Infrared laser magnetic resonance spectroscopy of the ν3 fundamental and associated hot bands of the NCO free radical

The v3 = 1 ← 0 (out-of-phase stretching vibration) transition wavenumbers of gas-phase X˜2Π NCO have been measured, in many cases with sub-Doppler resolution, by mid-infrared laser magnetic resonance (LMR) spectroscopy. In addition to the fundamental transition from the 2Π ground vibronic state, hot bands from the v2 = 1 2Σ and 2Δ and from the v2 = 2 2Φ vibronic states were detected and analyzed. The 2Σ vibronic level, with a spin-orbit coupling only partly quenched by the Renner-Teller effect, is characterised by complex and rapid tuning of energies in the magnetic field. This is the first successful analysis of magnetic resonance spectra for non-unique Renner-Teller vibronic states. The new LMR transitions were combined with data from previous studies and analysed, using an effective Hamiltonian in a 40-parameter fit to 660 transitions and combination differences. Several new coupling parameters are required, having magnitudes generally consistent with predictions from standard vibration-rotation theory.