 Their definitions from perturbation theory are 

where formula_70 is the degeneracy of ground state formula_71, formula_72 labels states other than formula_71 or formula_74, formula_27 and formula_76 and formula_77 label the levels within states formula_71 and formula_74 and formula_72 (respectively), formula_81 is the energy of unperturbed state formula_82, formula_83 is the formula_10 angular momentum operator, formula_85 is the formula_10 spin operator, and formula_87 indicates the real part of the expression.A term arises due to Zeeman splitting of the ground or excited degenerate states, like what is shown in the (2) in figure 1 The small Zeeman splitting would make the oppositely signed transitions RCP light and LCP light almost cancel out with each other, which leads to the derivative shape of the band.B term is due to the field-induced mixing of states.