X-ray photoelectron spectroscopy and diffraction in the hard X-ray regime: Fundamental considerations and future possibilities

The prospects for extending X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) measurements into the hard X-ray regime of 5–15 keV excitation energies are discussed from a fundamental point of view, in some cases using prior results obtained in the 1–2 keV range as starting points of discussion, together with theoretical estimates of behavior at higher energies. Subjects treated are: the instrumentation improvements needed to optimize peak intensities; the tuning of experimental conditions to achieve bulk or surface sensitivity; the use of grazing incidence to suppress spectral backgrounds; the use of standing waves created by Bragg reflection from crystal planes or synthetic multilayers to achieve position-sensitive densities of states, compositions, and magnetizations; photoelectron diffraction and Kikuchi-band effects as element-specific local structure probes; and valence-level measurements, including the role of non-dipole effects and mechanisms leading to complete Brillouin zone averaging and density-of-states like spectra. Several distinct advantages are found for such high-energy extensions of the XPS and XPD techniques.