Infrared Spectra of C2H6, C2H4, C2H2, and CO2 Aerosols Potentially Formed in Titan's Atmosphere

Pure and mixed aerosols of ethane, ethylene, acetylene and carbon dioxide were generated in a collisional cooling cell and characterized by Fourier transform infrared spectroscopy between 600 and 4000 cm(-1). Pure ethane, pure ethylene, and mixed ethane/ethylene initially form supercooled liquid droplets, which over time crystallize to their stable solid phases. These droplets are found to be long-lived (up to hours) for pure ethane and mixed ethane/ethylene, but short-lived (up to seconds) for pure ethylene. Acetylene and carbon dioxide form solid aerosol particles. Acetylene particles have a partially amorphous structure, while carbon dioxide particles are crystalline. The structure of the infrared bands of carbon dioxide is strongly determined by the particles' shape due to exciton coupling. The comparison of various mixed systems reveals that acetylene very efficiently induces heterogeneous crystallization. As reported earlier, the co-condensation of acetylene and carbon dioxide can lead to the formation of a metastable mixed crystalline phase. Our preliminary calculations show that this mixed phase has a monoclinic rather than the cubic structure proposed previously.