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Lifetime of the Z1 centre in KC1:Sr
0038-1098/81/020143-03502.00/0
Solid State Communications, Vol. 38, pp. 143-145. Pergamon Press Ltd. 1981. Printed in Great Britain.
LIFETIME OF THE Zt CENTRE IN KCI : Sr L. Bosi*, P. GaNiardelli and M. Nimis* Istituto di Fisica del Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy
(Received lOJuly 1980 by M. Cardona) Data on the radiative lifetime r(ZO of the relaxed excited state of the Zt centre in KCI :Sr in the 63--135 K range are reported for the first time It(Z,) = 194 ± 10nsec at 63 K]. The activation energy of deexcitation (zXE = 0.132 --+0.008 eV) for the luminescence process is in agreement with the value obtained by Paus from a quantum yield analysis. The results are discussed on the basis of an analogy with the F-centre behaviour. Z c C E N T R E EblISSION was studied in KC1 doped with Sr, Ca, Ba, in the classic work of Paus [ 1 ], who found that the emission bands are peaked below I eV (0.855 eV, half-width: 0.283 eV, in KC1 :Sr). In 1968, Bertolaccini et at, introduced the technique of measuring colour centre lifetime in alkali halides by determining the delay distribution of the sin~e photons emitted after a pulsed excitation [2, 3]. With this technique it is possible to investigate luminescence that is weak [4] or scarcely [5] detectable in view of the photomultiplier response, and so to overcome the difficulty of measuring the radiative lifetime, r(ZO, of the relaxed excited state of the Z l centre, due to the emission band position. We used a Philips 56 CVP photomultiplier, which is the best available in the spectral range between 1.3 and 1 eV, for use with our technique. The sample was a KC1 : SrC12 (0.06 mol.% in the melt), 3.87 mm thick, additively coloured, with about 4.3 x 10L6Fcm -3. The experimental apparatus for lifetime measurements was practically the same as that already described [6]. The exciting source was [6] a hydrogen spark lamp whose emission curve has a width o f ~ 6 n s e c at half maximum and ~ 16nsec at 10% of the maximum, with a working frequency of 5 Kc. The optical tilter used in the excitation channel was a BG18 Jenaer Glaswerke and in the detection channel a 87 A Wratten Kodak. Optical F--, Zt conversion was obtained at 243 K using a 150 W Philips 7158 lamp and a 525 nm Zeiss interference Fdter. The irradiation time was about 90 min.: even if absorption spectra indicated a complete F ~ Z 1 conversion, it always happened that F-luminescence [6, 7] was present in the decay curves, together with M-luminescence [6]. This fact is explained on the basis of the strong wavelength dependence of the * Gruppo Nazionale di Struttura della Materia del C.N.R.
photomultiplier response (F- and M-luminescence are at about 1 eV). In this way, a concentration o f F - and M-centres, practically undetectable by absorption measurements (an Ml-band of ~ 0.02 optical density was observed in our sample), produces strong components in the decay spectra. Moreover, an unknown slow component appears, whose relative intensity, with respect to the integrated area of the decay curves, increases as a function of the temperature, especially above 100 K (i.e. when the absolute quantum yield of the F- and Zl-centres decreases); its lifetime varies roughly from 1.1 ~sec to 100 nsec throughout the temperature range investigated. In spite of these difficulties, we have been able to investigate the ZI lifetime at different temperatures T (Fig. 1), even if data above 110 K are consequently less accurate. Figures 2(a) and (b) show some examples of the decay curves. Low temperature r(Zt)values are: 194 -+ 10 nsec at 63 K and 182 ± 10 nsec at near [aNT. It may be noted that the plot of r(Zl) vs T is similar to the quantum yield curve [ 1 ] and that the resulting activation energy of deexcitation AE (0.132 -+ 0.008 eV) is in close agreement