The characteristic lines in X-ray emission spectra correspond to atomic electronic transitions where an electron jumps down to a vacancy in one of the inner shells of an atom. Such a hole in an inner shell may have been produced by bombardment with electrons in an X-ray tube, by other particles as in PIXE, by other X-rays in X-ray fluorescence or by radioactive decay of the atom's nucleus.
Although still widely used in spectroscopy, this notation is unsystematic and often confusing. For these reasons, International Union of Pure and Applied Chemistry (IUPAC) recommends another nomenclature. The table below shows a few of the common electronic levels with their names in Siegbahn and IUPAC notation.
|High energy level||Low energy level||Name of the line||IUPAC notation|
|K (1s−1)||L3 (2p3/2−1)||Kα1||K-L3|
|L3 (2p3/2−1)||M5 (3d5/2−1)||Lα1||L3-M5|
|L2 (2p1/2−1)||M4 (3d3/2−1)||Lβ1||L2-M4|
|M5 (3d5/2−1)||N7 (4f7/2−1)||Mα1||M5-N7|