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Effects of uniaxial stress on self-trapped excitons in RbI
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 58 (1994) 347 349
Effects of uniaxial stress on self-trapped excitons in RbI H. Nishimura*, T. Tsujimoto, S. Morimoto, M. Nakayama Department ot Applied Physics, Osaka City University, Sumiyoshi-ku, Osaka 558, Japan
Abstract Spectral changes of three self-trapped exciton luminescence bands (o~,it and Ex bands) in Rb! have been investigated under uniaxial compressive stress. The it band weakens and the Ex band grows markedly under stress, accompanied by drastic changes of decay curves. The results are discussed in terms of a stress-induced change of the potential barrier between the it and Ex states.
1. Introduction According to recent theoretical and experimental investigations, there are at least two types of selftrapped excitons (STEs) in alkali halides [1,2]. One is an electron trapped by a Yk center, called an on-center STE, and the other is a nearest-neighbor pair of F and H centers, called an off-center STE. The latter also consists of two types: strong and weak off-center STEs. Three luminescence bands in RbI originate from these three STEs. The ~ (3.90 eV) band is assigned to the on-center STE, and the it (2.16eV) and Ex (3.07eV) bands are assigned to the strong and weak off-center STEs [2,3]. It is well known that the it and Ex states are separated by only about 1 meY, and thermal activation from the it state to the Ex state occurs easily even at very low temperature [4]. This situation seems to be sensitive to lattice deformation such as that induced by hydrostatic pressure or uniaxial stress. Applying hydrostatic pressure to an Rb! crystal, Kobayashi et al. have measured a spectral change of the STE luminescence bands and *
Corresponding author,
concluded that the strong off-center STE is unstable under a pressure larger than 2 kbar [5]. In the present study, we observed drastic changes of intensities and decay curves of the STE bands in Rb! under uniaxial compressive stress. We discuss the results in terms of a stress-induced change of the adiabatic potential energy surface for these STE states.
2. Experimental procedures RbI crystals were strained by applying a cornpressive stress in the <001> direction. For this purpose, we used a specially designed cryostat in which optical measurements under uniaxial stress are possible. To produce uniaxial stress, we utilized the principle of a lever. Samples with a bytypical 3 were heat dimension ofA 4deuterium conduction. x 4 x 1 mmlamp andcooled a nanosecond light pulser combined with a grating monochromator were used for luminescence excitation. A prism rnonochrornator was used to analyze luminescent photons. Decay curves were recorded with a 400 MHz digital oscilloscope. Luminescence
0022-2313/94/$07.OO © 1994 Elsevier Science WV. All rights reserved SSDI 0022-2313(93)E0182-W
348
H. Nishimura et al.
ii-
/ Journal of Luminescence
58 (1994) 347 349
RbI 4.5 K
~
\
Temperature (K)
Obar no polarizer
‘
RbI
Er
E
1
•~
~
218 bar
Photon Energy (eV)bar) at 4.51. KSTE Fig. under luminescence a compressive spectra stress (6.53eV (218 excitation) in the in <001> RbI ~o~~ 3:Eon
Fig. 2. a Decay times of the it(186bar) and Exin 1) bands (6.53 eVdirection. excitation) under compressive stress the2<001> Olo
spectra were corrected with a sensitivity of the detecting system. When necessary, a UV-type p0larizer was used to analyze the luminescence polarization.
3. Experimental results Figure 1 shows an intensity change of three STE luminescence bands measured at 4.5K under a compressive stress in the <00 1> direction. A stress of 218 bar causes the Ex band to grow at the expense of the it band intensity, keeping the peak energies of both bands unchanged, while the ~ band is rather insensitive to the stress. These STE bands are unpolarized in unstrained samples, but in the strained sample they are polarized; the Ex band is strong when observed in the <0 1 0> direction, and so is the c~band. Uniaxial stress also changes the decay curves of the it and Ex bands (Fig. 2). The open circles and triangles represent decay times of unstrained samples, which are almost the same as those reported [4]. At 186 bar stress, the decay time of the it band deviates from the value at 0 bar. The decay time is almost constant (21 t.ts) until 14K, but above 20K it coincides with the value at 0 bar. The decay time of the Ex band is also changed drastically; three
10~(K
3.0 Rb En (0 bar) .
0
.
.
