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Thermoluminescence and fluorescence emission of CsCl: Sm crystals
JOURNAL OF
LUMINESCENCE Journal of Luminescence 63 (1995) 137-142
ELSEWIER
Thermoluminescence
and fluorescence emission of CsCl : Sm crystals
J.K. Radhakrishnan, Department
of Physics, Bharathiar
S. Selvasekarapandian* University, Coimbatore-641046,
India
Received 8 February 1994; revised 21 July 1994; accepted 26 July 1994
Abstract Samarium enters C&l crystals at very low concentrations and is present partially in both divalent and trivalent states. The TL emission of C&l: Sm crystals is due to the F center electron - V center recombination and the excited Sm’+ ions. The fluoresence emission shows the presence of OH- ions in these crystals.
1. Introduction
Next to europium, samarium is the most predominantly studied divalent rare-earth impurity in ionic crystal lattices [l]. The spectroscopic properties of divalent samarium have been a subject of numerous investigations in alkali halide [2-S] crystals. Some researchers have observed Z-center formation in divalent samarium-doped alkali halides [6,8], which often occurs when the impurity does not change in its valence state upon irradiation. On the other hand, Fong and coworkers report that Sm2+ ions act as electron-trapping impurities in KCl, and are reduced to the monovalent state upon y-irradiation [S]. In the present work, the results of optical absorption, glow curve, TL spectral emission and fluorescence measurements made on CsCl : Sm crystals are presented and discussed.
Rare Earths Ltd., Udhyogamandal) by weight were grown in laboratory at room temperature, by slow evaporation of its water solution. Whereas pure CsCl crystallizes into dendrites, samarium-doped crystals are cubic in shape. These crystals were annealed at 400°C for four hours and then quenched to room temperature. Colouration of the samples were carried out in a 6oCo y-ray chamber with a dose rate of 1.03 krad/min. Other experimental details of measurement of optical absorption, glow curve and TL emission spectra have been described elsewhere [9]. The fluorescence spectra were recorded in a Hitachi 650-10s Fluorescence Spectrophotometer with a Himamatzu R928F photomultiplier tube.
3. Results 2. Experimental
details
Single crystals of cesium chloride (Analar, BDH, Poole, England) containing 5% of SmC13 (Indian *Corresponding author. 0022-2313/95/$09.50 0 1995 SSDI 0022-2313(94)00051-4
The uv-visible absorption spectra of uncoloured CsCl:Sm crystals were found to be similar to that of pure CsCl crystals [9]. Also similar to pure CsCl, y-irradiation on CsCl:Sm crystal produces the characteristic F-band centering around 603 nm,
Elsevier Science B.V. All rights reserved
138
J.K. Radhakrishnan,
S. Selvasekurapandian
/Journal
and a broad band in the uv-region over the absorption edge between 220 and 260 nm. However, a small apparent shift of about 6 nm to the lower wavelength side could be detected in the maximum of the uv-band, when compared with that of the pure CsCl crystal [9]. Fig. 1 shows the glow curves of a CsCl : Sm crystal irradiated with -y-rays for 1, 5, 10 and 20 min, recorded at a heating rate of 60”C/min. Glow peaks are observed at 368, 383, 398 and 408 K. The inset displays the growth of glow peak intensity at these temperatures with the time of y-irradiation. The effect of F-bleaching for two and five minutes on the glow curve of a 20 min y-irradiated crystal is (Fig. 2) a decrease in the intensity of the glow peaks. The presence of a glow peak around 343 K becomes apparent in F-bleached samples. The TL emission spectra were recorded at all the glow peak positions of a CsCl:Sm crystal irra-
Temperature
Fig. 1. Glow curves of a CsCl: Sm crystal glow peaks with the time of irradiation.
