Diode laser spectroscopy of the ν2 fundamental and hot bands of NH+3

Abstract

The nondegenerate ν2←0 and 2ν2←ν2 bands of the ammonia cation NH+3 have been observed using tunable diode lasers combined with the velocity modulation technique. The ion was produced in a water-cooled 6 kHz ac glow discharge cell with a gas mixture of [He]:[N2]:[H2]=9:0.06:0.3 (Torr). From the observed spectra, the band origins were determined to be ν1←0=903.3898 cm−1 and ν2←1=939.771 cm−1. Vibration–rotation interaction constants and potential constants were derived. The missing of K=0 R-branch transitions with N=odd in the ν2←0 band and N=even in the 2ν2←ν2 band clearly shows the characteristics of a D3h planar structure with a 2A″2 ground electronic state, just like its isoelectronic case CH3 radical. Spin–rotation doublets were well-resolved in R-branch transitions and spin–rotation interaction constants of the ground, ν2, and 2ν2 states were determined. A potential function consisting of a quadratic term and a quartic term was used for modeling the out-of-plane bending vibration of NH+3. In comparison with the methyl radical, the ammonia cation is more rigid and closer to a harmonic oscillator.