On the core hole screening mechanism in adsorbates in the absence of low lying π acceptor orbitals

Abstract

To investigate the effects of the absence of the low lying π* acceptor orbitals on the ligand core hole spectra of molecules adsorbed on metal surfaces, we considered the nitrogen 1s core hole spectrum of the NH 3/Ni system. We have performed ab initio 1h1p/2h1p and 2h2p/3h2p configuration interaction (CI) calculations of the nitrogen 1s core hole spectrum of NiNH 3 molecule, using an extended basis set. We considered several initial states. Despite the great variety of initial and final states considered, the present model suggests the presence of a satellite of significant intensity within a couple of eV from the main line (both of which can be actually made of several closely spaced components generated by additional many-body effects). The main active mechanism is relaxation of the Ni 4sp electron in an essentially d 94s(p) configuration towards NH 3 Rydberg and antibonding virtual orbitals. This is in line with the previous finding of Hermann and Bagus. The lack of the π* acceptor orbital in the ligand forbids the π charge transfer (CT) associated with dπ—π* backbonding, which is essential to the s—dσ promotion mechanism. Furthermore the absence of this π CT screening mechanism forces a pure σ CT screening from Ni sp to NH 3 Rydberg and antibonding orbitals, generating the appearance of significant σ—σ* satellites, albeit with a different origin.