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

Abstract

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.

Experimental 2CH excitation in acetylene-containing van der Waals complexes

“…The C 2 H 2 -CO 2 complex has been previously investigated experimentally in both microwave [94] and midinfrared [95] ranges and by ab initio methods [96][97][98], and also discussed within the context of Titan [99]. C 2 H 2 -N 2 O was similarly investigated in the microwave [100][101][102] and in the mid-infrared [103,104] regions.…”

First theoretical global line lists of ethylene (12C2H4) spectra for the temperature range 50−700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres

“…Again in Saturn, ethylene has recently been detected in the hot temperature beacon (Hesman et al 2012) region using the Composite Infrared Spectrometer (CIRS) with strong thermal emission at 10.5 µm. Various studies of ethylene and other hydrocarbons on Neptune, mini-Neptunes, and Titan were also reported (Schulz et al 1999;Hu & Seager 2014;Vinatier et al 2010;Coustenis et al 1991Coustenis et al , 1989Coustenis et al , 2007Coustenis & Bézard 1995;Vinatier et al 2007;Yang et al 2012;Canosa et al 2007;Roe et al 2004;Wang et al 2009). In particular, C 2 H 4 is the only molecular species on Titan whose mixing ratio decreases with height at latitudes below 46 • N (Vinatier et al 2010).…”

The weakly-bound CO2–acetylene complex: Fundamental and torsional combination band in the CO2 ν3 region

“…Additionally, less stable geometries were predicted by ab initio methods but these have not been observed experimentally. More recently the detection of the CO 2 -C 2 H 2 in the overtone C-H stretching region was reported [23] and the co-condensation of C 2 H 2 and CO 2 and possible importance for the cloud processes on Titan were discussed by Wang et al [24].…”