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

1991; American Physical Society; Volume: 43; Issue: 9 Linguagem: Inglês

10.1103/physrevb.43.6899

1095-3795

F. J. Himpsel, U. O. Karlsson, A. B. McLean, L. J. Terminello, Frank M. F. de Groot, M. Abbate, J. C. Fuggle, J. A. Yarmoff, B. T. Thole, G. A. Sawatzky,

Atomic and Molecular Physics

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.