K.J. Snowdon

Simulation of ballistic effects during scattering under glancing angles of incidence from crystal surfaces

A direct comparison of the results of molecular dynamics (MD) and sequential binary collision (BCA) simulations of the scattering of 3 keV light (He) and heavy (Xe) particles from a Cu(111) surface along the [110] direction under glancing incidence conditions is presented. Inelastic energy losses are neglected. Clear differences between the MD and BCA results are seen in the differential scattering distributions. The origin of these differences arises from approximations in the way “simultaneous” collisions between the projectile and several nearby surface atoms are treated in BCA-type simulations.

Transient adsorption or skipping motion of a 200–2000 eV Si+ beam on a Cu(111) surface

The experimental observation of transient adsorption or skipping motion of 200–2000 eV Si+ beams incident at 1–16° to a Cu(111) surface is reported. We propose that trapping occurs on an attractive excited state of the SiCu interaction potential. This state may be formed by an adiabatic charge rearrangement, or more generally, as a consequence of inelastic energy loss of the projectile to electronic degrees of freedom of the metal. Up to five vibrational oscillations in this potential could be detected, with an associated energy loss of 20% of the incident beam energy. The absolute energy losses, and their charge state dependences provide information about the charge changing interactions, the charge state of the projectile at the surface, and the time taken for the collision complex to achieve charge equilibrium.

Observation of transient adsorption or skipping motion in small angle ion-surface scattering

Transient adsorption or skipping motion of silicon ions scattered under specular conditions from a Cu(111) surface is reported. The phenomena is identified on the basis of the observation of discrete energy loss peaks in the scattered positive and negative ion kinetic energy distributions, together with their behaviour upon variation of the crystal azimuth, incidence angle of the beam to the surface, and scattering angular distributions. Up to five reflections in a binding potential were seen. We propose that the beam is trapped following an energy loss, by electron-hole pair or plasmon excitation, which exceeds the incident beam energy component normal to the surface. The results lend strong support to the thesis that ion beam techniques can probe the family of potential surfaces upon which adsorption and reaction, at thermal energies, proceed.

Theory of the momentum distribution of swift ions and its application to surface scattering

Abstract We propose a method to calculate the momentum distribution of swift ions. Since we treat the Hamiltonian by the real-space representation, the method is applicable to inhomogeneous media such as the surface system. We outline the theory and also present its applications to the surface system. The theory is useful for the study of the effect of electronic excitations on grazing-incident ions scattering from metal surfaces, and it points out a peculiarity of surface electronic excitations due to coupling between surface-parallel and -normal momenta of the surface electronic system. This coupling may be important for the skipping motion and related phenomena reported recently.

Skipping motion of low-velocity ( ν ⪡ νFermi) ions on metal surfaces

Abstract We have studied the scattering of 50–200 eV Na + and K + beams and 200–2000 eV Si + beams from Cu(lll) under angles of incidence from 1° to 20°. The Si + beams appear to access a transient bound excited state of the SiCu(111) electronic system, resulting in adsorption or trapping of the beam for up to 5 oscillations in this potential. The particles appear to detrap from the skipping state through a combination of elastic processes and momentum transfer between the perpendicular and parallel momentum components of the beam to the surface. Direct evidence for such momentum transfer is provided by the alkali scattering data, which exhibit negligible energy loss but scattered-particle angular distributions which peak within (2 ± 1)° of the surface, independent of the angle of incidence of the beam to the surface. These data, combined with data on the scattering of O 2 + from Ag(111), suggest that energetic ion beams can be adsorbed by both elastic and inelastic mechanisms into skipping states at metal surfaces.

Inelastic energy loss of fast ions for direct scattering and skipping trajectories on metal surfaces

Abstract We have measured the energy loss of 200–2000 eV silicon ions which have undergone both direct scattering and skipping motion on a Cu(111) surface. The inelastic energy losses associated with 1,2,…, 5 surface reflections for both positive and negative final charge states are identified. The energy loss is approximately proportional to the projectile energy and is independent of the crystal azimuth. A final charge state dependence is observed, as are extrapolated non-zero energy losses for Si− and Si+ production at zero beam energy. These latter losses agree quantitatively with the energy required to promote an electron from the Fermi level of Cu(111) to the affinity level of Si, and the energy required to reionize Si, respectively, thus providing direct information on the charge state during scattering. Comparison with theoretical calculations shows that energy loss theory must be extended to include charge redistributions characteristic of chemical bonding.

