X-ray standing-wave investigations of valence electronic structure - art. no. 125115

We have examined the valence-electron emission from Cu, Ge, GaAs, InP, and NiO single crystals under the condition of strong x-ray Bragg reflection; i .e., in the presence of the spatially modulated x-ray standing-wave interfe rence field that is produced by the superposition of the incident and refle cted x-ray beams. These crystals span the entire metallic, covalent, and io nic range of solid-state bonding. It is demonstrated that the valence-elect ron emission is closely coupled to the atomic cores, even for electron stat es close to a metallic Fermi edge. Using the bond-orbital approximation, th e x-ray standing-wave structure factor for valence-electron emission is der ived in terms of the bond polarities and photoionization cross sections of the atoms within the crystalline unit cell and compared to experiment. Addi tionally, we demonstrated that by exploiting the spatial dependence of the electric-field intensity under Bragg condition, site specific valence elect ronic structure may be obtained. The technique is demonstrated for GaAs and NiO.