Stefan Funkner

Communications: Polarity fluctuations of the protic ionic liquid ethylammonium nitrate in the terahertz regime

We have measured the complex dielectric function of the protic ionic liquid ethylammonium nitrate in the frequency range between 0.15 and 1.8 THz with a terahertz time domain spectrometer. The experiments reveal a terahertz mode which can be described as a damped harmonic oscillator with a central frequency of 1.3 THz. The terahertz mode is assigned to an intermolecular vibration, presumably associated with hydrogen-bond dynamics. The data are combined with microwave data to represent the dielectric spectrum from quasistatic conditions up to 1.8 THz.

Terahertz spectroscopic techniques for the study of proteins in aqueous solutions

Liquid water is a very strong absorber in the THz frequency range. We have set-up a unique germanium laser spectrometer consisting of a Ge:Be laser, tunable from 1 to 4 THz, and a sensitive Ge photoconductor detector. The spectrometer uses a measurement scheme alternating sample and reference signal while placed in an environmentally controlled housing for high stability of temperature and humidity. The laser system leads to a very small statistical error in the absolute absorption coefficient (400-500 cm-1) of less than 0.1% corresponding to 0.3 cm-1 while systematic errors due to filling of the sample cells become dominant. The high accuracy allows us to systematically investigate the effects of different solvates on water dynamics. Even a single point mutation in a protein can be measured in the THz absorption coefficient in the spectral range from 2 to 3 THz. The system has been recently used to study various solvates in liquid water like sugars and prototype proteins in aqueous buffer solutions in dependence of temperature, pH values, and denaturants. These studies are now augmented by time-resolved measurements using THz time-domain spectroscopy to analyze the kinetics of protein folding. We also discuss other THz sources and detection methods including the investigation of coherent synchrotron radiation at the synchrotron ANKA in Karlsruhe.

Using rattling ions to probe sub-ps water network dynamics

We present THz measurements on fifteen solvated alkali halide salts using narrow band THz absorption and broad band THz Fourier transform spectroscopies in order to shed new light on the controversy of salts as kosmotropes (structure breakers) or chaotropes (structure makers).

Terahertz spectroscopy of chemicals in the liquid phase

The multi-modal approach of GHz dielectric, terahertz and infrared spectroscopy revealed absorption and dispersion of an ionic liquid and resulted in a data set spanning 7 to 8 orders of magnitude in frequency and in absorption. Ionic liquids show comparable high absorption such as liquid water.