Molecular transition frequencies of CO2 near 1.6 µm with kHz-level uncertainties

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique, reporting vacuum transition frequencies of Doppler-broadened 12C16O2 in the 1.6 μm region for the (30012) ← (00001) and (30013) ← (00001) bands with an average combined standard uncertainty of ≈ 1 kHz. A global four-state model was fit to these data and literature values to provide spectroscopic parameters and a best-case fit precision of 4 kHz. We identified and quantified an interaction between the (30012) and (33301) states which was manifest as an observed Fermi-resonance-type perturbation in the (30012) ← (00001) band. This interaction was accounted for in the global fit, substantially reducing systematic uncertainties in the spectroscopic parameters of the (30012) state and altering many predicted transition frequencies in the (30012) ← (00001) band by 1 MHz or more relative to literature values. The accurate transition frequencies reported here may be considered as SI-traceable reference data for diverse applications including remote sensing and telecommunications.