X-Ray Standing-Wave Investigations of Valence Electronic Structure

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 interference field that is produced by the superposition of the incident and reflected x-ray beams. These crystals span the entire metallic, covalent, and ionic range of solid-state bonding. It is demonstrated that the valenceelectron emission is closely coupled to the atomic cores, even for electron states close to a metallic Fermi edge. Using the bond-orbital approximation, the x-ray standing-wave structure factor for valence-electron emission is derived in terms of the bond polarities and photoionization cross sections of the atoms within the crystalline unit cell and compared to experiment. Additionally, we demonstrated that by exploiting the spatial dependence of the electric-field intensity under Bragg condition, site specific valence electronic structure may be obtained. The technique is demonstrated for GaAs and NiO

The Si(111)/Cu interface studied with surface sensitive techniques

The formation of the Cu/Si interface is described on the basis of joint photoemission (valence band and Si 2p core levels) and Auger lineshape (SiL2,3VV) analysis. The system is characterized by an extended mixed phase where a silicidelike compound of average stoichiometry Cu3Si is formed and appears to be stable for an extended range of Cu coverages and annealing temperatures. The intermixing is strongly temperature dependent, but the chemical reaction between Cu and the top layers of Si can proceed even at 100 K

Evidence for [1s2p]3p shake-up channels in compounds and oxides of third-period elements

X-ray photoemission spectra (XPS) of NaF, MgO, Al2O3, SiO2, InP, and NaH2PO4 collected using synchrotron radiation in the 1900-3400-eV energy range are reported. Accurate XPS spectra have been collected around the Na, Mg, Al, Si, and P 1s photoemission peaks including a region extending for a few hundred eV on the high-binding-energy side. Clear features associated with additional excitations of 2p electrons, more evident in low-Z elements, have been identified. The fine structure of these satellites, dominated by the lower-upper splitting, has been studied in different compounds as a function of the photon energy. Energy positions and splitting are consistent with theoretical predictions

L-edge x-ray absorption resonances in palladium silicides and palladium metal

The Pd L2, 3 white line absorption has been measured for Pd metal, Pd2Si, and PdSi to probe the variations in occupation of the Pd 4d states due to the rehybridization of the d band in the silicides. A reduction of the density of occupied states of d character in Pd2Si and PdSi with respect to Pd metal is found, in agreement with partial-density-of-states calculations for the hybrid-bonded silicides