Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Alvin H. Nielsen

Committee Members

R. J. Lovell, W. E. Deeds, W. H. Fletcher


Introduction: The absorption or emission of electromagnetic radiation from 15 μ to 1000 μ is caused by energy changes in the rotations or the skeletal vibrations of molecules. This wavelength range is known as the far infrared spectral region and has in recent years become the object of extensive experimental activity. Much of the activity may be attributed to an increased awareness of the need for information which cannot be obtained elsewhere; to improved instrumentation and techniques; and to a natural desire to bridge the gap between the infrared and the microwave regions.

A bibliography of the far infrared compiled by Palik (1960) impressively indicates that the early history of the long wavelength region, from 1895 to 1920, is the history of the investigation of Heinrich Rubens and his co-workers. Their efforts were directed toward the determination of wavelengths, reflection, and transmission factors, index of refraction measurements, and polarization studies on a wide variety of materials such as quartz, mica, fluorite, rock salt, crown glass, and sulfur (Rubens and Nichols, 1897). Many of their results were immediately applied to new instrumentation and used for further research. For example, the discovery of suitable window materials while using the bolometer as a detector made possible the replacement of the bolometer by the more sensitive torsion radiometer.

Following Rubens a host of investigators led by Czerny, Badger, Cartwright, Strong, Randall, and Barnes further developed and improved the art of instrumentation; extended the applications; and refined the theory of the far infrared. Present day research is devoted to many kinds of physical phenomena such as the vibrations of long chain molecules found in polymers and organic substances, optical constants of liquids and solids, properties of semi-conductors, and magneto-optic effects in semi-conductors. These are in addition to extensive investigations on the pure rotational bands of gases.

Two of the most important examples in early years of observations on pure rotational spectra are those of M. Czerny (1927) on the hydrogen halides and R. M. Badger and C. H. Cartwright (1929) on ammonia. The first observation of hydrogen fluoride lines in the far infrared were reported in the paper by Czerny. Three lines were found lying between 45 μ and 125 μ. These lines were assigned to transitions in the ground state vibrational energy level between the rotational energy levels designated by the quantum numbers J(1) → J(2), J(3) → J(4), and J(4) → J(5). The pure rotational spectrum of HF then lay dormant until D. F. Smith and A. H. Nielsen (1956) concluded observations on lines corresponding to transitions originating with rotational levels J(10) through J(15) lying between 15 μ and 25 μ. Their frequency data were used in conjunction with data by Kupiers (1956), obtained in the fundamental vibration-rotation band, to determine the rotational constants of the hydrogen fluoride molecule.

For a few molecules such as NH3 and OCS, studies have been made in the microwave region of intensities, shapes, and widths of lines in pure rotational spectra (Bleaney and Penrose, 1948; Johnson and Slager, 1952). The tremendous resolving power of spectrometers working in this region makes possible the direct determination of true line shapes. Investigations recently completed in this laboratory (Herget, 1962) have demonstrated that with high resolution and a precisely measured split function it is possible to measure directly the shapes and widths of individual lines of the HF fundamental band in the near infrared.

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