3sphotoemission spectra of3dtransition-metal adatoms on graphite

Abstract

We report cluster model calculations on the $3s$ photoemission spectra of $3d$-transition-metal atoms adsorbed on graphite. The spectra are studied for the whole $3d$ series, for three different adsorption sites, and as a function of the adatom-surface distance h. For equilibrium h values, the adatoms are in a high spin (Hund's rule) ground state for all $3d$ elements and adsorption sites. Upon decreasing h, however, we observe in some cases a first-order spin transition to a low spin state. These spin transitions are accompanied by electronic transitions between different d orbitals and can be explained in an effective crystal-field model, where the crystal field is calculated in second-order perturbation in the adatom-surface hybridization. The photoemission spectra show intra-atomic exchange splitting as well as charge-transfer satellites from final-state core-hole screening by graphite $\ensuremath{\pi}$ electrons. The intensity of the charge-transfer satellites increases with d-electron number, i.e., from light to heavy $3d$ elements, which reflects the increasing electron-electron correlation. For the late $3d$ elements, charge transfer and exchange satellites are strongly mixed, and the spectra have essentially a three-peak structure. In this case, the observed splitting cannot directly be used as a measure of the magnetic moment of the adatom. Comparison is made with experimental data for V clusters on graphite.