Self-quenching effects of excitons in CaWO$_4$ under high density XUV FEL excitation

Using free electron laser excitation in the XUV range, CaWO$_4$ samples were exposed to ultra-short, intense photon pulses (photon energy 89.84 eV, 10 $\mu$J average pulse energy, 25 fs pulse length), and their luminescence was studied with time-resolved spectroscopy. In the decay curves measured in the temperature range 8–300 K, a non-exponential emission decay with shortening of the life times over the first few $\mu$s is observed, which depends on the excitation density. Using a model for dipole-dipole interaction of excitons under non-uniform excitation densities, the structure of the decay curves can be reproduced in good agreement with the experimental data, and parameters for the initial exciton interaction can be calculated.