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Emission properties of Ta-centres in KCl
Radiation Measurements 38 (2004) 731 – 734 www.elsevier.com/locate/radmeas
Emission properties of Ta -centres in KCl V. Topaa , T. Tsuboib , S. Polosanc;∗ a Center
for Advanced Studies in Physics of the Romanian Academy, St 13 Septembrie., No 13, Bucharest, Romania b Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 603-8555, Japan c National Institute of Materials Physics, P.O. Box MG-7, Bucharest R-76900, Romania Received 27 October 2003; received in revised form 6 January 2004; accepted 7 January 2004
Abstract Electrolytic colouration at about 600◦ C and 350 V=cm, for KCl crystal containing Pb2+ and for KCl : Pb2+ crystals co-doped with Li+ ; Na+ and Rb+ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0:86 eV emission band except for the Li+ -co-doped crystal which gives rise to a 0:80 eV emission band. These absorption bands are due to the same Ta -centre related to Pb− . The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres. c 2004 Elsevier Ltd. All rights reserved. Keywords: Emission spectra; Electrolytic coloring; Heavy metals negative ions; Ta -centres
1. Introduction The electrolytic colouration of alkali halide crystals containing heavy metal ions such as Cu+ and Ag+ leads to the formation of negative metal ions such as Cu− and Ag− (Topa 1967; Topa et al., 1972; Tsuboi et al., 2000). When the KCl crystals with Pb2+ ions are electrolytically coloured using a pointed cathode and a >at anode, negative lead ions (such as Pb− ) centres, called Ta -centres, are formed (Velicescu and Topa 1973; Topa et al., 1992). This paper studies the spectroscopic properties of Ta -centres in electrolytically coloured KCl : Pb2+ crystal and KCl : Pb2+ crystals co-doped with Na+ ; Li+ and Rb+ ions. Special attention was paid to: (1) the near-infrared emission, and (2) the in>uence of F-centres as well as co-doped alkali ions Na+ ; Li+ and Rb+ on the Ta -centres. 2. Experimental details Pure raw powder of KCl was heated in CCl4 in the melt in order to remove the OH− complexes. For ∗ Corresponding author. Optical and Spectroscopy, National Institute for Materials Physics, Fizicienilor St., no. 105 b BucharestMagurele 77900, Romania. Tel.: +40-214-930-195; fax: +40-2149-302-67. E-mail address: [email protected] (S. Polosan).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.021
crystal growth of doped KCl crystal, KCl powder containing 0:018 mol% PbCl2 salt was prepared, while for growth of co-doped crystal, KCl powder containing 0:93 mol% of NaCl, 0:96 mol% LiCl, or 0:5 mol% RbCl salt was prepared. Here doped crystal means KCl crystal doped with Pb2+ ions but without impurity alkali ions. Single crystals were grown by the Kyropoulos method in a dried nitrogen atmosphere. The electrolytic colouration was performed at about 600◦ C and 350 V=cm. The optical absorption and emission measurements have been described previously (Tsuboi et al., 2002). The intensity of emission was detected using a Ge or PbS photocell, and the emission spectra were recorded with an Advantest Q8381 a spectral analyzer.
3. Results and discussions When electrolytically coloured co-doped and doped KCl : Pb2+ crystals were excited by the 514:5 nm Ar + laser at 296 K, a broad emission band was observed at about 0:86 eV (1440 nm) for all crystals except KCl co-doped with Li+ ions. In the Li+ -co-doped crystal, the peak was at about 0:80 eV (1550 nm) as shown in Fig. 1. The same emission spectrum was obtained by excitation with 266, 355, 366, 488.0 and 632:8 nm laser lines. Unlike Na+ and Rb+ ; Li+ locates at an oG-centre position (Tsuboi et al.,
732
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Fig. 1. Emission spectra of electrolytically coloured doped KCl : Pb2+ crystal and KCl : Pb2+ crystals co-doped with Na+ ; Li+ and Rb+ ions at 296 K.
