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 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 (Na, K, Rb or 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 ⌬E = ␣⌬e⌽, where ␣ −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 K, Rb or Cs but not for Na. Based on the common behavior for the three heavier alkali metals we revise an earlier interpretation on results for only Na and K and suggest as an alternative that the secondary doublet is characteristic of an ordered 2 ϫ 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.

Dependence of electronic properties of epitaxial few-layer graphene on the number of layers investigated by photoelectron emission microscopy

“…It is well known that bulk graphite produces intense SE emission at around 3 eV above the vacuum level ͑ϳ7.5 eV above the Fermi level͒. [19][20][21][22][23][24] In Figs. 1͑e͒ and 2͑a͒, however, there is no such intense emission line at this energy.…”

Electronic structure and growth of K, Rb, and Cs monolayers on graphite studied by photoemission

“…Furthermore, it is found that the proportionality constant decreases with increasing size of the alkali metal atom. 19 The split and shift of the secondary peak thus occurs for all alkali metals studied ͑Na, K, Rb, and Cs͒. At a critical coverage a new secondary peak, additionally downshifted by FIG.…”

“…18 The energy shift and splitting of this peak upon AM adsorption and the strong coverage dependence of its intensity provides information regarding coverage and growth of the deposited alkali metal. 19 Based on the observations made at low photon energies we suggested that for K the initially formed condensate is a subsurface monolayer 9 but an adsorbed one for Na. 20 Our suggested interpretation of the K data is in conflict with the previous interpretation in which the K condensate is ascribed to adsorbed islands.…”

Unoccupied electronic structure of graphite probed by angle-resolved photoemission spectroscopy

“…The strongest (σ * ) band with + 1 symmetry (interlayer band) appears as a SEE feature at 3 eV kinetic energy. [18][19][20] However, we could not observe this feature as we did not probe such low kinetic energies.…”