Concentration-induced spectral variations of luminescence in AgBr:I

The author has measured the dependence of the photoluminescence spectrum of the isoelectronic trap AgBr:LI on the iodine concentration N in the range 4.3*10 14 cm -3 or=r s >or=0.97, where r s is practically the average distance between iodine impurities measured in Bohr radii (20 AA) of the iodine bound exciton. The following characteristic quantities have been considered in detail as a function of the concentration: position and halfwidth of the zero-phonon A line, position of the B line, the intensity ratio I B /I A of the two components of the zero-phonon line and the ratio of the luminescence intensities at 2.38 eV (position of the iodine pair line) and at 2.51 eV (maximum of the single-iodine phonon-assisted emission). The data are explained as being due to an interaction between pairs of iodine impurities. Either one iodine impurity is excited (i.e. carries a bound exciton) and interacts with an iodine impurity in its ground state, or less frequently, both iodine impurities are excited simultaneously. Good agreement is found between the resonant energy transfer model and the experimental data. For r s <1.4 an abrupt change in the single iodine spectrum occurs which is believed to be due to the appearance of a new phonon-assisted transition, whereby the phonon which is forbidden by symmetry in the single impurity bound exciton transitions becomes allowed due to the reduction in symmetry as a consequence of the interaction.