Terahertz Dynamics of LiCl Aqueous Solution Measured by Terahertz Time-Domain Spectroscopy

The boson peak (BP) is universal excitation of the terahertz (THz) band in glassy materials, and it appears in the display of g(ν)/ ν² obtained by dividing the vibrational density of states (VDOS) g(ν) by the square of the frequency ν. Also, the absorption coefficient α(ν) is associated with g(ν) through the infrared light vibration coupling constant C_IR(ν). Therefore, BP in infrared spectroscopy appears in the display of α(ν)/ ν².

Water is an ideal system for investigating the effect of hydrogen bonding on BP, but it crystallizes during normal cooling process. To overcome this problem, lithium chloride was mixed to prevent crystallization at low temperatures. Although there are several studies on “water BP”, the THz spectroscopic study has not been performed due to the difficulty of experimental techniques for making liquids extremely low-temperature.

In this study, we established terahertz time domain spectroscopy (THz-TDS) for measuring cryogenic liquids to detect BP in LiCl aqueous solution below T_g. And determined the infrared vibrational coupling constant C_IR(ν). Through this, the terahertz dynamics of hydrogen bonding was analyzed.