Colloquium: Ettore Majorana and the birth of autoionization

In some of the first applications of modern quantum mechanics to the spectroscopy of many-electron atoms, Ettore Majorana in 1931 solved several outstanding problems by developing the theory of autoionization. Later literature makes only sporadic references to this accomplishment. After reviewing his work in its contemporary context, we describe subsequent developments in understanding the spectra treated by Majorana and extensions of his theory to other areas of physics. Several puzzles are found concerning the treatment of Majorana's work in the subsequent literature and the way in which the modern theory of autoionization was developed.