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The stationary states of an atom in a combined static and rotating magnetic field have been investigated by using two frequency transitions in an atomic-beam machine. Very accurate measurements of a strong rf magnetic field may be made by inducing transitions between the stationary states with a small "tickling" rf field. The relaxation of the selection rules associated with the z component of angular momentum makes normally forbidden hyperfine transitions visible.
W. Happer (1964) studied this question.