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Ionization of Methane Clusters in Helium Nanodroplets



The electron ionization of helium droplets doped with methane clusters is investigated for the first time using high-resolution mass spectrometry. The dominant ion products ejected into the gas phase are the unprotonated (CH4)n+ cluster ions along with the protonated ions, CH5+(CH4)n−1. The mass spectra show clear evidence for magic numbers, which are broadly consistent with icosahedral shell closings. However, unusual features were observed, including different magic numbers for CH5+(CH4)n−1 (n=55, 148) when compared to (CH4)n+ (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type [C2Hx(CH4)n]+, which result from ion–molecule chemistry, are also observed and these too show clear magic number features. Finally, we report the first observation of (CH4)n2+ dications from methane clusters. The threshold for dication survival occurs at n≥70 and is in good agreement with a liquid droplet model for fission of multiply charged ions. Furthermore, we present evidence showing that these dications are formed by an unusual two-step mechanism which is initiated by charge transfer to generate a monocation and is then followed by Penning ionization to generate a dication.

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