Terahertz absorption spectra of supercritical CHF3 to investigate local structure through rotational and hindered rotational motions

Far infrared absorption spectra of neat supercritical fluoroform (CHF3) are measured with terahertz (THz) radiation. The spectra covering from 10 to 100 cm−1 are obtained at reduced temperature T/Tc=1.02 on densities varied by a factor of 200. As density increases, dominant component of spectra changes from rotational to hindered-rotational motions. However, the change is nonlinear to the density. Such specificity arises from difference between bulk and local densities, and the most enhanced local density is observed near the thermodynamic state of maximum density fluctuation. In the pure fluid system, the relationship between density fluctuation and local density enhancement is experimentally presented.