We establish a rigorous theoretical connection between measurements of the angular distribution of atomic photofragment alignment and the underlying dynamics of molecular photodissociation. We derive laboratory and molecular-frame angular momentum state multipoles as a function of photofragment recoil angles. These state multipoles are expressed in terms of alignment anisotropy parameters, which provide information on state symmetries, coherence effects, and nonadiabatic interactions. The method is intended for analysis of experimental data obtained with two-photon spectroscopy and ion imaging techniques, although it is readily modified for treating Doppler or time-of-flight mass spectrometer peak profiles. We have applied this method to the photodissociation of Cl2 at 355 nm, where we observe strong alignment in the ground state chlorine atom photofragments. Our analysis demonstrates that there are important contributions to the alignment from both incoherent and coherent perpendicular excitation. We also show that the existence of atomic alignment due to coherence requires that nonadiabatic transitions occur at long range.

The dependence of the angular momentum polarization (orientation and alignment) of the fragments on the direction of ejection k, is studied quantum mechanically for molecular photodissociation into two fragments of which one carries an angular momentum j. Explicit expressions in terms of the transition matrix elements for electronic excitation into the final dissociative states are given in the axial-recoil limit and for different photon polarizations. The importance of interference effects due to coherent excitation of dissociative states with different helicity quantum numbers (the projection n of j on the recoil direction k) is stressed. It is shown that not only absolute magnitudes but also relative phases of individual transition matrix elements can be determined separately if the spatial anisotropy of the angular momentum polarization is measured.