The modification of the surface electronic structure by an adsorbate is measured quantitatively with scanning tunneling microscopy and spectroscopy for the first time. The standing wave of the Cu surface-state electrons is utilized to probe the subtle change in the electronic structure on the Xe-covered Cu͑111͒. The observed Fermi wavelength on the Xe-covered surface is longer by 15% than on the bare Cu surface. The change upon Xe adsorption is explained with the observed modified dispersion of the Cu surface state; upward shift by (130Ϯ20) meV with almost same effective mass. RAPID COMMUNICATIONS R16 344 PRB 62 JI-YONG PARK et al.

The adsorption and growth of Xe layers on the Cu͑111͒ surface were studied with a low-temperature scanning tunneling microscope. Initially, Xe atoms preferentially adsorb at the step, revealing two different wetting behaviors at the upper and the lower step edges at the coverage of Ͻ0.1 monolayer. Three-dimensional island growth is followed on the terrace at the coverage of Ͼ0.2 monolayer when grown at Ͻ20 K. The island growth is attributed to inhomogeneous nucleation and lower diffusivity of Xe on the Xe monolayer than on the Cu͑111͒ surface. The diffusion barriers, the two-dimensional barrier on a terrace and the one-dimensional barrier along a step, and the step-down barrier determine the growth morphology of Xe layers as the substrate temperature was raised. ͓S0163-1829͑99͒00248-9͔

Selective excitations of specific vibronic modes in position space are realized in single naphthalocyanine molecules adsorbed on an ultrathin alumina film by a scanning tunneling microscope at low temperature. Distinct spatial distributions are imaged for the different vibronic modes, which are in accordance with spectra recorded over different points of the molecule and its orbital structure. These distinct vibronic images, together with the differential conductance images and calculated molecular orbitals, lead to vibrational excitations that are associated with the doubly degenerate lowest unoccupied molecular orbitals (LUMO)--LUMO-α and LUMO-β. These results reveal the presence of different molecular conformations on the surface and the nature of the electron-vibrational coupling.

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