F. Fulgêncio

Solid Eu(III) complexes studied by positron annihilation, optical and Mössbauer spectroscopies: insights on the positronium formation mechanism.

In this work, positron annihilation lifetime (PALS), Doppler broadening annihilation radiation lineshape (DBARL), Mössbauer and optical spectroscopies measurements were performed in Eu(III) dipivaloylmetanate complex, Eu(dpm)(3), at 295 and 80 K. The Eu(dpm)(3) complex is not luminescent at 298 K and does not form positronium. On the other hand, it is highly luminescent at 80K, but still does not form positronium. The absence of positronium formation at 80K cannot be explained by a ligand/metal charge transfer process. We found strong evidences that the electronic delocalization does not occur at both temperatures. Despite the Mössbauer results being inconclusive regarding the Eu(III)/Eu(II) reduction hypothesis, previous results showing positronium formation in other Eu(III) complexes suggest that this process is not occurring. Thus, more studies are needed to explain the absence of positronium in Eu(III) complexes.

Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Substitutional Tb1-xEux(dpm)3 Solid Solutions Studied by Optical and Positron Annihilation Spectroscopies

Positron annihilation lifetime (PALS) and photoluminescence spectroscopies measurements were performed in Tb(III) and Eu(III) dipivaloylmethanates, Tb(dpm)3 and Eu(dpm)3, and also on their binary solid solutions of general formula Tb1-xEux(dpm)3. A correlation between the 5D4 Tb(III) energy level lifetime and the positronium formation probability was observed, indicating that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition. From these results, a new model is proposed, showing that excited electronic states have a relevant role in the positronium formation mechanism.