Optical constants of exoplanet haze analogs from 0.3 to 30 µm: Comparative sensitivity between spectrophotometry and ellipsometry^★

¹ University of Paris Saclay, OVSQ, LATMOS, 11 Boulevard d'Alembert, 78280 Guyancourt, France
e-mail: thomas.drant@latmos.ipsl.fr
² Ludwig Maximilian University, Faculty of Physics, Observatory of Munich, Scheinerstrasse 1, Munich 81679, Germany
³ Ecole Polytechnique, LPICM, Route de Saclay, 91120 Palaiseau, France
⁴ NASA Ames Research Center, Space Science and Astrobiology Division, Code ST, Moffett Field, CA 94035, USA
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 5 place Jules Janssen, 92195 Meudon, France
⁶ Synchrotron SOLEIL, L’Orme des Merisiers, 91190 Saint-Aubin, France
⁷ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁸ University of Bern, ARTORG Center for Biomedical Engineering Research, Murtenstrasse 50, 3008 Bern, Switzerland
⁹ University of Warwick, Department of Physics, Astronomy & Astrophysics Group, Coventry CV4 7AL, UK

Received: 5 May 2023
Accepted: 18 September 2023

Abstract

We report new optical constants (refractive index, n, and extinction coefficient, k) for exoplanet haze analogs from 0.3 to 30 µm. The samples were produced in a simulated N₂-dominated atmosphere with two different abundance ratios of CO₂ and CH₄, using the PAMPRE plasma reactor at LATMOS. We find that our haze analogs present a significantly lower extinction coefficient in the optical and near-infrared (NIR) range compared to the seminal data obtained on Titan haze analogs. We confirm the stronger IR absorption expected for hazes produced in a gas mixture with higher CO₂ abundances. Given the strong impact of the atmospheric composition on the absorbing power of hazes, these new data should be used to characterize early-Earth and CO₂-rich exoplanet atmospheres. The data presented in this paper can be found in the Optical Constants Database. Using ellipsometry or spectrophotometry, the retrieved optical constants are affected by the sensitivity of the measurement and the accuracy of the calculations. A comparative study of both techniques was performed to identify limitations and better understand the discrepancies present in the previous data. For the refractive index n, errors of 1–3% are observed with both optical techniques and the different models, caused by the correlation with the film thickness. We find that UV-visible reflection ellipsometry provides similar n values, regardless of the model used; whereas the Swanepoel method on transmission is more subjected to errors in the UV. In the UV and mid-infrared (MIR), the different calculations lead to rather small errors on k. Larger errors of k arise in the region of weak absorption, where calculations are more sensitive to errors on the refractive index n.