Ettore Majorana and the birth of autoionization E. Arimondo∗,† Charles W. Clark,‡ and W. C. Martin§ National Institute of Standards and Technology Gaithersburg, MD 20899, USA (Dated: May 19, 2009) Abstract In some of the first applications of modern quantum mechanics to the spectroscopy of many-electron atoms, Ettore Majorana solved several outstanding problems by developing the theory of autoionization. Later literature makes only sporadic refer- ences to this accomplishment. After reviewing his work in its contemporary context, we describe subsequent developments in understanding the spectra treated by Ma- jorana, and extensions of his theory to other areas of physics. We find many puzzles concerning the way in which the modern theory of autoionization was developed. ∗ Permanent address: Dipartimento di Fisica E. Fermi, Universit`adi Pisa, Italy †Electronic address:
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[email protected] 1 Contents I.Introduction 2 II.TheState of Atomic Spectroscopy circa 1931 4 A.Observed Spectra 4 B.Theoriesof Unstable Electronic States 6 III.Symmetry Considerations for Doubly-Excited States 7 IV.Analyses of the Observed Double-Excitation Spectra 8 A.Double Excitation in Helium 8 B.TheIncomplete np2 3P Terms in Zinc, Cadmium, and Mercury 9 V.Contemporary and Subsequent Work on Autoionization 12 A.Shenstone’s Contemporary Identification of Autoionization 12 B.Subsequent foundational work on autoionization 13 VI.Continuing Story of P − P0 Spectroscopy for Zinc, Cadmium, and Mercury 14 VII.Continuing Story of Double Excitation in Helium 16 VIII.Autoionization as a pervasive effect in physics 18 Acknowledgments 19 References 19 Figures 22 I.