H.A. Gebbie

The determination of refractive index spectra by fourier spectrometry

Abstract If a dispersive medium is introduced into one arm of a Michelson interferometer irradiated by a wide band of radiation, the two-beam interferogram contains detailed information about the variation of the complex refractive index of the medium with wave-number. Recent experimental work in the infra-red spectral region has shown that this information may be recovered to give the complex refraction spectrum of the material. Details of the mathematical theory and the necessary observations and computations are given, and illustrated by the results of recent investigations of the refraction spectra of specimens in each of the three material phases.

Absorption and dispersion studies in the range 10–1000 cm−1 using a modular Michelson interferometer

Abstract A modular Michelson interferometer has been developed for use as a Fourier transform spectrometer providing both absorption and refraction spectra. The spectral range that it was intended to cover, 10–200 cm−1, represents only a part of its useful range which extends to 500 cm−1. It is shown that a few simple modifications enable the upper frequency limit to be raised to 1000 cm−1. There is no evidence for frequency errors greater than the resolution limit, nor are there any spurious features present in the spectra due to artefacts.

Absorption spectra of some inorganic complex halides by far infra-red interferometry

Abstract Stretching and bending frequencies associated with metal-halogen bonds have been observed in the range 40–400 cm −1 , using a far infra-red interferometer. The complexes, mostly of the type K 2 MX 6 (octahedral) and K 2 MX 4 (square-planar) were examined in the solid state.

Phase modulation in far infrared (submillimetre-wave) interferometers II—Fourier spectrometry and terametrology

Abstract The practical realisation of phase modulation in interferometers designed for use with broad-band submillimetre-wave radiation is described. The technique has been applied both in Fourier spectrometry and metrology. The jitter of path-difference necessary for the phase modulation was achieved by sinusoidal periodic displacement of one of the interferometer mirrors. Modulus and phase spectra and absorption and refraction spectra have been obtained with signal-to-noise ratios superior to those of amplitude modulated measurements. The application of submillimetre waves to the metrology of workpieces is shown to benefit from the use of phase modulation. The accuracy of displacement measurements in a workshop prototype is a few micrometres.

Submillimetre- and millimetre-wave absorptions of some polar and non-polar liquids measured by fourier transform spectroscopy

An assessment of the Fourier transform spectrometer indicates the advantages to be gained from its application to be the millimetre-wave region (10 to 2 cm–1). The attainable resolving power is adequate for the study of broad absorptions such as occur in liquids below 100 cm–1. With a mercury arc source, appropriate beam dividers, and an indium antimonide detector at 1 K, continuous refraction (n) and absorption (α) spectra have been obtained below 50 cm–1(to 2 cm–1 in some cases) for some typical polar and non-polar liquids. Results for water, aniline, 1,4-dioxan, cyclohexane, decahydronaphthalene, dimethyl acetylene (2-butyne) and 1-octyne are reported and considered in relation to earlier microwave dielectric data and far infra-red measurements. Water and aniline show strong, almost featureless, absorptions which are probably due to the superposition of three or four processes. The non-polar and weakly-polar liquids show much smaller absorptions : 1.4 dioxan has a profile consistent with a zero permanent electric dipole moment; 2-butyne and 1-octyne show α rising linearly with over the range 5-50 cm–1. The origin of the absorption in non-polar liquids is different from the dominant mechanism in polar liquids although the peak values for both lie in the submillimetre-wave regions. The non-polar absorption peaks occur at frequencies which correlate with those calculated for molecular collisions in the liquids using the formulations of Mie and Bradley.

Far infrared study of molecular crystals by interferometric methods

The use of a Michelson interferometer to study an extended frequency range in the far infrared (20–400 cm−1) is described, and details are given of the experimental techniques employed to study the absorptions of a number of molecular crystals at liquid nitrogen temperatures. Frequencies of maximum absorption, with qualitative descriptions of intensity and width, are listed for the following 16 crystals: HCl, DCl, HBR, DBr, Cl2, Br2, I2, CO2, COS, N2O, N2O4, SO2, H2S, D2S, NH3 and ND3. Most of these absorptions are lattice vibrations, but several intramolecular modes of the heavier molecules are also observed in this region. A correspondence between the Debye frequency and the lattice frequencies of many of these solids is briefly discussed.

Far infra-red spectra of liquid and crystaline tertiary butyl chloride (CH3)3CCl

Abstract Far infrared spectra of liquid and crystalline tertiary butyl chloride have been measured and considered in relation to possible rotation in the solid phases and motions in the liquid phase.

Far infra-red spectroscopy of benzene derivatives

Examples of the use of interferometers and Fourier transform spectroscopy for observation of the wide range far infra-red spectra (20–420 cm−1) of benzene derivatives are presented, and some assignments discussed.

Submillimetre dispersion of liquid tetrabromoethane

Abstract The Fourier method for measuring refractive index spectra using a Michelson interferometer has been applied to symmetrical tetrabromoethane in the liquid phase. The spectrum has been observed throughout the spectral range 20–190 cm −1 (500-53 μ), inclusive of regions of absorption, with a resolution of 2 cm −1 . From the data, band strengths and widths have been derived and the corresponding calculated refractive indices compared with the measured values.

The far infra-red spectrum of anthracene

Abstract Far infra red spectra of anthracene are reported. The spectra were obtained from solutions and oriented single crystals. Five new fundamental vibration frequencies have been identified and these have been assigned as the lowest frequency skeletal modes. The assignment is supported by the good agreement with a recent calculation [2].