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Inorganic Chemistry – Electronic Structure Group

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Electronic Structure:
Axel Knop-Gericke
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The ultimate surface and element sensitivity on the one hand and the possibility to vary the information depth on the other hand adds an extra quality to XPS when operated with a tuneable X-ray source like a synchrotron. In addition, X-ray absorption spectra (XAS) can be taken at a synchrotron. Thus, BESSY (Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H.) and the FHI recently installed the facility ISISS (Innovative Station for In Situ Spectroscopy) at the 3rd generation X-ray source located in the south east of Berlin. ISISS combines in a unique matter a state of the art soft X-ray beamline, a high pressure XPS endstation, and a proper, permanent infrastructure on site for experiments with a chemical background. The optical layout of the beamline is shown in Fig. 1.

(to see full size, click on the image)
Fig. 1: The optical layout of the ISISS beamline at BESSY.
The ISISS beamline is designed following the principle of a plane grating monochromator working in collimated light. The beamline exploits the synchrotron radiation of a bending magnet and it is optimised to deliver photons in the energy range between 80eV - 2000eV with a high photon flux of up to 6*1010 photons/ sec/ 0.1A ring current (compare to Fig. 2). It can be operated in different modes i.e. high flux, high spectral resolution, or higher order suppression (thereby increasing the spectral purity of X-ray beam).
Fig. 2: Photon flux after passing 100nm Si3N4 X-ray membrane of the high pressure XPS set-up recorded with a calibrated photo diode.
The increased spectral purity of the X-ray beam when operated in a different mode (c=1.6 instead of c=2.25) becomes obvious by the disappearance of 2nd and 3rd order Ga L-edge absorption features.
The photon beam is re-focused after the exit slit to ensure a high brilliant X-ray spot at the sample position. The spot size is typically 150µm x 80µm (h x v) as shown in Fig. 3.

More information about the high pressure XPS apparatus can be found here [link to METHODS/HP-XPS]. The XPS set-up is located in a hutch inside the experimental hall at BESSY (Fig. 4a and 4b). The installation of an elaborated ventilation and safety system makes it feasible to use hazardous substances (e.g. flammable, explosive, or poisonous) as feed gas. At ISISS you can find a chemical lab on site, too (Fig. 4c). This allows basic sample pre-treatments like washing, drying and pelletising.

Fig. 3: Typical X-ray focus size at ISISS

Fig. 4a: ISISS hutch at BESSY
Fig. 4b: HP-XPS inside hutch
Fig. 4c: Chemical lab
Homepage ISISS beamline ... ››
Homepage ISISS endstation ... ››
For more information please contact Dr. Axel Knop-Gericke or Dr. Michael Hävecker.
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