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Abstract

We present single-shot electron velocity-map images of nanoplasmas generated
from doped helium nanodroplets and neon clusters by intense near-infrared and
mid-infrared laser pulses. We report a large variety of signal types, most
crucially depending on the cluster size. The common feature is a two-component
distribution for each single-cluster event: A bright inner part with nearly
circular shape corresponding to electron energies up to a few eV, surrounded by
an extended background of more energetic electrons. The total counts and energy
of the electrons in the inner part are strongly correlated and follow a simple
power-law dependence. Deviations from the circular shape of the inner electrons
observed for neon clusters and large helium nanodroplets indicate non-spherical
shapes of the neutral clusters. The dependence of the measured electron
energies on the extraction voltage of the spectrometer indicates that the
evolution of the nanoplasma is significantly affected by the presence of an
external electric field. This conjecture is confirmed by molecular dynamics
simulations, which reproduce the salient features of the experimental electron
spectra.