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Experimental Evidence for the Existence of an Electronically Excited State of the Proposed Dihydrogen Radical Cation He‐H‐H‐He +
Author(s) -
Jaksch Stefan,
Ferreira da Silva Filipe,
Denifl Stephan,
Echt Olof,
Märk Tilmann D.,
Scheier Paul
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802545
Subject(s) - excited state , state (computer science) , atomic physics , chemistry , materials science , physics , computer science , algorithm
Survival of the weakest : The existence of a new class of centrosymmetric radical cations in which H 2 bridges two identical main group elements was recently proposed in this journal by Uggerud and co‐workers. 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‐H‐H‐He + (see picture), although in a metastable, electronically excited state.In a recent report, Uggerud and co‐workers (A. Krapp et al., Chem. Eur. J. 2008 , 14 , 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 2 molecules we observe various He x H y + cluster ions, including He 2 H 2 + , which would belong to the proposed class of radical cations. Mass‐analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field‐free region of the mass spectrometer. One reaction is into HeH 2 + + He with a low kinetic energy release of 15±4 meV. Surprisingly, another unimolecular reaction is observed, into HeH + + 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‐H‐H‐He + .