The Doublet Riddle and Atomic Physics circa 1924

Abstract

* University of Rochester. original version of this paper was read at the annual meeting of the History of Science Society in San Francisco, 29 December 1965. A subsequent version was awarded the 1966 Henry Schuman Prize. For criticism and advice I am indebted to Jack Felman, John L. Heilbron, Martin J. Klein, and Thomas S. Kuhn; in particular, I profited from the text of a talk on The Crisis of the Old Quantum Theory, 1922-1925 given by Prof. Kuhn at the American Philosophical Society in April 1966. developments discussed in the present paper are touched upon in a number of recent retrospective accounts. R. Kronig, The Turning Point, in Theoretical Physics in the Twentieth Century, ed. M. Fierz and V. F. Weisskopf (New York: Wiley, 1960), pp. 539; F. Hund, G6ttingen, Kopenhagen, Leipzig im Riickblick, in Werner Heisenberg und die Physik unserer Zeit, ed. F. Bopp (Braunschweig: Vieweg, 1961), pp. 1-7; S. Goudsmit, Entdeckung des Elektronspins (Max-Planck-Medaille lecture), Physikalische Bliitter, 1965, 21 :445-453. 1 G. E. Uhlenbeck and S. Goudsmit, Ersetzung der Hypothese vom unmechanischen Zwang durch eine Forderung beziglich des inneren Verhaltens jedes einzelnen Elektrons, Die Naturwissenschaften, 1925, 13:953-954 (published 20 Nov. 1925; dated 17 Oct. 1925). 2 spectral lines (whether in the optical or X-ray region) emitted by an atom of a given element are described as possessing a series and a fine, or complex, or structure; the same is said of the energy levels, or spectroscopic terms, which characterize the stationary states of the atomthe spectral lines being emitted in transitions between stationary states. Series structure and structure refer quite directly to characteristic patterns on a spectrogram, i.e., a series of relatively widely separated lines, each such line being in fact often composed of several closely spaced components, constituting the fine (or complex) of the line. simplest of the omplex structures, both in the X-ray and the optical spectra, is the doublet, a closely spaced pair of spectral lines. Doublets, for example, are characteristic of the of the optical spectra of the alkali metals. By 1922 the term structure was rather carefully restricted to the X-ray spectra and to the optical spectra of atoms with but one electron, while the term complex structure was applied particularly to the optical spectra of atoms with more than one electron, but was also used generically. By 1924 the burden of the particular reference had largely been assumed by the term multiplet structure, while complex structure for a time held on as the generic term for groups of lines which, considered as units, formed a series. Until mid-1924 it was generally accepted that the use of the two expressions (series and complex, or fine, structure) in connection with both the optical spectra of higher atoms, on the one hand, and the X-ray and hydrogen-like spectra on the other hand, merely reflected superficial similarities of the patterns on the spectrograms. alteration of this view, and the consequent difficulties for the Bohr theory, are described below.