Ultraviolet and infrared spectrum of C6H2 revisited and vapor pressure curve in Titan's atmosphere

Details Ultraviolet and infrared spectrum of C6H2 revisited and vapor pressure curve in Titan's atmosphere Ultraviolet and infrared spectrum of C6H2 revisited and vapor pressure curve in Titan's atmosphere

Jan 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003p%26ss...51....9s&link_type=abstract

Planetary and Space Science, Volume 51, Issue 1, p. 9-17.

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Scientific paper

The presence of the linear molecules called polyynes, (C2nH2, /n>2), in Titan's atmosphere is suggested by the signatures of acetylene C2H2 and of diacetylene C4H2 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 C4H2 depends on our ability to determine their spectra in the laboratory under low temperature and pressure conditions. We revisit here spectroscopic investigations on triacetylene, C6H2, 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 (185-320nm). In the IR range (220-4300cm-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 (170-200K) 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 (~100km) and the other in thermosphere (~700km). The condensation at an altitude of 700km 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.

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