The microwave spectrum of the NF radical in the second electronically excited (b ¹Σ⁺) state: Potentials of three low-lying states (X³Σ^-, a ¹A, b¹Σ⁺)

The pure rotational transition of the NF radical in the second electronically excited state, b ¹Σ⁺, was detected by microwave spectroscopy. The NF radical was generated by dc-discharge through a mixture of NF₃ and H² at around 90 K. Six rotational transitions for v= 0 and four for v = 1 were observed in the 73-442 GHz region. In addition to the rotational and centrifugal distortion constants, the quadrupole coupling constant of the nitrogen nucleus of NF(b ¹Σ⁺) was determined for the first time from the lowest two transitions of NF in the v = 0 level. The spectrum of NF(a ¹Δ) in the first vibrational excited state was also observed in order to derive highly accurate potential parameters for comparison with the data of NF(X ³Σ^-,b ¹Σ⁺).