irradiated
in y-rays
of Luminescence 63 (1995) 137-142
diated with y-rays for 10 min (Fig. 3). On account of low intensity, the resolution of the TL emission is poor for the 408 K glow peak. The emission spectra at all the glow peaks possess an intense band at 425 nm and a shoulder around 495 nm. The positions of these emissions do not change from peak to peak. Only the intensity relations are affected. But, in the emission spectra under 398 and 408 K glow peaks, in addition to the above-mentioned two bands another broad emission extending from 580 to 700 nm appears. Fig. 4 shows the excitation and emission spectra of a CsCl: Sm crystal. Excitation at 267 nm gives a broad emission band between 340 and 600 nm. Over the longer wavelength tail of the emission spectrum, two sharp line emissions at 620 and 630 nm, and a shallow broad band extending from 640 to 740 nm could be detected. Excitation corresponding to the emissions at 360 and 460 nm, is a sharp band at 267 nm. Excitation corresponding
(K)
for (a) 1 min, (b) 5 min, (c) 10 min and (d) 20 min. Inset: growth
of the
J.K. Radhakrishnan, S. Selvasekarapandian
/Journal
Temperature
qf Luminescence 63 (1995) 137-142
139
LK)
Fig. 2. Glow curves of a CsCI: Sm crystal irradiated in y-rays for (a) 20 min, (b) irradiated and (c) irradiated for 20 min followed by 5 min of F-bleaching.
for 20 min followed by 2 min of F-bleaching
61 I
a - 368K b-383K c - 398K d - 408K
3 9 x z P z
3
2
1
0
Fig. 3. Spectral 10 min.
resolution
of TL emission
at the different
glow
peak
positions
of a CsCl:Sm
crystal
irradiated
in y-rays
for
140
J.K. Radhakrishnan, S. Selvasekarapandian
Wavelength
/Journal
of Luminescence 63 (1995) 137-142
(nm)
Fig. 4. Fluorescence emission (excitation at 267 nm) and excitation (for emission at 360,460 and 680 nm) spectra of a CsCI:Sm crystal.
to the emission at 680 nm is a group of weak bands at 500,470,420,400 and 340 nm.
4. Discussion According to Przibram [lo], the trivalent form of the rare-earth impurity occurs, when an alkali halide crystal is grown from water solution composed of alkali halides and rare-earth trihalides. The trivalent rare-earth ions reduce to divalent state when these crystals are heated to temperatures above 200°C. These results have further been confirmed by Greuen et al. [l l] and Resfeld and Alasner [12]. In alkali metal halide crystals, the absorption spectrum of the divalent samarium ion [2-41 at 300 and 77 K, has been found to consist of a series of intense wide bands in the visible and ultraviolet regions. These have been associated with the allowed transitions of the Sm’ + ion, from the 4f6 shell to the levels of the 4f55d configuration. But, the uv-visible absorption spectra of the CsCl: Sm crystals here, were similar to that of pure CsCl
crystals. Even a considerable amount of thermal annealing at 400°C does not bring out the characteristic absorptions of the samarium impurity. But the addition of the SmCl, impurity has changed the growth habit of the CsCl crystals from dendritic form to cubic form. Also it is a proven fact [13] that, in order to change the habit of a growing crystal from dendritic to cubic form, the impurities must be incorporated in the growing crystal. Obviously, samarium impurity has entered the crystal, but in such low concentrations that it cannot be discovered by optical absorption. The small shift to the lower wavelength side, observed in the maximum of the V-band upon introducing samarium, may be due to the effect of the impurity. From Fig. 1 it could be observed that, in addition to the common glow peaks at 368, 383 and 398 K which are also present in y-irradiated undoped CsCl [9] crystals, a peak at 408 K is observed in the CsCl: Sm crystals. This glow peak at 408 K, observed in the samarium-doped crystals may be ascribed to the thermal decay of F-center traps in the neighbourhood of samarium impurity [ 143.
