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Fast decay behaviors of self-trapped exciton luminescence in ammonium halides
LUMINESCENCE
Journal of Luminescence 53 (1992) 121—124
JOURNAL OF
Fast decay behaviors of self-trapped exciton luminescence in ammonium halides Nobuhito Ohno
a
Minoru Itoh
b
and Satoshi Hashimoto
Department of Solid State Electronics, Faculty of Engineering, Osaka Electro-Communication Unii’ersiiy, Neyagawa 572, Japan ~‘ Department of Applied Science, Faculty of Engineering, Shinshu University, Nagano 380, Japan Department of Physics, Kyoto University of Education. Fushimi-ku, Kyoto 612, Japan
Decay behaviors of self-trapped exciton (STE) luminescence in ammonium halides has been examined at 10 K by use of a time-correlated photon counting method under a single-bunch operation of synchrotron radiation. The decay curves are found to Consist not only of a long-lived luminescence component (0.2—0.7 gs) but also of a fluorescent component with a short lifetime (2—3 ns). It is also found that the intensity ratio of the slow component to the fast one decreases in order of NH 4I, NH4Br and NH4CI. Lattice configurations of initial states of the STE are discussed in connection with the recent “on-center” and “off-center” model of the STE proposed for alkali halides. The present results strongly suggest that the STE in ammonium halides is situated in an on-center configuration just like the case of the STE in Nal and NaBr.
1. Introduction The self-trapped exciton (STE) in alkali halides has long been thought to consist of a self-trapped hole (VK center) and a bound electron, both being centered on the midpoint between two nearest-neighbor halide sites in a relaxed configuration of D2h symmetry [1]. This “on-center” model can reasonably account for various experimental facts. Recently, however, Song and coworkers [2—4]have proposed that the STE is unstable in the state of D2h symmetry, resulting in relaxation to an off-center configuration. The “off-center” model suggests mutual displacements of the VK core and the surrounding alkali ions. Recent experiments on luminescence and decay time of mixed alkali—halide crystals have revealed that the results obtained are in favor of the off-center model [5—7]. Ammonium halides are known to be an ionic crystal which undergoes an order—disorder transition. The low-temperature phase (called IV Correspondence to: Dr. N. Ohno, Department of Solid State Physics, Faculty of Engineering, Osaka Electro-Communication University, Neyagawa 572, Japan. 0022-2313/92/$05.00 © 1992
—
phase) of NH4C1 and NH4Br is of the cubic CsCI structure, and that of NH4I (called III phase) is of the slightly distorted CsCI structure. The fundamental absorption spectra of these halide crystals have been found to resemble closely those of alkali halides, since the tetrahedral NH~ ion acts like an alkali ion. Therefore, ammonium halides will be a suitable system for the extension of investigations on the exciton relaxation done for alkali halides. The intrinsic luminescence of ammonium halides was first observed by Marrone and Kabler under X-ray irradiation at low temperatures [8]. These luminescence bands are Stokes-shifted by 1.5—2.5 eV from the fundamental absorption edge, and have been attributed to the radiative annihilation of the STE. This supposition was confirmed through the luminescence measurements by Itoh under the UV-light excitation in the exciton absorption region [9,10]. The decaytime measurements of the intrinsic luminescence were also carried out by these authors [8—101,but the fast behaviors have not been reported so far. The purpose of the present study is to investigate the fast decay behaviors of the STE luminescence observed in ammonium halides. The lattice con-
Elsevier Science Publishers B.V. All rights reserved
I 22
A. Olino ci of. / I as! decay /xhai jars of .selt—! rapped evelton lio,,ou’scense
figurations of initial states of the STE are dis— cussed in connection with the recent “on-center” and “off-center model of the STE presented for
1.5 ,
~c
2. Experiments
—~-----——————r-———-
—,-
NH
4I
alkali halides.
ha/isles
it! (i!tlttiOJ!0110
NH4Br
NH4CI
10 ~0.5
I lie sample crystals of ammonium halides were grown at room temperature from a saturated aqueous solution containing urea. Details ol the crystal growth have been described in rets. RIO]. The specimens were cooled down to 1(1 K for optical measurements. For IiV-light excitation, a 0.4 m vacuum monochromator was used in cornbination with an MgF~-windowedD~lamp. Lurninescence from the sample was detected through a grating monochromator. Lifetime measurements were carried out by using a time-correlated photon counting method under the single-bunch operation of synchrotron radiation from UVSOR in the Institute for Molecular Science (Okazaki, Japan). The interval of successive pulses was 178 ns, and the pulse width was approximately O.~ns including time response of the detection system.