Fig. 3. Emission spectra of the electrolytically coloured doped KCl : Pb2+ crystal at various temperatures. Excitation was made with the 366 nm laser.
excited state, followed by the radiative transition from the relaxed excited state to the ground state. We irradiated an electrolytically coloured doped KCl : Pb2+ crystal with the 514:5 nm laser of 1 W at 296 K for 1 h. No change was observed in both the absorption spectrum and intensity of the 0:86 eV emission after irradiation. This indicates that the Ta -centre is optically stable and it is not destroyed by photo-ionization. Fig. 3 shows the 0:86 eV emission spectra of electrolytically coloured doped KCl : Pb2+ crystal at various temperatures. The emission band becomes narrow, shifts to a low energy side, and becomes a single Gaussian with the decreasing temperature. The half-width of the 0:86 eV emission band in doped KCl was plotted as a function of temperature. It was found that the temperature dependence is described by the following equation: Fig. 2. Absorption and excitation spectra of the electrolytically coloured KCl crystal doped with Pb2+ ions at 77 K. Excitation spectrum was recorded for the 0:86 eV emission.
2000), suggesting that Li+ can approach the negative Pb− ions more easily than Na+ and Rb+ . It seems that this in>uences the emission property of Li+ -co-doped crystal. Fig. 2 shows the absorption spectrum of an electrolytically coloured doped crystal and the excitation spectrum of the 0:86 eV emission. The measurement was performed at 77 K. The excitation spectrum was obtained using a 500 W Hg lamp and is quite similar to the absorption spectrum, suggesting that all the absorption bands give rise to the same 0:86 eV emission. This means that :(1) the Cve absorption bands shown in Fig. 2 are caused by the same centre, and (2) after the optical excitation into these bands the excited electrons relax from the excited states to the same lowest
H (T ) = H (0)coth1=2 (h!=4kT );
(1)
where H (T ) is the half-width at temperature T (K) and H (0) is the half-width at 0 K, and h; k and ! are the Planck constant, Boltzmann constant and phonon angular temporal frequency, respectively. The best Ct of experimental points to Eq. (1) was obtained with H (0) = 0:0731 eV and ! = 2:26 × 1013 s−1 . On the other hand, ! = 2:96 × 1013 s−1 was obtained from the temperature dependence of the half-width of the F absorption band in KCl. These close values indicate that phonons of almost the same frequency are responsible for both the Ta - and F-centres. Fig. 4 shows the dependence of the 0:86 eV emission intensity on the pump power of the 514:5 nm laser in doped crystal. The emission intensity increases linearly with the excitation intensity. This suggests that the emission is due to one-photon excitation and is not due to either two-photon absorption in the Ta -centre or energy transfer by cross-relaxation between two Ta -centres.
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Fig. 4. The 0:86 eV emission intensity Pemission of the electrolytically coloured doped KCl : Pb2+ crystal, which is plotted versus the 514:5 nm Ar + laser power Plaser . The excitation was performed at 296 K.
During the electrolytic colouration the F-centres are created because electrons injected from a point cathode are trapped by negative-ion vacancies in KCl crystal. The absorption band of the F-centre has a peak at about 560 nm at room temperature. This band is superimposed on the absorption bands of the Ta -centres. To avoid the overlap of the F band, after colouration the polarity of the electrodes was reversed, resulting in destruction of the F-centres (Topa 1967; Topa et al., 1972). The results of Figs. 1–4 were obtained in crystals after such an F-centre destruction. We examined the diGerence of optical properties of Ta -centres between the cases of coexistence and non-coexistence of F-centres. Immediately after we performed the electrolytic colouration for a doped KCl : Pb2+ crystal of 20 mm length at 600◦ C, we reversed the polarity of the electrodes, keeping the crystal temperature at 600◦ C. It was observed that the colour of the crystal was gradually changed from the anode side toward the cathode, indicating the destruction of F-centres. We stopped applying the voltage when the colour change was achieved for half of the crystal (i.e. region of 10 mm from the anode). In this way we obtained a crystal for which half contains no F-centres, but the other half still contains F-centres. Both parts (i.e. parts with and without F-centres), however, contain the same concentration of Ta -centres. We excited these parts with the 514:5 nm laser and observed that the 0:86 eV emission intensity is higher in the part with F-centres than in the part without F-centres, as shown in Fig. 5. This is understood as follows. The 514:5 nm laser can excite not only the Ta -centres but also F-centres in the part with F-centres because the F and Ta absorption bands extend to the 514:5 nm region. The F-centres release electrons by the excitation. The released electrons are
733
Fig. 5. Emission spectra of electrolytically coloured doped KCl : Pb2+ crystals with and without F-centres. The optical excitation was made using the 514:5 nm laser.
