The Cs adsorption on InP(100) surface is studied with synchrotron radiation photoelectron spectroscopy. The charge transfer from Cs to the InP substrate is observed from the Cs induced In 4d and P 2p components, and this charge transfer results in surface dipole formation and lowering of the work function. The Cs 4d intensity saturates at coverage of 1 ML. However, a break point is observed at 0.5 ML, which coincides with the achievement of the minimum work function. This break point is due to the different vertical placements of the first and the second half monolayer of Cs atoms. Based on this information, a simple bilayer structure for the Cs layer is presented. This bilayer structure is consistent with the behavior of the charge transfer from the Cs to the InP substrate at different Cs coverages. This, in turn, explains why the work function decreases to a minimum at 0.5 ML of Cs and remains almost constant beyond this coverage. The depolarization of the surface dipoles is attributed to the saturation of charge transfer to the surface In atoms and the polarization of the Cs atoms in the second half monolayer induced by the positively charged Cs atoms in the first half monolayer. (c) 2007 American Vacuum Society.