Organic Aerosols in the Presence of CO2 in the Early Earth and Exoplanets: UV–Vis Refractive Indices of Oxidized Tholins

Abstract : The goal of this experimental study is to investigate how the presence of atmospheric CO2 affects the optical properties of organic photochemical aerosols. To this end, we add CO2 to a N2:CH4 gas mixture used in a plasma typically used for Titan studies. We produce organic thin films (tholins) in plasmas where the CO2 / CH4 ratio is increased from 0 to 4. We measure these films via spectrometric ellipsometry in the ultraviolet, visible and near-infrared (270 nm to 2 µm). The ellipsometry parameters are fitted with a Tauc-Lorentz model used for optically transparent materials, to obtain the thickness of the thin film, its optical band gap, and the refractive indices. According to this optical mode, oxidized organic aerosols are transparent in the visible up to the near ultraviolet (k = 0 from 900-400 nm), while the fully reduced organic aerosol absorbs in the visible as well as in the near UV (k = 0 from 900-500 nm). As the CO2 /CH4 ratio is quadrupled, the position of the UV absorption resonance is shifted from ∼177 nm to 264 nm, and its strength is also quadrupled: oxidized tholins absorb more efficiently in the middle UV with respect to the far UV for reduced tholins. Our laboratory wavelength-tabulated refractive indices provide further constraints to atmospheric models of the early Earth and Earth-like exoplanets including photochemical hazes formed under increasingly oxidizing conditions.
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Lisseth Gavilan, Laurent Broch, Nathalie Carrasco, Benjamin Fleury, Ludovic Vettier. Organic Aerosols in the Presence of CO2 in the Early Earth and Exoplanets: UV–Vis Refractive Indices of Oxidized Tholins. The Astrophysical journal letters, Bristol : IOP Publishing, 2017, 848 (1), pp.L5. ⟨10.3847/2041-8213/aa8cc4⟩. ⟨insu-01808648⟩

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