trapped by Pb2+ ; Pb+ , and/or Pb◦ (Pb+ and Pb◦ are also created during the colouration), leading to the formation of Ta -centres. The increase of the number of Ta -centres results in an enhancement of the 0:86 eV emission. 4. Summary Absorption bands are observed in electrolytically coloured KCl : Pb2+ crystals in the visible-UV region. Excitation into these bands gives rise to the same broad 0:86 eV emission band, indicating that these bands are due to the same Ta -centre, related to Pb− . The electrolytically coloured KCl : Pb2+ crystals co-doped with Na+ and Rb+ ions show the same result, but the emission band of Li+ -co-doped crystal is a slightly shifted to low energy because of the oG-centred Li+ ions. It is observed that the infrared emission intensity is higher in the crystal with F-centres than in that without F-centres, suggesting that electrons released from F-centres by optical excitation give rise to an increase of Ta -centres. Acknowledgements The authors thank the CERES Program and the Romanian Academy for Cnancial support.
References Topa, V., 1967. Electrolytical coloured Ag+ doped alkali halides. Rev. Roum. Phys. 12, 781–784.
734
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Topa, V., Iliescu, B., Mateescu, I., 1972. Fotoluminiscentia T-tentrov b sciolocinih galoidnih K-kristallov. Izv. AN SSSR Sev. Fiz. 37, 591–594. Topa, V., Briat, B., Apostol, E., 1992. Magnetic circular dichroism and optical study of new Pb− complex centres in KCl. Proceedings of the XII International Conference of Defects in Insulating Materials, Nordkirchen, Germany, pp. 565–567. Tsuboi, T., Polosan, S., Topa, V., 2000. The Pb2+ (Li) and Pb2+ (Na) centers in KCl crystals. Phys. Stat. Sol. 217, 975–980.
Tsuboi, T., Polosan, S., Topa, V., 2002. Negatively charged Pb− ion produced by electrolytical colouration of KCl crystals containing Li+ ; Na+ and Rb+ ions. J. Phys.: Condens. Matter 14, 7265–7272. Velicescu, B., Topa, V., 1973. Coloration electrolitique des cristaux d’halogenures alcalines dopees au Pb2+ ; Sn2+ et Ge2+ . Phys. Stat. Sol. B 55, 793–799.
Emission properties of Ta -centres in KCl V. Topaa , T. Tsuboib , S. Polosanc;∗ a Center
for Advanced Studies in Physics of the Romanian Academy, St 13 Septembrie., No 13, Bucharest, Romania b Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 603-8555, Japan c National Institute of Materials Physics, P.O. Box MG-7, Bucharest R-76900, Romania Received 27 October 2003; received in revised form 6 January 2004; accepted 7 January 2004
Abstract Electrolytic colouration at about 600◦ C and 350 V=cm, for KCl crystal containing Pb2+ and for KCl : Pb2+ crystals co-doped with Li+ ; Na+ and Rb+ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0:86 eV emission band except for the Li+ -co-doped crystal which gives rise to a 0:80 eV emission band. These absorption bands are due to the same Ta -centre related to Pb− . The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres. c 2004 Elsevier Ltd. All rights reserved. Keywords: Emission spectra; Electrolytic coloring; Heavy metals negative ions; Ta -centres
1. Introduction The electrolytic colouration of alkali halide crystals containing heavy metal ions such as Cu+ and Ag+ leads to the formation of negative metal ions such as Cu− and Ag− (Topa 1967; Topa et al., 1972; Tsuboi et al., 2000). When the KCl crystals with Pb2+ ions are electrolytically coloured using a pointed cathode and a >at anode, negative lead ions (such as Pb− ) centres, called Ta -centres, are formed (Velicescu and Topa 1973; Topa et al., 1992). This paper studies the spectroscopic properties of Ta -centres in electrolytically coloured KCl : Pb2+ crystal and KCl : Pb2+ crystals co-doped with Na+ ; Li+ and Rb+ ions. Special attention was paid to: (1) the near-infrared emission, and (2) the in>uence of F-centres as well as co-doped alkali ions Na+ ; Li+ and Rb+ on the Ta -centres. 2. Experimental details Pure raw powder of KCl was heated in CCl4 in the melt in order to remove the OH− complexes. For ∗ Corresponding author. Optical and Spectroscopy, National Institute for Materials Physics, Fizicienilor St., no. 105 b BucharestMagurele 77900, Romania. Tel.: +40-214-930-195; fax: +40-2149-302-67. E-mail address: [email protected] (S. Polosan).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.021
crystal growth of doped KCl crystal, KCl powder containing 0:018 mol% PbCl2 salt was prepared, while for growth of co-doped crystal, KCl powder containing 0:93 mol% of NaCl, 0:96 mol% LiCl, or 0:5 mol% RbCl salt was prepared. Here doped crystal means KCl crystal doped with Pb2+ ions but without impurity alkali ions. Single crystals were grown by the Kyropoulos method in a dried nitrogen atmosphere. The electrolytic colouration was performed at about 600◦ C and 350 V=cm. The optical absorption and emission measurements have been described previously (Tsuboi et al., 2002). The intensity of emission was detected using a Ge or PbS photocell, and the emission spectra were recorded with an Advantest Q8381 a spectral analyzer.
