Coverage dependence of hydrogen absorption into Pd(111)

We investigate the absorption of H into Pd(111). In particular, we study the coverage dependence of the H-induced relaxation of the Pd lattice and the corresponding variation in the energy barrier for H absorption. We calculate, within density functional theory, the corresponding potential energy curves for the case where the H impinges on a frozen lattice surface, and on a lattice surface that is allowed to undergo H-induced relaxation. We consider the case where the final H coverage θ=1/9 and 1/4. From the results, it can be seen that, for both coverages, the energy barrier for H absorption takes a minimum value at a face-centered-cubic hollow site on the frozen lattice surface. Furthermore, the barrier is lowered by relaxation of the Pd lattice. The H-induced relaxation of the Pd lattice when θ=1/9 is more remarkable than when θ=1/4.