Interaction of hydrogen with a Pd(111) surface

The electronic properties of H chemisorbed on the Pd (111) surface are studied with the use of angle-resolved photoelectron spectroscopy and a self-consistent pseudopotential mixed-basis theoretical calculation. Adsorption of H onto a substrate cooled below 250 K results in an ordered (11) H overlayer, deduced from the symmetry of the dispersion of the H-Pd bonding band, which is split off from the Pd bulk bands. A comparison between theory and experiment gives very good agreement when the H atoms occupy threefold surface sites with a H-Pd bond length of 1. 69. The bonding is predominantly H1s-Pd4d as can been seen in the charge-density plots and the angular-momentum-decomposed local densities of states. Room-temperature adsorption of hydrogen or adsorption at low temperatures followed by a brief warming to 270 K produces a form of chemisorbed H which is nearly invisible in the photoelectron spectra. The possible bonding configurations of this invisible state are discussed.