arXiv subjects

M. S. Gravielle

Publications and source records attributed to M. S. Gravielle.

18 recordsLinked to original sources

arXiv2023

Decoherent phonon effects in fast atom-surface scattering

Understanding the influence of phonon-mediated processes on grazing-incidence fast atom diffraction (GIFAD) patterns is relevant for its use as a surface analysis technique. In this work, we apply the Phonon-Surface Initial Value Representation (P0-SIVR) approximation to study lattice vibration effects on GIFAD patterns for the He-LiF(001) system at room temperature. The P0-SIVR approach is a semiquantum method that describes the zero-phonon atom-surface scattering including the contribution of intermediate phonon transitions. The main features introduced by thermal lattice vibrations in the angular distributions of the scattered helium atoms are investigated by considering normal energies in the 0.1 -3 eV range. In all the cases, thermal fluctuations introduce a wide polar spread that transforms the interference maxima into elongated strips. We found that the polar width of these fringes does not depend on the normal energy, as it was experimentally observed. In addition, when the normal energy increases, not only the relative intensities of interference peaks are affected by the crystal vibrations, but also the visibility of the interference structures, which disappear completely for normal energies approximately equal to or higher than 3 eV. These findings agree fairly well with the experimental data reported in Phys. Chem. Chem. Phys. 237615 (2021). However, it should be noticed that the simulated polar widths underestimate the experimentally-derived limit, suggesting that there are other mechanisms, such as inelastic phonon processes, that contribute to the polar dispersion of the GIFAD patterns.

cond-mat.mtrl-sci↗

arXiv2022

Terrace effects in grazing-incidence fast atom diffraction from a LiF(001) surface

The effect produced by surface defects on grazing-incidence fast atom diffraction (GIFAD) patterns is studied by considering the presence of terraces in a LiF(001) sample. For helium atoms impinging along the <110> direction of the LiF surface, we analyze the influence of a monolayer terrace with its edge oriented parallel or perpendicular to the axial channel. We found that the presence of an outward transverse step introduces a diffuse background above the Laue circle, which displays additional peaked structures. For inward transverse steps, instead, such a background is placed below the Laue circle, showing a much weaker intensity. On the other hand, parallel steps give rise to asymmetric angular distributions, which are completely confined to the Laue circle. Therefore, these theoretical results suggest that GIFAD might be used to characterize terrace defects.

cond-mat.mtrl-sci↗

arXiv2021

The atomic ionization, capture, and stopping cross sections by multicharged ions satisfy the Benford law

The applicability of the Benford law for different data sets of atomic cross sections by ion impact is studied. We find that the data sets corresponding to theoretical ionization and capture cross sections of neutral targets by multicharged ions satisfy quite well the Benford law, not only for the first digits but also within a given order of magnitude. Experimental stopping power values from the International Atomic Energy Agency database were also scrutinized, but in this case the Benford conformity was not so satisfactory due to its small rank width. In all cases, errors, densities of prime numbers, and theorems of Nigrini and Pinkham are evaluated and discussed.

physics.atom-ph↗

arXiv2020

Thermal effects on helium scattering from LiF(001) at grazing incidence

Grazing-incidence fast atom diffraction (GIFAD) is an exceptionally sensitive method for surface analysis, which can be applied not only at room temperature but also at higher temperatures. In this work we use the He-LiF(001) system as benchmark to study the influence of temperature on GIFAD patterns from insulator surfaces. Our theoretical description is based on the Phonon-Surface Initial Value Representation (P0-SIVR) approximation, which is a semiquantum approach that includes the phonon contribution to the elastic scattering. Within the P0-SIVR approach the main features introduced by thermal lattice vibrations on the angular distributions of scattered projectiles are investigated as a function of the crystal temperature. We found that azimuthal and polar spectra are strongly affected by thermal fluctuations, which modify the relative intensities and the polar spread of the interference structures. These findings are relevant for the use of GIFAD in surface research. Moreover, the present results are contrasted with available experimental data at room temperature.

cond-mat.mtrl-sci↗

arXiv2019

Phonon contribution in grazing-incidence fast atom diffraction from insulator surfaces

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.

physics.atom-ph↗

arXiv2018

Surface analysis via fast atom diffraction: pattern visibility and spot-beam contribution

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.

physics.atom-ph↗

arXiv2016

Crystallographic orientation and induced potential effects in photoelectron emission from metal surfaces by ultrashort laser pulses

