- Email: [email protected]
Radiative decay times and kenetics of the luminescence from KI:Sn2+
Chemical
Physics 66 (1982) 51-55
North-Holland Publishing Company
DECAY TzM@S Al%D KDJETHCS LXJMXWXSCE~CE FROM- ~Snz+f
EMXA’hlvE
OF TME
Le Si DANCi Laboratooire de Spectmm&ie
Physique, University Scientifique et M.Zdica!e de Grenoble,
58&U. Grenoble, France
P.W.M. JACOBS& K. SCHMITT$., V. S. SIVASANKAIil Department of Chemistry, University of Western Otztario, London, Canada N6A 5B7
and D.J.
SIMKIN
Department Received
of Chemisny,
McGill University, Montreai, Quebec, Canada H3A ZK6
17 July 1981
The radiative decay time r of the emission from KI:Sn2+ excited at 350 nm in the A band has been measured as a function of temperature on the range 3-300 IL The kinetics of luminescence decay following pulse excitation have been analyzed in terms of a model developed previously to explain the emission spectra of KI:Sn’+. This model fits the observed temperature-dependence of z exceedingly well. A thoro;tgh search failed to disclose the presence of an additional fast-component and so we conclude that relaxation following absorption is entirely into the lower singlet from which emission is forbidden.
The spectroscopy of the emission from S$‘doped alkali halide phosphors has been investigated previously by Zazubovich and co-workers [l-4], by Fukuda and co-workers [5-81 and by ourselves [g-12]. In this paper we report detaiIed measurements of the temperaturedependence of the luminescence decay time 7 from very low temperatures (3.8 K) up to room temperature and the analysis of these data in terms of a theoretical. model for the luminescent centres. Our previous detailed paper 1121 on the luminescence of KI:Sn*’ included measurements + Contribution No. 263 from the Photochemistry Unit, University of Wes:ern %&trio. $ Associated with,the Centre for Interdisciplinary Studies in Chemical Physics, University of Western Ontario.
0301-0104/82/0000-0000/$02.75
of 5 up to about 20 K: the new measurements which were made by a different technique, overlap the old measurements and thus confirm their accuracy, while the extended temperature range permits a thorough test of the model formulated earlier [12].
2. Model for Sil’+ luminescent centres Each Sn2* ion introduced into a KI crystal is accompanied by a cation vacancy. During . quenching the Sn” ions and cation vacancies (CV) tend to associate with one another forming defect complexes with the cv in either a nearest neighbour (nn) or next nn (mm) position relative to the %n’+ ion. Thus the quenched crystal may co&in centres in cubic sites (itilated Sn*+-), tetragonal sites (nnncv), or sites of
@ 1982 North-Holland
orthorhombic symmetry (nncv). The splitting of the A= emission band into t‘x’o components at 2.21 and 2.40 eV is due to the electrostatic field caused by a nearby cation vacancy although one cannot determine from these data alone whether the vacancy occupies a nn or nnn cation site [12]. When the Jahn-Teller (JT) coupling to lattice modes of E, symme’uy is much stronger thn the spin-orbit coupling or vacancy perturbations, the relaxed excited state (RES) consists of three equivalent potential energy wells which correspond to JT distortions along the [loo]. [OlO] and [OOl] axes (fig. 1). A [loo] JT distortion lowers the energy of the X orbital state by .&* (the Jahn-Teller srabilization energy) and raises that of the Y and Z states by 2Err. Correspondingly [OlO] and [OOl] distortions lower the energy of the Y and Z orbitaI states respectively (fig. 1). Each of these orbital states has a potential three-fold spin degeneracy which is split by spin-orbit coupling in second order [13]. In the [loo] well the separation of the singlet Xx and the doublet Xv, Xz, where x, y, z denote the spin functions x =-2-“‘[a(l)a(2)-p(l)P(2)], )’ =2-“‘i[a(l)rr(2)
dnrjdt
= -(2klt +kl)nl + kz1n2= -nJ+;
(1)
dnzjdt
= 2klgzl-
(2)
(kzl+ kt)nz= --nz/r,
