Localized states in the Mott insulator kappa-(BEDT-TTF)₂Cu[N(CN)₂]Cl as probed by photoluminescence

We compare the photoluminescence spectra of the low-temperature Mott insulator \etcl (T_{MIT}=40 K) with spectra of metallic \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br, which is superconducting below T_c=11.8 K, in the temperature range between 300 and 20 K. In the Mott insulating state of \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl we observe a luminescence band at 1.95 eV due to the recombination of an exciton created by a HOMO-LUMO optical excitation. This luminescence is quenched both in the high-temperature bad metal state of \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl and in metallic \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br. The observation of the luminescence of an exciton provides evidence for the local character of excitations in the Mott insulating state.