A Molecular Beam Electric Resonance Study of the Hyperfine A Doubling Spectrum of OH, OD, SH, and SD W. L. MEERTS AND A. DYMANUS Fysisck Laboratorilrm, Katl~olieke Universiteit, Nijmeget~, The Netl1er1nnd.s Received April 9, 1975 The molecular beam electric resonance method was employed to obtain a complete set of hyperfine Adoubling transitions of the free radicals OH, OD, SH, and SD. The observed spectra could be explained very well by the degenerate perturbation theory adapted to the ZII state. The experimental results include fine and hyperfine coupling constants. the electric dipole moments could be explained very well by the degenerate perturbation theory adapted to the 2n state. The constants agree well with ab it~itio calculations. On a utilisC la methode de resonance Clectrique d'un faisceau molCculaire afin d'obtenir un ensemble complete de transitions hyperfines a dCdoublement A pour les radicaux libres OH, OD, SH et SD. Les spectres observCs pouvaient trts bien &tre expliques par la thCorie des perturba- tions pour un Ctat degenCrC applique B I'Ctat ZII. Les resultats expkrimentaux incluent les constantes du couplage de structure fine et de structure hyperfine, ainsi que le moment de dipBle Clectrique pour les quatre molCcules, de m&me que certaines proprietes magnktiques de SH. Les constantes de couplage de structure hyperfine dCduites des mesures sont en bon accord avec les rCsultats de calculs thtoriques. [Traduit par le journal] Can. J. Phys., 53,2123 (1975) 1. Introduction The structure and spectra of free radicals, defined here as molecules with one or more unpaired electrons, have been the subject of a large number of theoretical studies and of experimental investigations using practically all available spectroscopic techniques. In con- trast to the vast majority of stable molecules, the ground electronic state of many free radicals is a state with a nonzero electronic orbital and/or spin angular momentum. Because of these angular momenta, the energy levels of the radicals show fine structure and the effects of the coupling between rotation and electronic motion, such as A doubling in a 'n state and p doubling in a 'C state. The experimental investigations of the spectra of free radicals range from classical vacuum ultraviolet (UV) spectroscopy to gas phase electron paramagnetic resonance (EPR) and microwave spectroscopy. The present communication describes a study of the OH, OD, SH, and SD radicals by the molecular beam electric resonance (MBER) technique. These radicals all have the *n3/, ground electronic state. The most extensive measurements and anal- yses of the UV band spectra of OH were per- formed by Dieke and Crosswhite (1962). The UV spectrum of OD was measured recently by Clyne et al. (1973). The first measurements of the microwave A doubling spectra of O H and OD radicals were reported by Dousmanis et al. (1955). The observed spectra originate from direct transitions between the A doublet levels of rotational states in the ,FI,,, and the 'FI,,, electronic levels. The observed spectra are in the frequency range of 7.7 to 37 GHz. The experimental accuracy of the observed frequencies of Dousmanis et al. (1955) varied between 0.05 and 0.5 MHz. An extension (to the order of (E,,,/E,,)2 or (Efi,,/~,r)2) of Van Vleck's (1929) theory of molecular energies in the 'FI states was employed to explain the observed spectra. Subsequent investigations of OH by Poynter and Beaudet (1968) on the 'n3/,, J = 712, 912, and 1112 states, by Radford (1968) on the 211,12, J = 112 and 2n312, J = 512 states, and by Ball et al. (1970, 1971) on the 211,12, J = 312 and 512 states considerably improved the accuracy of the zero field transi- tions. Recently, ter Meulen and Dymanus (1972) used a beam-maser spectrometer to obtain very accurate hyperfine A doubling transition frequencies of the 2113,/t, J = 312 state of OH. The magnetic properties of O H in the 211,12, J = 312 and 512 states and in the ,Il3!,, J = 312, 512, and 712 states were in- vestigated by Radford (196 1, 1962) using the