Theory of terahertz pumping of chemical environments in the condensed phase

Newly emerged light-sources allow to generate fully synchronized, ultrashort and highly intense light pulses. With these light pulses, it is possible to initiate a process by a pump pulse and follow the dynamics via probe pulse in the femtosecond timescale. These pump-probe experiments play an important role for studying the chemical and biological processes in real time. Such techniques are also used to generate temperature-jump (T-jump) in ultrashort timescale to study the very fast kinetics of fundamental steps in chemical processes. Because of its biological and chemical relevance, T-jump experiments on liquid water have gained a lot of attention. Rather than acting as a passive environment, the dynamics of water during chemical and biological processes play a fundamental role in the solvation and stabilization of reaction intermediates. To target the O-H stretching mode of water with an infrared (IR) laser is a widely used mechanism to generate the T-jump in nanosecond to femtosecond timescales. With these techniques, T-jump has been limited only to few 10s of K so far. In this thesis, a new mechanism is investigated to generate T-jump up to few 100s of K in sub-ps timescale. The main portion of this thesis concentrates on the response of liquid water to sub-cycle THz pump pulses spectrally centered at 100 cm^-1 (∝3 THz). The THz pump pulse with intensity of 5 x 10¹² W/cm² transfers a large amount of energy to inter- and intramolecular vibrations of water in sub-ps timescale. After the pump pulse, water reaches to a quasiequilibrium state, which is a gas-like hot liquid. The large energy gain in water causes significant structural modifications and vibrational shifting, which can be probed by timeresolved coherent x-ray scattering and time-resolved IR spectroscopy, respectively. Here, the interaction of THz pulse with water molecules is investigated from clusters to bulk water. We find it to be mainly described via the interaction of electric field with the permanent dipole of water molecules. Further, we show that temperature and density of the water affect the energy transfer by THz pulse significantly. Another section of this thesis is focused on investigating the response of solutes to such hot and gas-like liquid water environment, created by the THz pump pulse. We show that an isolated phenol molecule, as example solute gains negligibly small energy directly from such pump pulse. However, the phenol in liquid water environment gains significantly large amount of energy due to the strong collisions of the highly mobile water molecules. The THz pump also modifies the solute-solvent dynamics substantially in sub-ps timescale. Thus, THz can potentially activate chemical processes long before the large amount of energy supplied to the system, leads to volume increase and vaporization of the medium. The last part of this thesis presents the state of art of existing THz sources for the possible pump-probe experiments. The response of water is analyzed for pulses of different central frequencies, pulse fluence and pule duration. The peak field amplitude of pulse plays crucial role in H-bond depletion. Once H-bond are broken, the energy transfer to water depends on the fluence of the pulse. A pump pulse of 20 THz frequency is also discussed as an adequate pumping mechanism for T-jump up to 1000 K or beyond.