HELIUM DIFFRACTION INVESTIGATIONS OF THE TRANSITION OF CHEMISORBED HYDROGEN INTO SUBSURFACE SITES ON PALLADIUM SURFACES

Palladium surfaces are well-suited model systems for hydrogen absorpti on and diffusion studies. In this paper we present - after reviewing p revious results on Pd(110) - new He-diffraction and thermal desorption spectroscopy (TDS) results on Pd(311). At 120 K, dissociative adsorpt ion occurs with an initial sticking coefficient near unity. With incre asing exposure, the formation of (2 x 1)H, (2 x 1)2H, (2 x 1)3H and c( 1 x 1) phases was observed, with coverages 0.25 ML, 0.50 ML, 0.75 ML a nd 1 ML, respectively. In all four phases, long-range order disappears at approximate to 170 It. These order-disorder transitions were found to be completely reversible for the (2 x 1)H and (2 x 1)2H phases upo n heating to 220 K and subsequent cooling to 120 K. For the (2 x 1)3H phase, however, a very different behaviour was observed: heating to 22 0 K leads to the migration of 0.25 ML chemisorbed H-atoms into subsurf ace sites, and the (2 x 1)2H phase is re covered after cooling down to 120 K. The fact that no evidence for a substrate reconstruction has b een found in any of tile three (2 x 1) phases suggests that the mechan ism involved in the population of subsurface sites on Pd(311) is very different from the one present on Pd(110), where it is driven by a pai ring-row reconstruction.