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Secondary ion emission induced by fission fragment impact in CO-NH <inf>3</inf> and CO-NH<inf>3</inf>-H<inf>2</inf>O ices: Modification in the CO-NH<inf>3</inf> ice structure

Abstract:

CO-NH3 and CO-NH3-H2O ices at 25-130 K were bombarded by 252Cf fission fragments (∼65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH3 molecules) emitted from CO-NH3 ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO-NH3 ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH3-H2O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH3 ice mixture generates molecular ion series such as (NH3)p-q(H2O)qCO + and replaces the cluster series (NH3)nNH 4+ emission by the hybrid series (NH3) I-i(H2O)i=1,2..1H+. The distribution of NH3 and H2O molecules within the cluster groups indicates that ammonia and water mix homogenously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH 3)nNH4+ is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH3 ice, formed by the condensation of the CO-NH3 gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure. Copyright © 2007 John Wiley & Sons, Ltd.