Phase transitions and excitation spectrum of submonolayer potassium on graphite

We have investigated both the structures and the excitation spectra produced by submonolayer adsorption of potassium on the surface of graphite at 90 K, using high-resolution low-energy electron diffraction in conjunction with high-resolution electron-energy-loss spectroscopy. We show that at very low coverages a dispersed potassium phase is formed with a giant K-K spacing (around 60 {}); this phase continuously compresses with increasing adatom density until a critical coverage where a close-packed structure nucleates. Below the critical coverage, charge is redistributed between the alkali and the substrate to increase the frequency of the substrate plasmon mode; above the critical coverage, excitations attributed to metallic potassium are observed.