Measurement of the rotational spectra of OH⁺ and OD⁺ by laser magnetic resonance

Far IR rotational transitions between the four lowest rotational levels in the X ³Σ^- vibronic ground states of OH⁺ and OD⁺ have been observed by laser magnetic resonance spectroscopy. Ground state molecular constants, including the three g factors, have been determined and employed in the calculation of a Born-Oppenheimer equilibrium geometry. The centrifugal distortion of the spin-rotation interaction is found to have a significant effect on the determination of other molecular constants. Hyperfine splittings have been resolved and analyzed for OH⁺, but could not be observed in OD⁺ spectra with a 6 MHz collision-broadened linewidth.