Note: Surface core level shifts are often observed for single crystals and nanoparticles and depend on the crystal surface exposed. Shifts can be to lower or higher binding energy, depending on the band structure of the bulk solid. For the outermost atom layer, the shift can be as high as \(\pu{0.4 eV}\). In X-ray photoelectron emission, the sampling depth is generally many atom layers when photoelectrons are detected with emission angles at, or close to, the surface normal. The shift for the outermost atom layer is thus seen most clearly at near-grazing emission angles. Separate energy shifts can be detected for the second and third atom layers from the surface.