2015ApJ...814...45A

The formation routes of two structural isomers–propenal (C₂H₃CHO) and cyclopropanone (c-C₃H₄O)–were investigated experimentally by exposing ices of astrophysical interest to energetic electrons at 5.5 K thus mimicking the interaction of ionizing radiation with interstellar ices in cold molecular clouds. The radiation-induced processing of these ices was monitored online and in situ via Fourier Transform Infrared spectroscopy and via temperature programmed desorption exploiting highly sensitive reflectron time-of-flight mass spectrometry coupled with single photon ionization in the post irradiation phase. To selectively probe which isomer(s) is/are formed, the photoionization experiments were conducted with 10.49 and 9.60 eV photons. Our studies provided compelling evidence on the formation of both isomers–propenal (C₂H₃CHO) and cyclopropanone (c-C₃H₄O)–in ethylene (C₂H₄)–carbon monoxide (CO) ices forming propenal and cyclopropanone at a ratio of (4.5±0.9):1. Based on the extracted reaction pathways, the cyclopropanone molecule can be classified as a tracer of a low temperature non-equilibrium chemistry within interstellar ices involving most likely excited triplet states, whereas propenal can be formed at ultralow temperatures, but also during the annealing phase via non-equilibrium as well as thermal chemistry (radical recombination). Since propenal has been detected in the interstellar medium and our laboratory experiments demonstrate that both isomers originated from identical precursor molecules our study predicts that the hitherto elusive second isomer–cyclopropanone–should also be observable toward those astronomical sources such as Sgr B2(N) in which propenal has been detected.