Tomas Hirschfeld

New trends in the application of Fourier transform infrared spectroscopy to analytical chemistry

Four major trends will characterize the evolution of Fourier transform spectroscopys analytical laboratory applications in the remaining seventies and early eighties: (1) A more thorough analysis of the nonidealities of real FT-IR systems will allow the quantitative accuracy of FT-IR to approach its SNR capability, or to be corrected to this level. (2) The power of FT methods is severely mismatched to current practice and habits in the infrared analytical laboratory. New operating procedures, combination techniques such as GCIR (a much better match to FT-IR capabilities) and more demanding spectroscopic techniques, will become more common. In the wake of this development, greater emphasis will be placed on automatic sample preparation and interpretation. (3) At the same time, the importance of spectra as the final output of spectroscopic measurements will decrease. Instead, higher order spectroscopic functions, or even final analytical data, will become a frequent instrumental output, with or without high level operator interaction. (4) The domain of application of Fourier transform spectroscopy will tee steady extension into new spectral regions, such as the near infrared and UV-VIS regions, while its extension to Raman spectroscopy still appears elusive. The analysis embodies a strong conservative bias, in that it excludes from consideration the consequences of new technology. This also insures its relevance to currently existing instrumentation.

Fourier Transform vs Hadamard Transform Spectroscopy

Recent articles have claimed a significant S/N advantage of Hadamard transform spectroscopy over Fourier transform spectroscopy. The scanty published data does not support this assertion, and the possibility that the claim is valid in theory is examined. Existing theory, as reported in the literature, is not consistent with the claims made for the technique. The advantages and drawbacks of Hadamard transform spectroscopy are examined in detail.

Improvements in photomultipliers with total internal reflection sensitivity enhancement.

An improved geometry for coupling a beam into a photomultiplier with total internal reflection sensitivity enhancement is described which allows this technique to be used with large sized beams. The equations giving the increase of the photocathodes absorption efficiency by the use of internal reflection within the window are discussed and conditions for best performance specified. The calculations, based on modifications of experimentally verified equations employed in internal reflection spectroscopy, predict that cathodes only a few monolayers thick could be made to absorb almost all the light falling on them in very few reflections. It then is shown that this will result in high quantum efficiencies with a much reduced spectral dependence. Particularly high increases may be expected for the less efficient cathodes such as S-1. The procedure is shown to relax the requirements on electron escape depth and absorption coefficient of the cathode material so much that the range of possible materials is considerably enlarged. Also, surface and defect level photoemitters will become practical. This and the change of work function with material thickness raise the possibility of extending the region of operation of photomultipliers more into the ir. Photomultipliers so built could also show a higher frequency response to modulated light beams. Finally, means of reducing the dark current and obtaining the multiplex gain in these photomultipliers are discussed.

Procedures for Attenuated Total Reflection Study of Extremely Small Samples

The conditions controlling the sensitivity of ATR toward extremely small samples are discussed with a view to their optimization. It is found that the formulas developed by Harrick and du Pré for thin film ATR apply to this case, with certain modifications. Optimum values for the indices of the optical material, sample, and backing, as well as for the incidence angle, are given for light of both polarizations. The advantages of reflecting the light beam several times on the same sample spot are pointed out, and three optical systems are proposed for this. These are a flat trapezoidal ATR plate with slant illumination, a cylinder with side-on illumination at an oblique angle to the axis, and a hemisphere ridged with retroreflecting teeth, wherein the beam repeatedly strikes the center with a rotating incidence plane. Modifications of the latter cell are proposed to make its use more convenient, as well as adaptations of its principle for conventional reflection, transmission, and ATR fluorescence work.

Multiple order spectra in Fourier transform infrared spectroscopy

A phenomenon akin to higher order spectra in grating spectroscopy has been found in Fourier transform spectroscopy. While its relative intensity is orders of magnitude down from similar effects in gratings, the high sensitivity of Fourier transform ir allows this perturbation to be detected.

Propagation of ultrashort pulses through lenses

Ordinary optical components may behave in extraordinary ways when illuminated with ultrashort optical pulses. In the cases of lenses, only the flat lenses such as Fresnel lenses, Fresnel zone plates, and holographic lenses display such anomalies. For them, the pulse may be both elongated temporally and spread spatially unless very high focal number lenses are used.

Optical communication at the source bandwidth limit

The possible limiting components on optical communication bandwidth are the source, the modulator, the propagation medium, and the detector. It is easy to show that the source bandwidth is the fundamental limit. The possibility of source bandwidth limited communication is demonstrated theoretically and experimentally. The basic principles involved are readily extendable to more practical partial approaches to this limit.