Energy-dependent Huang-Rhys factor of free excitons

We use quantum wells of the polar semiconductor ZnSe as a model system to demonstrate an energy dependence of the excitonic Huang-Rhys factor. Both the spectral shape of phonon-sideband emission and the intensity ratios of hot-exciton luminescence peaks yield identical results. We find that the Huang-Rhys factor decreases rapidly from 0.31 for cold excitons to less than 0.10 for hot excitons with a kinetic energy of about 15 meV, which is consistent with theoretical predictions. This behavior of the Huang-Rhys factor is confirmed to be independent of experimental conditions such as sample temperature and excitation wavelength. Previous (energetically integrating) experiments yielding such dependencies can be interpreted in terms of the influence of the exciton distribution.