Collective and single-particle excitations in thin layers of K on Ni(111)

Single-particle and collective excitations of K/Ni(111) were studied by high resolution electron energy loss spectroscopy (HREELS). Loss spectra were taken at 200 K as a function of K coverage over the 0-4 eV range of loss energy. Two peaks and a shoulder are characteristic of the submonolayer regime while, as the second atomic layer of potassium is formed, a single distinct feature dominates the loss spectrum. We assign the low-energy features in the submonolayer coverage regime to single-particle excitations: the loss at 0.5-0.6 eV is interpreted as due to electronic transitions from alkali a states to the Fermi level, while the one at about 1.68 eV (for a K coverage of 0.25 ML) is assigned to transitions from initial hybridized alkali s-Ni d(z)2 slates, to the Fermi level. We measured the dispersion curve omega(q(l)) of the single feature of the two layers of potassium. It follows the theoretical predictions for the surface plasmon of a thick K overlayer shelving an initial downward dispersion versus q(p)arallel to. The present results also give a hint of the nature of the alkali metal-to-substrate interactions from submonolayer regimes up to two layers of K on the Ni surface.