J.K. Radhakrishnan, S. Selvasekarapandian /Journal
The glow peak at 343 K detected in the F-bleached y-irradiated CsCl:Sm crystals is not to be confused with the formation of Zr center peak, as this peak has been observed as the prominent peak in X-irradiated CsCl [15] and CsCl: Sm crystals. The absence of the 343 K peak in y-irradiated crystals may be due to the slightly different LET of X and y rays [9], and its reappearance upon F-bleaching is probabily due to photostimulation [16]. The TL emission spectra of pure CsCl has two emission bands at 407 and 470 nm [9], and they have been attributed to the emissions resulting from the recombination of thermally detrapped F-center electrons with the V2 and V3 centers having absorption bands at 252 and 233 nm. Attention is drawn to the fact that the TL emission pattern of the CsCl: Sm crystals (Fig. 3), except for a small shift towards the higher wavelength region and the presence of the small additional band between 580 and 700 nm at the 398 and 408 K glow peaks, is similar to that of the pure CsCl. This shift to the higher wavelength side may be the effect of the impurity. The prominent TL spectral emissions at 425 and 495 nm are not characteristic of the samarium impurity. As in pure CsCl, these emission bands are due to the recombination of thermally detrapped F-center electrons with the V-type centers. The shift of the TL emission towards the higher wavelength (lower energy) can be accounted for by the small shift observed in the V-band (of approximately around 6 nm) towards lower wavelength (higher energy) side [17,18] in the impurity-doped samples, when compared with pure CsCl. The weak broad TL emission band observed between 580 and 700 nm may be due to the samarium impurity. Photoluminescence studies made on samarium-doped alkali halides [224] have shown that, when excited at the Sm2+ absorption bands, these crystals give luminescence emission due to radiative transitions from the levels of 4f5 5d configuration of the Sm2+ ions in the red and infrared regions of the spectrum. At room temperature [2,3] the luminescence emission consists of a single wide band (with maxima near 745 nm for NaCl, 765 nm for NaBr, 810 nm for NaI, 737 nm for KCl, 745 nm for KBr and 775 nm for KI). The broad weak TL emission band between 580 and 700 nm observed
qf’luminescence
63 (19951 137 -142
141
in the CsCl:Sm crystals may be attributed to the samarium ions in their divalent state. It is evident that samarium impurities are not in sufficient numbers so as to dominate the TL emission. Attention is drawn to the fact that in alkali halides a displacement of the Sm2+ emission band [224] to the shorter wavelength side is observed upon an increase in the lattice constant (from Na to K). This rule requires the Sm2’ emission band in CsCl to be at a wavelength position lowest among the alkali chlorides, as is the case observed here. The occurrence of the emission due to the divalent samarium ions along with the emission due to the F center electron ~ V center recombination can be explained by the following mechanism. Upon thermally annealing the y-irradiated crystal (i.e. during TL readout), the thermally activated F-center electrons recombine with the V-type centers. A part of this recombination energy is transferred nonradiatively to the neighbouring Sm2+ ions, raising them to their excited states. The excited Sm2 + ions upon their decay to the ground state give out their characteristic emission, observed between 580 and 700 nm. Some important informations with regard to interpretation of the TL process are given by the photoluminescence spectra of the CsCl: Sm crystals. The broad fluorescence emission band between 340 and 600 nm (Fig. 4) which has its excitation at 267 nm (and as well as an absorption band at 267 nm), is present in pure CsCl also, and has been attributed to the characteristic emission and absorption of OH- ions embedded in the lattice [16,19]. However, the remaining part of the emission spectrum starting from 620 nm could be observed only for the CsCl: Sm crystals. The emission spectra beyond 600 nm can be divided into two parts: (i) a line spectrum consisting of sharp line emissions at 620 and 630 nm, which are characteristic of Sm3’ ions 120-231 and (ii) a broad band emission between 640 and 740 nm which is due to the radiative transitions from the levels of the 4f55d configuration of the Srn” ions 12241. The presence of the characteristic emissions of both the Sm” and Sm 3 + ions in the fluorescence emission spectra of the thermally treated CsCl : Sm
142
J.K. Radhakrishnan,
S. Selvasekarapandian
crystals indicate that, the reduction of samarium ions from trivalent state to divalent state upon thermal annealing described by Przibram [lo] is incomplete here. The unirradiated crystal contains both Sm’+ and Sm3+ ions (as indicated by the fluorescence emission spectra). y-irradiation probably converts the remaining unreduced Sm3 + ions to Sm’ + state [20,21]. Upon TL readout, a little part of the thermally activated F-center electrons recombine with the Sm3 ’ ions, oxidizing them into Sm2 + state. This probably is the reason that the TL emission spectra show only the characteristic emission of the Sm2 + ions and none of the Sm3 + ions. 4. Conclusions Samarium impurity enters CsCl crystals at very low concentrations and is present in both divalent and trivalent states. y-irradiation, apart from creating F- and V-type centers, also converts the remaining unreduced Sm 3t ions into divalent state. The TL emission is the result of both F center electron - V center recombination and the excited Sm2+ ions. Acknowledgement One of the authors (J.K.R.) thanks CSIR, New Delhi for Senior Research Fellowship. References [l]
