The conversion of He(2³S) to He₂(a³ Sigma_u⁺) in liquid helium

P. Nijjar, A. I. Krylov, O. V. Prezhdo, A. F. Vilesov, and C. Wittig
J. Phys. Chem. Lett.  9, 6017 – 6023 (2018)

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(a³Sigma_u⁺)) 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.

Download this paper (PDF)

Related Research

Computational studies of electronically excited and open-shell species: Jahn-Teller systems, radicals, diradicals and triradicals