{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,13]],"date-time":"2025-10-13T15:20:30Z","timestamp":1760368830356,"version":"build-2065373602"},"reference-count":48,"publisher":"Wiley","issue":"16","license":[{"start":{"date-parts":[[2009,2,26]],"date-time":"2009-02-26T00:00:00Z","timestamp":1235606400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Chemistry A European J"],"published-print":{"date-parts":[[2009,4,14]]},"abstract":"<jats:title>Abstract<\/jats:title><jats:p><jats:bold>Survival of the weakest<\/jats:bold>: The existence of a new class of centrosymmetric radical cations in which H<jats:sub>2<\/jats:sub> bridges two identical main group elements was recently proposed in this journal by Uggerud and co\u2010workers. By growing complexes inside helium nanodroplets at subkelvin temperatures, we obtained experimental evidence for the existence of the most weakly bound member of this class, He\u2010H\u2010H\u2010He<jats:sup>+<\/jats:sup> (see picture), although in a metastable, electronically excited state.<jats:boxed-text content-type=\"graphic\" position=\"anchor\"><jats:graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" mimetype=\"image\/gif\" position=\"anchor\" specific-use=\"enlarged-web-image\" xlink:href=\"graphic\/mcontent.gif\"><jats:alt-text>magnified image<\/jats:alt-text><\/jats:graphic><\/jats:boxed-text><\/jats:p><jats:p>In a recent report, Uggerud and co\u2010workers (A. Krapp et\u2005al., <jats:italic>Chem. Eur. J.<\/jats:italic> <jats:bold>2008<\/jats:bold>, <jats:italic>14<\/jats:italic>, 4028) proposed the existence of a new class of radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H<jats:sub>2<\/jats:sub> molecules we observe various He<jats:sub><jats:italic>x<\/jats:italic><\/jats:sub>H<jats:sub><jats:italic>y<\/jats:italic><\/jats:sub><jats:sup>+<\/jats:sup> cluster ions, including He<jats:sub>2<\/jats:sub>H<jats:sub>2<\/jats:sub><jats:sup>+<\/jats:sup>, which would belong to the proposed class of radical cations. Mass\u2010analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field\u2010free region of the mass spectrometer. One reaction is into HeH<jats:sub>2<\/jats:sub><jats:sup>+<\/jats:sup> + He with a low kinetic energy release of 15\u00b14\u2005meV. Surprisingly, another unimolecular reaction is observed, into HeH<jats:sup>+<\/jats:sup> + HeH (or He + H). The probability of this reaction is an order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited state; they are consistent with the proposed linear, centrosymmetric ion structure of He\u2010H\u2010H\u2010He<jats:sup>+<\/jats:sup>.<\/jats:p>","DOI":"10.1002\/chem.200802545","type":"journal-article","created":{"date-parts":[[2009,2,26]],"date-time":"2009-02-26T07:51:30Z","timestamp":1235634690000},"page":"4190-4194","source":"Crossref","is-referenced-by-count":9,"title":["Experimental Evidence for the Existence of an Electronically Excited State of the Proposed Dihydrogen Radical Cation He\u2010H\u2010H\u2010He<sup>+<\/sup>"],"prefix":"10.1002","volume":"15","author":[{"given":"Stefan","family":"Jaksch","sequence":"first","affiliation":[]},{"given":"Filipe","family":"Ferreira\u2005da\u2005Silva","sequence":"additional","affiliation":[]},{"given":"Stephan","family":"Denifl","sequence":"additional","affiliation":[]},{"given":"Olof","family":"Echt","sequence":"additional","affiliation":[]},{"given":"Tilmann\u2005D.","family":"M\u00e4rk","sequence":"additional","affiliation":[]},{"given":"Paul","family":"Scheier","sequence":"additional","affiliation":[]}],"member":"311","published-online":{"date-parts":[[2009,4,3]]},"reference":[{"key":"e_1_2_6_1_2","doi-asserted-by":"publisher","DOI":"10.1016\/S0166-1280(98)00258-9"},{"key":"e_1_2_6_2_2","doi-asserted-by":"publisher","DOI":"10.1016\/S1387-3806(03)00135-0"},{"key":"e_1_2_6_3_2","doi-asserted-by":"publisher","DOI":"10.1002\/ange.19921040241"},{"key":"e_1_2_6_3_3","doi-asserted-by":"publisher","DOI":"10.1002\/anie.199201831"},{"key":"e_1_2_6_4_2","doi-asserted-by":"publisher","DOI":"10.1016\/S0301-0104(98)00299-7"},{"key":"e_1_2_6_5_2","doi-asserted-by":"publisher","DOI":"10.1002\/1521-3757(20010417)113:8<1591::AID-ANGE1591>3.0.CO;2-V"},{"key":"e_1_2_6_5_3","doi-asserted-by":"publisher","DOI":"10.1002\/1521-3773(20010417)40:8<1543::AID-ANIE1543>3.0.CO;2-6"},{"key":"e_1_2_6_6_2","doi-asserted-by":"publisher","DOI":"10.1063\/1.468980"},{"key":"e_1_2_6_7_2","doi-asserted-by":"publisher","DOI":"10.1007\/s004600050166"},{"key":"e_1_2_6_8_2","doi-asserted-by":"publisher","DOI":"10.1063\/1.477713"},{"key":"e_1_2_6_9_2","doi-asserted-by":"publisher","DOI":"10.1063\/1.1418249"},{"key":"e_1_2_6_10_2","doi-asserted-by":"publisher","DOI":"10.1140\/epjd\/e2004-00109-0"},{"key":"e_1_2_6_11_2","doi-asserted-by":"publisher","DOI":"10.1080\/01442350600625092"},{"key":"e_1_2_6_12_2","doi-asserted-by":"publisher","DOI":"10.1063\/1.1665271"},{"key":"e_1_2_6_13_2","doi-asserted-by":"publisher","DOI":"10.1021\/ja030653q"},{"key":"e_1_2_6_14_2","doi-asserted-by":"publisher","DOI":"10.1063\/1.1982790"},{"key":"e_1_2_6_15_2","unstructured":"F. 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