A high-pressure scanning tunneling microscope for studying heterogeneous catalysis

We describe an experimental setup for studying gas adsorption and chemical surface reactions by scanning tunneling microscopy (STM) at gas pressures ranging from ultrahigh vacuum (UHV) to 1 bar. The apparatus is designed for experiments to bridge the so-called pressure gap in catalysis research by obtaining atomic scale information about catalytic reaction mechanisms under steady-state conditions. It combines a UHV chamber for sample preparation and post-reaction surface analysis with a small high-pressure cell (volume 1.5 l) which contains the STM. Several concepts to improve the variable-pressure performance of existing high-pressure STM designs are described. These include access to the entire pressure range between UHV and 1 bar without triggering gas discharges, the potential for high-speed scanning and for variable temperature measurements. The design also features a fast transfer mechanism from the reactor to UHV, thus allowing for fast analysis of surface species after high-pressure experiments. First results with atomic resolution were obtained on a Ru(0001) surface at high oxygen pressures. The images show an O(1x1) adsorbate layer not observed in UHV experiments at room temperature