In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C${}_2$H${}_2$ ice

Abstract

Acetylene (C${}_2$H${}_2$) is commonly observed in star-forming regions, young stellar objects, and the Solar System. Laboratory and theoretical studies have linked this simplest alkyne to volatile hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) through UV- or cosmic-ray-driven chemistry, but it remains unclear whether refractory material can efficiently form through solid-state reactions of C${}_2$H${}_2$ on dust grains. We experimentally investigate the chemical complexity induced by UV irradiation of pure C${}_2$H${}_2$ ice and characterize both volatile and nonvolatile photoproducts. Experiments were performed with MATRI${}^2$CES under ultra-high-vacuum conditions at 15 K using 7.2--10.2 eV photons. UV-processed ices were monitored in situ by laser desorption post-ionization reflection time-of-flight mass spectrometry (LDPI ReTOF-MS) combined with pulsed ion deflection (PID). Volatile and refractory products were measured in situ at 15 and 300 K, respectively. After a fluence of $3\times 10^{17}$ photons cm${}^{-2}$, corresponding to about $10^6$ years in dense clouds, large saturated and unsaturated hydrocarbons containing up to 13 carbon atoms are formed. After sublimation of the volatile products, the 300 K residue shows a rich and distinct mass spectrum consistent with refractory material containing conjugated triple (-C$\equiv$C-) and double (-C=C-) bonds. These results demonstrate that UV processing of pure C${}_2$H${}_2$ ice can produce substantial molecular complexity and refractory hydrocarbons under astronomically relevant conditions, with possible implications for unidentified infrared emission bands.

arXiv comment25 pages, 9 figures

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