Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Substitutional Tb_1-<i>x</i>Eu_<i>x</i>(dpm)₃ Solid Solutions Studied by Optical and Positron Annihilation Spectroscopies

Abstract

Positronium formation in the bimary molecular solid solutions Tb 1Àx Eu x (dpm) 3 (dpm = dipivaloylmethanate) has been investigated. A strong linear correlation between the 5 D 4 Tb(III) energy level excited state lifetime and the positronium formation probability has been observed. This correlation indicates that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition, as previously proposed. A kinetic mechanism is proposed to explain this correlation and shows that excited electronic states have a very important role in the positronium formation mechanism.

Electronic structure of solid molecular complexes evaluated by positronium formation studies at low temperature

“…In order to explain this result, our group developed the CCSKM correlating the presence of low LMCTS and Ps formation, involving the participation of the ligand's excited states in the Ps formation [12,13]. According to this mechanism, the Ps is formed from the interaction between a positron and a ligand's excited electron.…”

“…Concerning that this phenomenon depend on electronic availability, resulting from ionization (second Spur model [4]) or excitation (second Cybotatic model [12,13]), this parameter is used to evaluate the positronium formation and probe the electronic structure at low temperature. Thus, I 3 was the main PALS parameter used in the present study.…”

“…Thus, the I 3 parameter should not be sensitive to the nature of the chemical bond (ionic or covalent) or the intermolecular interactions present in the condensed phase. In other way, CCSKM [12,13] regards that an epithermal positron collides with the compound's electronic cloud, thus exciting an electron and forming a cybotatic correlated system (CCS) composed by the colliding positron and the excited molecule. This CCS can decay by different channels, two of them leading to the Ps formation or the molecule electronic relaxation.…”

“…A correlation was obtained between probability, I 3 , and the covalence of complexes, evaluated by molar electrical conductivity in dimethylformamide (DMF), corroborating our hypothesis about the influence of electronic availability on the positronium formation parameter -I 3 . Moreover, these results were analyzed in terms of the spur model [4] and of the recently proposed mechanism named cybotatic correlated system kinetic mechanism (CCSKM) [12,13].…”