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Flash photolysis in Eu2+ doped NaCl
JoutnalofMolecuhr Structure, 143(1986)219-222 ElsevierSciencePubHshersB.V.,Amsterdam -PrintedinTheNetherkmds
219
2+ FLASH PHOTOLYSIS IN Eu DOPED NaCl P. ACEITUNO (a', J.I. DEL EARRIO (b', F. CUSS0 (a', J. GARCIA SOLE (a) , F. JAQUE 'a), F.J. LOPEZ(a), J.R. REHATO (b) and F.M.G. TAEiLAS(b) Dptos. Optica y Estructura de la Materia (a) and Electroquimica (b), Universidad Autonoma de Madrid, 28049 Madrid, Spain.
ABSTRACT The luminescence of NaCl:Eu under flash-lamp and N2-laser excitation has been studied. It has been found an anomalous non-exponential luminescence decay which followsa dependence I=Io/tm with m=l at low (90 K) and room temperatures. The observed kinetics is consistent with a tunneling mechanism involving recombination of electrons and holes (V,) at Eu sites.
INTRODUCTION Alkali halides doped with different impurities have been recently used in laser technology. For instance, in color center lasers, impurities have been additionaly introduced in order to improve the stability of the active centres (1). Also, alkali halide crystals are preferred as windows in high power CO2 lasers with the addition of impurities in order to increase the mechanical strength of the window (2). Damage (radiolysis) in alkali halides under ‘r(or X-ray irradiation has been exhaustively studied during the last decades but much less information is available under intense visible or near UV irradiation (3). The purpo2+ se of this work is to study the optical damage (photolysis) of NaCl:Eu crystals using flash lamp or N2-pulsed laser excitation, taking advantage on the 2+ previous detailed studies on the luminescence properties of Eu doped crystals.
RESULTS A blue phosphorescencewhich lasts for several minutes has been observed af* ter flash lamp or laser excitation . The emission spectrum of the phosphorescence is presented in Fig. 1 for an as-grown sample of NaC1:Eu2+ excited at room temperature with the N2 laser. It consists on a broad band at 450 runwhich aggrees with that reported for precipitated Eu in NaCl under continuous low intensity excitation (4). This spectrum
A concentrical quartz flash lamp filled with Xe or Ar at 20 torr was used to produce a ~5 pulse by dischirging a 2.5 bF,capacitor (low inductance) charged at 15 kV. Alternatively a N2-laser (0.6 mJ/pulse) was also used. 0022-2860/86/$03.50 0 1986 Elsevier Science Publishers B.V.
220
150
400
500 WAVELENGTH
550 (nm 1
Fig. 1. Emission spectrum under flash-lamp or N2-laser excitation of NaCl:Eu and room temperature.
has been attributedtothe superposition of several bands associated with different kinds of precipitates (4). It is noteworthy that the photoluminescence excitation band associated with isolated Eu (not precipitatedj is lacking in Fig. 1. In this way, it has been found that after quenching the samples from 700 K
in
order
to solve the precipitates, the phosphorence is supressed. All
these results indicate that the observed phosphorence is related to the presence of the precipitates. The time dependence of the phosphorescence intensity is presented in Fig. 2a.
100
150
200
250
300
TEMPERATURE(K)
Fig. 2. (a) Time dependence of the phosphorescence intensity at peratures. (b) Evolution of the exponent m vs. temperature.
different tem-
221 It follows a law I Nl/tm
with m=l at low (90 K) and room temperatures and
m#l for temperatures around 160 K. A l/t dependence has been previously detected after X-ray excitation and associated to tunneling recombination of trapped electrons and holes (5). The temperature variation of the exponent m is given in detail in Fig. 2b. The range where m>l corresponds with the temperature of motion of the holes (Vkcenter). Vk centers consist on C12- molecules along direction. Their stabilization implies the trapping of an electron in an anion vacancy (F center) or in an impurity (Eu*f
Eu+). These electron-hole pairs are
produced after irradiation with photons of energy hr,> 7 eV; this value being far above the energy of the photons used in this work (< 3.8 eV). Nevertheless, this energy could be enough if a two photon process were involved in the damage sequence. In order to ascertain this possibility the dependence of the phosphorescence intensity with the excitation intensity is given in Fig. 3. showing a clear linear dependence, which discards a two-photon origin in the damage process. Therefore, the phosphorescence observed is attributed to a photolysis localized at the Eu precipitates followed by a recombination procedure involving tunneling recombination between electrons and Vk -centers which regenerate the per71 feet lattice and produce simultaneouly excited EuLT centers.
EXCITATION
INTENSITY,
100 I (%
0
5000
I
I2
10000 I”/. I4
Fig. 3. Dependence of the emission intensity vs. the pumping power of the N2 laser.
ACKNOWLEDGMENTS Financial support by the Comision Asesora de Investigation Cientifica y Tecnica (C.A.I.C. y T.) is gratefully acknowledged.
222 REFERENCES 1. L.F. Mollenauer, Opt. Lett. 5, 118 (1980). 2. J.B. Wolfenstine and T.G. Stoebe, "Laser induced damage in optical materials" National Bureau of Standards (1977). 3. F. A&lo-Lopez, F.J. Lopez and P. Jaque, Cryst. Latt. Def. and Amorph. Mat. 9, 227 (1982). 4. F.3. Lopez, H. Murrieta S., J. Hernandez A. and J. Rubio O., Phys. Rev. G, 6428 (1980). 5. C.J. Delbecq, Y. Toyozawa and P.H. Yuster, Phys. Rev. B9, 4497 (1974).
