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Luminescence of a thallium-perturbed on-centre self-trapped exciton in CsCl:Tl crystal
11 April 1997
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 268 (1997) 280-284
Luminescence of a thallium-perturbed on-centre self-trapped exciton in CsCI:T1 crystal V. Nagimyi a, M. Nikl b, G.P. Pazzi c, S. Zazubovich
a
a Institute of Physics, Riia 142, EE2400, Tartu, Estonia b Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 16200 Prague, Czech Republic c IROE del CNR, Via Panciatichi 64, 50127 Florence, Italy
Received 12 July 1996
Abstract In the luminescence spectrum of a thallium centre in CsCI:T1 crystal, besides two ultraviolet bands of the Tl + ion and two visible bands belonging to the off-centre self-trapped exciton perturbed by the Tl + ion, an additional band peaking at 4.56 eV has been observed at the 6.1 eV excitation. The decay kinetics and polarization of this emission have been studied. On the basis of the results obtained, the 4.56 eV emission is ascribed to a type I (on-centre) self-trapped exciton perturbed by the T1 + ion.
1. Introduction The luminescence of thallium-doped cesium halides has been studied in Refs. [1-5] by the methods of time-resolved polarization spectroscopy at 0 . 4 - 5 0 0 K. Four emission bands have been found to belong to the same thallium centre, and a new model of its relaxed excited states (RES) has been proposed in Ref. [1]. In this model, two ultraviolet bands are ascribed to the electronic transitions from the trigonal (A x) and the tetragonal (A T) Jahn-Teller minima of the triplet RES of Tl+, whereas two visible bands, A~r and g x , are ascribed to the radiative decay of the self-trapped excitons (STE) of two different off-centre configurations perturbed by the Tl + ion. In CsCI:T1 the maxima of these four emission bands are at 3.88, 3.65, - - 3 . 0 and 2.55 eV, respectively. In Refs. [5,6] an additional emission band peaking at 4.56 eV has been found in CsCl:T1 crystal at the 6.1 eV excitation. In Ref. [6] this band has been
interpreted as the C emission of the T1 + ion, whereas Ref. [5] interprets it as a result of the charge transfer between the Tl + ion and the nearest-neighbouring chlorine ions. In this Letter, the decay kinetics and polarization of this emission are studied at 4 - 3 0 0 K. On the basis of the results obtained the 4.56 eV emission is ascribed to a type I (on-centre) STE perturbed by the Tl + ion. In CsCl crystal the luminescence of the on-centre STE has not been detected before.
2. Experimental The same CsCI:T1 crystal grown from extremely pure CsC1 salt as the one described in Refs, [4,5] was investigated. The samples studied were cut out parallel to the (100) crystal planes. The apparatus used was analogous to the one described in Ref. [5].
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All tights reserved PII S0009-2614(97)00186-3
V. Nagirnyi et al./ Chemical Physics Letters 268 (1997) 280-284
Polarization of the emission was studied under excitation by unpolarized light observing in the direction normal to the direction of the excitation light propagation. The polarization degree observed under such experimental conditions (Pob~) is connected with the true value ( P ) by an equation P = 2Pobs/(1 + Pob~)" Only P values are given in this Letter. The decay kinetics were measured with a 199S spectrofluorimeter (Edinburgh Instrument) using the single photon counting method (see Ref. [7]).
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The 4.56 eV emission is excited in a narrow band peaking at 6.1 eV (at 4.2 K) and 6.02 eV (at 80 K) (Fig. 1). At 4.2 K its half-width (0.34 eV) is larger than that of the A x and A T emissions of the T1+ ion (0.28 and 0.22 eV), but smaller than the half-widths of the "~T and A~x emission bands of the off-centre STE perturbed by the TI ÷ ion (-~ 0.56 and 0.60 eV). The Stokes shift (1.58 eV) is slightly larger than in the case of the A x and A T emission (1.29 and 1.50 eV), but much smaller than in the case of the A~T and '~x emissions (-- 3.14 and 3.59 eV) (cf. Table 1 in Ref. [4]).
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281
5 Photon energy (eV)
Fig. I. (1, I') Emission, (2, 2') excitation and (3') polarization spectra of the 4.56 eV emission of CsCi:T! measured at 4.2 K (curves I, 2) and 80 K (curves 1'-3').
