Hydrogen-adsorption-induced phase transitions on Pt(100)-hex and the surface structure of Pt(100)-(1 x 1)H.

While adsorption of 100 L (1 L = 10(-6) Torr s) H-2 molecules at room temperature does not cause any phase transitions from a clean reconstructed Pt(100)-hex surface, a 10-L dose of dissociated H-2 can induce a phase transition from the Pt(100)-hex to an atomic-hydrogen-stabilized Pt(100)-(1 X 1)structure as observed using low-energy electron diffraction (LEED), This Pt(100)-(1 X 1) surface was subsequently cooled down using liquid nitrogen to a temperature around 80 K and LEED I-V (intensity vs electron energy) curves were measured at normal incidence. Dynamical tenser LEED calculations were performed using different surface structure models including neglecting hydrogen, ordered (1 X 1) hydrogen at atop, bridge, and fourfold hollow sites. It has been found that within the converging range of atomic displacements, the minimum overall Pendry R factors (R(p)) for the above models are 0.28, 0.35, 0.31, and 0.28, respectively. For the first model, Pt(100) interlayer relaxations down to the fifth layer were calculated. The fourfold hollow site model which also gave an overall small R(p) showed scattered R(p) for different beams and it is likely that the surface contained disordered hydrogen.