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

Abstract

Using helium atom scattering to identify adsorbate interactions and separate the effects of long and short range forces.

Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene

“…Helium atoms scatter coherently and the effects of correlations due to long-range forces between the monomers play a role in the scattering, especially at values of ΔK < 1 Å. We discuss the full picture later but first we identify the energylandscape for motion by removing the effects of pair-correlations among the adsorbates using established methods [40][41][42] . We use an approximate form for the scattering form factor 42 and estimates of the quasi-elastic structure factor (see Supplementary Note 4) to obtain the result for incoherent scattering and the corresponding single-particle dephasing rate in Fig.…”

Surface diffusion within the Caldeira–Leggett formalism

“…As can be clearly seen from eqn (41), the decoherence function is negative since the width s t , given by eqn (16), is always greater than the initial width s 0 . Furthermore, when D = 0 or T = 0, the width s t is…”

“…In diffusion of adsorbates/adparticles on surfaces sampled by He atoms instead of neutrons, the scattering is fully coherent 5 , that is, only I (Δ k , t ) and S (Δ k , ω ) are observables; I s (Δ k , t ) and S s (Δ k , ω ) being relevant only when the surface coverage is really very small since correlations between any of two particles are almost negligible. In this context, the momentum transfer Δ k as well as adsorbate trajectories R ( t ) are written in capital letters meaning that they are quantities parallel to the surface.…”

“…For example, this is the case for sodium atoms diffusing on a Cu(111) surface. 16 However, within the range of applicability of this approximation, one is able to provide analytical expressions for I and S , overcoming the explicit knowledge of I d or S d . Two extreme time regimes are well characterized in this open dynamics, the ballistic regime, at very short times, where the dynamics is friction and noise free and the Brownian or diffusive regime, at very long times, where the thermal equilibrium is reached with the surface.…”

Ultrafast Diffusion at the Onset of Growth: O/Ru(0001)

“…The fcc site may be relevant in explaining the peak. We do not believe that A is strongly influenced by the intensity scattering form factor, given that the form factor of isolated O atoms, which dominate the diffusion signature, is expected to decay monotonically [21,38].…”