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Luminescence quantum efficiency of europium aggregates in sodium chloride
JOURNAL OF
LUMINESCENCE EISEVIER
Journal
of Luminescence
72-74
(1997) 233-235
Luminescence quantum efficiency of europium aggregates in sodium chloride J.A. Mufioz”,*, E. Rodriguez”, J.O. Tochob, F. CussY’ “Dpto. de Fisica de Materiales, C-IV, Universidad Autbnoma de Madrid, 28049 Madrid, Spain ‘Centro de Investigaciones Opticas. La Plata, Argentina
Abstract
In this work, the luminescent quantum efficiency of aggregate states of europium in sodium chloride from simultaneous photoacoustical and optical (emission and absorption) measurements. Keywords:
Alkali halides; Eu; Photoacoustic
spectroscopy;
Quantum
The absorption spectrum of Eu’ ’ in NaCl, consists of two broad bands attributed to transitions from the 4f’ (sS7,2) ground state to the tZg and e, components of the 4f65d excited state. After excitation, only one luminescence band is observed [l]. This luminescence has a very high quantum efficiency (@ = 100%) as it has been determined from simultaneous photoacoustic and luminescence experiments in quenched alkali halides [2]. Nevertheless, except in freshly quenched samples, Eu2+ ions in NaCl undergo a relatively fast clustering process which produces different aggregates/precipitates with well characterised changes in the absorption and emission spectra [l, 33. Associated to this precipitation process a decrease in the overall luminescence intensity has been reported [4], although a quantification of the luminescence quantum efficiency of the different aggregates/precipitates is still lacking. In the present work the photoacoustic and
author. *Corresponding Fcus@vm 1.sdi.uam.es.
Fax:
34-l-3978579;
e-mail:
0022-2313/97/$17.00 ((i 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00332-8
are determined
efficiency
luminescence technique previously applied to quenched samples has been extended to samples with precise precipitation characteristics in order to obtain the luminescent quantum efficiencies of Eu precipitates. Optical absorption and luminescence measurements were performed by using a Cary-14 spectrophotometer and a Jobin-Yvon JY3CS spectrofluorimeter. For quantum efficiency measurements, pulsed excitation is achieved by using a Nd : YAG laser linked to a harmonic generator. Photoacoustic and luminescence signals were detected simultaneously using a resonant piezoelectric and an EM1 9558QB photomultiplier tube. Both signals are collected and averaged by using a digital oscilloscope Tektronix 2440. Fig. l(a) shows the absorption and emission spectra of a NaCl: Eu2+ sample which has been annealed at 600°C for 1 h and then slowly (24 h) cooled to room temperature. With this thermal treatment a single type of Eu2+ aggregates (Eu2) is produced [S]. The luminescence quantum efficiency can be obtained following a procedure similar to
234
JA. Mutioz et al. J Journal of Luminescence
72-74 (1997) 233-235
Table 1 Notation
Precipitate
Y.
Eul Eu2 Eu3 Eu4
Dipolar europium Distorted precipitates (3 10) plate-like CI,Eu
0.05 0.10 0.30 0.70
@
ij Eu2 0.6 g c 0,4 p 5 %
1.00 0.80 0.50 < 0.10
0,2 = 1,o 200
300 400 500 0,O 0,2 0,4 0,6 0,s Wavelength (nm) LUM (arb. units)
0.0 I,0
I,0 h exe = 266 nm
(a)
Fig. 1. (a) Absorption and emission spectra for slow cooling sample.(b) Photoacoustic versus luminescent signals at 4~ and 30, excitations for this sample.
that used with quenched samples, where dipolar Eu2+ (Eul) is formed. Fig. l(b) shows the photoacoustic (PAS) and luminescence (LUM) signals detected after pulsed excitation at 2 = 355 nm (squares) or I = 266 nm (circles), which correspond to the third (30,) and fourth (4~~) harmonics of the Nd : YAG laser. The different experimental points correspond to different excitation powers. As it has been described elsewhere [2], the luminescence quantum efficiency (@) is readily obtained from the expression: @ _ 300 (A - (4/3))/(1 - a) (A 1) ’ wemi where n = ml/m2 is the ratio between the two PAS vs. LUM slopes, experimentally determined from Fig. l(b), o. is the fundamental frequency of the Nd : YAG laser, cC),,ithe frequency of the Eu photoluminescence and X, which is also experimentally determined, represents the fraction of Eu2+ ions which suffer non-radiative relaxation to the ground state when excited to the upper electronic level [2]. The values of the quantum efficiency (@) and the non-radiative connection (cx)obtained for Eul and Eu2 are listed in Table 1. Once these two states have been characterised it is now possible to prepare samples with only one additional Eu2+ precipitate, either the metastable (3 10) precipitates formed by annealing at low
Abs. Power (arb. units) Fig. 2. Photoacoustic (d) and luminescent (0) signals versus absorbed light power for (a) 401, and (b) 30~6 excitations for 30 min annealed samples at 200°C.
