VUV photochemistry of cyclopropenone (c-C₃H₂O): formation rate and abundance ratios of propynal (HCCCHO) and propadienone (CH₂CCO)

The distribution of isomeric species in the interstellar medium cannot be directly related to their relative energetic stabilities but more to their mechanisms of formation and evolution. The abundances of the three isomers of C₃H₂O, cyclopropenone, propynal and propadienone, are an example among many other interstellar species wherein kinetic effects control their presence in astrophysical regions. To date, only propynal and cyclopropenone, the two less stable isomers of propadienone, have been detected in the interstellar medium. In this work, we examined the vacuum ultraviolet (VUV) photochemistry of cyclopropenone (c-C₃H₂O), an unsaturated ketone and aromatic compound, which is the most thermodynamically unstable form of the three isomers of C₃H₂O. Using Fourier transform infrared spectroscopy and mass spectrometry to probe the photoproducts, we investigated the photolysis of cyclopropenone ice and cyclopropenone trapped in a neon matrix. We showed that the VUV photolysis of c-C₃H₂O ice at 10 K led mainly to CO and C₂H₂ fragments, in addition to propynal HCCCHO and propadienone CH₂CCO. The distribution of the two isomers depended on the irradiation time. Within a short irradiation time, HCCCHO was formed in twice the abundance of the most stable isomer CH₂CCO, while upon increasing the irradiation time, the same abundances were observed, showing that equilibrium was reached between the two isomers. The photolysis of c-C₃H₂O isolated in the neon matrix allowed the characterization of HCCCO and CCCO as reaction intermediates, resulting from the VUV-induced dehydrogenation of cyclopropenone, which are the precursors of propynal and propadienone through the CCCO + 2H reaction.