Cx (295 bar)
ir
2.0 0
20 40 60 Temperature (K) Fig. 3. Peak energies of the it and Ex bands under a compressive stress (295bar) in the <001> direction.
exponential decay components are replaced by a single component of 7 ~.ts.This value is unchanged until 35 K and then coincides with the value at 0 bar. We measured the peak energies of the it and Ex bands at 295 bar (Fig. 3). The it band, showing an anomalous blue shift at 0 bar, shows only a weak blue shift at 295 bar. Above 15K, however, the peak energies move very steeply to higher energy to coincide with the value at 0 bar. The Ex band, on the other hand, does not show such a blue shift nor a change under stress.
H. Nishimura et al.
/ Journal of Luminescence 58
.~
0~SingIet Ex
U
~
Trip’eS I
-1
TEx
0 Configuration CoordInate Q2 Fig. 4. Schematic diagram of adiabatic potential energy surfaces of the triplet and singlet STE states in RbI.
4. Discussion Figure 4 is a diagram of the adiabatic potential energy surfaces of the triplet and singlet STE states, depicting what we suppose is happening in RbI under stress. The energy surface is cut along the <110> off-center axis Q2. According to Kayanuma, the singlet surface has a minimum at the on-center configuration Q2 = 0 from which the ~ band arises, while the triplet surface does not have a minimum at Q2 = 0, but at Q2 > 0 it has double minima sharing a common ground state [6]. The two mmima are separated from each other by a very small potential barrier (‘-.~1 meV). The energy difference of the it and Ex bands is thus ascribed to the difference in the ground state energy between these two configurations. The results obtained in the present study can be interpreted by assuming the potential barrier to enlarge under uniaxial compressive stress: (1) the enlarged potential barrier prevents the excitons from relaxing from the Ex state to the it state which enhances the Ex band at the expense of the it band intensity; (2) the potential barrier prevents STEs from commuting between the it and Ex states, so that both STEs decay with their own radiative rates, 21 !~‘~ for the it band and 7 ~.tsfor the Ex band; (3) the enlarged potential barrier also prevents the it band from shifting to higher energy by making the curvature of the potential surface at the it configuration sharp. At high temperatures, thermal energy overcomes this enlarged potential barrier,
(1994) 347 349
349
so that the decay times and peak energies coincide with the values at 0 bar. Next we discuss the polarization aspect of the STE luminescence bands. The Ex band is stronger in the strained sample, and it is strongest when observed in the <0 1 0> direction (Fig. 1). STEs formed optically are distributed uniformly among the six <110> directions, and the it and Ex bands with it character are polarized perpendicularly to each <11 0> off-center axis. A compression of Rb! is known to enhance the Ex band at the expense of the it band intensity [5], and vice versa for the expansion of Rb!. Taking this into account, we deduce the following: under a compressive stress in the <0 0 1> direction, the STEs formed in the (1 00) and (0 1 0) faces suffer a compressive strain which enhances the intensity of the Ex band. Although the STEs formed in the (1 00) face make equal contributions to the enhanced intensity of the Ex band in the <001> and <0 1 0> directions, the STEs formed in the (0 1 0) face make a stronger contribution to that in the <0 1 0> directions. The STEs formed in the (00 1) face, on the other hand, suffer an expansive strain, which obscures the Ex band. It is thus reasonable that the Ex band is stronger when observed in the <0 1 0> direction than in the <00 1> direction. Finally we remark that the <00 1> compressive stress of 250 bar reduces the lattice constant by 0.1% in the <00 1> direction and expands it by 0.01% in the <0 1 0> and <1 00> directions. It should be noted that this very small strain gives rise to drastic changes of the luminescence spectra and decay times. Probably this is because the it and Ex states are on a very flat potential energy surface with a very small potential barrier (‘— 1 meV).
References [1] C..H. Leung, G. Brunet and KS. Song, J. Phys. C 18 (1985) [2] K. Kan’no, K. Tanaka and T. Hayashi, Rev. Solid State Sci. 4 (1990) 383.
[3] R.T. Williams, Hanli Liu, G.P. Williams, Jr. and Kevin, J. Platt, Phys. Rev. Lett. 66 (1991) 2140. [4] J.U. Fischbach, D. Frohlich and MN. Kabler, J. Lumin. 6 (1973) 29.
[5] M. Kobayashi, T. Hirose and H. Nishimura, J. Lumin. 48&49 (1991) 98. [6] Y. Kayanuma, Rev. Solid State Sci. 4 (1990) 403.
