We summarize and discuss some of the available experimental and theore
tical data important for understanding the role played by subsurface s
ites in dissociative chemisorption calculations for the H-2/Pd(111) sy
stem. Then we use a semi-empirical potential energy surface (PES) to m
odel 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 two-dimensional wave
packet calculation is used to find qualitative trends in the direct su
bsurface absorption and to reveal the time scales involved. We suggest
that a partial in-plane relaxation occurs for the slowest incoming pa
rticles, thus resulting in a higher direct subsurface absorption proba
bility for low energies. (C) 1996 American Institute of Physics.