title: Experimental evidence for the existence of an electronically excited state of the proposed dihydrogen radical cation He-H-H-He+.
abstract: 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(+).
authors.affiliation: Institut für Ionenphysik und Angewandte Physik, Leopold Franzens Universität, Technikerstrasse 25, 6020 Innsbruck, Austria.
authors.forename: Stefan
authors.initials: S
authors.lastname: Jaksch
authors.forename: Filipe
authors.initials: F
authors.lastname: Ferreira da Silva
authors.forename: Stephan
authors.initials: S
authors.lastname: Denifl
authors.forename: Olof
authors.initials: O
authors.lastname: Echt
authors.forename: Tilmann D
authors.initials: TD
authors.lastname: Märk
authors.forename: Paul
authors.initials: P
authors.lastname: Scheier