The first observations were for atomic spectra, let George Pake take over the story. An excellent theoretical discussion of the Breit Rabi energy levels is in Neil Atherton's "Principles of Electron Spin Resonance", Ellis Horwood, 1993, pp114.	The Breit Rabi formulae described the energy levels for H· atom, linking nuclear and electron spin angular momentum : Phys. Rev.,38,2082.