  The electronic structure of ultra-small Au clusters on thin MgO/Ag(001) films
has been analyzed by scanning tunneling spectroscopy and density functional
theory. The clusters exhibit two-dimensional (2D) quantum well states, whose
shapes resemble the eigen-states of a 2D electron gas confined in a parabolic
potential. From the symmetries of the HOMO and LUMO of a particular cluster,
its electron filling and charge state is determined. In accordance to a DFT
Bader-charge analysis, aggregates containing up to twenty atoms accumulate one
to four extra electrons due to a charge transfer from the MgO/Ag interface. The
HOMO - LUMO gap is found to close for clusters containing between 70 and 100
atoms.
