Photoelectron spectroscopy allows to determine  binding energy of core levels in solids. Each element has its own set of energies of core levels (alike a "fingerprint"), herewith energy of core levels, corresponding to different elements, quite well energetically separated. This technique allows to identify various elements by the photoelectron spectra, i.e., get information about elemental composition of investigated system. By defining energy peaks in the photoelectron spectra it is possible to get information not only about atoms of which elements are located at the solid surface, but also in which are they chemical state. The formation of chemical bonds between atoms of a solid body, accompanied by a redistribution of the electron density, can lead to a change in the energy of electrons that will, certainly, appear in the change of kinetic energy of photoelectrons.