Photoemission from surface-atom core levels, surface densities of states, and metal-atom clusters: A unified picture

Abstract

A model is presented which relates the observed narrowing and shifting of the surface density of states, reported in the preceding paper, with the sign and magnitude of the surface-atom core-level shift (SCS). This relationship, along with the identical line shapes observed for the bulk and surface-atom core-level photoemission peaks, is shown to provide strong support for a predominantly initial-state interpretation of the SCS. Inclusion of final-state screening energy differences between surface and bulk atoms is discussed within the framework of a more quantitative description of the SCS. General consideration of the effects responsible for the modified surface band structure, i.e., reduced coordination number and the resulting redistribution of charge between and within the $s,p \mathrm{and} d$ bands, allows our model to be extended to the transition metals and allows apparently different theoretical explanations of the SCS to be unified within a single picture. The special case of those rare-earth metals in which narrow corelike $f$ levels appear close to the Fermi energy is also discussed. Our previously described measurements and analytical procedures are compared with other recently reported surface-atom core-level studies, and all experimental results are compared with self-consistent and semiempirical thermodynamic calculations. Generally encouraging qualitative agreement is obtained, particularly the sign of the SCS which changes from element to element across the Periodic Table. Quantitative agreement is most lacking for the more-open single-crystal surfaces and for the metals near the ends of the transition series. Finally, the closely related photoemission experiments from small metal-atom clusters (akin to supported catalysts) have been reinterpreted. Consistency between these and the present results is obtained, provided that a proper reference level is employed.