Grazing Incidence Diffraction of keV Helium Atoms on a Ag(110) Surface (original) (raw)
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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.
Fast-atom diffraction at surfaces
2009
Fast helium atoms diffracted at alkali-halide surfaces under grazing angles of incidence exhibit intriguing diffraction patterns. The persistence of quantum coherence is remarkable, considering high surface temperatures and high (keV) kinetic energies of the incident atoms. Dissipative and decohering effects such as the momentum transfer between the incident helium atoms and the surface influence the diffraction patterns and control the width of the diffraction peaks, but they are weak enough to preserve the visibility of the diffration patterns. We perform an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the ⟨110⟩ direction. Our results agree well with recent experimental diffraction data.
Decoherence in fast atom diffraction from surfaces
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2011
Diffraction of fast atoms from crystal surfaces at grazing incidence (GIFAD) has now been observed on all types of materials, from wide band gap insulators to metals, including semiconductors. Since mainly the (slow) motion normal to the surface is important diffraction patterns are comparable to those obtained in thermal energies atomic diffraction (TEAS), however, the specific scattering geometry of GIFAD has a strong influence on decoherence phenomena. The contribution of atomic vibrations is much less pronounced than in TEAS but other sources of decoherence such as electronic excitations, clearly observed on metals, can participate due to the comparatively large projectile velocity parallel to the surface. We present here simple models that describe these decoherence effects. The results are in good agreement with the experimental results.
Surface analysis via fast atom diffraction: pattern visibility and spot-beam contribution
arXiv: Atomic Physics, 2018
Grazing incidence fast atom diffraction (GIFAD or FAD) is a sensitive tool for surface analysis, which strongly relies on the quantum coherence of the incident beam. In this article the influence of the incidence conditions and the projectile mass on the visibility of the FAD patterns is addressed. Both parameters determine the transverse coherence length of the impinging particles, which governs the general features of FAD distributions. We show that by varying the impact energy, while keeping the same collimating setup and normal energy, it is possible to control the interference mechanism that prevails in FAD patterns. Furthermore, we demonstrate that the contribution coming from different positions of the focus point of the incident particles, which gives rise to the spot-beam effect, allows projectiles to explore different zones of a single crystallographic channel when a narrow surface area is coherently lighted. In this case the spot-beam effect gives also rise to a non-coherent background, which contributes to the gradual quantum-classical transition of FAD spectra. Present results are compared with available experimental data, making evident that the inclusion of focusing effects is necessary for the proper theoretical description of the experimental distributions.
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.
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.
Phonon contribution in grazing-incidence fast atom diffraction from insulator surfaces
Physical Review A, 2019
We study the effect of crystal lattice vibrations on grazing-incidence fast atom diffraction (GIFAD) from insulator surfaces. To describe the phonon contribution to GIFAD we introduce a semiquantum method, named Phonon-Surface Initial Value Representation (P-SIVR), which represents the surface with a harmonic crystal model, while the scattering process is described by means of the Surface Initial Value Representation approach, including phonon excitations. Expressions for the partial scattering probabilities involving zero-and one-phonon exchange are derived. In particular, the P-SIVR approach for zero-phonon scattering is applied to study the influence of thermal lattice vibrations on GIFAD patterns for Ne/LiF(001) at room temperature. It is found that the thermal lattice fluctuations introduce a polar-angle spread into the projectile distributions, which can affect the relative intensities of the interference maxima, even giving rise to interference sub-patterns depending on the incidence conditions. Present results are in agreement with the available experiments.
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.
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.