HYPERFINE STRUCTURE

IT is well known that hyperfine structure in spectra arises from the interaction of nuclear magnetic dipoles and the magnetic field due to the electron cloud. In the case of hydrogen the interaction energy is appreciably greater than the natural width of the energy levels only in the case of the ground level and the abnormally narrow (metastable) 2²S_½ level. The hyperfine structure of both these levels has been measured with great precision by atomic beam magnetic resonance methods. The ground level in particular has been repeatedly examined by this technique, and also by the method of paramagnetic absorption of microwaves.

Calculation of the hyperfine structure requires knowledge of the magnetic moment of the proton. If this were a Dirac particle, its moment would be eh/4πMc (one nuclear magneton), which stands in conflict with the value 2.79275 nuclear magnetons measured by a variety of methods. This experimental value, used in the simplest hyperfine structure formula, leads to substantial agreement between experiment and theory. Even so, there remains a discrepancy of about 0.1 per cent, which is greatly in excess of the experimental errors. This discrepancy is closely related to the interactions which give rise to the Lamb shift, as we shall now see.