Condensation of Na metal on graphite studied by photoemission

Above a submonolayer threshold coverage of around 0.2 monolayer (ML) Na deposited on graphite forms adsorbed islands. Via photoemission from quantum well states (QWS's) the islands are detected down to less than 1% surface coverage and are characterized with respect to thickness via the QWS binding energies. Below 90 K monolayer islands form while at around 110 K the thickness of the initially observed islands is 3 ML. Below 90 K and for coverages slightly above the submonolayer threshold the islands form slowly enough from a dispersed phase that the process can be monitored in real time. Below the threshold coverage the existence of a dispersed Na phase is indicated by the changes of the work function and of the energies for graphite emission lines. One graphite line, which is due to a final state populated by secondary electrons, allows observation of atomic-layer-dependent energy shifts for the substrate. The shifts indicate that Na valence electrons are transferred predominantly to the topmost carbon layer. The work function change and the energy shifts saturate near below the coverage needed for islands to form. The QWS energies for 1 ML Na are well understood in terms of a quasi-two-dimensional free-electron gas having the density expected for a nearly neutral monolayer of Na metal.