¹²CO₂ transition frequencies with kHz-accuracy by saturation spectroscopy in the 1.99-2.09 μm region

Saturation spectroscopy has been used to determine the absolute frequencies of 107 ro-vibrational transitions of the two strongest ¹²CO₂ bands of the 2 μm region. The considered 20012-00001 and 20013-00001 bands are of importance for the CO₂ monitoring in our atmosphere. Lamb dips were measured using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator or to an ultra-stable optical frequency. The comb-coherence transfer (CCT) technique was applied to obtain a RF tunable narrow-line comb-disciplined laser source using an external cavity diode laser and a simple electro-optic modulator. This setup allows obtaining transition frequency measurements with kHz-level accuracy. The resulting accurate values of the energy levels of the 20012 and 20013 vibrational states are reproduced with a (1σ)-rms of about 1 kHz using the standard polynomial model. The two upper vibrational states appear thus to be highly isolated except for a local perturbation of the 20012 state leading to an energy shift of 15 kHz at J = 43. A recommended list of 145 transition frequencies with kHz accuracy is obtained providing secondary frequency standards across the 1.99-2.09 μm range. The reported frequencies will be valuable to constrain the zero-pressure frequencies of the considered transitions in ¹²CO₂ retrieval from atmospheric spectra.