Surface-atom x-ray photoemission from clean metals: Cu, Ag, and Au

Surface-atom core-level shifts from evaporated noble metals are reported using angle-dependent x-ray photoemission with monochromatized $\mathrm{Al}K\alpha$ radiation. The absence of line broadening with increasing takeoff angle for the case of aluminum metal, where the surface shift is known to be small, confirms that the shifts observed in the noble metals are real surface phenomena. An analysis procedure is developed which establishes that the effect of the vacuum-solid interface in these systems is confined to the first atomic layer. The asymmetric (final-state—related) line shapes of the surface- and bulk-atom photopeaks are also shown to be identical within experimental error. The surface core-level shifts are -0.40±0.02 eV for Au, -0.08±0.03 eV for Ag, and -0.24±0.02 eV for Cu, with the surface contributions occurring at lower electron binding energy. The analysis additionally yields electron mean free paths of 19±3 Å at 1400 eV in Au and 14±3 Å at 550 eV in Cu. Using the surface-to-bulk intensity ratios of the $\mathrm{Au}4f$ core levels, it was possible to isolate the contribution of the surface-atom valence electrons. The width of the surface density of states is narrowed (8±2)% with respect to the bulk density of states and its center of gravity is shifted by -0.5±0.1 eV. The analysis procedures and conclusions presented here should be applicable to core and valence surface-atom photoemission from a wide variety of other systems.