Neat Liquids

Our earliest work with the THz spectrometer involved measuring the frequency-dependent absorption coefficient and index of refraction of highly absorbing neat liquids. There were two reasons for doing this. First, we had to develop the experimental protocol and methodology for accurately and reproducibly measuring the spectra of liquids. The spectra of the neat liquids could then be compared to literature values, which then allowed us to study binary mixtures of highly absorbing liquids in the far-IR region of the spectrum for the first time. Second, it is necessary to know the optical properties of the neat liquid in order to understand the changes that occur upon photoexcitation of a dissolved molecule in solution (TRTS experiments).

A schematic description of the spectrometer is shown here. A variable pathlength sample cell is used to avoid ambiguities or numerical artifacts when working up the data. This is done because reflections exist at the interface of two media with differing absorption coefficients and indices of refraction. Because the absorption and index of the sample under study are not known a priori, it is best to use a variable pathlength cell. With this method, reflectivity losses will be the same for all pathlengths, and the power absorbed and phase retardation introduced will only depend on the sample thickness. It is then a straightforward matter to extract the absorption coefficient and index of refraction. The pathlengths must be chosen such that the pulse is transmitted through the sample and that enough power is absorbed to attenuate multiple reflections (etalon effects).