Welcome to the Division for Soft X-ray Physics
Introduction
The research activities include spectroscopic studies of electronic structure and excitation dynamics of molecules and solids. Both basic studies and applied research is carried out. Investigations are conducted of fundamental phenomena in simple systems in order to further the understanding of electron correlation, and new experimental methods are developed and applied to problems of technical relevance.
At the heart of the activity is soft X-ray spectroscopy. Soft X-rays originate in transitions between shallow quasi-atomic core levels and the outermost electron shell, and they offer information about the local electronic structure at specific atomic sites. The finite penetration of soft X-rays allows bulk properties and buried structures to be studied, and it facilitates in situ studies at real ambient conditions. In addition, the short life time of the core vacancy facilitates studies of dynamics on the femtosecond time scale.
Instrumentation
Soft X-ray emission is a low-yield process, which presents a challenge for spectroscopy studies; in particular since soft X-ray photons require grazing incidence for their reflection in optical systems. Therefore, necessary compromises regarding energy resolution and efficiency of the instruments have been compulsory in the past making the technique less attractive and for many years pursued only by a small number of groups.
Recent advances in techniques for producing VUV and soft X-ray radiation, notably the advent of synchrotron radiation, have changed the picture completely. This group made the first soft X-ray emission measurements with monochromatized synchrotron radiation in 1987, using a newly designed grazing incidence Rowland spectrometer. This instrument, with subsequent upgrades has been put to the market by a Swedish company, and some ten instruments are presently used in various research laboratories around the world. This instrument, in a modified version to fully exploit the momentum transfer in the X-ray process, will be adapted to take advantage of the extraordinary brilliance at the shortly upcoming PETRA III synchrotron radiation facility in Hamburg. Aiming for exceptional energy resolution as well as high efficiency we have developed a new type of spectrometer based on parabolic optics and plane grating. We are also investigating the potential of interferometric methods for spectroscopic purposes. The first VUV Fourier transform spectrometer is now being commissioned in our laboratory.
The group is using synchrotron radiation sources all over the world, such as ALS (Berkeley), BESSY, (Berlin), ELETTRA (Trieste), SPring-8 (Hyogo), and ESRF (Grenoble), and it has built a beamline at MAX-II (Lund) for soft X-ray spectroscopy. This beamline is presently being rebuilt to become a dedicated source for the new spectrometers.
Using ultrathin windows soft X-ray spectroscopy can be pursued on gases, liquids, and samples under pressure. Thus, the application range of soft X-ray spectroscopy is significantly extended, and we are developing new variations of gas/liquid cells.
In addition to soft X-ray spectroscopy the group pursues related method developments. These include various coincidence techniques, such as X-ray emission-threshold-electron coincidence spectroscopy (XETECO), and multi-electron coincidence spectroscopy using magnetic bottle spectrometers.
Laser-based activities include development of excimer lasers based on rare-gas dimers, and fundamental studies of these systems, e.g., using REMPI spectroscopy.
Theoretical modeling is an important and necessary element in spectroscopic research, and model calculations using various codes developed by dedicated computational physics groups are used within the group, as well as in collaboration projects with theory groups.
Scientific Projects
Soft X-ray spectroscopic methods have applications in a wide range of fields from the most fundamental aspects of electronic structure to applied materials science. Basic research pursued by this group include studies of transition metal and rare earth compounds, which exhibit strong electron correlation and therefore are of general fundamental interest. Due to their special properties regarding e.g. magnetism, chemical activity or other, such materials are often important components in technical applications. Electron correlation is also the main study object in the group’s spectroscopic work on the simplest systems, like atomic He and other few-electron systems.
The nature of the applied research carried out by the group is not very different than the basic research the difference being that there has been a defined technical use of the knowledge obtained. The current projects of this kind have a direct connection to the basic studies performed, and they concern actinide chemistry related to long term nuclear waste management; materials for nuclear technology, e.g. oxygen-free corrosion of waste cannisters; details of chemical processes in atmospheric corrosion, measured at real conditions; properties and optimization of Li-ion battery electrodes.