Surface-state Stark shift in a scanning tunneling microscope

We report a quantitative low-temperature scanning tunneling spectroscopy (STS) study on the Ag(111) surface state over an unprecedented range of currents (50 pA to 6 muA) through which we can tune the electric field in the tunnel junction of the microscope. We show that in STS a sizable Stark effect causes a shift of the surface-state binding energy E-0. Data taken are reproduced by a one-dimensional potential model calculation, and are found to yield a Stark-free energy E-0 in agreement with recent state-of-the-art photoemission spectroscopy measurements.