The influence of the crystallographic orientation of a typical metal surface, like aluminum, on electron emission spectra produced by grazing incidence of ultrashort laser pulses is investigated by using the band-structure-based-Volkov (BSB-V) approximation. The present version of the BSB-V approach includes not only a realistic description of the surface interaction, accounting for band structure effects, but also effects due to the induced potential that originates from the collective response of valence-band electrons to the external electromagnetic field. The model is applied to evaluate differential electron emission probabilities from the valence band of Al(100) and Al(111). For both crystallographic orientations, the contribution of partially occupied surface electronic states and the influence of the induced potential are separately analyzed as a function of the laser carrier frequency. We found that the induced potential strongly affects photoelectron emission distributions, opening a window to scrutinize band structure effects.

physics.atm-clus↗

arXiv2016

The interaction of atoms with LiF(001) revisited

Pairwise additive potentials for multielectronic atoms interacting with a LiF(001) surface are revisited by including an improved description of the electron density associated with the different lattice sites, as well as non-local electron density contributions. Within this model, the electron distribution around each ionic site of the crystal is described by means of an onion approach that accounts for the influence of the Madelung potential. From such densities, binary interatomic potentials are then derived by using well-known non-local functionals for the kinetic, exchange and correlation terms. Rumpling and long-range contributions due to projectile polarization and van der Waals forces are also included in an analogous fashion. We apply this pairwise additive approximation to evaluate the interaction potential between closed-shell - He, Ne, Ar, Kr, and Xe - and open-shell - N, S, and Cl - atoms and the LiF surface, analyzing the relative importance of the different contributions. The performance of the proposed potentials is assessed by contrasting angular positions of rainbow and supernumerary rainbow maxima produced by fast grazing incidence with available experimental data. The good agreement found for normal energies in the eV- range represents a meaningful evidence of the quality of the present description.

physics.atom-ph↗

arXiv2016

Van der Waals effects on grazing incidence fast atom diffraction for H/LiF(001)

We theoretically address grazing incidence fast atom diffraction (GIFAD) for H atoms impinging on a LiF(001) surface. Our model combines a description of the H-LiF(001) interaction obtained from Density Functional Theory calculations with a semi-quantum treatment of the dynamics. We analyze simulated diffraction patterns in terms of the incidence channel, the impact energy associated with the motion normal to the surface, and the relevance of Van der Waals (VdW) interactions. We then contrast our simulations with experimental patterns for different incidence conditions. Our most important finding is that, for normal energies lower than 0.5 eV and incidence along the <100> channel, the inclusion of Van der Waals interactions in our potential energy surface yields a greatly improved accord between simulations and experiments. This agreement strongly suggests a non-negligible role of Van der Waals interactions in H/LiF(001) GIFAD in the low-to-intermediate normal energy regime.

cond-mat.mtrl-sci↗

arXiv2016

Single- and double-slit collimating effects on fast-atom diffraction spectra

Diffraction patterns produced by fast He atoms grazingly impinging on a LiF(001) surface are investigated focusing on the influence of the beam collimation. Single- and double- slit collimating devices situated in front of the beam source are considered. To describe the scattering process we use the Surface Initial Value Representation (SIVR) approximation, which is a semi-quantum approach that incorporates a realistic description of the initial wave packet in terms of the collimating parameters. Our initial wave-packet model is based on the Van Cittert-Zernike theorem. For a single-slit collimation the width of the collimating aperture controls the shape of the azimuthal angle distribution, making different interference mechanisms visible, while the length of the slit affects the polar angle distribution. Additionally, we found that by means of a double-slit collimation it might be possible to obtain a wide polar angle distribution, which is associated with a large spread of the initial momentum perpendicular to the surface, derived from the uncertainty principle. It might be used as a simple way to probe the surface potential for different normal distances.

physics.atom-ph↗

arXiv2015

Influence of the lighting on Fast Atom Diffraction studied via a semi-quantum approach

The influence of the collimating conditions of the incident beam on diffraction patterns produced by grazing scattering of fast atoms off crystal surfaces is studied within a semi-quantum approach, named Surface Initial Value Representation (SIVR) approximation. In this approach we incorporate a realistic description of the incident particle in terms of the collimating parameters, which determine the surface area that is coherently illuminated. The model is applied to He atoms colliding with a LiF(001) surface after passing through a rectangular aperture. As it was experimentally observed [1], SIVR spectra as a function of the azimuthal angle are very sensitive to the width of the collimating slit. We also found that the length of the collimating aperture affects polar angle distributions, introducing additional interference structures for the longer collimating slits.

physics.atom-ph↗

arXiv2014

Trajectory-dependent energy loss for swift He atoms axially scattered off a silver surface