temperature (T < 200°C) which we shall assign as Eu3, or the stable EuCl, precipitates formed by high temperature annealing which we shall label as Eu4. With only one new contribution to the photoacoustic and luminescence signals it is possible then to extract the contribution of these precipitates and determine the quantum efficiency parameters. They are summarized in Table 1. It should be mentioned that in samples with precipitated Eu3 or Eu4 (( 3 10) platelets or EuCl,) the values reported in Table 1 correspond to the limit of low excitation power, because in these cases, while the linear response of the photoacoustic signal with the excitation power is maintained, the luminescence exhibits a clear deviation from linearity. This is illustrated in Fig. 2 which corresponds to a sample with Eu3 precipitates. This fact occurs for both 30, and 4w,-,excitation, although it is clearer at the latter excitation, and it is an indication that two photon processes and excited state absorption appear under UV excitation in aggregate samples [S].
JA. Murioz et al. / Journal of Luminescence
Acknowledgements Work partially supported by the European Community under programme CII*-CT93-0316. References [l] F.J. Lbpez, H. Murrieta, J. HernCndez and J. Rubio, Phys. Rev. B 22 (1980) 6428.
72-74 (1997) 233-235
235
I21 E. Rodriguez, J.A. Mufioz, J.O. Tocho and F. Cusso, J. Phys.: Condens. Matter 6 (1994) 10625. c31F.J. Lbpez, H. Murrieta, J. HernCndez and J. Rubio, Lumin. 26 (1980) 129. r41 J.E. Murioz-Santiuste and J. Garcia-Sol& Phys. Rev. B (1988) 10874. [51 C. Zaldo, E.M. Orozco, A.A. Mendoza and J. Rubio, Phys. D 18 (1985) 247. t-61L.D. Merkle and P.K. Bandyopadhyay, Phys. Rev. B (1989) 6939.
J. 38 J. 39
LUMINESCENCE EISEVIER
Journal
of Luminescence
72-74
(1997) 233-235
Luminescence quantum efficiency of europium aggregates in sodium chloride J.A. Mufioz”,*, E. Rodriguez”, J.O. Tochob, F. CussY’ “Dpto. de Fisica de Materiales, C-IV, Universidad Autbnoma de Madrid, 28049 Madrid, Spain ‘Centro de Investigaciones Opticas. La Plata, Argentina
Abstract
In this work, the luminescent quantum efficiency of aggregate states of europium in sodium chloride from simultaneous photoacoustical and optical (emission and absorption) measurements. Keywords:
Alkali halides; Eu; Photoacoustic
spectroscopy;
Quantum
The absorption spectrum of Eu’ ’ in NaCl, consists of two broad bands attributed to transitions from the 4f’ (sS7,2) ground state to the tZg and e, components of the 4f65d excited state. After excitation, only one luminescence band is observed [l]. This luminescence has a very high quantum efficiency (@ = 100%) as it has been determined from simultaneous photoacoustic and luminescence experiments in quenched alkali halides [2]. Nevertheless, except in freshly quenched samples, Eu2+ ions in NaCl undergo a relatively fast clustering process which produces different aggregates/precipitates with well characterised changes in the absorption and emission spectra [l, 33. Associated to this precipitation process a decrease in the overall luminescence intensity has been reported [4], although a quantification of the luminescence quantum efficiency of the different aggregates/precipitates is still lacking. In the present work the photoacoustic and
author. *Corresponding Fcus@vm 1.sdi.uam.es.
Fax:
34-l-3978579;
e-mail:
0022-2313/97/$17.00 ((i 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00332-8
are determined
efficiency
luminescence technique previously applied to quenched samples has been extended to samples with precise precipitation characteristics in order to obtain the luminescent quantum efficiencies of Eu precipitates. Optical absorption and luminescence measurements were performed by using a Cary-14 spectrophotometer and a Jobin-Yvon JY3CS spectrofluorimeter. For quantum efficiency measurements, pulsed excitation is achieved by using a Nd : YAG laser linked to a harmonic generator. Photoacoustic and luminescence signals were detected simultaneously using a resonant piezoelectric and an EM1 9558QB photomultiplier tube. Both signals are collected and averaged by using a digital oscilloscope Tektronix 2440. Fig. l(a) shows the absorption and emission spectra of a NaCl: Eu2+ sample which has been annealed at 600°C for 1 h and then slowly (24 h) cooled to room temperature. With this thermal treatment a single type of Eu2+ aggregates (Eu2) is produced [S]. The luminescence quantum efficiency can be obtained following a procedure similar to
234
JA. Mutioz et al. J Journal of Luminescence
72-74 (1997) 233-235
Table 1 Notation
Precipitate
Y.
