Abstract

The evolution of electronic structure across the actinide series is unique in the periodic table. At the beginning of the series the 5f electrons are clearly in involved in bonding and are well described by band theory; beyond Pu, however, localization occurs and magnetic and other physical properties indicate a transition to atomic-like behavior has taken place. In the valence-band photoemission spectra this change is manifested by the recession of 5f spectral weight from the Fermi energy and the appearance of multiplet structure. The latter results from the coupling of electron spin and angular momentum in the 5f-ionized final state. The situation is in principle analogous to 4f photoemission in the lanthanides which are well understood, and for which the final-state multiplet intensities can be calculated accurately from atomic theory. Similar calculations have been carried out for the actinides, but to date the only opportunity for comparison with experiment has been Am. The structure observed here was not consistent with that expected for a 5f⁶-to-5f⁵ photo-emission process prompting the proposal that the surface layer is actually divalent (5f⁷) and exhibits a slightly different binding energy than atoms in the bulk. The observed spectrum is then the sum of three contributions. Both the tunability and resolution provided by an XUV-FEL would be immensely beneficial in sorting out problems of this kind. Being able to change the photon energy at which spectra are obtained allows controlling the escape depth of the photoelectrons and hence the relative contributions from the bulk and surface atoms. The increased resolution of an FEL over even He resonance sources could well reveal additional multiplet structure, further simplifying the analysis.

© 1988 Optical Society of America