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

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


19

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.


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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:


*

* *

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