Spot-beam effect in grazing atom-surface collisions: from quantum to classical

Abstract

Grazing incidence fast atom diffraction (GIFAD) is a sensitive tool for surface analysis, which strongly relies on the quantum coherence of the incident beam. In this article we study the spot-beam effect, due to contributions coming from different positions of the focus point of the incident particles, which affects the coherence of GIFAD spectra. We show that the influence of the spot-beam effect on GIFAD patterns depends on the width of the surface area that is coherently lighted by the atomic beam. While for extended illuminations the spot-beam contribution plays a minor role, when a narrow surface area is coherently lighted, the spot-beam effect allows projectiles to explore different zones of a single crystallographic channel, bringing to light intra-channel interference structures. In this last case the spot-beam effect gives also rise to a non-coherent background, which deteriorates the visibility of the interference structures. We found that by varying the impact energy, while keeping the same collimating setup, it is possible to switch gradually from quantum to classical projectile distributions. 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 spectra.

Anomalous KCl(001) Surface Corrugation from Fast He Diffraction at Very Grazing Incidence

“…The two-dimensional angular distributions of scattered projectiles were collected by placing a detector formed by a Microchannel plate, a phosphor-coated screen and a CCD camera in the forward direction at a distance of 1.27 m. In Figure 1 we depict a GIFAD pattern for 600 eV 4 He atoms with E ⊥ =21 meV. The simulated pattern, that shows good accord with the experiment, was obtained with the Surface Initial Value Representation (SIVR) [20,21], a wellestablished semi-quantum approach to describe the scattering dynamics [22,23]. Note that collimation [21], surface defects and thermal vibrations may contribute to the elongation of the Bragg spots along the polar angle, though the latter two are not included in the simulations.…”

Coherence-Length Effects in Fast Atom Diffraction at Grazing Incidence

“…Finally, notice that the predicted dependence of GIFAD patterns on the transverse coherence length of the projectiles has been successfully contrasted with experimental data in Refs. [16,19]. However, extensive experimental research on the topic should be desirable.…”

“…, which provide information of local zones of the atom-surface potential inside a single channel. In a simplified picture, each atom probes the region of the effective equipotential contour (i.e., averaged along the axial channel) that is around the turning point of its classical trajectory, with l tr ≈ Na y being approximately the transverse length of the explored zone [19]. Thence, for l tr values about or lower than the half of the channel width, the partial distributions A…”

“…However, in addition, the spot-beam effect originates a non-coherent background, centered at Θ ≈ 0, which modifies the relative intensities of the interference structures, affecting their visibility. The visibility V (n), associated with the supernumerary maximum labelled with n in Figure 7a, with n = 0, ±1, ±2, ..., can be defined as [19,38]…”

“…Finally, for narrow coherent illuminations of the crystal surface, we analyze the contribution of the spot-beam effect, which is associated with random-distributed focus points of the incident particles. Such a spot-beam effect introduces a non-coherent background in GIFAD spectra, modifying the visibility of the interference signatures and contributing to the transition from quantum to classical projectile distributions [19].…”