Enhanced photoemission from localized inhomogeneities on Cu(001) characterized by laser-assisted photoemission electron microscopy and low-energy electron microscopy

Multiphoton photoelectron emission from a state-of-the-art prepared Cu(001) surface has been investigated with photoemission electron microscopy. Randomly distributed, spatially confined regions (about $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter) of intense electron yield (hot spots) are observed. The hot spots give rise to distortions of the corresponding low-energy electron microscopy image. They are identified as polycrystallinelike protrusions embedded in the surface. The density of these inhomogeneities is about $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ and cannot be manipulated by ion bombardment or by homoepitaxial growth. The response of hot spots to illumination under ultraviolet light produced by an arc lamp and illumination by blue and infrared femtosecond laser light for different polarizations is recorded from the same surface region, allowing for a direct comparison. Hot spots respond very efficiently and even stronger to $s$-polarized light as compared to $p$-polarized light, and show apparently a nonresonant behavior with respect to the exciting wavelength. We discuss some of the conceivable mechanisms underlying this anomalous photoemission, also in view of the particular characteristics that are specific to our experimental setup. Our data are not compatible with an interpretation which relies exclusively on the excitation of (localized) surface plasmon modes, but rather with a nonresonant near-zone light field enhancement.