J.W. Fleming

High resolution far infrared fourier transform spectrometry using Michelson interferometers with and without collimation

Abstract A two-beam interferometer of the Michelson type has been used to measure the absorption spectra of carbon monoxide, nitrous oxide and nitric oxide in the spectral region 15–40 cm−1, at a nominal resolution of 0.05 cm−1. The interferometer was used in both a collimated and uncollimated mode, and a detailed comparison of measured wavenumbers was made for carbon monoxide. The wavenumber contractions in the collimated interferometer are as expected from finite aperture considerations, and the larger contractions observed with the uncollimated interferometer can be related to the geometry of the collecting optics. Once the wavenumber scale given by the collimated interferometer is calibrated in terms of calculated absorption positions for CO. measurements made on NO and N2O show agreement with expected positions better than 0.003 cm 1. Distortions of the spectral profiles are observed in both cases; they are more marked for the uncollimated interferometer. The theories of Parshin and Steel are discussed, and attempts made to relate these to our results.

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.

Convergence corrections for absorption measurements in the far infra-red

Abstract The effect of the use of converging radiation in absorption coefficient measurements is considered in some detail, and the results applied specifically to the NPL Cube Interferometer. Experiment and calculations show that the error introduced may be quantitatively expressed as due to contributions from a defocussing effect, and from the non-uniform path length through the sample.

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.

The far infrared rotational spectrum of nitric acid vapour

Abstract The pure rotational absorption spectrum of nitric acid vapour has been observed in the region 8-40 cm −1 at a resolution of 0.05 cm −1 , using the technique of Fourier transform spectrometry. The structure of the spectrum is of considerable interest, since it corresponds closely to the normally forbidden perpendicular pure rotation band of a symmetric top. In the limit of a planar oblate near symmetric rotor, the band simply shows a series of equispaced sharp absorption lines.