Some Weisskopf Contributions to Atomic Physics

Kurt Gottfried and David Jackson (Physics Today, February 2003, page 43) cited the width of spectral lines and the theory of resonance radiation as being Viktor Weisskopf's first contributions to physics. Although the authors subsequently focused on more fundamental problems, two research papers related to Viki's work have had a significant continuing impact on atomic physicists. One of those papers, based on Viki's thesis, deals with the Hanle effect and, more generally, with radiation from atomic levels that become degenerate in magnetic fields.¹

In the mid-1920s, polarization of resonance radiation was studied intensively in the US and Europe. Arnold Sommerfeld, an atomic physicist widely known for the Bohr- Sommerfeld theory, went to Göttingen, Germany, and visited quantum physicist James Franck, familiar for the Franck-Hertz experiment. Sommerfeld showed Franck a letter from Robert W. Wood and Alexander Ellett,² who had obtained some puzzling results on polarization in fluorescence that they could not explain by the Zeeman effect. Wood was well known for his work in physical optics in the early part of the 20th century. Franck called in his graduate student, Wilhelm Hanle, who worked in physical optics, and asked if he could understand Wood's findings. Hanle immediately gave his explanation in terms of the classical precession of a radiating electron oscillator in a magnetic field--the basis of what is now called the Hanle effect.

Franck and Sommerfeld did not quite believe it, so Hanle went to the laboratory. He worked all night, took pictures of the appropriate observations, came to Franck the next day, and declared, "This is my thesis!" Hanle was invited to present his work in Innsbruck, Austria, at a joint meeting of the Austrian and German physical societies. Before the meeting, Franck received much criticism and skepticism from colleagues who tried to explain the observations by the Faraday effect. Not wishing to have his student suffer humiliation, Franck canceled the presentation. To console Hanle, Franck invited him to lunch with some other physicists, after which one of them, an older gentleman, took Hanle aside and said, "Don't worry about this, young man. We all make mistakes, and I am sure that you will do something worthwhile in the future." Later, Hanle found out that man was Max Planck!

Viktor Weisskopf, in the early 1990s, standing beside the CERN apparatus designed to measure the Bohr-Weisskopf effect.

Another contribution Viki made to atomic physics is our understanding of the Bohr-Weisskopf effect, also known as the hyperfine-structure anomaly.⁵ Viki and Aage Bohr's theory accounts for the effect of the finite size of the nuclear magnetization on the hyperfine-structure interaction. Hans Kopfermann suggested the effect's existence in the first edition of his book on nuclear moments,⁶ but qualified the effect as much too small ever to be observed.

Francis Bitter's experiments⁷ at MIT stimulated the work of Bohr and Viki. Bitter observed that the interaction of the nuclear magnetization measured by nuclear magnetic resonance in a homogeneous magnetic field differs from its interaction with the slightly inhomogeneous magnetic field set up by the electron at the site of the nucleus. The interaction in the inhomogeneous field is dependent on the distribution of the nuclear magnetization. For many ordinary atoms, the effect is small; nonetheless, highly precise experiments allow extraction of nuclear wavefunction data beyond those provided by nuclear moments alone. For muonic atoms, the Bohr-Weisskopf effect can account for half of the hyperfine structure interaction. Experiments on the hyperfine structure of hydrogen-like ions of heavy elements also require knowing the Bohr-Weisskopf effect in order to extract the influence of quantum electrodynamics.

The information in this letter is based on Wilhelm Hanle's recollections and on my remarks in 1983 at the Sixth International Conference on Laser Spectroscopy in Interlaken, Switzerland, on the occasion of the 60th anniversary of the Hanle effect (with permission of Comments in Atomic and Molecular Physics).

1. 1. V. F. Weisskopf, Ann. Phys. (Leipzig) 9, 23 (1931), equation 54.
2. 2. R. W. Wood, A. Ellett, Phys. Rev. 24, 243 (1924) ; Proc. R. Soc. 103, 396 (1923).
3. 3. G. Breit, Rev. Mod. Phys. 5, 91 (1933) , equation 159.
4. 4. F. D. Colgrove, P. A. Franken, R. R. Lewis, R. H. Sands, Phys. Rev. Lett. 3, 420 (1959) [CAS].
5. 5. A. Bohr, V. F. Weisskopf, Phys. Rev. 77, 94 (1950) [CAS].
6. 6. H. Kopfermann, Kernmomente, Akademische Verlagsgesellschaft, Leipzig, Germany (1940), p. 17.
7. 7. F. Bitter, Phys. Rev. 75, 1326 (1949).

I very much enjoyed Kurt Gottfried and J. David Jackson's appreciation of Victor Weisskopf. Another illustration of Viki's approach to life may be of interest to readers.

When Viki arrived at the University of Rochester in 1937, he found himself somewhat lonely and disconnected, as might have been expected. As a result, he spent substantial time with Arthur Gibson, who for many years had been in charge of the physics department's machine shop. Gibson undertook to assist Viki in becoming settled in his new city. Over time, they became close friends, and it was Gibson who served as Viki's formal sponsor when he applied for American citizenship.

Leaving Rochester for Los Alamos in 1943, and then moving on to MIT in 1946, Viki maintained his contact with Gibson. Whenever Viki visited Rochester--as he did frequently in the late 1940s and the 1950s while I was there--he would go talk with Gibson in the machine shop before contacting any of the physicists. Gibson was extremely proud of their relationship, and the visits demonstrated that Viki never forgot who his real friends were.