A determination of the spin-rotation parameters for NO₂ in the X̃²A₁ state by microwave-optical double resonance

Magnetic dipole transitions between the spin components of rotational levels of NO₂ in its ground state have been observed by the technique of microwave-optical double resonance. Fluorescence induced by an argon ion laser was used to monitor the process. Observations were made in the range 4 to 12 GHz and provide information on levels with a wide spread of N and K _a values. The results have been used in conjunction with the known electric dipole transition frequencies and some recent data from radio frequency-infrared double resonance to determine the rotational and spin-rotational parameters for the molecule in its ground state. It has proved possible to determine all the three quadratic and six quartic parameters in the spin-rotational Hamiltonian. The improvement in knowledge of the spin-rotation intervals allow a refinement of the Zeeman parameters for NO₂ by refitting the data from the EPR and LMR spectra. The results are in good agreement with the relationships g_S^aa=g _s-ε_aa/2B_aa, etc.