Cluster emission and chemical reactions in oxygen and nitrogen ices induced by fast heavy-ion impact

Two ices, O-2 and a mixture of O-2 and N-2, are bombarded by Cf-252 fission fragments (FF) (similar to 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N-2 sublimation, in which hybrid chemical species are formed, (2) before 02 sublimation, in which the TOF mass spectrum is dominated by low-mass (O-2)(p) cluster ions and (3) after O-2 Sublimation, in which (N-2)(p) or (O-2)(p) cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2 +/-, and N4On-4-. In the second region, two positive and negative ion series are identified: (O-2)(p)O-+/- and (O-2)(p)O-2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O-3 or 3O(2), units in the cluster structure, respectively. Copyright (C) 2008 John Wiley & Sons, Ltd.