Dynamical core-hole screening in weak chemisorption systems

Abstract

Adsorbate core-hole screening in weak chemisorption systems is studied by the example of the ${N}_{2}/Ni(100)$ system. Fourth-order Green's-function calculations have been carried out on the $\mathrm{N}1s$ x-ray photoelectron spectra (XPS) of a linear ${\mathrm{NiN}}_{2}$ cluster used as a model of the chemisorption system. Good agreement with the experimental spectra is obtained. A powerful method, based on exact effective Hamiltonians (EEH's) constructed by means of a technique for a unique block diagonalization of Hermitian matrices [J. Phys. A 22, 2427 (1989)] is used for a detailed analysis of many-electron core-hole states. Two lowest-energy states in the $\mathrm{N}1s$ XPS of the ${\mathrm{NiN}}_{2}$ cluster, split by 1.47 eV (the experimental splitting in the chemisorption system is 1.3 eV), are classified as $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\pi}}{\ensuremath{\pi}}^{*}$ shakedown satellites, whereas two states dominating the region of the so-called ``giant satellite'' are attributed to the main states. The EEH technique allows us to study in detail the dynamical screening of core holes, which is shown to be responsible for the formation of the strong shakedown satellites. A comparative EEH analysis of the core-hole states of the ${\mathrm{NiN}}_{2}$ and NiCO clusters explains in simple terms the origin of the very different intensities of the giant satellites observed experimentally in adsorbate spectra of weak and strong chemisorption systems.