Fine structure of the Ca 2px-ray-absorption edge for bulk compounds, surfaces, and interfaces

The fine structure of the Ca 2p soft-x-ray-absorption edge is studied for a variety of bulk compounds (Ca metal, ${\mathrm{CaSi}}_2$, CaO, and ${\mathrm{CaF}}_2$), for surfaces and interfaces [${\mathrm{CaF}}_2$(111), ${\mathrm{BaF}}_2$ on ${\mathrm{CaF}}_2$(111), Ca and ${\mathrm{CaF}}_2$ on Si(111)], and for defects (F centers in ${\mathrm{CaF}}_2$). The observed multiplet structure is explained by atomic calculations in a crystal field [cubic ${\mathit{O}}_{\mathit{h}}$ for the bulk and threefold ${\mathit{C}}_{3\mathit{v}}$ for the (111) surfaces and interfaces]. While the bulk spectra are isotropic, the surface and interface spectra exhibit a pronounced polarization dependence, which is borne out by the calculations. This effect can be used to become surface and/or interface selective via polarization-modulation experiments, even for buried interfaces. A change in valence from ${\mathrm{Ca}}^{2+}$ to ${\mathrm{Ca}}^{1+}$ causes a downwards energy shift and extra multiplet lines according to the calculation. The energy shift is observed for F centers at the ${\mathrm{CaF}}_2$ surface and for the ${\mathrm{CaF}}_2$/Si(111) interface.