Direct subsurface absorption of hydrogen on Pd(111)
We summarize and discuss some of the available experimental and theoretical data important for understanding the role played by subsurface sites in dissociative chemisorption calculations for the H$_2$/Pd(111) system. Then we use a semi-empirical potential energy surface (PES) to model the interaction of a H$_2$ molecule impinging on a Pd(111) surface. The London-Eyring-Polanyi-Sato (LEPS) construction has been extended to make direct subsurface absorption possible. A 2-dimensional wave packet calculation is used to find qualitative trends in the direct subsurface absorption and to reveal the time scales involved. We suggest that a partial in-plane relaxation occurs for the slowest incoming particles, thus resulting in a higher direct subsurface absorption probability for low energies.