Knowledge Bank

Microwave optical double resonance lines have been observed by monitoring the fluorescence of a sample of $NO2$ which was irradiated with a strong microwave field. As the microwave frequency is swept through a resonance between excited state rotational levels, the spatial distribution of the fluorescence is altered and signals the absorption of microwave energy by the sample. The spectrum has been observed at both zero and finite magnetic field, and it has been shown that the microwave transitions are $\Delta M=\pm 1$ electric dipole transitions. The decoupling of the electron spin by a fairly weak magnetic field indicates that the molecule is bent.