Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H n + . The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H n + . The use of high-resolution mass spectrometry allows the unambiguous identification

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... f even-numbered H n + up to n Х 120 by their mass excess that distinguishes them from He n + , mixed He m H n + , and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n = 6, pronounced drops in the abundance of even-numbered cluster ions are seen at n = 30 and 114, which suggest icosahedral shell closures at H 6 + ͑H 2 ͒ 12 and H 6 + ͑H 2 ͒ 54 . Possible isomers of H 6 + are identified at the quadratic configuration interaction with inclusion of single and double excitations ͑QCISD͒/aug-cc-pVTZ level of theory