  Photoemission electron microscopy was used to image the electrons
photoemitted from specially tailored Ag nanoparticles deposited on a Si
substrate (with its native oxide SiO$_{x}$). Photoemission was induced by
illumination with a Hg UV-lamp (photon energy cutoff $\hbar\omega_{UV}=5.0$ eV,
wavelength $\lambda_{UV}=250$ nm) and with a Ti:Sapphire femtosecond laser
($\hbar\omega_{l}=3.1$ eV, $\lambda_{l}=400$ nm, pulse width below 200 fs),
respectively. While homogeneous photoelectron emission from the metal is
observed upon illumination at energies above the silver plasmon frequency, at
lower photon energies the emission is localized at tips of the structure. This
is interpreted as a signature of the local electrical field therefore providing
a tool to map the optical near field with the resolution of emission electron
microscopy.
