Avoided level crossing and population trapping in atoms

The interference phenomena between two neighbour resonance states depend on the type of interaction between the two states. Level repulsion along the real axis appears if the interaction is mainly a direct one while a bifurcation of the widths appears if the interaction occurs mainly via the continuum of decay channels. If both types of interactions are of comparable strength then further interferences may appear. An example for such a case is the population trapping observed in atoms.