Aggregation processes in NaCl:Eu2+ system

The Eu2+ aggregates formed, after a long storage at room temperature, in two NaCl:Eu2+ samples (doped with a Eu2+ concentration of 100 and 500 ppm, respectively) are examined by luminescence and time decay spectra. In both the samples, aggregates emitting at 431, 444, and 469 nm are formed. These aggregates show the same characteristics in both the samples, seeming to be independent of the overall Eu2+ concentration; their decay scheme show a behaviour similar to that of isolated Eu2+ ions. In the more concentrated samples a further aggregate can be detected, the emission band of which is peaking at 414 nm; its decay scheme is different from that measured for the above mentioned aggregates. Finally for low doped samples an energy transfer process between the dipoles and/or clusters emitting in the region 410 to 430 nm and aggregates emitting in the region 440 to 470 nm is evidenced. (author)