The conversion of He(23S) to He2(a3 Sigmau+) in liquid helium
The report of an anomalously intense He4+ peak in mass spectra of large helium droplets created a stir three decades ago that continues to this day. When the electron kinetic energy exceeds 41 eV, an additional pathway to He4+ 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. The creation of He4+ has been conjected to proceed via a pair of He2* (He2(a3Sigmau+)) precursors, whose formation mechanism remains poorly understood. High level ab initio theory combined with classical molecular dynamics has been applied to systems comprising small numbers of He atoms. Conversion of He* to He2* in such systems was found to be rapid and efficient. We conclude that He2* is a likely participant in the production of He4+ on surfaces of large helium droplets.