VUV photodynamics of free tholins nanoparticles investigated by imaging Angle- Resolved Photoemission with the Synchrotron Radiation
Résumé
Thanks to the Cassini Huygens mission, it is now established that the aerosols appear from an altitude of 1,000 km in Titan’s atmosphere. Once they are formed and through their descent towards the surface, those grains will still interact with persistent UV/VUV radiations, at different energies, that can reach lower atmospheric layers. This inter- action has some impact, for example on the radiative transfer or on the ionization yield of the atmospheric compounds.
Models are a good way to study those processes, but the lack of data on the refractive index or the absolute absorption/ionization cross subsections of the aerosols can be an obstacle.
In order to shed some light and quantify those processes, we ionize analogs of aerosols produced with the PAMPRE experiment (LATMOS) on the SAPHIRS platform from the DESIRS VUV beamline at the synchrotron SOLEIL, equipped with an aerodynamic lens. The aerosols are injected directly under vacuum as isolated free nanoparticles and do not need to take the form of a film deposited on a substrate. The generated photoelectrons are then collected with a Velocity Map Imaging detector and their energetic and angular signatures are analyzed using the ARPES method (Angle-Resolved PhotoElectron Spectroscopy).
Both the nanoparticles size distribution and the incident wavelength determine the parameters governing the pho- toemission process (intra-particles electron mean free path, photon penetration depth) as revealed by the angular distribution of the photoelectron showing in same cases a marked forward/backward asymmetry with respect to the photon axis. Those parameters may provide us with information on the optical behavior of the aerosols. In addition we can extract the ionization potential in direct connection with the absorption cross subsections of the aerosol, from which altitude dependent photodynamics can be unraveled.
We will present here the experiments performed, at different VUV energies, on Titan’s aerosol analogs with the ARPES method and show how the first results can have implications regarding Titan’s atmosphere overall optical characteristics.