A microwave plasma source for VUV atmospheric photochemistry

International audienceMicrowave plasma discharges working at low pressure are nowadays a well-developed technique mainly used to provide radiations at different wavelengths. The aim of this work is to show that those discharges are an efficient windowless VUV photon source for planetary atmospheric photochemistry experiments. To do this, we use a surfatron-type discharge with a neon gas flow in the mbar pressure range coupled to a photochemical reactor. Working in the VUV range allows to focus on nitrogen-dominated atmospheres (λ < 100 nm). The experimental setup makes sure that no other energy sources (electrons, metastable atoms) than the VUV photons interact with the reactive medium. Neon owns two resonance lines at 73.6 and 74.3 nm which behave differently regarding the pressure or power conditions. In parallel, the VUV photon flux emitted at 73.6 nm has been experimentally estimated in different conditions of pressure and power and varies in a large range between 2×10 13 ph/s/cm 2 and 4×10 14 ph/s/cm 2 which is comparable to a VUV synchrotron photon flux. Our first case study is the atmosphere of Titan and its N2-CH4 atmosphere. With this VUV source, the production of HCN and C2N2, two major Titan compounds, is detected, ensuring the suitability of the source for atmospheric photochemistry experiments. PACS. 50 Physics of gases, plasmas, and electric discharges 52.70.Kz Optical (ultraviolet, visible, infrared) measurements 52.80.Pi High-frequency and RF discharges 52.80.Yr Discharges for spectral sources (including inductively coupled plasma