Low-Pressure EUV Photochemical Experiments: Insight on the Ion Chemistry Occurring in Titan's Atmosphere

Thanks to the Cassini spacecraft onboard instruments, it has been known that Titan's ionospheric chemistry is complex and the molecular growth is initiated through the photolysis of the most abundant species directly in the upper atmosphere. Among the pool of chemical compounds formed by the photolysis, N-bearing species are involved in the haze formation, but the chemical incorporation pathways need to be better constrained. In this work, we performed low-pressure extreme ultraviolet (EUV) photochemical laboratory experiments. The APSIS reactor was filled with a N-2/CH4 (90/10%) gas mixture relevant to the upper atmosphere of Titan. The cell was irradiated by using a EUV photon source at 73.6 nm, which has been difficult to produce in the laboratory for previous studies. The photoproducts (both neutral and ionic species) were monitored in situ with a quadrupole mass spectrometer. The chemical pathways are explained by confronting experimental observations and numerical predictions of the photoproducts. The most interesting result in this work is that methanimine was the only stable N-bearing neutral molecule detected during the experiments, and it relies on N2+ production. This experimental result is in agreement with the relatively high abundance predicted by 1-D photochemical models of Titan's atmosphere and comforts methanimine as an intermediate toward the formation of complex N-bearing organic molecules. This experiment is only testing one part of the overall chemical scheme for Titan's upper atmosphere due to the selective wavelength but demonstrates the capability to probe the chemical pathways occurring in Titan's atmosphere by minimizing bias coming from wall surface reactions.