On Simulating the Proton-Irradiation of O$_2$ and H$_2$O Ices Using
  Astrochemical-type Models, with Implications for Bulk Reactivity
  Many astrochemical models today explicitly consider the species that comprise
the bulk of interstellar dust grain ice-mantles separately from those in the
top few monolayers. Bombardment of these ices by ionizing radiation - whether
in the form of cosmic rays, stellar winds, or radionuclide emission -
represents an astrochemically viable means of driving a rich chemistry even in
the bulk of the ice-mantle, now supported by a large body of work in laboratory
astrophysics. In this study, using an existing rate equation-based
astrochemical code modified to include a method of considering radiation
chemistry recently developed by us, we attempted to simulate two such studies
in which (a) pure O$_2$ ice at 5 K and, (b) pure H$_2$O ice at 16 K and 77 K,
were bombarded by keV H$^+$ ions.
  Our aims are twofold: (1) to test the capability of our newly developed
method to replicate the results of ice-irradiation experiments, and (2) to
determine in such a well-constrained system how bulk chemistry is best handled
using the same gas-grain codes that are used to model the interstellar medium
(ISM). We find that our modified astrochemical model is able to reproduce both
the abundance of O$_3$ in the 5 K pure O$_2$ ice, as well as both the abundance
of H$_2$O$_2$ in the 16 K water ice and the previously noted decrease of
hydrogen peroxide at higher temperatures. However, these results require the
assumption that radicals and other reactive species produced via radiolysis
react quickly and non-diffusively with neighbors in the ice.
