Molecular vibration and rotation constants are obtained for the metastable a1Δg electronic state of molecular oxygen. The constants are given for the u = 0 and u = 1 vibrational levels by combining recent high-resolution laboratory measurements with high-resolution observatory-based planetary and solar spectra of the magnetic dipole transitions of the system a1Δg ← X3Σ-g.
Laurence S. Rothman, "Magnetic dipole infrared atmospheric oxygen bands," Appl. Opt. 21, 2428-2431 (1982)