Abstract

Helium-3 surface spin echo spectroscopy (HeSE) is an experimental method capable of measuring atomic-scale surface processes associated with nano- to picosecond timescales. A standard application is the study of adsorbed species diffusing on a periodic corrugated surface, a description that applies to the diffusion of carbon and hydrogen on Ru(001), two recent and completely separate experiments on the Cambridge instrument.

Surface carbon was slowly precipitated from residual carbon in the Ru bulk, a process known to result eventually in epitaxial graphene. At elevated temperatures moiré diffraction features characteristic of graphene/Ru(001) typically develop over the course of hours; concurrently the spin echo method reveals jump diffusion of surface carbon in the nano- to picosecond window.

The quantum diffusion of H/Ru(001) has previously been characterised in detail with HeSE. A crossover from activated to tunneling-dominated diffusion occurs at a surface temperature of around 140K. The story continues for 2H (deuterium), in the wider context of isotope effects in quantum diffusion.