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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: (√3 × √3)R30°, ΘNa=⅓ monolayer (ML) and (2×2), ΘNa=½ ML. At normal incidence, we find peaks at energies 1.1 and 2.1 eV above the Fermi energy EF for the low-coverage phase and 1.8 and 2.7 eV for the high-coverage phase. These peaks disperse away from EF 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 EF. 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.
