Abstract
Coherence properties of projectiles, found relevant in ion-atom collisions, are investigated by analyzing the influence of the degree of coherence of the atomic beam on interference patterns produced by grazing-incidence fast-atom diffraction (GIFAD or FAD). The transverse coherence length of the projectiles, which depends on the incidence conditions and the collimating setup, determines the overall characteristics of GIFAD distributions. We show that for atoms scattered from a LiF(001) surface after a given collimation, we can modify the interference signatures of the angular spectra by varying the total impact energy, while keeping the normal energy as a constant. Also, the role played by the geometry of the collimating aperture is analyzed, comparing results for square and circular openings. Furthermore, we study the spot-beam effect, which is due to different focus points of the impinging particles. We show that when a region narrower than a single crystallographic channel is coherently illuminated by the atomic beam, the spot-beam contribution strongly affects the visibility of the interference structures, contributing to the gradual quantum-classical transition of the projectile distributions.
Highlights
The coherence conditions of the incident beam have been recently found to play an important role in atomic collisions involving crystal surfaces [1,2,3] and molecules [4] as targets, and atoms [5,6]
In this article we present an overview of coherence-length effects in GIFAD, illustrating how the incidence conditions, that is, the energy and mass of the projectiles [18], as well as the width of the incidence channel, affect the general shape of GIFAD patterns obtained by employing a given collimating setup
We have analyzed the influence of the total energy, the incidence channel, and the projectile mass on the general characteristics of GIFAD patterns produced by an atomic beam that collides grazingly on a LiF(001) surface, after passing through a fixed collimating setup
Summary
The coherence conditions of the incident beam have been recently found to play an important role in atomic collisions involving crystal surfaces [1,2,3] and molecules [4] as targets, and atoms [5,6]. [3,16,17] it was shown that the experimental collimating scheme noticeably affects GIFAD distributions, allowing one to examine two different interference mechanisms—inter-channel or intra-channel interferences—by varying the size of the collimating aperture This behavior is related to the transverse length of the surface area that is coherently illuminated by the incident beam, whose determination is indispensable for an appropriate description of the experimental spectra. To derive the extent of the surface region that is coherently illuminated by the atomic beam after collimation we resort to the Van Cittert-Zernike theorem [17,38], which is here extended to consider different geometries of the collimating slit This information is used to determine the size of the coherent initial wave packet to be evolved within the SIVR approach.
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