Role of photoionization in the formation of complex organic molecules in Titan's upper atmosphere

Abstract

Recent observations by the Cassini Orbiter suggest that complex organic chemistry occurs in the upper atmosphere of Titan. To understand the role of EUV-VUV light in the complex organic chemistry of Titan's upper atmosphere, we investigate the formation of gaseous species from a N₂/CH₄ gas mixture as a function of irradiation wavelengths from 50 nm to 150 nm. A N₂/CH₄ gas mixture (= 95/5) at 0.066 mbar in a windowless photocell is irradiated using a narrowband synchrotron radiation source. The analysis of gaseous products by quadrupole mass spectrometry reveals the formation of heavy organics up to C₇ to C₈ by EUV light irradiation. In particular, the efficient formation of benzene and toluene is observed at wavelengths less than 80 nm, which is well correlated to photoionization of N₂. In Titan's upper atmosphere, EUV radiation may play an important role in the formation of complex organic molecules through photoionization of N₂.