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

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%N-2-5%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 370 nm to 900 nm wavelength using spectroscopic ellipsometry. The values of n decrease from n = 1.64 (at 370 nm) to n = 1.57 (at 900 nm) as well as the values of k which feature two behaviors: an exponential decay from 370 nm to 500 nm, with k = 12.4 x e(-0.018 lambda) (where lambda is expressed in nm), followed by a plateau, with k = (1.8 +/- 0.2) x 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. (C) 2011 Elsevier Inc. All rights reserved.