Irradiation-enhanced adsorption and trapping of O2 on nanoporous water ice

We have found that irradiation with 50–150 keV protons enhances gas adsorption in nanoporous amorphous ice by creating high-energy binding sites. If irradiation is done in vacuum, the ice is compacted and does not adsorb significantly in subsequent exposure to gas. If irradiation occurs while the ice is exposed to an ambient O2 pressure, adsorption is enhanced by a factor as high as 5.5 compared to unirradiated ice. The maximum concentration of adsorbed O2 increased with increasing pressure and decreasing ion flux, achieving 6% throughout the ion-penetration depth in the low-flux limit at 7.2 10−5 mbar and 50 K . After simultaneous irradiation and oxygen exposure, the adsorbed O2 could be retained in the ice when the ambient oxygen was removed, indicating that ion impacts close nanopores containing O2. Similar results, but with somewhat lower gas/ice ratios where obtained with Ar, which has a lower binding energy than O2. The experiments suggest a mechanism for gas trapping in comets and icy satellites in the outer Solar system.