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A. Jolly, Y. Benilan, F. Shindo, F. Raulin (LISA, Paris XII)
The temperature dependence of absorption coefficients can lead to large discrepancies on the measured abundance of organic molecules in solar system atmospheres. Ultraviolet spectra are mostly obtained at room temperature, few are measured at lower temperature (down to 180 K). But absorption coefficients corresponding to the temperature of Titanís or Jupiterís atmosphere (100-180 K) are only accessible by spectroscopic modelling.
In this work, we have first measured experimentally the Mid-UV absorption coefficient of three important compounds : C2H2, C2N2 and NH3. The comparison of spectra at room and low temperature (180 K) shows a pronounced temperature dependence in the wavelength range 200-250 nm due to the appearance of hot bands. Since these molecules are relatively simple (not to many vibrational modes) and their spectroscopic constants relatively well known, it is possible to compute synthetic spectra. If these spectra compare well with the experimental ones, extrapolation to lower temperature gets possible.
The good results obtained with these molecules will enable us to use synthetic spectra in the temperature range 100-180 K to analyse future HST observations of Titan and Jupiter. Also, these new spectra should be introduced in photochemical models in order to reduce the errors due to the use of inadequate absorption coefficients