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I. I. RABI 313 choosing and with a novel technique which greatly simplified the experiments. The day after he sent in his doctoral thesis, he married Helen Newmark, who remained his lifelong companion and became the mother of his two daughters, Nancy Lictenstein and Margaret Beels. The Rabis soon went on a traveling fellowship to Europe, where he worked intermittently with Sommerfeld, Heisenberg, Bohr, and Pauli. The Stern-Gerlach experiment demonstrating the reality of space quantizations had earlier sparked Rabi's keen interest in quantum mechanics and so, while working in Hamburg with Pauli, Rabi became a frequent visitor to Stern's molecular beam laboratory. During one of these visits Rabi suggested a new form of deflecting magnetic field; Stern in characteristic fashion invited Rabi to work on it in his laboratory, and Rabi in an equally characteristic fashion accepted. Rabi's work in Stern's laboratory was decisive in turning his interest toward molecular beam research. RESEARCH AND TEACHING AT COLUMBIA Rabi returned from Europe to join the faculty at Columbia University and to begin atomic beam research in his own laboratory. In 1931 he and Gregory Breit developed the important Breit-Rabi formula, which showed how the magnetic energy of an atom and its effective magnetic moment vary with the strength of the external magnetic field. These changes occur because the atomic configuration varies from the electron angular momentum being primarily coupled to the nucleus at a low external field to being principally coupled to the external magnetic field at a high field. Utilizing the Breit-Rabi formula and an atomic beam apparatus which deflected the atomic magnetic moments with inhomogeneous magnetic fields, Rabi, V. Cohen, and others3 were able to determine the strengths of the electron
I. I. RABI 314 nucleus interaction and the magnitudes of nuclear spins and magnetic moments. Rabi further improved the precision of the measurements by noting from the Breit-Rabi formula that the effective magnetic moments are zero at certain magnetic fields, which give marked identifiable rises in the intensity of the undeflected atoms passing through an inhomogeneous field. Since the strengths of the fields giving zero moments depended on the hyperfine interactions and nuclear spins, Rabi, Fox, and other students and associates determined a number of hyperfine interactions by measuring the zero moment magnetic fields. Although the zero moment method did not work for atoms with nuclear spin 1/2, Rabi devised an alternative refocusing technique which did. Rabi also showed that the molecular beam deflection method could be adapted to measurements of the signs of nuclear magnetic moments by determining which transitions occurred when atoms went through a region of space in which the directions of the magnetic fields were successively reversed.3 Rabi developed the theory of such transitions in his important paper entitled "Space Quantization in a Gyrating Magnetic Field" (1937). In this paper Rabi assumed for simplicity, that the applied field changed its direction ("gyrated") at a fixed frequency. As a result, this paper provides the theoretical basis for all subsequent magnetic resonance experiments. Rabi initially applied his theory to fields which changed only in space rather than in time. A few months after the publication of that paper, following a visit by C. J. Gorter, Rabi directed the major efforts of his laboratory toward the development of the molecular beam magnetic resonance method with the magnetic fields oscillating in time. As shown in Figure 1, a molecular beam was deflected by
I. I. RABI 315 one inhomogeneous magnetic field and refocused by a similar field. In passing between the two fields, the molecules were subjected to a weak oscillatory magnetic field at frequency v. When v equaled the Bohr frequency ν0 = (Wf-Wi)/ h, transitions could take place with a consequent refocusing failure and a reduction in beam intensity. By measuring the beam intensity as a function of frequency, one could thereby determine the spacing of the molecular energy levels. FIGURE 1 The first successful molecular beam magnetic resonance experiment was that of Rabi, S. Millman, P. Kusch, and J. R. Zacharias in 1938, which determined the nuclear magnetic movement of Li. Soon thereafter, J. M. B. Kellogg, Rabi, N. F. Ramsey, and Zacharias applied the method to molecular hydrogen and discovered a multiplicity of resonance lines, whose separation arose from the magnetic interactions of the nuclear moments with each other and with the magnetic field caused by the rotation of the molecule. They found that the separations of the resonances