J. Rubio.
0, J. Phys. Chem. Solids. 52 (1991) 101.
/Journal
of Luminescence 63 (1995) 137-142
121V.E.
Karapetyan, B.I. Maksakov and P.P. Feofilov, Opt. Spectrosc. 14 (1963) 236. and P.P. Feofilov, Opt. Spectrosc. 16 c31A.A. Kaplyanski (1964) 144. M W.E. Bron and W.R. Heller, Phys. Rev. 136 (1964) A 1433. I51 F.K. Fong, J. A. Cape and E.Y. Wong, Phys. Rev. 151 (1966) 299. and B.V.R. Chowdari, Phys. Stat. Sol. (A) 161S. Radhakrishna 12 (1972) 557. 171A.J. Ramponi and J.C. Wright, Phys. Rev. B 35 (1987) 2413. 181K.N. Reddy, M.L. Roa and V.H. Haribabu, Phys. Stat. Sol. (A). 70 (1982) 335. and S. Selvasekarapandian, Phys. c91J.K. Radhakrishnan Stat. Sol. (A) 136 (1993) 229. Cl01K. Przibram, Nature (London) 139 (1937) 329. and R.D. McLanghlin, c111D.M. Gruen, J.G. Conway J. Chem. Phys. 25 (1956) 1102. Cl21R. Reisfeld and A. Alasner, J. Opt. Sot. Amer. 54 (1964) 331. Dendritic Crystallization, Consultants v31 D.D. Saratovkin, Bureau Inc., New York (1959) (Translated from Russian by J.E.S. Bradley). Cl41Y.V.G.S. Murti, K.R.N. Murthy and C. Ramasastry, J. Phys. C 4 (1971) 1606. and S. Selvasekarapandian, Cryst. Cl51J.K. Radhakrishnan Res. Tech. 27 (1992) K95. Ph.D. Thesis, Bharathiar University, Cl61J.K. Radhakrishnan, Coimbatore (1993) submitted. Phys. Stat. Sol. Cl71I. Katz, B. Chenfoux and N. Kristianpoller, (A) 12 (1972) 307. WI R.K. Gartia, B.S. Acharya and V.V. Ratnam, Phys. Stat. Sol. (A) 48 (1978) 235. and S. Selvasekarapandian, J. Phys.: Cl91J.K. Radhakrishnan Condensed Matter 6 (1994) 1. cw J.L. Merz and P.S. Pershan, Phys. Rev. 162 (1967) 217 & 235. C2ll M. Becker, J. Kiessling and A. Scharmann, Phys. Stat. Sol. (A) 15 (1973) 515. P-d N. Rabinner, Phys. Rev. 130 (1963) 502. S.A. Sazonova, A.V. Dolgopolova and ~231B.S. Skorobogatov, L.V. Kovaleva, Opt. Spectrosc. 16 (1964) 292.
LUMINESCENCE Journal of Luminescence 63 (1995) 137-142
ELSEWIER
Thermoluminescence
and fluorescence emission of CsCl : Sm crystals
J.K. Radhakrishnan, Department
of Physics, Bharathiar
S. Selvasekarapandian* University, Coimbatore-641046,
India
Received 8 February 1994; revised 21 July 1994; accepted 26 July 1994
Abstract Samarium enters C&l crystals at very low concentrations and is present partially in both divalent and trivalent states. The TL emission of C&l: Sm crystals is due to the F center electron - V center recombination and the excited Sm’+ ions. The fluoresence emission shows the presence of OH- ions in these crystals.
