Abstract

Quantum effects are expected in the dynamics of light adsorbates at surfaces, however they present a formidable challenge to experimentalists. We describe two recent studies demonstrating quantum effects in surface scattering and diffusion measured using the helium Spin-Echo technique, which allows resolution of atomic scale motion on pico- to nano-second timescales. We first consider the absorption and diffusion of hydrogen on the Ru(0001) surface where we observe a transition from classical behaviour to a deep tunnelling regime at the relatively high temperature of 140 K. Path Integral Molecular Dynamics, proton band-structure calculations, and Monte Carlo simulations, give insights into phenomena such as temperature-dependent friction and strong inter-adsorbate interactions. Secondly, we present the first measurements of surface dynamics of the Ionic Liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C2C1Im][Tf2N] on an Au(111) substrate, where quantum scattering is seen.