Effects of $150-1000$ eV Electron Impacts on Pure Carbon Monoxide Ices using the Interstellar Energetic-Process System (IEPS)

Pure CO ice has been irradiated with electrons of energy in the range $150-1000$~eV with the Interstellar Energetic-Process System (IEPS). The main products of irradiation are carbon chains C$_n$ ($n=3$, 5, 6, 8, 9, 10, 11, 12), suboxides, C$_n$O ($n=2$, 3, 4, 5, 6, 7), and C$_n$O$_2$ ($n=1$, 3, 4, 5, 7) species. \ce{CO2} is by far the most abundant reaction product in all the experiments. The destruction cross-section of CO peaks at about 250 eV, decreases with the energy of the electrons and is more than one order of magnitude higher than for gas-phase CO ionization. The production cross-section of carbon dioxide has been also derived and is characterized by the competition between chemistry and desorption. Desorption of CO and of new species during the radiolysis follows the electron distribution in the ice. Low energy electrons having short penetration depths induce significant desorption. Finally, as the ice thickness approaches the electron penetration depth the abundance of the products starts to saturate. Implications on the atmospheric photochemistry of cold planets hosting surface CO ices are also discussed.