Unoccupied electronic states of Ge(001) 2 × 1 by constant initial band photoemission

The unoccupied electronic band structure of the Ge(001)2 × 1 surface along the \̄gGJ̄J̄' line of the surface Brillouin zone is investigated in an energy regime from the vacuum level up to about 22 eV above the valence band maximum (VBM) by applying a new constant initial band (CIB) mode of angle-resolved photoelectron spectroscopy using synchrotron radiation. In this mode final energy and photon energy are simultaneously scanned in accordance with energy and momentum conservation using a preselected dispersive initial band. We have chosen the well known occupied dangling-bond band as initial band which lies close to the uppermost bulk band at k⊥ = 0. Several bulk emission features are observed, showing good agreement with pseudopotential bands. In addition, about 13 eV above VBM we observe an emission revealing the periodicity of the reconstructed surface being attributable to a surface state. Constant initial energy spectra taken at normal emission further support its two-dimensionality