Two different THz-TDS systems were used for the accurate study of broadband terahertz dynamics between 0.1 and 5.0 THz. These systems enabled to observe the whole spectral structure of the broad peak at 1.5 THz in propylene glycol. At high-frequency side of the broad peak in the dielectric spectrum, the small shoulder peak at 3.6 THz was also observed. It is assigned to the inter-molecular vibration, which is supported by the precedent report and the Gaussian calculation. In contrast, the low-frequency side of the broad peak includes the contribution of the relaxation processes which exist in the MHz and GHz region. Furthermore, we compared the dielectric spectrum with the Raman spectrum and concluded that the mode at 3.6 THz is IR active while Raman inactive.
International Letters of Chemistry, Physics and Astronomy (Volume 62)
S. Koda et al., "Broadband Terahertz Time-Domain Spectroscopy and Low-Frequency Raman Scattering of Propylene Glycol", International Letters of Chemistry, Physics and Astronomy, Vol. 62, pp. 15-20, 2015