with the value (0.131 eV) obtained from a quantum yield analysis [I]. It is worth mentioning that the radiative lifetime estimation from the quantum yield analysis suggests [ i ] a value (60 nsec) smaller than our experimental one. Let us now discuss the r(Z 0 value we found in the low-temperature region. In this context, we recall that the Zccentre is considered a perturbed F-centre on account of many similarities [ 1, 8 - 1 0 ] . These include the one [1] between the effective lattice frequency were of the Zvcentre emission band and the Were for the F-centre in KCI [ 1], and that between the emission halfwidths, which are proportional [ 11 ] to coem. The diffuse nature of the relaxed excited state of the F-centre has already been experimentally evidenced (see, for example,
143
144
LIFETIME OF THE Zt CENTRE IN KC1 : Sr
effective fields: (2J + I) factors arise from degeneracy of the levels (J angular momentum). Taking into account our suggestions, we also made the foilowing assumptions:
r
L
"{CI Sect,
Z~ cen{re decay
4
,&E = O~3Z ":-0008 aV
!0
50
i
I0
]
80
I
'
90 100 110 IEM?ERAIURE (K)
'
120
130
[
ILO
Fig. 1. Lifetime of the Zx-centre as a function of temperature. The activation energy of deexcitation is calculated by assuming the usual two-channel decay scheme. [7, 12-15] ) and theoretically justified [ 16, 17]. The experimental [11] statement ¢Oem COT, the lattice transverse optical frequency, has been tentatively interpreted [11] as further evidence of the diffuse nature of the F-centre relaxed excited state (indeed, coT is referred to K = 0). Since the effective lattice frequencies of the absorption and emission bands are almost the same for the Z~-centre (unlike what happens with the F-centre) and since its absorption and emission band halfwidtbs are not too different, we made the drastic assumption that, for the Z~ -centre, the ground state, is a diffuse one. This assumption is not justified a priori, even though the above similarities and difference in the F- and ZI -centre behaviour might support it: however, we tried to use it t(> calculate the r(Z t) value from the Fowler-Dexter formula (relationship 21 of [17]), which gives us the radiative lifetime of luminescent centres in ionic solids: =
1 I. r,~
-
fmk
¢o
ke~,.r(E,~m)J
.12 h2c3m(*) Ern,~ ~ "
"
x
l(rz,a>!2(2J~ + 1) ×
Vot. 38, No. 2
(i) t), = 0.58, on the basis of the t~ value (0.68), obtained by Bosi e/ al. [18]. and of the ¢),/j'}-ratio [1]: (ii) e~ef" eo, which is generally valid [17] when the centre is diffuse; (iii) m (*) -~ 0.5 m~ [ l 9] : this is also the value used in the F-centre case [13] ; (iv) I(ra-r)l'- "" [(r-r6)[a, based on the similarity of the absorption and emission properties for the Zl-centre and on the drastic assumption regarding the diffuse nature of the involved states; (v) (2J~ + l)(2J,~ + 1)-' = 3, as in the case of the F-centre [I7]. In this way we calculated r(Zl) = 135 nsec. The discrepancy between this value and our experimental one is not marked. In our opinion, it may be due to the above drastic assumption concerning the [(r)i" values, which strongly depend on the wave function parameters of the ground and excited states [13, 16]. Incidentally, we note that the tong F-centre radiative lifetime (795 nsec at LHeT in KCI [7]) has been explained by assuming [13, 16] that the relaxed excited state is diffuse, and that the ground state is rather compact and, consequently, that i(r8~l)I2 --~ I(r.rs)12; this has been supported by ESR measurements [15]. We should also like to stress that the calculated lifetime value may be better matched with the experimental one, by assuming [(r~.r)l 2 to be only somewhat lower than I(r-~,)1"-• In conclusion, we believe that our technique may be useful for performing lifetime measurements for all types of Z-centre when photomultipliers with a better response curve in the wavelength range in question become available.
