Energy transfer between UO22+ and Eu3+ in POCl3–SnCl4 solutions

Journal of Alloys and Compounds 341 (2002) 283–287

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Energy transfer between UO 21 and Eu 31 in POCl 3 –SnCl 4 solutions 2 E.A. Seregina*, A.A. Seregin, G.V. Tikhonov Institute of Physics and Power Engineering, Sq. Bondarenko 1, 249020 Obninsk, Russia

Abstract The results of the study of interaction between UO 21 and Eu 31 in solutions of POCl 3 –SnCl 4 with a constant europium concentration 2 and a variable uranyl concentration are represented. The change of branching factors for transitions 5 D 0 → 7 F 2 (increase) and 5 D 0 → 7 F 4 (reduction) is revealed in the Eu 31 luminescence spectra. The intense europium luminescence associated with uranyl sensitizing is registered in excitation spectra of Eu 31 ions in the range of 350–500 nm. The quenching rate constant of UO 21 fluorescence with Eu 31 is 2 5 21 21 derived and its value is determined to be (562)310 M s . It is estimated that the rate constant of the quenching process with subsequent excitation and fluorescence of europium ion is equal to (3.760.5)310 5 M 21 s 21 .  2002 Elsevier Science B.V. All rights reserved. Keywords: Phosphor; Liquid quenching; Crystal structure and symmetry; Luminescence

1. Introduction The solutions of rare-earth elements in complex POCl 3 – SnCl 4 aprotic inorganic solvent are of considerable interest as media for lasers and optical quantum amplifiers. In POCl 3 –SnCl 4 the active ions of rare-earth elements form mononuclear discrete complexes, which poorly interact with each other and with a solvent [1]. Due to this particular feature, it is possible to reach a high quantum output of luminescence (more than 90%) and in such solutions a concentration quenching process is practically absent. However, the same feature prevents the increase of the pumping efficiency of active ions due to the energy transfer from any other ion-sensitizer. Our recent investigations have shown that under certain conditions polynuclear complexes can arise in POCl 3 –SnCl 4 solutions containing Nd 31 - or Er 31 ions and uranyl. It was found that rather an effective transfer of the excitation energy is possible in the polynuclear complexes. Trivalent lanthanide ion and uranyl are probably bridged by dichlorophosphate group, which facilitates excitation energy transfer [2–6]. The search for a sensitizer for Eu 31 ions is important as they have low-intensity absorption bands. The phenomenon of transfer of the excitation energy from UO 21 to 2 Eu 31 was studied earlier in such inorganic solvents as D 2 O

*Corresponding author. Tel.: 17-843-998-839; fax: 17-958-833-112. E-mail address: [email protected] (E.A. Seregina).

[7,8], polyphosphoric [9] and sulfuric [10] acids. The Eu 31 ion luminescence sensitized by UO 21 was detected in all 2 these solutions. This paper presents the results of studies of the interaction between europium ion and uranyl in POCl 3 –SnCl 4 – 31 UO 21 solutions by the spectral–luminescent meth2 –Eu ods.

2. Experiment The solutions were prepared by dissolving europium and uranium compounds in POCl 3 –SnCl 4 solvent with fixed europium concentration and variable uranium concentration, following the procedure described in [2,3]. Absorption quartz cells with an optical path length of 1 cm were filled up with the prepared liquids. The samples were sealed hermetically. The europium and uranium concentrations were determined by both gravimetric and spectrophotometric methods. The absorption spectra of solutions have been recorded on a SF-20M high-resolution spectrophotometer interfaced to an IBM PC computer and operated in the on-line mode. Fluorescence and excitation spectra have been measured on a SDL-2 spectrofluorimeter with a 150-W xenon lamp. The required wavelength of the exciting radiation was selected with an MDR-12 monochromator to excite test solutions. Fluorescence photons were then detected with a FEU-100 single-electron photomultiplier tube after passing through a MDR-23 monochromator. The wavelength response of the registering

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00025-7

E. A. Seregina et al. / Journal of Alloys and Compounds 341 (2002) 283 – 287

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apparatus has been measured by using a TRSh-2050 calibrated tungsten lamp. The excited state lifetime of Eu 31 or UO 221 has been recorded with an ILGI-503 nitrogen pulse laser ( llas 5337 nm, tpulse 520 nc), an MDR-23 monochromator with a FEU-100 photomultiplier tube and time-to-pulse converter unit operating on-line with an IBM PC.

