X-ray standing-wave investigations of valence electronic structure

Abstract

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.

Ab initioabsorption spectrum of NiO combining molecular dynamics with the embedded cluster approach in a discrete reaction field

“…Moreover, the indirect measure of the electronic levels offered by PES/IPES techniques are inherently delocalized, giving no information on local transitions which could be important to get the complete picture of the electronic structure of NiO. A study with site-specific x-ray photoelectron spectroscopy (SSXPS) localized on Ni and O sites showed that the top of the valence band has a higher O(2p)-Ni(3d) mixing than previously thought 26 and the comparison of these techniques with x-ray absorption spectroscopy (XAS) proved that this kind of data obtained for the valence band is dependent on the methodology applied. 27 Furthermore, all these spectroscopic methods produce core holes in the sample during the measure, and these hole levels could be not completely innocent, requiring a sophisticated theoretical model to interpret the experimental results.…”

Making use of x-ray optical effects in photoelectron-, Auger electron-, and x-ray emission spectroscopies: Total reflection, standing-wave excitation, and resonant effects

“…The more conventional hard x-ray standing-wave technique involving Bragg reflection from crystal planes has been used extensively in studies of atomic structure at the Å scale, in both bulk crystals and near surfaces. 15,16 However, beginning with work by Kim and Kortright 19 and by Yang et al, 18,20 it has been shown that the standing-wave method can be extended into the soft x-ray regime ($0.5-1.5 keV) and it has subsequently been combined with high-resolution x-ray photoelectron spectroscopy to develop a non-destructive and element specific depth-resolved spectroscopic tool. 18,[20][21][22][23][24][25][26][27][28][29][30][31][32] This new soft x-ray technique has several significant advantages: for soft x-rays, the photoelectric cross sections are significantly larger than those in the hard x-ray regime, and the standing wave can be scanned through the layers, thus highlighting/de-highlighting a certain depth of a layer by either varying the photon incidence angle around the Bragg angle or varying the incidence angle around the Bragg condition, as noted earlier.…”

“…This method is similar in philosophy to that used in prior standing-wave photoemission studies making use of singlecrystal planar Bragg-reflection studies. 16 E. Standing-wave localization, total reflection, and waveguide effects in tailored multilayer configuration…”

“…13 Beyond these total reflection studies, x-ray optical effects have not been rigorously accounted for in the analysis of most photoemission or x-ray emission data, with the assumption being made that the x-rays penetrate much more deeply, and essentially without attenuation or modulation, into the sample, as compared to the depths from which the photoelectrons or fluorescent x-rays are emitted. Yet x-ray optical effects can play a particularly significant role for measurements done at grazing incidence angles, 9-13 near absorption-edge resonances, 14 in Bragg-like reflection from single-crystal planes [15][16][17] and synthetic multilayers, 8,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and with variable polarization so as to produce magnetic and other dichroic effects. 19,21,30 In beginning this discussion, it is worthwhile to discuss in more detail the specific types of x-ray optical effects that need to be considered: first are optical interference effects that modify the total electric field intensity vs. depth into the sample.…”

Photoelectronic characterization of heterointerfaces.

“…The beamline and end station are discussed extensively in other works. [20][21][22][23] Incident energies were selected from 2.2 keV to 4.0 keV as indicated in the respective figures. XPS data were analyzed using CasaXPS.…”