X-ray notation is a method of labeling atomic orbitals that grew out of X-ray science.
Also known as IUPAC notation, it was adopted by the International Union of Pure and Applied Chemistry in 1991 as a simplification of the older Siegbahn notation.
[1] In X-ray notation, every principal quantum number is given a letter associated with it.
In many areas of physics and chemistry, atomic orbitals are described with spectroscopic notation (1s, 2s, 2p, 3s, 3p, etc.
), but the more traditional X-ray notation is still used with most X-ray spectroscopy techniques including AES and XPS.