## 5 Spectroscopy Analysis
### Hydrogen Bonds and Hydration State  
Hydrogen bond water network perturbations are referred to as hydration dynamics. The timescales for the dynamics of water molecules affected by the presence of a solute fall in the sub-picosecond region correspond to the THz region in the frequency domain. THz-ATR spectroscopy has been proved as a sensitive tool to analyze the water network in an aqueous solution. The absence of solute can affect the original dynamics of the surrounding water molecules and will contribute to the change of the dielectric spectra. For further qualitative and quantitative study of the response characteristic of aqueous solutions, analysis on hydrogen bonds and calculations around hydration were further carried out, with the absorption spectra and dielectric spectra as the following.  
Hydrogen bonds have been demonstrated in various aqueous liquids. [PERSON] et al. [53] found that they significantly influenced the Coulomb systems structure in the imidazolium-based ionic liquids. [PERSON] et al. [39] analyzed the dielectric dispersion of water in the THz region, indicating that the high-frequency vibrational modes should a make contribution to the dielectric dispersion. Then, [PERSON] et al. [54; 55] determined the dielectric character of water (Figure 5a) and its isotopes with different temperature, and decomposed them into four components: slow relaxation, fast relaxation at, intermolecular stretching vibration, and intermolecular libration. This research shows the unparalleled superiority of THz-ATR for evaluating the hydrogen-bonding system.  
Figure 4: ATR absorbance of four explosives with distinguished character. (**a**) HNIW: Hexanitrohexaazaisowurtzitane, (**b**) TEX: Tetraoxadinitroisowurtzitane, (**c**) GUAZ: Guanidinium azotetrazolate, (**d**) TAGAZ: Triaminoguanidinium azotetrazolate. Reproduced with permission from [27], Copyright © 2018, The Optical Society.  
Analysis on the hydration state is an attractive subject in exploring the structures and assessing the functions of biomolecules or chemical molecules in the aqueous matrix. Figure 5b shows the simple model of water molecules and solutes in aqueous, the water molecules are segmented into two stations, bulk water and solvation shell (hydrated water). These components collectively contribute to the response of the dielectric loss spectra, as shown in Figure 5c. Based on these, the concentration-dependent hydration state in saccharide solutions was successfully evaluated [57; 58; 59], which further shows the strong relationship with the structure and groups of the saccharide molecule. For biomolecules, like phospholipid bilayers, amino acids, short peptides, and protein and carbohydrate polymers, their structure also are found to be related to the hydration state through the analysis on hydration numbers [56; 58; 60; 61]. For chemical molecules, 2-butoxyethanol (2 BE), shows a temperature-dependent relationship with the hydration number [31]. The hydration state of the surfactant was also found to be changed by the structural transitions [62]. Thus, the hydration state could be evaluated through calculating the hydration numbers from dielectric responses in the THz-ATR system.