Fragmentation dynamics of Ar4He1000 upon electron impact ionization: Competition between ion ejection and trapping

Abstract

The fragmentation upon electron impact ionization of Ar 4 He 1000 is investigated by means of mixed quantum-classical dynamics simulations. The Ar + 4 dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple Ar + 4 electronic surfaces and possible transitions between them. Helium atoms are treated individually using the zeropoint averaged dynamics (ZPAD), a method based on the building of an effective He-He potential. Fast electronic relaxation is observed, from less than 2 ps to ∼ 30 ps depending on initial conditions. The main fragments observed are Ar + 2 He q and Ar + 3 He q (q ≤ 1000), with a strong contribution of the bare Ar + 2 ion, and neither Ar + nor Ar + He q fragments are found. The smaller fragments (q ≤ 50) are found to mostly come from ion ejection whereas larger fragments (q > 500) originate from long-term ion trapping. Although the structure of the trapped Ar + 2 ions is the same as in the gas phase, trapped Ar + 3 and Ar + 4 are rather slightly bound Ar + 2 • • • Ar and Ar + 2 • • • Ar • • • Ar structures (i.e., an Ar + 2 core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release Ar + 3 He q or Ar + 4 He q (q ≤ 50) in the gas phase and/or induce strong helium droplet evaporation. Finally, the translational energy of the fragment center of mass was found suitable to provide a clear signature of the broad variety of processes at play in our simulations.

SF6+: Stabilizing Transient Ions in Helium Nanodroplets

“…Ejection leads to small ions He n X + where n ≪ N ; trapping leads to large ions He N–n X + (X is the dopant or a fragment thereof). The two outcomes have been nicely illustrated in a recent mixed quantum-classical dynamics simulation of He 1000 Ar 4 by Halberstadt and Bonhommeau . Experiments on undoped HNDs (where a tightly bound He 2 + takes on the role of the dopant) also demonstrate the coexistence of these two channels, although in this situation Coulomb repulsion between multiple charges also plays a role (see the Supporting Information and ref ).…”

“…The two outcomes have been nicely illustrated in a recent mixed quantum-classical dynamics simulation of He 1000 Ar 4 by Halberstadt and Bonhommeau. 36 Experiments on undoped HNDs (where a tightly bound He 2 + takes on the role of the dopant) also demonstrate the coexistence of these two channels, although in this situation Coulomb repulsion between multiple charges also plays a role (see the Supporting Information and ref ( 37 )). The relative yields of the two competing channels will depend on the nature of the dopant, the size N , and other factors, but coexist they will.…”

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

“…Magic numbers and shell closures in cluster size distributions are the result of fragmentation where less stable cluster sizes are depleted and decay into more stable ones that more likely survive [30]. For heliophilic dopants such as gold, charge transfer from He + or a small He n + ionic core to a dopant cluster is the dominant ionization mechanism upon electron irradiation of doped HNDs at sufficiently high electron energies [27,[31][32][33][34]. The high ionization energy of helium compared to all dopants makes this reaction highly exothermic and the excess energy released into the internal degrees of freedom of the ionized dopant cluster is expected to lead to fragmentation and subsequently to the observed intensity anomalies in the cluster size distributions.…”

A Path Integral Molecular Dynamics Simulation of a Harpoon-Type Redox Reaction in a Helium Nanodroplet

“…In fact, once the charge transfer happens, a large amount of energy is distributed over the kinetic energy of the helium atoms, which subsequently leads to atomic helium evaporation (see, e.g., Ref. [ 61 ]).…”