Undamped kinematical excitations in molecular chains

The possibility of the existence of undamped kinematical excitations in one-dimensional molecular crystals is examined. It is shown that such excitations exist although not for all wave vector values in the first Brillouin zone but only in the interval -2π5⩽ka⩽2π5. It is found that for the positive exciton effective mass, undamped kinematical levels are more populated than normal exciton levels since they have substantially lower energies.