FUV Photoionization of Titan Atmospheric Aerosols

Thanks to the Cassini-Huygens mission, it is now established that the first aerosols in Titan's upper atmosphere are found from an altitude of ∼1200 km. Once they are formed and through their descent toward the surface, these nanoparticles are submitted to persistent far-ultraviolet (FUV) radiation that can reach lower atmospheric layers. Such an interaction has an impact, especially on the chemistry and charge budget of the atmospheric compounds. Models are useful to understand this photoprocessing, but they lack important input data such as the photoemission threshold or the absolute photoabsorption/emission cross sections of the aerosols. In order to quantify the photoemission processes, analogs of Titan's aerosols have been studied with the DESIRS FUV beamline at the synchrotron SOLEIL as isolated substrate-free nanoparticles. We present here the corresponding angle-resolved photoelectron spectroscopy data recorded at different FUV photon energies. The results show a very low photoionization threshold (6.0 0.1 eV ∼ 207 nm) and very high absolute ionization cross sections (∼106 Mb), indicating that FUV photoemission from aerosols is an intense source of slow electrons that has to be taken into account in photochemical models of Titan's atmosphere.