Inter-adsorbate forces and coherent scattering in helium spin-echo experiments.

David J Ward,Arjun Raghavan,Salvador Miret-Artès,William Allison,John Ellis,Anton Tamtögl,Andrew P Jardine,Emanuel Bahn

doi:10.1039/d0cp04539j

Abstract

In studies of dynamical systems, helium atoms scatter coherently from an ensemble of adsorbates as they diffuse on the surface. The results give information on the co-operative behaviour of interacting adsorbates and thus include the effects of both adsorbate-substrate and adsorbate-adsorbate interactions. Here, we discuss a method to disentangle the effects of interactions between adsorbates from those with the substrate. The result gives an approximation to observations that would be obtained if the scattering was incoherent. Information from the experiment can therefore be used to distinguish more clearly between long-range inter-adsorbate forces and the short range effects arising from the local lattice potential and associated thermal excitations. The method is discussed in the context of a system with strong inter-adsorbate interactions, sodium atoms diffusing on a copper (111) surface.

Highlights

Helium atom scattering is well established as a sensitive probe of surface processes and for studies of the dynamics of pristine and adsorbate covered surfaces.[1]
Helium atoms are sensitive to changes in the electron density as adsorbed atoms and molecules move on an otherwise flat surface.[5,6,7]
We are concerned with the motion of adsorbates on a surface and the charge distribution from which the helium atoms scatter has a formfactor that must be retained if, as here, the intensity distribution is important to the analysis

Summary

Introduction

Helium atom scattering is well established as a sensitive probe of surface processes and for studies of the dynamics of pristine and adsorbate covered surfaces.[1]. In neutron scattering the moving particles are point-like and the form-factor is independent of DK to a good approximation.[11,12,13] We are concerned with the motion of adsorbates on a surface and the charge distribution from which the helium atoms scatter has a formfactor that must be retained if, as here, the intensity distribution is important to the analysis. Since co-operative motion requires some degree of interaction between the adsorbates, differences between the coherent and incoherent intensities reflect the nature of inter-particle forces. We consider sodium atoms moving on a copper (111) surface, for which both a model landscape and an interaction model are available.[20] We use simulations based on the Langevin molecular-dynamics framework to deduce the coherent and incoherent scattering intensities, upon the assumption of point scatterers. The form-factor allows us to obtain an incoherent scattering intensity from the measurements of coherent scattering

Analysis of coherent and incoherent scattering

Numerical simulations of coherent and incoherent scattering

Experimental measurements and the form-factor