Vacuum ultraviolet photochemistry of solid acetylene: a multispectral approach

Abstract

Aims. Gas phase acetylene (C 2 H 2 ) and polyynes (H(-C≡C-) m H) are ubiquitous in the interstellar medium. However, astrochemical models systematically underestimate the observed abundances, supporting the idea that enrichment from the solid state takes place. In this laboratory-based study, we investigate the role C 2 H 2 plays in interstellar ice chemistry and we discuss the way its photoproducts may affect gas phase compositions. Methods. C 2 H 2 ice is investigated under vacuum ultraviolet (VUV) irradiation in its pure form as present in the atmosphere of Titan and in a water-dominated ice as present on grain mantles in molecular clouds and on comets. To disentangle the photochemical network, a unique, complementary combination of infrared and ultraviolet-visible (UV-VIS) spectroscopy is used. Results. From the experimental results, it can be concluded that the VUV-induced solid state C 2 H 2 reaction network is dominated by polymerization resulting in the formation of polyynes at least up to C 20 H 2 and larger polyyne-like molecules. At low temperatures, this process takes place very efficiently and suggests low barriers. When extending this reaction scheme to a water-rich environment, the dominant reaction products are CO and CO 2 but the simultaneous detection of polyyne like molecules is evidence that the reactions as observed in pure C 2 H 2 ice persist. Conclusions. From the spectroscopic evidence as presented in this laboratory study, it is concluded that the formation of polyynes upon VUV irradiation of interstellar ices is a process that may contribute to at least part of the observed gas phase enrichment in space.

Photochemistry and Astrochemistry: Photochemical Pathways to Interstellar Complex Organic Molecules

“…Two prototypical PAHs (pyrene and coronene) were photolyzed using PAH:H 2 O concentrations in the range of 1:30 000 to pure PAH at temperatures between 12 and 125 K. Both the photoproduct composition and the ionization efficiency depends on PAH concentration; the ionization efficiency is 60% in dilute ices, but only 15% in a 1:1000 ice. 150 An increased PAH concentration results in a lower ionization yield due to a combination of an increased recombination rate with increased PAH clustering, and a decreased solvation efficiency of electrons as the OH/e-ratio decreases. OH has a higher electron affinity than H 2 O and the lower PAH-to-H 2 O concentration the higher number of photoproduced electrons will become trapped by OH, increasing the ionization efficiency.…”

“…Perhaps by chance, most of them involve simple mixtures of a small hydrocarbon and water, i.e. photolyzed mixtures of CH 4 :H 2 O,137,149 and C 2 H 2 :H 2 O ices 150.…”

Ab initio dynamics and photoionization mass spectrometry reveal ion–molecule pathways from ionized acetylene clusters to benzene cation

“…It has also been suggested that the formation of benzene can proceed via barrierless radical reaction of ethynyl radical and 1,3-butadiene (30,32). Other studies focused on the formation of benzene and other organic molecules upon irradiation of acetylene ices (22,33). Notwithstanding this progress, the formation of benzene (or its cation) in the ISM is still not fully understood.…”

“…In soot formation, extensive work has been done on the formation of the first aromatic molecule (7). Among the proposed reactions, recombination of propargyl radicals is believed to be an important step that leads (22) to cyclic products such as benzene and C 6 H 5 (23,24). Other important radical reactions include reactions of C 4 H 3 and C 4 H 5 with acetylene (25)(26)(27).…”

Photoreactivity of condensed acetylene on Titan aerosols analogues

“…Direct solid-state photochemistry of C 2 H 2 required higher energy photons than the 355 nm used in this experiment, as absorption of the acetylene (for the first excited S 0 -S 1 state) starting at wavelength shorter than 237 nm (Couturier-Tamburelli et al, 2015;Malsch et al, 2001). Moreover, the direct acetylene ice photochemistry in the VUV has been experimentally studied, highlighting the formation of longer hydrocarbons molecules, including the formation of polymer (Compagnini et al, 2009;Cuylle et al, 2014). They were not observed in our experiments, excluding direct photochemistry in the acetylene layer, also in agreement with our observation that no significant depletion of C 2 H 2 was detected when pure C 2 H 2 was deposited on a sapphire window.…”