Theory of terahertz pumping in the condensed phase
Traditionally, pump-probe experiments of chemical processes are constrained to the area of photochemistry. In solution or in biological environments an ensemble of molecular systems is pumped into an excited electronic state by UV or visible light, and the vibrational and electronic dynamics of the ensemble of molecules in the bulk are followed by a probe pulse on time scales of femto- to picoseconds.
One should, however, bear in mind that most chemical processes in the condensed phase are thermally activated. In such processes, reactants and products are separated by an energy barrier region in coordinate space—the transition state—which in most cases is larger than the thermal energy ΔE≫kBT. Under such conditions, the passage through the barrier region becomes a “rare event”, meaning that it occurs on time scales that are orders of magnitude larger than other characteristic times of the system, such as intra- and intermolecular vibrational periods.
Once a member of the ensemble starts a rare event, however, it usually takes only tens of femtoseconds up to picoseconds to complete, i.e. similar times as for photoinitiated reactions. Two main problems must be overcome in order to be able to probe thermal chemical reactions in real time. First, only a very small fraction of molecules is reacting at any given time, and second, there is no common time zero with respect to which the probe time can be defined. Our goal in this project is to theoretically investigate ways of initiating thermal reactions in a controlled fashion by terahertz (THz) and IR pumping of the molecular environment, such that a significant fraction of molecules of the ensemble start undergoing the chemical reaction at a well-defined time.
The goal of the Project is to develop a quantitative theory of THz pumping of thermal chemical reactions and specific vibrational modes in condensed phase systems.
There will be collaborations with the Chapman group (anomalous diffraction at high X-ray intensity, P4), and the Küpper group (X-ray multiphoton ionization of laser-aligned molecules, P1). The theoretical approach to be developed will be applicable to the problems posed in projects P1, P2, P3, P7, P9.