Abstract
Describing diffraction of atomic and molecular projectiles at fast grazing incidence presents a real challenge for quantum theoretical simulations due to the high incidence energy (100eV–1keV) used in experiments. This is one of the main reasons why most theoretical simulations performed to date are based on reduced dimensional models. Here we analyze two alternatives to reduce the computational effort, while preserving the real dimensionality of the system. First, we show that grazing incidence conditions are already fulfilled for incidence angles ⩽5°, i.e., incidence angles higher than those typically used in experiments. Thus, accurate comparisons with experiment can be performed considering diffraction at grazing incidence, but with smaller total incidence energies, whilst keeping the same experimental normal energy in the calculations. Second, we show that diffraction probabilities obtained at fast grazing incidence are fairly well reproduced by simulations performed at slow normal incidence. This latter approach would allow one to simulate several experimental spectra, measured at the same normal incidence energy for several incidence crystallographic directions, with only one calculation. This approach requires to keep the full dimensionality of the system.
Highlights
Since 2007, when first diffraction experimental results obtained using grazing incidence fast atoms diffraction (GIFAD) techniques were published [1,2], these techniques have highly improved [3]
It is worth pointing out that within this formalism the equations of motions for the expansion coefficients and the singleparticle functions (SPFs) are derived from the Dirac–Frenkel variational principle, which leads to a set of coupled equations that can be solved with less computational effort than in standard time-dependent wave packet (TDWP) propagation methods
We have performed Multi Configuration Time-Dependent Hartree (MCTDH) quantum dynamics calculations, based on a first-Principles 3D potential energy surface, aimed at evaluating several options to reduce the computational effort required to analyze experimental spectra obtained at fast grazing incidence conditions
Summary
Since 2007, when first diffraction experimental results obtained using grazing incidence fast atoms diffraction (GIFAD) techniques were published [1,2], these techniques have highly improved [3]. With the aim of proposing a method to reduce the computation effort, while keeping the accuracy of the theoretical analysis, we have tested to what extend quantum calculations at slow normal incidence could be used to simulate GIFAD experiments and by extension GIFMD (grazing incidence fast molecules diffraction) experiments. Our results indicate that grazing incidence conditions are reached at incidence angles higher than the ones used experimentally, and that results at slow normal incidence could be used, as a first approximation, to analyze fast grazing incidence experimental results At this point, we should remark that the two methods we propose to save computing time do not represent per se a substantial improvement respect to the ASC approach, they just represent other approaches to the problem. To keep the full dimensionality of the system presents an advantage over
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