3. Results and discussion The concentration and some other characteristics of the prepared and tested solutions are given in Table 1. The typical absorption spectra of POCl 3 –SnCl 4 –Eu 31 , POCl 3 – SnCl 4 –UO 221 and POCl 3 –SnCl 4 –UO 221 –Eu 31 solutions are shown in Fig. 1. The Eu 31 absorption bands coincide with the UO 21 absorption bands in a broad wavelength 2 31 region. Therefore, in POCl 3 –SnCl 4 –UO 21 the anal2 –Eu 31 ysis of the Eu absorption intensity is hampered even for hypersensitive transition 7 F 0 → 5 D 2 ( l5464 nm). In the visible region the electron absorption spectrum of uranyl is connected with transition from the highest molecular orbital to the uranium 5 f orbital. It is the molecular nature of uranyl that causes high sensitivity of uranyl absorption to the coordination surroundings. From Fig. 1 it can be seen that the intensity ratio in the fine structure of UO 21 absorption changes in the solution with 2 uranyl and europium. Eu 31 fluorescence spectra in POCl 3 –SnCl 4 solutions with and without UO 221 are shown in Fig. 2. Here the spectra are normalized to the same integrated intensity of 5 D 0 → 7 F 1 magnetic dipole transition in the wavelength region from 582 to 602 nm. The Eu 31 fluorescence spectra were used for determination of the branching ratio for 5 D 0 → 7 F j transitions. The error of the fluorescence branching ratio determination was less than 2%. The dependence of the fluorescence branching ratio of Eu 31 ion on UO 21 concentration in the solvent is found. As is 2 seen from Table 1, the branching ratio of 5 D 0 → 7 F 2 transition has slightly increased and that of 5 D 0 → 7 F 4 transition has decreased. This is an indication of changes appearing in the coordination of the Eu 31 ion complex.

Fig. 1. Absorption spectra of solutions POCl 3 –SnCl 4 –Eu 31 (1) POCl 3 – 31 SnCl 4 –UO 21 (2) and POCl 3 –SnCl 4 –UO 21 (3). [Eu 31 ]50.4 M (1, 2 2 –Eu 21 3); [UO 2 ]50.10 M (2, 3); l51 cm.

Excitation spectra of Eu 31 fluorescence were recorded at a wavelength of 69961 nm. In this wavelength region uranyl has no fluorescence and cannot make a contribution to the excitation spectrum either. Fig. 3 gives the Eu 31 ion excitation spectra in POCl 3 –SnCl 4 solutions with and without UO 21 2 . It is seen that in solutions with uranyl the Eu 31 ion fluorescence intensity increases sharply in the wavelength region of UO 21 absorption bands. On the 2 whole in the exciting wavelength region from 350 to 600 nm the integrated intensity of Eu 31 ion fluorescence increases approximately by an order in POCl 3 –SnCl 4 – 31 UO 21 solution, containing 0.1 M of uranyl. 2 –Eu The uranyl absorption bands are much more intense than the europium absorption bands. Besides, a low-energy edge of the uranyl absorption band lies in the region of 496 nm (20 160 cm 21 ), i.e. it is only 2860 cm 21 higher than the 5 D 0 metastable level (17 300 cm 21 ) of Eu 31 ion. The above features are favorable for non-radiative energy transfer from UO 21 (donor of energy) to Eu 31 (acceptor of 2 energy). The fluorescence lifetime tlum of the 5 D 0 metastable level of Eu 31 ion has been determined from the time decay of fluorescence at wavelength of 699 nm. The decay of

Table 1 31 Characteristics of solutions POCl 3 –SnCl 4 –UO 21 2 –Eu No. sample

CEu 31 (M)

CUO 221 (M)

tlum (ms)

I(0)a (rel. units)

The fluorescence branching ratio of Eu 31 ion b 5

1p 2p 3p 4p 5p

0.40 0.42 0.42 0.37 0.43 a b

lex 5337 nm. lex 5394 nm.

– 0.04 0.07 0.10 0.14

4300 3880 3640 3040 3400

600 65 000 92 000 121 000 148 000

D 0 → 7F 1

0.235 0.235 0.235 0.235 0.230

5

D 0 → 7F 2

0.370 0.390 0.410 0.400 0.440

5

D 0 → 7F 3

0.015 0.015 0.016 0.017 0.016

5

D 0 → 7F 4

0.380 0.360 0.335 0.345 0.315

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Fig. 2. Eu 31 ion fluorescence spectra in POCl 3 –SnCl 4 solutions without (dashed line) and with UO 21 (solid line). The 5 D 0 → 7 F 0 transition intensity is 2 increased by the factor of 100 times. Spectra are corrected for wavelength response of registering apparatus. [UO 221 ]50.10; lex 5394 nm.

Eu 31 ion fluorescence intensity is described by an exponential law with a high accuracy I(t) 5 I(0) exp(2t /tlum ) where I(0) is the fluorescence intensity at the starting time when the solution was excited by pulse laser radiation. As

is seen from Table 1 (column 5), the intensity of europium luminescence has increased by more than 200 times in the presence of 0.1 M uranyl in POCl 3 –SnCl 4 –UO 221 –Eu 31 solution. At the same time Table 1 shows that the presence of uranyl in the solutions reduces Eu 31 ion lifetime to some extent. The reduction of the fluorescence lifetime could be

31 Fig. 3. Eu 31 ion excitation spectra in POCl 3 –SnCl 4 solution without (1) and with (2) UO 21 ]50.4 M (1, 2); [UO 21 2 . [Eu 2 ]50.10 M (2).

