Production of Hydronium Ion (H3O)+ and Protonated Water Clusters (H2O)nH+ after Energetic Ion Bombardment of Water Ice in Astrophysical Environments

Water ice exists on many objects in space. The most abundant icy species, among them water, are present in the icy satellites of the outer Solar System giant planets. The nuclei of comets, which are mainly composed of water ice, give another example of its abundance. In the interstellar medium (ISM), ice mantles, formed by molecular species sticking on dust grains, consist mainly of water ice. All these objects are exposed to ionizing radiation like ions, UV photons, and electrons. Sputtering of atoms, molecules, ions, and radicals from icy surfaces may populate and maintain exospheres of icy objects in the Solar System. In other respects, ionized hydrides such as OH+, H2O+, and H3O+ have been detected in the gas phase in star-forming regions. Interactions with cosmic rays could be an additional explanation to the current models for the formation of those species. In fact, laboratory simulations showed that the main components of the sputtered ionic species from water ice are oxygen hydrides. In this work, water ice targets were irradiated at several temperatures (10-200 K) by 90 keV O6+ ions, yielding an electronic stopping power of about 12 eV/Å, when the nuclear stopping power is comparable to the electronic stopping power. Sputtering of secondary ions after bombardment of the ice target was analyzed by time-of-flight mass spectrometry (TOF-SIMS). Besides hydrogen ions (H+, H2+, H3+), also O+, O2+, OH+, (H2O)+, and clusters of (H2O)nH+ with n = 1-8 are emitted. Our results show a progressive yield decrease with increasing temperature of all of the detected species. This is related to the structure of the ice: the ionic sputtering yield for crystalline ice is much lower than for an amorphous ice. For instance, amorphous ice at 10 K exhibits a yield of the order of 2 × 10-6 secondary (H2O)nH+ hydride ions/projectile (with n = 1-8). As the temperature is increasing toward the phase transition to crystalline ice, the yields decrease by about one order of magnitude