positron annihilation lifetime spectroscopy – pals at 294 k. resolution: 220 ps
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Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb 1- x Eu x ( dpm ) 3 solid solutions. - PowerPoint PPT PresentationTRANSCRIPT
Evidence of the Participation of Electronic Excited States
in the Mechanism of Positronium Formation in
Tb1-xEux(dpm)3 solid solutions
Welington F. MAGALHÃES1§, F. FULGÊNCIO1, D. WINDMÖLLER1,
J. C. MACHADO1, F. C. de OLIVEIRA2, H. F. BRITO3, O. L. MALTA4 and 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 - LEAP2 Centro Federal de Educação Tecnológica de Minas Gerais, Timóteo, MG, Brazil
3 Instituto de Química, Universidade de São Paulo, 05508-900, São Paulo, SP, Brazil4 Departamento de Química Fundamental, UFPE, 50670-901 Recife, PE, Brazil
Welington F. MAGALHÃES: [email protected]
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• 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 Eu dipivaloylmetanates, Tb(dpm)3 and Eu(dpm)3, and their binary solid solutions, Tb1-xEux(dpm)3.
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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.
544 nm
615 nm
484-489 nm
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Fig. 3 – Luminescence excitation spectraTb: 485 nm
Tb Eu Energy transfer
Tb: lemission = 544 nm
Eu: lemission = 615 nm
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Fig. 4 – Partial energy level diagram for the relevant photophysical process associated with photoluminescence in
Tb1-xEux (dpm)3.
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xEu τ 3 / ns τ2 / ns I3 / % I2 / %tLTb* / ms at
298 KtLTb* / ms at
77 K
0.000 1.40 ± 0.05 0.611 ± 0.035 39.6 ± 3.4 31.6 ± 3.7 0.763 0.821
0.025 1.65 ± 0.09 0.53 ± 0.02 33.1 ± 1.9 39.2 ± 3.1 ---- ----
0.050 1.57 ± 0.09 0.56 ± 0.04 32.4 ± 2.4 38.5 ± 2.0 ---- ----
0.075 1.87 ± 0.37 0.49 ± 0.02 28.9 ± 1.9 44.6 ± 0.1 ---- ----
0.100 1.74 ± 0.03 0.511 ± 0.004 23.1 ± 0.6 49.7 ± 0.6 0.645 0.795
0.150 1.75 ± 0.05 0.50 ± 0.01 20.2 ± 1.3 53.5 ± 1.4 ---- ----
0.200 1.64 ± 0.08 0.46 ± 0.02 19.9 ± 3.3 53.1 ± 2.4 ---- ----
0.300 1.68 ± 0.04 0.444 ± 0.009 15.0 ± 0.7 59.0 ± 0.6 0.529 0.608
0.500 1.92 ± 0.22 0.40 ± 0.01 11.1 ± 0.9 67.4 ± 1.1 0.517 0.615
0.700 2.06 ± 0.23 0.368 ± 0.005 7.7 ± 2.0 74.7 ± 2.4 ---- ----
1.000 2.20 ± 0.55 0.341 ± 0.004 2.4 ± 0.2 83.3 ± 1.4 ---- ----
Table 1 – PALS parameters (lifetimes and intensities) at (294 1) K, t1 fixed at 0.120 ns, and the luminescence lifetimes for Tb1‑xEux(dpm)3 solid solutions.
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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.
298 K
77 K
kQlum
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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)
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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
xEu = 0.1
xEu = 0.3xEu = 0.5
Calculated data from eq. (16), eq. (19) and fit (a) in Table 2.Fitted line, R2 = 0.9796
Experimental data: Table 1,.Fitted line, R2 = 0.9749
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Fig. 8 – Scheme for the kinetic mechanism of the Ps formation from ligand excited states in Tb1-xEux(dpm)3 solid solutions, showing the Ps inhibition formation and the luminescence quenching, due to energy transfer between Tb(III) and Eu(III) ions.
Direct Ps precursor
Indirect Ps precursor
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Kinetic rate equations for Tb side of the mechanism
(12)
(13)
(14) (15)
(16) (17)
*
** *
1 5 L Tb e
L Tb eLTb LTb e L Tb eTb Tb
dk k
dtl
*
** * *
4 8 LTb e
LTb eL Tb e LTb LEu e LTb eTb Eu
dk k
dtl
* *
*
06L Tb e L Tb e
L Tb e
1 LEuTbkl lt
* *
*
08 9LTb e LTb e
LTb e
1 LEuTb Tbk kl lt
*
*
02 3 4 110L Tb e
L Tb e
1Tb Tb Tb Tbk k k kl
t
*
*
05 7 100LTb e
LTb e
1Tb Tb Tbk k kl
t
kQlumFig.5
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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:
* *
* *
*1 5 8L Tb e LTb e*
4 5 L Tb e LTb e
LEu e LTbL Tb e
1
Tb Tb Eu
Tb Tb
k k k
k k
t t
t t
1 1Tb Tbk k e
2nd parcel1st parcel
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*
* *
*
* *
1 L Tb e* +*
4 5 L Tb e LTb e
01 8 9LTb e
0 06 8 9 4 5L Tb e LTb e
LTbL Tb e
1
LEu 1 LEu
LEu LEu
Tb
Tb Tb
Tb Tb Tb
Tb Tb Tb Tb Tb
k
k k
k k k
k k k k k
t
t t
l
l l
(19)1 2 3
456
7
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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.
Fit / ns–1 / ps / s–1 / msk´1Tb / ns–1
k4Tb / ns–1
k5Tb / ms–1
k6Tb / ns–1
kQlum / s–1
sfit / %
(a)10.060
0.085
99.40
0.841310.6 0.763
95.82.8
0.10.1000
0.0012
0.01.1
1899.2 2.315
(b)10.080
0.078
99.21
0.771310.6 0.763
95.02.6
0.10.1000
0.0011
–0.90
0.941899.2 2.159
(c)10.080
0.12
99.211.2
1310.6 0.76395.03.6
0.10.1000
0.0017
–0.901.1
1254.3 3.227
(d)10.060
0.073
99.40
0.721310.6 0.763
98.02.4
0.10.1000
0.0011
–6.0
0.66
4.00103
0.32103
1.925
*0L Tb e
l *0L Tb e
t *0LTb e
l *0LTb e
t
*0L Tb e
l *0L Tb e
t *0LTb e
l *0LTb e
t
1 2 3 4 5 6 7 8 9 10 11
Fig.6
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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 Tb 5D4 luminescent excited state of the Tb(dpm)3 complex, the indirect Ps formation precursor.
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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: [email protected]
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Thanks for your attention
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09Sep11 Friday 10h00: Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb1-xEux(dpm)3 solid
solutions.Tb Eu Energy transfer
Luminescence excitation spectra
Ps formation inhibition fitted by the deduced model:
model prediction:
Proposed kinetic model
experimental values:
Welington F. MAGALHÃES: [email protected]
*
* *
01 8 9LTb e* +*
0 06 8 9 4 5L Tb e LTb e
LEu 1 LEuL Tb e
LEu LEu
Tb Tb Tb
Tb Tb Tb Tb Tb
k k k
k k k k k
l
l l
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