Electrons occupying surface states on the closepacked faces of noble metals form a two-dimensional (2-d) nearly-free electron gas. Because this system is accessible to the scanning tunneling microscope (STM), it provides a unique opportunity to study the local properties of electrons in reduced dimensions. When these surface state electrons scatter off of surface defects or steps, a standing wave due to the interference of incident and reflected waves is created. We have discovered that this standing wave pattern can be directly imaged with the STM (Fig. 1). Analysis of the energy dependence of the periodicity of the standing waves gives an independent measure of the surface state dispersion (Fig. 2).
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