  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.
