Electron-metal-surface interaction potential with vacuum tunneling: Observation of the image force

It is shown that the classical image potential plays an important role in the correct interpretation of vacuum-tunneling experiments. The logarithmic derivative of the tunnel current with respect to the electrode separation is nearly constant and approximately equal to the square root of the average work function. From the experimental tunnel-voltage dependence with the electrode separation we obtain the barrier height as a function of distance. © 1984 The American Physical Society.