SIMBAD references

The strong ν₈ band of diacetylene at 627.9/cm has been investigated to improve the spectroscopic line data used to model the observations, particularly in Titan's atmosphere by Cassini/Composite Infrared Spectrometer. Spectra have first been recorded in the laboratory at 0.5 and 0.1/cm resolution and temperature as low as 193 K. Previous analysis and line lists present in the GEISA database appeared to be insufficient to model the measured spectra in terms of intensity and hot band features. To improve the situation and in order to be able to take into account all rovibrational transitions with a non-negligible intensity, a global analysis of C₄H₂ has been carried out to improve the description of the energy levels up to E_v= 1900/cm. The result is a new extensive line list which enables us to model very precisely the temperature variation as well as the numerous hot band features observed in the laboratory spectra. One additional feature, observed at 622.3/cm, was assigned to the ν₆ mode of a ¹³C isotopologue of diacetylene. The ν₈ bands of both ¹³C isotopomers were also identified in the 0.1/cm resolution spectrum. Finally, a ¹³C/C₄H₂ line list was added to the model for comparison with the observed spectra of Titan. We obtain a clear detection of ¹³C monosubstituted diacetylene at 622.3/cm and 627.5/cm (blended ν₈bands), deriving a mean ¹²C/¹³C isotopic ratio of 90 ±8. This value agrees with the terrestrial (89.4, inorganic standard) and giant planet values (88±7), but is only marginally consistent with the bulk carbon value in Titan's atmosphere, measured in CH₄ by Huygens GCMS to be 82±1, indicating that isotopic fractionation during chemical processing may be occurring, as suggested for ethane formation.