3. Results and discussions When electrolytically coloured co-doped and doped KCl : Pb2+ crystals were excited by the 514:5 nm Ar + laser at 296 K, a broad emission band was observed at about 0:86 eV (1440 nm) for all crystals except KCl co-doped with Li+ ions. In the Li+ -co-doped crystal, the peak was at about 0:80 eV (1550 nm) as shown in Fig. 1. The same emission spectrum was obtained by excitation with 266, 355, 366, 488.0 and 632:8 nm laser lines. Unlike Na+ and Rb+ ; Li+ locates at an oG-centre position (Tsuboi et al.,
732
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Fig. 1. Emission spectra of electrolytically coloured doped KCl : Pb2+ crystal and KCl : Pb2+ crystals co-doped with Na+ ; Li+ and Rb+ ions at 296 K.
Fig. 3. Emission spectra of the electrolytically coloured doped KCl : Pb2+ crystal at various temperatures. Excitation was made with the 366 nm laser.
excited state, followed by the radiative transition from the relaxed excited state to the ground state. We irradiated an electrolytically coloured doped KCl : Pb2+ crystal with the 514:5 nm laser of 1 W at 296 K for 1 h. No change was observed in both the absorption spectrum and intensity of the 0:86 eV emission after irradiation. This indicates that the Ta -centre is optically stable and it is not destroyed by photo-ionization. Fig. 3 shows the 0:86 eV emission spectra of electrolytically coloured doped KCl : Pb2+ crystal at various temperatures. The emission band becomes narrow, shifts to a low energy side, and becomes a single Gaussian with the decreasing temperature. The half-width of the 0:86 eV emission band in doped KCl was plotted as a function of temperature. It was found that the temperature dependence is described by the following equation: Fig. 2. Absorption and excitation spectra of the electrolytically coloured KCl crystal doped with Pb2+ ions at 77 K. Excitation spectrum was recorded for the 0:86 eV emission.
2000), suggesting that Li+ can approach the negative Pb− ions more easily than Na+ and Rb+ . It seems that this in>uences the emission property of Li+ -co-doped crystal. Fig. 2 shows the absorption spectrum of an electrolytically coloured doped crystal and the excitation spectrum of the 0:86 eV emission. The measurement was performed at 77 K. The excitation spectrum was obtained using a 500 W Hg lamp and is quite similar to the absorption spectrum, suggesting that all the absorption bands give rise to the same 0:86 eV emission. This means that :(1) the Cve absorption bands shown in Fig. 2 are caused by the same centre, and (2) after the optical excitation into these bands the excited electrons relax from the excited states to the same lowest
H (T ) = H (0)coth1=2 (h!=4kT );
(1)
where H (T ) is the half-width at temperature T (K) and H (0) is the half-width at 0 K, and h; k and ! are the Planck constant, Boltzmann constant and phonon angular temporal frequency, respectively. The best Ct of experimental points to Eq. (1) was obtained with H (0) = 0:0731 eV and ! = 2:26 × 1013 s−1 . On the other hand, ! = 2:96 × 1013 s−1 was obtained from the temperature dependence of the half-width of the F absorption band in KCl. These close values indicate that phonons of almost the same frequency are responsible for both the Ta - and F-centres. Fig. 4 shows the dependence of the 0:86 eV emission intensity on the pump power of the 514:5 nm laser in doped crystal. The emission intensity increases linearly with the excitation intensity. This suggests that the emission is due to one-photon excitation and is not due to either two-photon absorption in the Ta -centre or energy transfer by cross-relaxation between two Ta -centres.
