Cesium core level binding energy shifts at the O 2/Cs/Si(113) surface

Abstract

We have studied the cesium core level binding energy shifts at the O 2/Cs/Si(113) surface at 150 K using X-ray photo-electron spectroscopy. It was observed that the cesium core levels, such as Cs 3d, 5s and 5p, of theCs/Si(113) surface were shifted towards lower binding energy upon oxidation. However, the Si 2p core level and the valence band of silicon were not shifted and the Cs MNN Auger peaks were drastically shifted in the opposite direction (lower kinetic energy) for the oxygen exposure, at which the negative binding energy shifts of the Cs3 d 5/2 core level were observed. It was found that the Auger parameter decreased by about 3.7 eV upon oxidation. The negative binding energy shifts of the cesium core levels are thus not due to the shift of the Fermi level position, sample charging effect, and the increase in final state screening. The negative binding energy shift of the Cs3 d 5/2 core level occurred with the drastic increase in the ionicity of cesium and with the concurrent negative binding energy shift of the O 1s core level at the same exposure. Therefore, the negative binding energy shifts of the Cs3 d 5/2 core level seem to be due to the Madelung potential by the increasing negative oxygen ions around the positive cesium ion.