Temperature-Dependent Terahertz Spectroscopy of Liquid n-alkanes

We describe measurements of the terahertz dielectric properties of normal alkanes. We study all of the liquid alkanes from pentane (C-5) to hexadecane (C-16) over the temperature range from 20-80A degrees C, and obtain the absorption coefficients and refractive indices in the spectral range from 0.1 to 2.5 THz (3-83 cm(-1)). The mean molecular polarizability is found to vary linearly with chain length at all temperatures, indicating that an additive model for polarizability, with distinct contributions from the methylene groups and the methyl end groups, provides an accurate description. The absorption coefficients of these non-polar liquids, arising from transient induced dipoles, are essentially featureless in this spectral range, with an almost linear dependence on frequency and negligible temperature dependence. These results provide a baseline for far infrared spectroscopic studies of inter-molecular interactions in non-polar hydrocarbons.