In spectroscopy experiments, a sample is illuminated with light and the various product particles (electrons, ions, or fluorescent photons) are detected and analyzed. The unifying feature is that some “property” of a material is measured as the x-ray (photon) energy is swept though a range of values. At the most basic level, one measures the absorption, transmission, or reflectivity of a sample as a function of photon energy.
Probes that use the vacuum ultraviolet (VUV) region of the spectrum (10–100 eV) are very well matched to the elucidation of bonding in solids, surfaces, and molecules; to the investigation of electron–electron correlations in solids, atoms, and ions; and to the study of reaction pathways in chemical dynamics. At the lowest end of this energy range (below 1 eV) we have infrared, far-infrared, and terahertz spectroscopies, which are well matched to vibrational modes and other modes of excitation.
Soft x-ray spectroscopies employ the excitation of electrons in relatively shallow core levels (100–2000 eV) to probe the electronic structure of various kinds of matter. Elemental specificity is the watchword for this kind of spectroscopy. Each element has its own set of core levels that occur at characteristic energies. The photon-energy tunability of synchrotron radiation is essential.
Hard x-ray spectroscopy is applied in a wide variety of scientific disciplines (physics, chemistry, life sciences, and geology) to investigate geometric and electronic structure. The method is element-, oxidation-state-, and symmetry-specific. It is a primary tool in the characterization of new and promising materials. It is also used in the elucidation of dilute chemical species of environmental concern.