Abstract
Book review: E. Back and A. Landé. Zeeman Effect and Multiplet Structure of Spectral Lines.
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Bibliography
E. Back and A. Landé. Zeemaneffekt und Multiplettstruktur der Spektrallinien. Berlin, J. Springer, 1925, pp. XII + 213.
E. Back and A. Landé. The Zeeman Effect and the Multiple Structure of Spectral Lines.
The first issue of the new series Struktur der Materie, edited by Born and Franck, is devoted to one of the most burning questions in contemporary atomic physics: the Zeeman effect and the multiple structure of spectral lines. It would be hard to find another field of atomic physics whose development in recent years has proceeded at such a stormy, continuously increasing pace and has been so rich in fruitful results.
Only three years ago, only the simplest types of multiple structure of spectral lines were known—doublets (for example, in sodium) and triplets (for example, in mercury). At the present time, however, not only lines of higher multiplicity have been studied (up to eightfold lines, the so-called octets), but essentially new types of the so-called multiplets1 of second degree have also been found. A connection has been established between the multiplicity of lines and the position of the element in the periodic system. New types of the Zeeman effect (up to 25 components), studied to a considerable extent through the work of Back, have been investigated. An exact connection has been established between the multiplicity of lines and the character of their splitting in a magnetic field. A coherent theory of all these interrelated phenomena has been created, to a considerable extent through the work of Landé—a theory which, to be sure, has a formal-arithmetical character (Zahlenmysterium, in Sommerfeld’s apt expression), but whose predictions have nevertheless been brilliantly confirmed by experiment. With its aid it has been possible to “unravel,” i.e. to discover, serial regularities and to give a quantum interpretation to spectra before whose extraordinary complexity we were quite powerless only recently (for example, the spectra of iron, neon, manganese, etc.).
However, in our opinion the special importance of studying this field of physics lies not in the abundance—however interesting—of still nevertheless partial—
of results, but in the meaning which it acquires for solving the fundamental problem of principle in constructing a rational physics of the atom. Here the now generally recognized inadequacy of the contemporary quantum theory and the necessity of its substantial deepening and modification have become apparent with the greatest clarity. The particular sharpness of the accumulated contradictions allows one to hope that, by studying precisely this group of phenomena, it will be possible to uncover their causes and to outline the foundations of a new mechanics of the atom1. On the other hand, even now one can point to a number of conclusions whose significance goes far beyond the limits of the group of phenomena that interests us. To illustrate the latter statement I shall give only two examples.
Until recently, quantum theory could approach the question of the intensity of spectral lines only by means of considerations based on the correspondence principle and having a provisional and purely qualitative character. The study of the relative intensity of multiple lines last year led for the first time to the establishment of exact quantitative regularities, bearing an arithmetical character characteristic of quantum theories. At present analogous regularities have also been found in the lines of the X-ray spectrum.
Another example. The distances (difference of frequencies) between the corresponding lines of spectral doublets and triplets, as was found last year, obey the same law as the distances between the lines of X-ray doublets and the lines of the fine structure in the spectrum of hydrogen. Meanwhile it is generally recognized that in the fine structure and in X-ray doublets there appears the dependence of the electron mass on its velocity (the so-called relativistic correction), whereas the splitting of the lines of the optical spectrum is due to different orientations of the orbit of the outer part of the atom relative to the inner part. In order to reduce identical consequences to a single cause, it is necessary fundamentally to modify our basic views either on the nature of the optical spectrum or on the nature of the X-ray spectrum. In recent months this dilemma has acquired an ever more acute character.
What has been said is sufficient to characterize the full importance of the phenomena to which the book by Back and Landé is devoted. Meanwhile, in view of the novelty and complexity of these phenomena and of their interpretation, they remain comparatively little known to broad circles of physicists. Therefore one must warmly welcome the appearance of a book giving the first systematic exposition of the new and complex material scattered through journals.
material, and belongs to the pen of two persons, an experimentalist and a theorist, to whose joint labors this field of physics owes such rapid and successful development.
The book is divided into two almost equal parts—the theoretical and the experimental. The first begins with an exposition of the well-known foundations of the classical and quantum theory of the Zeeman effect. The fundamentals of the quantum classification of spectra are set forth in detail. There are many numerical examples and graphical schemes facilitating understanding. The exposition is distinguished by simplicity and clarity. The results of investigations of the Zeeman effect available up to the autumn of 1924 are included almost in their entirety; the results of investigations of multiple spectra are presented less fully. The applications of the theory of the Zeeman effect to related phenomena are briefly characterized (8 pages) (the magneton, the Stern and Gerlach experiments, paramagnetism, the magneto-mechanical effect). Unfortunately, there is no exposition of Sommerfeld’s systematics of multiplets, which formally differs from Landé’s systematics and theory.
The experimental part is written vividly and interestingly. The description of the apparatus and arrangements used in the investigation of the Zeeman effect is of independent interest. Especially interesting is the description (37 pages) of the complex path leading from the raw results of direct measurement to the determination of the ideal “type” of magnetic splitting of lines.
The book is supplied with a valuable index of the literature from 1914 to the autumn of 1924. I have noticed only one significant omission—an important work by Sommerfeld in Annalen der Physik, vol. 73, p. 209. Two plates of photographs of spectral lines split in a magnetic field are magnificently executed.
Il. Tamm.