From Matrix Isolation Spectroscopy to First Observation of the CO2 628 Isotopologue (_2 + _3) Band in the Atmosphere Of Venus

Due to its interest for atmospheric studies, carbon dioxide is probably the molecule whose IR spectrum has been the most extensively studied. However, recent observation and identification of a new absorption band, at 3.35 µm, detected by SOIR on board ESA Venus Express mission was not straight forward. It was finally attributed to the 628 isotopologue of carbon dioxide, as the result of a collaboration between different teams involving different techniques 1, 2, 3, 4 . One of these, matrix isolation spectroscopy, coupled to gas phase spectroscopic methods, provides a useful and easy way to help identifying spectral features of individual absorbers from dense spectra. The process requires evaluation of the matrix effect environnement on the electric potential driving the nuclear motions, calculation of the ro-vibrational energies and finally determination of the selection rules from group theory, for allowed transitions. Comparison of observed and calculated transitions from Laser Induced fluorescence validated this method for carbon dioxide and ozone5 . More elaborate methods can then be used to determine the line parameters such as intensity and widths from observed spectra to fully characterize the observed absorptions 6 . Taking into account those newly observed transitions may help refining the modeling of green house effect on Venus and on Mars, it could improve the retrieval of methane, whose main absorption features overlap with the 628 CO2 lines. The presentation will focus on the methods described above applied to the retrieval of the line parameters, positions and intensities of the observed rovibrational transitions from spectra recorded by SOIR in the atmosphere of Venus above 70 km. We will briefly describe the instrument and recall the SOIR technique. Then we will focus on the retrieval of the line positions from matrix isolation spectroscopy and analysis of symmetry to determine the allowed selection rules. Finally, results of calculations of the line parameters and the Hermann-Wallis factors for the observed band will be presented with a glance at results obtained for other isotopologues detected on Mars from earth observations 7 .