ANOMALOUS SURFACE CORE-LEVEL SHIFTS ON BE METAL

An unusual behavior of the surface core-level shifts on the three low-index surfaces of Be metal has been revealed. Three surface-shifted Be 1s components were identified in photoemission studies of both the (0001) and the (10 ($) over bar 10) surfaces while for the (11 ($) over bar 20) surface only one surface-shifted component was resolved. On the close-packed (0001) surface, components shifted by -825, -570, and -265 meV relative to the bulk component were revealed and interpreted to originate from the three outermost surface layers. For the (10 ($) over bar 10) surface, components shifted by -700, -500, and -220 meV were determined and interpreted to originate from the first, second, and third plus fourth atomic layers. For the (11 ($) over bar 20) surface only one component shifted by -410 meV was resolved and interpreted to originate from atoms in the outermost layer. These findings are most unusual since two subsurface core-level shifts have previously not been observed on a close-packed metal surface, and larger surface shifts are determined for the close-packed (0001) surface compared to the more open (10 ($) over bar 10) and (11 ($) over bar 20) surfaces. This latter is contrary to all previous findings on other metal surfaces and also contrary to the predictions of the partial-shift model, which previously has been successfully applied to several metallic systems. The experimental findings are reviewed and reasons for this unusual behavior of the surface shifts are discussed in view of recent theoretical results.