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Beamline Description

The Electron Analyzer

Since March 2008, a new electron energy analyzer, a SPECS Phoibos 150 with a 2D-CCD detector system, is operative in the BaDElPh end station. The analyzer is mounted on a fixed geometry with an angle of 50° relative to the synchrotron radiation direction. The angular dispersive plane of the analyzer coincides with the polarization plane of the synchrotron light in first harmonic (horizontal plane). The use of a 2D-CCD detector offers the possibility of simultaneous acquisition of the energy as well as the angular distributions of the photoelectrons. The span of  k|| vectors simultaneously probed by the analyzer is defined by the angular acceptance of the particular lens mode and by the photon energy. Four different angular resolved modes of the lens operation have been specifically designed for ARPES measurements reaching a maximum angular acceptance of about 26°; moreover, an ultimate angular resolution of about 0.1° could be achieved. From gas phase photoemission, an ultimate energy resolution of less than 4 meV has been certified by the supplier. The energy resolution in photoemission experiments from solids can be conveniently determined by measuring the Fermi edges of polycrystalline noble metals at low temperature (see Fig. 1). The fitting function was the product of a linear background with the Fermi–Dirac distribution (FDD) at the experimental temperature convoluted with a Gaussian. As a result of the fit the full width at half maximum (FWHM) of the Gaussian, i.e., the combined experimental resolution of the beamline and the analyzer, was found to be 5.4 meV. Under the above conditions, the calculated energy resolution of the BaDElPh NIM is 2.4 meV. Therefore, an energy resolution of 4.8 meV can be estimated for the electron analyzer.


Figure 1: Photoemission spectrum of the Fermi edge of polycrystalline silver at 10 K acquired at the photon energy of 7.8 eV together with the fit of the data and its components. The exit slit of the beamline was set to 0.3 mm, while the entrance slit and the pass energy of the analyzer were set to 0.5 mm and 1 eV, respectively.
Under the above conditions, an energy resolution of 4.8 meV can be estimated for the electron analyzer.

Ultima modifica il Lunedì, 15 Gennaio 2018 18:05