Formation of He4+via electron impact of helium droplets

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Doping with multiple cations and failure of charge transfer in large ionized helium droplets

We report experimental observations of aniline (A) cations and He₂ ⁺ when aniline is doped into ionized helium droplets. Large droplets containing 10⁸ atoms are bombarded by energetic electrons, resulting in more than one positive charge in one droplet. When aniline encounters the charged droplets, some are ionized via charge transfer, while others can remain neutral in the presence of He₂ ⁺ when the mass-to-charge ratio (m/z) of the droplet is sufficiently large. Upon resonant excitation of the dopant A_n or A_n ⁺ (n ≥ 1), He₂ ⁺ can be ejected. The excitation spectrum of He₂ ⁺ becomes a juxtaposition of the spectra of A_n and A_n ⁺. Moreover, an anticorrelation between the yields of He₂ ⁺ and A⁺ is observed with increasing energies of the ionizing electrons. We attribute this result to the combined effect of reduction in m/z of the droplets and the different locations of He₂ ⁺ and neutral A_n. Limited by the penetration depths of the ionizing electrons and further assisted by the Coulomb repulsion of coexisting cations, He₂ ⁺ is located within 20 nm of the surface, while neutral A_n has an average position inside a large droplet. Upon resonant excitation of the interior A_n, He₂ ⁺ is preferentially ejected. With increasing energies of the colliding electrons, the m/z of the droplets are reduced, leading to less effective charge shielding and more effective charge transfer, until ultimately, all He₂ ⁺ can be neutralized to form A⁺.

Conversion of He(23 S) to He2( a3Σ 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*( a³Σ _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.