Secondary Ion Emission from Water Ice at 10-130 K Induced by MeV N2+ Ions

Secondary ion emission from vapor-deposited water ice has been investigated in exploring the influence of phase transitions on the desorption yield of H2O ions, particularly, the hydronium H3O+ and the cluster ions (H2O) H3O+. For 1.5 MeV N2+ ion bombardment on water ice at a temperature of similar to 80 K, the sum of the yields of H3O+ and of the cluster ions represents 80% of the positive ion yield (i.e., 0.23 ions/impact) and 0.33% of the total H2O molecular sputtering yield. When the beam energy of N2+ is varied between 0.5 and 1.7 MeV, the shape of the cluster mass distributions does not change and the total cluster ion yield scales with the third power of the electronic energy loss. Time-of-flight mass spectra were taken during the warming up of the ice from 10 to 216 K. Up to 30 K, the cluster ion yields are approximately constant, whereas between 30 and 75 K, they decrease gradually by a factor 6.5 and remain constant until 130 K. This is in analogy with modifications in the ice structure: in the 38-78 K region, the amorphous ice phases change gradually from, the high-density to low-density form. The high ion yields below 30 K are supposedly caused by a reduction of charge carrier transport in high-density amorphous ice. The yield ratio of Y((H2O)H3O+)/Y(H3O+), measured as a function of time/temperature, exhibits clearly the three amorphous regimes and, above 130 K, the change in the composition of the top layer of the ice due to enrichment with organic material. This procedure offers a reliable method to identify structural changes of water ice, such as phase transitions.