Comment on “Spectroscopic database of CO2 line parameters: 4300–7000 cm–1”

The “Spectroscopic database of CO₂ line parameters: 4300–7000 cm^–1” constructed by Toth et al., has been considered in relation with our previous and current studies of the absorption spectrum of carbon dioxide (CO₂) by high-sensitivity CW-cavity ring down spectroscopy (CW-CRDS) in the 5850–7000 cm^?1 region. Part of the line parameters of the database are based on accurate spectroscopic measurements by Fourier transform spectroscopy (FTS) but Toth et al. have chosen to fix to a very low value (4×10^?30 cm/molecule) the lower intensity cut off. This value which is far below the FTS detection limit has led to long range extrapolations to high J values and to the inclusion of weak unobserved bands which were theoretically predicted. In the 5850–7000 cm^?1 region, most of these calculated transitions were previously observed by CW-CRDS. The comparison with the CW-CRDS ¹³CO₂ spectrum in this region, has evidenced that (i) many weak bands above the intensity cut off are missing; (ii) there are important deviations between the line parameters provided in the database and our previous observations both for line positions (up to 1.7 cm^?1) and line intensities (up to a factor 80). Our discussion was limited to the three ¹³C species (¹³C¹⁶O₂, ¹⁶O¹³C¹⁸O and ¹⁶O¹³C¹⁷O) but the conclusions should apply to the other isotopologues in particular ¹²C¹⁶O₂ and to the full spectral range of the database.Alternatively, the global effective operators models for CO₂ can reproduce satisfactorily all the experimental line positions and line intensities available in the literature. This polyad model, which has been developed for most of the CO₂ isotopologues, constitutes an interesting alternative for the most accurate and complete CO₂ database. In particular, very weak bands, accidental resonances, intensity transfers and extra lines are accurately accounted for and predicted by this polyad model.