Fig. 2. Temperature dependences of the intensities (1,2) and polarization degree (3) measured for the 4.56 eV emigsion (curves 1, 3) and for the ~ x emission (curve 2) of CsCI:TI.
The position of this band is almost temperatureindependent, but the intensity of the 4.56! eV emission increases several times as the temperature rises from 4.2 to 100 K (Fig. 2, curve 1). Irt the same temperature range the intensity of the P~X emission decreases (curve 2). At 100 K the 4.56 eV emission is about twice as weak as the Pgx emission. At temperatures nearing 2 l0 K the reverse redistribution of the 4.56 eV and the A~x emission intensities is observed. These results point to the thermal connection between the corresponding states. The 4.56 eV emission is polarized about - 1 0 % at 80 K (Fig. 1, curve 3'). At 4.2 K the polarization degree seems to be the same. It does not! change up to 200 K and then starts to decrease (Fig. 2, curve 3). At 4 K three components are obserVed in the decay kinetics of the 4.56 eV emission with decay times of 7.5 ns, 6.5 Ixs and about 200 IxS. The light sum of the fastest component makes up about 4% of the total light sum of the 4.56 eV emission. Its emission spectrum (Fig. 3, curve l) is shifted about 0.1 eV to the higher energy side with respect to that of the slower components (curves 2) and has the maximum near 4.65 eV. The light sum off the 200 p~s component makes up only 2-3% of the total light sum of the 4.56 eV emission. Due to the gmall initial amplitude of this component it turns out to be impossible to measure its spectrum precisely. Thus, the 6.5 Ixs component is the main one in the delay kinetics of the 4.56 eV emission. Its spectrum co!ncides with
V. Nagirnyi et al. / Chemical Physics Letters 268 (1997) 280-284
282
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Photon energy (eV) Fig. 3. Time-resolvedemission spectra measured at 4.2 K under the 6.1 eV excitation for the time intervals 0-50 ns (curve l) and 1-20 p.s (curve 2) (mainly for the 7.5 ns and the 6.5 p.s decay component, respectively).Solid curves show the fitting by gaussians with the peak energies 4.65 and 4.56 eV and the half-widths 0.19 eV.
that of the steady-state 4.56 eV emission, and the dependence shown in Fig. 2, curve 1 for the stationary 4.56 eV emission reflects the temperature dependence of the light sum of this main component. As the temperature rises, the decay time of the second component decreases slightly from 6.5 I~s at 4 K to 4.5 i~s at 72 K and to 2.2 txs at 155 K and its light sum increases about 5 times. The decay time of the fastest component remains almost unchanged up to 100 K. At higher temperatures this component is not observed partly because it becomes masked by the increasing amplitude of the slower component and, probably, due to the non-radiative decay of the corresponding excited state.
I(T)
4. Discussion
In Refs. [6,8-11] the luminescence of undoped extremely pure CsCI crystal has been studied in detail. It has been concluded (see Refs. [ 11,12]) that only the triplet emission of the off-centre STE (having a maximum at 2.92 eV, half-width 0.7 eV and decay time 12 ms) exists in this crystal. The emis-
sion peaking at 4.6 eV and having a half-width at about 0.65 eV and the decay time t < 2 ns at low temperature has been detected in CsCI as well (see e.g. review in Ref. [13]). As the threshold energy of its excitation has been shown to be about 14 eV (Ref. [14]), the 4.6 eV emission has been interpreted as the crossluminescence of CsC1 crystal. Thus, no on-centre STE luminescence has been detected before in CsC1. However, the luminescence of on-centre STE has been found in some other alkali halides (see e.g. Refs. [15-20]). At low temperatures its decay kinetics consist of up to three components: a fast one with ~'0 = 1.6-3.8 ns, which is connected with the radiative decay of the singlet state of the on-centre STE (the tr emission), and of two much slower components (FC and SC) connected with the radiative decay of the emitting (ZFc = 0.2-0.5 l~s) and the metastable (rsc = 200 i~s in NaBr) levels of the triplet state (the ~r emission). Due to the stronger overlapping of the electron and hole wavefunctions in the case of the on-centre STE, the radiative decay probabilities for the on-centre STE are much higher than those for the off-centre STE. According