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Fig. 4. The 0:86 eV emission intensity Pemission of the electrolytically coloured doped KCl : Pb2+ crystal, which is plotted versus the 514:5 nm Ar + laser power Plaser . The excitation was performed at 296 K.
During the electrolytic colouration the F-centres are created because electrons injected from a point cathode are trapped by negative-ion vacancies in KCl crystal. The absorption band of the F-centre has a peak at about 560 nm at room temperature. This band is superimposed on the absorption bands of the Ta -centres. To avoid the overlap of the F band, after colouration the polarity of the electrodes was reversed, resulting in destruction of the F-centres (Topa 1967; Topa et al., 1972). The results of Figs. 1–4 were obtained in crystals after such an F-centre destruction. We examined the diGerence of optical properties of Ta -centres between the cases of coexistence and non-coexistence of F-centres. Immediately after we performed the electrolytic colouration for a doped KCl : Pb2+ crystal of 20 mm length at 600◦ C, we reversed the polarity of the electrodes, keeping the crystal temperature at 600◦ C. It was observed that the colour of the crystal was gradually changed from the anode side toward the cathode, indicating the destruction of F-centres. We stopped applying the voltage when the colour change was achieved for half of the crystal (i.e. region of 10 mm from the anode). In this way we obtained a crystal for which half contains no F-centres, but the other half still contains F-centres. Both parts (i.e. parts with and without F-centres), however, contain the same concentration of Ta -centres. We excited these parts with the 514:5 nm laser and observed that the 0:86 eV emission intensity is higher in the part with F-centres than in the part without F-centres, as shown in Fig. 5. This is understood as follows. The 514:5 nm laser can excite not only the Ta -centres but also F-centres in the part with F-centres because the F and Ta absorption bands extend to the 514:5 nm region. The F-centres release electrons by the excitation. The released electrons are
733
Fig. 5. Emission spectra of electrolytically coloured doped KCl : Pb2+ crystals with and without F-centres. The optical excitation was made using the 514:5 nm laser.
trapped by Pb2+ ; Pb+ , and/or Pb◦ (Pb+ and Pb◦ are also created during the colouration), leading to the formation of Ta -centres. The increase of the number of Ta -centres results in an enhancement of the 0:86 eV emission. 4. Summary Absorption bands are observed in electrolytically coloured KCl : Pb2+ crystals in the visible-UV region. Excitation into these bands gives rise to the same broad 0:86 eV emission band, indicating that these bands are due to the same Ta -centre, related to Pb− . The electrolytically coloured KCl : Pb2+ crystals co-doped with Na+ and Rb+ ions show the same result, but the emission band of Li+ -co-doped crystal is a slightly shifted to low energy because of the oG-centred Li+ ions. It is observed that the infrared emission intensity is higher in the crystal with F-centres than in that without F-centres, suggesting that electrons released from F-centres by optical excitation give rise to an increase of Ta -centres. Acknowledgements The authors thank the CERES Program and the Romanian Academy for Cnancial support.
References Topa, V., 1967. Electrolytical coloured Ag+ doped alkali halides. Rev. Roum. Phys. 12, 781–784.
734
V. Topa et al. / Radiation Measurements 38 (2004) 731 – 734
Topa, V., Iliescu, B., Mateescu, I., 1972. Fotoluminiscentia T-tentrov b sciolocinih galoidnih K-kristallov. Izv. AN SSSR Sev. Fiz. 37, 591–594. Topa, V., Briat, B., Apostol, E., 1992. Magnetic circular dichroism and optical study of new Pb− complex centres in KCl. Proceedings of the XII International Conference of Defects in Insulating Materials, Nordkirchen, Germany, pp. 565–567. Tsuboi, T., Polosan, S., Topa, V., 2000. The Pb2+ (Li) and Pb2+ (Na) centers in KCl crystals. Phys. Stat. Sol. 217, 975–980.
Tsuboi, T., Polosan, S., Topa, V., 2002. Negatively charged Pb− ion produced by electrolytical colouration of KCl crystals containing Li+ ; Na+ and Rb+ ions. J. Phys.: Condens. Matter 14, 7265–7272. Velicescu, B., Topa, V., 1973. Coloration electrolitique des cristaux d’halogenures alcalines dopees au Pb2+ ; Sn2+ et Ge2+ . Phys. Stat. Sol. B 55, 793–799.