Photo-conductivity measurements of water-rich ices at cryogenic temperatures

Abstract

Solid water (H_2O) ice is found in high abundances in a large variety of astrophys. environments: e.g. icy mantles coating interstellar dust grains, in icy bodies like comets and satellites like Europa where exposure to energetic photons, ions and electrons is significant. In these conditions, solid H O is known to dissoc. into H and OH fragments under VUV irradn. while promoting ionization of org. impurities such as polycyclic arom. hydrocarbons (PAHs). This was explained in terms of the relatively high electron affinity of the H_2O mols. and of the OH fragments which, in synergy with the thermodn. stability assocd. with the solvation of ions in solid H_2O, enhances the ionization of pos. charged PAHs. If such ions are indeed produced, these will change the dielec. properties of the ice and can move inside the H_2O matrix when a bias is applied, depending on the ice temp. Our work reports on the investigation of photocurrents in millimeter-sized, water-rich ices contg. org. impurities (e.g. toluene) and aims at understanding how these measurements correlate with the desorption of volatiles during vacuum UV (VUV, e.g. 121.6 nm) and electron (2 keV) irradn.