Optical constants from 370nm to 900nm of Titan tholins produced in a low pressure RF plasma discharge

• Published in: Icarus (New York, N.Y. 1962) Vol. 218; no. 1; pp. 356 - 363
• Main Authors: Sciamma-O’Brien, E., Dahoo, P.-R., Hadamcik, E., Carrasco, N., Quirico, E., Szopa, C., Cernogora, G.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-03-2012
• Subjects: Aerosols; Albedo; Aluminum; Atmospheric composition; Computer simulation; Constants; Discharge; Mathematical models; Photometry; Titan; Titan; Photometry
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2011.12.014

► Atmospheric aerosols are important in the radiative transfer balance of Titan. ► Here we produce analogues with a RF plasma simulating the upper atmospheric reactivity. ► Their optical properties are determined by ellipsometric spectroscopy.
Determining the optical constants of Titan aerosol analogues, or tholins, has been a major concern for the last three decades because they are essential to constrain the numerical models used to analyze Titan’s observational data (albedo, radiative transfer, haze vertical profile, surface contribution, etc.). Here we present the optical constant characterization of tholins produced with an RF plasma discharge in a (95%N2–5%CH4) gas mixture simulating Titan’s main atmospheric composition, and deposited as a thin film on an Al–SiO2 substrate. The real and imaginary parts, n and k, of the tholin complex refractive index have been determined from 370nm to 900nm wavelength using spectroscopic ellipsometry. The values of n decrease from n=1.64 (at 370nm) to n=1.57 (at 900nm) as well as the values of k which feature two behaviors: an exponential decay from 370nm to 500nm, with k=12.4×e−0.018λ (where λ is expressed in nm), followed by a plateau, with k=(1.8±0.2)×10−3. The trends observed for the PAMPRE tholins optical constants are compared to those determined for other Titan tholins, as well as to the optical constants of Titan’s aerosols retrieved from observational data.