Rovibrational Energies of Carbon Dioxide with Kilohertz Accuracy

High-precision ro-vibrational spectroscopy of carbon dioxide isotopologues is essential for atmospheric remote sensing, climate studies, and fundamental molecular physics. Accurate determination of CO2 vibrational-rotational energy levels is crucial for improving the reliability of relevant spectroscopic databases. We report a cavity-enhanced double-resonance spectroscopy technique stabilized by an optical frequency comb for precision measurement of weak rovibrational transitions in CO2 isotopologues. By pumping a strong transition while probing a weak counterpart, the method enables sub-Doppler detection of 18 weak transitions in the 41 101–00 001 band with uncertainties of 4–52 kHz. Using complementary rovibrational models, we derived 394 and 133 experimental energy levels for 12C16O2 and 13C16O2, respectively, up to 14 170 cm−1. These results, all from sub-Doppler measurements, provide significantly more accurate benchmarks than current spectroscopic databases and will improve atmospheric remote-sensing retrievals for climate and planetary science. The approach is generally applicable to other polyatomic molecules and opens a route to systematic precision spectroscopy of weak and forbidden transitions.