In the liquid phase, water molecules are connected to one another via hydrogen bonds. Fluctuations in the water network occur on the sub-ps and ps timescale. THz absorption measurements (1 THz = 1012 Hz = 1 ps-1) "see" these dynamic reorientations of dipole moments. This makes THz spectroscopy a sensitive tool to probe solute-induced changes in the collective water network motions.
Water molecules in bulk hydrogen bond, on average, 3 to 4 other water molecules at any given time, as deduced from neutron diffraction studies, for example. However, these hydrogen bonds are in a constant state of flux. The hydrogen-bonded environment hinders the free rotation of water molecules in solution, forcing water molecules to undergo a librational motion on a sub-picosecond timescale. This means that within a picosecond, a hydrogen bond between any two molecules may break and reform many times.
The diffusion of water molecules occurs on a picosecond timescale. Over this longer timescale, a given hydrogen bond between two water molecules may no longer exist, as reorientation and translation of a specific water molecule could favour new bond formation. All these motions lead to a fluctuation in the water network and, thus, to fluctuations of the water dipole moments on the sub-picosecond and picosecond timescales.
Image in: Novelli, F.; Guchhait, B.; Havenith, M. Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths. Materials 2020, 13, 1311. https://doi.org/10.3390/ma13061311
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What can THz spectroscopy measure?
Terahertz absorption measurements are sensitive to these dynamic reorientations of dipole moments. The rearrangements occur on the picosecond timescale (1 THz = 1012 Hz = 1 ps-1). This makes THz spectroscopy a sensitive tool to probe solute-induced changes in the fast collective water network motions.
Examples are investigations of:
How does the technique work?
To measure liquid samples, a uniquely tailored flat-jet system is used. A water film is produced with variable thickness between 5-50 µm. The system is temperature stabilized with a selectable temperature range beween 0 to 40 °C. Because no cuvette is needed, signals from window materials are avoided and the measurements are not restricted by any cuvette window absorption.
Enlarge imageTo capture these ultrafast solvation dynamics in real-time, we use ultrafast spectroscopic techniques and expand them to the THz spectral region.
Central component of the optical system is a high energy pulsed infrared laser (amplified Ti:sapphire laser). The laser emits high energy light pulses centered around 800 nm wavelength with a pulse length of 50 fs (0.00000000000005 s). We then use nonlinear optical techniques to convert these infrared pulses to much longer wavelengths in the far-infrared – the so called THz spectral region. In addition to that, the optical system is also equipped with optical parametric amplifiers. In this way, a very broad spectral range all the way from the ultraviolet down to the THz region is accessible for the experiments.
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Last update: Feb 22, 2022
