  We study the light emission from a Ag(111) surface when the bias voltage on a
scanning tunneling microscope (STM) junction is ramped into the field emission
regime. Above the vacuum level, scanning tunneling spectroscopy (STS) shows a
series of well defined resonances associated with the image states of the
surface, which are Stark shifted due to the electric field provided by the STM
tip. We present photon--energy resolved measurements that unambiguously show
that the mechanism for light emission is the radiative decay of surface
localized plasmons excited by the electrons that tunnel inelastically into the
Stark shifted image states. Our work illustrates the effect of the tip radius
both in the STS spectrum and the light emission maps by repeating the
experiment with different tips.
