The diatomic radical CH (methylidyne), containing carbon and hydrogen, was one of the first interstellar molecules to be identified, in 1937 by T. Dunham and also by P. Swings and L. Rosenfeld. It is widespread in the diffuse interstellar medium, where it produces absorption lines in the visible spectrum of background stars. It is an important intermediary in the production and destruction of interstellar organic molecules. Methylidyne is also present in the coma (atmosphere) of comets, where it is presumably a photodissociation product of organic molecules sublimating from the cometary nucleus.
After the first detection of the interstellar OH radical by radio astronomers in 1963, several groups looked for radio emission from interstellar CH. The search was difficult, however, because no laboratory measurements of the lowest frequency transitions were available, and theoretical predictions of the relevant frequencies were not very accurate. Radio...