Surface Core Level Shifts of the Lanthanide Metals Ce⁵⁸-Lu⁷¹: A Comprehensive Experimental Study

Synchrotron radiation excited photoelectron spectra of the 4 f emission region for all lanthanide metals Ce 58 to Lu 71 (except Pm 61 ) have been recorded. Photon energies ranging from 30 eV to 200 eV have been used to excite the 4 f electrons with maximum surface sensitivity. The 4 f emission spectra of all metals studied, with the exception of Ce, show 4 f emission from the surface layer atoms and the bulk atoms as clearly separated structures with different binding energies. For Ce no unambiguous separation of the 4 f structure into bulk and surface emission could be made. We fitted the experimental spectra with calculated 4 f emission spectra obtained by adding a bulk and a shifted surface multiplet for each metal. The intensity ratios within the multiplets were obtained from an atomic calculation utilizing the intermediate coupling scheme. The intensity ratio between bulk and surface emission ( I B / I S ) and the magnitude of the surface core level shift (SCS) were used as fit parameters. The deduced SCSs are all positive (towards higher binding energy), 0.40 eV-0.77 eV, and in fair agreement with calculated SCSs using the Johansson-Mårtensson-Rosengren model. For the trivalent lanthanides we found a systematic variation of the SCSs through the series, which can be attributed to differences in the actual electron distribution between s, p and d -like valence states. For the trivalent Sm we found the surface to be completely divalent and deduced a surface shift of > 0.46 eV, being the energy necessary to change the surface valency.