The transitions in such circumstances were quite different from those for which the effects of the nuclear spins could be neglected. Rabi pointed out that these discrepancies arose from the effects of the nuclear magnetic moments since some of the transitions were performed in such weak fields that strong or intermediate coupling between the nuclei and the electrons prevailed. However, some of the results of Frisch and Segre were not consistent with theoretical expectations. Transitions did not take place when the rate of change of the direction of H was small compared to the Larmor frequency. Which is the classical frequency of precession of a classical magnetized top with the same ratio γ 1 of magnetic moment to angular momentum. Frisch and Segre continued atomic beam experiments with adiabatic and nonadiabatic transitions of paramagnetic atoms and found, in agreement with Guttinger’s and Majorana’s theories, that transitions took place when the rate of change of the direction of the field was larger than or comparable to the Larmor frequency, Guttinger and Majorana developed further the theory of such experiments. Inspired by Darwin’s theoretical discussion, Phipps and Stern in 1931 performed the first experiments on paramagnetic atoms passing through weak magnetic fields whose directions varied rapidly in space. Physicist Sir Charles Darwin -the grandson of the great evolutionist-discussed theoretically the nonadiabatic transitions that make it possible for an atom’s angular momentum components along the direction of a magnetic field to be integral multiples of h/2 π both before and after the direction of the field is changed an arbitrary amount. The molecular beam magnetic resonance method arose from a succession of ideas, the earliest of which can be traced back to 1927, although that idea was rather remote from the principle of magnetic resonance.

The early molecular beam experiments did not use oscillatory fields and were of limited accuracy.

Rabi and his associates at Columbia in the 1930’s. The development of molecular beams as a valuable research technique was largely due to the work of Otto Stern and his collaborators in Hamburg in the 1920’s and early 1930’s, and to contributions from I. The earliest molecular beam experiment was that of Dunoyer 1 to show that Na atoms travel in straight lines in an evacuated tube.

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