Nonlinear THz spectroscopy to study the solvent dynamics in water

Abstract

In recent years terahertz (THz) technology has been stablished as a novel tool to study the dynamics of the most important solvent: water. Water has to be attributed a much more active role in the function of hydrated molecules, which we begin to understand via THz studies accompanied with novel theoretical insights. Due to a lack of powerful THz sources, precise THz absorption measurements have been for a long time an enormous challenge since water is a strong THz absorber. We have overcome this difficulty by developing instrumentation which made it possible to record THz absorption coefficients of solvated samples with a precision of 0.2 %. This allowed to detect subtle changes in the dynamical (sub-psec) orientation of water molecules that are obscured in radial distribution functions from scattering experiments. These measurements unravelled unknown phases of water under nanoconfinement as well as provided a local, label free probe on the protonation state of amino acids [1] , [2] . Both parameters are of importance to understand the enormous reactivity in cleft of enzymes compared to chemically designed catalyst. The availability of high-power THz sources, which become possible due to progress in the development of THz sources allows to carry out non-linear THz experiments. Thus, we are able to show that water can be aligned via strong THz fields [3] and we are able to carry out the pump probe experiments to probe a solvent quake in real time.