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.