Irradiation of condensed CO reveals a new pathway for the formation of aromatic molecules in astrochemical ices

Given the importance of carbon monoxide in the interstellar medium (ISM) both in the gas and ice phase, the condensed phase CO has been studied by the irradiation of energetic particles, UV irradiation and the products formed from CO dissociation have been investigated for nearly four decades. However, our understanding on the physical nature of the residue made from CO ice irradiation is limited to-date. Hence we irradiated CO ice with 2 keV electrons and probed the ice using vacuum ultraviolet / ultraviolet (VUV/UV) spectroscopy techniques. The in-situ VUV/UV spectral analysis of irradiated product provided compelling evidence (peak at ∼ 240 nm) for the presence of a refractory residue made of carbon atoms. The ex-situ analysis carried out using high resolution transmission electron microscopy (HR-TEM) revealed the presence of ordered carbon atoms viz-a-viz graphene, graphitic carbon and graphitic quantum dots. The direct ring closure from carbon atoms released from CO reveal a new pathway to be considered in the bottom-up formation of polycyclic aromatic hydrocarbon (PAH) molecules, on cold dust in the ISM, via the hydrogenation of graphene / graphitic carbon.