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Positron Annihilation Lifetime Spectroscopy – PALS at 294 K. Resolution: 220 ps

Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb 1- x Eu x ( dpm ) 3 solid solutions.

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Positron Annihilation Lifetime Spectroscopy – PALS at 294 K. Resolution: 220 ps

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  1. Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb1-xEux(dpm)3solid solutions

  2. Welington F. MAGALHÃES1§, F. FULGÊNCIO1, D. WINDMÖLLER1,J. C. MACHADO1, F. C. de OLIVEIRA2, H. F. BRITO3, O. L. MALTA4and G. F. de SÁ4 1§Departamento de Química, ICEx, Universidade Federal de Minas Gerais - UFMG, Av Antônio Carlos, 6627, 31270-901 Belo Horizonte, Brazil Laboratório de Espectroscopia de Aniquilação de Pósitrons - LEAP 2Centro Federal de Educação Tecnológica de Minas Gerais, Timóteo, MG, Brazil 3Instituto de Química, Universidade de São Paulo, 05508-900, São Paulo, SP, Brazil 4 Departamento de Química Fundamental, UFPE, 50670-901 Recife, PE, Brazil

  3. Positron Annihilation Lifetime Spectroscopy – PALS at 294 K. Resolution: 220 ps • Time resolved Photoluminescence Spectroscopy – TPhoS at 294 K and 77 K. • Studied system: The molecular complexes Tb and Eudipivaloylmetanates, Tb(dpm)3 and Eu(dpm)3, and their binary solid solutions, Tb1-xEux(dpm)3. Welington F. MAGALHÃES: welmag@ufmg.br

  4. 484-489 nm 544 nm Fig. 2 – Photoluminescence emission spectra obtained at 77 K, excited at 340 nm for: a) Tb(dpm)3, b) Eu(dpm)3, c) Tb(0.9)Eu(0.1)(dpm)3, d) Tb(0.7)Eu(0.3)(dpm)3, and e) Tb(0.5)Eu(0.5)(dpm)3. 615 nm Welington F. MAGALHÃES: welmag@ufmg.br

  5. Fig. 3 – Luminescence excitation spectra Tb: 485 nm Tb Eu Energy transfer Tb: lemission = 544 nm Eu: lemission = 615 nm Welington F. MAGALHÃES: welmag@ufmg.br

  6. Fig. 4 – Partial energy level diagram for the relevant photophysical process associated with photoluminescence in Tb1-xEux(dpm)3. Welington F. MAGALHÃES: welmag@ufmg.br

  7. Table 1 – PALS parameters (lifetimes and intensities) at (294  1) K, 1 fixed at 0.120 ns, and the luminescence lifetimes for Tb1‑xEux(dpm)3 solid solutions. Welington F. MAGALHÃES: welmag@ufmg.br

  8. 298 K Fig. 5 – Luminescence decay constants, lLTb* = 1/tLTb*, for the luminescent 5D4 excited state level (l = 544 nm) of Tb(III) ion in the Tb(dpm)3 complex versus the mole fraction of Eu(dpm)3 complex in the Tb1-xEux(dpm)3 solid solutions. kQlum 77 K Welington F. MAGALHÃES: welmag@ufmg.br

  9. Fig. 6: Inhibition of the o-Ps intensity as a function of the mole fraction of Eu(III). The lines shows the fits (a), (c) and (d) of the equation (19) with parameters in Table 2. (c) (d) (a) Welington F. MAGALHÃES: welmag@ufmg.br

  10. Fig. 7: The strong linear correlation between the o-Ps intensity I3 and the luminescence lifetime of the Tb(III)5D4 energy level. I3calc: 39.4, 23.2, 14.0, 13.2%. xEu = 0 Experimental data: Table 1,. Fitted line, R2 = 0.9749 xEu = 0.1 Calculated data from eq. (16), eq. (19) and fit (a) in Table 2. Fitted line, R2 = 0.9796 xEu = 0.3 xEu = 0.5 Welington F. MAGALHÃES: welmag@ufmg.br

  11. Fig. 8 – Scheme for the kinetic mechanism of the Ps formation from ligand excited states in Tb1-xEux(dpm)3solid solutions, showing the Ps inhibition formation and the luminescence quenching, due to energy transfer between Tb(III) and Eu(III) ions. DirectPs precursor IndirectPs precursor Welington F. MAGALHÃES: welmag@ufmg.br

  12. Kinetic rate equations for Tb side of the mechanism (12) (13) (14) (15) (16) (17) Fig.5 kQlum Welington F. MAGALHÃES: welmag@ufmg.br

  13. Applying the steady-state hypothesis in the equations (12) and (13) leads to: (18) k’1Tb is a pseudo first order reaction rate constant for the ligand excitation by epithermal positrons: 2nd parcel 1st parcel Welington F. MAGALHÃES: welmag@ufmg.br

  14. (19) 1 2 3 4 6 5 7 Welington F. MAGALHÃES: welmag@ufmg.br

  15. Table 2: Values of the fitted parameters of equation (19) on the positronium yields I3 in Table 1 and shown in Fig. 6. The parameters without uncertainties are fixed values. The uncertainties were obtained by a numerical procedure [Bevington 2003]. kQlum = k8Tb + k9Tb. Fig.6 1 2 3 4 5 6 7 8 9 10 11 Welington F. MAGALHÃES: welmag@ufmg.br

  16. CONCLUSIONS • From the proposed mechanism a equation was deduced, and it describes very well the inhibition of Ps formation performed by the Eu(dpm)3 complex, as well as the linear correlation between the Ps formation probability, I3, and the lifetime of the Tb5D4 luminescent excited state of the Tb(dpm)3 complex, the indirect Ps formation precursor. Welington F. MAGALHÃES: welmag@ufmg.br

  17. CONCLUSIONS • The proposed mechanism raises strong evidences of the participation of electronic excited states as precursors for the Ps formation, at the positron molecule scattering, what is a characteristic of the Ore and resonant models. • As in the spur model the proposed mechanism presents various competitive reactions that can reduce the probability of positronium formation, in a way completely consistent with the Stern-Volmer behavior. Welington F. MAGALHÃES: welmag@ufmg.br

  18. Thanks for your attention ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Welington F. MAGALHÃES: welmag@ufmg.br

  19. Luminescence excitation spectra Proposed kinetic model Tb Eu Energytransfer 09Sep11 Friday 10h00: Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb1-xEux(dpm)3 solid solutions. Ps formation inhibition fitted by the deduced model: experimental values: model prediction: Welington F. MAGALHÃES: welmag@ufmg.br

  20. Welington F. MAGALHÃES: welmag@ufmg.br

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