Partial screening in Ca Silicides measured by Ca 2 p electron-energy -loss spectroscopy

We report Ca 2p electron-energy-loss spectra (EELS) of Ca silicides (Ca2Si, CaSi, CaSi2) for various primary electron-beam energies (Ep=565, 800, 1046, and 1500 eV). For fixed primary energy, the Ca 2p profiles are found to be very similar regardless of the site-specific environment. The high-energy excited EELS spectra bear a strong resemblance to the Ca 2p x-ray-absorption spectra (XAS) of Ca metal and CaSi2, while the low-primary-energy spectra are characterized by a strong transfer of spectral weight toward the threshold and an inversion of the 2p1/2 vs 2p3/2 branching ratio. Whatever the primary energy is, the EELS spectra are found to strongly deviate from Ca 2p spectra calculated in a pure bandlike approach (ground-state calculation). An insight into the role of screening effects is achieved via the calculation of the absorption profile of Ca metal excited by the presence of the 2p core hole. While the ground-state calculation accounts for the nonscreened system, the excited one describes a fully screened atom. We show that both the XAS and the high-energy EELS spectra reflect a configuration of partial screening of the 2p core hole. The strong transfer of spectral weight toward the threshold found in the low-primary-energy EELS spectra are interpreted in terms of a more complete screening due to the participation of the slower injected electron. Therefore, the degree of screening can be driven by varying the final-state energy of the injected electron.