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

Abstract

We present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along 110 . When the He normal energy decreases below 100 meV, i.e. He-surface distances Z > 2Å, the corrugation unexpectedly increases up to an impressive > ∼ 85%. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with Z. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignement properties of weakly-interacting overlayers.

Grazing incidence fast atom diffraction, similarities and differences with thermal energy atom scattering (TEAS)

“…A comparable effect has been observed qualitatively after scattering of helium on a graphene layer 77 using the HCW or on KCl using a semi-classical approach. 78 The strategy proposed here is to present the data in a form that is model-independent and which displays a high contrast.…”

Dynamical effects as a window into van der Waals interactions in grazing-incidence fast He-atom diffraction from KCl(001)

“…Physisorption wells are a well known feature of Hesurface systems [1], traditionally attributed to polarization and vdW attractive terms in the potential. Despite their typical shallowness (depth ∼ 10 meV), a recent study of grazing incidence fast atom diffraction (GIFAD) for the He-KCl(001) system showed that this potential feature results in a sharp increase of the surface corrugation and rainbow angle for incidence along the 110 channel with very low normal energy (E ⊥ < 60 meV) [2]. The key factor enabling this unexpected connection, besides GIFAD's high sensitivity to details of the surface electronic density, was the soft character of the He-surface interaction, i.e.…”

“…An additional appealing aspect of physisorption wells is their location, at He-surface distances where both polarization and van der Waals (vdW) interactions are expected to significantly contribute to the potential, a fact that has been mentioned in recent GIFAD studies for He-LiF(001) [3], Ne-LiF(001) [4] and He-KCl(001) [2]. However, all three cases focused on the 110 incidence channel, where the leading order dispersive force is likely given by polarization [5], which may hide vdW contributions.…”

“…In fact, in Ref. [2], calculations performed with a density functional theory (DFT) potential in which the Perdew-Burke-Ernzenhof (PBE) [6] exchangecorrelation functional implicitly includes polarization but neglects vdW contributions, quantitavely captured the experimental corrugation and rainbow features, while alternative functionals that included vdW yielded no significant improvement on the theory-experiment accord.…”

“…We perform a thorough analysis of the role dynamical effects play as probes of the physisorption well. The relevant physical quantities for this study are the surface corrugation and the rainbow angle, given their already reported sensitivity to dynamical effects which, for normal energy E ⊥ < 60 meV, are particularly enhanced by the attractive potential [2].…”