First identification of the a1Δg-X3Σg− electric quadrupole transitions of oxygen in solar and laboratory spectra

Electric quadrupole transitions in the a1Δg-X3Σg− band of 16O2 near 1.27 μm are reported for the first time. They were first detected in atmospheric solar spectra acquired with a ground-based Fourier transform spectrometer (FTS) in Park Falls, WI. Subsequently high-sensitivity CW-cavity ring down spectroscopy (CW-CRDS) experiments were carried out at Grenoble University in the 7717-7917 cm−1 region in order to provide quantitative intensity information for the electric quadrupole transitions. Measured intensities were used as input data for the calculation of the complete list of electric quadrupole transitions with ΔJ=±2, ±1 and 0. The calculation was carried out for the intermediate coupling case and assuming that these transitions are possible only through mixing of the Ω=0 component of the ground electronic state and b1Σg+ state induced by spin-orbit coupling. The calculated line list agrees well with experimental measurements and was used to improve the residuals of the fitted solar atmospheric spectrum. Emission probability for the electric quadrupole band was determined to be (1.02±0.10)×10−6 s−1.