Experimental Observation of Direct Transitions in Angle-Resolved UV Photoemission from the Al(111) Surface

Photon energies of 11.7, 16.85 and 21.2 eV were used to observe weak but strongly dispersive features in the angular photoemission in the (-10) and (-11) planes from an Al (111) surface. Structures seen in both detection planes can be excellently accounted for by direct bulk transitions of assisting reciprocal lattice vectors G002 and G220. The 11.7 eV spectrum for angles between 35° and 60° towards the [001] axis is highly reminiscent of that previously reported for this surface at 10.2 eV, which was interpreted in terms of a surface resonance band associated with the sp gap at the X-point. We confirm such a surface component in the spectra by discerning an emission in the opposite k||-direction where bulk transitions are absent. Following a recent suggestion by Rosei et al., we have used Fermi surface data and our own measurements to determine the momentum k of transitions originating at the Fermi level. Agreement with standard band-structures is good at the Fermi energy, while considerable discrepancy and uncertainty exists for the final energies.