PHOTOEMISSION STUDIES OF SURFACES AND THIN FILM SYSTEMS OF SILVER AND GOLD.

摘要

Angle-integrated photoemission spectroscopy was used to study the properties of Au 4f(,7/2) core level in several systems: Au overlayers on Ag(111), Au overlayers on Ag(100), and an Au monolayer buried in Ag(111). Properties of a zone-center surface state were also investigated with the use of angle resolved photoemission spectroscopy for many related systems: the clean Ag(111) surface, Ag overlayers on Au(111), and Au overlayers on Ag(111). For Au epitaxial overlayers on Ag, the binding energies of Au 4f(,7/2) core level were measured as a function of overlayer thickness. The Au 4f(,7/2) core level associated with the top layer is shifted relative to the underlayers. With increasing Au coverages on Ag(100) beyond 5 monolayers, the surface reconstruction changes from (1 x 1) to (5 x 20), and the binding energies change correspondingly. The measured binding energies are related to the variation of the one-electron potential within the overlayer. For the Au monolayer buried in Ag(111), the Au 4f(,7/2) core-level binding energies were measured as a function of the depth below the surface. The measured Au 4f(,7/2) binding energy converges asymptotically to a constant value for depths greater than about three atomic monolayers. The measured spatial range of the surface effect is significantly greater than what is expected from a jellium calculation. This result implies that significant subsurface-core-level shifts can occur for close-packed metal surfaces. For the studies of the zone center surface state of Au overlayers on Ag(111), it was observed that this surface state evolves continuously from Ag(111)-like to Au(111)-like for increasing Au coverages on Ag, and a similar behavior was also observed for Ag on Au(111). The shifts of the binding energies as a function of overlayer thickness can be related to the z-dependence of the surface state wave function. The envelope function of this surface state in Ag(111) was further determined by use of Ag(111) samples with one subsurface atomic layer substituted by a matched Au atomic layer. Using this technique, the decay length of this surface state is determined to be about 28 (ANGSTROM). The photoemission cross section of the Ag(111) surface state was also measured over the photon energy range of 36 - 65 eV. A resonance at 54 eV was observed. The energy position of this resonance is in agreement with the prediction of a theoretical model based on an extension of the direct-transition model for bulk states.