Abstract
The investigation of liquid surfaces and interfaces with the powerful toolbox of ultra-high vacuum (UHV)-based surface science techniques generally has to overcome the issue of liquid evaporation within the vacuum system. In the last decade, however, new classes of liquids with negligible vapor pressure at room temperature-in particular, ionic liquids (ILs)-have emerged for surface science studies. It has been demonstrated that particularly angle-resolved X-ray Photoelectron Spectroscopy (ARXPS) allows for investigating phenomena that occur at gas-liquid and liquid-solid interfaces on the molecular level. The results are not only relevant for IL systems but also for liquids in general. In all of these previous ARXPS studies, the sample holder had to be tilted in order to change the polar detection angle of emitted photoelectrons, which restricted the liquid systems to very thin viscous IL films coating a flat solid support. We now report on the concept and realization of a new and unique laboratory "Dual Analyzer System for Surface Analysis (DASSA)" which enables fast ARXPS, UV photoelectron spectroscopy, imaging XPS, and low-energy ion scattering at the horizontal surface plane of macroscopically thick non-volatile liquid samples. It comprises a UHV chamber equipped with two electron analyzers mounted for simultaneous measurements in 0° and 80° emission relative to the surface normal. The performance of DASSA on a first macroscopic liquid system will be demonstrated.
Highlights
Despite this undisputable progress, some issues have remained when working at elevated pressures, that is, the need for permanent replacement of the evaporating liquid in the jet experiments excluding static and flat liquid surfaces with a well-defined equilibrium temperature, and the potential effects of beam damage, when working with an extremely focused photon beam at near ambient pressures.In contrast to molecular liquids, ionic liquids (ILs), which are salt melts with melting points below 100 ◦C, are perfectly suited for XPS investigations under ultra-high vacuum (UHV) conditions
It has been demonstrated that angle-resolved X-ray Photoelectron Spectroscopy (ARXPS) allows for investigating phenomena that occur at gas-liquid and liquid-solid interfaces on the molecular level
We report on the concept and realization of a new and unique laboratory “Dual Analyzer System for Surface Analysis (DASSA)” which enables fast ARXPS, UV photoelectron spectroscopy, imaging XPS, and low-energy ion scattering at the horizontal surface plane of macroscopically thick non-volatile liquid samples
Summary
Some issues have remained when working at elevated pressures, that is, the need for permanent replacement of the evaporating liquid in the jet experiments excluding static and flat liquid surfaces with a well-defined equilibrium temperature, and the potential effects of beam damage, when working with an extremely focused photon beam at near ambient pressures. Mimicking CO2 chemisorption by primary amines using chain-functionalized ILs, ARXPS demonstrated that the adsorption state within the surface layer is different from that in the bulk.[25] There have been many other examples within the last ten years demonstrating that ARXPS allows for studying IL interfaces and ionic liquid chemistry phenomena in great detail.[23,27,47–54] These studies can serve as model studies of reference systems for liquid surfaces in general At this point, one should note, that all ARXPS studies on ILs reported so far involved sample rotation with respect to a fixed electron analyzer in order to change the electron emission angle. First results on liquid systems that could not be studied before will be shown demonstrating the ability of this unique setup
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