Ab Initio Potential Energy Curves for the Ground and Low Lying Excited States of NH^– and the Effect of ²Σ^± States on Λ‑Doubling of the Ground State X²Π

Complete basis set extrapolated ab initio potential energy curves obtained from multireference configuration interaction (MRCI) level calculations for the ground state (X²Π) and the a⁴Σ^– state of NH^– and the ground state (X³Σ^–) of NH are reported. The potential energy curves for the A′²Σ^– and A²Σ⁺ states of NH^– have been computed using the V6Z basis set at the MRCI level. Λ-doubling parameters <i>p</i> and <i>q</i> are calculated for the ground and the first excited vibrational states of the ground electronic state of NH^– using second-orderperturbation theory. The effect of the ²Σ⁺ and ²Σ^– states on the Λ-doubling values is discussed. Earlier experiments had not considered the influence of the ²Σ^– state on <i>p</i> and <i>q</i> while fitting the spectral data. Using the computed potential energy curves and the ro-vibrational spectra including the fine splitting, we have computed the threshold for electron detachment. The result is in agreement with the experimental values of Neumark et al. [J. Chem. Phys. 1985, 83, 4364] and Farley et al. [Phys. Rev. A 1987, 35, 1099]