Molecular Transition Frequences Of Co₂ Near 1.6 μ With Khz-level Uncertainties

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique [1], reporting vacuum transition frequencies of Doppler-broadened $^{12}$C$^{16}$O$_{2}$ in the 1.6 $\mu$m region for the (30012) $\leftarrow$ (00001) and (30013) $\leftarrow$ (00001) bands with an average combined standard uncertainty of ~1 kHz. A global multi-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 assigned an interaction between the (30012) and (33301) states which was manifest as an observed Fermi resonance in the (30012) $\leftarrow$ (00001) band. This interaction was accounted for in the global fit, significantly reducing uncertainties in the spectroscopic parameters of the (30012) state. We find excellent agreement with literature values, including better than 1 kHz RMS deviation with a recent saturation cavity ring-down study of the (30013) $\leftarrow$ (00001) band [2]. Agreement is excellent with CDSD 2019 [3] and HITRAN 2016 [4] for the (30013) $\leftarrow$ (00001) band, but much poorer for the (30012) $\leftarrow$ (00001) band. [1] Z. D. Reed, D. A. Long, H. Fleurbaey, and J. T. Hodges, Optica 7, 1209-1220 (2020). [2] H. Wu, et al., Phys. Chem. Chem. Phys. 22, 2841-2848 (2020). [3] S. A. Tashkun, et al., J. Quant. Spectros. Rad. Trans. 228, 124-131 (2019). [4] I. E. Gordon, et al. J. Quant. Spectros. Rad. Trans. 203, 3-69 (2017)