  Atoms and molecules attached to rare gas clusters are ionized by an
interatomic autoionization process traditionally termed 'Penning ionization'
when the host cluster is resonantly excited. Here we analyze this process in
the light of the interatomic Coulombic decay (ICD) mechanism, which usually
contains a contribution from charge exchange at short interatomic distance, and
one from virtual photon transfer at large interatomic distance. For helium (He)
nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range
and short-range contributions to the interatomic autoionization can be clearly
distinguished by detecting electrons and ions in coincidence. Surprisingly, ab
initio calculations show that even for alkali metal atoms floating in dimples
at large distance from the nanodroplet surface, autoionization is largely
dominated by charge exchange ICD. Furthermore, the measured electron spectra
manifest ultrafast internal relaxation of the droplet into mainly the 1s2s 1^S
state and partially into the metastable 1s2s 3^S state.
