Hole Burning in Long Chain Molecular Aggregates

The salts of pseudoisocyanine (PIC) have very specific optical properties. Under certain conditions (concentration, temperature) the PIC-molecules form linear aggregates in solution. Since the coupling between the molecules comprising the aggregate is very strong, the excited states are delocalized over a large range. A coherence length on the order of 1000 Å is not uncommon. These excitonic states carry momentum and, hence, the physics of these systems shows novel phenomena as compared to small probe systems. Such phenomena are , for instance, extreme motional narrowing of inhomogeneous line broadening and a very specific temperature dependence of the homogeneous width due to exciton-phonon-scattering processes in which both, energy as well as momentum, have to be conserved.