Solid state studies by means of Fourier transform spectroscopy

New designs for far-infrared beam splitters

We explore theoretically the design of multilayer beam splitters for the far-infrared that are based on coated pellicles as well as on solid substrates. The specific design criterion considered is as high a value as possible of the efficiency E = 4RT throughout the 10-140-µm spectral region. Here R and T are the reflectance and the transmittance of the beam splitter, respectively. In the numerical study the refractive indices of the substrates and coating materials varied between 1.50 and 4.00. To survey the range of designs, we make a number of simplifying assumptions, the significance of which is later investigated. Various potential manufacturing problems are considered. It is shown that the performance of the beam splitters is not sensitive to the accuracy with which the layer thicknesses can be controlled. However, it does depend strongly on the lowest available refractive index of the coating materials. The performance is particularly sensitive to the extinction coefficients of solid substrate materials. Multilayer designs presented should be useful for use in Fourier-transform spectrometers, as well as in other applications that do not require as high a spectral resolution.

Working resolution of 0.010 cm(-1) between 20 cm(-1) and 1200 cm(-1) by a Fourier spectrometer

The original double-beam Michelson-type Fourier spectrometer at the University of Oulu has been partly modified so that a maximum resolution of slightly better than 0.010 cm(-1) between 20 cm(-1) and 1200 cm(-1) can be obtained. The resolution is determined as a halfwidth of an observed line. The accuracy of the line position is 0.0005 cm(-1) in favorable conditions. The Fourier spectrometer is connected on-line to a minicomputer, in order to make the fast Fourier transforms of interferograms of up to 10(6) sampling points. The maximum optical path difference of the interferometer is about 1.5 m, giving a maximum theoretical resolution of 0.004 cm(-1).

High-resolution lamellar-grating Fourier-transform spectrometer for the submillimeter region

A lamellar-grating Fourier-transform spectrometer has been constructed for high-resolution studies and for general measurements in the submillimeter region. The instrument has a maximum working resolution of 0.034 cm(-1) and an accuracy of frequency of +/-0.0015 cm(-1). To illustrate the performance of the instrument, the results of high-resolution measurements of H(2)O, CO, N(2)O, and CH(3)CN gases, and, in addition, the magnetic resonance measurements of some biological substances and the refractive index measurements of some III-V compounds are presented.

Double-beam high resolution Fourier spectrometer for the far infrared

The construction and the performance of a double-beam Michelson-type Fourier spectrometer are described. The instrument covers a wavenumber range from 10 cm(-1) to 450 cm(-1) with a maximum resolution of 0.025 cm(-1) and an accuracy of frequency within +/-0.001 cm(-1). The optical path difference is measured by means of an He-Ne gas laser. To illustrate the general performance of the instrument, the pure rotational spectra of H(2)O and CO have been recorded.

Michelson-type fourier spectrometer for the far infrared

This paper describes the construction, performance, and applications of a Michelson-type Fourier spectrometer designed for spectroscopy in the 10-cm(-1) to 200-cm(-1) spectral region. Development of the arc-stabilized mercury lamp, adequate stabilization of the cooled detector, and selection of optimum sampling time have provided reproducible and high S/N interferograms. The evaluated reproducibility and accuracy of intensity of the spectrometer are better than +/-2%. An apodized resolution of 0.1 cm(-1) and accuracy of frequency within +/-0.015 cm(-1) have also been obtained. The electron spin resonance of some biological substances and the refractive index of solids, for example, have been measured with this spectrometer.

A Fourier Transform Spectrometer for the 10-10,000 cm(-1) Spectral Region

A Michelson interferometer spectrometer has been constructed for the investigation of ir emission spectra from weakly emitting sources such as thermally excited gases, fluorescing solids, and gas discharges at any wavenumber between 10-10,000 cm(-1). Auxiliary apparatus permits the study of absorption spectra of gases, liquids, or solids and of samples at liquid helium or nitrogen temperatures. The spectrometer is described and the design parameters are discussed. The absorption and the thermally excited emission spectra of the v(2) forbidden band of methane and the inversion rotation emission spectra of ammonia are shown as examples of the degree to which the design conditions are met.

Magnetooptical studies of solids using fourier transform spectroscopy

An interferometric spectrometer manufactured by the Research and Industrial Instruments Company has been adapted for use in polarized far ir magnetooptical investigations. The system includes a superconducting magnet and light pipe optics. The use of an indium antimonide free electron bolometer and an extrinsic germanium photoconductive detector with the system is described. As an example of the operation of the spectrometer, some investigations of donor impurity states in indium antimonide have been outlined.