Multi-photon photoelectron spectromicroscopy of supported polystyrene spheres

Multi-photon photoemission excited by 100 fs, 400 nm laser pulses leads to an unexpected high contrast in photoelectron images of polystyrene spheres on a platinum substrate. The total, energy-integrated photoelectron yield shows clear signatures of two-photon photoemission from the substrate while photoemission from polystyrene is dominated by one-photon processes for low laser power and multi-photon processes for higher laser power. For excitation with UV light from a conventional Hg arc lamp, we observe a marked energy shift of the photoelectron spectrum of polystyrene with respect to that of the substrate. This shift is related to the different surface potentials of the conductive substrate and the dielectric spheres in the strong electric field of the objective lens of the microscope. Laser illumination causes photoconductivity in polystyrene by efficient two-photon excitation of long-lived states and induces a shifting of the surface potential of the polystyrene spheres. Pump-probe experiments support our conclusion that photoemission from polystyrene takes place from these long-lived intermediate states via a one-photon process for sufficiently low laser power. We suggest that photoelectron spectromicroscopy might be useful as a non-scanning method for fast height profiling of supported dielectric structures.