Experiment has shown that a plot of differential conductance vs. applied voltage exhibits large oscillations when a scanning tunneling microscope is operated in constant current mode. These oscillations are an interference effect caused by multiple reflections of the tunneling electron between the sample and the classically forbidden barrier region in the vacuum gap. A theoretical study of these oscillations has been performed. The potential, taken to be one dimensional, included multiple image effects. A free-electron model was used for the metals of the probe and sample. The electron transmission probability was found, using a numerical integration scheme. The amplitude of the oscillations has been shown to be sensitive to the shape of the surface potential of the sample, giving a new method to study surfaces with the scanning tunneling microscope.
