6 Autograph letters signed to Samuel Goudsmit. 13 pp. total

His Scientific Archives Were Destroyed in a Bombing Raid; Only 189 Letters Survive, of which the Five Offered Here are Previously Unknown

Paschen, Friedrich (1865-1947). 6 Autograph letters signed to Samuel Goudsmit (1902-78). Charlottenburg or Berlin, January 15, 1932-June 13, 1935. 13pp. total on 7 sheets measuring either 237 x 208 mm. or 286 x 223 mm. Creased where previously folded, with minor soiling and wear along creases, otherwise fine. English translations included.

Excellent series of very technical scientific letters to Goudsmit discussing his ongoing investigations in spectroscopy, particularly problems of hyperfine structure and the Zeeman effect. Described by one biographer as "probably the greatest experimental spectroscopist of his time" (quoted in DSB), Paschen, in the words of Niels Bohr, had a "happy intuition" that led him always to "pursue experimentally those problems the investigation of which proved to be of decisive significance for the extension of general theoretical conceptions" (quoted in DSB).

Paschen's experimental work in spectroscopy provided some of the most revealing clues to atomic structure; it was particularly useful to Arnold Sommerfeld, whose modifications of Bohr's theory of atomic structure had impressed Paschen so much that he devoted six months to confirming the theory's predictions. During his tenure as professor of physics at the University of Tubingen (1901-24), Paschen made Tubingen into Germany's most important center of atomic spectroscopy. In 1924 he succeeded Nernst as president of the Physikalische-Technische Reichsanstalt, the highest post to which a German experimental physicist could aspire; on May 1, 1933, after the Nazi's seizure of power in Germany, he was forced to retire in favor of the pro-Nazi physicist Johannes Stark. Despite this setback, Paschen was able to continue his laboratory research for a few years afterward-although at the cost of considerable difficulty and personal humiliation, some of which is hinted at in the letters offered here.

These six letters from Paschen to Goudsmit are almost exclusively concerned with the scientific investigations performed by the two men during the years 1932-1935; they exemplify the interplay between theoretical and experimental physics so characteristic of Paschen's career. This is particularly apparent in the last four letters (August 22, 1933-June 13, 1935), in which Paschen provided Goudsmit with detailed information on the results of his latest researches: Thank you so much for kindly sending me your and Mr. Bacher's work on anomalies in hyperfine structures. At the same time please accept my cordial congratulations on your wonderful quantitative settlement of the disputed questions regarding Al II and Hg I. For Al everything now appears to be in perfect order. Also, in the meantime I have become convinced that two fine structure components of the two lines 4 1,3F3-nG are simple and do not have any weak companions. Accordingly, the term nG must be split with 0.48 cm-1, as you conclude, and not with 0.35 as I assumed. . . . It is fascinating how the same problem is approached independently from several angles. However, your work is very important because you derive quantitative, definitive results. And in his letter of Aug. 22, 1933: You were so kind to write me concerning the theoretical fundamentals for comprehending the fine structures of the lines of Al II. Since you went to such trouble, I assume that the progress of my work on this topic would be of interest to you and would like to inform you that the publication thereof will not be forthcoming for some time. . . . I have made a study of extensive new material and have progressed to the point that the entire spectrum can be based quite soundly on the values of the fine structure terms. . . . All triplet terms now have two different numerical values corresponding to j + 1/2 and j - 1/2, which are based on the fine structure splitting values that you provided. . . .

Paschen's letter of April 28, 1935 praised Goudsmit and Pauling's The Structure of Line Spectra (1930) and discussed Sch¸ler's article on the hyperfine structures of europium: The book by Pauling and Goudsmit is today the "bible" of the practical spectroscopist. However, your experimental studies, the fundamentals of hyperfine structures, conducted in collaboration with Back, belong to the classic works of spectroscopy. . . . An article by Sch¸ler and a colleague on hyperfine structures of lines of europium recently appeared in Zeitschrift fur Physik [Journal of Physics]. The authors find violations of the Laude rule and attribute this to the geometric arrangement of nucleus and electron shell. This will certainly be of interest to you. I would like to point out that in this article results of my measurements are also mentioned and are presented in support of the proposed hypothesis. The spectrum In II contains a great many deviations from the Laude rule. However, in any event they involve detectable disturbances of the type that you discussed in the case of mercury two years ago.

And in the final letter of June 13, 1935: Your kind letter of May 17 is of great value to my current research and I am very grateful for the effort you put into it. At this time I cannot say very much about the calculations you provided, but you have inspired a renewed testing of fine splittings that will require much time before it is ultimately conclusive. [A detailed numerical analysis of the fine splittings of various terms follows.] . . . I am conveying these details to you since you appear to hold the key for the peculiar splittings of the 1,3F3 terms. I myself am so deeply involved with the measurements and thereby so overloaded with numbers that I cannot yet delve into the theory. However, clarifying the F terms interests me and does not seem to be difficult for you. . . . Similar to the Runge rule for the anomalous Zeeman effects, only whole-number fragments of the splitting factor A = a or the span S = will thus always be present. In this regard the fine structure is indicated as Zeeman effect of the nuclear moment. This is my interpretation as an experimental physicist, even if it is theoretically self-evident. The conclusion I was inclined to draw, that namely absolutely no fine structure splittings can occur other than such whole-number fragments, is however much more difficult to substantiate than with the Zeeman splittings, since the splittings are too small and the measurements lack the required accuracy.

The first two letters in this series, although briefer, are also of interest. In the first (January 15, 1932), Paschen gave Goudsmit permission to include the Rydberg spectrographic tables in a proposed study, and praised Goudsmit for "the wonderful and renowned articles you have published to advance theoretical and practical spectroscopy. The discovery of electron spin was a very great accomplishment. . . ." In the second (April 9, 1933), Paschen acknowledged Goudsmit's gift of a copy of his and Bacher's Atomic Energy States (1932): "I would like to express my highest gratitude for this valuable dedication and at the same time express my delight in seeing that you undertook this effort of producing this valuable but laborious work. All spectroscopists will be greatly indebted to you. So far I have read only the introduction and I have been delighted by the precise and clear presentation. I will now use the tables quite extensively, since they render the extremely tedious bibliographic search superfluous."

According to the Dictionary of Scientific Biography, all of Paschen's scientific manuscripts perished in November 1943 when his house was destroyed in a bombing raid; thus his letters to scientific colleagues such as Goudsmit represent his only surviving unpublished work in physics. The DSB gives an inventory of 184 known letters by Paschen to other physicists, taken from Kuhn's Sources for the History of Quantum Physics and the catalogues of the Niels Bohr Institute and the American Institute of Physics; of these 184 letters, only one to Goudsmit (the Jan. 15, 1932 letter included in the present collection) is cited. The collection of letters we are offering here, replete with formulae and scientific data, thus marks an important and valuable addition to the Paschen canon.