- Email: [email protected]
Luminescence excitation of barium fluorohalides by using synchrotron radiation
Nuclear Instruments and Methods in Physics Research A282 (1989) 607-609 North-Holland, Amsterdam
607
LUMINESCENCE EXCITATION OF BARIUM FLUOROHALIDES BY USING SYNCHROTRON RADIATION V.V . MIKHAILIN Moscow State University, Moscow 117234, USSR
M.A . TEREKHIN
I V. Kurchatov Institute of Atomic Energy, Moscow 123182, USSR
Luminescence excitation spectra of barium fluorohalides are studied using synchrotron radiation of the Siberia-1 electron storage ring m the energy range from 4 to 35 eV . The energy position of edge and core exciton formation as well as that of the electronic excitation multiplication threshold have been established. 1. Introduction Luminophors on the basis of fluorohalides, in particular BaFCI-Eu and BaFBr-Eu, have been studied intensely for 15 years [1-3] . Still more new properties are discovered, suggesting that these luminophors are of considerable interest both from the research point of view and that of applications . In particular, they found their practical application to the novel image-plate technique [4,5]. 2. Experimental procedure We studied the luminescence excitation spectra (LES) of barium fluorochlorides (BaF2-BaC1 2) and fluorobromtdes (BaF2-BaBr2), both Eu-activated (2 mol%) and without doping . In the case of undoped phosphors, the intrinsic radiation due to annihilation of self-trapped excitons [6] was extracted. Samples were synthesized at the Moscow Scientific Research X-Ray Radiological Institute [7]. The LES investigations were made in the 4-35 eV range at the SR source, i.e . the 450 MeV electron storage ring at the IN . Kurchatov Institute of Atomic Energy [8]. The measurements were performed on the VUV spectroscopy station [9] comprising the vacuum SR extraction channel, a normal-incidence monochomator with a working range of 4-40 eV, a cryostat and an automated system of recording. 3. Results and discussion Fig. 1 presents the LESs of BaFCI and BaFBr both Eu e+-activated and undoped. In the case of the Eu-
activation, the LESs of both the compounds in the 4-5.5 eV range are characterized by a wide band with initiated splitting into two bands (being particularly noticeable in BaFCI-Eu) assigned to the 5d( 2eg )- and 5d( 2 t 2,)-states of the Eu z+ ion [10] . Moreover, a wide excitation band is observed with maxima at E'= 6 .7 eV for BaFCl-Eu and E'= 6.0 eV for BaFBr-Eu, which we believe to be due to Eu z+ ionization . The ionization of the Eu z+ center is verified experimentally by two facts. First, there is an E'-shift by 0.2 eV to the shortwave range in cooling down to T= 80 K, since the fundamental absorption edge (FAE) is shifted to the same direction, and, secondly, there is the presence of photoconductivity in the given crystals at E >_ E' [3]. In the LESS of unactivated phosphors near the FAE, the band connected with the valence-electron excitation is observed in the vicinity of anion vacancies (the a-band) (see figs . la, c) . Comparison of the LESs for the crystals under study with the absorption and reflection spectra in the range of 7-10 eV, measured in ref. [11], shows that the intensive absorption (reflection) peaks related to the I'-exciton formation correspond to pronounced minima in the LESS at 8.4 eV for BaFCI and a doublet (7 .4 and 7.9 eV) for BaFBr, appearing due to spin-orbit splitting of the valence band formed in the ground 4p-state of Br - (the so-called halogen doublet [11]). For BaFCI-Eu the halogen doublet is manifested in the LES by the sample cooling down to T= 80 K and appears as a result of the CI - 3p-state splitting. The minima in the LESS are partially due to the fact that no corrections for the reflection were introduced. That is also possible at large absorption coefficients K(E) (in the exciton range K= 10 5-10 6 cm - ' [11]) due to lessening of the light penetration depth into the crystal and strengthening of the part played by the near-surface V. MATERIALS SCIENCE
608
V. V Mikhadm, M.A
Terekhm / Luminescence excitation of barium fluorohahdes
0.3-
2 -
W Io Fig. 1. Luminescence excitation spectra : (a) undoped BaFCl; (b) BaFCI-Eu; (c) undoped BaFBr; (d) BaFBr-Eu. The relative (compared with that of sodium salicylate) quantum yield is plotted on the ordinate axis .
