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arXiv:1510.04909 · Physical Review A

Influence of beam collimation on fast-atom diffraction studied via a semiquantum approach

M. S. Gravielle, J. E. Miraglia

Published Dec 11, 2015• 21 citations
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Abstract

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 semiquantum approach, called the 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 was experimentally observed [Nucl. Instrum. Methods Phys. Res., Sect. B 350, 99 (2015)], 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.

DOI
10.1103/physreva.92.062709

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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 semiquantum approach, called the 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 was experimentally observed [Nucl. Instrum. Methods Phys. Res., Sect. B 350, 99 (2015)], 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.
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Diffraction of fast helium atoms at alkali-halide surfaces under grazing angles of incidence shows intriguing diffraction patterns. The persistence of quantum coherence is remarkably strong, even though high surface temperatures and high (keV) kinetic energies of the incident atoms would strongly suggest the dominance of dissipative and decohering processes. The main source of decoherence is the excitation or absorption of surface vibrations upon impact. The momentum transfer between the surface and the incident helium atom depends on the amplitude of the thermal vibrations of the surface atoms and the energy of the incident particle. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the (110) direction, and compare with recent experimental diffraction data.
Scattering of fast neutral atoms with keV kinetic energies at alkali-halide surfaces under grazing angles displays intriguing diffraction patterns. The surprisingly strong persistence of quantum coherence despite the impulsive interaction with an environment at solid state density and elevated temperatures raises fundamental questions such as to the suppression of decoherence and of the quantum-to-classical crossover. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the 110 direction and compare with recent experimental diffraction data. From the quantitative reconstruction of diffraction images the vertical LiF-surface reconstruction, or buckling, can be determined.
Typically one expects that when a heavy particle collides with a surface, the scattered angular distribution will follow classical mechanics. The heavy mass usually assures that the coherence length of the incident particle in the direction of the propagation of the particle (the parallel direction) will be much shorter than the characteristic lattice length of the surface, thus leading to a classical description. Recent work on molecular interferometry has shown that extreme collimation of the beam creates a perpendicular coherence length which is sufficiently long so as to observe interference of very heavy species passing through a grating. Here we show, using quantum mechanical simulations, that the same effect will lead to quantum diffraction of heavy particles colliding with a surface. The effect is robust with respect to the incident energy, the angle of incidence, and the mass of the particle.
Scattering of fast neutral atoms with keV kinetic energies at alkali-halide surfaces under grazing angles displays intriguing diffraction patterns (Schuller, Wethekam and Winter 2007, Roisseau et al. 2007). In spite of the impulsive interaction with a surface at elevated temperature, which results in phonon excitation, quantum coherence evidently persists. To quantitatively study this remarkable survival of coherence, we present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the 〈110〉 direction and compare with recent experimental diffraction data. Decoherence is analyzed employing a quantum trajectory Monte Carlo method (Minami, Reinhold and Burgdorfer 2003), calculating the ensemble average over solutions of a stochastic linear Schrodinger equation. The evolution of the atomic wavepacket is governed by a sequence of stochastic collisions and continuous propagation in the He-LiF surface potential. We find near-perfect agreement between the resulting diffraction patterns and experimental results (Schuller, Wethekam and Winter 2007) without using any adjustable parameters. The question, whether the LiF surface features “buckling”, (i.e. vertical surface reconstruction where the F atoms are displaced relative to the Li atoms) has been addressed in the past (de Wette, Kress and Schroder 1985). However, experimental uncertainties were of the same order as the displacement amplitude itself. In atom-surface diffraction at a grazing angle, surface reconstruction drastically changes the diffraction patterns. By comparison of numerical and experimental results, the buckling amplitude can be determined with unprecedented accuracy.
The semiclassical perturbation theory formalism of Hubbard and Miller [J. Chem. Phys. 78, 1801 (1983)] for atom surface scattering is used to explore the possibility of observation of heavy atom diffractive scattering. In the limit of vanishing ℏ the semiclassical theory is shown to reduce to the classical perturbation theory. The quantum diffraction pattern is sensitive to the characteristics of the beam of incoming particles. Necessary conditions for observation of quantum diffraction are derived for the angular width of the incoming beam. An analytic expression for the angular distribution as a function of the angular and momentum variance of the incoming beam is obtained. We show both analytically and through some numerical results that increasing the angular width of the incident beam leads to decoherence of the quantum diffraction peaks and one approaches the classical limit. However, the incoherence of the beam in the parallel direction does not destroy the diffraction pattern. We consider the specific example of Ar atoms scattered from a rigid LiF(100) surface.