Terahertz Spectroscopic Research On Alkali Metal Salts And Amino Acid Salts

"Terahertz radiation" is the most common term used to refer to the frequency band between 0.1 and 10 THz,analogous to microwaves,infrared radiation,and x-rays.It is rather awkward to use a frequency unit for naming a spectral band.An alternative and seemingly better terminology is "T-rays",where "T" stands for terahertz.There are many absorption spectra of crystal locating in the THz band which can be regarded as fingerprint characteristic spectra.The abundant structure information revealed by these spectra can give us a better understanding of these crystals.In this thesis,dozens of common inorganic salts and two amino acid salts(L-histidine hydrochloride monohydrate(LHHM)and L-cysteine hydrochloride monohydrate(LCHM))were characterized by terahertz time-domain spectroscopy(THz-TDS).The low-temperature THz spectra of LHHM and LCHM presented the blue-shift and splitting as the temperature decreased.Solid-state density function theory(DFT)calculation of the vibrational modes of LHHM and LCHM were performed for better understanding the THz characteristic spectrum,which indicates that the THz spectral technique is a powerful tool in structure detection of substances.The main works are list here:1.Dozens of alkali metal salts were examined using terahertz time-domain spectroscopy(THz-TDS)at room temperature,including sodium salt,potassium salt,and so on.2.THz spectroscopy was employed to investigate the powder crystal LHHM and LCHM.The low-frequency absorption spectrum of LHHM and LCHM was observed over 0.5-4.0 THz and then compared with the THz spectrum of amino acid LHis and LCys.The distinctive spectra of LHHM and LHis indicate that the characteristic THz fingerprints can be used in detection and identification applications.The temperature dependence of the vibrational modes of LHHM and LCHM were monitored.It was found that as the temperature decreased,the observed absorption bands became sharper and shifted towards higher frequencies.The observation manifests that THz spectroscopy is sensitive to the variation in temperature.The structure of LHHM and LCHM were verified by powder X-ray diffraction(PXRD)examination in preparation for the next calculation.3.The frequency calculation and individual vibrational modes assignment of LHHM and LCHM were performed by adopting DFT method.The results show that THz absorption spectra mainly reflect the collective vibrational motions of the whole molecule.Intermolecular hydrogen bonds were also found to have the contribution to the low-frequency vibrations.