Side group addition to the polycyclic aromatic hydrocarbon coronene by proton irradiation in cosmic ice analogs

Ices at 15 K consisting of the polycyclic aromatic hydrocarbon coronene (C24H12) condensed either with H2O, CO2, or CO in the ratio of 1:100 or greater have been subjected to MeV proton bombardment from a Van de Graaff generator. The resulting reaction products have been examined by infrared transmission-reflection-transmission spectroscopy and by microprobe laser-desorption laser-ionization mass spectrometry. Just as in the case of UV photolysis, oxygen atoms are added to coronene, yielding, in the case of H2O ices, the addition of one or more alcohol (-OH) and ketone (>C=O) side chains to the coronene scaffolding. There are, however, significant differences between the products formed by proton irradiation and the products formed by UV photolysis of coronene containing CO and CO2 ices. The formation of a coronene carboxylic acid (-COOH) by proton irradiation is facile in solid CO but not in CO2, the reverse of what was previously observed for UV photolysis under otherwise identical conditions. This work presents evidence that cosmicray irradiation of interstellar or cometary ices should have contributed to the formation of aromatics bearing ketone and carboxylic acid functional groups in primitive meteorites and interplanetary dust particles.