## 1 Introduction
Their chemical structures are reported in Figure 1.  
Figure 1: The chemical structures of the studied saccharides (the abbreviations used to denote the compounds are explained in the Section 2.1 Sample). Black, red, and white spheres represent, respectively, carbon, oxygen, and hydrogen atoms. Saccharides have been investigated since the beginning of THz spectroscopy [11; 13]. The crystallized form of \(\alpha\)-lactose monohydrate (the nomenclature given in [14] is adopted) has become a sort of \"gold standard\" for THz spectroscopy due to its wide availability, low cost, and the typical absorption spectrum with two strong and narrow peaks at 0.53 and 1.37 THz [6; 15; 16; 17; 18; 19; 20]. Another lactose polymorph, \(\beta\)-lactose, was studied in the frequency range of approximately 0.2-2 THz in a mixture with the \(\alpha\)-form to quantitatively evaluate the anomer content in a non-destructive way. [17]. The influence of water content in the lactose molecular crystal THz spectra was also studied recently, reporting complex phase transformations as a function of temperature [6]. Using temperature-dependent THz spectroscopy, the temperature-induced molecular decomposition of \(\alpha\)-lactose monohydrate was studied [21], and the reaction rates and Arrhenius activation energy of the thermoactivated solid-state reaction were determined [22]. In a recent study, the temperature dependence of the vibrational spectra and the resonance parameters of \(\alpha\)-lactose monohydrate and other saccharides, including galactose and glucose monohydrate, was investigated. Anharmonicity in the vibrational modes appears as coupling between resonances and the creation of wide asymmetric bands, revealing a temperature-dependent square relationship for resonance eigenfrequencies in saccharides [23; 24]. Anhydrorus and monohydrate glucose spectra were experimentally investigated in the frequency range 0.3-3.7 THz [20] and compared to solid-state DFT simulations [25]. A 6 cm\({}^{-1}\) (0.18 THz) overestimation of the predicted vibrational frequencies was observed for anhydrous glucose and interpreted as vibrational anharmonicity effects. In another study, the THz spectral transformation of monohydrate glucose under dehydration was measured in the range 0.8-2.2 THz [26]. The THz absorption spectrum and refractive index of galactose were measured in the frequency range of 0.2-3 THz. Simulations using the Gaussian 03 package were conducted on isolated galactose molecules, and the predicted vibrational frequencies partially matched the experimental ones. The authors suggested that the non-predicted resonances might be due to interactions between galactose molecules. In a subsequent study, the absorption spectrum of galactose was measured in the range of 0.5-4 THz. The structural analyses and normal mode calculations were performed by using solid-state and gas-phase DFT simulations. The results demonstrated good agreement between experimental spectra and simulations, with differences within 0.2 THz [27]. The THz absorption spectrum of cellobiose was measured in the 0.5-3.8 THz interval [28]. The cellulose THz spectrum was first investigated with the main goal to determine the crystallinity of the sample [29]. It was later studied in more detail in single freely standing paper sheets made of cotton cellulose, mainly in the 1\({}_{\beta}\) phase, in the 0.2-3.5 THz range [30]. By eliminating the Fabry-Perot (FP) interference effects from the transmittance data, the complex refractive index was numerically extracted [31]. The cellulose spectrum consists of two main peaks at approximately 2.1 and 3.1 THz superimposed to the background due to the amorphous phase. A more recent study was focused on distinguishing the THz spectra of cellulose I allomorph (I\({}_{a}\) and I\({}_{\beta}\)), which exhibits differences in its crystal structure [32].