We demonstrate the use of a novel design of a photoelectron microscope in combination to an imaging energy filter for momentum resolved photoelectron detection. Together with a time resolved imaging detector, it is possible to combine spatial, momentum, energy, and time resolution of photoelectrons within the same instrument. The time resolution of this type of energy analyzer can be reduced to below 100 ps. The complete ARUPS pattern of a Cu(111) sample excited with He I, is imaged in parallel and energy resolved up to the photoelectron emission horizon. Excited with a mercury light source (h nu=4.9 eV), the Shockley surface state at the energy threshold is clearly imaged in k-space. Electron-electron interactions are observed in momentum space as a correlation hole in two-electron photoemission. With the high transmission and the time resolution of this instrument, possible new measurements are discussed: Time and polarization resolved ARUPS measurements, probing change of bandstructure due to chemical reaction, growth of films, or phase transitions, e.g., melting or martensitic transformations.
A novel instrument for imaging ESCA is described. It is based on a tandem arrangement of two
hemispherical energy analysers used as an imaging energy filter. The main spherical aberration
(α2-term) of the analyser is corrected by the antisymmetry of the tandem configuration. The
kinetic energy range useable for imaging extends up to 1.6 keV; this is compatible with Mg and
Al Kα
laboratory x-ray sources. First experiments on the chemical surface composition of a
Cu0.98Bi0.02
polycrystal, a GaAs/AlGaAs heterostructure and Ag crystallites on Si(111) have been
performed using synchrotron radiation. The results reveal an energy resolution of
190 meV and a lateral resolution (edge resolution) of 120 nm. Besides elimination
of the analyser’s spherical aberration, the tandem arrangement largely retains
the time structure of the electron signal, unlike a single hemispherical analyser.
The properties of molecular films are determined by the geometric structure of the first layers near the interface. These are in contact with the substrate and feel the effect of the interfacial bonding, which particularly, for metal substrates, can be substantial. For the model system 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(110), the geometric structure of the first monolayer can be modified by preparation parameters. This leads to significant differences in the electronic structure of the first layer. Here, we show that, by combining angle-resolved photoelectron spectroscopy with low-energy electron diffraction, we cannot only determine the electronic structure of the interfacial layer and the unit cell of the adsorbate superstructure, but also the arrangement of the molecules in the unit cell. Moreover, in bilayer films, we can distinguish the first from the second layer and, thus, study the formation of the second layer and its influence on the buried interface.
We measured the work function of micrograins at the surface of a polycrystalline copper sample using NanoESCA, a photoelectron microscope equipped with two analysers in series. This allowed us to obtain accurate spectroscopic information with energy-filtered images showing lateral resolutions of 40 nm. Reconstructed microspectra directly show variations in the local work function of grains having different crystalline orientation.
We demonstrate dark field imaging in photoelectron emission microscopy (PEEM) of heterogeneous few layer graphene (FLG) furnace grown on SiC(000-1). Energy-filtered, threshold PEEM is used to locate distinct zones of FLG graphene. In each region, selected by a field aperture, the k-space information is imaged using appropriate transfer optics. By selecting the photoelectron intensity at a given wave vector and using the inverse transfer optics, dark field PEEM gives a spatial distribution of the angular photoelectron emission. In the results presented here, the wave vector coordinates of the Dirac cones characteristic of commensurate rotations of FLG on SiC(000-1) are selected providing a map of the commensurate rotations across the surface. This special type of contrast is therefore a method to map the spatial distribution of the local band structure and offers a new laboratory tool for the characterisation of technically relevant, microscopically structured matter.
Using nanoESCA, a novel energy-filtered PEEM instrument equipped with an aberration corrected energy filter, we have imaged micro-sized (1 µm) polished copper grains with UV (4.9 eV) and VUV (21.1 eV) radiation. Energy-filtered UV-PEEM images allow to reach a lateral resolution of 40 nm. From series of energy-filtered images, reconstructed VUV microspectra taken over typical areas of 600 nm 2 , directly show up variations in the local work function, on an absolute energy scale, of grains having different crystalline orientation. Work function values measured from fitting of the photoemission threshold are consistent with < 111 >, < 100 > or < 110 > crystalline orientations (work function of 4.9, 4.6, and 4.5 eV, respectively). This experiment opens new possibilities in the local characterization of surfaces in the sub-micron range with high energy-and lateral resolutions.
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