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Conversion of He(23S) to He2(a3Σu+) in Liquid Helium

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posted on 01.10.2018, 00:00 by P. Nijjar, A. I. Krylov, O. V. Prezhdo, A. F. Vilesov, C. Wittig
The report of an anomalously intense He4+ peak in electron impact mass spectra of large helium droplets created a stir 3 decades ago that continues to this day. When the electron kinetic energy exceeds 41 eV, an additional pathway opens that yields He4+ predominantly in an electronically excited metastable state. A pair of He*­(23S) atoms has been implicated based on the isolated He* energy of 19.82 eV and the 41 eV threshold, and the creation of He4+ has been conjectured to proceed via a pair of He2*­(a3Σu+) precursors. The mechanism whereby He* converts to He2* in liquid helium has remained a mystery, however. High level ab initio theory combined with classical molecular dynamics has been applied to systems comprising small numbers of He atoms. The conversion of He* to He2* in such systems is shown to be due to a simple many-body effect that yields He2* rapidly and efficiently.