Subsurface hydrogen at Pd(100) induced by gas-phase atomic hydrogen

We have studied the interaction of gas-phase atomic H(g) (D(g)) with H(a) ( D(a)) atoms preadsorbed on the Pd(100) surface. The preadsorbed atoms are f orced by the impinging atoms to be absorbed into the Pd substrate. The abso rbed hydrogen, H(b), was found to occupy the subsurface site just beneath t he surface and its saturation coverage was determined to be 1.06 +/- 0.16 m onolayer (ML). The cross sections for the absorption were found to be diffe rent between the cases with and without subsurface H atoms just beneath the surface species, and are estimated to be 2.1 +/- 0.6 and 5.3 +/- 1.5 Angst rom(2), respectively. Upon annealing the sample, H atoms in the subsurface sites diffuse into the bulk at 210 K, from which the activation energy for in-diffusion of 0.56 +/- 0.03 eV was determined, and desorb recombinatively at similar to 280 K. Upon further exposure to H(g) atoms, as much as 8 MLs of bulk hydride form near the surface region which are decomposed at 200 K giving rise to H-2 desorption.