Photoemission from surface states and surface resonances on the (100), (110), and (111) crystal faces of aluminum

The angular-resolved energy distributions for photoelectrons emitted from the (100), (110), and (111) crystal faces of aluminum are presented for photon energies in the range 7.0-11.6 eV. The emission in this photon energy range is dominated by surface photoemission excited by the normal optical field or the surface-plasmon field. Three structures in the spectra are identified as emission from electronic surface states that have been predicted in theoretical calculations. Two of these surface-state bands, seen in emission from the (100) and (110) crystal faces, are associated with the $\mathrm{sp}$ band gap around the $X$ point in $\overrightarrow{\mathrm{k}}$ space. We interpret a structure in the emission from the (111) crystal face as due to a surface resonance band also associated with the $\mathrm{sp}$ band gap around the $X$ point. No conclusive evidence for the existence of structures in the spectra due to direct transitions in the bulk has been obtained.