Local electronic states in the topmost surface layer probed by metastable atom electron spectroscopy: N2 adsorbed and condensed on Ni(111)

Electron emission spectra obtained by thermal collisions of He*(2(3)S) metastable atoms with N-2 Molecules on a Ni(111) surface in the chemisorbed, physisorbed, and condensed phases were measured together with the gas-phase spectrum. In the gas-phase collision where the metastable atoms approach randomly oriented N-2 molecules, Penning ionization rates for the 3sigma(g) and 2sigma(u) states are almost identical reflecting their spatial extent, while that for the 1 pi(u) state is rather small. In the saturated overlayer at similar to50 K where the N-2 molecules are chemisorbed with the molecular axes perpendicular to the surface, the 3sigma(g)-derived state is drastically enhanced relative to the 2sigma(u)-derived state. This indicates that, upon chemisorption, two sigma states are strongly modified in space by mixing with each other to yield a strong charge localization, i.e., an outer N atom in the 3sigma(g)-derived state gains considerable charge whereas an outer N atom in the 2sigma(u)-derived state loses it. The orbital mixing was confirmed by crystal orbital overlap population obtained using density functional theory within a generalized gradient approximation. Our spectra at similar to20 K show that the physisorbcd N-2 molecules are located on the chemisorbed species with a parallel orientation in the monolayer region and then condense to form a multilayer. We also found a characteristic band shift and narrowing with increasing layer thickness and these findings are discussed in terms of two final-state relaxation effects.