Angle- and energy-loss- resolved distributions of helium atoms grazingly scattered from a Ag(110) surface along low indexed crystallographic directions are investigated considering impact energies in the few keV range. Final projectile distributions are evaluated within a semi-classical formalism that includes dissipative effects due to electron-hole excitations through a friction force. For mono-energetic beams impinging along the $[1\bar{1}0]$, $[1\bar{1}2]$ and $[1\bar{1}0]$ directions, the model predicts the presence of multiple peak structures in energy-loss spectra. Such structures provide detailed information about the trajectory-dependent energy loss. However, when the experimental dispersion of the incident beam is taken into account, these energy-loss peaks are completely washed out, giving rise to a smooth energy-loss distribution, in fairly good agreement with available experimental data.

cond-mat.mtrl-sci↗

arXiv2014

Semi-quantum approach for fast atom diffraction: solving the rainbow divergence

In this work we introduce a distorted wave method, based on the Initial Value Representation (IVR) approach of the quantum evolution operator, in order to improve the semiclassical description of rainbow effects in diffraction patterns produced by grazing scattering of fast atoms from crystal surfaces. The proposed theory, named Surface Initial Value Representation (SIVR) approximation, is applied to He atoms colliding with a LiF(001) surface along low indexed crystallographic channels. For this collision system the SIVR approach provides a very good representation of the quantum interference structures of experimental projectile distributions, even in the angular region around classical rainbow angles where common semiclassical methods diverge.

physics.atom-ph↗

arXiv2013

Influence of the surface band structure on electron emission spectra from metal surfaces

Electron distributions produced by grazing impact of fast protons on Mg(0001), Cu(111), Ag(111) and Au(111) surfaces are investigated, focusing on the effects of the electronic band structure. The process is described within the Band-Structure-Based approximation, which is a perturbative method that includes an accurate representation of the electron-surface interaction, incorporating information of the electronic band structure of the solid. For all the studied surfaces, the presence of partially occupied surface electronic states produces noticeable structures in double differential - energy- and angle- resolved - electron emission probabilities from the valence band. These structures remain visible in electron emission spectra after adding contributions coming from core electrons, which might make it possible their experimental detection.

physics.atom-ph↗

arXiv2013

Grazing incidence fast atom diffraction for He atoms impinging on a Ag(110) surface

Experimental diffraction patterns produced by grazing scattering of fast helium atoms from a Ag(110) surface are used as a sensitive tool to test both the scattering and the potential models. To describe the elastic collision process we employ the surface eikonal (SE) approximation, which is a semi-classical method that includes the quantum interference between contributions coming from different projectile paths. The projectile-surface potential is derived from an accurate density-functional theory (DFT) calculation that takes into account the three degrees of freedom of the incident projectile. A fairly good agreement between theoretical and experimental momentum distributions is found for incidence along different low-indexed crystallographic directions.

physics.atom-ph↗

arXiv2009

He-LiF surface potential from fast atom diffraction under grazing incidence

Diffraction patterns produced by grazing scattering of fast atoms from insulator surfaces are used to examine the atom-surface interaction. The method is applied to He atoms colliding with a LiF(001) surface along axial crystallographic channels. The projectile-surface potential is obtained from an accurate DFT calculation, which includes polarization and surface relaxation. For the description of the collision process we employ the surface eikonal approximation, which takes into account quantum interference between different projectile paths. The dependence of projectile spectra on the parallel and perpendicular incident energies is experimentally and theoretically analyzed, determining the range of applicability of the proposed model.

cond-mat.other↗

arXiv2009

Study of the induced potential produced by ultrashort pulses on metal surfaces

The influence of the induced potential on photoelectron emission from metal surfaces is studied for grazing incidence of ultrashort laser pulses. To describe this process we introduce a distorted wave method, the Surface Jellium Volkov approach, which includes the perturbation on the emitted electron produced by both the laser and the induced fields. The method is applied to an Al(111) surfaces, contrasting the results with the numerical solutions to the time-dependent Schrodinger equation (TDSE). We found that SJV approach reproduces well the main features of emission spectra, accounting properly for effects originated by the induced potential.

cond-mat.other↗

arXiv2008

Influence of the polarization in grazing scattering of fast hellium atoms from LiF(001) surfaces

Grazing scattering of neutral atoms from insulator surfaces is investigated in the intermediate velocity range, in which interference effects have been recently observed. To describe this process we introduce a distorted-wave method, based on the use of the eikonal wave function, which takes into account the phase of the scattering state along the classical projectile path. The eikonal theory is applied to evaluate the angular distribution of few keV helium atoms after impinging on a LiF(001) surface along low-index crystallographic directions. The interest focuses on the role played by the projectile polarization produced by cations and anions of the crystal surface. For the considered collision system we found a polarization channel, corresponding to the direction <110>, which is affected by this effect, while for incidence in the direction <100> the polarization contribution is nearly negligible. The proposed eikonal approach, including polarization effects, provides angular projectile spectra in good agreement with the experimental data.

physics.atom-ph↗