—
0 30
35
40 PHOTON
50
45
ENERGY
55
(eV)
lig. I . luminescence spectra ot ammonium halides measured at II) K under the excitation with liv light of the hand-to-hand energy. Each curve has been
normalized
to unity ill the
maximum.
1 NH4I
-~
0.1
C 3
-U
3. Results Figure 1 shows luminescence spectra of ammonium halides under the UV-light excitatioti into the interhand transition at It) K. A broad luminescence hand with a Gaussian line shapeappears at 3.74 eV for NH4I. 4.18 eV for NH4Br and 4.91 eV for NH4CI, arising from the radiative annihilation of the STE [8—10].These hands are efficiently stimulated for the excitation into the exciton absorption region. It is worth noting that ammonium halides exhibit a single intrinsic hand in the ultra-violet region. Besides the main hand. a weak luminescence hand was sometimes observed in each halide, and has been attributed to some trace impurity [9,10]. Figure 2 shows decay profiles of the STE luminescence hands at 10 K under the excitation with synchrotron radiation at 6.9 eV for NH I and NH4Br and at 7.8 eV for NH4CI. In HN4I and NH4Br. one can see a piling-up effect stemming
1
~
NH~Br
0.1
1 NH4CI 0.1
0.01
0.001 0
20
40
60
80
TIME (ns) Fig. 2. Decay profiles of the STE luminescence bands in ammonium halides at It) K under the single—hunch operation of synchrotron radiation from UVSOR at 6.9 eV for NI 141 and NI-I 1Br and at 7.8 cV for N}14C1. The emission intensity has been normalized to unity at t
=
N. Ohno ci al
/
Fast decay behaviors of self-trapped exciton luminescence in ammonium halides
Table I Peak positions and decay times of the STE luminescence bands observed in ammonium halides
________________________________________________ Present work (at 10 K)
Ref. [8] (at LHeT)
peak position decay time [eV] Ens]
decay time [ns]
fast NH
4I
3.74
NH4Br 4.18 NH4CI 4.91
slow
2.0±0.2 200±20
fast —
2.0±0.2 700±50 5-10 2.7 ±0.2 s-io
slow —
770±40 —
from superposition of successive events caused by the repeated pulse excitation. it appears that the STE producing these luminescence bands of ammonium halides has two decay components; they consist not only of a long-lived component but also of a fast decay component. The fast decay component has a lifetime of 2—3 ns in the three cases. The slow component is longer in NH4Br (700 ns) than in NH4I (200 ns). The decay times obtained in the present study are listed in table 1, together with the previous results of ref. [8]. It should be noticed that the short-lived component increases in intensity in order of NH4I, NH4Br and NH4C1, while the long-lived component decreases in this order. In NH4C1, it was difficult to distinguish the existence of a slow component from the noise level.
