Strongly allowed transitions

For a Kramers doublet the strongly allowed transitions are those in which the nuclear magnetic quantum number does not change. The frequency at which the transition occurs is not altered by the presence of the term given by eq. (14), because the value of P is the same for each component of the doublet. However, in the presence of anisotropy, transitions in which the nuclear magnetic quantum number changes by 1 or 2 may be weakly allowed if these are observable, they provide additional information from which the value of P may be derived. 

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