Surface-Photoelectric-Yield Spectrum of Indium below the Plasmon Energy

The surface photoelectric yield of indium has been obtained by measuring in situ the total yield and the reflectivity for $s$ and $p$ polarizations on ultrasmooth evaporated films with preferential (111) orientation. With use of a theoretical calculation of Feibelman, it is shown that it is possible to extract the surface potential from the data in the energy range $0.6<\mathrm{}\frac{\omega }{{\omega }_p}<1\mathrm{}$. It deviates strongly from the corresponding Lang-Kohn result.