Lambda-doubling and rotational transition frequencies for the OH radical in the ν = 0 level of its X 2Π state have been fitted close to the limit imposed by experimental error for a very wide range of J values. High power centrifugal distortion correction terms (up to N12) were required in the effective Hamiltonian to achieve this. The traditional power series expansion of the rotational Hamiltonian is only very slowly convergent for the highest N values included (up to N = 47). Nevertheless, reliable interpolation of rotational transition frequencies for intermediate N values is possible from the results, giving values which are superior to those measured directly in the solar spectrum. A refined and extended table of rotational term values for OH in its ground state has been computed from the molecular parameters.

The far-infrared (FIR) Laser Magnetic Resonance (LMR) spectrum of the 17OH radical in the v = 0 level of the X2Π state has been studied in detail. The measurements have been analyzed and, in combination with some earlier EPR measurements, subjected to a single least-squares fit using an effective Hamiltonian. A full set of 17O nuclear hyperfine parameters has been determined. Some implications of these parameter values for the electronic wavefunction of OH are considered.

By employing two different CO-L.M.R.-spectrometers we have observed 52 vibration rotation transitions in five vibrational bands of five isotopic forms of the SiH free radical. A set of ground state vibrational rotational constants has been determined including a precise value of the spin orbit constant A 0 = 142·8957(8) cm-1 from the detection of spin forbidden transitions. The comparison of relative signal amplitudes yields a vibrational temperature near 1600 K for SiH in a discharge of helium, hydrogen and silane in which the walls are at room temperature.

Transitions between Rydberg states of atomic chlorine have been detected between 1477 and 1507 cm-1 by the technique of carbon monoxide laser magnetic resonance (LMR). Transitions have been observed in both emission and absorption. The spectra have been assigned for those transitions involving levels which have been identified previously. However, several more resonances are unassigned, revealing the presence of previously unidentified states of atomic chlorine. Many of the lines show resolved nuclear hyperfine structure.

Three rotational transitions of CO in the Ω = 1 and Ω = 2 spin components of the metastable (τ ∼ 7 ms) a 3п state have been measured by far infrared laser magnetic resonance spectroscopy of a positive column discharge. The L.M.R. spectra were analysed in terms of an effective hamiltonian to yield an improved set of molecular parameters.