Electronic structure of chemisorbed chalcogen atoms on Ni ( hkl ) surfaces

Spectroscopic and bonding properties of sulfur, selenium, and tellurium atoms chemisorbed on different crystallographic faces of Ni are studied using self-consistent local-density molecular-cluster models. Adsorbate-substrate interactions lead to significant modification of the Ni $3d$, $4s$, $4p$ conduction bands in addition to forming the $\ensuremath{\sigma}$, $\ensuremath{\pi}$ adatom levels located at \ensuremath{\sim}5 eV below the Fermi energy. Adatom-adatom interactions are treated by ${\mathrm{S}}_{2}$${\mathrm{Ni}}_{9}$ coupled clusters. Adatom bonding to the substrate is seen to be dominated by near-neighbor interactions through $\mathrm{sp}$ hybridization of the low-lying chalcogen $\mathrm{ns}$ level, and by $\ensuremath{\pi}$-bonding interaction of the $\mathrm{np}$ level. The (110) surface appears to have more ionic character than either (001) or (111). The calculated variation of adsorbate levels with adatom height above the metal surface supports low-energy-electron diffraction structural analyses. Partial densities of states derived from cluster orbital atomic populations are used to discuss features of photoelectron and Auger spectra.