Temperature dependence of HC₃N, C₆H₂, and C₄N₂ mid-UV absorption coefficients. Application to the interpretation of Titan's atmospheric spectra

Three organic compounds (HC3N, C6H2, and C4N2) relevant of Titan's atmosphere have been studied within the framework of the SIPAT (Spectroscopie UV d'Interet Prebiologique dans l'Atmosphere de Titan) program. Since this facility is still unable to reach the very low temperatures (similar to 170 K) of Titan's high atmosphere, spectra have to be obtained at several absorption-cell temperatures, and the data extrapolated towards lower temperatures. Previously published HC3N and C6H2 absorption coefficient data are reviewed, while new spectroscopic data are presented on C4N2. Integrated intensity calculations over the vibrational bands are performed apart from the background continuum. Thus, only the band contrast is considered here. While, the temperature dependence of the hot-band integrated intensity follows a Boltzmann distribution, we have enhanced the fit through an empirical parametrisation to account for the observed temperature dependence of the C4N2 and HC3N absorption coefficients, and to extrapolate those data to the low temperature conditions of Titan's high atmosphere. Finally, we discuss the implications of the results to possible detection by remote sensing observations of these minor compounds in Titan's atmosphere.