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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. [ABSTRACT FROM AUTHOR]

Femtosecond pump-probe electron and ion spectroscopy is applied to study the development of a helium nanoplasma up to the nanosecond timescale. Electrons, bound by the deep confining mean-field potential, are elevated toward the vacuum level in the nanoplasma expansion. Subsequent electron recombination gives rise to transitions between He+ states, resulting in autoionization. The time-resolved analysis of the energy transfer to quasifree electrons reveals a transient depletion of the Auger emission, which allows for a temporal gate to map the distribution of delocalized electrons in the developing mean field. Furthermore, we trace the recombination of delocalized electrons near the vacuum level into highly excited Rydberg states. Transient above-threshold ionization is introduced as a diagnostic tool to resolve the dynamics. Thus, the development of the electron distribution in the nanoplasma mean-field potential can be monitored via the features observed in the emission spectra. [ABSTRACT FROM AUTHOR]