Many fundamental physical and chemical processes are governed by the properties of surfaces or interfaces. Especially interfaces between solids and gases or solids and liquids play a key role in many applications, such like heterogeneous catalysis, energy conversion and storage, or environmental science. XPS is a very well established and versatile technique for the investigation of surfaces and can provide quantitative information about the elemental composition and the chemical state of the surface. In standard XPS systems the sample needs to be kept under UHV conditions. The main reasons for this requirement are the interaction of the photoemitted electrons with surrounding gas molecules resulting in a very short mean free path and the operating principle of electron detectors as well as X-ray excitation sources that require vacuum conditions. This limitation strongly restricts the type of samples that can be investigated mainly to solid samples or liquids with a very low vapor pressure.
This limitation is not present anymore in NAP-XPS systems due to the technical developments done over the last 45 years, but with a stron focus in the last 10 years. As a consequence, nearly any type of sample can be measured, e.g. powders and solids, humid samples, insulating samples, and even liquids. Furthermore, these samples can also be investigated in contact with controlled gas atmospheres at different temperatures. In this way, in-situ and even in-operando studies under realistic working conditions are possible.
A NAP-XPS system mainly consists of three building blocks, the analysis chamber/reaction cell that accommodates the sample under controlled conditions, a special electron analyzer with a dedicated differential pumping concept that collects as many photoelectrons as possible at a distance that is shorter than their mean free path and the excitation source, typically a high flux small spot laboratory X-ray or UV source and/or a gas-tight synchrotron beam entrance stage.
There are basically three system layouts for NAP-XPS systems which mainly differ in the way the sample environment is realized. All designs have specific advantages for specific experimental tasks or conditions.
(i) In the Backfilling System configuration the entire vacuum chamber, accommodating the sample, excitation source, and analyzer, serves as the reaction cell. In this setup the entire chamber is filled with desired gas atmosphere. The advantages of this setup are its flexibility and simplicity. Different excitation or preparation sources can be installed that point towards the sample in measurement position. Furthermore, the electron analyzer and, even more important, the sample manipulator can be mounted in different orientations. In this way, a horizontal oriented sample enables measurements on loose powder samples or even liquids.
For detailed information on the available NAP components and different NAP systems please refer to the following product homepages of our NAP solutions in this section.