Isidor Isaac Rabi - Molecular Beam Laboratory

Molecular Beam Laboratory

On 26 March 1929, Rabi received an offer of a lectureship from Columbia, with a good annual salary of $3,000. The Dean of Columbia's Physics Department, George Pegram, was looking for a theoretical physicist to teach statistical mechanics and an advanced course in the new subject of quantum mechanics, and Heisenberg had recommended Rabi. Helen was now pregnant, so Rabi needed a regular job, and this job was in New York. He accepted, and returned to the United States in August on the SS President Roosevelt. Rabi became the only Jewish faculty member at Columbia at the time.

As a teacher, Rabi was underwhelming. Leon Lederman recalled that after a lecture, students would head to the library to try and figure out what Rabi had been talking about. Irving Kaplan rated Rabi and Harold Urey as "the worst teachers I ever had". Norman Ramsey considered Rabi's lectures "pretty dreadful", while William Nierenberg felt that he was "simply an awful lecturer". Despite his shortcomings as a lecturer, his influence was great. He inspired many of his students to pursue careers in physics, and some became famous physicists.

Rabi's first daughter, Helen Elizabeth, was born in September. A second girl, Margaret Joella, followed in 1934. Between his teaching duties and his family, he had little time for research, and published no papers in his first year at Columbia, but was nonetheless promoted to assistant professor at its conclusion. He subsequently became a professor in 1937.

In 1931 Rabi returned to particle beam experiments. In collaboration with Gregory Breit, he developed the Breit-Rabi equation, and predicted that the Stern-Gerlach experiment could be modified to confirm the properties of the atomic nucleus. The next step was to do so. With the help of Victor W. Cohen, Rabi built a molecular beam apparatus at Columbia. Their idea was to employ a weak magnetic field instead of a strong one, with which they hoped to detect the nuclear spin of sodium. It was predicted that if the nuclear spin were I, then the magnetic field would split the beam into (2I + 1) beamlets. When the experiment was conducted, four beamlets were found, indicating that the nuclear spin of sodium was 3⁄₂.

Rabi's Molecular Beam Laboratory began to attract others, including Sidney Millman, a graduate student who studied lithium for his doctorate. Another was Jerrold Zacharias, who, believing that the sodium nucleus would be too difficult to understand, proposed studying the simplest of the elements, hydrogen. Hydrogen's Deuterium isotope had only recently been discovered at Columbia in 1931 by Urey, who received the 1934 Nobel Prize in Chemistry for this work. Urey was able to supply them with both heavy water and gaseous deuterium for their experiments. Despite its simplicity, Stern's group in Hamburg had observed that hydrogen did not behave as predicted. Urey also helped in another way; he gave Rabi half his prize money to fund the Molecular Beam Laboratory. Other scientists whose careers began at the Molecular Beam Laboratory included Norman Ramsey, Julian Schwinger, Jerome Kellogg and Polykarp Kusch. All were men; Rabi did not believe that women could be physicists. He never had a woman as a doctoral or postdoctoral student, and generally opposed women as candidates for faculty positions.

At the suggestion of C. J. Gorter, the team attempted to use an oscillating field. This became the basis for the nuclear magnetic resonance method. In 1937, Rabi, Kusch, Millman and Zacharias used it to measure the magnetic moment of several lithium isotopes with molecular beams, including LiCl, LiF and dilithium. Applying the method to hydrogen, they found that the moment of a proton was 2.785±0.02 nuclear magnetons, and not 1 as predicted by the current theory, while that of a deuteron was 0.855±0.006 nuclear magnetons. Rabi suggested that the results implied that the deuteron was a magnetic quadrupole. For the creation of the molecular-beam magnetic-resonance detection method, Rabi was awarded the Nobel Prize for Physics in 1944.