In the course of developing a method for determining the height of the atmospheric ozone layer, it became necessary to determine the effect of total pressure on the absorption of radiation by the ozone band at 9.6µ. Accordingly, the transmission of ozone was measured with a reststrahlen receiver whose response curve was centered near 9.6µ. This was done at seven pressures between 6 mm. and 722 mm. with ozone thicknesses ranging up to 5 mm.
The proportionality of absorption to the square root of the thickness at low pressures suggested that the mechanism was Lorentz broadening. The possibility of using the dependence of band absorption on thickness as an ultra-spectrometer technique made it desirable to repeat the measurements with spectral resolution.
With these experimental results, it was found possible to deduce a value of the effective spacing of the lines in the band, and in this way a "line-density contour" of the ozone 9.6µ band was obtained. This density contour indicated that the band had a central branch, which served to strengthen the isosceles triangle model of the ozone molecule.
The name "ultra-spectrometer" was adopted to emphasize that the technique yields the spacing of the lines in a band when that spacing is far smaller than the available spectral resolving power. It was suggested by analogy with the ultra-microscope, an instrument that detects particles smaller than the available optical resolving power.
The importance of this technique as an aid in the identification of band types is readily appreciated when it is realized that all molecules but the simplest possess spectra unresolvable by present-day spectrometers. Without adequate resolving power it is difficult to deduce the rotationalstructure of a band, and unless this is known, it is impossible to assign the bands in a spectrum to specific vibrational modes. Hence, a "line-density contour" obtained with the ultra-spectrometer method may be decisive in the analysis of an infra-red spectrum.
