Energetic Electron Irradiation of Ices
Many types of radiation fields ubiquitous in interstellar and circumstellar regions can chemically process carbon monoxide, giving rise to a variety of effects going from chemical evolution to photodesorption. In our recent study, we exploit electrons in the middle-range energy (150-1000 eV), to explore the effects at energies closer to mean energies of primary electrons produced by cosmic-rays and X-rays impacting with molecular gas. Our results of CO ice irradiated with electrons of energy in the range150−1000 eV show that the main products of irradiation are carbon chains Cn (n = 3,5, 6, 8, 9, 10, 11, 12), suboxides, CnO (n = 2, 3, 4, 5, 6, 7), and CnO2 (n = 1, 3, 4, 5, 7). The inventory of reaction products does not change with the energy of the impinging electrons. As for other energetic sources (e.g., VUV photons or X-rays), the most abundant species is CO2. When the thickness of the ice is comparable with the penetration depth (obtained from CASINO code), the product abundances decreases with respect to the case of thicker ice.
During the periods of energetic CO ice irradiation, the parent CO and products and some products desorbed from the ice. Among these products, the main reaction products, CO2, also lead to the strongest desorption signal. The positive correlation of CO and CO2 desorption yields suggests that CO2 co-desorbs with the much more abundant CO. The accumulated counts of desorbing species decrease with the energy of the impacting electrons. For electrons with higher energies, the penetration depth increases significantly, thus less electrons contribute the energy in the top few MLs, we may expect a significantly lower desorption (per eV), consequently a richer chemical production, and the CO2 column density peak located at a much larger absorbed energy.
The chemistry that we observe as the result of CO ice electron processing may have implications in the atmospheric photochemistry of cold planets hosting surface CO ices.