where T is the measured decay time of the emission foliowing pulsed excitation in the Pband. The one-phonon transition rates between the levels are controlled by the rate constants ktt = MB,
(3)
k21 = #(E + i),
(4)
where ii = [eexp (D/kT) - l]-‘,
+p(l)p(2)],
(5)
is the average number of phonons in the mode with energy ho =D. The decay time T satisfies the quadratic equation
2 =2-*“[a(l)P(2)Ccu(2)P(l)], is D =
in the static JT limit, where 4 is the spin-orbit coupling constant for a Sp electron in Sn” in the crystal field. The dashed arrows from the Xx, Yy and Zz levels in fig. 1 emphasize that optical transitions to the ground state from these lower singlet 1eveIs are symrretry- and spin-forbidden, whereas transitiorfi from the doublet levels are symmetry allowed. Let nl denote the occupation number of level 1 (the singlet) and n2 that of the doublet (fig. 2). Then in the notation of fig. 2,
T-2+bf-1+c=o,
(~/2)‘/3Err,
(6)
where JT axis :I
Km
b=-(klskz+2k12+k21),
food
Y
1. The ‘P
‘. (8)
2
AT
AESO~TIOH Fig.
c =2klzkz+k21klfklk2.
(7)
(I=
0; 1.2)
state in a cubic f&Id immediately
following absorption of a photon and after relaxation. supposed that JT*SO.
It is
Fig.t.‘EnergyIevelsand transition probabilities
in the FES.
:
L.S. Dang et al. / Luminescence from Ki : Sn”
3. Experimental rest&s and data analysis The Kf:St?* crystals.used were cut from the same specimens as had been used in an earlier investigation [12]. Cleaves were quenched from 450°C to rooxh temperature before mounting in an Oxford Instrument Co. Cl04 cryostat. The cryostat could be mounted directly in the sample chamber of a Photochemical Research Associates (PRA) System 3000 photon counting fluorescence lifetime instrument, equipped with a PRA SlOC nanosecond excitation Iamp. A typical decay curve ,measured at 7f) K is shown in fig. 3. This shows the background counts, the lamp spike, and the best fit (a single exponential decay) to the experimental data after deconvolution of the lamp profile. The values of the decay time T derived from these measurements were combined with lowtemperature data (which they overlapped) measured earlier [12] using a pulsed nitrogen laser and a Biomation 805 transient recorder, and then least-squares fitted to the (larger) root of eq. (6). This has two real roots 7-l corresponding to fast and slow decay modes. The fit of the theoretical equation to the combined sets of data is shown in figs. 4 and 5; numerical
CHANNEL NUMBER 80 120 160 200
40
240
4:
E z z .- !F3
.,
:.. .;y_
;.
I\___:.-:.-.
2 G
,. .i-. :. . i.....:: ._j _ -: .._ _,.,..
2
2 _.,,i
p
: .:.-
3
‘.:
s 1; 0
.
..
1 I-, 7
2 3 4 TiMEIMICROSECONDS
5
6
Fig. 3. Emission from KI:SnZ’ following pulse excitation at 70 EC.The straight Iine is the best fit to the data and shows that the decay follows a single expoilential law.
53
lo-3l D = 33.67cm-’
I
I
I
10-S
x
cc
ii
Y
id6
jf____---___._’
I
GO
0.1
02
0.3
0.4
0.5
I 0.6
(l/T) K-’
Fig. 4. Plot of emission decay time T of KI:Sr?’ against T-‘, showing that r becomes constant at low temperature. The continuous and dashed lines were calculated from eq. (6) using the values of the parameters shown in the figure.
values of pvameters are included in fig. 4. Plots of 7 against both T and T-’ are necessary to show the excellent fitting of the data and the levelling off of r at both high and low temperatures, since a plot against T compresses the low temperature results while T-’ does the same to the high-temperature end. The dashed lines in both figures show the predicted fast decay. This is well within the range of the PRA instrument but a thorough search failed to reveal any evidence for a fast decay mode. This imphes that at least 99% of the relaxation is into the XX, Yy, Zr singlets, in contrast to KBr:Ga’ [14] where relaxation occurs predominantly into the radiative doub!et in the A-r RES and equally into the three levels of- the AX RES. Figs. 4 and 5 emphasize the excellent correspondence of the decay time measurements made by different experimenters in difierent laboratories using different techniques.
