  We present a low-temperature scanning tunneling spectroscopy study of the
Au(111) and of the Cu(111) surface states showing that their binding energy
increases when the tip is approached towards the surface. This result,
supported by a one-dimensional model calculation and by a comparison to
existing photoemission spectroscopy measurements, confirms the existence of a
tip-induced Stark effect as previously reported for Ag(111) [L. Limot et al.,
Phys. Rev. Lett. 91, 196801 (2003)], and suggests that this effect is a general
feature of scanning tunneling spectroscopy.
