the production of X-ray emission and absorption spectra and the use of such spectra for investigating the electronic energy structure of atoms, molecules, and solids. In addition, X-ray spectroscopy includes electron spectroscopy for chemical analysis (ESCA), which is the spectroscopy of photoelectrons and Auger electrons whose emission is induced by X-radiation. Also classified under X-ray spectroscopy are excitation potential spectroscopy and studies based on the method of isochromates, that is, the investigation of the intensity of continuous and characteristic spectra as a function of the voltage across the X-ray tube.

There are two types of X-ray emission spectra: primary spectra and fluorescence spectra. Primary spectra are obtained by bombarding with accelerated electrons the substance under study, which forms the target in the X-ray tube. Fluorescence spectra are obtained by irradiating the substance with primary rays. The emission spectra are detected by means of X-ray spectrometers, and their investigation is based on the dependence of the radiation intensity on the energy of the X-ray photons. The shape and region of X-ray emission spectra give information on the energy distribution of the state density of valence electrons and permit experimental determinations to be made of the symmetry of the valence electrons’ wave functions and the distribution of the valence electrons between the strongly bound localized electrons of an atom and the collective electrons of a solid.

An X-ray absorption spectrum is formed when a narrow section of the bremsstrahlung spectrum is transmitted through a thin layer of the substance under study. Data on the energy distribution of the density of free electron states are obtained by investigating the dependence of the substance’s absorption coefficient for X-radiation on the energy of the X-ray photons. The spectral positions of the boundaries of the absorption spectrum and of its fine-structure maxima permit the ion charge multiplicity in compounds to be found; in many cases, the multiplicity can also be determined from shifts in the principal lines of the emission spectrum.

X-ray spectroscopy also permits scientists to establish the symmetry of the immediate vicinity of an atom and to investigate the nature of the chemical bond. The X-ray spectra that arise when the atoms of a target are bombarded with high-energy heavy ions yield information on the distribution of the radiating atoms with respect to the multiplicity of internal ionizations. ESCA is made use of in determining the energy of the inner levels of atoms, in chemical analysis, and in determining the valence states of atoms in chemical compounds.