New coherence effects in the polarization of light emitted by photofragments: Theory

A quantum-mechanical treatment of the degree of polarization of photofragment fluorescence in the case where two dissociative electronic states of different symmetry with the same limit are excited coherently is presented. This results in an interference effect which depends both on the photoabsorption probability amplitudes in the different excited states as well as on the phase differences of their vibrational wave functions at large distances. Oscillations of the polarization ratio as a function of the photon energy are predicted. Applications to some simple cases involving a diatomic molecule initially in a ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }$ state are presented. The case of ${\mathrm{Ca}}_2$ is discussed. The role of fine (or hyperfine) structure, and in particular the case of unresolved fluorescence is treated. The classical model in terms of oscillating electric dipole moments properly averaged over the initial orientations of the molecule is presented and allows the interpretation of the quantum results.