Filling the spectroscopic gap between microwaves and the infra-red. Part II

Bearing in mind the great success of spectroscopy, in its several branches, in testing and forming our theoretical models of the structures of matter, it is remarkable that there are four octaves in the middle of the electromagnetic spectrum which have hardly been exploited at all. This range lies between wavelengths of 0.05 mm and 1–5 mm, bounded by the infra-red on the short wavelength side and the microwave region on the long wavelength side. Little use had been made of such wavelengths until very recently because of the peculiar experimental difficulties, but several research groups are now making progress here. In this and the succeeding article, I shall describe the steps which have been taken to develop techniques and the several kinds of spectra which have been discovered, mainly in the last three or four years. I believe that this spectral range, which may be called the extreme-infra-red or alternatively the submillimetre range, is going to be even more fruitful than the conventional inf...