4. Discussion As is well known, annihilation of the STE in alkali halides results in one or more luminescence bands. In most cases, fluorescence arising from a singlet state and phosphorescence from a triplet state, usually called the r and the ii band, respectively, appear as separate luminescence bands. According to the recent morphology of the STE [2—7],the Cr band, which has a small Stokes shift, has been attributed to a radiative transition of an on-center STE. On the other hand, a large Stokes shift of the i~ band has been explained by introducing an additional off-center relaxation of
123
the VK core of the STE. The off-center model can also account for the mechanism of low-ternperature defect formation such as an F—H pair creation [3]. Among the alkali halides, Nal and NaBr are quite particular materials. They have a single STE luminescence band (m- band) with an anomalously small Stokes shift in the ultra-violet region. Moreover, its transient behavior has been found to consist of two decay components of 1.0 ns (1.5 ns) and 102 ns (475 ns) in NaI (NaBr) [11]. From these results, the STE luminescence of Nal and NaBr has been explained as due to the radiative annihilation of the lowest STE with an on-center configuration, or a nearly on-center configuration [7,11,12]. As can be seen in fig. 1, ammonium-halide crystals exhibit a single intrinsic luminescence band with a small Stokes shift. These bands have been confirmed to arise from the lowest STE state on the basis of the excitation spectra; they are intensely stimulated with UV light in the n = 1 exciton region [9,10]. In addition, the present study reveals that their decay times consist of a long-lived component and of a fluorescent component with a short lifetime. These results of ammontum halides indicate a close resemblance to those of Nal and NaBr. Therefore, it is very likely that the STE in ammonium halides is situated in an on-center configuration. The initial state of the STE would be a pair of the triplet ~ state and the singlet ~ state, separated slightly by the exchange interaction [9]. According to recent arguments about the STE relaxation in alkali halides [2—7],whether the STE occupies an on-center or off-center configuration at equilibrium, the choice is sensitive to geometric parameters such as the lattice constant and ionic radius, e.g., the Rabin—Klick parameter. In ammonium halides, cations surrounding the VK core are the NH~ ions oriented in parallell (“ferro”-ordering) or in antiparallel (“antiferro”-ordering) to each other. The NH~ ion would work as a restraint on the off-center displacement of the VK core because of its tetrahedral structure. The low temperature phase of ammonium halides crystallizes in the CsCl structure, which is more close-packed compared with
I 24
\. (.!hno ci of. ,/ I—as! ilciar lie/ialiors of seff-trapped eieiton /ooiuic,see/ise in a?nttiirtuisiii ha/ides
the NaCI-type structure. This may he another reason why the on-center STE is stable in ammonium halides. From fig. 2. it is obvious that the slow coniponent is reduced in intensity and has a longer lifetime in order of NH I, NH Br and NH Cl. I hese trcnds tic reason thly explained by consid ering that the spin—orbit interaction of the hole in VK center decreases in this order, reducing the mixing of a higher excited Il state into the lowest ~ state. In conclusion, the present results suggest that the STE in ammonium halides does not relax off center, and is situated in the symmetric [VK+ ci configuration. This is in accordance with the fact that no F center is produced in ammonium halides, although the VK center does exist. -,
.
~.
Studies Program of the Institute for Molecular Science.
References [I] M - N - Kabler, Phys. Rev. 136 (1964) A I 296. [2] ( 11 Lcun6 C Brunrt md K S Sons, I
1 lix, (
IS
(1985) 4459 [3] R.T. Williams. KS. Song. WI-.. Faust and (.11. teung. Ph~s. Rev. 13 33 (1986) 7232. [4] KS. Song and Cli. Lcung. Rcv. Solid State Sci. 4 (990) [5] K Kanno. K. i:maka and T I lavashi. Rev. Solid State Sci. 4(199))) 353.
[6] M. Itoh. N. Ohno and S. I lashimoto. J. t’hys. Soc. Jpn. 59 (1990) 4534. [7] M. Itoh, S. ilashimoto, N. Ohno and K. Kanno. .1. Phvs. Soc. Jpn. 6)) (1991) 61. [5] Mi Marrone and M N Kabler. Phys Rev 176 (1968) 11)7)).
Acknowledgements The authors are grateful to Professors M. Watanahe and M. Kamada for their favorable supports in lifetime measurements under the Joint
[9] M. Itoh. J. Phys. Soc. ipn. 57 )I98S) 372. [10] M. Itoh. i. Phys. Soc. Jpn. 58 11989) 2994. Ill] K. Kanno. K. Tanaka. H. Kosaka, T. Mukai, Y. Nakai. M. Itoh. T. Miyanaga. K. Fukui and M. Watanahe. Physca Scripta 41(199)1)12(1 [12] M. Itoh, S. Ilashimoto and N. Ohno, J. Phys. Soc. Jpn. 6)) (1991) 4357.
