Ultrafast electron dynamics as a route to explore chemical processes

These lecture notes give a concise overview of the problem of describing quantum-mechanically the correlated motion of electrons and nuclei in a molecule. The focus is put on the methodology allowing to study the ultrafast, pure electron dynamics triggered by ionization of a molecule. It is shown that due to the electron correlation the removal of an electron from a molecular orbital can create electronic coherences manifesting in the migration of the positive charge throughout the system on a few-femtosecond time scale; a phenomenon known as correlation-driven charge migration. Some interesting perspectives for designing schemes to influence the chemical reactivity of the molecule by manipulating the charge migration dynamics are also briefly discussed.

Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission

Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

Ultrafast nonthermal heating of water initiated by an X-ray Free-Electron Laser

The bright ultrafast pulses of X-ray Free-Electron Lasers allow investigation into the structure of matter under extreme conditions. We have used single pulses to ionize and probe water as it undergoes a phase transition from liquid to plasma. We report changes in the structure of liquid water on a femtosecond time scale when irradiated by single 6.86 keV X-ray pulses of more than 106 J/cm2. These observations are supported by simulations based on molecular dynamics and plasma dynamics of a water system that is rapidly ionized and driven out of equilibrium. This exotic ionic and disordered state with the density of a liquid is suggested to be structurally different from a neutral thermally disordered state.

Ultrafast charge dynamics in glycine induced by attosecond pulses

Photoionization of biomolecules upon interaction with an attosecond pulse leads to ultrafast charge fluctuations in the sub-femtosecond time scale. The ultrafast charge migration process in glycine, resulting from the coherent superposition of cationic states, is described using the time-dependent static-exchange DFT method.