to Refs. [6,8-11 ], no luminescence can be excited in the extremely pure CsC1 crystal by photons of E < 7.5 eV energy. Thus, the 4.56 eV emission can be caused only by the presence of TI ÷ ions in the CsCI:T1 crystal studied. However, the 4.56 eV emission of CsCI:T1 cannot be connected with the C or B excited state of the T1 ÷ ion as it has a negative value of the polarization degree, which is not characteristic of the isotropic Tl÷-like centres in alkali halides (see e.g. Ref. [21]). As a matter of fact, a high positive polarization degree and the presence of only one fast component in the decay kinetics are the characteristics of the C emission. The B emission cannot have a ns-component in the decay kinetics. However, the characteristics of the 4.56 eV emission are similar to those observed for the type I (on-centre) STE in some other alkali halides (see e.g. Refs. [15-20]). Its half-width and the Stokes shift are smaller than those observed for the STE emission. The decay component with % = 7.5 ns may be connected with the radiative decay of the higher, the singlet state of the STE. The component with ~'FC= 6.5 IXS is most probably caused by the radiative transitions from the emitting minima of the triplet
V. Nagirnyi et al. / Chemical Physics Letters 268 (1997) 280-284
RES, while the third component with the decay time ~'sc = 200 Ixs may be caused by the radiative decay of the metastable minimum. The difference in the maxima positions of the emission spectra shown in Fig. 3 (curves 1 and 2) may be caused by the relatively large energy distance between the singlet and triplet RES. It may point to a larger exchange energy for the on-centre STE as compared with that of the off-centre STE (see Tables 3 and 4 in Ref. [4]). The decay times ~'Fc = 6.5 Ixs and ~'sc--200 Ixs of the 4.56 eV emission are much smaller than those observed in Ref. [4] for the ~ and ?~x emissions of the CsCi:T1 crystal (~'FC: 750 and 850 izs, Zsc: 2.4 and 3.6 ms, respectively), which is characteristic of the on-centre STE emission. The decay probability of the singlet state is of the order of 10 s s- i. The ns component and the txs components of the STE emission have to be polarized in mutually perpendicular directions. Unfortunately, we were not able to measure the polarization for the separate decay components of the 4.56 eV emission. Thermal redistribution of the intensities between the .~ x and the 4.56 eV emission band may indicate to the fact that both these bands belong to the same luminescence centre. In view of the results obtained in Refs. [4,5], one may conclude that five minima of different types coexist in the RES of the thallium centre in CsCI:TI crystal. At T < 80 K, where thermal redistribution of the intensities occurs between the P~x and 4.56 eV emission bands, no big changes are observed either in the polarization degree or in the decay kinetics of the emission. This refers to the fact that the redistribution of the intensities shown in Fig. 2 is caused by some process in the non-relaxed excited state of the luminescence centre (similar intensity redistribution has been found between the A x and the Ax state in all thallium-doped cesium halide crystals and discussed in more detail in Refs. [2-5]). Although at 100-200 K the intensity of the 4.56 eV emission is comparable with that of the other emission bands of the thallium centre in CsCI:TI crystal observed at the 6.1-5.9 eV excitation, the fast (nanosecond) component of this emission is extremely weak, which sets limits on the possible practical applications of the 4.56 eV emission. Thus, the 4.56 eV emission of CsCI:T1 crystal is
283
probably the luminescence of the type I (on-centre) STE perturbed by the T1 ÷ ion. As has been mentioned above, the 4.6 eV emission band is observed in the luminescence spectrum of a pure CsCI crystal. In Ref. [19] it has been shown that the 4.3 eV emission of a pure CsI crystal (peaking at 4.1 eV at room temperature, Ref. [20]) is the emission of the on-centre STE, although in some papers (see e.g. Refs. [22,23]) its excitation-threshold energy has been found to be high. One may assume that the 4.6 eV emission of a pure CsC1 crystal is also caused by the radiative decay of the on-centre STE. A detailed study of its low-temperature decay kineticsl is necessary for a precise conclusion concerning its nature.
