Hyperfine quadruplet

Electron Spin Resonance (ESR) spectroscopy was used to characterize sodium atoms in the highly non-stoichiometric Na Siigg clathrates [28, 88, 89]. In addition to an intense and broad line centered at g = 2.0021 due to the presence of Na clusters, four sharp lines of a hyperfine quadruplet with an isotropic hf splitting of 13.3 mT was observed, corresponding to isolated Na atoms (I = 3/2). From the observed hf splitting value, the spin density in the 3 s orbitals was estimated to be 45 % of one unit spin density [88]. With decreasing x values, the intensity of the cluster line decreases, whereas those of the hyperfine quadruplet lines increased. For the lowest x content, the contribution of the cluster line to the total intensity was very low and a quantitative determination of the residual x content was possible by comparison of the observed intensities with those of standards (diluted solutions of Cr " doped alum). The ultimate e sodium content in Na Siise was found to be close to 0.006, i.e. 35 ppm [89]. 

These quadruplet states lie from higher to lower energy, as shown in Fig. 11. The hyperfine interaction is effectively expressed by... 

The level-crossing and anticrossing phenomena have been observed for the first time for hyperfine components of atomic energy levels.The level-anticrossing spectroscopy was then applied to the study of very weak singlet-triplet (or doublet-quadruplet) coupling in atoms" and in diatomic molecules (see, e.g.. Refs. 124 and 125 and references therein). 

---