Ion irradiation of asphaltite as an analogue for solid hydrocarbons in the interstellar medium

Abstract

We present the results of experiments of low temperature ion irradiation of pure asphaltite samples and those covered with water ice layers. Asphaltite (a natural bitumen) was used as an analogue of carbonaceous interstellar grains on which water-rich icy mantles can be formed in the dense and cold phases of the interstellar medium. We simulated irradiation by cosmic ions in the laboratory to study, by in situ infrared spectroscopy, the formation of molecular species in the asphaltite samples and at the interface between the refractory solid (asphaltite) and the water ice mantle. New molecular species were formed at the expense of water molecules and atoms (mainly carbon and hydrogen) from the asphaltite substrate. The vibration modes we detected at low temperature (16 K) include: C-H stretch (carbynoids), C-H asymmetric stretch (acetylene), C-H aromatic stretch, C=O stretch in CO₂, C≡O stretch in CO, and C≡C stretch (carbynoids). Some of these species remain stable after warming of the irradiated samples to room temperature.