In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C 2 H 2 ice

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 squared 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 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.