1. Introduction
Next to europium, samarium is the most predominantly studied divalent rare-earth impurity in ionic crystal lattices [l]. The spectroscopic properties of divalent samarium have been a subject of numerous investigations in alkali halide [2-S] crystals. Some researchers have observed Z-center formation in divalent samarium-doped alkali halides [6,8], which often occurs when the impurity does not change in its valence state upon irradiation. On the other hand, Fong and coworkers report that Sm2+ ions act as electron-trapping impurities in KCl, and are reduced to the monovalent state upon y-irradiation [S]. In the present work, the results of optical absorption, glow curve, TL spectral emission and fluorescence measurements made on CsCl : Sm crystals are presented and discussed.
Rare Earths Ltd., Udhyogamandal) by weight were grown in laboratory at room temperature, by slow evaporation of its water solution. Whereas pure CsCl crystallizes into dendrites, samarium-doped crystals are cubic in shape. These crystals were annealed at 400°C for four hours and then quenched to room temperature. Colouration of the samples were carried out in a 6oCo y-ray chamber with a dose rate of 1.03 krad/min. Other experimental details of measurement of optical absorption, glow curve and TL emission spectra have been described elsewhere [9]. The fluorescence spectra were recorded in a Hitachi 650-10s Fluorescence Spectrophotometer with a Himamatzu R928F photomultiplier tube.
3. Results 2. Experimental
details
Single crystals of cesium chloride (Analar, BDH, Poole, England) containing 5% of SmC13 (Indian *Corresponding author. 0022-2313/95/$09.50 0 1995 SSDI 0022-2313(94)00051-4
The uv-visible absorption spectra of uncoloured CsCl:Sm crystals were found to be similar to that of pure CsCl crystals [9]. Also similar to pure CsCl, y-irradiation on CsCl:Sm crystal produces the characteristic F-band centering around 603 nm,
Elsevier Science B.V. All rights reserved
138
J.K. Radhakrishnan,
S. Selvasekurapandian
/Journal
and a broad band in the uv-region over the absorption edge between 220 and 260 nm. However, a small apparent shift of about 6 nm to the lower wavelength side could be detected in the maximum of the uv-band, when compared with that of the pure CsCl crystal [9]. Fig. 1 shows the glow curves of a CsCl : Sm crystal irradiated with -y-rays for 1, 5, 10 and 20 min, recorded at a heating rate of 60”C/min. Glow peaks are observed at 368, 383, 398 and 408 K. The inset displays the growth of glow peak intensity at these temperatures with the time of y-irradiation. The effect of F-bleaching for two and five minutes on the glow curve of a 20 min y-irradiated crystal is (Fig. 2) a decrease in the intensity of the glow peaks. The presence of a glow peak around 343 K becomes apparent in F-bleached samples. The TL emission spectra were recorded at all the glow peak positions of a CsCl:Sm crystal irra-
Temperature
Fig. 1. Glow curves of a CsCl: Sm crystal glow peaks with the time of irradiation.
irradiated
in y-rays
of Luminescence 63 (1995) 137-142
diated with y-rays for 10 min (Fig. 3). On account of low intensity, the resolution of the TL emission is poor for the 408 K glow peak. The emission spectra at all the glow peaks possess an intense band at 425 nm and a shoulder around 495 nm. The positions of these emissions do not change from peak to peak. Only the intensity relations are affected. But, in the emission spectra under 398 and 408 K glow peaks, in addition to the above-mentioned two bands another broad emission extending from 580 to 700 nm appears. Fig. 4 shows the excitation and emission spectra of a CsCl: Sm crystal. Excitation at 267 nm gives a broad emission band between 340 and 600 nm. Over the longer wavelength tail of the emission spectrum, two sharp line emissions at 620 and 630 nm, and a shallow broad band extending from 640 to 740 nm could be detected. Excitation corresponding to the emissions at 360 and 460 nm, is a sharp band at 267 nm. Excitation corresponding
(K)
for (a) 1 min, (b) 5 min, (c) 10 min and (d) 20 min. Inset: growth
of the
J.K. Radhakrishnan, S. Selvasekarapandian
/Journal
Temperature
qf Luminescence 63 (1995) 137-142
139
LK)
Fig. 2. Glow curves of a CsCI: Sm crystal irradiated in y-rays for (a) 20 min, (b) irradiated and (c) irradiated for 20 min followed by 5 min of F-bleaching.
