Organic Aerosols in the Presence of CO_2 in the Early Earth and Exoplanets: UV-Vis Refractive Indices of Oxidized Tholins

摘要

In this experimental study we investigate the role of atmospheric CO_2 on the optical properties of organic photochemical aerosols. To this end, we add CO_2 to a N2:CH_4 gas mixture used in a plasma typically used for Titan studies. We produce organic thin films (tholins) in plasmas where the CO_2/CH_4 ratio is increased from 0 to 4. We measure these films via spectrometric ellipsometry and apply 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 in the UV-visible (270-600 nm). All samples present a significant absorption band in the UV. According to the Tauc- Lorentz model, as the CO_2/CH_4 ratio is quadrupled, the position of the UV band is shifted from ~177 nm to 264 nm while its strength is quadrupled. Consequently, we infer that oxidized organic aerosols absorb more efficiently at longer UV wavelengths than reduced aerosols. Our laboratory wavelength-tabulated UV-vis refractive indices provide new constraints to atmospheric models of the early Earth and Earth-like exoplanets including photochemical hazes formed under increasingly oxidizing conditions.