Axial recoil approximation

For photodissociation, when it is assumed that the dissociation is rapid relative to the rotational period of the molecule (axial recoil approximation) (Zare and Herschbach, 1963) a limiting value of (3 = +2 is obtained when dissociation is induced by a transition (AQ = 0, /i parallel to the internuclear axis) and (3 = — 1 when the dissociative transition is -L (Afi = 1, /i perpendicular to the internuclear axis). 

If the dissociation process is not direct (see the section on predissociation below) and the molecule has time to rotate (a few picoseconds), then the angular distribution will be smeared out (i.e. it becomes isotropic) and the jS value will be zero. Intermediate values of jS can also arise from a breakdown of the axial recoil approximation (i.e. when the tangential velocity of the rotating parent molecule is greater than the recoil velocity), which can occur close to the threshold for dissociation, or when mixed parallel/ perpendicular transitions are encountered. 

The spectrum of [Au7(PPh3)7][OH] shows two doublets thought to correspond to the axial and equatorial Au atoms of a pentagonal bipyramidal cluster. However, the area ratio of the two subspectra is approximately 1.4 1 instead of the expected 0.4 1. Even when the expected relative intensities are treated to take the effective masses of the recoiling units into account, as described by Parish, Salter, and co-workers (111), the expected ratio increases to only 0.48 1. 

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