CN- MOLECULAR-IONS IN SILVER-HALIDES - OPTICAL-PROPERTIES OF A NEW ISOELECTRONIC EXCITON TRAP

Low-temperature optical absorption and luminescence spectra of AgCl an d AgBr doped with CN- molecular defects are investigated. While in AgB r an amalgamation type of exciton is formed, CN- in AgCl is found to a ct as an isoelectronic trap, producing a bound exciton state with a lo calization energy of about 350 meV. Evidence for strong lattice relaxa tion to stabilize this state is provided by the positions and lineshap es of the spectra. An exciton model is discussed to account for the st rong temperature dependence of the decay time and intensities. The res ults are compared with those for atomic-type isoelectronic impurities in AgCl, like Br-and I-.