Bonding of an Isolated K atom to a Surface: Experiment and Theory

We present a new and general technique with which the local properties of the unoccupied states of an adsorbed alkali atom in the low coverage limit can be studied. The method, based on a combination of experimental core level spectroscopy data and calculations, is demonstrated for the K/graphite system. We show that core excitation and decay spectra of adsorbed Ar combined with the calculations form a most powerful tool in order to determine the unoccupied density of states for an isolated K adatom. We find that the $4s$ level of an isolated K atom on graphite is placed 2.7 eV above $E_F$, with a hybridization width of the order of only 0.1 eV.