Auger emission from the Coulomb explosion of helium nanoplasmas

The long-time correlated decay dynamics of strong-field exposed helium nanodroplets is studied by means of photoelectron spectroscopy. As a result of the adiabatic expansion of the laser-produced, fully inner-ionized nanoplasma, delocalized electrons in the deep confining mean field potential are shifted towards the vacuum level. Meanwhile, part of the electrons localize in bound levels of the helium ions. The simple hydrogenlike electronic structure of He⁺ results in clear signatures in the experimentally observed photoelectron spectra, which can be traced back to bound-free and bound-bound transitions. Auger electron emission takes place as a result of the transfer of transition energy to weakly bound electrons in the quasifree electron band. Hence, the spatial and temporal development of the nanoplasma cloud is encoded in the experimental spectra, whereas the electronic properties of He⁺ help resolve the different contributions.