High resolution observation of microwave-optical double resonance in NO2

A previously reported microwave-optical double resonance observed in the fluorescence of NO2 excited by the 4880 Å argon ion laser has been observed through a high resolution spectrograph. The common level in the double resonance is the 524 rotational level of a vibrational state of B2 symmetry of the 2B2 electronic state of NO2. The optical transition exciting this level originates from the 431 level of the ground vibrational state of the 2A1 ground electronic state of NO2. The most likely explanation of the double resonance is that the microwave transition is to the 523 level of another vibrational state of the 2B2 electronic state with A1 vibrational symmetry. This level structure could be produced by an inversion splitting if the molecule had unequal N–O bond lengths in its equilibrium geometry or by an accidental near coincidence between vibrational levels of a symmetrical molecule.