First observation of the M 2Σ+ → E2Σ+ (0—0) transition in NO

The emission spectrum of NO excited by electric discharge has been recorded with a high-resolution Fourier transform interferometer. Strong perturbations are observed in the spectrum of the transition M²Σ⁺→ E²Σ⁺ (0—0), due to mixing of the Rydberg state M²Σ⁺(v = 0) with valence states B²Π(v = 22, 23) and L²Π(v = 3). Accurate energies for the M²Σ⁺ rotational levels are given.