8,3'
REFERENCES
Y ll'-(:J~ + t) ,5,y
The subscripts k and m denote electronic levels: the first letter refers to the electronic level of the system before a transition takes place, and the second refers to the level into which the transition is made. The dipole matrix element I(rs@[", is expressed in terms of the electronic wave functions of the initial and final states/5 and 7; .¢" is the oscillator strength, r~z(*/, for diffuse centres, is the effective mass and, for very tightly-bound centres, the electronic mass me [17] ;Ern~, and Ek, n are the absolute values of the energies of emission and absorption, respectively; n(Ek~ ) is the refraction index evaluated at peak emission energy; eo and eerr are the average and the
1. 2. 3. 4. 5. 6. 7. 8.
H.J. Paus, Z. Phys. 218, 56 (1969). M. Bertolaccini, L. Bosi, S. Corn, C. Bussolati & G. Spinolo, 1968 Int. S)'mp. opz Color Ce~zters in Alk'ali Halides Roma, Italy (Paper 20). S. Cova, M. Bertolaccini & C. Bussolati, P/,ys. Statz~s Solidi(a) 18, 11 (1973). L. Bosi, A. Longoni & M. Nimis. Phys. Srarz,s Solidi (b) 89,221 (1978). L. Bosi & M. Nimis, Phys. Statzts Solidi (b) 85, K27 (1978). L. Bosi, C. Bussolati & G. Spinolo, Ph_vs. Rev. BI, 890 (1970). L. Bosi, S. Cova & G. Spinolo, PIzvs. Rex,. B9, 4542 (1974). H. I-l~irtel & F. I_£tty, Z. Phys. 182. 111 (1964).
Vol. 38, No. 2
145
LIFETIME OF THE Zt CENTRE LN KC1 : Sr
It:
~ ,
T : 11g K
tJ~
g o t.)
u. 102 o
z
le
I
I
50
100
I
I
I
I
150 CHANNELS
I*
200
250
I
I
I
KC[ :SrC[ z
10 4
T=79K
t ( M ) = 30 *-3ns z(Z 0 =182 ± 10ns t ( F ) =540 z 20ns
103
21 74 ns/channels
o
10 2 c~ bJ D Z
1! 0
F
/M 50
I 100
{ 150 CHANNELS
÷÷
200
*L 250
Fig. 2. (a) and (b). Some examples of the decay curves in additively coloured KCISrC12, after F-+ Z 1 conversion; (little crosses represent averages over 8 channels). 9. I0. 11. 12. 13. 14.
J.C. Bushnell, Thesis Univ. Illinois (1964);Int. Syrup. on Color Centers in Alkali Halides, Abstract 25. Urbana (1965). H.J. Paus & F. Uuty,Phys. Rev. Lett. 20, 57 (1968). L. Bosi & M. Nimis, Phys. Status Solidi(b) 98, KI51 (1980). F. LiJty, Physics of Color Centers (Edited by W.B. Fowler), Chapter 3. Academic Press, New York and London (1968). L. Bosi, P. Podini & G. Spinolo,Phys. Rev. 175, 1133 (1968); L. Bosi, S. Cova & G. Spinolo, Phys. Status Solidi (b) 68,603 (1975). L. Bosi & M. Nimis, Phys. Status Solidi (b) 95, 615 (1979).
15. 16. 17. 18.
19.
H.J. Reyher, K. Hahn, Th. Vetter & A. Winnacker, Z. Phys. B33,357 (1979). W.B. Fowler, Phys. Rev. 135, A1725 (1964). W.B. Fowler&D.L. Dexter, Phys. Rev. 128,2154 (1962). L. Bosi, A.L. Fantola-Lazzarini & E. Lazzarini, Phys. Status Solidi (b) 66,285 (I 974); L. Bosi, A.L. Fantola-Lazzarini, E. Lazzarini, V. Marigliano-Ramaglia & A. Tagliacozzo, Phys. Stares Solidi (b) 69, 519 (1975). G.W. Hodby, J,A. Borders, F.C. Brown & S. Foner, Phys. Rev. Lett. 19, 952 (1967").
