The spectra of many diatomic free radicals like those of CH, OH, NH, C
2
, CN have been known and identified as such almost since the beginning of molecular spectroscopy. They occur readily in emission in a variety of flames (including the ordinary Bunsen burner) and in electric discharges. It was through the study of these spectra that our knowledge of the structure of diatomic molecules was developed and in particular that the different coupling conditions between the electron spin and the other angular momenta were recognized. Most of the resonance transitions of the diatomic free radicals lie in the visible or near ultra-violet region where a detailed high-resolution study is easily possible. It is for this reason also that these spectra play a most prominent part in the spectra of astrophysical sources: comets, low-temperature stars and the interstellar medium. The spectra of free
polyatomic
radicals have become recognized and analyzed only in the last ten or fifteen years. The reason for this delay is the much greater complexity of the spectra of polyatomic as compared to the spectra of diatomic molecules combined with the greater difficulty of exciting these spectra in emission. Ordinary polyatomic molecules can easily be studied in infra-red absorption or by means of the Raman effect. The infra-red and Raman spectra are simpler than the electronic spectra and their interpretation has been established for a long time; but up to now no infra-red (or Raman) spectrum of a free polyatomic radical has been obtained. One is entirely dependent on electronic spectra for the study of the structure of polyatomic free radicals, and these electronic spectra exhibit many complexities.
