Symmetry breaking of angular momentum distribution of diatomic molecules in an external field

If an ensemble of atoms or molecules is excited by linearly polarized light , then usually one expects linearly polarized fluorescence form the ensembl e. This reflects a symmetry of the excitation process, namely the fact that the electric field in a light wave and consequently the angular momentum d istribution of atoms or molecules in an excited state can be characterized by a double headed arrow double left arrow double right arrow. An ensemble of particles in an excited state possesses a net alignment. In this paper s ome processes that for molecules in an external field can break this symmet ry are reviewed. As a result of symmetry breaking the ensemble of molecules acquires a net orientation of angular momentum, that can be characterized by a single headed arrow double right arrow, and one can expect a circularl y polarized fluorescence from this ensemble.