Electron impact ionization of argonclusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe12+ is unambiguously confirmed; the spectra also prove the formation of Ar2Hen+ complexes that had been claimed to fragment into pure Ar2+. Distributions of larger argoncluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argonclusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argoncluster ions. Intriguing abundance anomalies are observed in distributions of argoncluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar55H2O+, Ar54O2+ and Ar54N2+ contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes suggest structural differences.
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Physical Chemistry Chemical Physics
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