Journal of Luminescence 53 (1992) 121—124
JOURNAL OF
Fast decay behaviors of self-trapped exciton luminescence in ammonium halides Nobuhito Ohno
a
Minoru Itoh
b
and Satoshi Hashimoto
Department of Solid State Electronics, Faculty of Engineering, Osaka Electro-Communication Unii’ersiiy, Neyagawa 572, Japan ~‘ Department of Applied Science, Faculty of Engineering, Shinshu University, Nagano 380, Japan Department of Physics, Kyoto University of Education. Fushimi-ku, Kyoto 612, Japan
Decay behaviors of self-trapped exciton (STE) luminescence in ammonium halides has been examined at 10 K by use of a time-correlated photon counting method under a single-bunch operation of synchrotron radiation. The decay curves are found to Consist not only of a long-lived luminescence component (0.2—0.7 gs) but also of a fluorescent component with a short lifetime (2—3 ns). It is also found that the intensity ratio of the slow component to the fast one decreases in order of NH 4I, NH4Br and NH4CI. Lattice configurations of initial states of the STE are discussed in connection with the recent “on-center” and “off-center” model of the STE proposed for alkali halides. The present results strongly suggest that the STE in ammonium halides is situated in an on-center configuration just like the case of the STE in Nal and NaBr.
1. Introduction The self-trapped exciton (STE) in alkali halides has long been thought to consist of a self-trapped hole (VK center) and a bound electron, both being centered on the midpoint between two nearest-neighbor halide sites in a relaxed configuration of D2h symmetry [1]. This “on-center” model can reasonably account for various experimental facts. Recently, however, Song and coworkers [2—4]have proposed that the STE is unstable in the state of D2h symmetry, resulting in relaxation to an off-center configuration. The “off-center” model suggests mutual displacements of the VK core and the surrounding alkali ions. Recent experiments on luminescence and decay time of mixed alkali—halide crystals have revealed that the results obtained are in favor of the off-center model [5—7]. Ammonium halides are known to be an ionic crystal which undergoes an order—disorder transition. The low-temperature phase (called IV Correspondence to: Dr. N. Ohno, Department of Solid State Physics, Faculty of Engineering, Osaka Electro-Communication University, Neyagawa 572, Japan. 0022-2313/92/$05.00 © 1992
—
phase) of NH4C1 and NH4Br is of the cubic CsCI structure, and that of NH4I (called III phase) is of the slightly distorted CsCI structure. The fundamental absorption spectra of these halide crystals have been found to resemble closely those of alkali halides, since the tetrahedral NH~ ion acts like an alkali ion. Therefore, ammonium halides will be a suitable system for the extension of investigations on the exciton relaxation done for alkali halides. The intrinsic luminescence of ammonium halides was first observed by Marrone and Kabler under X-ray irradiation at low temperatures [8]. These luminescence bands are Stokes-shifted by 1.5—2.5 eV from the fundamental absorption edge, and have been attributed to the radiative annihilation of the STE. This supposition was confirmed through the luminescence measurements by Itoh under the UV-light excitation in the exciton absorption region [9,10]. The decaytime measurements of the intrinsic luminescence were also carried out by these authors [8—101,but the fast behaviors have not been reported so far. The purpose of the present study is to investigate the fast decay behaviors of the STE luminescence observed in ammonium halides. The lattice con-
Elsevier Science Publishers B.V. All rights reserved
I 22
A. Olino ci of. / I as! decay /xhai jars of .selt—! rapped evelton lio,,ou’scense
figurations of initial states of the STE are dis— cussed in connection with the recent “on-center” and “off-center model of the STE presented for
1.5 ,
~c
2. Experiments
—~-----——————r-———-
—,-
NH
4I
alkali halides.
ha/isles
it! (i!tlttiOJ!0110
NH4Br
NH4CI
10 ~0.5
I lie sample crystals of ammonium halides were grown at room temperature from a saturated aqueous solution containing urea. Details ol the crystal growth have been described in rets. RIO]. The specimens were cooled down to 1(1 K for optical measurements. For IiV-light excitation, a 0.4 m vacuum monochromator was used in cornbination with an MgF~-windowedD~lamp. Lurninescence from the sample was detected through a grating monochromator. Lifetime measurements were carried out by using a time-correlated photon counting method under the single-bunch operation of synchrotron radiation from UVSOR in the Institute for Molecular Science (Okazaki, Japan). The interval of successive pulses was 178 ns, and the pulse width was approximately O.~ns including time response of the detection system.
