Size of three-dimensional objects measured by means of photoemission electron microscopy

Abstract

Pb particles on a Si(111) substrate were investigated using a photoemission electron microscope (PEEM). The electrons have been excited by a linearly polarised femtosecond laser beam with a wavelength of λ = 400 nm. The PEEM images exhibit an unusually high contrast. They arise due to photoemission being of a nontrivial nature because the photon energy is lower than the work function of the sample. It may be caused by the following processes: two- or multi-photon absorption, tunnel-photoeffect induced by a strong lowering of the potential barrier owing to enhancement of the electrical field of the powerful incident light wave on a rough surface, generation of second harmonic with its following absorption, or electron gas heating. Images obtained with orthogonal directions of the photon polarisation were compared to estimate the shape of different Pb particles. The size of the islands was determined by two methods, either from the shape of the spatial brightness distribution or from the area under the curve of this distribution.