The electronic states of Na on Cu(111) have been studied by high-resol
ution two-photon photoemission. Image states which are well separated
from alkali states are observed at all coverages. An abrupt change of
the occupancy of the alkali state is observed at coverages between 0.4
0 and 0.45 ML. We interpret this as the transition from a more ionic b
onding of alkali atoms on the surface to the metallic bond in alkali i
slands. Other alkali metals on close-packed surfaces show a very simil
ar electronic structure as a function of alkali coverage.