Adsorption and diffusion of Li on an Ru(001) surface: an NMR study

Abstract

Adsorption, diffusion and desorption of lithium atoms on and from an Ru(001) surface have been investigated for various lithium coverages by β-decay-detected nuclear magnetic resonance (β-NMR), by thermal desorption spectroscopy and by work function measurements. For low lithium coverages, β-NMR has been employed to measure the spin lattice relaxation rate between 200 and 1250 K surface temperature. The local density of states at the Fermi energy (LDOS( E F)) extracted from these data at 800 K surface temperature turns out to be coverage independent up to 0.15 of a monolayer. Thus, up to this coverage the adsorbate wave functions do not overlap. The extracted value of LDOS( E F)=(0.13±0.01) eV −1 Å −3 agrees with recent theoretical predictions and excludes a complete ionic bond. Diffusion has been studied through observation of spin lattice relaxation rates for low coverages as a function of surface temperature between 200 and 1250 K and at 200 K, as a function of coverage, up to a full monolayer. The analysis of the data exhibits the existence of two distinctly different diffusion barriers on the surface, valued at 0.46 and 0.20 eV. The former is attributed to jumps from step to terrace sites, whereas the latter is the barrier between adjacent terrace sites. The desorption energy of lithium atoms, deduced from thermal desorption spectroscopy data, decreases from 3.2 eV at a coverage of 10 −3 of a monolayer to about 2.5 eV at percent coverages. This strong variation shows most likely the preferred population of defect sites at very low coverages.