Terahertz absorption in non-polar condensed matter systems

Abstract

Terahertz time-domain spectroscopy (THz-TDS) is a powerful non-contact optical technique for characterizing materials and sub-picosecond dynamic behavior in the far-infrared region (100 GHz --- 10 THz) of the electromagnetic spectrum. As a young field (less than 25 years old), the terahertz spectral properties of many materials and physical processes remain unexplored. One of the most interesting, and fundamentally important, uncharacterized phenomena is the phase behavior of condensed matter systems. Normal ( n)-alkanes are molecules of particular interest since they are linear, straight-chain hydrocarbons that are structurally simple and have melting points near room temperature. Using a custom designed spectrometer and sample cell, we characterize the terahertz optical properties of various liquid branched and n-alkanes (C5H12 - C16H34) and use this as the basis to conduct imaging studies of a two-component surfactant/octane system to discern the content across the boundary layer. This prefaced our characterization of the phase behavior of longer n-alkanes where we observed novel temperature-dependent absorption features in the equilibrium non-liquid phases of n-tricosane (C23H48) through n-hexacosane (C26H54). These spectroscopic features have never previously been observed in non-polar (infrared inactive) materials, and provide new insight into the crystallization dynamics of organic molecular solids.