Acknowledgements This work was supported by the NATO High Technology Linkage Grant No. 931435, the Estonian Science Foundation Grant No. 2273 and Czech Academy of Sciences Grant No. AI010505.
References [1] V. Nagimyi, S. Zazubovieh, V. Zepelin, M. Ntkl and G.P. Pazzi, Chem. Phys. Lett. 227 (1994) 533. [2] V. Nagirnyi, A. Stolovich, S. Zazubovich, V. Zepelin, E. Mihokova, M. Nikl, G.P. Pazzi and L. Salvini, J. Phys. Condens. Matter 7 (1995) 3637. [3] V. Nagimyi, A. Stolovich, S. Zazubovich, V . Zepelin, M. Nikl, E. Mihokova and G.P. Pazzi, Radiat. Eff. Def. Sol. 135 (1995) 379. [4] E. Mihokova, V. Nagirnyi, M. Nikl, A. Stolovich, G.P. Pazzi, S. Zazubovich and V. Zepelin, J. Phys. Condens. Matter, in press. [5] V. Nagirnyi, S. Zazubovich and N. Jaanson, Phys. Status Solidi B 175 (1993) 155. [6] K.I. Ibragimov, A.Ch. Lushchik, Ch.B. Lushchik, A. Baimakhanov, E.A. Vassiltchenko and T.I. Savikhina, Sov. Phys. Solid State 34 (1992) 1831 (Fiz. Tverd. Tela 34 (1992) 3421). [7] K. Polak, D.J.S. Birch and M. Nikl, Phys. Status Solidi B 145 (1988) 741. [8] A. Lushchik, E. Feldbach, A. Frorip, K. lbragimov, i. Kuusmann and Ch. Lushchick, J. Phys. Condens. Matter 6 (1994) 2357. [9] M. Kirm, 1. Martinson, A. Lushchik, K. Kalder i R. Kink, Ch. Lushchik and A. LShmus, Solid State CommUn. 90 (1994) 741.
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[10] K.I. Ibragimov, A.Ch. Lushchik, Ch.B. Lushchik, A.G. Frorip and N. Jaanson, Sov. Phys. Solid State 34 (1992) 1690 (Fiz. Tverd. Tela 34 (1992) 3161). [11] A. Lushchik, E. Feldbach, A. Frorip, K. Ibragimov, F. Savikhin and Ch. Lushchik, J. Lumin. 63 (1995) 273. [12] M. ltoh, K. Tanimura and N. ltoh, J. Phys. Soc. Jpn. 62 (1993) 2904. [13] C.W.E. van Eijk, J. Lumin. 60/61 (1994) 936. [14] I. Kuusmann, T. Kloiber, W. Laasch and G. Zimmerer, Radiat. Eft. Def. Sol. 119/121 (1991) 21. [15] Ch.B. Lushchik and A.Ch. Lushchik, Decay of Electronic Excitations with Defect Formation in Solids (Nauka, Moscow, 1989) (in Russian). [16] K.S. Song and R.T. Williams, Self-trapped Excitons, eds. M. Cardona et al. (Springer, Berlin, 1993), and references therein.
[17] T. Matsumoto, T. Kawata, A. Miyamoto and K. Kan'no, J. Phys. SOC. Jpn. 61 (1992) 4229. [18] T. Kawata, T. Mukai, T. Matsumoto and K. Kan'no, in: Proceedings of the 12th Int. Conf. on Defects in Insulating Materials, Vol. 2, eds. O. Kanert and J.-M. Spaeth (World Scientific, Singapore, 1993) p. 1232. [19] T. Tsujimoto, H. Nishimura, M. Nakayama, H. Kurisu and T. Komatsu, J. Lumin, 60/61 (1994) 798. [20] H. Nishimura, M. Sakata, T. Tsujimoto and M. Nakayama, Phys. Rev. B 51 (1995) 2167. [21] S. Zazubovich, Int. J. Mod. Phys. B 8 (1994) 985. [22] S. Kubota, S. Sakuragi, S. Hashimoto and J. Ruan (Gen), Nucl. Instrum. Meth. A 268 (1988) 275. [23] A.N. Belsky, A.N. Vasil'ev and V.V. Mikhailin, Phys. Rev. B 49 (1994) 13197.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 268 (1997) 280-284
Luminescence of a thallium-perturbed on-centre self-trapped exciton in CsCI:T1 crystal V. Nagimyi a, M. Nikl b, G.P. Pazzi c, S. Zazubovich
a
a Institute of Physics, Riia 142, EE2400, Tartu, Estonia b Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 16200 Prague, Czech Republic c IROE del CNR, Via Panciatichi 64, 50127 Florence, Italy
Received 12 July 1996
Abstract In the luminescence spectrum of a thallium centre in CsCI:T1 crystal, besides two ultraviolet bands of the Tl + ion and two visible bands belonging to the off-centre self-trapped exciton perturbed by the Tl + ion, an additional band peaking at 4.56 eV has been observed at the 6.1 eV excitation. The decay kinetics and polarization of this emission have been studied. On the basis of the results obtained, the 4.56 eV emission is ascribed to a type I (on-centre) self-trapped exciton perturbed by the T1 + ion.
