Aromatic Formation Promoted by Ion-Driven Radical Pathways in EUV Photochemical Experiments Simulating Titan’s Atmospheric Chemistry

Abstract

In the atmosphere of Titan, Saturn's main satellite, molecular growth is initiated by 85.6 nm-Extreme UltraViolet (EUV) photons triggering a chemistry with charged and free-radical species. However, the respective contribution of these species to the complexification of matter is far from being known. This work presents a chemical analysis in order to contribute to a better understanding of aromatic formation pathways. A gas mixture N 2 /CH 4 (90/10 %) within the closed SURFACAT reactor was irradiated at relatively low pressure (0.1 mbar) and room temperature for six hours by EUV photons (∼ 85.6 nm). The neutral molecules formed at the end of the irradiation were condensed in a cryogenic trap and analyzed by electron ionization mass spectrometry. An analysis of the dominant chemical pathways highlights the identification of benzene and toluene and underlies the importance of small ion and radical reactions. On the basis of the experimental results, a speculative mechanism based on sequential Helimination/CH 3 -addition reactions is proposed for the growth of aromatics in Titan's atmosphere. Elementary reactions to be studied are given to instill future updates of photochemical models of Titan's atmosphere.

Coincidence study of core-ionized adamantane: site-sensitivity within a carbon cage?

“…Our results show that the adamantane molecule fragments into small hydrocarbons after exposure to soft X-rays and could partially play a part in the deficiency of diamondoids in space. In addition, diamondoids could contribute as a source of reactive species for carbonaceous cyclic compounds formation 60 . Therefore, this study could be used to model the X-ray driven photodissociation of adamantane and perhaps other diamondoids in accompaniment to the X-ray driven photodissocation of PAHs.…”

“…In addition, diamondoids could contribute as a source of reactive species for carbonaceous cyclic compounds formation. 60 Therefore, this study could be used to model the X-ray driven photodissociation of adamantane and perhaps other diamondoids in accompaniment to the X-ray driven photodissocation of PAHs.…”

“…It is also worth mentioning that the combined branching fractions of ion pairs that include CH + 3 , C 2 H + 3 , and C 3 H + 3 , account for almost 40% thus could contribute to the astrochemistry of certain regions 59 , for instance planetary atmospheres such as Titan's one where formation of large polycyclic aromatic hydrocarbons (PAHs) are observed 60 .…”

Time-resolved photoelectron spectroscopy: the continuing evolution of a mature technique

“…For example, at 166 kHz, the stability of high-power third-order harmonic at 85.7 nm (14.5 eV) of a 257 nm driving field, selected by a single SiO 2 plate under 30° grazing incidence, combined with 150 nm thick indium foil, has been tested for several hours with a flux of 4 × 10 11 photons s −1 on molecular targets. 91 Obviously, other VUV wavelengths can be produced based on high harmonic generation with lower cascaded harmonics of the 1 μm driving field or its second harmonic. 92 The main challenge is however to handle appropriately the drifting pointing of the EUV beam resulting from thermal effects in the monochromatisation stage.…”

Titan Atmospheric Chemistry Revealed by Low-Temperature N2–CH4 Plasma Discharge Experiments

“…Plasma discharges may be able to simulate the chemistry induced by the charged particles. Moreover, the energy distribution of the plasma discharges provides a closer simulation of the solar spectrum energy distribution than that offered by UV photon sources. , Laboratory experiments with UV photons usually employ a typical Lyman alpha emission lamp ,, or EUV photons at selective wavelengths (60/82.5 nm, 73.6 nm, 85.6 nm, and 89.2 nm). The plasma discharges are not only energetic enough to dissociate N 2 directly (while a typical Lyman alpha emission lamp could not), but also offer a wider energy distribution than EUV sources.…”