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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.

EMIL, the Energy Materials In situ Laboratory Berlin is a concerted effort of the Helmholtz-Zentrum Berlin (HZB) and the Max-Planck-Society with Fritz-Haber-Institute and the MPI for Chemical Energy Conversion as the MPG institutes in charge. A unique beamline at BESSY II that covers a very wide continuous photon energy range from the soft energy region up to the tender X-ray region, i.e. 80eV – 8000eV has been installed during at the 2nd generation synchrotron BESSY II of the HZB within the EMIL project. The photon sources for these beamlines are two canted undulators (soft: APPLE II type elliptical undulator UE48; tender: cryogenic in-vacuum undulator U17). Fig. 1 displays the X-ray intensity in the whole photon energy range resulting from ray tracing simulations. First results obtained during beamline commission confirm the values for photon flux and spectral resolution from the simulations.
Figure 1: X-ray photon flux as obtained by ray tracing simulations
(Stefan Hendel, HZB).
Three combined foci of the soft and tender X-ray beamlines are located in a newly constructed laboratory and office building outside of but directly connected to the experiment hall of BESSY (Fig. 2). The MPG will operate a high kinetic energy ambient pressure X-ray photoelectron spectrometer (AP-XPS) at one focus inside the CAT lab. CAT offers about 150m2 of lab space and all infrastructure that is required for in situ experiments with a chemical background like for instance gas supply and management, lab and local ventilation systems, exhaust lines, gas sensors, safety cabinets, and a glove box for air sensitive materials.

Figure 2: The EMIL building is an annex
to the BESSY II experiment hall.
First soft X-rays could be successfully transmitted from UE48 to the CAT focus in front of the entrance aperture (diameters available: 300µm, 1000µm) of the differentially pumped AP-XPS spectrometer at the beginning of November 2017 (Fig. 3a, 3b). This spectrometer is capable to cover a kinetic energy range of photoelectrons up to 7000eV nicely matching the available photon energy range for excitation at the CAT beamline.
Figure 3a: High resolution picture of the
soft X-ray spot on a fluorescence screen
at the CAT focus.
Figure 3b: X-ray spot in front of the
entrance aperture of the AP-XPS visible
on a fluorescence sample.
The performance of the spectrometer at the CAT beamline has been evaluated by measuring a standard test sample (Ar sputter cleaned Au foil) in vacuum at different excitation energies (Fig. 4a). Also, first in situ measurements in the presence of gas could be performed. Fig. 4b shows the Au4f core level region (Ekin=336eV) in vacuum, 50Pa of Argon gas and 200Pa of Argon, respectively, verifying its capability for XPS spectroscopy under gas pressure. Please note the appearance of extra peaks at the low kinetic energy side of the spectrum (i.e. apparent higher binding energy) only in the presence of a gas. These peaks are caused by inelastic scattering of the primary electrons released by excitation of the Au4f core levels with the Ar molecules in the gas phase. Inelastic scattering of the photoelectrons reduces the XPS intensity and does not only result in changing background of the spectra but also other electron impact excitations of the gas molecules below the ionisation level to continuum as possible.
Figure 4a: Wide range survey XPS spectra
of Ar sputter cleaned Au foil with different
excitation energies as indicated.
Figure 4b: Detailed XPS scans of the Au
4f core level region in vacuum, 50Pa Ar
and 200 Pa Ar, respectively.
Further commissioning of the CAT beamlines and the AP-XPS endstation is scheduled for the first half of 2018. The commissioning phase will slowly transpose into test and friendly user operation of the AP-XPS spectrometer at CAT in the course of 2018.
For more information please contact Dr. Axel Knop-Gericke or Dr. Michael Hävecker.
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