34
L.S. Dang er al. i Luminescence from Ii;l : Sn”
5. Coadusion When the Jahn-Teller coupling is much stronger than spin-orbit coupling, the appropriate model for the RES of letragonal symmetry of an sz ion in an afkaii halide crystal is one described by the wavefunctions: singlet Kjc and doublet Xy, Xz, when the 3T axis is parallel to [IOO](with appropriate cyclic permutations for. [OlO] and [OOlj). This model has been verified quantitatively for KI:Sn” by fitting the temperature-dependence of the emission decay. times measured between 3.8 R and room temperature.
Acknowledgement
Fig. 5. Plot of emission decay time : of KI:Sn2’ against z showing that the slow decay becomes constant at high temperatures.
D.S. and P.W.M.J. thank the Natural Sciences and Engineering Iiesearch Ccuncil of Canada for financial support in the form of individual operating grants and a COOP grant. We are also indebted to the Quebec Ministry of Intergovernmental Affairs and the Ministry of Education of Ontario for support under the auspices of Quebec-Ontario exchange program.
4. Discussion References
The radiative lifetimes of the singlet and doublet k;’ and k;’ are reasonable considering that the transitions occur only because of spinorbit coupling; in addition thz transition from the lower spin-singlet is symmetry forbidden, Three temperature re@mes may be recognized in F&s. 4 and 5. At high temperatures relaxation within the wells is so rapid that the observed decay time is that for the allowed transition JC;‘, multiplied bv the factor (g + U/g, with the degeneracy g = ?-in this case. At intermediate temperatures the decay time is controlled by relaxation between the spin states and log P is a linear function of T-’ with slope equal to D/k. Below 4 K the attainment of thermal equilibrium is so slow (2k12~r-‘) that emission occurs from the singlet with : = k;‘.
[l] S.G. Zazubovich, Opt Spktry. 37 (1974) 711 [Optika i Sepkroskopya (USSR) 37 (1974) 4041. [2] V. Hizhnayakov, S. Zazubovich and ‘I’. Soovik, Phys. Stat. Sol. B 66 (1974) 727. [3] V. Hizhnayakov and S. Zazubovich, Phys. Stat. Sol. B 86 (1978) 733. [4] E. Realo and S. Zazubovich. Phys. Stat. Sol. B 57 (1973) 69. [5] A. Fukuda, K. Inohara and R. Onaka, J. Phys. Sot. Jpn. 19 (1964) 1274. [6] A. Fukuda, Phys. Rev. Letters 26 (1971) 314. [7] A. Fukuda and P. Yuster, Phys. Rev. Letters 28 (1972) 1032. [S] A. Fukuda, Solid State Commun. 12 (1973) 1039. [9] P.W.M. Jacobs. Y. Kamishina, L.L. Coatsworth and M.J. Stillman, J. Luzninescence 18/19 (1979) ?19. [lo] DJ. Simkin. K.O. Gannon, J.P. Martin, Y. Kamishina and P.W.M. Jambs, J. Luminescence N/19 (1979) 623.
