High-resolution measurement of the infrared rotation-vibration spectrum of the negative molecular ionNH−14

The authors have performed an extensive series of measurements of the infrared rotation-vibration spectrum of the negative molecular ion ${}_{}{}^{14}{}_{}{}^{}\mathrm{NH}_{}^{\mathrm{-}}{}_{}{}^{}$. A total of 114 transitions in the P, Q, and R branches of the fundamental band near 3000 ${\mathrm{cm}}^{\mathrm{-}1}$ have been measured with an accuracy (1 standard deviation) of 0.01 ${\mathrm{cm}}^{\mathrm{-}1}$. The resolution is much higher (7 MHz), enabling us to resolve hyperfine structure. Autodetachment linewidths have also been measured. The electron affinity of NH has been measured with a precision a thousand times higher than previous work: $E_{\mathrm{EA}}$(NH)=0.374 362±0.000 005 eV. Our data agree with earlier measurements of the R branch and include the first high-resolution measurements of the Q and P branches. We have observed rotationally excited metastable states as much as 1.8 eV above the autodetachment threshold. A least-squares fit to the transition frequencies has been performed, using an effective Hamiltonian with a sextic term. The predictions of the best previous work, extrapolated to high J, disagree by as much as 10 ${\mathrm{cm}}^{\mathrm{-}1}$, or one thousand times the experimental uncertainty. This paper more than doubles the number of measured transition frequencies in this species.