—
0 30
35
40 PHOTON
50
45
ENERGY
55
(eV)
lig. I . luminescence spectra ot ammonium halides measured at II) K under the excitation with liv light of the hand-to-hand energy. Each curve has been
normalized
to unity ill the
maximum.
1 NH4I
-~
0.1
C 3
-U
3. Results Figure 1 shows luminescence spectra of ammonium halides under the UV-light excitatioti into the interhand transition at It) K. A broad luminescence hand with a Gaussian line shapeappears at 3.74 eV for NH4I. 4.18 eV for NH4Br and 4.91 eV for NH4CI, arising from the radiative annihilation of the STE [8—10].These hands are efficiently stimulated for the excitation into the exciton absorption region. It is worth noting that ammonium halides exhibit a single intrinsic hand in the ultra-violet region. Besides the main hand. a weak luminescence hand was sometimes observed in each halide, and has been attributed to some trace impurity [9,10]. Figure 2 shows decay profiles of the STE luminescence hands at 10 K under the excitation with synchrotron radiation at 6.9 eV for NH I and NH4Br and at 7.8 eV for NH4CI. In HN4I and NH4Br. one can see a piling-up effect stemming
1
~
NH~Br
0.1
1 NH4CI 0.1
0.01
0.001 0
20
40
60
80
TIME (ns) Fig. 2. Decay profiles of the STE luminescence bands in ammonium halides at It) K under the single—hunch operation of synchrotron radiation from UVSOR at 6.9 eV for NI 141 and NI-I 1Br and at 7.8 cV for N}14C1. The emission intensity has been normalized to unity at t
=
N. Ohno ci al
/
Fast decay behaviors of self-trapped exciton luminescence in ammonium halides
Table I Peak positions and decay times of the STE luminescence bands observed in ammonium halides
________________________________________________ Present work (at 10 K)
Ref. [8] (at LHeT)
peak position decay time [eV] Ens]
decay time [ns]
fast NH
4I
3.74
NH4Br 4.18 NH4CI 4.91
slow
2.0±0.2 200±20
fast —
2.0±0.2 700±50 5-10 2.7 ±0.2 s-io
slow —
770±40 —
from superposition of successive events caused by the repeated pulse excitation. it appears that the STE producing these luminescence bands of ammonium halides has two decay components; they consist not only of a long-lived component but also of a fast decay component. The fast decay component has a lifetime of 2—3 ns in the three cases. The slow component is longer in NH4Br (700 ns) than in NH4I (200 ns). The decay times obtained in the present study are listed in table 1, together with the previous results of ref. [8]. It should be noticed that the short-lived component increases in intensity in order of NH4I, NH4Br and NH4C1, while the long-lived component decreases in this order. In NH4C1, it was difficult to distinguish the existence of a slow component from the noise level.
