EUV Photochemical Production of Unsaturated Hydrocarbons: Implications. to EUV Photochemistry in Titan and Jovian Planets

The EUV photochemistry of methane is one of the dominant chemical processes in the upper atmospheres of Titan and Jovian planets. The dilution of CH4 with N-2 significantly changes the subsequent hydrocarbon chemistry initiated by EUV photoionization. At wavelengths below 80 rim, the presence of the dominant N-2 species in a N-2/CH4 gas mixture (=95/5) selectively enhances the formation of unsaturated hydrocarbons, such as benzene and toluene, while pure CH4 gas leads to a wide mixture of saturated/unsaturated hydrocarbon species. This enhanced formation of unsaturated hydrocarbons is most likely initiated by the generation of CH3+ via a dissociative charge-transfer reaction between N-2(+) and CH4. This mechanism was further confirmed with the dilution of CH4 with Ar, which shows similarly enhanced formation of unsaturated species from an Ar/CH4 (=95/5) gas mixture. In contrast, the depleted generation of unsaturated species from a H-2/CH4 gas mixture (=95/5) suggests that the CH5+ ion generated via a proton-transfer reaction is not an important precursor in the production of complex unsaturated hydrocarbons. Therefore, it is the dissociative charge-transfer reaction of CH4 that initiates the formation of unsaturated complex hydrocarbons through production Of C2H5+ with subsequent dissociative recombination. Implications regarding photochemistry in the upper atmospheres of Titan and the Jovian planets are discussed.