20-EPN-005: Cosmic-ray-induced chemistry in pure ices
September 5, 2021

20-EPN-005: Cosmic-ray-induced chemistry in pure ices

Virtual visit by Alexei Ivlev, Max Planck Institute for Extraterrestrial Physics (MPE) (Germany) to TA2.11 Atomki Ice Chamber for Astrophysics / Astrochemistry (ICA) (Hungary).
Dates of visit: 23 February – 05 July 2021

Report Summary: The principal aim of the project was a dedicated study of generic effects induced in pure astrophysical ice analogs due to their bombardment by cosmic rays with energies E in the vicinity of the maximum of electronic stopping power. It is known that the energy of ejected electrons, which are produced in primary ionization events, has a significant dependence on E in this energy range. 

Thus, by selecting pairs of beam energies on both sides of the Bragg peak, such that the corresponding stopping-power values are equal, we were able to probe the effect of electron-impact excitations of ice molecules. We selected CO films as the best irradiation target, for which the biggest variety of radiolysis products was expected and the most detailed predictions of chemical models were available. 

We found that the first radiolysis products, detected at the astrophysically relevant values of ion fluence, are very different from predictions of chemical models. At the same time, the reaction kinetics shows no statistically significant difference between ion beams of same stopping power. This rules out the importance of electron-impact excitation in radiolysis chemistry of CO, and suggests that this process may generally be negligible compared to the chemistry driven by CR heating (determined by the stopping power value). On the other hand, by comparing the sputtering yields measured for beams of same stopping power, we discovered a significant asymmetry, with the yield at lower energies being up to a factor of two larger that at higher energies.

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