G.W. Chantry

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.

High-precision dielectric measurements on liquids and solids at millimeter and submillimeter wavelengths

A brief outline is given of the improvements which have been made in the measurement techniques available for the determination of the complex relative permittivity of liquids and low-loss solids in the frequency range v = 100 GHz to 18 THz. These improvements involve very stable interferometric systems with either dielectric beam splitters for the upper frequency end or else free standing wire grid beam splitters for the lower frequency end, novel cell designs, the use of sensitive detectors, and of new computational procedures. The results can be checked by independent spot frequency measurements using HCN and H2O lasers coupled to an improved Mach-Zehnder interferometer. The optical properties of the polymeric (polyethylene, TPX, polypropylene, PTFE) and single crystal (quartz, silicon germanium) windows now assume an increased importance and we have determined refractive index n (v) and power absorption coefficient α(v) to much improved precision. With our present technique we can now determine the complex permittivity of very heavily absorbing liquids such as primary alcohols and water as far as 6 THz and beyond in favorable cases as functions of frequency over a wide temperature range.

Dielectric relaxation and far infrared dispersion in pure liquid chloroform

Abstract Accurate measurements of the complex permittivity of pure liquid chloroform are reported for discrete frequencies within the broad range 0 to 112 GHz. The measurements were taken at temperatures of 25 and −60°C. In addition, continuous far infrared dielectric spectra (3.3 tp 140 cm −1 , 100 to 4200 GHz) are presented for room temperature. The data are analysed by means of a Cole-Cole plot. This is found to be re-entrant, in agreement with recent theoretical expectations.

Absorption spectra of polypropylene in the millimetre and submillimetre regions

Abstract The absorption spectra of various samples of polypropylene have been determined in the range 2–400 cm −1 . The spectrum contains a number of moderately sharp bands which have been assigned to the normal modes of the macromolecular helix. Differences in the overall absorption level have been attributed to the presence in the material of catalyst residues and of antioxidants and other additives. A pure form of polypropylene in which these contaminants have been reduced to low levels is fairly transparent apart from the vibrational resonances throughout the range studied. The observed spectrum of this material has been combined with radio frequency dielectric data to give tan δ values for the frequency range 10 0 –10 13 Hz. There is clearly present a strong submillimetre absorption in addition to the radio frequency relaxations and the vibrational resonances and this broad feature is tentatively assigned to liquid-like motion in the amorphous regions of the polymer.

The far infra-red spectrum of crystalline polytetrafluoroethylene

Abstract The far infra-red spectrum of highly crystalline polytetrafluoroethylene has been shown to contain a new band near 33cm−1, the band at 55cm−1 has been shown to be a close doublet and two further bands at 291 and 308cm−1 have been observed. These results strongly support the conclusions of Boerio and Koenig from Raman observations that the unit cell contains at least two molecular segments.

Temperature effects and the observation of the B2u lattice mode in the far infrared absorption spectrum of polyethylene

Abstract Using high precision radiometry in conjuction with a Michelson far infrared spectrometer the B 2u lattice mode of polyethylene has been observed at both room and liquid-nitrogen temperatures. The continuum absorption of polyethylene in the far infrared has been shown to be temperature dependent from whick it is concluded that difference type multiphonon bands make a considerable contribution.

On the vibrational assignment problem for polytetrafluoroethylene: 1. The far infra-red spectrum

Studies of the polarized far infra-red spectra of drawn specimens of polytetrafluoroethylene (PTFE) and of the spectrum of the oligomer C14F30 both at room and liquid nitrogen temperatures have given results which strongly confirm the general form of recently published dispersion diagrams. However the absolute values of band intensities and their variation with specimen morphology suggest that some minor changes to the optical intersections are necessary. Intensity arguments also strongly indicate that the labelling, adopted by most recent workers, for the branches B6 and B7 should be interchanged.

Resolution limits in Fourier Transform Spectrometry

Abstract It is shown by mathematical analysis, and by close inspection of experimentally determined spectra, that the frequently quoted result Δv= 0.5/D, where D is the maximum path difference introduced, for the resolution in unapodized Fourier transform spectrometry is over optimistic. Even when the two lines are of equal strength the resolution is found to be Δv =0.7/D whilst if they differ markedly in strength the resolution is even worse (∼ 0.8/D). The loss in resolution in going to apodized spectrometry is not as severe as has previously been thought with Δv = 0.9/D for equal strength components and Δv = 1/D for the components having an intensity ratio of 2:1.

Temperature effects and the observation of lattice bands in the far infrared spectrum of polytetrafluorethylene

Abstract Four discrete bands have been observed in the region 40–90 cm−1 for polytetrafluorethylene samples at low temperatures. These are assigned as lattice bands and the observation confirms the deduction from earlier Raman studies that the unit cell contains more than one molecular segment. Continuum absorption in the far infrared for polytetrafluorethylene has been shown to be a function of crystallinity and temperature.

A far infra-red study of conformational disorder in PTFE

Abstract The examination of the 250–350 cm −1 region of the infra-red spectrum of polytetrafluoroethylene (PTFE) has been extended by studies of thermally degraded and sintered material. We conclude that a band at 277 cm −1 does not arise from a true mode of the crystal but has its origin in regions where the chain has a bent conformation. Our previous speculation that bands at 292 cm −1 and 312 ∗ cm −1 (Raman active only, for the isolated chain) acquire their weak activity in the infra-red through conformational disorder is confirmed. It is likely that the conformational disorder in question arises from chain folding in the polymer crystal.