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

The fragmentation upon electron impact ionization of Ar₄He₁₀₀₀ is investigated by means of mixed quantum–classical dynamics simulations. The $Ar4+$ dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple $Ar4+$ electronic surfaces and possible transitions between them. Helium atoms are treated individually using zero-point averaged dynamics, 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 $Ar2+Heq$ and $Ar3+Heq$ (q ≤ 1000), with a strong contribution of the bare $Ar2+$ 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 $Ar2+$ ions is the same as in the gas phase, trapped $Ar3+$ and $Ar4+$ are rather slightly bound $Ar2+⋯Ar$ and $Ar2+⋯Ar⋯Ar$ structures (i.e., an $Ar2+$ core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release $Ar3+Heq$ or $Ar4+Heq$ (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 to be suitable to provide a clear signature of the broad variety of processes at play in our simulations.