LUMINESCENCE ELSEVIER
Journal of Luminescence 58 (1994) 347 349
Effects of uniaxial stress on self-trapped excitons in RbI H. Nishimura*, T. Tsujimoto, S. Morimoto, M. Nakayama Department ot Applied Physics, Osaka City University, Sumiyoshi-ku, Osaka 558, Japan
Abstract Spectral changes of three self-trapped exciton luminescence bands (o~,it and Ex bands) in Rb! have been investigated under uniaxial compressive stress. The it band weakens and the Ex band grows markedly under stress, accompanied by drastic changes of decay curves. The results are discussed in terms of a stress-induced change of the potential barrier between the it and Ex states.
1. Introduction According to recent theoretical and experimental investigations, there are at least two types of selftrapped excitons (STEs) in alkali halides [1,2]. One is an electron trapped by a Yk center, called an on-center STE, and the other is a nearest-neighbor pair of F and H centers, called an off-center STE. The latter also consists of two types: strong and weak off-center STEs. Three luminescence bands in RbI originate from these three STEs. The ~ (3.90 eV) band is assigned to the on-center STE, and the it (2.16eV) and Ex (3.07eV) bands are assigned to the strong and weak off-center STEs [2,3]. It is well known that the it and Ex states are separated by only about 1 meY, and thermal activation from the it state to the Ex state occurs easily even at very low temperature [4]. This situation seems to be sensitive to lattice deformation such as that induced by hydrostatic pressure or uniaxial stress. Applying hydrostatic pressure to an Rb! crystal, Kobayashi et al. have measured a spectral change of the STE luminescence bands and *
Corresponding author,
concluded that the strong off-center STE is unstable under a pressure larger than 2 kbar [5]. In the present study, we observed drastic changes of intensities and decay curves of the STE bands in Rb! under uniaxial compressive stress. We discuss the results in terms of a stress-induced change of the adiabatic potential energy surface for these STE states.
2. Experimental procedures RbI crystals were strained by applying a cornpressive stress in the <001> direction. For this purpose, we used a specially designed cryostat in which optical measurements under uniaxial stress are possible. To produce uniaxial stress, we utilized the principle of a lever. Samples with a bytypical 3 were heat dimension ofA 4deuterium conduction. x 4 x 1 mmlamp andcooled a nanosecond light pulser combined with a grating monochromator were used for luminescence excitation. A prism rnonochrornator was used to analyze luminescent photons. Decay curves were recorded with a 400 MHz digital oscilloscope. Luminescence
0022-2313/94/$07.OO © 1994 Elsevier Science WV. All rights reserved SSDI 0022-2313(93)E0182-W
348
H. Nishimura et al.
ii-
/ Journal of Luminescence
58 (1994) 347 349
RbI 4.5 K
~
\
Temperature (K)
Obar no polarizer
‘
RbI
Er
E
1
•~
~
218 bar
Photon Energy (eV)bar) at 4.51. KSTE Fig. under luminescence a compressive spectra stress (6.53eV (218 excitation) in the in <001> RbI ~o~~ 3:Eon
Fig. 2. a Decay times of the it(186bar) and Exin 1) bands (6.53 eVdirection. excitation) under compressive stress the2<001> Olo
spectra were corrected with a sensitivity of the detecting system. When necessary, a UV-type p0larizer was used to analyze the luminescence polarization.
3. Experimental results Figure 1 shows an intensity change of three STE luminescence bands measured at 4.5K under a compressive stress in the <00 1> direction. A stress of 218 bar causes the Ex band to grow at the expense of the it band intensity, keeping the peak energies of both bands unchanged, while the ~ band is rather insensitive to the stress. These STE bands are unpolarized in unstrained samples, but in the strained sample they are polarized; the Ex band is strong when observed in the <0 1 0> direction, and so is the c~band. Uniaxial stress also changes the decay curves of the it and Ex bands (Fig. 2). The open circles and triangles represent decay times of unstrained samples, which are almost the same as those reported [4]. At 186 bar stress, the decay time of the it band deviates from the value at 0 bar. The decay time is almost constant (21 t.ts) until 14K, but above 20K it coincides with the value at 0 bar. The decay time of the Ex band is also changed drastically; three
10~(K
3.0 Rb En (0 bar) .
0
.
.
