Alkali-metal-deposition-induced energy shifts of a secondary line in photoemission from graphite

Abstract

Photoemission spectra from graphite recorded along the surface normal reveal a state $7.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level populated by secondary electrons. We have studied this emission line both for pristine graphite and after deposition of different alkali metals ($\mathrm{Na}$, $\mathrm{K}$, $\mathrm{Rb}$ or $\mathrm{Cs}$). Common for all the alkali metals is that initially upon deposition the work function decreases rapidly and then saturates at a submonolayer coverage. In the low coverage range the secondary line splits into two components one of which remains at nearly constant energy. The other shifts to lower energy upon deposition in proportion to the work function change $\ensuremath{\Delta}E=\ensuremath{\alpha}\ensuremath{\Delta}e\ensuremath{\Phi}$, where ${\ensuremath{\alpha}}^{\ensuremath{-}1}$ is approximately proportional to the radius of the alkali metal atom. Of main interest is the interpretation of two secondary lines which appear as the evaporation is continued beyond saturation of the work function for $\mathrm{K}$, $\mathrm{Rb}$ or $\mathrm{Cs}$ but not for $\mathrm{Na}$. Based on the common behavior for the three heavier alkali metals we revise an earlier interpretation on results for only $\mathrm{Na}$ and $\mathrm{K}$ and suggest as an alternative that the secondary doublet is characteristic of an ordered $2\ifmmode\times\else\texttimes\fi{}2$ atomic layer of alkali metal. The additional emission lines are then due to electrons occupying final states at the midpoints of the boundaries of the hexagonal Brillouin zone and which are diffracted into the normal direction.