Ab initiopotential for the He-Ag(110) interaction investigated using grazing-incidence fast-atom diffraction (original) (raw)
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Physical Review A, 2013
Experimental diffraction patterns produced by grazing scattering of fast helium atoms from a Ag(110) surface are used as a sensitive tool to test an ab initio potential model derived from accurate density-functional theory (DFT) calculations. The scattering process is described by means of the surface eikonal approximation, which is a distorted-wave method that includes the quantum interference between contributions coming from different projectile paths, taking into account the complete corrugation of the three-dimensional projectile-surface potential. A fairly good agreement between the theoretical and experimental momentum distributions is found for incidence along different low-indexed crystallographic directions. This agreement is indicative of the quality of the DFT potential. The effective corrugation of the interaction potential across the incidence channel is also investigated.
Grazing Incidence Diffraction of keV Helium Atoms on a Ag(110) Surface
Physical Review Letters, 2008
Diffraction of fast atoms at grazing incidence has been recently demonstrated on the surface of alkali halides and wide band gap semiconductors, opening applications for the online monitoring of surface processes such as growth of ultrathin layers. This Letter reports energy resolved diffraction of helium on Ag(110) metal surface showing that a band gap is not mandatory to restrict the decoherence due to electron-hole pair excitations by the keV projectile. Measurement of the energy loss, which is in the eV range, sheds light on the scattering process.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2009
Grazing incidence diffraction of fast atoms (GIFAD) on surfaces has first been discovered on ionic insulators where electronic excitations are strongly reduced due to the large band gap. At variance no threshold exists for electronic excitations close to the Fermi edge of a metal surface. New results of energy resolved diffraction of keV helium atoms on a Ag(1 1 0) surface are presented which considerably extend the application range of GIFAD. The combined analysis of the energy loss and diffraction data could help providing a detailed description of the collision of helium with the surface electrons.
Diffraction of swift atoms after grazing scattering from metal surfaces: N/Ag(111) system
Physical Review A, 2010
Diffraction patterns produced by grazing scattering of fast N atoms from a Ag(111) surface are investigated by employing the surface eikonal approximation. This method is a distorted-wave theory that takes into account the coherent addition of contributions coming from different projectile paths. In the model the projectile-surface potential is obtained from an accurate density-functional theory calculation. The dependence of the scattered projectile spectra on impact energy and incidence channel is analyzed, and possible incident direction and energy range for the observation of the interference patterns are predicted. In addition, it is found that as a result of the high reactivity of N atoms, asymmetries of the surface potential might be detected through their effects on diffraction patterns.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003
We compare the azimuthal distribution of H atoms after scattering off Ag(1 1 0) obtained by molecular dynamics with different H-Ag(1 1 0) potential energy surfaces (PES) and experimental results. We use grazing incident H atoms and low energies (up to 4 keV). Density functional theory (DFT) calculations are performed for the static case of an H atom in front of an Ag(1 1 0) surface. The surface is represented by an 8-atom slab, and the H atoms form 1 Â 1 and 2 Â 2 supercells. The generalized gradient approximation is used. Classical trajectories are evaluated on the obtained PES, and the azimuthal distribution of the scattered atoms is calculated. Good agreement with experiment is obtained which gives us some confidence in the correct description of the system at low energies by the static DFT calculations. These results are also compared with pair-potential calculations. The accuracy of trajectories may be important for the correct evaluation of charge transfer, energy loss and straggling during ion-surface collisions.
Out-of-plane scattering of Ar from Ag(111)
Surface Science, 1996
We present the resulting out-of-plane scattering data for the collisions of Ar with an Ag(111) surface, using a classical molecular dynamics simulation with a potential energy surface based on the local density approximation. The incidence energies for Ar vary from 0.03 eV up to 22 eV with an incidence angle of 40 °. Results are presented for a static surface and for a surface with a finite temperature of 600 K. For the low incidence energies, the peak of the angular scattering distributions lies in the plane of incidence and the in-plane width is larger than the out-of-plane one. In the high energy range, the peaks of the scattering distributions appear not in the plane of incidence and for the static surface, clear out-of-plane surface rainbow peaks appear. For a surface temperature of 600 K, the surface rainbow peaks diminish and the angular distributions smooth. The total angular scattering distributions yield the relative amount of in-plane scattering and that ranges from 10 to 40%, depending on the incidence energy. A maximum of the in-plane scattering occurs for an incidence energy of 1 eV, where the effective surface corrugation has a minimum.
Energy-loss contribution to grazing scattering of fast He atoms from a silver surface
The energy lost by helium atoms axially scattered from a Ag(110) surface is studied in order to investigate the influence of dissipative processes on fast atom diffraction spectra. In this work inelastic projectile distributions are evaluated within a semiclassical formalism that includes dissipative effects due to electron-hole excitations through a friction force. For incidence along the [112] and [110] directions the model predicts the presence of multiple peaks in the energy-loss spectrum for a given impact energy. But these structures are completely washed out when the experimental dispersion of the incident beam is taken into account, giving rise to a smooth energy-loss distribution. Simulations including the experimental energy spread are in fairly good agreement with available experimental data for the [112] channel. In addition, our results suggest that inelastic processes produce an almost constant background in the transverse momentum distribution, except in the extremes of the momentum range where classical rainbow maxima appear. By adding elastic and inelastic contributions, experimental diffraction patterns are well reproduced.
Calculation of the Debye-Waller factor for atom-surface scattering: He on Ag(111)
Physical Review B, 1987
By a consistent application of the distorted-wave Born approximation, the Debye-Wailer exponent for the scattering of He atoms from the Ag(111) surface is directly computed. When diffraction is negligible, as in this case, the decrease in specular intensity is simply obtained by summing the scattering due to all possible one-phonon processes. Using one-phonon computed intensities that agree with experiment along high-symmetry directions of the surface Brillouin zone, we find that the computed Debye-Wailer exponent agrees with experiment and corresponds to an effective Debye temperature of 241 K. All corrections to the eikonal-type formula, 2 8' =4p, (tt,), are automatically included. In particular, the contribution of phonons with high parallel momentum is sharply cut off.
Low energy He atom scattering from Ag(110) and Ag(111): Is there an effective two-body potential?
Surface Science, 1987
A rather wide range of scattering potentials is consistent with the extensive data for He scattering from Ag(ll0). This illustrates the nonuniqueness of potentials obtained by fitting scattering data for weakly corrugated surfaces. However, the corrugation of the equipotential surface for these potentials only differs by-20% and is therefore quite well determined by the data. An effective pair potential between the He atom and a Ag atom can be found which is consistent with scattering data for both the (110) and the (111) face of Ag, but it is unrealistic in some other respects.