  Cesium adsorption on 2x4 GaAs (001) was studied by photoemission and low
energy electron diffraction. The different Cs induced changes of the As 3d and
Ga 3d core level spectra show that charge transfer is almost complete for Ga
surface sites, but is negligible to surface As at a coverage smaller than 0.3
ML. The situation is opposite for a coverage larger than 0.3ML, at which
transfer occurs to As but no longer to Ga. Charge transfer to As atoms leads to
disordering and destabilization and induces surface conversion from the As-rich
surface to the Ga-rich 4x2 one after annealing at a reduced temperature of 450
C.
