Hard X-ray photoelectron spectroscopy: sensitivity to depth, chemistry and orbital character

Photoelectron spectroscopy is growing in importance as a tool for characterizing not only the surface but also the bulk of solids. Photon fluxes of modern synchrotron radiation sources compensate the cross-section lowering of photoelectrons with kinetic energy in the hard X-ray range, the only electrons able to escape from bulk regions. We present examples of photoelectron emission experiments where we measured core and valence photoelectrons up to ∼5 keV kinetic energy. We studied the evolution of photoelectron emission spectra collected at increasing incident photon energy: core-level spectra of Samarium and its compounds indicate increased bulk-sensitivity, valence band spectra reveal significant cross-section changes of electrons with different orbital character. The chemical and depth sensitivity given by the energy dependence of the attenuation length has been quantitatively assessed in the case of layered GaAs/AlAs/GaAs. These are examples of the wide scientific importance that hard X-ray photoelectron spectroscopy is acquiring in many aspects of the study of solids.