Angular distributions and anisotropic scattering

Returning to Equation (15.1), it is clear that a further challenge arises if more than one electronic state of the atoms is produced (e.g. if more than one repulsive PES is accessed), as is often the case. In such cases, two or more flight times are observed which vary with the pump (photolysis) frequency. Photofragment imaging is particularly powerful in this area, as illustrated for the photodissociation of DI in Chapter 9. Information of this type can be used to characterize the potential surfaces and the couplings between surfaces at curve crossings (see Section 15.3). 

For electric dipole transitions the probability of molecular excitation (i.e. the probability of absorption) is proportional to fi-Ep. Thus, the strongest 

Clearly, the fragments will recoil in a direction either parallel or perpendicular to the direction of polarization E, depending on the orientation of the transition moment in the molecule. The laboratory frame angular distribution of fragments is derived by averaging the distribution of p about and is given by 

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. 

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