Ultrafast Pump-Probe Experiments on Condensed Phase Systems In-silico

• Applicant: Professor Dr. Thomas D. Kühne, since 3/2018
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 352773479

Newly emerged light sources such as FLASH in Hamburg, LCLS in Stanford and the upcoming European XFEL in Hamburg allow to generate fully synchronized, ultra-short and highly intense light pulses. With these light pulses, it is becoming possible to initiate chemical and biological process by a pump pulse and follow the dynamics with the probe pulse on a femtosecond timescale. The pump probe experiments play an important role in the real time study of chemical and biological processes. Such techniques are also used to generate temperature jumps (T jump) on ultrashort timescales in order to study the very fast kinetics of chemical processes. Because of its biological and chemical relevance, T jump experiments in liquid water have gained a lot of attention. Rather than just acting as a passive environment, the dynamics of water during chemical and biological processes play an important role in the solvation and stabilization of reaction intermediates. To target the OH stretching mode of water with an infrared (IR) laser is a widely used mechanism to generate the T jump on a nano- to femtosecond timescale. With this mechanism, T jumps have been limited only to few 10s of K so far. In my previous works, I have proposed a new mechanism to generate T-jumps of up to few 100s of K on a sub ps timescale by means of computer simulations. The THz pump pulse of 3 THz with intensity of 5×10^12 W/cm^2 transfers a relatively large amount of energy to the inter- and intramolecular vibrations of water within less than ps. The large energy transfer to water causes significant structural and vibrational modifications, which can be measured by different time-resolved probe methods. In this project, I will focus on investigating the modifications in the H-bond network of liquid water due to strong THz pump pulses by means of ab initio Molecular Dynamics combined with a recently developed energy decomposition technique for extended systems (ALMO EDA). Leveraging my expertise of time-dependent electric fields by means of electronic structure calculations, I will extent the second generation Car Parrinello Molecular Dynamics scheme for pump-probe studies on much more complex systems than previously thought feasible. This development will open the door to investigate the time resolved evolution of chemical and biological processes. Recent developments of novel THz sources with the possibility to produce pulses on a wide range of frequencies and intensities are opening a completely new dimension to transfer energy and modify the structure. Keeping this in mind, I will focus on probing the pumped systems be means of time-resolved IR, Raman and X ray spectra in order to put experimentally measured spectra and theoretically computed properties on the same page. To that extent my focus is on developing new computational methods to open new experimental dimensions.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Pankaj Kumar Mishra, Ph.D., until 2/2018