Crystal-field splitting in coadsorbate systems: c(2X2) CO/K/Ni(100)

It is demonstrated how the crystal field splitting (CFS) fine structure can be used to characterize a coadsorbate system. We have applied K 2p x-ray a bsorption spectroscopy (XAS) to the c(2X2) CO/K/Ni(100) system. The CFS fin e structure is shown to be sensitive to the the local atomic environment, t he level of interaction, and the chemical state of the alkali atoms. From a ngle dependent XAS measurements, combined with x-ray photoelectron spectros copy, a significant K-CO electrostatic adsorbate-adsorbate interaction is f ound, whereas the K-Ni interaction is substantially weaker. The present res ults provide evidence for a coadsorbed overlayer best described in terms of the properties associated with an ionic (two-dimensional) crystal.