ISISS (Innovative Station for In Situ Spectroscopy) is a project of the Inorganic Chemistry department of the FHI (Fritz-Haber-Institut der Max-Planck-Gesellschaft) and HZB/BESSY II in Berlin. The purpose of the ISISS facility is to provide access for a large community to surface sensitive gas/solid interface characterisation methodologies in the presence of a reactive gas, i.e. in situ under conditions equal to or close to reality. The working area of the facility is material science in general and catalysis in particular. Our approach includes 3 units that have to complement on another: a state of the art soft X-ray beamline, an endstation for ambient pressure X-ray photoelectron (NAP-XPS) and X-ray absorption spectroscopy (XAS), and an infrastructure on site to perform experiments with a chemical background. In contrast to standard vacuum surface science experiments, in situ experiments require the installation of a complex gas feed and an elaborated gas analytic to follow the conversion of the gas phase during the reaction.
Fig. 1: Scheme of ISISS approach
Fig. 2: Optical beamline layout
Fig. 3: Typical photon flux at sample position (with 50nm SiNx pressure resistant membrane)
Fig. 4: Impact of higher diffraction orders at varying c-value (simulation)
The beamline has to accommodate for a variety of user requirements resulting from the scientific approach as outlined above. The basic design considerations are as follows:
A variety of materials with a large diversity in composition should be characterised and diverse scientific problems should be tackled. This requires a beamline that is adaptable to the needs of the users. This comprises a high flexibility concerning the provided photon energy range and spectral resolution.
Due to the compounds usually studied in catalyst research, the available photon flux at energies between 400 eV and 1200 eV is the main concern covering e.g. the C1s, N1s, O1s and transition metal 2p core levels.
The X-ray spot size at the sample position is optimised for a best fit to the electrostatic lens system of the in situ apparatus and the electron analyser.
The beamline should be easy to handle to allow for multi-user operation without elaborated knowledge of technical details.
An accurate and reliable energy calibration should be feasible as an essential part of high quality XPS studies.
The design considerations resulted in the selection of a plane grating monochromator (PGM). This PGM design is a development based on the Petersen type monochromator at BESSY at which the light is colliminated in the dispersive plane in front of the grating by the mirror M1. In this design, the fix focus constant c = (cos β / cos α) (α: angle of incidence, β: angle of diffraction relative to the grating normal, respectively) is kept constant during the scanning of the photon energy. This allows the free adjustment of the fix-focus constant without movement of the exit slit which can be used to easily optimise the monochromator to the requirements of the users.
Obviously, the understanding of the interaction of a catalyst surface with the reactants plays a key role in a detailed description of catalytic processes. X-ray photoelectron spectroscopy (XPS) is a well-established powerful tool to study in detail the outermost surface of solids but it was traditionally restricted to high vacuum and low pressure conditions. However, recently a methodology based on a differentially pumped electrostatic lens system has gained much interest.
The Fritz-Haber-Institut der MPG has operated at BESSY such an instrument since 2002 at different undulator based beamlines. In 2007 a beamline (ISISS) dedicated to near ambient pressure XPS (NAP-XPS) experiments has been implemented at HZB/BESSY II. A further improved version of this instrument is installed as the ISISS beamline since June 2013. A picture of the set-up can be seen as Fig. 1 while Fig. 2 shows a sketch of the main components.
Fig. 1: picture of NAP-HE-XPS (courtesy of SPECS GmbH, Berlin)
Fig. 2: Scheme of the NAP-HE-XPS endstation installed at the ISISS beamline.
The spectrometer (right site) is displayed retracted from the XPS cell module (left side).
Table 1: Gas analytics
Table 2: Laboratory facilities at ISISS