The adsorption of a monolayer of Xe on an Ag(111) surface is of special interest because of a two dimensional electron surface state which is occupied close to the energy minimum at -67 meV with respect to the Fermi energy for the clean Ag(111) surface and which becomes completely unoccupied with a minimum energy of +52 meV after Xe adsorption. Therefore the signal of the surface state disappears in the photoelectron spectrum after Xe adsorption. However by using scanning tunneling spectroscopy, which gives access also to unoccupied electron states, this modification can be studied in detail. Combining a point-by-point mapping of spectroscopic data with a lock-in detection of dI/dU for the determination of the local density of states, we have imaged the standing wave pattern formed by the scattering of the surface state at a step edge for 14 different energies covering the range below as well as above the Fermi energy from E = -0.06 eV to E = +0.20 eV (a similar measurement for the clean Ag(111) surface can be found here).
The results arranged in a film sequence (click Figure) visualize the wave properties of the surface state and its different energy dependence E(k) on the clean Ag(111) area (bottom/right) and the Xe covered area (top/left). On the right side of the figure the experiment is explained with a schematic drawing of the E(k) dispersion.
(Click to start animation (0.8 Mb))
Maps of dI/dU with U = -0.06 ... +0.20 V of an Ag(111) surface partially covered with a Xe monolayer (50 nm x 50 nm, on the left side). The periodicity of the wave pattern shows two different energy dependencies E(k) (sketched on the right side). The 14 images are arranged in a continuous loop. The colour scaling in the STS maps is optimized for a smooth film display.
Reference:
Modification of the Shockley-Type surface state on Ag(111) by an Adsorbed Xenon Layer, H. Hövel, B. Grimm, B. Reihl, Surface Science 477, 43 (2001) .
