Electronic structure study of Eu intermetallic compounds by photoelectron spectroscopy

We have studied photoelectron spectra of Eu4f, 3d and 4d core levels of several divalent Eu intermetallic compounds (EuPd, EuAg5 and EuAu5) and trivalent Eu compounds (EuPd3 and EuPt5) using synchrotron radiation in the photon energy range 40-140 eV and a conventional X-ray source. We find that at the surface the Eu atoms of trivalent compounds show the valence transition to the divalent state, and those of divalent compounds show the 4f core-level energy shifts to higher binding energies. These phenomena are consistent with the predictions based on Johansson and Martensson's fully screened core-hole model with Miedema's semiempirical cohesive energy estimation scheme. The apparent ''shakedown'' and ''shakeup'' satellites reported in the core-level spectra of Eu compounds are found to be due to this surface valence transition and the <(3d)under bar>3d4f(7) multiplet structures (underline denotes a hole), respectively. This interpretation resolves the apparent inconsistency between the results of low energy probes such as Mossbauer isomer shift and those of high energy probes such as photoelectron spectroscopy regarding the valence of Eu in these intermetallic compounds. We also obtain the change of the electron mean free path with the electron kinetic energy in these Eu intermetallic compounds from the intensity ratio between the surface and bulk peaks in the photoemission spectra as a function of incident photon energy.