An investigation of the interaction between close-lying2Σ and2Π states of diatomic molecules

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Abstract
It is shown that the perturbation calculation has to be carried to the sixth order to obtain a satisfactory theoretical description of the large Λ-doubling in the A 2Π levels of SrH, BaH and BaD. In these cases a fit of Mulliken and Christy's formula to the 2Π1/2 levels leads to results which seriously disagree with the observed Λ-doubling of the 2Π3/2 levels. This inconsistency is reduced by working through (extended) second and fourth order formulae, until rather excellent agreement is obtained by the introduction of sixth order terms. Finally the sixth order values of the parameters p and q are inserted in Van Vleck's secular determinant, and a numerical solution of the cubic equation yields results in good agreement with both the observed splitting and the sixth order values.