H2/LiF(001) diffractive scattering under fast grazing incidence using a DFT-based potential energy surface

Abstract

Grazing incidence fast molecule diffraction (GIFMD) has been recently used to study a number of surfaces, but this experimental effort has not been followed, to present, by a subsequent theoretical endeavor. Aiming at filling this gap, in this work, we have carried out GIFMD simulations for the benchmark system H 2 /LiF(001). To perform our study, we have built a six-dimensional potential energy surface (6D-PES) by applying a modified version of the corrugation reducing procedure (CRP) to a set of density functional theory (DFT) energies. Based on this CRP interpolated PES, we have conducted quantum dynamics calculations using both the multiconfiguration time-dependent Hartree and the time-dependent wave packet propagation methods. We have compared the results of our GIFMD simulations with available experimental spectra. From this comparison, we have uncovered a prominent role of the interaction between the quadrupole moment of H 2 and the electric field associated with LiF(001) for specific incidence crystallographic directions. We show that, on the one hand, the molecule's initial rotation strongly affects its diffractive scattering and, on the other hand, the scattering is predominantly rotationally elastic over a wide range of incidence conditions typical for GIFMD experiments.

Stereodynamics effects in grazing-incidence fast-molecule diffraction

“…These findings have been rationalized in terms of the interaction between the quadrupole moment of the molecule and the electric field associated with the ionic crystal, which differs from one crystallographic direction to another. 65 To verify that the conclusions drawn for H 2 /LiF(001) are valid for other surfaces with a marked ionic character, in this work, we have studied GIFMD for H 2 /KCl(001) focusing on the role played by the initial rotational state ( J , M J ) of the molecule. The comparison between our simulated diffraction patterns and the experimental ones available in the literature 67 reveals a puzzling stereodynamics effect.…”

“…Using the modified-CRP version described in detail in ref. 65, the 6D PES is written as: V 6D ( R ) = I 6D ( R ) + V 3D ( r 1 ) L z 0,δ z ( r 1 ) + V 3D ( r 2 ) L z 0,δ z ( r 2 ),where I 6D ( R ) is a 6D smooth function ( R representing the molecular DOFs, X CM , Y CM , Z CM , r , θ , φ – see Fig. 1) that can be easily interpolated.…”

“…As expected, these findings are, generally speaking, similar to those observed for H 2 /LiF(001). 65,66,75,76…”

“…Thus experimentally by measuring the atomic reflectivity probabilities one could infer the dissociative adsorption probabilities. Focusing on diffracted molecules, our previous studies for H 2 /LiF(001) 64,65 have revealed a strong dependence of the diffraction patterns, and diffractograms, on both the initial rotational state of the molecules ( J and M J ) and the crystallographic incidence direction. A similar dependence has also been found for normal incidence at low impact energy.…”

Thermal effects on helium scattering from LiF(001) at grazing incidence

“…This system has been extensively investigated with GIFAD at room temperature [16,[20][21][22][23][24][25], becoming a prototype of the GIFAD phenomenon. However, most of the theoretical descriptions were based on static crystal models [5,6,23,26,27], with the crystal atoms at rest at their equilibrium positions, while thermal vibration effects were studied in much less extent [14,16,17]. Furthermore, to our knowledge there are no available results of He-LiF GIFAD at temperatures higher than room temperature.…”