Dissolution studies about the aggregation-precipitation stages of the divalent europium in KBr crystals by photoacoustic technique

Abstract
Four different aggregation-precipitation (AP) stages of the divalent europium (Eu²⁺) ions have been fixed in KBr single crystals by means of heat treating processes. To examine the dissolution of the ions configuration (AP stages), the samples are continuously heated for activating the ions diffusion. This process is monitored in situ by the pulsed photoacoustic (PA) technique. To follow the ions configuration dissolution, the PA signals at each temperature are successively acquired. AP stage of ions changes are recovered by a numerical procedure based on the correlation coefficients. This procedure applied to digitally recorded PA signals allowed the construction of an AP profile N_S(T_i), where is easier to analyze the ions behaviour into the crystal host. The results agree with those obtained by the optical absorption analysis, under similar conditions. The PA signal is generated as a consequence of the non-radiative processes that have taken place after the excitation of the Europium ion. This excitation, which corresponds to an electronic transition to the low energy absorption band T_2g of the Eu²⁺ ions, is achieved by means of a laser pulse at 355 nm focused on the sample.