Conversion of He(23 S) to He2(a 3Σ u +) in Liquid Helium

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*­(23 S) 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*­(a 3Σ u +) 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.