L.S. Dnng et al. / Luminescence [ill
D.J. Simkin. J.P. Martin; L.S. Dang and Y. Kamishina,. Chem. PhF. Letters 65 (1979) 569. .[12] Y. KFishina, P.W.M. Jacobs, D.J. Simkin, J.-P. Martin, K.O. Gannon and L.S. Dang, Phys. Rev. B 22 (1980) 3010_
-_
frim KT:Sn’+
1131 F.S. Ham, Phys:Rev. 138 (1965) A1727. [14] L.S. Dang, R. Romestain and D. Siiin, Phys. Rev. B18 (1978) 2989.
5s
Physics 66 (1982) 51-55
North-Holland Publishing Company
DECAY TzM@S Al%D KDJETHCS LXJMXWXSCE~CE FROM- ~Snz+f
EMXA’hlvE
OF TME
Le Si DANCi Laboratooire de Spectmm&ie
Physique, University Scientifique et M.Zdica!e de Grenoble,
58&U. Grenoble, France
P.W.M. JACOBS& K. SCHMITT$., V. S. SIVASANKAIil Department of Chemistry, University of Western Otztario, London, Canada N6A 5B7
and D.J.
SIMKIN
Department Received
of Chemisny,
McGill University, Montreai, Quebec, Canada H3A ZK6
17 July 1981
The radiative decay time r of the emission from KI:Sn2+ excited at 350 nm in the A band has been measured as a function of temperature on the range 3-300 IL The kinetics of luminescence decay following pulse excitation have been analyzed in terms of a model developed previously to explain the emission spectra of KI:Sn’+. This model fits the observed temperature-dependence of z exceedingly well. A thoro;tgh search failed to disclose the presence of an additional fast-component and so we conclude that relaxation following absorption is entirely into the lower singlet from which emission is forbidden.
The spectroscopy of the emission from S$‘doped alkali halide phosphors has been investigated previously by Zazubovich and co-workers [l-4], by Fukuda and co-workers [5-81 and by ourselves [g-12]. In this paper we report detaiIed measurements of the temperaturedependence of the luminescence decay time 7 from very low temperatures (3.8 K) up to room temperature and the analysis of these data in terms of a theoretical. model for the luminescent centres. Our previous detailed paper 1121 on the luminescence of KI:Sn*’ included measurements + Contribution No. 263 from the Photochemistry Unit, University of Wes:ern %&trio. $ Associated with,the Centre for Interdisciplinary Studies in Chemical Physics, University of Western Ontario.
0301-0104/82/0000-0000/$02.75
of 5 up to about 20 K: the new measurements which were made by a different technique, overlap the old measurements and thus confirm their accuracy, while the extended temperature range permits a thorough test of the model formulated earlier [12].
2. Model for Sil’+ luminescent centres Each Sn2* ion introduced into a KI crystal is accompanied by a cation vacancy. During . quenching the Sn” ions and cation vacancies (CV) tend to associate with one another forming defect complexes with the cv in either a nearest neighbour (nn) or next nn (mm) position relative to the %n’+ ion. Thus the quenched crystal may co&in centres in cubic sites (itilated Sn*+-), tetragonal sites (nnncv), or sites of
@ 1982 North-Holland
orthorhombic symmetry (nncv). The splitting of the A= emission band into t‘x’o components at 2.21 and 2.40 eV is due to the electrostatic field caused by a nearby cation vacancy although one cannot determine from these data alone whether the vacancy occupies a nn or nnn cation site [12]. When the Jahn-Teller (JT) coupling to lattice modes of E, symme’uy is much stronger thn the spin-orbit coupling or vacancy perturbations, the relaxed excited state (RES) consists of three equivalent potential energy wells which correspond to JT distortions along the [loo]. [OlO] and [OOl] axes (fig. 1). A [loo] JT distortion lowers the energy of the X orbital state by .&* (the Jahn-Teller srabilization energy) and raises that of the Y and Z states by 2Err. Correspondingly [OlO] and [OOl] distortions lower the energy of the Y and Z orbitaI states respectively (fig. 1). Each of these orbital states has a potential three-fold spin degeneracy which is split by spin-orbit coupling in second order [13]. In the [loo] well the separation of the singlet Xx and the doublet Xv, Xz, where x, y, z denote the spin functions x =-2-“‘[a(l)a(2)-p(l)P(2)], )’ =2-“‘i[a(l)rr(2)
dnrjdt
