Preface to the Special Issue of Topics in Catalysis on Ambient Pressure X-ray Photoelectron Spectroscopy

This Special Issue of Topics in Catalysis focuses on the application of Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) for the investigation of heterogeneous interfaces. The topics of the papers included in this issue range from heterogeneous catalysis, including electrocatalysis, to the chemistry of interfaces in the environment, to solid/liquid and liquid/vapor interfaces. This broad array of applications reflects the utility of APXPS for investigations in a wide range of disciplines. The first APXPS instrument was conceived and installed by the Siegbahn group in Uppsala in the early 1970s and applied in particular to the investigation of concentration gradients across liquid/vapor interfaces, a topic that is as important nowadays as it was then. Several other APXPS instruments were installed over the following decades: At Cardiff at the end of the 1970s, at Novosibirsk in the mid-1980s, and at Orono/ME at the end of the 1980s. All of these instruments used laboratory sources for the incident X-rays, and a set of differentially pumped apertures to separate the high pressure in the sample compartment from the vacuum in the electron analyzer, to minimize the path length of the electrons through the high pressure region and prevent arcing due to large potential differences in the electron analyzer at elevated pressure. The design of a differentially-pumped lens system at Berkeley Lab in the late 1990s helped to overcome the trade-off between efficient differential pumping using small apertures and highthroughput electron detection by placing electrostatic lenses between the differentially-pumped apertures, which increased the solid angle of electron collection in APXPS and thus the pressure limit from previously 1 mbar by at least an order of magnitude. The first two instruments at the Advanced Light Source in Berkeley and at BESSY in Berlin that used this design produced the vast majority of APXPS publications that appeared in the first decade of the twentieth century. At the end of that decade commercial APXPS instruments became available, many of which were installed at other 3rd generation synchrotron sources around the world. At the time of writing 11 synchrotrons either have already operating instruments or are in the process of installing them. The availability of instruments at synchrotrons has dramatically increased access to APXPS measurements for researchers from a wide range of disciplines, including heterogeneous catalysis, geosciences, atmospheric science and electrochemistry. Recently, a growing number of laboratory-based instruments have been installed; these instruments greatly benefit from the availability of small-spot, monochromatized X-ray anode sources. Future growth in the number of APXPS instruments is expected to be mainly due to the installation of additional laboratory-based instruments as opposed to those at synchrotrons. While the growth of the APXPS community and the breadth in the application of the technique over the past 15 years has been an important contribution to the development of new tools for the operando investigation of interfaces, there are many open problems that need to be addressed by the APXPS community over the coming years. Chief among these is the preparation of surfaces and interfaces with the same control over impurity levels and crystallography as one is used to from ultra-high vacuumbased surface science investigations. Sample preparation is in particular challenging for liquids with high vapor pressures, where exact control of the thermodynamic & Hendrik Bluhm hbluhm@lbl.gov