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connected with an increase in both radiative and nonradiative rate of 5 D 0 level relaxation. The 5 D 0 level radiative lifetime of the Eu 31 ion was calculated using the branching ratio data from Table 1. We assumed that the relaxation rate of 5 D 0 → 7 F 1 magnetic dipole transition is constant and equal A 1 515.1 n 3 [11]. Here n is the refractive index of the solution. It was found that the 21 radiative lifetime is independent of the UO 2 concentration and it is equal to 4985654 ms. Thus the reduction of the fluorescence lifetime is connected with the increase of the non-radiative rate of the 5 D 0 level relaxation. Plausible reasons of increase in non-radiative relaxation can be both the addition of quenching impurities, for example OH 2 , which often accompanies uranyl compounds, and interaction presses between excited UO 21 and 2 Eu 31 ions. The uranyl fluorescence lifetime t was measured in solutions with and without europium. The measurements of t were performed at a wavelength of 517 nm, corresponding to the uranyl fluorescence maximum in POCl 3 – SnCl 4 . The decay of UO 221 fluorescence intensity is described by exponential law with a high accuracy too. It has been found that the uranyl fluorescence lifetime is equal to t0 52064 ms in solution without Eu 31 ion and t 5461 ms in solutions with europium. The latter data are obtained by averaging the results of t measurements for all POCl 3 –SnCl 4 –UO 221 –Eu 31 solutions. The sharp increase of europium fluorescent intensity and the diminution of the uranyl fluorescent lifetime in solutions are indicative of the excitation energy transfer from UO 21 to Eu 31 . The 2 21 quenching rate constant of UO 2 fluorescence in solutions

with Eu 31 ions, k q , was determined by the Stern–Volmer equation [12]: 1 k q 5 ] s1 /t 2 1 /t0d CEu where CEu is the europium concentration in a solution. We obtained the quenching rate constant equal to k q 5(562)1 10 5 M 21 s 21 . Part of the excitation energy absorbed by UO 21 ion 2 level leads to Eu 31 ion fluorescence due to energy transfer as shown by the equation: k1

hn

31 UO 21 →UO 221 1 Eu 31 * →UO 221 1 Eu 31 2 * 1 Eu

The rest of the energy may be lost in the environment by radiation-less processes. Constant k 1 was evaluated directly from build-up of the europium ion fluorescence. The fluorescence intensity as a function of a donor–acceptor pair concentration, CB , is given by the relation [7]:

H

1 1 1 ]] 5 ] 1 1 ]] G k 1t0 CB I(0)

J

Here, G is a factor including geometrical and other conditions of the experiment. The donor–acceptor pair concentration was assumed to be equal to the uranyl concentration in the solution. Fig. 4 shows the inverse value of the europium intensity from column 5 of Table 1 as a function of the inverse value of the donor–acceptor pair concentration. From the curve slope we obtained the information about k 1 . It turned out that the value of k 1 is 5 21 21 (3.760.5)110 M s , in other words it is practically

Fig. 4. I(0)21 of the Eu 31 fluorescence as a function of C 21 B .

E. A. Seregina et al. / Journal of Alloys and Compounds 341 (2002) 283 – 287

equal to k q . Thus energy transfer from uranyl to europium occurs mainly with the europium ions being excited and a small fraction of the energy is lost by the UO 21 ion in the 2 non-radiative processes. Attention is drawn to the fact that the quenching rate constant of UO 21 fluorescence in 2 solutions with Eu 31 , Nd 31 or Er 31 ions has the same order of magnitude [4]. Apparently, the mechanism of UO 21 2 fluorescent quenching by these lanthanide ions has the same nature. Based on the experimental results obtained, it is possible to draw the following conclusions: • It has been determined that the intensity ratio in the structure of UO 21 absorption changes in the solution 2 with uranyl and europium; • it has been found that uranyl affects the fluorescence branching ratio of the Eu 31 ion; • it has been estimated that the quenching rate constant of UO 21 fluorescence with Eu 31 , i.e. k q , is equal to 2 (562)110 5 M 21 s 21 and the rate constant of the energy transfer from excited UO 21 to Eu 31 with 2 subsequent fluorescence of europium ion, i.e. k 1 , is equal to (3.760.5)110 5 M 21 s 21 ; • the effect of sensitizing Eu 31 ions by UO 21 is reliably 2 21 31 established in POCl 3 –SnCl 4 –UO 2 –Eu solutions: 31 the intensity of Eu ion fluorescence increase more than 200 times under excitation by a laser radiation with llas 5337 nm and approximately 10 times under excitation by Xe-lamp light in the wavelength region from 350 to 600 nm.

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Acknowledgements The authors are grateful to Dr P.P. D’yachenko for his interest in this work and useful discussions. This work was supported by the Russian Foundation for Basic Research.

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