Direct subsurface absorption of hydrogen on Pd(111): Quantum mechanical calculations on a new two-dimensional potential energy surface

We have calculated a two-dimensional ~2D! potential energy surface ~PES! for H 2 interacting with aP d ~111! surface. The geometry considered is for H 2 approaching a bridge site and dissociating into neighboring hollow sites and the subsurface sites directly below these. Density functional calculations were performed using both the local density approximation ~LDA! and the generalized gradient approximation ~GGA!. The LDA PES gives the usual overbinding and shows no barrier ~relative to the bottom of the H2 potential! to subsurface absorption, while the GGA PES agrees with the experimental adsorption energies and has a large barrier. We have performed quantum mechanical wave packet calculations on the GGA PES to obtain the direct subsurface absorption probability. We have also calculated the barrier height’s dependence on a coordinate that can be associated with a local surface vibrational mode and the results suggest that this degree of freedom should be taken into account in the dynamical calculations. © 1997 American Institute of Physics. @S0021-9606~97!01722-4#