Both the clean and Cs-exposed Si(111)root 3X root 3R30 degrees:B surfa
ces have been investigated by high-resolution photoemission. In the sp
ectra from the Si 2p level, contributions were identified from the bul
k, the adatoms plus the first layer, the second and 2/3 ML of the thir
d-layer atoms not binding to the B atoms, and, finally, the 1/3 ML of
third-layer atoms binding to the boron atoms. The interaction between
the Cs atoms and the surface is found to be dependent on the coverage.
At low coverages the geometric and electronic structure of the Si(111
)root 3X root 3R30 degrees:B interface is only to a minor degree affec
ted by the presence of Cs on the surface. At high coverage the Cs atom
s react with the surface and alter the binding configuration of the Si
adatoms.