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A. Jolly, Y. Benilan, F. Raulin (LISA, Univ. Paris 12, France)
We have measured C2H2 absorption coefficients in the 185-235 nm range at 295 and 173 K. These coefficients were poorly known above 200 nm and were not taken into account when trying to model the photodissociation of C2H2. We have shown that more than 25 to dissociation above 200 nm. This result can be explained by the fact that, even though the acetylene absorption decreases by four order of magnitude between 150 and 220 nm, the solar flux increases rapidly in the same wavelength range. The temperature dependence of the absorption coefficients is very important in this wavelength range. We observe that the background continuum, on which the band system is superimposed, decreases rapidly with the temperature. Large variations in the band structures due to temperature effects are also observed. In order to have a better understanding of these effects and to extrapolate variation to lower temperatures, we have undertaken a spectroscopic modelling of the acetylene absorptions bands. The attribution of all the bands can be made down to 200 nm where congestion of the bands makes the identification impossible. We were able to calculate and reproduce fairly well the experimental spectra at both temperatures. The main differences between both spectra comes from hot bands involving the first excited level of the n4’’ bending mode which are very strong at 298 K and completely disappear at 173 K. The shape and structure variations are also well reproduced by the calculations, especially the relative intensity changes between the P/Q and the R branch maxima. The good agreement between experimental and calculated absorption spectra opens the way to the calculations of synthetic spectra at any temperature and any resolution. This will be of great help in future analysis of observations of giant planets and Titan.