Eul Eu2 Eu3 Eu4
Dipolar europium Distorted precipitates (3 10) plate-like CI,Eu
0.05 0.10 0.30 0.70
@
ij Eu2 0.6 g c 0,4 p 5 %
1.00 0.80 0.50 < 0.10
0,2 = 1,o 200
300 400 500 0,O 0,2 0,4 0,6 0,s Wavelength (nm) LUM (arb. units)
0.0 I,0
I,0 h exe = 266 nm
(a)
Fig. 1. (a) Absorption and emission spectra for slow cooling sample.(b) Photoacoustic versus luminescent signals at 4~ and 30, excitations for this sample.
that used with quenched samples, where dipolar Eu2+ (Eul) is formed. Fig. l(b) shows the photoacoustic (PAS) and luminescence (LUM) signals detected after pulsed excitation at 2 = 355 nm (squares) or I = 266 nm (circles), which correspond to the third (30,) and fourth (4~~) harmonics of the Nd : YAG laser. The different experimental points correspond to different excitation powers. As it has been described elsewhere [2], the luminescence quantum efficiency (@) is readily obtained from the expression: @ _ 300 (A - (4/3))/(1 - a) (A 1) ’ wemi where n = ml/m2 is the ratio between the two PAS vs. LUM slopes, experimentally determined from Fig. l(b), o. is the fundamental frequency of the Nd : YAG laser, cC),,ithe frequency of the Eu photoluminescence and X, which is also experimentally determined, represents the fraction of Eu2+ ions which suffer non-radiative relaxation to the ground state when excited to the upper electronic level [2]. The values of the quantum efficiency (@) and the non-radiative connection (cx)obtained for Eul and Eu2 are listed in Table 1. Once these two states have been characterised it is now possible to prepare samples with only one additional Eu2+ precipitate, either the metastable (3 10) precipitates formed by annealing at low
Abs. Power (arb. units) Fig. 2. Photoacoustic (d) and luminescent (0) signals versus absorbed light power for (a) 401, and (b) 30~6 excitations for 30 min annealed samples at 200°C.
temperature (T < 200°C) which we shall assign as Eu3, or the stable EuCl, precipitates formed by high temperature annealing which we shall label as Eu4. With only one new contribution to the photoacoustic and luminescence signals it is possible then to extract the contribution of these precipitates and determine the quantum efficiency parameters. They are summarized in Table 1. It should be mentioned that in samples with precipitated Eu3 or Eu4 (( 3 10) platelets or EuCl,) the values reported in Table 1 correspond to the limit of low excitation power, because in these cases, while the linear response of the photoacoustic signal with the excitation power is maintained, the luminescence exhibits a clear deviation from linearity. This is illustrated in Fig. 2 which corresponds to a sample with Eu3 precipitates. This fact occurs for both 30, and 4w,-,excitation, although it is clearer at the latter excitation, and it is an indication that two photon processes and excited state absorption appear under UV excitation in aggregate samples [S].
JA. Murioz et al. / Journal of Luminescence
Acknowledgements Work partially supported by the European Community under programme CII*-CT93-0316. References [l] F.J. Lbpez, H. Murrieta, J. HernCndez and J. Rubio, Phys. Rev. B 22 (1980) 6428.
72-74 (1997) 233-235
235
I21 E. Rodriguez, J.A. Mufioz, J.O. Tocho and F. Cusso, J. Phys.: Condens. Matter 6 (1994) 10625. c31F.J. Lbpez, H. Murrieta, J. HernCndez and J. Rubio, Lumin. 26 (1980) 129. r41 J.E. Murioz-Santiuste and J. Garcia-Sol& Phys. Rev. B (1988) 10874. [51 C. Zaldo, E.M. Orozco, A.A. Mendoza and J. Rubio, Phys. D 18 (1985) 247. t-61L.D. Merkle and P.K. Bandyopadhyay, Phys. Rev. B (1989) 6939.
J. 38 J. 39