Radiation effects in water ice: A near-edge x-ray absorption fine structure study.

The changes in the structure and composition of vapor-deposited ice films irradiated at 20 K with soft x-ray photons (3–900 eV) and their subsequent evolution with temperatures between 20 and 150 K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO₂ radicals, as well as the oxygen O₂ and hydrogen peroxide H₂O₂ molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (I_cryst) ice films. The evolution of their concentrations with the temperature indicates that HO₂, O₂, and H₂O₂ result from a simple step reaction fuelled by OH, where O₂ is a product of HO₂ and HO₂ a product of H₂O₂. The local order of ice is also modified, whatever the initial structure is. The crystalline ice I_cryst becomes amorphous. The high-density amorphous phase (I_ah) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (I_al). The phase I_ah is thus peculiar to irradiated ice and does not exist in the as-deposited ice films. A new “very high density” amorphous phase—we call I_avh—is obtained after warming at 50 K the irradiated p-ASW ice. This phase is stable up to 90 K and partially transforms into crystalline ice at 150 K. [ABSTRACT FROM AUTHOR]