Measurement of Hyperfine Structure in the Infrared Rotation-Vibration Spectrum of NH−

Hyperfine structure (hfs) has been observed in the $P$, $Q$, and $R$ branches of the fundamental rotation-vibration spectrum of the $X{}_{}{}^2{}_{}{}^{}\Pi$ electronic state of the molecular anion N${\mathrm{H}}^{-}$. A coaxial ion-beam-laser-beam spectrometer is employed in which rotation-vibration transitions are induced by a color-center laser operating near 3000 ${\mathrm{cm}}^{-1\mathrm{}}$. The vibrationally excited ions autodetach, and the fast neutral NH is detected. The structure has been analyzed and the magnetic dipole hfs parameters obtained in both ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ ${\mathrm{H}}^{-}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ ${\mathrm{H}}^{-}$. This is the first analyzed case of hfs in any molecular anion.