Fig. 2.

Energy level diagram displaying fine and hyperfine structure splitting of ⁸⁷Rb. The fine structure splitting (left of panel) is a result of the interaction between the electron's orbital and spin angular momentum. The dimensionless number J defines the magnitude of the total angular momentum of the electron ($\left|\overrightarrow{J}\right|$) in a similar way to which $S$ defines the magnitude of the spin angular momentum of the electron. The hyperfine structure splitting is a result of the interaction between the total angular momentum of the electron (J) and the spin angular momentum of the nucleus (I), resulting in possible values for total angular electron momentum (F) of 2 and 1.