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Hybrid molecular ions emitted from CO-NH<inf>3</inf> ice bombarded by fission fragments

Abstract:

CO-NH3 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 NH3 molecules) and of the hybrid ion species (formed from both CO and NH3 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, NH3 specific ions and hybrid molecular ions, respectively. The observed spectrum after CO sublimation is very similar to that of a pure NH3 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 NH3 containing ions and 80% are ions having the CnOmHl+ structure. The ions Cn≤3Hl,2,3+, COH+, NO+, NH3OH+ and NH3CO+ 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)n indicates the formation of hydrogen cyanide. © 2006 Elsevier B.V. All rights reserved.