nonradiative recombinations . The dip attributed to the F-exciton formation is followed by a relatively
edge
smooth shape of the LES, modulated by the absorption coefficient and corresponding to the range of optical interband electron transitions. We hold it that the shoulder at E = 8.9 eV for BaFCl and at E = 8.3 eV for BaFBr in the LES is connected with the onset of the interband transitions . Thus, the forbidden gap width Eg at T= 298 K, determined by us from the LES, amounts to 8.9 eV for BaFCl and to 8.3 eV for BaFBr. At T= 100 K the FAE shaped by the F-exciton formation
is shifted by 0.2 eV in the short-wave direction. At the excitation quantum energy, - 16 eV
for
BaFCI and --- 15 eV for BaFBr, a steplike buildup of the luminescence efficiency is observed in the LES. We
interpret the dips observed at 16 .7, 17 .5, 18 .2 and 19 .4 eV for BaFCl and at 16 .8, 17 .5, 18 .1 and 19 .4 eV for BaFBr as the formation of the core-cation excitons (CE) of the
p5
s- and ps d-types, appearing as a result of the
Ba t+ ion 5p-shell excitation . The dips in the LES ap-
pear at the places of the CE formation, due to the same reasons as to the production of the edge Beginning from
E = 24
F-excitons .
eV, in the given systems the
quantum yield becomes almost proportional to E, i .e . the energy yield In
71
remains practically constant.
BaFz-enriched Eu-activated
reduced
EU Z+
fluorochlorides the
luminescence and strong buildup of Eu 3+
emission are displayed, as compared with those of fluorochlorides. The reduction of
EU Z+
radiation is accom-
believe that such a behaviour of the LES in this region
is due to the onset of electron-excitation multiplication (EEM), when a hot electron, as a result of scattering by
o,s _I
valence-band electrons, can generate a secondary exci-
ton or a secondary electron-hole pair. The fact that the EEM begins earlier than
(Eg + Ee,)
in the given com-
pounds (where E., is the edge exciton formation en-
0.3_l
I
.., J
ergy) is explained by the same reasons which are re-
sponsible for the FAE smearing. The much higher buildup of luminescence observed in the LES of BaFBr-Eu, as compared with that of BaFCI-Eu, is explained by the fact that in the second
case it is practically overlapped with the beginning of possible excitations of the Ba t+ -cation 5p-shell electrons of the basic material, which may, to a considerable extent, affect the luminescence effectiveness [12] . In particular, in analogy to the BaFZ crystal [13], we
E,ev 20
1 30
E; Fig. 2. Luminescence excitation spectra m BaF16 Cl o4 -Eu phosphor, measured through CC-15 (solid curve) and OC-11 (dotted curve) light filters. The relative (compared with that of sodium salicylate) quantum yield is plotted on the ordinate axis .
V. V Mikhaihn, M .A . Terek{un / Luminescence excitation of barium fluorohalides
partied by the gain of the intrinsic luminescence up to a level characterizing a nonactivated luminophor . Hence, one may conclude that the excess of BaF2 decreases the solubility or, in any case, the activation ability of twovalent Eu. As a result, the undoped luminophor luminescence starts to manifest itself or even dominate in this case . Fig. 2 presents the LES of Eu 3+ m the BaFi 6 Cl,, matrix (the dotted line) and the LES of the intrinsic radiation of the same luminophor (the solid line). Up to Eg the intrinsic radiation LESS display a band being due to excitation of valence electrons in the vicinity of anion vacancies. The LESS exhibit the steplike buildup of the Eu 3+-ion emission efficiency (see fig. 2) at E 3 15 eV from X19 = 3% at E = 13 eV to il q = 30% at E = 18 eV, where rl q is the luminescence quantum yield. We believe in the steplike buildup to the production threshold of the core-cation excitons (the minimal binding energy of 5p-Ba being Ec,,, =14.7 eV [14]) which effectively transfer the absorbed energy to the Eu 3+ centers. At E = Eg + Eco,e = 24 eV there is also a buildup which we interpret as that due to the electronexcitation multiplication by hot conductivity-band electron scattering on 5p-Ba2+ -state core-cation zone electrons. It is interesting to note that the LESS of the Eu 3+ centres in the cation-exciton production region (15-20 eV) is antibate to that of the intrinsic luminescence . This fact suggests that there are practically no nearsurface losses in the case of the core-cation exciton energy transfer to the Eu3+ centres in BaFi 6Cl 04 Eu . 4. Conclusion On the basis of studying the general behaviour of the LESs at T = 298 K, one may conclude that in the fundamental absorption region the crystal phosphors are characterized by the high efficiency of the energy