= -(2klt +kl)nl + kz1n2= -nJ+;
(1)
dnzjdt
= 2klgzl-
(2)
(kzl+ kt)nz= --nz/r,
where T is the measured decay time of the emission foliowing pulsed excitation in the Pband. The one-phonon transition rates between the levels are controlled by the rate constants ktt = MB,
(3)
k21 = #(E + i),
(4)
where ii = [eexp (D/kT) - l]-‘,
+p(l)p(2)],
(5)
is the average number of phonons in the mode with energy ho =D. The decay time T satisfies the quadratic equation
2 =2-*“[a(l)P(2)Ccu(2)P(l)], is D =
in the static JT limit, where 4 is the spin-orbit coupling constant for a Sp electron in Sn” in the crystal field. The dashed arrows from the Xx, Yy and Zz levels in fig. 1 emphasize that optical transitions to the ground state from these lower singlet 1eveIs are symrretry- and spin-forbidden, whereas transitiorfi from the doublet levels are symmetry allowed. Let nl denote the occupation number of level 1 (the singlet) and n2 that of the doublet (fig. 2). Then in the notation of fig. 2,
T-2+bf-1+c=o,
(~/2)‘/3Err,
(6)
where JT axis :I
Km
b=-(klskz+2k12+k21),
food
Y
1. The ‘P
‘. (8)
2
AT
AESO~TIOH Fig.
c =2klzkz+k21klfklk2.
(7)
(I=
0; 1.2)
state in a cubic f&Id immediately
following absorption of a photon and after relaxation. supposed that JT*SO.
It is
Fig.t.‘EnergyIevelsand transition probabilities
in the FES.
:
L.S. Dang et al. / Luminescence from Ki : Sn”
3. Experimental rest&s and data analysis The Kf:St?* crystals.used were cut from the same specimens as had been used in an earlier investigation [12]. Cleaves were quenched from 450°C to rooxh temperature before mounting in an Oxford Instrument Co. Cl04 cryostat. The cryostat could be mounted directly in the sample chamber of a Photochemical Research Associates (PRA) System 3000 photon counting fluorescence lifetime instrument, equipped with a PRA SlOC nanosecond excitation Iamp. A typical decay curve ,measured at 7f) K is shown in fig. 3. This shows the background counts, the lamp spike, and the best fit (a single exponential decay) to the experimental data after deconvolution of the lamp profile. The values of the decay time T derived from these measurements were combined with lowtemperature data (which they overlapped) measured earlier [12] using a pulsed nitrogen laser and a Biomation 805 transient recorder, and then least-squares fitted to the (larger) root of eq. (6). This has two real roots 7-l corresponding to fast and slow decay modes. The fit of the theoretical equation to the combined sets of data is shown in figs. 4 and 5; numerical
CHANNEL NUMBER 80 120 160 200
40
240
4:
E z z .- !F3
.,
:.. .;y_
;.
I\___:.-:.-.
2 G
,. .i-. :. . i.....:: ._j _ -: .._ _,.,..
2
2 _.,,i
p
: .:.-
3
‘.:
s 1; 0
.
..
1 I-, 7
2 3 4 TiMEIMICROSECONDS
5
6
Fig. 3. Emission from KI:SnZ’ following pulse excitation at 70 EC.The straight Iine is the best fit to the data and shows that the decay follows a single expoilential law.
53
lo-3l D = 33.67cm-’
I
I
I
10-S
x
cc
ii
Y
id6
jf____---___._’
I
GO
0.1
02
0.3
0.4
0.5
I 0.6
(l/T) K-’
Fig. 4. Plot of emission decay time T of KI:Sr?’ against T-‘, showing that r becomes constant at low temperature. The continuous and dashed lines were calculated from eq. (6) using the values of the parameters shown in the figure.
values of pvameters are included in fig. 4. Plots of 7 against both T and T-’ are necessary to show the excellent fitting of the data and the levelling off of r at both high and low temperatures, since a plot against T compresses the low temperature results while T-’ does the same to the high-temperature end. The dashed lines in both figures show the predicted fast decay. This is well within the range of the PRA instrument but a thorough search failed to reveal any evidence for a fast decay mode. This imphes that at least 99% of the relaxation is into the XX, Yy, Zr singlets, in contrast to KBr:Ga’ [14] where relaxation occurs predominantly into the radiative doub!et in the A-r RES and equally into the three levels of- the AX RES. Figs. 4 and 5 emphasize the excellent correspondence of the decay time measurements made by different experimenters in difierent laboratories using different techniques.
