  We analyze topographic scanning force microscopy images together with Kelvin
probe images obtained on Pb islands and on the wetting layer on Si(111) for
variable annealing times. Within the wetting layer we observe negatively
charged Si-rich areas. We show evidence that these Si-rich areas result from
islands that have disappeared by coarsening. We argue that the islands are
located on Si-rich areas inside the wetting layer such that the Pb/Si interface
of the islands is in line with the top of the wetting layer rather than with
its interface to the substrate. We propose that the Pb island heights are one
atomic layer smaller than previously believed. For the quantum size effect
bilayer oscillations of the work function observed in this system, we conclude
that for film thicknesses below 9 atomic layers large values of the work
function correspond to even numbers of monolayers instead of odd ones. The
atomically precise island height is important to understand ultrafast
"explosive" island growth in this system.
