What is it about?
Helium nanodroplets are cryogenic nanolaboratories ideal to investigate the rotational spectroscopy of embedded molecules, the active cooling of ions whose production in the gas phase would be impossible, or the growth of nanoclusters. However, the quantum nature of these large systems retricted former theoretical studies to a few hundred helium atoms. For the first time, the fragmentation of argon-doped helium droplets composed of a thousand of helium atoms enabled us to characterize not only the mechanims of ion ejection but also those of ion trapping and recombination induced by the helium environment.
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Why is it important?
The ZPAD (zero-point averaged dynamics) method used in this work includes quantum effects in a mostly classical dynamical approach, making it easily applicable to much larger helium nanodroplets and a broad variety of dopants. Typical experimental sizes of helium nanodroplets range from one thousand to billion of atoms on time scales much longer than typical quantum simulations. A simple and versatile method is therefore required to investigate the dynamics of such large systems on the nanosecond time scale. By using a droplet composed of one thousand helium atoms we confirmed the existence of recombinations of argon atoms trapped within the helium droplet that stimulate the production of large ions. Recombinations were experimentally confirmed for a number of dopants but never proved theoretically until now.
Perspectives
As a direct extension of my PhD work, I had planned to do this work since more than a decade. My next goal is to explore the dynamics of Arn+ clusters (n < 10) embedded in helium nanodroplets by the ZPAD approach in an attempt to shed some light on magic numbers observed in experimental mass spectra. In the future, rare-gas dopants are expected to be replaced by other kinds of atomic clusters, molecules, or molecular assemblies.
David Bonhommeau
Universite de Reims Champagne-Ardenne
Read the Original
This page is a summary of: Fragmentation dynamics of Ar4He1000 upon electron impact ionization: Competition between ion ejection and trapping, The Journal of Chemical Physics, June 2020, American Institute of Physics,
DOI: 10.1063/5.0009363.
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Resources
Modeling the fragmentation dynamics of ionic clusters inside helium nanodroplets: The case of He100Ne4+
The first paper that describes how to build effective He-He potentials included in the ZPAD method. A small neon-doped helium cluster is used as prototype system.
Fragmentation of ionized doped helium nanodroplets: Theoretical evidence for a dopant ejection mechanism
Second paper in line on the ZPAD method applied to small neon-doped helium clusters. The influence of the dopant size is tackled and a mechanism of ion ejection is observed theoretically for the first time in such systems. No trapping and recombination could be observed due to the small size of the helium nanodroplet.
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