  Alkali-metal adsorption on the surface of materials is widely used for in
situ surface electron doping, particularly for observing unoccupied band
structures by angle-resolved photoemission spectroscopy (ARPES). However, the
effects of alkali-metal atoms on the resulting band structures have yet to be
fully investigated, owing to difficulties in both experiments and calculations.
Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films
with first-principles calculations of the electronic charge densities and
demonstrate a simple method to evaluate alkali-metal induced band deformation.
We reveal that deformation of bismuth surface bands is directly correlated with
vertical charge-density profiles at each electronic state of bismuth. In
contrast, a change in the quantized bulk bands is well described by a
conventional rigid-band-shift picture. We discuss these two aspects of the band
deformation holistically, considering spatial distributions of the electronic
states and cesium-bismuth hybridization, and provide a prescription for
applying alkali-metal adsorption to a wide range of materials.