1. Introduction The luminescence of thallium-doped cesium halides has been studied in Refs. [1-5] by the methods of time-resolved polarization spectroscopy at 0 . 4 - 5 0 0 K. Four emission bands have been found to belong to the same thallium centre, and a new model of its relaxed excited states (RES) has been proposed in Ref. [1]. In this model, two ultraviolet bands are ascribed to the electronic transitions from the trigonal (A x) and the tetragonal (A T) Jahn-Teller minima of the triplet RES of Tl+, whereas two visible bands, A~r and g x , are ascribed to the radiative decay of the self-trapped excitons (STE) of two different off-centre configurations perturbed by the Tl + ion. In CsCI:T1 the maxima of these four emission bands are at 3.88, 3.65, - - 3 . 0 and 2.55 eV, respectively. In Refs. [5,6] an additional emission band peaking at 4.56 eV has been found in CsCl:T1 crystal at the 6.1 eV excitation. In Ref. [6] this band has been
interpreted as the C emission of the T1 + ion, whereas Ref. [5] interprets it as a result of the charge transfer between the Tl + ion and the nearest-neighbouring chlorine ions. In this Letter, the decay kinetics and polarization of this emission are studied at 4 - 3 0 0 K. On the basis of the results obtained the 4.56 eV emission is ascribed to a type I (on-centre) STE perturbed by the Tl + ion. In CsCl crystal the luminescence of the on-centre STE has not been detected before.
2. Experimental The same CsCI:T1 crystal grown from extremely pure CsC1 salt as the one described in Refs, [4,5] was investigated. The samples studied were cut out parallel to the (100) crystal planes. The apparatus used was analogous to the one described in Ref. [5].
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All tights reserved PII S0009-2614(97)00186-3
V. Nagirnyi et al./ Chemical Physics Letters 268 (1997) 280-284
Polarization of the emission was studied under excitation by unpolarized light observing in the direction normal to the direction of the excitation light propagation. The polarization degree observed under such experimental conditions (Pob~) is connected with the true value ( P ) by an equation P = 2Pobs/(1 + Pob~)" Only P values are given in this Letter. The decay kinetics were measured with a 199S spectrofluorimeter (Edinburgh Instrument) using the single photon counting method (see Ref. [7]).
60
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50
2 Ii2 i
40 30 20
-10
~m~
• 3'
6
~.
z o
~2~ 10
•
•
" 200
300
Temperature (K)
The 4.56 eV emission is excited in a narrow band peaking at 6.1 eV (at 4.2 K) and 6.02 eV (at 80 K) (Fig. 1). At 4.2 K its half-width (0.34 eV) is larger than that of the A x and A T emissions of the T1+ ion (0.28 and 0.22 eV), but smaller than the half-widths of the "~T and A~x emission bands of the off-centre STE perturbed by the TI ÷ ion (-~ 0.56 and 0.60 eV). The Stokes shift (1.58 eV) is slightly larger than in the case of the A x and A T emission (1.29 and 1.50 eV), but much smaller than in the case of the A~T and '~x emissions (-- 3.14 and 3.59 eV) (cf. Table 1 in Ref. [4]).
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3. Experimental results
281
5 Photon energy (eV)
Fig. I. (1, I') Emission, (2, 2') excitation and (3') polarization spectra of the 4.56 eV emission of CsCi:T! measured at 4.2 K (curves I, 2) and 80 K (curves 1'-3').
