Ion desorption from frozen H 2O irradiated by MeV heavy ions

Abstract

Nitrogen (0.13–0.85 MeV) and 252Cf fission fragments (∼65 MeV) beams are employed to sputter positive and negative secondary ions from frozen water. Desorption yields are measured for different ice temperatures and projectile energies. Target surface is continuously refreshed by condensed water while the target temperature varies and ice thickness changes. In both projectile energy ranges, the preferentially ejected ions are H +, H 2 + and (H 2O) n H +-cluster ions. The yields of the corresponding negative ions H − and (H 2O) n O − or (H 2O) n OH − are 1–2 orders of magnitude lower. The (H 2O) n H + desorption yields decrease exponentially as the cluster size, n, increases. In the low energy range, the desorption of positive ion clusters may occur in a two-step process: first, desorption of preformed H 2O clusters and, then, ionization by H + or H 3O + capture. For 0.81 MeV N + projectile ions, the cluster ion emission contributes with 0.05% to the total H 2O desorbed yield. There are indications that emission of the (H 2O) n H + disappears for an electronic energy loss lower than 20 eV/Å. For the high energy range, desorption of small ion clusters is particularly enhanced, revealing that a fragmentation process also exists.