Infrared diode laser spectroscopic study of the active nitrogen. The electronic transition between the lowest singlet metastable states a 1Πg–a′ 1Σ−u, 0–0 band

The a 1Πg–a′ 1Σ−u, 0–0 band of the molecular nitrogen was observed in an electric discharge plasma of nitrogen using infrared diode laser spectroscopy at Doppler-limited resolution. Precise values were obtained for the rotational constants and the energy separation between the two electronic states. The effect of Λ-type doubling in the a 1Πg state was observed as small shifts of Q branch and P/R branch transitions to higher and lower frequencies, respectively, than those expected if Λ doubling were absent.