Fig. 2. Temperature dependences of the intensities (1,2) and polarization degree (3) measured for the 4.56 eV emigsion (curves 1, 3) and for the ~ x emission (curve 2) of CsCI:TI.
The position of this band is almost temperatureindependent, but the intensity of the 4.56! eV emission increases several times as the temperature rises from 4.2 to 100 K (Fig. 2, curve 1). Irt the same temperature range the intensity of the P~X emission decreases (curve 2). At 100 K the 4.56 eV emission is about twice as weak as the Pgx emission. At temperatures nearing 2 l0 K the reverse redistribution of the 4.56 eV and the A~x emission intensities is observed. These results point to the thermal connection between the corresponding states. The 4.56 eV emission is polarized about - 1 0 % at 80 K (Fig. 1, curve 3'). At 4.2 K the polarization degree seems to be the same. It does not! change up to 200 K and then starts to decrease (Fig. 2, curve 3). At 4 K three components are obserVed in the decay kinetics of the 4.56 eV emission with decay times of 7.5 ns, 6.5 Ixs and about 200 IxS. The light sum of the fastest component makes up about 4% of the total light sum of the 4.56 eV emission. Its emission spectrum (Fig. 3, curve l) is shifted about 0.1 eV to the higher energy side with respect to that of the slower components (curves 2) and has the maximum near 4.65 eV. The light sum off the 200 p~s component makes up only 2-3% of the total light sum of the 4.56 eV emission. Due to the gmall initial amplitude of this component it turns out to be impossible to measure its spectrum precisely. Thus, the 6.5 Ixs component is the main one in the delay kinetics of the 4.56 eV emission. Its spectrum co!ncides with
V. Nagirnyi et al. / Chemical Physics Letters 268 (1997) 280-284
282
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Photon energy (eV) Fig. 3. Time-resolvedemission spectra measured at 4.2 K under the 6.1 eV excitation for the time intervals 0-50 ns (curve l) and 1-20 p.s (curve 2) (mainly for the 7.5 ns and the 6.5 p.s decay component, respectively).Solid curves show the fitting by gaussians with the peak energies 4.65 and 4.56 eV and the half-widths 0.19 eV.
that of the steady-state 4.56 eV emission, and the dependence shown in Fig. 2, curve 1 for the stationary 4.56 eV emission reflects the temperature dependence of the light sum of this main component. As the temperature rises, the decay time of the second component decreases slightly from 6.5 I~s at 4 K to 4.5 i~s at 72 K and to 2.2 txs at 155 K and its light sum increases about 5 times. The decay time of the fastest component remains almost unchanged up to 100 K. At higher temperatures this component is not observed partly because it becomes masked by the increasing amplitude of the slower component and, probably, due to the non-radiative decay of the corresponding excited state.
I(T)
4. Discussion
In Refs. [6,8-11] the luminescence of undoped extremely pure CsCI crystal has been studied in detail. It has been concluded (see Refs. [ 11,12]) that only the triplet emission of the off-centre STE (having a maximum at 2.92 eV, half-width 0.7 eV and decay time 12 ms) exists in this crystal. The emis-
sion peaking at 4.6 eV and having a half-width at about 0.65 eV and the decay time t < 2 ns at low temperature has been detected in CsCI as well (see e.g. review in Ref. [13]). As the threshold energy of its excitation has been shown to be about 14 eV (Ref. [14]), the 4.6 eV emission has been interpreted as the crossluminescence of CsC1 crystal. Thus, no on-centre STE luminescence has been detected before in CsC1. However, the luminescence of on-centre STE has been found in some other alkali halides (see e.g. Refs. [15-20]). At low temperatures its decay kinetics consist of up to three components: a fast one with ~'0 = 1.6-3.8 ns, which is connected with the radiative decay of the singlet state of the on-centre STE (the tr emission), and of two much slower components (FC and SC) connected with the radiative decay of the emitting (ZFc = 0.2-0.5 l~s) and the metastable (rsc = 200 i~s in NaBr) levels of the triplet state (the ~r emission). Due to the stronger overlapping of the electron and hole wavefunctions in the case of the on-centre STE, the radiative decay probabilities for the on-centre STE are much higher than those for the off-centre STE. According to Refs. [6,8-11 ], no