Observation of skipping motion or transient adsorption in small angle scattering of silicon ions from a Cu(111) surface

Abstract Transient adsorption or skipping motion of 200–2000 eV silicon ions scattered under specular conditions from a Cu(111) surface is reported. The phenomena are identified on the basis of the observation of discrete energy loss peaks in the scattered Si+ and Si− ion kinetic energy distributions, together with their behaviour upon variation of the crystal azimuth, incidence angle of the beam to the surface, and scattering angular distributions. Up to five reflections, with a total energy loss of 20% of the incident beam energy, could be identified. We propose that the beam is trapped following energy loss, by electron-hole pair and plasmon excitation, which exceeds the incident beam energy component normal to the surface. Trajectory calculations which include this coupling of the energy loss and interaction potential satisfactorily reproduce the experimental observations.

Energy dissipation of fast neutral beams scattered at glancing angles from crystal surfaces

We provide a model calculation of the contribution of the non-adiabatic response of the substrate electronic degrees of freedom to the energy dissipated by a fast neutral particle scattered under a glancing angle of incidence from a metal surface. These degrees of freedom couple to the monopole charge or static induced dipole of the moving particle. The coupling strength is additionally modulated by the corrugation of the surface electronic density profile. We calculate the mean energy loss and mean number of electron-hole pairs created due to the interaction with an Al(111)-surface, via both types of coupling, of neutral Ar atoms as a function of the parallel velocity of the incident particles in the velocity range 0.02–1 vF (where νF is the Fermi velocity of electrons in the metal). We find that the corrugation-induced oscillation of the coupling strength between the traversing particle and the surface can provide a significant contribution to the energy loss, irrespective of the nature of the coupling assumed.

Observation of perpendicular to parallel momentum transfer in small angle alkali-ion-metal-surface scattering

We have studied the scattering of 50–200 eV Na+ and K+ beams from Cu(111) under angles of incidence to the surface from 4 to 12°. Energy losses, if they occur at all, are less than 0.14 eV at a beam energy of 185 eV. The scattered particle angular distributions, measured in the plane defined by the incident beam direction and the surface normal, peak within 2 ± 1° of the surface. The results seem compatible with energy loss theories based either on a generalized Anderson-Newns Hamiltonian or electrodynamic response formalisms, which both predict a coupling between the perpendicular and parallel momentum components of the projectile during its interaction with the surface.

Ro-vibrational excitation, alignment and orientation distributions of fast non-dissociatively scattered molecules

Abstract The ro-vibrational distribution of fast diatomic molecules scattered from an uncorrugated surface under strongly dissipative glancing incidence conditions is calculated. The classical trajectory simulation includes potential surface switching associated with hot-electron scattering processes. Both ro-vibrational excitation and strong alignment of the classical angular momentum vector in the surface plane (“cartwheel motion”) are observed, independent of the occurrence of potential surface switching. Ro-vibrational excitation is enhanced strongly by transitions between potential surfaces. The resultant larger proportion of molecules in highly rotationally excited states leads to a higher fraction of cartwheel-aligned molecules in the scattered molecule ensemble. The molecules which dissociate in the simulation are characterised by surface normal peaked internuclear axis orientation distributions. This is in agreement with the results of recent experiments [A. Nesbitt et al., Surf. Sci. 331–333 (1995) 321]. We observe, in addition, an enhanced rotational population of “topspin” oriented molecules, which arises from differences in the surface parallel oriented friction forces acting on each atom of the molecule. Glancing incidence scattering from well-prepared close-packed metal surfaces would appear to provide an efficient, general method to obtain a beam of preferentially aligned fast neutral diatomic molecules.