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 coadsor-bate system. We have applied K 2p x-ray absorption spectroscopy (XAS) to the c(2x2) CO/K/Ni(100) system. The CFS fine structure is shown to be sensitive to the the local atomic environment, the level of interaction, and the chemical state of the alkali atoms. From angle dependent XAS measurements, combined with x-ray photoelectron spectroscopy, a significant K-CO electrostatic adsorbate-adsorbate interaction is found, whereas the K-Ni interaction is substantially weaker. The present results provide evidence for a coad-sorbed overlayer best described in terms of the properties associated with an ionic (two-dimensional) crystal.