Site-specific X-ray photoelectron spectroscopy using X-ray standing waves

We describe how X-ray photoelectron spectroscopy (XPS) measurements when conjoined with dynamical X-ray Bragg diffraction can determine site-specific electronic structure information. This result depends on the dipole approximation for the valence photo-excitation process in the low-energy X-ray limit that we validate through angle-integrated X-ray standing-wave (XSW) measurements of single-crystal Cu. We demonstrate the site-specific XPS technique for the single-crystal rutile TiO2 valence band and extract the individual Ti and O photoelectron partial density of states. First principles density functional calculations agree with our data after the angular-momentum-dependent photo-ionization cross-sections are properly accounted for.