Tunneling spectroscopy of Stark-shifted image potential states on Cu and Au surfaces

Using low-temperature scanning tunneling microscopy (STM) distance-voltage characteristics at constant current are measured on Cu(111), Cu(110), Au(111), and Au(100) surfaces. Resonant tunneling features are observed that arise from image potential states distorted by the strong electric field of the STM tip. Distance-voltage characteristics are measured over a wide range of current setpoints and the energetic position and width of the resonant features show a significant dependence on this parameter. The lowest energy feature is found to be nearly independent of STM tip condition, while higher energy resonances show substantial tip dependence. The results are discussed and compared with recent calculations.