First detection of transitions in the second quadrupole overtone band of nitrogen near 1.44 μm by CW-CRDS with 6 × 10−13 cm−1 sensitivity

The extremely weak S(4) and S(6) electric quadrupole transitions of the 3-0 band of nitrogen have been detected by very high sensitivity CW-Cavity Ring Down spectroscopy near 6950 cm−1. A noise equivalent absorption on the order of αmin ∼ 6 × 10−13 cm−1 was achieved by averaging several thousand 0.5 cm−1 wide spectra recorded successively over several days. The studied 14N2 lines are among the weakest ever measured in the laboratory (intensity on the order of 1.5 × 10−31 cm/molecule). The positions determined with a 0.002 cm−1 uncertainty and the line intensities are compared with theoretical values included in the HITRAN2012 database.