Attosecond Science, driven by the advance in laser technology, is one of physics’ most exciting frontiers. Attosecond physicists usually aim at time-resolving the motion of electrons on their natural timescale. Such dynamics derives from the creation and evolution of coherence between different electronic states and proceeds on sub-femtosecond timescales (1 attosecond = 10-18 s, 1 femtosecond = 10-15 s). In contrast, molecular and chemical dynamics involves position changes of atomic centers and functional groups and typically proceeds on a slower, femtosecond timescale inherent to nuclear motion. Nonetheless, we believe that there are exciting ways in which dynamics studies of (slightly) more complex molecules, chemical reactions, and material- and nano-science can hugely benefit from the technological developments pushed forward in the vibrant field of Attosecond Science.
Laser-Driven Electron Rescattering in Molecules
An intense infrared laser field can lead to tunnel-ionization of the valence electron shell in a molecule. The liberated electron wavepaket(s) can be accelerated in the oscillating field of the laser and driven back to its origin within a fraction of the optical cycle, where it (they) can recollide (see image). Due to the well-defined source of the electron and the locking of the process to the optical laser cycle a high current is achieved at the location of the molecule, comparable to a conventional electron beam.
However, there are a lot of open questions since to date none of the applied spectroscopic techniques has directly delivered the time-dependent transition state structure. Hence, from analysis of the elastically diffracted, energetic electrons structural information on the molecule can be obtained, such as in conventional electron diffraction, but now with inherent time resolution. Laser-Induced Electron Diffraction (LIED), often described by self-imaged of a molecule by one of its own electrons, is one of the most promising emerging techniques to make the molecular movie. LIED is capable of recording structural changes in molecules, for instance during a chemical reaction, in real time.
This project is funded by a Grant by the German Science Foundation (DFG).
Time-Resolved Structural Imaging of Chemical Transition State Dynamics
In a chemical reaction two molecules come so close that bonds between atoms are broken and new bonds are formed. The configurations during the chemical transformation, being neither reactants nor products, are called the transition state. Transition state dynamics is closely related to reaction mechanisms and of fundamental importance in chemistry. Much work has been done to unravel these dynamics, which often involve major structural rearrangement of atoms.
Efforts to depict these structural transformations during chemical reactions have thus far fallen short due to a conceptual experimental problem: The start-time dilemma. In conventional samples the reactants are distributed over a wide range of spatial configurations and even with an ultrashort laser pulse there is no external control over the precise moment when a reaction takes place. We aim to solve this dilemma by bringing together two key ingredients: First, reaction partners are held closely together, in a well-defined initial configuration, within a reaction precursor. Such a complex allows initiating the chemical reaction at a defined time with a femtosecond laser pulse. Moreover, the tunable wavelength of the laser pulse allows controlling the speed with which the two reactions partners encounter each other. Second, as a function of delay after the initiating laser pulse, the three-dimensional structure of the transition state is imaged with Timed Coulomb Explosion Imaging. Coulomb explosion is a tool from the toolbox of Attosecond Science: Within a very short time the binding electrons are removed with an extremely short and very intense laser or X-ray pulse, such that the positively charged atomic fragments repel each other. An experimental determination of the fragment momenta in coincidence allows constructing the evolving chemical structure.
This project is funded by the Consolidator Grant ‘c-TSD-p’ by the European Research Council (ERC).
Recombination of a tunnel-ionized valence electron within half a period of the driving laser field can lead to the generation of attosecond duration light flashes in the XUV / soft X-ray spectral region. Such pulses enable new forms of spectroscopy.