The conversion of light energy into other forms of energy is of uttermost importance in a vast range of fields, such as natural and artificial photosynthesis, photoelectrochemistry, or optoelectronic device development. The absorption of a photon by a material can lead to the population of an electronically excited state with enhanced chemical reactivity or to the switching of a material between two states with vastly different properties. Excited-state energy dissipates by a variety of mechanisms, including intersystem crossing, electron-phonon coupling, luminescence, charge/energy transfer, or non-radiative decay in the form of heat. Depending on the specific application, the aim is to optimize one or several of these relaxation pathways, while avoiding others. A fundamental atomic-scale and microscopic understanding of the dynamics after photon absorption is thus pivotal for the rational control and bottom-up design and synthesis of light-harvesting and photoswitching materials and devices.
HZB/R. van der Veen
While ultrafast optical spectroscopies are very powerful for probing the electronic population of excited states with attosecond (as) to nanosecond (ns) temporal resolution, they often lack the spatial resolution necessary to resolve individual atoms. Our department specializes in the development and application of time-resolved pump-probe techniques based on X-ray and fast-electron probes. The Å-nm-scale de Broglie wavelength of these probe pulses renders them directly sensitive to the atomic-scale structure. Knowledge of the structural dynamics that accompany light-induced relaxation processes is crucial to uncover the mechanisms of bond-breaking and -making in photochemical reactions, to derive intricate structure-property-photoactivity relationships, and to enhance the predictive ability of theoretical atomic-scale models in photovoltaics, photocatalysis, and optoelectronics.
Our department consists of the following groups:
Ultrafast soft and hard X-ray spectroscopy of nanoscale and molecular materials
X-ray absorption spectroscopy (XAS) is particularly appealing for the study of metal-organic and heterogeneous (nano)materials, as it is an element-specific local probe of both the electronic and geometric structure, and it can be implemented in any medium. In our lab, we use short (fs-ps) X-ray pulses from synchrotron radiation facilities or X-ray free-electron lasers (XFEL) to probe the dynamics after excitation with a short laser pulse. We have a particular interest in heterostructured nanomaterials for photo-(electro)catalysis and solar energy conversion, molecular photocatalysts based on first-row transition metals, and strongly cooperative spin-crossover nanomaterials.
Ultrafast electron microscopy
In collaboration with the CatLab and the group of Prof. Christoph Koch at the Humboldt University we are developing a unique ultrafast electron microscope (UEM). UEM combines the high temporal resolution of ultrafast laser spectroscopy with the superb spatial resolution of transmission electron microscopy (TEM). Compared to ultrafast optical and X-ray methods, UEM exhibits a superior spatial imaging resolution (sub-nm), and it offers the possibility to characterize morphology (via imaging), geometric structure (via diffraction), and electronic structure (via spectroscopy, EELS) of materials - all within the same table-top setup. The UEM apparatus is based on a Hitachi H8110 TEM, which is being modified to enable two pulsed laser beams to enter the ultrahigh vacuum environment of the microscope. One laser beam is used for excitation of the sample with tunable UV/visible wavelengths and pulse durations between ~200 fs and tens of ns. A second UV laser beam is used for the generation of short electron packets by making use of the photoelectric effect at the cathode tip in the microscope gun. By finely tuning the relative time delay between laser excitation (pump) pulses and the electron probing pulses using an optical delay line (fs time scale) or digital delay generators (ns time scale), we are able to make an atomic-scale movie of the photoinduced structural dynamics in individual nanostructures.
Photoinduced phase transitions and ultrafast magnetism in solids using the femtosecond X-ray slicing source
Materials with complex phase diagrams like magnetic compounds exhibit macroscopic functionalities that may be used in future electronic or storage devices to make them faster, smaller, and more energy efficient. The aim of our research is the understanding and control of these functionalities on a fundamental microscopic level. To this end, we explore the complex interplay between materials’ spin, orbital, charge, and lattice degrees of freedom upon photoexcitation on an ultrafast timescale. An excellent tool to probe these in a time-resolved manner are ultrashort soft X-ray pulses. Soft X-rays allow probing the material response in a highly selective and local manner. In particular spectroscopy employing circular polarized X-rays is unique sensitive to spin and orbital magnetic moments.
With the BESSY II Femtoslicing Facility we have established a unique storage ring based source of ultrashort soft X-ray pulses. As the only fs soft X-rays source providing circular polarization in a wide photon energy range, this facility has been instrumental in understanding magnetic effects on ultrashort time scales. Even today, very few such sources exist, while ours provides a unique experimental environment for time-resolved X-ray absorption spectroscopy, X-ray magnetic circular dichroism and for other photon-in-photon-out techniques like (resonant) soft X-ray (magnetic) scattering, and (resonant) soft X-ray (magnetic) reflectivity. The materials’ response on a 100 fs time scale provides insight into the coupling of various degrees of freedom and our suite of methods and instrumentation allows investigating a large variety of complex materials.