Recoil anisotropy following multiphoton dissociation via near-resonant intermediate states

Expressions are derived for the moments beta(n) of the anisotropy of fragment recoil following multiphoton dissociation. The results for all possible two- and three-photon excitation pathways from a sigma or pi initial state, with prompt dissociation, are listed in tabular and graphical form, thereby facilitating the identification of pathways compatible with experimental data. The consequence of possible interference between multiple-excitation pathways is discussed. In addition, allowance is made for rotation of the parent molecule in the dissociating state on the time scale of its dissociation, with specific equations for three cases: fully resolved rotation on excitation, significant but unresolved rotation, and the intermediate case of partially resolved excitation. Finally, expressions are given for the consequence of significant rotation in a near-resonant intermediate state. It is pointed out that the effects of rotation can lead to a strong variation of the anisotropic moments over an excitation band, and that the absence of rotational features does not necessarily imply that the effects of rotation on recoil anisotropy can be ignored. The application of the theory is illustrated for a number of model systems, restricted to diatomic molecules with possible rotation but subsequent axial recoil. However, the expressions are easily adapted to polyatomic systems, provided that any fragment rotation is unresolved.