luminescence can be excited in the extremely pure CsC1 crystal by photons of E < 7.5 eV energy. Thus, the 4.56 eV emission can be caused only by the presence of TI ÷ ions in the CsCI:T1 crystal studied. However, the 4.56 eV emission of CsCI:T1 cannot be connected with the C or B excited state of the T1 ÷ ion as it has a negative value of the polarization degree, which is not characteristic of the isotropic Tl÷-like centres in alkali halides (see e.g. Ref. [21]). As a matter of fact, a high positive polarization degree and the presence of only one fast component in the decay kinetics are the characteristics of the C emission. The B emission cannot have a ns-component in the decay kinetics. However, the characteristics of the 4.56 eV emission are similar to those observed for the type I (on-centre) STE in some other alkali halides (see e.g. Refs. [15-20]). Its half-width and the Stokes shift are smaller than those observed for the STE emission. The decay component with % = 7.5 ns may be connected with the radiative decay of the higher, the singlet state of the STE. The component with ~'FC= 6.5 IXS is most probably caused by the radiative transitions from the emitting minima of the triplet
V. Nagirnyi et al. / Chemical Physics Letters 268 (1997) 280-284
RES, while the third component with the decay time ~'sc = 200 Ixs may be caused by the radiative decay of the metastable minimum. The difference in the maxima positions of the emission spectra shown in Fig. 3 (curves 1 and 2) may be caused by the relatively large energy distance between the singlet and triplet RES. It may point to a larger exchange energy for the on-centre STE as compared with that of the off-centre STE (see Tables 3 and 4 in Ref. [4]). The decay times ~'Fc = 6.5 Ixs and ~'sc--200 Ixs of the 4.56 eV emission are much smaller than those observed in Ref. [4] for the ~ and ?~x emissions of the CsCi:T1 crystal (~'FC: 750 and 850 izs, Zsc: 2.4 and 3.6 ms, respectively), which is characteristic of the on-centre STE emission. The decay probability of the singlet state is of the order of 10 s s- i. The ns component and the txs components of the STE emission have to be polarized in mutually perpendicular directions. Unfortunately, we were not able to measure the polarization for the separate decay components of the 4.56 eV emission. Thermal redistribution of the intensities between the .~ x and the 4.56 eV emission band may indicate to the fact that both these bands belong to the same luminescence centre. In view of the results obtained in Refs. [4,5], one may conclude that five minima of different types coexist in the RES of the thallium centre in CsCI:TI crystal. At T < 80 K, where thermal redistribution of the intensities occurs between the P~x and 4.56 eV emission bands, no big changes are observed either in the polarization degree or in the decay kinetics of the emission. This refers to the fact that the redistribution of the intensities shown in Fig. 2 is caused by some process in the non-relaxed excited state of the luminescence centre (similar intensity redistribution has been found between the A x and the Ax state in all thallium-doped cesium halide crystals and discussed in more detail in Refs. [2-5]). Although at 100-200 K the intensity of the 4.56 eV emission is comparable with that of the other emission bands of the thallium centre in CsCI:TI crystal observed at the 6.1-5.9 eV excitation, the fast (nanosecond) component of this emission is extremely weak, which sets limits on the possible practical applications of the 4.56 eV emission. Thus, the 4.56 eV emission of CsCI:T1 crystal is
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probably the luminescence of the type I (on-centre) STE perturbed by the T1 ÷ ion. As has been mentioned above, the 4.6 eV emission band is observed in the luminescence spectrum of a pure CsCI crystal. In Ref. [19] it has been shown that the 4.3 eV emission of a pure CsI crystal (peaking at 4.1 eV at room temperature, Ref. [20]) is the emission of the on-centre STE, although in some papers (see e.g. Refs. [22,23]) its excitation-threshold energy has been found to be high. One may assume that the 4.6 eV emission of a pure CsC1 crystal is also caused by the radiative decay of the on-centre STE. A detailed study of its low-temperature decay kineticsl is necessary for a precise conclusion concerning its nature.
