Institute for Ion Physics und Applied Physics, University of Innsbruck, Innsbruck, Austria.
Chemphyschem. 2012 Feb;13(2):469-76. doi: 10.1002/cphc.201100880. Epub 2011 Dec 8.
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 (CH(4))(n)(+) cluster ions along with the protonated ions, CH(5)(+)(CH(4))(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 CH(5)(+)(CH(4))(n-1) (n=55, 148) when compared to (CH(4))(n)(+) (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type C(2)H(x)(CH(4))(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 (CH(4))(n)(2+) 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.
氦液滴掺杂甲烷团簇的电子电离首次采用高分辨质谱进行了研究。气相中主要的离子产物是未质子化的（CH(4))(n)(+)团簇离子以及质子化的离子 CH(5)(+)(CH(4))(n-1)。质谱图清楚地显示了幻数的存在，这些幻数与二十面体壳层的闭合大致一致。然而，观察到了一些不寻常的特征，包括 CH(5)(+)(CH(4))(n-1) (n=55, 148) 与 (CH(4))(n)(+) (n=54, 147) 的不同幻数。对其中一些差异提出了可能的解释。还观察到了 C(2)H(x)(CH(4))(n) 类型的产物，这些产物是离子-分子化学的结果，也显示出明显的幻数特征。最后，我们报告了甲烷团簇中首次观察到的 (CH(4))(n)(2+) 二价阳离子。二价阳离子存活的阈值出现在 n≥70，与多电荷离子裂变的液滴模型一致。此外，我们提出的证据表明，这些二价阳离子是通过一种不寻常的两步机制形成的，该机制由电荷转移引发，生成单电荷，然后通过彭宁电离生成二价阳离子。
