Abstract

Electrons in Shockley-type metal surface states act as a quasi-two-dimensional free-electron gas (2DFEG). We show that scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to study the interactions of this 2DFEG with features such as individual steps and adsorbed or embedded atoms^1,2. Two approaches were utilized in our studies. One is based on the fact that an incident surface electron-wave when partially reflected by localized potentials such as those produced by steps, can interfere with the incident wave. This interference leads to an oscillatory local density-of-states near the step. Such oscillations were observed in dI/dV maps of Au(111) and Ag(111) surfaces. From dI/dV images as a function of the bias voltage the dispersion of the surface states and the scattering phase-shifts were obtained. Point-defects lead to isotropic scattering which appears in the form of concentric circles surrounding the defect. Because of the finite penetration depth of the surface state, not only adsorbed but also ebbeded species were found to act as scatterers. The electron oscillations observed at 300K decay exponentially as a function of the distance from the scattercr. We attribute this to the temperature-dependent Δk-width of the STS measurement which leads to the dephasing of the thus formed wave-packet. Another approach we used to study the interactions of the 2DFEG involves the spectroscopy of the onset region of the surface state band as a function of the distance from the scatterer². In this way, the range of strong perturbation of the surface state was estimated to be 15-20 Å, and evidence was found for defect-induced bulk-surface state mixing.

© 1995 Optical Society of America