Excitons, or no excitons, that is the question

We study the population of excitons in GaAs/AlGaAs quantum wells in the tim e span between optical excitation and optical recombination. We distinguish between Is excitons and free electrons and holes by tracing the electron-h ole exchange energy and thereby the overlap of electron and hole. The exper iments prove that at many experimental conditions rather electrons and hole s than Is excitons are present during this time span although recombination takes place at the Is exciton energy of free or localized excitons. The Is exciton states are only populated under stringent experimental conditions.