Solid State Communications, Vol. 38, pp. 143-145. Pergamon Press Ltd. 1981. Printed in Great Britain.
LIFETIME OF THE Zt CENTRE IN KCI : Sr L. Bosi*, P. GaNiardelli and M. Nimis* Istituto di Fisica del Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy
(Received lOJuly 1980 by M. Cardona) Data on the radiative lifetime r(ZO of the relaxed excited state of the Zt centre in KCI :Sr in the 63--135 K range are reported for the first time It(Z,) = 194 ± 10nsec at 63 K]. The activation energy of deexcitation (zXE = 0.132 --+0.008 eV) for the luminescence process is in agreement with the value obtained by Paus from a quantum yield analysis. The results are discussed on the basis of an analogy with the F-centre behaviour. Z c C E N T R E EblISSION was studied in KC1 doped with Sr, Ca, Ba, in the classic work of Paus [ 1 ], who found that the emission bands are peaked below I eV (0.855 eV, half-width: 0.283 eV, in KC1 :Sr). In 1968, Bertolaccini et at, introduced the technique of measuring colour centre lifetime in alkali halides by determining the delay distribution of the sin~e photons emitted after a pulsed excitation [2, 3]. With this technique it is possible to investigate luminescence that is weak [4] or scarcely [5] detectable in view of the photomultiplier response, and so to overcome the difficulty of measuring the radiative lifetime, r(ZO, of the relaxed excited state of the Z l centre, due to the emission band position. We used a Philips 56 CVP photomultiplier, which is the best available in the spectral range between 1.3 and 1 eV, for use with our technique. The sample was a KC1 : SrC12 (0.06 mol.% in the melt), 3.87 mm thick, additively coloured, with about 4.3 x 10L6Fcm -3. The experimental apparatus for lifetime measurements was practically the same as that already described [6]. The exciting source was [6] a hydrogen spark lamp whose emission curve has a width o f ~ 6 n s e c at half maximum and ~ 16nsec at 10% of the maximum, with a working frequency of 5 Kc. The optical tilter used in the excitation channel was a BG18 Jenaer Glaswerke and in the detection channel a 87 A Wratten Kodak. Optical F--, Zt conversion was obtained at 243 K using a 150 W Philips 7158 lamp and a 525 nm Zeiss interference Fdter. The irradiation time was about 90 min.: even if absorption spectra indicated a complete F ~ Z 1 conversion, it always happened that F-luminescence [6, 7] was present in the decay curves, together with M-luminescence [6]. This fact is explained on the basis of the strong wavelength dependence of the * Gruppo Nazionale di Struttura della Materia del C.N.R.
photomultiplier response (F- and M-luminescence are at about 1 eV). In this way, a concentration o f F - and M-centres, practically undetectable by absorption measurements (an Ml-band of ~ 0.02 optical density was observed in our sample), produces strong components in the decay spectra. Moreover, an unknown slow component appears, whose relative intensity, with respect to the integrated area of the decay curves, increases as a function of the temperature, especially above 100 K (i.e. when the absolute quantum yield of the F- and Zl-centres decreases); its lifetime varies roughly from 1.1 ~sec to 100 nsec throughout the temperature range investigated. In spite of these difficulties, we have been able to investigate the ZI lifetime at different temperatures T (Fig. 1), even if data above 110 K are consequently less accurate. Figures 2(a) and (b) show some examples of the decay curves. Low temperature r(Zt)values are: 194 -+ 10 nsec at 63 K and 182 ± 10 nsec at near [aNT. It may be noted that the plot of r(Zl) vs T is similar to the quantum yield curve [ 1 ] and that the resulting activation energy of deexcitation AE (0.132 -+ 0.008 eV) is in close agreement with the value (0.131 eV) obtained from a quantum yield analysis [I]. It is worth mentioning that the radiative lifetime estimation from the quantum yield analysis suggests [ i ] a value (60 nsec) smaller than our experimental one. Let us now discuss the r(Z 0 value we found in the low-temperature region. In this context, we recall that the Zccentre is considered a perturbed F-centre on account of many similarities [ 1, 8 - 1 0 ] . These include the one [1] between the effective lattice frequency were of the Zvcentre emission band and the Were for the F-centre in KCI [ 1], and that between the emission halfwidths, which are proportional [ 11 ] to coem. The diffuse nature of the relaxed excited state of the F-centre has already been experimentally evidenced (see, for example,
143
144
LIFETIME OF THE Zt CENTRE IN KC1 : Sr
effective fields: (2J + I) factors arise from degeneracy of the levels (J angular momentum). Taking into account our suggestions, we also made the foilowing assumptions:
r
L
"{CI Sect,
Z~ cen{re decay
4
,&E = O~3Z ":-0008 aV
!0
50
i
I0
]
80
I
'
90 100 110 IEM?ERAIURE (K)
'
120
130
[
ILO
Fig. 1. Lifetime of the Zx-centre as a function of temperature. The activation energy of deexcitation is calculated by assuming the usual two-channel decay scheme. [7, 12-15] ) and theoretically justified [ 16, 17]. The experimental [11] statement ¢Oem COT, the lattice transverse optical frequency, has been tentatively interpreted [11] as further evidence of the diffuse nature of the F-centre relaxed excited state (indeed, coT is referred to K = 0). Since the effective lattice frequencies of the absorption and emission bands are almost the same for the Z~-centre (unlike what happens with the F-centre) and since its absorption and emission band halfwidtbs are not too different, we made the drastic assumption that, for the Z~ -centre, the ground state, is a diffuse one. This assumption is not justified a priori, even though the above similarities and difference in the F- and ZI -centre behaviour might support it: however, we tried to use it t(> calculate the r(Z t) value from the Fowler-Dexter formula (relationship 21 of [17]), which gives us the radiative lifetime of luminescent centres in ionic solids: =
1 I. r,~
-
fmk
¢o
ke~,.r(E,~m)J
.12 h2c3m(*) Ern,~ ~ "
"
x
l(rz,a>!2(2J~ + 1) ×
Vot. 38, No. 2
(i) t), = 0.58, on the basis of the t~ value (0.68), obtained by Bosi e/ al. [18]. and of the ¢),/j'}-ratio [1]: (ii) e~ef" eo, which is generally valid [17] when the centre is diffuse; (iii) m (*) -~ 0.5 m~ [ l 9] : this is also the value used in the F-centre case [13] ; (iv) I(ra-r)l'- "" [(r-r6)[a, based on the similarity of the absorption and emission properties for the Zl-centre and on the drastic assumption regarding the diffuse nature of the involved states; (v) (2J~ + l)(2J,~ + 1)-' = 3, as in the case of the F-centre [I7]. In this way we calculated r(Zl) = 135 nsec. The discrepancy between this value and our experimental one is not marked. In our opinion, it may be due to the above drastic assumption concerning the [(r)i" values, which strongly depend on the wave function parameters of the ground and excited states [13, 16]. Incidentally, we note that the tong F-centre radiative lifetime (795 nsec at LHeT in KCI [7]) has been explained by assuming [13, 16] that the relaxed excited state is diffuse, and that the ground state is rather compact and, consequently, that i(r8~l)I2 --~ I(r.rs)12; this has been supported by ESR measurements [15]. We should also like to stress that the calculated lifetime value may be better matched with the experimental one, by assuming [(r~.r)l 2 to be only somewhat lower than I(r-~,)1"-• In conclusion, we believe that our technique may be useful for performing lifetime measurements for all types of Z-centre when photomultipliers with a better response curve in the wavelength range in question become available.
