Unoccupied electron band structure of Na overlayers on Al(111)

Abstract

Using the technique of inverse photoemission spectroscopy in the isochromat mode, we have determined the unoccupied part of the electron band structure of Na/Al(111) for two ordered overlayers: (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}, ${\ensuremath{\Theta}}_{\mathrm{Na}=(1/3}$ monolayer (ML) and (2\ifmmode\times\else\texttimes\fi{}2), ${\ensuremath{\Theta}}_{\mathrm{NA}=1/2}$ ML. At normal incidence, we find peaks at energies 1.1 and 2.1 eV above the Fermi energy ${E}_{F}$ for the low-coverage phase and 1.8 and 2.7 eV for the high-coverage phase. These peaks disperse away from ${E}_{F}$ as a function of increasing parallel momentum of the incident electron. At larger angles a new state is observed in each phase which disperses towards ${E}_{F}$. We assign these peaks to unoccupied p and d levels of the Na/Al(111) overlayers and compare our measurements to a recent band-structure calculation of isolated alkali-metal monolayers.