First-principles study of hydrogen adsorption on Ru(0001): Possible occupation of subsurface sites

We have performed an ab initio pseudopotential total-energy study of hydrogen adsorption on the Ru(0001) surface. It is found that the surface threefold hollow sites have the lowest energy, as expected for close-packed metal surfaces. However, at higher coverage, the occupation of the subsurface octahedral sites is made possible by the repulsive interaction between nearest-neighbor surface hollow sites. Our results resolve the previous controversy of two hydrogen-binding states observed in the measurements of work-function changes and sticking coefficients.