High-Resolution Fourier Transform Spectroscopy in the Far-Infrared

Abstract

The construction and performance of three far-infrared spectrometers are described. These are a large-aperture diffraction-grating monochromator using six 28×35-cm interchangeable echelette gratings to span the frequency range from 10 to 150 cm−1, a set of 30-cm-square lamellar grating plates which can be inserted into the grating monochromator to convert it into a lamellar grating interferometer, and an 18-cm Michelson interferometer. The two interferometers are used with an automatic digital data recording system which records the interferograms on punched cards so that the spectra can be obtained by numerical transformation on a digital computer. All three instruments have been operated with the same detector and the same source, thus providing, for the first time, a controlled test of the relative merits of these three types of spectrometers. As expected theoretically, the two interferometers performed similarly except for differences due to beamsplitter efficiency and mechanical accuracy. Owing to their ability to look at all parts of the spectrum simultaneously and to achieve high resolution with large aperture, however, both interferometers proved far superior to the grating spectrometer, giving some of the highest resolution spectra yet obtained in the far-infrared. Examples are presented demonstrating resolution of ∼0.1 cm−1 over the frequency range from 3 to 80 cm−1.