for 20 min followed by 2 min of F-bleaching
61 I
a - 368K b-383K c - 398K d - 408K
3 9 x z P z
3
2
1
0
Fig. 3. Spectral 10 min.
resolution
of TL emission
at the different
glow
peak
positions
of a CsCl:Sm
crystal
irradiated
in y-rays
for
140
J.K. Radhakrishnan, S. Selvasekarapandian
Wavelength
/Journal
of Luminescence 63 (1995) 137-142
(nm)
Fig. 4. Fluorescence emission (excitation at 267 nm) and excitation (for emission at 360,460 and 680 nm) spectra of a CsCI:Sm crystal.
to the emission at 680 nm is a group of weak bands at 500,470,420,400 and 340 nm.
4. Discussion According to Przibram [lo], the trivalent form of the rare-earth impurity occurs, when an alkali halide crystal is grown from water solution composed of alkali halides and rare-earth trihalides. The trivalent rare-earth ions reduce to divalent state when these crystals are heated to temperatures above 200°C. These results have further been confirmed by Greuen et al. [l l] and Resfeld and Alasner [12]. In alkali metal halide crystals, the absorption spectrum of the divalent samarium ion [2-41 at 300 and 77 K, has been found to consist of a series of intense wide bands in the visible and ultraviolet regions. These have been associated with the allowed transitions of the Sm’ + ion, from the 4f6 shell to the levels of the 4f55d configuration. But, the uv-visible absorption spectra of the CsCl: Sm crystals here, were similar to that of pure CsCl
crystals. Even a considerable amount of thermal annealing at 400°C does not bring out the characteristic absorptions of the samarium impurity. But the addition of the SmCl, impurity has changed the growth habit of the CsCl crystals from dendritic form to cubic form. Also it is a proven fact [13] that, in order to change the habit of a growing crystal from dendritic to cubic form, the impurities must be incorporated in the growing crystal. Obviously, samarium impurity has entered the crystal, but in such low concentrations that it cannot be discovered by optical absorption. The small shift to the lower wavelength side, observed in the maximum of the V-band upon introducing samarium, may be due to the effect of the impurity. From Fig. 1 it could be observed that, in addition to the common glow peaks at 368, 383 and 398 K which are also present in y-irradiated undoped CsCl [9] crystals, a peak at 408 K is observed in the CsCl: Sm crystals. This glow peak at 408 K, observed in the samarium-doped crystals may be ascribed to the thermal decay of F-center traps in the neighbourhood of samarium impurity [ 143.
J.K. Radhakrishnan, S. Selvasekarapandian /Journal
The glow peak at 343 K detected in the F-bleached y-irradiated CsCl:Sm crystals is not to be confused with the formation of Zr center peak, as this peak has been observed as the prominent peak in X-irradiated CsCl [15] and CsCl: Sm crystals. The absence of the 343 K peak in y-irradiated crystals may be due to the slightly different LET of X and y rays [9], and its reappearance upon F-bleaching is probabily due to photostimulation [16]. The TL emission spectra of pure CsCl has two emission bands at 407 and 470 nm [9], and they have been attributed to the emissions resulting from the recombination of thermally detrapped F-center electrons with the V2 and V3 centers having absorption bands at 252 and 233 nm. Attention is drawn to the fact that the TL emission pattern of the CsCl: Sm crystals (Fig. 3), except for a small shift towards the higher wavelength region and the presence of the small additional band between 580 and 700 nm at the 398 and 408 K glow peaks, is similar to that of the pure CsCl. This shift to the higher wavelength side may be the effect of the impurity. The prominent TL spectral emissions at 425 and 495 nm are not characteristic of the samarium impurity. As in pure CsCl, these emission bands are due to the recombination of thermally detrapped F-center electrons with the V-type centers. The shift of the TL emission towards the higher wavelength (lower energy) can be accounted for by the small shift observed in the V-band (of approximately around 6 nm) towards lower wavelength (higher energy) side [17,18] in the impurity-doped samples, when compared with pure CsCl. The weak broad TL emission