In Situ X-ray Absorption Spectroscopy in the Soft Energy Range: Novel Prospects for the Chemical Characterization of Sol
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In Situ X-ray Absorption Spectroscopy in the Soft Energy Range: Novel Prospects for the Chemical Characterization of Solid State Surfaces at High Pressure And High Temperature Th. Schedel-Niedrig, M. Hävecker1, A. Knop-Gericke1, P. Reinke2, R. Schlögl1, and M. Ch. Lux-Steiner Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, 14109 Berlin, Germany 1 Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin, Germany 2 Universität Basel, Georg-August-Universität Göttingen, II. Physikalisches Institut, Bunsenstrasse 7-9, 37073 Göttigen, Germany ABSTRACT An instrument equipped with total electron yield detectors was designed and constructed for in situ X-ray absorption spectroscopy (XAS) investigations in the soft X-ray range (100 eV ≤ hν ≤ 1000 eV) at elevated pressures (mbar range) and sample temperatures (T ≤ 1000 K) [1]. This allows, for the first time, XAS studies in a surface-sensitive mode of the light elements (Z = 3-15). Furthermore, the gas phase XAS and the surface-related XAS of the solid state phase can be collected simultaneously in order to correlate the gas/solid reaction rate with the surface electronic structure under working conditions in a flow-through mode. The novel experimental tool represents a contribution to the experimental overcoming of the "pressure gap" in material science. In this work examples are presented belonging to the field of heterogeneous catalysis [2-4] and to the reactivity of diamond surfaces [5]. Additionally, prospects for in situ studies in material science will be given. INTRODUCTION Understanding gas/solid reactions and the resulting products often requires the use of spectroscopic techniques that minimise the perturbation of the specimen and allow to use the appropriate reaction (environment) conditions during the investigation. X-ray absorption spectroscopy (XAS) investigations have long been performed due to their value in the study of the local structure of solid state materials, but it is only within the last decade that the development of in situ techniques has been progressed most significantly [6]. An in situ investigation means a study performed under real practical conditions which is necessary since the structure of solid state materials - e.g., a catalyst- can be very different from that found after the reaction or ex situ, i.e., under vacuum conditions. Photons in the VUV range (100 – 1000 eV) are suitable probes for the electronic structure of reacting surfaces. Their interaction with solid matter leads to photoabsorption processes which can be detected via the Auger electrons created by the relaxation of the core holes. Photons of this energy interact also moderately strong with gas molecules giving rise to the same type of photoabsorption-electron emission processes. The gas phase absorption limits the useful pressure range of the experiments to about 1–10 mbar depending on the nature of the gas phase and the photon flux of the excitation source. The detection of the signal from the solid surface is less complex as long as an element is investigated which
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