  Helium nanodroplets are widely used as a cold, weakly interacting matrix for
spectroscopy of embedded species. In this work we excite or ionize doped He
droplets using synchrotron radiation and study the effect onto the dopant atoms
depending on their location inside the droplets (rare gases) or outside at the
droplet surface (alkali metals). Using photoelectron-photoion coincidence
imaging spectroscopy at variable photon energies (20-25 eV), we compare the
rates of charge-transfer to Penning ionization of the dopants in the two cases.
The surprising finding is that alkali metals, in contrast to the rare gases,
are efficiently Penning ionized upon excitation of the (n=2)-bands of the host
droplets. This indicates rapid migration of the excitation to the droplet
surface, followed by relaxation, and eventually energy transfer to the alkali
dopants.