34
L.S. Dang er al. i Luminescence from Ii;l : Sn”
5. Coadusion When the Jahn-Teller coupling is much stronger than spin-orbit coupling, the appropriate model for the RES of letragonal symmetry of an sz ion in an afkaii halide crystal is one described by the wavefunctions: singlet Kjc and doublet Xy, Xz, when the 3T axis is parallel to [IOO](with appropriate cyclic permutations for. [OlO] and [OOlj). This model has been verified quantitatively for KI:Sn” by fitting the temperature-dependence of the emission decay. times measured between 3.8 R and room temperature.
Acknowledgement
Fig. 5. Plot of emission decay time : of KI:Sn2’ against z showing that the slow decay becomes constant at high temperatures.
D.S. and P.W.M.J. thank the Natural Sciences and Engineering Iiesearch Ccuncil of Canada for financial support in the form of individual operating grants and a COOP grant. We are also indebted to the Quebec Ministry of Intergovernmental Affairs and the Ministry of Education of Ontario for support under the auspices of Quebec-Ontario exchange program.
4. Discussion References
The radiative lifetimes of the singlet and doublet k;’ and k;’ are reasonable considering that the transitions occur only because of spinorbit coupling; in addition thz transition from the lower spin-singlet is symmetry forbidden, Three temperature re@mes may be recognized in F&s. 4 and 5. At high temperatures relaxation within the wells is so rapid that the observed decay time is that for the allowed transition JC;‘, multiplied bv the factor (g + U/g, with the degeneracy g = ?-in this case. At intermediate temperatures the decay time is controlled by relaxation between the spin states and log P is a linear function of T-’ with slope equal to D/k. Below 4 K the attainment of thermal equilibrium is so slow (2k12~r-‘) that emission occurs from the singlet with : = k;‘.
[l] S.G. Zazubovich, Opt Spktry. 37 (1974) 711 [Optika i Sepkroskopya (USSR) 37 (1974) 4041. [2] V. Hizhnayakov, S. Zazubovich and ‘I’. Soovik, Phys. Stat. Sol. B 66 (1974) 727. [3] V. Hizhnayakov and S. Zazubovich, Phys. Stat. Sol. B 86 (1978) 733. [4] E. Realo and S. Zazubovich. Phys. Stat. Sol. B 57 (1973) 69. [5] A. Fukuda, K. Inohara and R. Onaka, J. Phys. Sot. Jpn. 19 (1964) 1274. [6] A. Fukuda, Phys. Rev. Letters 26 (1971) 314. [7] A. Fukuda and P. Yuster, Phys. Rev. Letters 28 (1972) 1032. [S] A. Fukuda, Solid State Commun. 12 (1973) 1039. [9] P.W.M. Jacobs. Y. Kamishina, L.L. Coatsworth and M.J. Stillman, J. Luzninescence 18/19 (1979) ?19. [lo] DJ. Simkin. K.O. Gannon, J.P. Martin, Y. Kamishina and P.W.M. Jambs, J. Luminescence N/19 (1979) 623.
L.S. Dnng et al. / Luminescence [ill
D.J. Simkin. J.P. Martin; L.S. Dang and Y. Kamishina,. Chem. PhF. Letters 65 (1979) 569. .[12] Y. KFishina, P.W.M. Jacobs, D.J. Simkin, J.-P. Martin, K.O. Gannon and L.S. Dang, Phys. Rev. B 22 (1980) 3010_
-_
frim KT:Sn’+
1131 F.S. Ham, Phys:Rev. 138 (1965) A1727. [14] L.S. Dang, R. Romestain and D. Siiin, Phys. Rev. B18 (1978) 2989.
5s