609
transfer from the basic substance of BaFCI or BaFBr to the Eu 2+ luminescence centers, separated by electronhole pairs, as well as by excitons . Finally, it should result in a high yield of activation luminescence in the X-ray region . Indeed, according to the data of ref. [151, in the X-ray excitation q amounts to 12% for BaFCI-Eu . It follows from our measurements that in the range of 20-30 eV it reaches 10% for BaFCI-Eu and 16% for BaFBr-Eu (without taking into account the losses due to the exciting radiation reflection and the emitted light reabsorption) . References
[21
[4] [5] [6]
[81 [9] [10] [111 [121 [131 [14] [151
A.L .N . Stevels and F. Pingault, Philips Res. Rep. (1975) 277. A.M . Gurvich, R.V . Katomina and N.P . Soshchm, Izv Akad . Nauk SSSR, Set. Fiz. 41 (1977) 1372 . K. Takahashi, K. Kohda and J. Miyahara, J. Lumin. 31/3 2 (1984) 266. M. Sonoda et al ., Radiology 148 (1983) 833. J. Miyahara et al ., Nucl. Instr. and Meth . A246 (1986) 572. M. Yuste and L. Taurel, Solid. State Commun . 17 (1975) 1435 . A.M . Gurvich et al., m: Luminescent Receivers and Ionizing Radiation Transducers (Nauka, Novosibirsk, 1985) p. 72 . A.N . Artemyev et al., Nucl . Instr. and Meth . A261 (1987) 18 . A.N . Belsky et al ., Nucl . Instr and Meth . A261 (1987) 85 . A.M . Gurvich et al ., Zh. Prikl. Spektrosk. 38 (1983) 765 . E. Nicklaus, Phys . Status Solidi A53 (1979) 217. A.N . Belsky et al ., Voprosy Atomnoy Nauki i Tekhmki, Ser. Obshch. i Yadern . Fiz. 4 (40) (1987) 63 . G.W . Rubloff, Phys . Rev, B5 (1972) 662 J.C. Fuggel and N. Martensson, J. Electron . Spectrosc . Relat. Phenom . 21 (1980) 275. A.M . Gurvich, Radiologa Diagnostica B20 (1979) 519
V. MATERIALS SCIENCE
607
LUMINESCENCE EXCITATION OF BARIUM FLUOROHALIDES BY USING SYNCHROTRON RADIATION V.V . MIKHAILIN Moscow State University, Moscow 117234, USSR
M.A . TEREKHIN
I V. Kurchatov Institute of Atomic Energy, Moscow 123182, USSR
Luminescence excitation spectra of barium fluorohalides are studied using synchrotron radiation of the Siberia-1 electron storage ring m the energy range from 4 to 35 eV . The energy position of edge and core exciton formation as well as that of the electronic excitation multiplication threshold have been established. 1. Introduction Luminophors on the basis of fluorohalides, in particular BaFCI-Eu and BaFBr-Eu, have been studied intensely for 15 years [1-3] . Still more new properties are discovered, suggesting that these luminophors are of considerable interest both from the research point of view and that of applications . In particular, they found their practical application to the novel image-plate technique [4,5]. 2. Experimental procedure We studied the luminescence excitation spectra (LES) of barium fluorochlorides (BaF2-BaC1 2) and fluorobromtdes (BaF2-BaBr2), both Eu-activated (2 mol%) and without doping . In the case of undoped phosphors, the intrinsic radiation due to annihilation of self-trapped excitons [6] was extracted. Samples were synthesized at the Moscow Scientific Research X-Ray Radiological Institute [7]. The LES investigations were made in the 4-35 eV range at the SR source, i.e . the 450 MeV electron storage ring at the IN . Kurchatov Institute of Atomic Energy [8]. The measurements were performed on the VUV spectroscopy station [9] comprising the vacuum SR extraction channel, a normal-incidence monochomator with a working range of 4-40 eV, a cryostat and an automated system of recording. 3. Results and discussion Fig. 1 presents the LESs of BaFCI and BaFBr both Eu e+-activated and undoped. In the case of the Eu-
