ATOMIC MOMENTS AND HYPERFINE STRUCTURES

Abstract

The earliest magnetic resonance experiments were with non-paramagnetic molecules in 1Σ states which have no net electronic magnetic moment. They were later applied to paramagnetic atoms, which gave multiple energy levels due to the interactions of different electron and nuclear orientation states from which the hyperfine separations, Δνhfswere obtained. The paramagnetic atoms also permitted measurements of the electron magnetic moment and the magnetic moment due to the electrons orbital motion. To obtain the correctly calibrated values of nuclear magnetic moments, one needs to know the absolute value of both the frequency and the applied magnetic field as briefly discussed in Chapter 6. There are well known procedures for frequencies, but the problem is much more difficult for magnetic fields. In the first experiments the magnetic fields were calibrated with a conventional ballistic galvanometer. To obtain better precision, the next experiments measured the fields in terms of the magnetic moment of the electron, which was presumed to have the exact value predicted by Dirac. However, atomic beam experiments later showed this assumption was incorrect, but the fields could be correctly calibrated in terms of the orbital magnetic moment of the electron being one Bohr magneton. Nuclear magnetic moments are accurately expressed in nuclear magnetons in the tables of Chapter 6 and values of the hyperfine separations are given Chapter 9. The discovery that the hyperfine structure for atomic hydrogen differed from the predicted value stimulated the development of QED and the discovery of the anomalous electron magnetic moment. Accurate measurements of Δνhfs, are given in tables for many atoms.