The CO2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (I) Rovibrational lines

Abstract

The absorption of carbon dioxide is very weak near 2.3µm which makes this transparency window of particular interest for the study of Venus’ lower atmosphere. As a consequence of the weakness of the transitions located in this region, previous experimental data are very scarce and spectroscopic databases provide calculated line lists which should be tested and validated by experiment. In this work, we use the Cavity Ring Down Spectroscopy (CRDS) technique for a high sensitivity characterization of the CO2 absorption spectrum in two spectral intervals of the 2.3µm window: 4248–4257 and 4295–4380cm−1 which were accessed using a Distributed Feed Back (DFB) diode laser and a Vertical External Cavity Surface Emitting Laser (VECSEL) as light sources, respectively. The achieved sensitivity (noise equivalent absorption, αmin, on the order of 5×10−10cm−1) allowed detecting numerous new transitions with intensity values down to 5×10−30cm/molecule.The rovibrational assignments were performed by comparison with available theoretical line lists in particular those obtained at IAO Tomsk using the global effective operator approach. Hot bands of the main isotopologue and 16O12C18O bands were found to be missing in the HITRAN database while they contribute importantly to the absorption in the region.Additional CRDS spectra of a CO2 sample highly enriched in 18O were recorded in order to improve the spectroscopy of this isotopologue. As a result about 700 lines of 16O12C18O, 16O12C17O, 17O12C18O, 12C18O2 and 13C18O2 were newly measured.The status of the different databases (HITRAN, CDSD, variational calculations) in the important 2.3µm transparency window is discussed. Possible improvements to correct evidenced deficiencies are suggested.