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 σ, π adatom levels located at {~}5 eV below the Fermi energy. Adatom-adatom interactions are treated by S₂{Ni}₉$ coupled clusters. Adatom bonding to the substrate is seen to be dominated by near-neighbor interactions through $sp$ hybridization of the low-lying chalcogen $ns$ level, and by ${π}$-bonding interaction of the $np$ 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.