Bombardment of CO Ice by Cosmic Rays. I. Experimental Insights into the Microphysics of Molecule Destruction and Sputtering

Ivlev, Alexei V. and Giuliano, Barbara M. and Juhász, Zoltán and Herczku, Péter and Sulik, Béla and Mifsud, Duncan and Kovács, Sándor and Rácz, Richárd Péter and Biri, Sándor and Rajta, István and Vajda, István K. and Mason, Nigel J. (2023) Bombardment of CO Ice by Cosmic Rays. I. Experimental Insights into the Microphysics of Molecule Destruction and Sputtering. ASTROPHYSICAL JOURNAL, 944 (2). ISSN 1538-4357

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Abstract

We present a dedicated experimental study of microscopic mechanisms controlling radiolysis and sputtering of astrophysical ices upon bombardment by cosmic-ray ions. Such ions are slowed down owing to inelastic collisions with bound electrons, resulting in ionization and excitation of ice molecules. In experiments on CO ice irradiation, we show that the relative contribution of these two mechanisms of energy loss to molecule destruction and sputtering can be probed by selecting ion energies near the peak of the electronic stopping power. We have observed a significant asymmetry, in both the destruction cross section and the sputtering yield, for pairs of ion energies corresponding to the same values of the stopping power on either side of the peak. This implies that the stopping power does not solely control these processes, as usually assumed in the literature. Our results suggest that electronic excitations represent a significantly more efficient channel for radiolysis and, likely, for sputtering of CO ice. We also show that the charge state of incident ions and the rate for CO + production in the ice have a negligible effect on these processes.

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