activation, the LESs of both the compounds in the 4-5.5 eV range are characterized by a wide band with initiated splitting into two bands (being particularly noticeable in BaFCI-Eu) assigned to the 5d( 2eg )- and 5d( 2 t 2,)-states of the Eu z+ ion [10] . Moreover, a wide excitation band is observed with maxima at E'= 6 .7 eV for BaFCl-Eu and E'= 6.0 eV for BaFBr-Eu, which we believe to be due to Eu z+ ionization . The ionization of the Eu z+ center is verified experimentally by two facts. First, there is an E'-shift by 0.2 eV to the shortwave range in cooling down to T= 80 K, since the fundamental absorption edge (FAE) is shifted to the same direction, and, secondly, there is the presence of photoconductivity in the given crystals at E >_ E' [3]. In the LESS of unactivated phosphors near the FAE, the band connected with the valence-electron excitation is observed in the vicinity of anion vacancies (the a-band) (see figs . la, c) . Comparison of the LESs for the crystals under study with the absorption and reflection spectra in the range of 7-10 eV, measured in ref. [11], shows that the intensive absorption (reflection) peaks related to the I'-exciton formation correspond to pronounced minima in the LESS at 8.4 eV for BaFCI and a doublet (7 .4 and 7.9 eV) for BaFBr, appearing due to spin-orbit splitting of the valence band formed in the ground 4p-state of Br - (the so-called halogen doublet [11]). For BaFCI-Eu the halogen doublet is manifested in the LES by the sample cooling down to T= 80 K and appears as a result of the CI - 3p-state splitting. The minima in the LESS are partially due to the fact that no corrections for the reflection were introduced. That is also possible at large absorption coefficients K(E) (in the exciton range K= 10 5-10 6 cm - ' [11]) due to lessening of the light penetration depth into the crystal and strengthening of the part played by the near-surface V. MATERIALS SCIENCE
608
V. V Mikhadm, M.A
Terekhm / Luminescence excitation of barium fluorohahdes
0.3-
2 -
W Io Fig. 1. Luminescence excitation spectra : (a) undoped BaFCl; (b) BaFCI-Eu; (c) undoped BaFBr; (d) BaFBr-Eu. The relative (compared with that of sodium salicylate) quantum yield is plotted on the ordinate axis .
nonradiative recombinations . The dip attributed to the F-exciton formation is followed by a relatively
edge
smooth shape of the LES, modulated by the absorption coefficient and corresponding to the range of optical interband electron transitions. We hold it that the shoulder at E = 8.9 eV for BaFCl and at E = 8.3 eV for BaFBr in the LES is connected with the onset of the interband transitions . Thus, the forbidden gap width Eg at T= 298 K, determined by us from the LES, amounts to 8.9 eV for BaFCl and to 8.3 eV for BaFBr. At T= 100 K the FAE shaped by the F-exciton formation
is shifted by 0.2 eV in the short-wave direction. At the excitation quantum energy, - 16 eV
for
BaFCI and --- 15 eV for BaFBr, a steplike buildup of the luminescence efficiency is observed in the LES. We
interpret the dips observed at 16 .7, 17 .5, 18 .2 and 19 .4 eV for BaFCl and at 16 .8, 17 .5, 18 .1 and 19 .4 eV for BaFBr as the formation of the core-cation excitons (CE) of the
p5
s- and ps d-types, appearing as a result of the
Ba t+ ion 5p-shell excitation . The dips in the LES ap-
pear at the places of the CE formation, due to the same reasons as to the production of the edge Beginning from
E = 24
F-excitons .
eV, in the given systems the
quantum yield becomes almost proportional to E, i .e . the energy yield In
71
remains practically constant.
BaFz-enriched Eu-activated
reduced
EU Z+
fluorochlorides the
luminescence and strong buildup of Eu 3+
emission are displayed, as compared with those of fluorochlorides. The reduction of
EU Z+
radiation is accom-
believe that such a behaviour of the LES in this region
is due to the onset of electron-excitation multiplication (EEM), when a hot electron, as a result of scattering by
o,s _I
valence-band electrons, can generate a secondary exci-
ton or a secondary electron-hole pair. The fact that the EEM begins earlier than
(Eg + Ee,)
in the given com-
pounds (where E., is the edge exciton formation en-
0.3_l
I
.., J
ergy) is explained by the same reasons which are re-
sponsible for the FAE smearing. The much higher buildup of luminescence observed in the LES of BaFBr-Eu, as compared with that of BaFCI-Eu, is explained by the fact that in the second
case it is practically overlapped with the beginning of possible excitations of the Ba t+ -cation 5p-shell electrons of the basic material, which may, to a considerable extent, affect the luminescence effectiveness [12] . In particular, in analogy to the BaFZ crystal [13], we
E,ev 20
1 30
E; Fig. 2. Luminescence excitation spectra m BaF16 Cl o4 -Eu phosphor, measured through CC-15 (solid curve) and OC-11 (dotted curve) light filters. The relative (compared with that of sodium salicylate) quantum yield is plotted on the ordinate axis .