band observed between 580 and 700 nm may be due to the samarium impurity. Photoluminescence studies made on samarium-doped alkali halides [224] have shown that, when excited at the Sm2+ absorption bands, these crystals give luminescence emission due to radiative transitions from the levels of 4f5 5d configuration of the Sm2+ ions in the red and infrared regions of the spectrum. At room temperature [2,3] the luminescence emission consists of a single wide band (with maxima near 745 nm for NaCl, 765 nm for NaBr, 810 nm for NaI, 737 nm for KCl, 745 nm for KBr and 775 nm for KI). The broad weak TL emission band between 580 and 700 nm observed
qf’luminescence
63 (19951 137 -142
141
in the CsCl:Sm crystals may be attributed to the samarium ions in their divalent state. It is evident that samarium impurities are not in sufficient numbers so as to dominate the TL emission. Attention is drawn to the fact that in alkali halides a displacement of the Sm2+ emission band [224] to the shorter wavelength side is observed upon an increase in the lattice constant (from Na to K). This rule requires the Sm2’ emission band in CsCl to be at a wavelength position lowest among the alkali chlorides, as is the case observed here. The occurrence of the emission due to the divalent samarium ions along with the emission due to the F center electron ~ V center recombination can be explained by the following mechanism. Upon thermally annealing the y-irradiated crystal (i.e. during TL readout), the thermally activated F-center electrons recombine with the V-type centers. A part of this recombination energy is transferred nonradiatively to the neighbouring Sm2+ ions, raising them to their excited states. The excited Sm2 + ions upon their decay to the ground state give out their characteristic emission, observed between 580 and 700 nm. Some important informations with regard to interpretation of the TL process are given by the photoluminescence spectra of the CsCl: Sm crystals. The broad fluorescence emission band between 340 and 600 nm (Fig. 4) which has its excitation at 267 nm (and as well as an absorption band at 267 nm), is present in pure CsCl also, and has been attributed to the characteristic emission and absorption of OH- ions embedded in the lattice [16,19]. However, the remaining part of the emission spectrum starting from 620 nm could be observed only for the CsCl: Sm crystals. The emission spectra beyond 600 nm can be divided into two parts: (i) a line spectrum consisting of sharp line emissions at 620 and 630 nm, which are characteristic of Sm3’ ions 120-231 and (ii) a broad band emission between 640 and 740 nm which is due to the radiative transitions from the levels of the 4f55d configuration of the Srn” ions 12241. The presence of the characteristic emissions of both the Sm” and Sm 3 + ions in the fluorescence emission spectra of the thermally treated CsCl : Sm
142
J.K. Radhakrishnan,
S. Selvasekarapandian
crystals indicate that, the reduction of samarium ions from trivalent state to divalent state upon thermal annealing described by Przibram [lo] is incomplete here. The unirradiated crystal contains both Sm’+ and Sm3+ ions (as indicated by the fluorescence emission spectra). y-irradiation probably converts the remaining unreduced Sm3 + ions to Sm’ + state [20,21]. Upon TL readout, a little part of the thermally activated F-center electrons recombine with the Sm3 ’ ions, oxidizing them into Sm2 + state. This probably is the reason that the TL emission spectra show only the characteristic emission of the Sm2 + ions and none of the Sm3 + ions. 4. Conclusions Samarium impurity enters CsCl crystals at very low concentrations and is present in both divalent and trivalent states. y-irradiation, apart from creating F- and V-type centers, also converts the remaining unreduced Sm 3t ions into divalent state. The TL emission is the result of both F center electron - V center recombination and the excited Sm2+ ions. Acknowledgement One of the authors (J.K.R.) thanks CSIR, New Delhi for Senior Research Fellowship. References [l]
J. Rubio.
0, J. Phys. Chem. Solids. 52 (1991) 101.
/Journal
of Luminescence 63 (1995) 137-142
121V.E.
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