We report on a new effect which is observed when the small polyatomic molecule NO2 is prepared under collision-free conditions into an electronically excited state. As the intensity of the exciting laser light is varied, the degree of polarization of the fluorescence light may undergo a change in sign. The analysis of the experimental results confronts us with an interesting problem concerning the time evolution of an isolated polyatomic system. In this paper we present the experimental results and a simple model which successfully describes these results. According to this model, the inversion of the polarization of the fluorescence light is a consequence of light-induced stability of the optically prepared state. zation of the light beam and the polarization of the polarizers can be chosen appropriately. To specify these polarizations we introduce the polarization vectors e", e", and e, which describe linear polarizations along the x, y, and z axes, respectively. Additionally, a static magnetic field B and a rf field with amplitude Bi can be applied. The B field can be directed either along the z or the x axis. The Bi field is linear polarized along the y axis.The following experiments are performed.(i) Zero-field level crossing (Hanle effect). The B field is parallel to the z axis and is swept through B =0. No B& field is present. The laser light has polarization e" and the two polarizers have polarizations e"and e", 30 270
The investigations reported here aim to resolve a controversy concerning the excited state lifetimes and g factors. The results reveal a correlation between the values of lifetime and g-factor measurements, which was not seen before.
The Hanle effect signal and the optical radio-frequency double-resonance signal on NO2 may reveal an inversion effect under some particular conditions. However, the experimental results are not satisfactorily explained within the standard description of molecules. Recent experimental results suggest the overlap of Delta F=O transitions with Delta F=+or-1 transitions as a possible explanation. It is shown that this explanation fails to describe the experimental results for NO2.
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