Infrared spectroscopy of NH+: An analysis of the perturbation between the X 2Π and a 4Σ− states

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Abstract
The v=1–0 vibration–rotation transitions in the X 2Π and a 4Σ− states as well as those between the two electronic states were observed with a difference frequency laser as a radiationsource. The two electronic states (X 2Π and a 4Σ−) lie close together and interact each other strongly through the spin–orbit coupling. A merged least‐squares fit was carried out with the present infrared transition wave numbers, some of the previous optical term values, and the recent far‐infrared rotational transition frequencies to determine the spectroscopic parameters precisely. The equilibrium internuclear distance was obtained to be 1.0692±0.0002 and 1.0924±0.0001 Å for the X and a states, respectively. The Λ‐type doubling transition frequencies were calculated for several of the lowest J states with the molecular constants obtained and the hyperfine coupling constants determined from the far‐infrared transitions.