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A. Jolly, Y. Benilan, F. Shindo, F. Raulin (LISA, Universités Paris XII, 94010 Creteil Cedex, France)
Band intensities of most molecules are temperature dependent. However, most of the experimental studies in the infrared have been done at room temperature which is incompatible with the conditions found on Titan or the outer planets atmosphere. This temperature dependence is mainly influenced by hot bands intensities. The hot band effect starts to be appreciable for relatively light molecules like acetylene and gets more and more important when molecules get heavier. Our aim is to study C4H2, C6H2, and C8H2 for which hot bands effects are very strong. These polyynes will be very interesting molecules to study with the IR spectrometer on board Cassini because they are expected to be linked to the formation of the organic aerosols seen on Titan. But it is necessary to have correct low temperature band intensities in order to retrieve the right abundance in low temperature atmospheres like Titan. The hot bands intensities can in principle be simply predicted by a Boltzman calculation but statistical weight of degenerate vibration levels and l-doubling intensity factor makes it much more complex. Measurements are also possible but very few high resolution experiments, usually dedicated to the determination of spectroscopic parameters, include measurements of the absolute transition strength. We will review here the different studies made on acetylene and see how various experimentators have dealt with this problem. We will also compare our recent C3H4 (allene) spectra with a high resolution low temperature study done by Wang et al.(1). Finally, we will show how recently obtained experimental data for polyynes can be extrapolated to low temperature using our knowledge obtained on lighter molecules.
(1) W.F. Wang, J.M. Sirota and D.C. Reuter, J. Mol. Spec 198, 201-208 (1999)
The author(s) of this abstract have provided an email address for comments about the abstract: jolly@lisa.univ-paris12.fr