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Dielectric Relaxation of HCl and NaCl Solutions Investigated by Terahertz Time-Domain Spectroscopy Wenchao Qiao & Kejian Yang & Arne Thoma & Thomas Dekorsy

Received: 2 April 2012 / Accepted: 13 June 2012 / Published online: 23 June 2012 # Springer Science+Business Media, LLC 2012

Abstract We have used Terahertz time-domain spectroscopy (THz-TDS) to investigate the complex dielectric function of water solutions containing different ions. Using HCl and NaCl solutions with different concentrations we study the changes of the dielectric response introduced by the ions. We find a linear increase of the real and imaginary part of the dielectric function compared with pure water with increasing ion concentrations. We use an expanded model for fitting the dielectric function based on a combination of a Debye relation and damped harmonic oscillators for the anions and cations. A good agreement between the model and the experimental results is obtained. Keywords Teraherz time-domain spectroscopy . Water solution . Dielectric function . Oscillation

1 Introduction As one of the most important substances on our planet water has been studied extensively in the past. Many different methods have been applied for the studies of water and water based solutions. Among those techniques, Terahertz time-domain spectroscopy (THz-TDS) has become important, since the relaxational response associated with intermolecular fluctuations of polar liquids can be observed with THz spectroscopy. Hence this method provides access to the understanding of the physical and chemical properties of liquids on a molecular level. Rønne et al. and Jepsen et al. have used THz-TDS in reflection geometry to study the dielectric response of liquid water including the temperature dependence and also some organic solutions [1–5]. Hirori et al. and Ngai et al. have built an attenuated total reflection THz-TDS system to study water dynamics and biological solutions at THz frequencies [6– 9]. Tielrooij et al. have built a THz-TDS system in transmission geometry to study the THz dynamics of ions in water [10, 11]. W. Qiao (*) : K. Yang : A. Thoma : T. Dekorsy Department of Physics and Center for Applied Photonics, University of Konstanz, 78457 Constance, Germany e-mail: [email protected]

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J Infrared Milli Terahz Waves (2012) 33:1029–1038

Ions solutions such as acid, alkali and salt solutions are of great relevance in biological systems. Compared to pure water, ions provide electrical conductivity to the solution and cause a rearrangement in the hydrogen bond network which makes intermolecular interactions much more complicated. Charged ions can induce electric fields which will reorient the dipoles of the surrounding water molecules, hence the charge will be screened and longrange interaction between cations and anions is reduced [12]. Protons (H+) play different roles compared with regular positive ions. It was revealed by de Grotthuss in 1806 that hydronium (H3O+) can form covalent and hydrogen-bonds (H-bonds) with adjacent molecules, through whi