X-ray absorption at the ${\mathit{L}}_{2,3}$ edge of an anisotropic single crystal: Cadmium (0001)

Surface extended x-ray-absorption fine structure (EXAFS) is a very powerful technique for the crystallographic characterization of adsorbed atoms, layers, and thin films. The linear polarization of the synchrotron radiation is used to infer direct information about the crystallographic anisotropy of these systems. For the K absorption edge, the polarization dependence of the EXAFS signal is simple: the contribution of each bond is weighted by a factor ${\cos }^2$θ, where θ is the angle between the direction of the bond and the linear polarization of the x rays. For ${\mathit{L}}_{2,3}$ absorption edges, the polarization dependence is more complicated, since the initial p state can be excited to a final state of s or d symmetry. The angular dependences of these two terms are strongly different. We show here, with a study on a bulk anisotropic system, cadmium, that the two terms have to be considered and that neglecting the p to s transitions in the analysis of angular-dependent spectra leads to errors in bond-length determination as high as 0.1 Å. © 1996 The American Physical Society.