Methoden

Infrared absorption spectroscopy is a powerful method to study various excitations in matter below ~1 eV. In the context of molecules, the various vibrations specific to a molecular system are like a fingerprint that encodes the structural arrangement of atoms in molecules in the infrared absorption spectrum. Therefore, this type of spectroscopy has been used for more than a century as an analytical tool to study matter.

       The advent of ultrafast lasers having pulse durations below 1 picosecond (= 0.000 000 000 001 s) triggered a development that also lead to infrared pulses of less than 0.1 picoseconds (= 100 femtoseconds) duration. 100 femtoseconds is the typical timescale on which atoms can be displaced which means that femtosecond infrared spectroscopy can follow structural rearrangements in matter by 'watching' atoms move, in real time so to speak. Two-dimensional infrared (2DIR) spectroscopy is a more elaborate version of 'linear' infrared spectroscopy which allows to follow concerted atom motion, i.e. two spectral dimensions visualize which vibrations (and hence atomic motions) are coupled. Accordingly, 2DIR spectroscopy is sensitive to loss and growth of coupled vibrations which makes it a prime tool for structural studies of solvated molecules and increasingly so in solid-state physics to follow the relevant complex dynamics of low-energy excitations.

       We use 2D-Fourier Transform infrared spectroscopy as one implementation of 2DIR spectroscopy in which temporal and spectral resolution are decoupled. In addition, use of a (visible) excitation pulse can trigger (photo)chemistry during which 2DIR spectroscopy is used to follow structural dynamics and characterize structures of short-lived intermediate states. Transient 2DIR spectroscopy is very versatile because current technology allows to study the response of matter over a wide range of excitations from 600 meV to as low as 60 meV, equivalent to wavelengths from 2 µm to 20 µm. We are currently developing a suite of experiments to exploit the this spectral range while planning to extend the spectral capabilities into the THz range.

X-ray spectroscopy utilizes photons with energies ranging from ~100 eV to tens of keV. to excite core-shell electrons of atoms into unoccupied bound or continuum states, either measuring absorption, emission, or photoemitted electrons. As every element has a characteristic energy spectrum (essentially mapping out in the periodic table of elements), core-level transition energies are unique to each element. Moreover, the core-excited electron is localized close to the atom's core (or nucleus) in a state of well-defined symmetry and extent, rendering X-ray spectroscopy chemically highly specific with intrinsic atomic resolution. By projecting a core-electron's wavefunction onto a valence state or into the continuum (above the ionization threshold) a variety of spectroscopic methods can be devised which report in a unique manner on the atomic structure, the extent and symmetries of the valence charge density ('gluing' the atoms together), electronic correlations, and spin-states.

       Time-resolved X-ray spectroscopy probes dynamics of transforming matter and transient states of matter by exploiting the characteristics of core-level transitions. Due to the high photon energies associated with X-rays, the corresponding wavelengths range from a few nanometers to a fraction of a nanometer (typically a few Ångströms), ultrashort X-ray pulses can be generated from Laser-based sources and more recently in free-electron laser with pulse durations below 1 femtosecond. Ultrafast X-ray spectroscopy is part of the exciting new and rapidly growing field of ultrafast X-ray science, probing states of matter over a wide range of timescales, capable of even following sub-femtosecond motions of excited electrons.

       We focus on solvated molecular systems to develop a microscopic picture of physical and chemical processes in liquid phase, thereby exploring ways of solving experimental challenges in sample preparation, exploiting new spectral windows, and in collaboration with theoreticians, develop ab initio models that can reproduce our experimental results.