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