219
2+ FLASH PHOTOLYSIS IN Eu DOPED NaCl P. ACEITUNO (a', J.I. DEL EARRIO (b', F. CUSS0 (a', J. GARCIA SOLE (a) , F. JAQUE 'a), F.J. LOPEZ(a), J.R. REHATO (b) and F.M.G. TAEiLAS(b) Dptos. Optica y Estructura de la Materia (a) and Electroquimica (b), Universidad Autonoma de Madrid, 28049 Madrid, Spain.
ABSTRACT The luminescence of NaCl:Eu under flash-lamp and N2-laser excitation has been studied. It has been found an anomalous non-exponential luminescence decay which followsa dependence I=Io/tm with m=l at low (90 K) and room temperatures. The observed kinetics is consistent with a tunneling mechanism involving recombination of electrons and holes (V,) at Eu sites.
INTRODUCTION Alkali halides doped with different impurities have been recently used in laser technology. For instance, in color center lasers, impurities have been additionaly introduced in order to improve the stability of the active centres (1). Also, alkali halide crystals are preferred as windows in high power CO2 lasers with the addition of impurities in order to increase the mechanical strength of the window (2). Damage (radiolysis) in alkali halides under ‘r(or X-ray irradiation has been exhaustively studied during the last decades but much less information is available under intense visible or near UV irradiation (3). The purpo2+ se of this work is to study the optical damage (photolysis) of NaCl:Eu crystals using flash lamp or N2-pulsed laser excitation, taking advantage on the 2+ previous detailed studies on the luminescence properties of Eu doped crystals.
RESULTS A blue phosphorescencewhich lasts for several minutes has been observed af* ter flash lamp or laser excitation . The emission spectrum of the phosphorescence is presented in Fig. 1 for an as-grown sample of NaC1:Eu2+ excited at room temperature with the N2 laser. It consists on a broad band at 450 runwhich aggrees with that reported for precipitated Eu in NaCl under continuous low intensity excitation (4). This spectrum
A concentrical quartz flash lamp filled with Xe or Ar at 20 torr was used to produce a ~5 pulse by dischirging a 2.5 bF,capacitor (low inductance) charged at 15 kV. Alternatively a N2-laser (0.6 mJ/pulse) was also used. 0022-2860/86/$03.50 0 1986 Elsevier Science Publishers B.V.
220
150
400
500 WAVELENGTH
550 (nm 1
Fig. 1. Emission spectrum under flash-lamp or N2-laser excitation of NaCl:Eu and room temperature.
has been attributedtothe superposition of several bands associated with different kinds of precipitates (4). It is noteworthy that the photoluminescence excitation band associated with isolated Eu (not precipitatedj is lacking in Fig. 1. In this way, it has been found that after quenching the samples from 700 K
in
order
to solve the precipitates, the phosphorence is supressed. All
these results indicate that the observed phosphorence is related to the presence of the precipitates. The time dependence of the phosphorescence intensity is presented in Fig. 2a.
100
150
200
250
300
TEMPERATURE(K)
Fig. 2. (a) Time dependence of the phosphorescence intensity at peratures. (b) Evolution of the exponent m vs. temperature.
different tem-
221 It follows a law I Nl/tm
with m=l at low (90 K) and room temperatures and
m#l for temperatures around 160 K. A l/t dependence has been previously detected after X-ray excitation and associated to tunneling recombination of trapped electrons and holes (5). The temperature variation of the exponent m is given in detail in Fig. 2b. The range where m>l corresponds with the temperature of motion of the holes (Vkcenter). Vk centers consist on C12- molecules along
Eu+). These electron-hole pairs are
produced after irradiation with photons of energy hr,> 7 eV; this value being far above the energy of the photons used in this work (< 3.8 eV). Nevertheless, this energy could be enough if a two photon process were involved in the damage sequence. In order to ascertain this possibility the dependence of the phosphorescence intensity with the excitation intensity is given in Fig. 3. showing a clear linear dependence, which discards a two-photon origin in the damage process. Therefore, the phosphorescence observed is attributed to a photolysis localized at the Eu precipitates followed by a recombination procedure involving tunneling recombination between electrons and Vk -centers which regenerate the per71 feet lattice and produce simultaneouly excited EuLT centers.
EXCITATION
INTENSITY,
100 I (%
0
5000
I
I2
10000 I”/. I4
Fig. 3. Dependence of the emission intensity vs. the pumping power of the N2 laser.
ACKNOWLEDGMENTS Financial support by the Comision Asesora de Investigation Cientifica y Tecnica (C.A.I.C. y T.) is gratefully acknowledged.
222 REFERENCES 1. L.F. Mollenauer, Opt. Lett. 5, 118 (1980). 2. J.B. Wolfenstine and T.G. Stoebe, "Laser induced damage in optical materials" National Bureau of Standards (1977). 3. F. A&lo-Lopez, F.J. Lopez and P. Jaque, Cryst. Latt. Def. and Amorph. Mat. 9, 227 (1982). 4. F.3. Lopez, H. Murrieta S., J. Hernandez A. and J. Rubio O., Phys. Rev. G, 6428 (1980). 5. C.J. Delbecq, Y. Toyozawa and P.H. Yuster, Phys. Rev. B9, 4497 (1974).