ANGLE-RESOLVED PHOTOEMISSION AND LEED FROM RARE-EARTH-METALS

Angle-resolved photoemission and LEED studies of single-crystal surfaces of the rare-earth metals are still relatively few in number. The reasons for this are outlined and comparisons are made between the procedures used to obtain clean and well-ordered surfaces suitable for study. Valence-band photoemission spectra are compared with surface electronic structure and photocurrent calculations in order to identity surface and bulk features. Core-level spectroscopy indicates that the 5p levels of the lanthanides show intensity variations with emission angle that are not explicable in terms of a simple core-level shift. Although the 4p levels of Y have some band character, detailed comparison of the 4p photoemission lineshape with those produced by photocurrent calculations enables the surface core-level shift to be determined. Quantitative LEED studies of close-packed (0001) surfaces show essentially a bulk-terminated structure for Sc, a structural reconstruction of the divalent-surface of Sm and a magnetic reconstruction of ferromagnetic Gd. The more open (1120BAR) and (1010BAR) surfaces of all the rare-earth metals studied undergo extensive reconstructions into a close-packed structure almost identical to that of a (0001) surface. Angle-resolved photoemission and LEED have vet to distinguish between these structures, indicating a reconstruction depth of at least five atomic layers.