4. Discussion As is well known, annihilation of the STE in alkali halides results in one or more luminescence bands. In most cases, fluorescence arising from a singlet state and phosphorescence from a triplet state, usually called the r and the ii band, respectively, appear as separate luminescence bands. According to the recent morphology of the STE [2—7],the Cr band, which has a small Stokes shift, has been attributed to a radiative transition of an on-center STE. On the other hand, a large Stokes shift of the i~ band has been explained by introducing an additional off-center relaxation of
123
the VK core of the STE. The off-center model can also account for the mechanism of low-ternperature defect formation such as an F—H pair creation [3]. Among the alkali halides, Nal and NaBr are quite particular materials. They have a single STE luminescence band (m- band) with an anomalously small Stokes shift in the ultra-violet region. Moreover, its transient behavior has been found to consist of two decay components of 1.0 ns (1.5 ns) and 102 ns (475 ns) in NaI (NaBr) [11]. From these results, the STE luminescence of Nal and NaBr has been explained as due to the radiative annihilation of the lowest STE with an on-center configuration, or a nearly on-center configuration [7,11,12]. As can be seen in fig. 1, ammonium-halide crystals exhibit a single intrinsic luminescence band with a small Stokes shift. These bands have been confirmed to arise from the lowest STE state on the basis of the excitation spectra; they are intensely stimulated with UV light in the n = 1 exciton region [9,10]. In addition, the present study reveals that their decay times consist of a long-lived component and of a fluorescent component with a short lifetime. These results of ammontum halides indicate a close resemblance to those of Nal and NaBr. Therefore, it is very likely that the STE in ammonium halides is situated in an on-center configuration. The initial state of the STE would be a pair of the triplet ~ state and the singlet ~ state, separated slightly by the exchange interaction [9]. According to recent arguments about the STE relaxation in alkali halides [2—7],whether the STE occupies an on-center or off-center configuration at equilibrium, the choice is sensitive to geometric parameters such as the lattice constant and ionic radius, e.g., the Rabin—Klick parameter. In ammonium halides, cations surrounding the VK core are the NH~ ions oriented in parallell (“ferro”-ordering) or in antiparallel (“antiferro”-ordering) to each other. The NH~ ion would work as a restraint on the off-center displacement of the VK core because of its tetrahedral structure. The low temperature phase of ammonium halides crystallizes in the CsCl structure, which is more close-packed compared with
I 24
\. (.!hno ci of. ,/ I—as! ilciar lie/ialiors of seff-trapped eieiton /ooiuic,see/ise in a?nttiirtuisiii ha/ides
the NaCI-type structure. This may he another reason why the on-center STE is stable in ammonium halides. From fig. 2. it is obvious that the slow coniponent is reduced in intensity and has a longer lifetime in order of NH I, NH Br and NH Cl. I hese trcnds tic reason thly explained by consid ering that the spin—orbit interaction of the hole in VK center decreases in this order, reducing the mixing of a higher excited Il state into the lowest ~ state. In conclusion, the present results suggest that the STE in ammonium halides does not relax off center, and is situated in the symmetric [VK+ ci configuration. This is in accordance with the fact that no F center is produced in ammonium halides, although the VK center does exist. -,
.
~.
Studies Program of the Institute for Molecular Science.
References [I] M - N - Kabler, Phys. Rev. 136 (1964) A I 296. [2] ( 11 Lcun6 C Brunrt md K S Sons, I
1 lix, (
IS
(1985) 4459 [3] R.T. Williams. KS. Song. WI-.. Faust and (.11. teung. Ph~s. Rev. 13 33 (1986) 7232. [4] KS. Song and Cli. Lcung. Rcv. Solid State Sci. 4 (990) [5] K Kanno. K. i:maka and T I lavashi. Rev. Solid State Sci. 4(199))) 353.
[6] M. Itoh. N. Ohno and S. I lashimoto. J. t’hys. Soc. Jpn. 59 (1990) 4534. [7] M. Itoh, S. ilashimoto, N. Ohno and K. Kanno. .1. Phvs. Soc. Jpn. 6)) (1991) 61. [5] Mi Marrone and M N Kabler. Phys Rev 176 (1968) 11)7)).
Acknowledgements The authors are grateful to Professors M. Watanahe and M. Kamada for their favorable supports in lifetime measurements under the Joint
[9] M. Itoh. J. Phys. Soc. ipn. 57 )I98S) 372. [10] M. Itoh. i. Phys. Soc. Jpn. 58 11989) 2994. Ill] K. Kanno. K. Tanaka. H. Kosaka, T. Mukai, Y. Nakai. M. Itoh. T. Miyanaga. K. Fukui and M. Watanahe. Physca Scripta 41(199)1)12(1 [12] M. Itoh, S. Ilashimoto and N. Ohno, J. Phys. Soc. Jpn. 6)) (1991) 4357.