Acknowledgements This work was supported by the NATO High Technology Linkage Grant No. 931435, the Estonian Science Foundation Grant No. 2273 and Czech Academy of Sciences Grant No. AI010505.
References [1] V. Nagimyi, S. Zazubovieh, V. Zepelin, M. Ntkl and G.P. Pazzi, Chem. Phys. Lett. 227 (1994) 533. [2] V. Nagirnyi, A. Stolovich, S. Zazubovich, V. Zepelin, E. Mihokova, M. Nikl, G.P. Pazzi and L. Salvini, J. Phys. Condens. Matter 7 (1995) 3637. [3] V. Nagimyi, A. Stolovich, S. Zazubovich, V . Zepelin, M. Nikl, E. Mihokova and G.P. Pazzi, Radiat. Eff. Def. Sol. 135 (1995) 379. [4] E. Mihokova, V. Nagirnyi, M. Nikl, A. Stolovich, G.P. Pazzi, S. Zazubovich and V. Zepelin, J. Phys. Condens. Matter, in press. [5] V. Nagirnyi, S. Zazubovich and N. Jaanson, Phys. Status Solidi B 175 (1993) 155. [6] K.I. Ibragimov, A.Ch. Lushchik, Ch.B. Lushchik, A. Baimakhanov, E.A. Vassiltchenko and T.I. Savikhina, Sov. Phys. Solid State 34 (1992) 1831 (Fiz. Tverd. Tela 34 (1992) 3421). [7] K. Polak, D.J.S. Birch and M. Nikl, Phys. Status Solidi B 145 (1988) 741. [8] A. Lushchik, E. Feldbach, A. Frorip, K. lbragimov, i. Kuusmann and Ch. Lushchick, J. Phys. Condens. Matter 6 (1994) 2357. [9] M. Kirm, 1. Martinson, A. Lushchik, K. Kalder i R. Kink, Ch. Lushchik and A. LShmus, Solid State CommUn. 90 (1994) 741.
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[10] K.I. Ibragimov, A.Ch. Lushchik, Ch.B. Lushchik, A.G. Frorip and N. Jaanson, Sov. Phys. Solid State 34 (1992) 1690 (Fiz. Tverd. Tela 34 (1992) 3161). [11] A. Lushchik, E. Feldbach, A. Frorip, K. Ibragimov, F. Savikhin and Ch. Lushchik, J. Lumin. 63 (1995) 273. [12] M. ltoh, K. Tanimura and N. ltoh, J. Phys. Soc. Jpn. 62 (1993) 2904. [13] C.W.E. van Eijk, J. Lumin. 60/61 (1994) 936. [14] I. Kuusmann, T. Kloiber, W. Laasch and G. Zimmerer, Radiat. Eft. Def. Sol. 119/121 (1991) 21. [15] Ch.B. Lushchik and A.Ch. Lushchik, Decay of Electronic Excitations with Defect Formation in Solids (Nauka, Moscow, 1989) (in Russian). [16] K.S. Song and R.T. Williams, Self-trapped Excitons, eds. M. Cardona et al. (Springer, Berlin, 1993), and references therein.
[17] T. Matsumoto, T. Kawata, A. Miyamoto and K. Kan'no, J. Phys. SOC. Jpn. 61 (1992) 4229. [18] T. Kawata, T. Mukai, T. Matsumoto and K. Kan'no, in: Proceedings of the 12th Int. Conf. on Defects in Insulating Materials, Vol. 2, eds. O. Kanert and J.-M. Spaeth (World Scientific, Singapore, 1993) p. 1232. [19] T. Tsujimoto, H. Nishimura, M. Nakayama, H. Kurisu and T. Komatsu, J. Lumin, 60/61 (1994) 798. [20] H. Nishimura, M. Sakata, T. Tsujimoto and M. Nakayama, Phys. Rev. B 51 (1995) 2167. [21] S. Zazubovich, Int. J. Mod. Phys. B 8 (1994) 985. [22] S. Kubota, S. Sakuragi, S. Hashimoto and J. Ruan (Gen), Nucl. Instrum. Meth. A 268 (1988) 275. [23] A.N. Belsky, A.N. Vasil'ev and V.V. Mikhailin, Phys. Rev. B 49 (1994) 13197.