The aromatic benzene molecule (C 6 H 6 )-a central building block of polycyclic aromatic hydrocarbon molecules-is of crucial importance for the understanding of the organic chemistry of Saturn's largest moon, Titan. Here, we show via laboratory experiments and electronic structure calculations that the benzene molecule can be formed on Titan's surface in situ via non-equilibrium chemistry by cosmic-ray processing of low-temperature acetylene (C 2 H 2 ) ices. The actual yield of benzene depends strongly on the surface coverage. We suggest that the cosmic-ray-mediated chemistry on Titan's surface could be the dominant source of benzene, i.e., a factor of at least two orders of magnitude higher compared to previously modeled precipitation rates, in those regions of the surface which have a high surface coverage of acetylene.

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We investigated the formation of two C 3 H 2 O isomers, i.e., cyclopropenone (c-C 3 H 2 O) and propynal (HCCCHO), in binary ice mixtures of carbon monoxide (CO) and acetylene (C 2 H 2 ) at 10 K in an ultrahigh vacuum machine on highenergy electron irradiation. The chemical evolution of the ice samples was followed online and in situ via a Fourier transform infrared spectrometer and a quadrupole mass spectrometer. The temporal profiles of the cyclopropenone and propynal isomers suggest (pseudo-) first-order kinetics. The cyclic structure (c-C 3 H 2 O) is formed via an addition of triplet carbon monoxide to ground-state acetylene (or vice versa); propynal (HCCCHO) can be synthesized from a carbon monoxideYacetylene complex via a [HCO. . .CCH] radical pair inside the matrix cage. These laboratory studies showed for the first time that both C 3 H 2 O isomers can be formed in low-temperature ices via nonequilibrium chemistry initiated by energetic electrons as formed in the track of Galactic cosmic ray particles penetrating interstellar icy grains in cold molecular clouds. Our results can explain the hitherto unresolved gas phase abundances of cyclopropenone in star-forming regions via sublimation of c-C 3 H 2 O as formed on icy grains in the cold molecular cloud stage. Implications for the heterogeneous oxygen chemistry of Titan and icy terrestrial planets and satellites suggest that the production of oxygen-bearing molecules such as C 3 H 2 O may dominate on aerosol particles compared to pure gas phase chemistry.

Dissociative electron attachment to nicotine, pyridine, and N-methyl-pyrrolidine was studied in the gas phase in order to assess their stability with respect to low-energy electron interactions. Anion yield curves for different products at electron energies ranging from zero to 15 eV were measured, and the molecular fragmentation pathways were proposed. Nicotine does not form a stable parent anion or a dehydrogenated anion, contrary to other biological systems. However, we have observed complex dissociation pathways involving fragmentation at the pyrrolidine side accompanied by isomerization mechanisms. Combining structure optimization and enthalpy calculations, performed with the Gaussian09 package, with the comparison with a deuterium-labeled N-methyl-d3-pyrrolidine allowed for the determination of the fragmentation pathways. In contrast to nicotine and N-methylpyrrolidine, the dominant pathway in dissociative electron attachment to pyridine is the loss of hydrogen, leading to the formation of an [M-H] anion. The presented results provide important new information about the stability of nicotine and its constituent parts and contribute to a better understanding of the fragmentation mechanisms and their effects on the biological environment.

The chemical processing of ethylene ices (C 2 H 4 ) by energetic electrons was investigated at 11 K to simulate the energy transfer processes and synthesis of new molecules induced by secondary electrons generated in the track of galactic cosmic ray particles. A combination of Fourier transform infrared spectrometry (solid state) and quadrupole mass spectrometry (gas phase) resulted in the identification of six hydrocarbon molecules: methane (CH 4 ), the C2 species acetylene (C 2 H 2 ), ethane (C 2 H 6 ), the ethyl radical (C 2 H 5 ), and-for the very first time in ethylene irradiation experiments-the C4 hydrocarbons 1-butene (C 4 H 8 ) and n-butane (C 4 H 10 ). By tracing the temporal evolution of the newly formed molecules spectroscopically online and in situ, we were also able to fit the kinetic profiles with a system of coupled differential equations, eventually providing mechanistic information, reaction pathways, and rate constants on the radiolysis of ethylene ices and the inherent formation of smaller (C1) and more complex (C2, C4) hydrocarbons involving carbon-hydrogen bond ruptures, atomic hydrogen addition processes, and radical-radical recombination pathways. We also discuss the implications of these results on the hydrocarbon chemistry on Titan's surface and on ice-coated, methane-bearing interstellar grains as present in cold molecular clouds such as TMC-1.