Knowledge Bank

This review lecture is concerned with the study of the radiofrequency and microwave spectra of molecules in open-shell ground or excited electronic states. Attention will be restrited to diatomic and linear triatomic species, since very little work on non-linear species has been reported. The first free radical to be studied by pure microwave spectroscopy was OH, produced by a discharge in water vapour. Detection of the gas phase electron resonance spectrum followed shortly after, and combination of the results obtained by the two techniques yield detailed information about the $\Lambda$ doubling and nuclear hyperfine parameters. More recently the electron resonance spectra of a number of diatomic radicals have been detected by electron resonance. The radicals are produced by reacting atoms with secondary gases, mixing occurring inside the spectrometer cavity. In a number of cases the pure microwave spectra have subsequently been detected, and the results obtained will be reviewed. The linear triatomic radicals NCO and NCS have also been studied, and the observation of excited vibronic states by electron resonance enables one to establish some details of the Renner effect. Molecular beam magnetic resonance has been an important technique for the study of atoms for some time, but as yet no short-lived ground state open-shell molecules (free radicals) have been studied by beam methods. Important experiments have, however, been performed on $H2$ and CO in their metastable excited $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ states. In both cases microwave spectra have been detected by means of beam intensity techniques. Another important recent development has been that of radiofrequency-optical double resonance; applications to the radicals OH and CS will be described. Finally, the different techniques will be compared and future possibilities for development considered.