Helium Nanoplasma Dynamics
Doped He nanodroplets are widely used as inert, transparent, and cold matrix for spectroscopy of embedded molecules and clusters. However, when exposed to strong laser fields, a few dopant atoms (for example Xe) are enough to “ignite” avalanche-like ionization that turns the whole droplet into a strongly absorbing nanoplasma [1,2]. As a result, the complete He droplet fully ionizes and explodes, see the snapshots from a MD simulation above. Highly energetic electrons, He+, He++, as well as highly charged dopant ions (up to Xe21+) are produced.
The fundamental questions, which we want to investigate, are:
- How does ignition of the He nanoplasma occur in detail?
- What processes determine the nanoplasma evolution?
- Why is one type of dopant particle better suited than the other for igniting a He nanoplasma?
- What is the role of resonances (collective oscillation of the electron cloud driven by the laser field) when the nanoplasma absorbs light while it builds up and then expands?
- What kind of applications could such nanoplasmas be used for?
We approach these questions experimentally using intense, ultrashort laser pulses, which we shine on a beam of He nanodroplets doped with atoms and molecules of various species. The ejected electrons and ions are detected using imaging techniques [4], which provide information about the number of electrons/ions ejected, their energy and angular distribution.
[1]S.R. Krishnan, L. Fechner, M. Kremer, V. Sharma, B. Fischer, N. Camus, J. Jha, M. Krishnamurthy, T. Pfeifer, R. Moshammer, J. Ullrich, F. Stienkemeier, M. Mudrich, Phys. Rev. Lett. 107, 173402 (2011)