Laser spectroscopy of perturbed levels in N2(B 3Πg,v=10) and the first experimental determination of the N2(A′ 5Σ+g) term energy

Using both a pulsed and a narrow-band cw dye laser, laser-induced fluorescence excitation spectra of N2(B,v=10) were observed on the N2(B 3Πg←A 3Σ+u) transition from the metastable N2(A) component of a molecular beam. Lifetime measurements were made for some of the B,v=10 rotational/fine structure levels, including one which was recently shown to exhibit very specific kinetic effects due to a perturbation by the N2(A′ 5Σ+g) state [Ch. Ottinger, L. G. Smirnova and A. F. Vilesov, J. Chem. Phys. 100, 4848 (1994)]. The perturbation manifested itself in the present work by a significantly longer lifetime of the 3Πe2(12) level relative to other B,10 levels, as well as by line shifts. The S/O interaction matrix element is obtained to be 0.35 cm−1. From the required very close accidental coincidence between well-identified levels in the B and A′ states the term energy of the latter could be precisely determined as Te(A′ 5Σ+g)=75 990.0 cm−1. This is the first measurement of this value, and the result is ≂440 cm−1 lower than was predicted by ab initio calculations.