V. V Mikhaihn, M .A . Terek{un / Luminescence excitation of barium fluorohalides
partied by the gain of the intrinsic luminescence up to a level characterizing a nonactivated luminophor . Hence, one may conclude that the excess of BaF2 decreases the solubility or, in any case, the activation ability of twovalent Eu. As a result, the undoped luminophor luminescence starts to manifest itself or even dominate in this case . Fig. 2 presents the LES of Eu 3+ m the BaFi 6 Cl,, matrix (the dotted line) and the LES of the intrinsic radiation of the same luminophor (the solid line). Up to Eg the intrinsic radiation LESS display a band being due to excitation of valence electrons in the vicinity of anion vacancies. The LESS exhibit the steplike buildup of the Eu 3+-ion emission efficiency (see fig. 2) at E 3 15 eV from X19 = 3% at E = 13 eV to il q = 30% at E = 18 eV, where rl q is the luminescence quantum yield. We believe in the steplike buildup to the production threshold of the core-cation excitons (the minimal binding energy of 5p-Ba being Ec,,, =14.7 eV [14]) which effectively transfer the absorbed energy to the Eu 3+ centers. At E = Eg + Eco,e = 24 eV there is also a buildup which we interpret as that due to the electronexcitation multiplication by hot conductivity-band electron scattering on 5p-Ba2+ -state core-cation zone electrons. It is interesting to note that the LESS of the Eu 3+ centres in the cation-exciton production region (15-20 eV) is antibate to that of the intrinsic luminescence . This fact suggests that there are practically no nearsurface losses in the case of the core-cation exciton energy transfer to the Eu3+ centres in BaFi 6Cl 04 Eu . 4. Conclusion On the basis of studying the general behaviour of the LESs at T = 298 K, one may conclude that in the fundamental absorption region the crystal phosphors are characterized by the high efficiency of the energy
609
transfer from the basic substance of BaFCI or BaFBr to the Eu 2+ luminescence centers, separated by electronhole pairs, as well as by excitons . Finally, it should result in a high yield of activation luminescence in the X-ray region . Indeed, according to the data of ref. [151, in the X-ray excitation q amounts to 12% for BaFCI-Eu . It follows from our measurements that in the range of 20-30 eV it reaches 10% for BaFCI-Eu and 16% for BaFBr-Eu (without taking into account the losses due to the exciting radiation reflection and the emitted light reabsorption) . References
[21
[4] [5] [6]
[81 [9] [10] [111 [121 [131 [14] [151
A.L .N . Stevels and F. Pingault, Philips Res. Rep. (1975) 277. A.M . Gurvich, R.V . Katomina and N.P . Soshchm, Izv Akad . Nauk SSSR, Set. Fiz. 41 (1977) 1372 . K. Takahashi, K. Kohda and J. Miyahara, J. Lumin. 31/3 2 (1984) 266. M. Sonoda et al ., Radiology 148 (1983) 833. J. Miyahara et al ., Nucl. Instr. and Meth . A246 (1986) 572. M. Yuste and L. Taurel, Solid. State Commun . 17 (1975) 1435 . A.M . Gurvich et al., m: Luminescent Receivers and Ionizing Radiation Transducers (Nauka, Novosibirsk, 1985) p. 72 . A.N . Artemyev et al., Nucl . Instr. and Meth . A261 (1987) 18 . A.N . Belsky et al ., Nucl . Instr and Meth . A261 (1987) 85 . A.M . Gurvich et al ., Zh. Prikl. Spektrosk. 38 (1983) 765 . E. Nicklaus, Phys . Status Solidi A53 (1979) 217. A.N . Belsky et al ., Voprosy Atomnoy Nauki i Tekhmki, Ser. Obshch. i Yadern . Fiz. 4 (40) (1987) 63 . G.W . Rubloff, Phys . Rev, B5 (1972) 662 J.C. Fuggel and N. Martensson, J. Electron . Spectrosc . Relat. Phenom . 21 (1980) 275. A.M . Gurvich, Radiologa Diagnostica B20 (1979) 519
V. MATERIALS SCIENCE