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Hybrid molecular ions emitted from CO-NH3 ice bombarded by fission fragments

Abstract:

CO-NH<inf>3</inf> ice at 25 K is bombarded by 65 MeV fission fragments and the emitted secondary ions are analyzed by time-of-flight mass spectrometry. The yields of the specific ion species (those formed only from CO or from NH<inf>3</inf> molecules) and of the hybrid ion species (formed from both CO and NH<inf>3</inf> molecules) are determined as a function of the ice temperature. The time-temperature dependence of desorption yields has been used for secondary ion identification because its behavior characterizes the ion's origin around the sublimation temperature of CO ice (∼30 K). The mass spectrum of positive ions measured before CO sublimation is decomposed into three spectra corresponding to CO specific ions, NH<inf>3</inf> specific ions and hybrid molecular ions, respectively. The observed spectrum after CO sublimation is very similar to that of a pure NH<inf>3</inf> specific spectrum. The total yield of all positive hybrid molecular ions over 600 u mass range is found to be about 2 ions/impact: 20% of this is attributed to N and NH<inf>3</inf> containing ions and 80% are ions having the C<inf>n</inf>O<inf>m</inf>H<inf>l</inf>+ structure. The ions C<inf>n≤3</inf>H<inf>l,2,3</inf>+, COH+, NO+, NH<inf>3</inf>OH+ and NH<inf>3</inf>CO+ were identified. At T = 25 K, the total yield of negative ions is about 20 times lower than that of positive ions, the most abundant species being the cyanide ion CN-. Observation of cluster ions based on (HCN)<inf>n</inf> indicates the formation of hydrogen cyanide. © 2006 Elsevier B.V. All rights reserved.