  Using a first-principles approach, we investigate the origin of the fine
structure in Si 2$p$ photoelectron spectra at the Si(100)2$\times$1 surface and
at the Si(100)-SiO$_2$ interface. Calculated and measured shifts show very good
agreement for both systems. By using maximally localized Wannier functions, we
provide an interpretation in which the effects due to the electronegativity of
second nearest neighbor atoms and due to the local strain field are
distinguished. Hence, in combination with accurate modeling, photoelectron
spectroscopy can provide a direct measure of the strain field at the atomic
scale.
