Abstract
Quantum coherence of incident waves results essential for the observation of interference patterns in grazing incidence fast atom diffraction (FAD). In this work we investigate the influence of the impact energy and projectile mass on the transversal length of the surface area that is coherently illuminated by the atomic beam, after passing through a collimating aperture. Such a transversal coherence length controls the general features of the interference structures, being here derived by means of the Van Cittert-Zernike theorem. The coherence length is then used to build the initial coherent wave packet within the Surface Initial Value Representation (SIVR) approximation. The SIVR approach is applied to fast He and Ne atoms impinging grazingly on a LiF(001) surface along a low-indexed crystallographic direction. We found that with the same collimating setup, by varying the impact energy we would be able to control the interference mechanism that prevails in FAD patterns, switching between inter-cell and unit-cell interferences. These findings are relevant to use FAD spectra adequately as a surface analysis tool, as well as to choose the appropriate collimating scheme for the observation of interference effects in a given collision system.
Highlights
Nowadays grazing-incidence fast atom diffraction (GIFAD or FAD) can be considered as one of the most sensitive methods to investigate the morphological and electronic characteristics of ordered surfaces [1, 2]
Such a behavior is related to the transversal length of the surface area that is coherently lighted by the incident wave packet, whose knowledge becomes crucial for an appropriate comparison between experiments and simulations
We consider a collimating configuration similar to the one depicted in Fig. 1, with a square collimating aperture with size d = 0.2 mm, placed at a distance Lc = 25 cm from the surface plane. These values are in agreement with ordinary collimating setups for FAD [13], while the source parameters were chosen within the validity range of Eq (2) [15]
Summary
Nowadays grazing-incidence fast atom diffraction (GIFAD or FAD) can be considered as one of the most sensitive methods to investigate the morphological and electronic characteristics of ordered surfaces [1, 2]. In recent articles [13,14,15] it was shown that the size of the collimating aperture strongly affects FAD distributions, making it possible to observe two different mechanisms - Bragg diffraction or supernumerary rainbows - by varying the width of the collimating slit. Such a behavior is related to the transversal length of the surface area that is coherently lighted by the incident wave packet, whose knowledge becomes crucial for an appropriate comparison between experiments and simulations. All the theoretical results reported in Refs. [14, 15] were obtained by considering the same incidence
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