The group's main experimental approach to study the electronic structure and its excitations in materials is X-ray spectroscopy. We are specialized in inelastic X-ray scattering, and have close ties with many international synchrotron laboratories (ESRF, MaxLAB, APS etc.).

The experimental activities are complemented by strong emphasis on computational work. We use quantum chemistry, band structure, atomic multiplet and molecular dynamics simulations methods to interpret and predict the experimental data. Most of our projects involve both the experimental and computational side. Supercomputing has a significant role in the computational work.

Our Academy of Finland projects:

  • Advanced materials studied by novel hard x-ray methods
  • In situ spectroscopic and imaging studies of hierarchical structures using synchrotron radiation
  • Computational methods for x-ray research on advanced materials
  • Quantum simulations of molecular optical properties in the condensed phase

Our CSC projects:

  • Inelastic x-ray spectroscopy (the group's main project)
  • Electronic and optical properties of perovskite-based photovoltaic materials (Grand Challenge project from April 2015)

Fundamentals of electronic structure and excitations

In the gas phase, we use X-rays to reveal the complex excitation mechanisms of molecules and molecular clusters. Working in either resonant or non-resonant scattering conditions, we obtain new insight into the detailed atomic structures and electronic excited states. One of our important research topic in this field is understanding the binding patterns in atmospheric aerosol particles and in the corresponding extended condensed systems via X-ray experiments. Understanding in general the dynamics of the chemical bond is an unsolved issue, and our research contributes in this area.

In solids, non-resonant inelastic X-ray scattering probes the dynamic structure factor, which directly measures the fundamental dielectric properties of the material. In the so-called Compton scattering regime we obtain unique information on the ground-state correlations in the electronic system. The measured properties both in the gas and the solid phases can be compared with predictions from, for example, advanced quantum chemistry or many-body perturbation theory, to better understand the structure and excitations of the systems and validate the theoretical descriptions.

Highlights

Advanced materials and new techniques

This research line covers studies on energy, photovoltaic and superconducting materials. For advanced battery and energy storage materials, we selectively probe elements by core-specific X-ray techniques to map their chemical environment, which is relevant for understanding the system's or device's operation. In the field of photovoltaics, we study 3rd generation thin film systems, which include chalcopyrite and perovskite materials, to get more insight into their sunlight absorption properties. We also study dye sensitized solar cells, aiming to understand the binding of the antenna molecules on the surface, which is known to influence the photovoltaic efficiency.

Probing the dynamic structure factor of these materials provides the fundamental information on the possible energy-momentum excitations of the system. In resonant scattering conditions we employ the RIXS mapping technique to understand localized excitations, which are related to, for example, the emergence of high-temperature superconductivity in cuprates. In the field of new X-ray techniques, we have developed a direct tomography method for 3D X-ray imaging.

Highlights

Complex liquids

We use core-level X-ray scattering (X-ray Raman scattering) to reveal the short-range molecular order (that is, the molecular arrangement at the length scales of roughly 1-5 Å) in various complex liquids. There are a lot of open questions even in the case of neat liquid water of its microscopic structure, which shows that advanced probing methods are needed. The systems we study include neat and multicomponent liquids, hydrates, ionic solvation, and larger solvated molecules or polymers.

The information extends the understanding obtained by more traditional X-ray or neutron diffraction methods. The X-ray spectra are particularly sensitive to the hydrogen bond network. In these studies, we typically investigate the same system also theoretically, with molecular dynamics simulations and spectral calculations, for a full experiment-theory comparison. Another, complementary technique is X-ray Compton scattering, which we use to detect subtle changes in the covalent bond lengths - at the sub-picometer scale - and in the hydrogen bond network (see example in the figure below).

Highlights

Figure: Schematic representation of structural changes in the two solvation regimes in water-ethanol mixtures compared to pure liquids. X-ray Compton scattering was used in the study, see the Phys. Rev. Lett. article. (Figure: Iina Juurinen)

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