|
The development of interferometry and Fourier transform interferometry with its application to many problems shall be discussed by several of the lecturers; however, it should be noted that the first chemical applications of FT-IR spectroscopy were made in the far infrared region of the spectrum (approximately 400-10 cm-1) because the instrumentation required to study this region was much simpler than that needed for the mid-infrared or near infrared regions. The instrumental advantages in the far infrared region included a lower tolerance for the mirror drive of a Michelson interferometer, a smaller dynamic range of the interferogram, and a longer sampling time which resulted in a reduced number of data points. However, far infrared spectroscopy also had its apparent difficulties such as low-energy sources, poor detectors, a lack of suitable optical materials, and spectral interference of water. Researchers in this area have overcome most of these difficulties and therefore investigations in the far infrared spectral region have become widespread. In this presentation some of the applications of Fourier transform far infrared spectroscopy will be examined. The three areas in which research was initially carried out using Fourier transform far infrared spectroscopy were investigations of pure rotational spectra, studies of molecules in the solid state, and studies of heavy atom vibrations and skeletal bending modes. These initial studies did not show a significant departure from the mainstream of far infrared research since a majority of them could be carried out utilizing dispersive instrumentation, but in the field of solid state research, the interferometrically obtained spectra provided data which were not easily obtained by grating instruments. Thus, physicists have put interferometers to use in the study of the solid state to determine optical constants such as the index of refraction, complex indices, phase angle transmission coefficients, and the electronic processes in insulating crystals, as well as the intermolecular vibrations of molecular crystals. Such studies have been important in the development of interpretative theories of solid state behavior. For the sake of brevity I shall mainly discuss current uses of far infrared interferometry for the study of molecules in the gas phase which contain very anharmonic vibrations, such as torsional vibrations and ring-puckering motions in some small ring compounds, and for conformational analyses by the spectral assignment of the asymmetric rotor transitions. The interpretation of the far infrared data for several molecules will be illustrated and many examples will be provided.
|
