Ultraviolet and infrared spectrum of C 6H 2 revisited and vapor pressure curve in Titan's atmosphere

Abstract

The presence of the linear molecules called polyynes, (C 2 n H 2, n>2), in Titan's atmosphere is suggested by the signatures of acetylene C 2H 2 and of diacetylene C 4H 2 in Voyager spectra. Both atmospheric simulations and photochemical modelling support polyynes implication in Titan's chemistry as an interface between the gaseous phase and the solid phase visible in aerosols form. However, the detection of polyynes higher than C 4H 2 depends on our ability to determine their spectra in the laboratory under low temperature and pressure conditions. We revisit here spectroscopic investigations on triacetylene, C 6H 2, since previous UV and IR measurements suffered from great uncertainty, respectively, due to an impurity contribution and saturation effects. We point out the importance of studying a pure sample and we underline the strong temperature dependency of UV absolute absorption coefficients (185– 320 nm ). In the IR range (220– 4300 cm −1 ), our determination of the absolute intensity of the main bands is 30% higher than previous measurements. For the first time, the vapor pressure law of triacetylene is investigated in a limited temperature range (170– 200 K ) allowing a calculation of its enthalpy of sublimation. Those results applied to Titan's atmospheric conditions show the possible existence of two condensation regions: one located in the low stratosphere (∼100 km) and the other in thermosphere (∼700 km) . The condensation at an altitude of 700 km is consistent with the observation of an upper haze layer. This could imply the presence of a heterogeneous chemistry but also an inhibition of the polyynes formation, not included in available photochemical models.