Cx (295 bar)
ir
2.0 0
20 40 60 Temperature (K) Fig. 3. Peak energies of the it and Ex bands under a compressive stress (295bar) in the <001> direction.
exponential decay components are replaced by a single component of 7 ~.ts.This value is unchanged until 35 K and then coincides with the value at 0 bar. We measured the peak energies of the it and Ex bands at 295 bar (Fig. 3). The it band, showing an anomalous blue shift at 0 bar, shows only a weak blue shift at 295 bar. Above 15K, however, the peak energies move very steeply to higher energy to coincide with the value at 0 bar. The Ex band, on the other hand, does not show such a blue shift nor a change under stress.
H. Nishimura et al.
/ Journal of Luminescence 58
.~
0~SingIet Ex
U
~
Trip’eS I
-1
TEx
0 Configuration CoordInate Q2 Fig. 4. Schematic diagram of adiabatic potential energy surfaces of the triplet and singlet STE states in RbI.
4. Discussion Figure 4 is a diagram of the adiabatic potential energy surfaces of the triplet and singlet STE states, depicting what we suppose is happening in RbI under stress. The energy surface is cut along the <110> off-center axis Q2. According to Kayanuma, the singlet surface has a minimum at the on-center configuration Q2 = 0 from which the ~ band arises, while the triplet surface does not have a minimum at Q2 = 0, but at Q2 > 0 it has double minima sharing a common ground state [6]. The two mmima are separated from each other by a very small potential barrier (‘-.~1 meV). The energy difference of the it and Ex bands is thus ascribed to the difference in the ground state energy between these two configurations. The results obtained in the present study can be interpreted by assuming the potential barrier to enlarge under uniaxial compressive stress: (1) the enlarged potential barrier prevents the excitons from relaxing from the Ex state to the it state which enhances the Ex band at the expense of the it band intensity; (2) the potential barrier prevents STEs from commuting between the it and Ex states, so that both STEs decay with their own radiative rates, 21 !~‘~ for the it band and 7 ~.tsfor the Ex band; (3) the enlarged potential barrier also prevents the it band from shifting to higher energy by making the curvature of the potential surface at the it configuration sharp. At high temperatures, thermal energy overcomes this enlarged potential barrier,
(1994) 347 349
349
so that the decay times and peak energies coincide with the values at 0 bar. Next we discuss the polarization aspect of the STE luminescence bands. The Ex band is stronger in the strained sample, and it is strongest when observed in the <0 1 0> direction (Fig. 1). STEs formed optically are distributed uniformly among the six <110> directions, and the it and Ex bands with it character are polarized perpendicularly to each <11 0> off-center axis. A compression of Rb! is known to enhance the Ex band at the expense of the it band intensity [5], and vice versa for the expansion of Rb!. Taking this into account, we deduce the following: under a compressive stress in the <0 0 1> direction, the STEs formed in the (1 00) and (0 1 0) faces suffer a compressive strain which enhances the intensity of the Ex band. Although the STEs formed in the (1 00) face make equal contributions to the enhanced intensity of the Ex band in the <001> and <0 1 0> directions, the STEs formed in the (0 1 0) face make a stronger contribution to that in the <0 1 0> directions. The STEs formed in the (00 1) face, on the other hand, suffer an expansive strain, which obscures the Ex band. It is thus reasonable that the Ex band is stronger when observed in the <0 1 0> direction than in the <00 1> direction. Finally we remark that the <00 1> compressive stress of 250 bar reduces the lattice constant by 0.1% in the <00 1> direction and expands it by 0.01% in the <0 1 0> and <1 00> directions. It should be noted that this very small strain gives rise to drastic changes of the luminescence spectra and decay times. Probably this is because the it and Ex states are on a very flat potential energy surface with a very small potential barrier (‘— 1 meV).
References [1] C..H. Leung, G. Brunet and KS. Song, J. Phys. C 18 (1985) [2] K. Kan’no, K. Tanaka and T. Hayashi, Rev. Solid State Sci. 4 (1990) 383.
[3] R.T. Williams, Hanli Liu, G.P. Williams, Jr. and Kevin, J. Platt, Phys. Rev. Lett. 66 (1991) 2140. [4] J.U. Fischbach, D. Frohlich and MN. Kabler, J. Lumin. 6 (1973) 29.
[5] M. Kobayashi, T. Hirose and H. Nishimura, J. Lumin. 48&49 (1991) 98. [6] Y. Kayanuma, Rev. Solid State Sci. 4 (1990) 403.