High sensitivity CRDS of CO2 in the 1.74 µm transparency window. A validation test for the spectroscopic databases

Highlights: • High sensitivity CRDS of CO₂ in the 1.74 µm transparency window of interest for Venus. • More than 1100 transitions assigned to 26 bands of six isotopologues. • Very weak lines with intensity value as low as 10⁻³⁰ cm/molecule are detected. • The observations validate ab initio intensity values. • Some deficiencies are evidenced in the HITRAN2016 list and analyzed. The very weak absorption spectrum of natural CO₂ near 1.74 µm (5702–5879 cm⁻¹) is studied at high sensitivity. The investigated region corresponds to a transparency window of very weak opacity which is of particular interest for Venus. Very weak lines with intensity value as low as 10⁻³⁰ cm/molecule at 296 K are detected by Cavity Ring Down Spectroscopy. On the basis of the predictions of effective Hamiltonian models, 1135 lines of six carbon dioxide isotopologues - ¹²C¹⁶O₂, ¹³C¹⁶O₂, ¹⁶O¹²C¹⁸O, ¹⁶O¹²C¹⁷O, ¹⁶O¹³C¹⁸O and ¹⁶O¹³C¹⁷O - were rovibrationnally assigned to 26 bands. The accurate spectroscopic parameters of 16 bands are determined from standard band-by-band analysis (typical rms deviations of the line positions are 8 × 10⁻⁴ cm⁻¹). These newly observed bands include perturbed bands, weak hot bands and bands of minor isotopologues (in particular ¹⁶O¹²C¹⁸O in natural abundance) and provide critical validation tests for the most recent spectroscopic databases. The comparison to the Carbon Dioxide Spectroscopic Databank (CDSD), HITRAN2016 database and recent ab initio line lists is presented. Deficiencies are evidenced for some weak perpendicular bands of the HITRAN2016 list and identified as due to inaccurate CDSD intensities which were preferred to ab initio intensities. While Ames and UCL ab initio intensities are believed to be accurate within a few % for the strong unperturbed bands, the reported measurements allow testing important (>50%) differences between ab initio values of some weak perturbed bands.