TEMPERATURE-DEPENDENT BEHAVIOR IN THE ADSORPTION OF SUBMONOLAYER POTASSIUM ON GRAPHITE

The adsorption of submonolayer potassium on graphite at 50 and 30 K has been studied using HREELS and LEED. The work function change as a function of potassium coverage at 90 K is also presented, and the shape of the profile related to the recently discovered discontinuous phase transition at a potassium coverage of about 0.1 ML. The results at different temperatures are compared and contrasted, and discussed in terms of the kinetics and thermodynamics of the system. New diffraction features are observed at 50 and 30 K, which are absent at 90 K. These are attributed to metastable phases which arise because of the reduced mobility of the potassium atoms at these lower temperatures. This interpretation is consistent with the comparatively greater disorder found at 30 K. In the HREELS spectrum, we find that excitations at 1.2, 1.5 and 2.2 eV, clearly observed at 50 and 90 K, are absent at 30 K. Our results indicate that these discrete excitations may be characteristic of the well defined edges of close-packed islands, absent at 30 K as a result of the greater structural disorder.