Warm dense matter created by isochoric laser heating

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation of the nanoparticles produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yields measured for single nanoparticles and the surrounding flat surface. For p-polarized, 400 nm (∼3.1 eV) femtosecond radiation, a distribution of enhancement factors is found with a large percentage (67%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution are characterized by spherical shapes and relatively smooth silver film morphologies. In contrast, the largest enhancement factors were produced by a small percentage (7%) of particles that displayed silver coating defects that altered the overall particle structure. Comparisons are made between the experimentally measured enhancement factors and previously reported calculations of the localized near-field enhancement for isolated silver nanoparticles.

Plasmonic field enhancement of individual nanoparticles by correlated scanning and photoemission electron microscopy.

This thesis describes photoelectron emission measurements made at the surface of Highly Oriented Pyrolytic Graphite (HOPG) using ultrashort laser pulses. It concentrates on the observation and understanding of a new phenomenon whereby infrared laser pulses of just 1.5eV photon energy can be used to generate photoelectrons with kinetic energies of up to 80eV. Intensity dependence measurements depict a highly nonlinear excitation process and for p-polarised light observations can be explained by a high-order multiphoton excitation mechanism. Comparisons with photoelectron spectra taken using XUV pulse trains show a striking resemblance suggesting that the same final states excited by multiple IR pulses can also be reached by a single XUV photon. Interferometric autocorrelation measurements of the photoemission signal show increasingly high nonlinearity at greater photoelectron energies and a simulation of the interferometric data constructed using Optical Bloch Equations agreed with experiments showing that in the highly non-linear regime the autocorrelation shape depends almost exclusively on the nonlinearity of the excitation. XUV-IR pumpprobe measurements are also presented and the technical difficulties of such measurements discussed. Finally a novel technique of velocity map imaging of photoemission from a surface has been demonstrated for the first time at the HOPG surface.

/pdf/photoelectron-spectroscopy-of-highly-oriented-pyrolytic-h4na6g1mzy.pdf

Photoelectron Spectroscopy of highly oriented pyrolytic graphite using intense ultrashort laser pulses

The adsorption of silver on the Cu(110)-(2×1)O surface was studied by Auger electron spectroscopy and scanning tunneling microscopy. For deposition at room temperature the CuO layer is overgrown by a highly stressed and therefore rough silver layer. If the sample is annealed, the oxygen will desorb via formation of temporary Ag-O compounds. At the same time silver will diffuse into the copper bulk. The situation is quite different if silver is deposited at 660 K: For the deposition of up to one monolayer of silver no significant desorption of oxygen is observed by Auger electron spectroscopy. It is found that domains of a Ag rich Ag/Cu alloy and the bare CuO phase are spatially separated. Only upon further Ag deposition, oxygen desorption takes place. [DOI: 10.1380/ejssnt.2010.32]

Ag on Cu(110)-(2×1)O: Desorption of Oxygen versus Diffusion of Ag

This chapter first discusses studies of graphene, whose intriguing physical properties and promising technological applications has attracted widespread attention. An area of similar interest, both from the point of view of basic physics and technology, plasmon physics is addressed next. While in the graphene studies all available imaging methods have been used with great success, the study of plasmons has been a domain of PEEM. Surface microscopy with slow electrons has also been used in a wide range of more technology-oriented applications ranging from minerals to electronic devices, and these are discussed next, followed by a short section describing the limited amount or work done in biology. A study of an unknown material, in which many methods had to be combined, completes this chapter.

Applications in Other Fields

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 $1\phantom{\rule{03em}{0ex}}\ensuremath{\mu}\mathrm{m}$ 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 $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{03em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ 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

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

The effects of pulsed laser irradiation on the morphology of the Cu(110) surface were investigated by means of reflectance difference spectroscopy (RDS) and spot profile analysis low-energy electron diffraction (SPA-LEED). The laser light induces surface defects (adatoms and islands) as well as subsurface dislocation lines. The high surface mobility leads to efficient annealing of the surface defects even at room temperature, whereas the subsurface dislocation lines persist up to temperatures T>800 K. SPA-LEED profiles of the (00) diffraction spot from the laser irradiated surface suggest an anisotropic distribution of the subsurface line defects related to the geometry of the fcc easy glide system, which is corroborated by STM measurements. Comparative experiments using conventional Ar ion bombardment point out the distinctiveness of the morphological changes induced by laser irradiation.

Effects of laser irradiation on the morphology of Cu(110)

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

Effects of laser irradiation on the morphology of Cu(110)