Difference frequency laser spectroscopy of the nu 3 fundamental band of NH + 2

The ν₃ band of NH⁺₂ in the X∼ ³B₁ ground electronic state was observed in direct absorption with a tunable difference frequency laser spectrometer in the 3 μ region, using velocity modulation detection. NH⁺₂ and NH⁺₃ ions were generated in an ac discharge of He and NH₃, or of He, N₂, and H₂. Fifty-three rovibrational transitions were measured and fit to a triplet A-reduced Hamiltonian to determine rotational, centrifugal distortion, and spin-rotation constants. The band origin was found to be ν₀=3359.932 cm^-1, in excellent agreement with a recent calculation of Jensen, Bunker, and McLean. Indirect evidence from the spectrum suggested that NH⁺₂ is quasilinear, but selection rules prevented a determination of the A rotational constant.