Selection rules for collisional energy transfer in homonuclear diatomics elastic collisions and the influence of nuclear spin

Quantitative study of the circular polarisation of resonance fluorescence doublets in molecular iodine as a function of rotational quantum number permits the formulation of a selection rule on the space-fixed rotational projection quantum number. This rule states that for elastic collisions of homonuclear diatomic molecules, changes in M as a result of the collision are forbidden. Essential to the elucidation of this rule is the realisation of the influence of nuclear spin on the polarisation ratios and when this effect is considered, there is no part of the depolarisation which may be attributed to collisional interactions. (Auth.)