Energy and angular momentum transfer in homonuclear diatomic molecules from polarized laser fluorescence

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Abstract
Polarized fluorescence studies of elastic and inelastic collisions between iodine molecules indicate that these interactions show characteristics of both strong and weak collisions. Very large changes in rotational angular momentum are accompanied by a strong memory of the original orientation. The polarization of the resonance doublets from rotational states J′ = 2 to J′ = 21 of ν′ = 16 have been measured and compared with values calculated by either neglecting or including the effects of nuclear spin. Only the latter model provides an accurate description, particularly for low J′ values, and it is clear that nuclear spin plays an important role in determining the polarization behaviour of molecular iodine in the 3Π ou + excited state. When this effect is properly included there is no depolarization which may be attributed to collisions. This, together with earlier evidence that resonance features are not depolarized at very high foreign gas pressures, leads to the proposal that in homonuclear diatomics, which possess nuclear spin, the projection quantum number is not changed by elastic collisions. Rotationally inelastic transfer also exhibits marked persistence of initial orientation and two simple models are used to compare theoretical and experimental behaviour.