The determination of infrared optical constants of some liquids and thin-film solids

Abstract

Fourier transform infrared techniques have been applied in order to determine the optical constants of several liquid and thin-film solid systems over infrared frequencies. In the far infrared region, dispersive interferometry has been employed leading to the derivation of both real and imaginary parts of the complex refractive index in one experiment. As an extension to this, a method has been devised for the analysis of ‘full interferograms' involving multiple internal reflections that inevitably arise for normal transmission through thin parallel specimens. A program has been written which constrcts a theoretical complex insertion loss for the system understudy and then performs an iteration on estimated values for absorption coefficient and refractive index. Results are presented for several liquids analysed using this technique. The bandshapes of pure acetonitrile and benzene have been studied in the far infrared region of the spectrum. This was as a consequence of claims in the literature [83,84] that discernable structure in such bandshapes indicated the presence of long-ranged order in molecular liquids. Bandshapes have been produced using both non-dispersive and dispersive transmission, and spectra from both experiments have been averaged. These are presented together with the standard deviation on the mean. It is concluded that there is no evidence for real structure above the level of the noise in the spectra. The collision-induced spectrum of iodine in benzene has been studied in the far infrared region. Subsequent analysis has been done in both the frequency domain and the time domain in order to examine the nature of the intermolecular interactions and the time evolution of the relaxation processes. Experimental intensity and second moment data have been found to agree well with those predicted by a theory based only on classical electrostatic forces of attraction. Spectra have been modelled using two approaches for solving the generalised Langevin equation. The fits were found to be reasonable and the fitted parameters have been used to study the interactions involved in the system. A mid infrared study of Langmuir-Blodgett films has been undertaken, involving layers of w-tricosenoic acid on a silicon substrate. Anomalies in the carbonyl intensities for the first few layers have been attributed to the partially polarised nature of the incident radiation, and have been used to study the orientation of the fatty acid molecules relative to the substrate surface. A variable temperature cryostat with sample-holder has been interfaced with a Beckmann FS720 interferometer. This has been used to measure the optical constants of several solids over a limited frequency range in the far infrared.