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Cavity-enhanced saturated absorption spectroscopy of the (30012) - (00001) band of 12C16O2.

The (30012) ← (00001) band of 12C16O2 in the 1.6 μm region is used for satellite observation of carbon dioxide in the Earth's atmosphere. Here, we report a Doppler-free spectroscopy study of this band with comb-locked wavelength-modulated cavity-enhanced absorption spectroscopy. Frequencies of 18 transitions with the rotational quantum numbers up to 42 were determined with sub-kHz accuracy, corresponding to a fractional uncertainty at the 10-12 level. With this precision, we revealed an anomalous decrease of the line shift and an increase of the line broadening for the Lamb dips of CO2 in the low-pressure regime compared to values obtained from Doppler-limited spectra at higher pressures.