The authors are experts in the use of spectroscopy for the study of intrinsic and collision-induced perturbations in diatomic iodine. They introduced a new method of  three-step optical population of the iodine ion-pair states. The iodine molecule has 23 valence states correlating with three dissociation limits, 20 so-called ion-pair states, nestled in four tiers and a multitude of Rydberg states. All the states have different angular momenta, parities and very dense rovibronic levels. Moreover, perturbations caused by atomic or molecular partners lead to effective nonadiabatic transitions. For these reasons the authors propose this molecule as a model system for spectroscopic studies of intra- and intermolecular perturbations in other diatomic molecules. weiterlesen