PICOSECOND AND EXCITATION SPECTROSCOPY OF LOCALIZED ELECTRONIC STATES IN SILVER-HALIDES

In AgCl and AgBr at low temperature, an intense resonant secondary emission with no shift in regard to the excitation energy is observed below the indirect exciton absorption edge. It is attributed to resonance fluorescence from localized electronic states that are tentatively ascribed to small non-metallic silver clusters. Its dependence on excitation photon energy, light polarization and temperature as well as its time decay proceeding on a picosecond scale are analyzed and discussed in terms of a degenerate two-level model in which energy- and cross-relaxation are taken into account. The parallel reduction of intensity and decay time above the exciton edge is interpreted in terms of a resonant energy transfer between the electronic states of the clusters and the indirect free exciton of the host crystal. The interpretation of this process via the dipole-dipole interaction enables us to estimate average numbers of silver atoms forming the clusters.