Λ-type doubling in the molecules14NH+,15NH+, and14ND+

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Abstract
Ab initio calculations of the Λ-type doubling in the X 2II states of 14NH+, 15NH+, and 14ND+ are reported for rotational levels with J = ½ to . It is found that to account successfully for the Λ-type doubling in these levels Van Vleck's theory of Λ-type doubling in 2II states has to be extended to include the effect of interactions between the X 2II state and a low-lying 4Ω- state. The interaction between the X 2II state and the a 4Ω- state was treated using an exact matrix diagonalization, and the interactions between the X 2II state and the A 2Ω- and C 2Ω+ states, which are the only interactions considered in Van Vleck's theory, were treated using second-order perturbation theory. Excellent agreement was achieved with existing experimental values. The X 2II-a 4Ω- interaction was found to make an important contribution to the calculated values of the Λ-type doubling. The calculated values of the Λ-type doubling should aid the search for the interstellar radio spectrum of NH+.