8,3'
REFERENCES
Y l
The subscripts k and m denote electronic levels: the first letter refers to the electronic level of the system before a transition takes place, and the second refers to the level into which the transition is made. The dipole matrix element I(rs@[", is expressed in terms of the electronic wave functions of the initial and final states/5 and 7; .¢" is the oscillator strength, r~z(*/, for diffuse centres, is the effective mass and, for very tightly-bound centres, the electronic mass me [17] ;Ern~, and Ek, n are the absolute values of the energies of emission and absorption, respectively; n(Ek~ ) is the refraction index evaluated at peak emission energy; eo and eerr are the average and the
1. 2. 3. 4. 5. 6. 7. 8.
H.J. Paus, Z. Phys. 218, 56 (1969). M. Bertolaccini, L. Bosi, S. Corn, C. Bussolati & G. Spinolo, 1968 Int. S)'mp. opz Color Ce~zters in Alk'ali Halides Roma, Italy (Paper 20). S. Cova, M. Bertolaccini & C. Bussolati, P/,ys. Statz~s Solidi(a) 18, 11 (1973). L. Bosi, A. Longoni & M. Nimis. Phys. Srarz,s Solidi (b) 89,221 (1978). L. Bosi & M. Nimis, Phys. Statzts Solidi (b) 85, K27 (1978). L. Bosi, C. Bussolati & G. Spinolo, Ph_vs. Rev. BI, 890 (1970). L. Bosi, S. Cova & G. Spinolo, PIzvs. Rex,. B9, 4542 (1974). H. I-l~irtel & F. I_£tty, Z. Phys. 182. 111 (1964).
Vol. 38, No. 2
145
LIFETIME OF THE Zt CENTRE LN KC1 : Sr
It:
~ ,
T : 11g K
tJ~
g o t.)
u. 102 o
z
le
I
I
50
100
I
I
I
I
150 CHANNELS
I*
200
250
I
I
I
KC[ :SrC[ z
10 4
T=79K
t ( M ) = 30 *-3ns z(Z 0 =182 ± 10ns t ( F ) =540 z 20ns
103
21 74 ns/channels
o
10 2 c~ bJ D Z
1! 0
F
/M 50
I 100
{ 150 CHANNELS
÷÷
200
*L 250
Fig. 2. (a) and (b). Some examples of the decay curves in additively coloured KCISrC12, after F-+ Z 1 conversion; (little crosses represent averages over 8 channels). 9. I0. 11. 12. 13. 14.
J.C. Bushnell, Thesis Univ. Illinois (1964);Int. Syrup. on Color Centers in Alkali Halides, Abstract 25. Urbana (1965). H.J. Paus & F. Uuty,Phys. Rev. Lett. 20, 57 (1968). L. Bosi & M. Nimis, Phys. Status Solidi(b) 98, KI51 (1980). F. LiJty, Physics of Color Centers (Edited by W.B. Fowler), Chapter 3. Academic Press, New York and London (1968). L. Bosi, P. Podini & G. Spinolo,Phys. Rev. 175, 1133 (1968); L. Bosi, S. Cova & G. Spinolo, Phys. Status Solidi (b) 68,603 (1975). L. Bosi & M. Nimis, Phys. Status Solidi (b) 95, 615 (1979).
15. 16. 17. 18.
19.
H.J. Reyher, K. Hahn, Th. Vetter & A. Winnacker, Z. Phys. B33,357 (1979). W.B. Fowler, Phys. Rev. 135, A1725 (1964). W.B. Fowler&D.L. Dexter, Phys. Rev. 128,2154 (1962). L. Bosi, A.L. Fantola-Lazzarini & E. Lazzarini, Phys. Status Solidi (b) 66,285 (I 974); L. Bosi, A.L. Fantola-Lazzarini, E. Lazzarini, V. Marigliano-Ramaglia & A. Tagliacozzo, Phys. Stares Solidi (b) 69, 519 (1975). G.W. Hodby, J,A. Borders, F.C. Brown & S. Foner, Phys. Rev. Lett. 19, 952 (1967").