The NanoESCA is a combination of PEEM and aberration-corrected tandem hemispherical analyser that allows spectromicroscopy (i.e. energy-filtered imaging), microspectroscopy (small spot analysis) and momentum microscopy in k-space. It was especially designed for use in the X-ray range and has been developed in a cooperation project between two university groups (Universität Mainz and Universität des Saarlandes, Saarbrücken, group of Prof. Stefan Hüfner) and two companies (FOCUS GmbH and OMICRON GmbH). For details on the instrument and its performance, see here and here.
The instrument allows the three modes of operation:
The first three modes are discussed and applied in the papers by Escher et al. (see below), here we explain the fourth mode of operation in some detail:
This mode constitutes a very efficient way to obtain the full photoelectron energy-vs-momentum data set as recently demonstrated in the group of Prof. J. Kirschner MPI für Mikrostrukturphysik, Halle (B.Krömker et al.) that we may term “momentum microscopy”. Instead of angular scanning by a moving electron spectrometer one can take a whole 2D plot in reciprocal space at a given energy in a parallel imaging microscope. The basic principle is clear from simple geometrical optics, see schematic figure.
Imaging of energy-cuts through the Brillouin zone by “momentum microscopy”
Electrons emitted parallel to the electron-optical axis of a photoemission electron microscope are focused to the centre of the back focal plane BFP, whereas electrons ejected under off-normal angles are focused in the same plane, but to points away from the centre (7 different angles have been simulated in the left panel of the figure). The distance from the z-axis is a direct measure for k|| at the given energy. Thus, the BFP shows an image of the angular distribution of the photoelectrons in the kx-ky-plane (kz chosen perpendicular to the surface). The subsequent imaging energy filter (IDEA: Imaging double hemispherical energy analyser) sets the desired band pass and a projective optics generates a magnified electron-optical image on the multichannel plate / fluorescent screen image detection unit. In this way, a full section of the selected E = const.–surface in k-space is obtained within less than 1 minute using a lab source and within few seconds using Synchrotron radiation. The multitude of images taken throughout the energy region of interest yields the same information content as a full 2-axes ARUPS plot, but in a much shorter time. The resolution in k-space depends on the slit widths of the energy analyser and on the field of view on the sample. A good k-resolution of 0.034 Å-1 has been obtained in the first experiments, with potential for improvement. The figure shows an example: Imaging of the Fermi surface of Cu.
B. Krömker, M. Escher, D. Funnemann, D. Hartung, H. Engelhard, J. Kirschner
P. Bernhard, J. Maul, U. Ott, Ch. Sudek, M. Escher, N. Weber, M. Merkel, B. Krömker, D. Funnemann, G. Schönhense
M. Escher, N. Weber, M. Merkel, Ch. Ziethen, P. Bernhard, G. Schönhense, S. Schmidt, F. Forster,
F. Reinert, B. Krömker, D. Funnemann
M. Escher, N. Weber, M. Merkel, B. Krömker, D. Funnemann, S. Schmidt, F. Reinert, F. Forster, S. Hüfner, P. Bernhard, Ch. Ziethen, H. J. Elmers, G. Schönhense
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