefunction in the number of particle-hole pairs (we keep terms up to and including a single particle-hole pair). We extend their earlier work by including level crossings, excited neutrals and negative ions. The full set of equations of motion are integrated numerically, without further approximations, to obtain the many-body amplitudes as a function of time. The velocity and work-function dependence of final state quantities such as the distribution of ion charges and excited atomic occupancies are compared with experiment. In particular, experiments that scatter alkali ions off clean Cu(001) surfaces in the energy range 5 to 1600 eV constrain the theory quantitatively. The neutralization probability of Na$^+$ ions shows a minimum at intermediate velocity in agreement with the theory. This behavior contrasts with that of K$^+$, which shows ... (7 figures, not included. Figure requests: jbm@yollabolly.physics.brown.edu), Comment: 43 pages, plain TeX, BUP-JBM-2

Sputtering yields of solid SO2 by high energy ions were measured in order to study the mechanism for sputtering dielectrics with ions in the electronic stopping power region. The incident ions were helium and fluorine with energies ranging from 1.5 MeV to 25 MeV. Yields as high as 7000 SO2 molecules/incident F ion were measured; the 1.5 MeV He-4 beam had a sputtering yield of 50. The data are compared to yield measurements made on UF4 and H2O targets. There is a striking similarity in the yield as a function of the incident F energy for all three targets. The data compare favorably with theoretical yield curves based on a new model for the sputtering which considers the electronic excitations induced in the target by the incident beam. Measurements and calculations of this sort are also useful in understanding processes which occur on the surface of Jupiter's satellite Io, which is covered with SO2 frost and bombarded by energetic ions trapped in the Jovian magnetosphere.

The erosion of frozen SO2 due to bombardment by both light and heavy ions (He and F) was measured for bombarding energies of 0.08 to 1.3 MeV/amu. The number of SO2 molecules ejected from the target per incident ion (i.e., the sputtering yield) was 50 for 1.5 MeV He ions and 7300 for 6 MeV F ions. Ion bombardment followed by heating produced an oxygen/sulfur residue which was much more stable against subsequent ion bombardment than the initial frozen SO2. The erosion rate of SO2 frost on Jupiter's moon Io depends strongly on the elemental composition and energy spectra of the magnetospheric ion flux which bombards the surface. The combined effects of ion bombardment and heating which produced residues on the target substrates may also occur on Io from magnetospheric ion bombardment and heating by volcanism. The experimental results compare favorably with a new model of the sputtering process which considers the energy loss of the incident ion to electronic excitation in the target.

In this paper recent experimental results on sputtering of UF4 and H2O (ice) by fast heavy ions are reviewed. Measurements have been made of the dependence of the sputtering yield on the incident ion type, charge state, and energy. In the case of UF4, the energy spectra of neutral sputtered particles have been obtained as well. There is a clear dependence of the sputtering yield on the electronic part of the stopping power, and the yield is strongly affected by the charge state of the incident ion, which shows that in the near-surface region from which sputtered particles arise, the charge state of the incident ion has not reached equilibrium. The shape of the energy spectra observed for UF4 targets is that expected from a thermal distribution rather than that of the collision cascade form typical of ordinary sputtering. A model of the sputtering-track registration process that has arisen from these data is shown to provide a framework of understanding the close relationship of these observations to the 'plasma desorption' of marcromolecules.