X-rays have as ionizing radiation a number of other unique properties beyond their very short wavelength. One important property for our science is their element selectivity. By choosing and investigating the spectra of particular elements that are placed at unique places in complex molecules we have a localized "atomic sensor". By investigating these atoms at different times after the structure was excited with light we can trace the development of electronic and structural changes even in very complex systems or in other words follow the electron through the molecule and over interfaces.
X-ray Absorption Spectroscopy (XAS)
In x-ray absorption spectroscopy (XAS) the energy of the absorbed photon raises an electron from a deeply bound state into unoccupied bound states or it gains enough energy to escape the atom. The absorption spectrum thus contains detailed information about the density of empty states and allows conclusions about coordination, oxidation state and many more information about the local structure. If the energy of the the photon is sufficient to overcome the binding potential the electron the absorption probability is influenced by a electron scattering process from the local environment of the surrounding atoms. This technique called EXAFS and can be used to determine the local structure around the absorbing atoms.
X-ray Emission Spectroscopy
The electrons filling the core hole that is generated during the absorption process emit x-ray photons whose energy can be analyzed. The fine structure of this x-ray emission (XES) give detailed information on the density of filled states and can be used due to the influence of spin-orbit coupling to track the oxidation and total spin state of the emitting atom.
X-ray diffuse scattering (XDS) / X-ray wide angle scattering (WAXS)
Photons that are not absorbed can be elastically scattered. Since our interest lays primarily in the dynamics of molecules in solution this process can be used to study the relation of points of high electron density to each other. With other words we can use wide angle x-ray scatter (WAXS) also named x-ray diffuse scattering (XDS) to study structural dynamics in materials. This technique does, like the techniques above, not require that there is a relation between different molecules. Since we are missing the coherent amplification of crystals we can primarily study changes in structures with this technique.
Combination of techniques
XAS, XES and XDS give complementary information and are used in combination depending on the capabilities of the instruments. Many of our studies use large scale research facilities like synchrotron- and free-electron-laser facilities. With these techniques we have successfully traced the electron transfer in complex molecules like the shown Ruthenium - Platinum complex with very high time-resolution. We are investigating novel iron-carbene complexes for solar light sensitization, light activated catalysts, novel polymer structures combining light sensitization and catalytic properties and novel nano dots for light sensitization and catalysis.
Table top ultra-fast x-ray spectroscopy
A large part of our research efforts is aimed to develop table-top systems capable of measuring x-ray spectra with sufficient time-resolution to perform these studies in-house. We have developed a x-ray plasma source based on a water that produces x-rays from the XUV to beyond 80 keV. We have developed detection technology that is capable to analyse spectra from x-ray absorption and x-ray emission with extreme efficiency in the range from >300eV to 10 keV with better 3eV at 6keV, which is sufficient for many applications. Our secret is the use of direct detection, energy dispersive schemes using CCD's and Microcalorimeter. A particular focus of these setup is the spectroscopy in the "tender" x-ray range from 1keV to 5keV including the science of sulfur and phosphor based polymers.
Other techniques e.g. x-ray photo electron spectroscopy (XPS)
We also support the development of local facilities at the MaxIV laboratories and at the Lund Laser Center by e.g. advising and designing the experimental spectroscopy end-station at FEMTOMAX.
In support of many other groups we also use x-ray photo electron spectroscopy to analyze the binding energy of electrons directly. The surface sensitivity of this technique makes it very suitable to investigate the binding and e.g. sensitization of surfaces or catalytic activities of materials.