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Anti-Stokes fluorescence of Gd3+ in K2GdF5
Journal of Luminescence 45 (1990) 363—365 North-Holland
363
ANTI-STOKES FLUORESCENCE OF Gd~IN K2GdF5 R. MAHIOU, J. METIN and J.C. COUSSEINS Laboratoire de Chimie des Solides (URA 444), Université Blaise Pascal (Clerinont-Ferrand) et ENSCCF, 63177 Aubiere Cedex, France
ofW the of K 2GdF5 whenwethe is absorbed into the 6PWe report on the observation3~. ithintrinsic the aid anti-Stokes of a doublefluorescence frequency pulsed dye laser, arelaser ablelight to carefully analyse excitation 7/2 first and excited emission state spectra of Gd as well as the decay time at very low temperatures. Three anti-Stokes fluorescences appear efficiently. Two are assigned to the 6G 8S 7/2 617/2 —~ 7/2 transitions6P while the fluorescence observed around 2420 A could be due to an emitting level of an impurity or to the coupling of the 7/2 state with an impurity or a color center. The high excitation density effect on the intrinsic decay as well as the decay of the anti-Stokes fluorescences are well described by a model involving an Photon Addition by Energy Transfer process.
1. Introduction
a!u
In an earlier paper, we reported the
/\
observation of
the UV Stokes fluorescence of the one-dimensional compound: K2GdF5 [1]. 8S One photon absorption was used to characterize the 7/2 ~6 P7/2 energy transitions. Measurements at low excitation densityand (N0the 1014 3) show that the intrinsic imexcited ions/cm purities induced traps fluorescence dynamics are well described in terms of diffusion-limited energy transfer. The non-exponential (T < 20 K) intrinsic fluorescence decay consists of a short time component (;) corresponding to diffusion and a long time component (T
/ /
a
~
//
‘N
\ ~
~G7,2
\
as 7/2
4.4K
-—-TRS
~:
:~:‘:~~ ‘
2048
—‘.~—-
U
2049
I
1)
coming from thermal back transfer (;= 2.7 ms and = 6.9 ms at 4.4 K). In the following we will give a brief description of the experimental results obtained in high pumping regime. The anti-Stokes fluorescence spectra of K2GdF5 will be discussed.
4AK b
~TRS
~
2422
2423
2424
2. Experimental 1u
2.1. Emission
a
The UV excitation in one of the stark components of the 6P 712 multiplet (— 3120 A) [1] produces fluorescences around 2047 A, 2420 A and 2798 A. The emis-
61
7/2
a~
7/2
44K
I
-—-TRS d•Iay 0.5 ~
C
sion spectra at 4.4 K are reported in fig. 1. By comparison with an absorption spectrum, the fluorescence lines at 2047.2 A and 2798.1 A are attributed to the resonant 8
6
8
9at.
50 is
6
transitions S7/2 — G712 and S7/2 —~ 17/2, respectively. The lines on the low energy side of the resonant transition 3~trap have emission the already same origin reported of impurity in refs. [1—3] induced for Gd 6P the 7/2 level for various compounds. However the 0022-2313/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
2797.5 2798.5 1 Fig. 1. Anti-Stokes fluorescences when absorbing the laser light 6P into the spectra integrated 7/2 stateand at the 4.4 dot—dashed K. The solidcurves curve represent the
time resolved spectra.
364
R. Mahiou et at.
/ Anti-Stokes fluorescence of Gd3
fluorescence around 2420 A does not correspond to8Sany 3~.In addition the 6D absorbing level of Gd 9/2 —* 7/2
+
in K,GdF 5
a~i A1
A2
2520 A) fluorescence observed at low temperature in GdC13 [3] and RbGd3F10 [41compounds is not observed in K2GdF5 studied here. 2.2. Excitation
A3
is ~
(—
712
712 -
a
The excitation spectra at 4.4 K of the UV anti-Stokes fluorescences arising from ‘7/2 and 2420 A (fig. 2(a)) correlates fairly well with the absorption spectrum 8S 6P of the 6G 7/2 — 7/2 transition. Excitation spectrum of the 7/2 8 S7/2 transition registered in the 8S same conditions exhibits the general features of the 7/2 _~6 P7/2 excitation spectrum except for the linewidth and the intensity of the peaks (fig. 2(b)). This spectrum is identical to that obtained under two (red) photon excitation conditions at the same temperature [5].
3115
3120
I
A
a.u
4
as
6~ 112
712 4.4 K
2.3. Fluorescence decay
At 4.4 3), K, when under thelow laserpumping excitation condition stands at(N0 3123.7 — 10~ A ions/cm (line A 4 on fig. 2), the fluorescence decay 8Sof the emission line centered at 2798.1 A (617/2 —* 7/2) exhibits an initial rise time (i~ — 187 ~s) followed by an exponential (T1 — 6G 1.3 ms). A8Ssimilar behavior is observed for the intrinsic 7/2 —* 7/2 line at 2047.2 A(~ 122 ps). On the other hand, the fluorescence decay of the 2421.5 A emission line is nonexponential (r1 3.7 ms) and does not exhibit any rise time. When increasing the 3), three effects are pumping power (N0 > iO~ions/cm
b
~1
~2
3115 3120 A 6P Fig. 2. Excitation8Sspectra in the 7/2 multiplet at 4.4. K of: (a) the 617/2 —96G7/2 and 8S — 2420 A fluorescence, (b) the 7/2 - 7/2 fluorescence.
1. 7/2 4
8
12
mi
7/2
3). The dotted line represents the experimental
data while the solid line represents a fit Fig. 3. Decays of the intrinsic anti-Stokes fluorescence at 4.4. K to (N0the— data 1016 derived ions/cmfrom eq. (1) in the text.
3 + in K,GdF
R. Mahiou et at. / Anti-Stokes fluorescence of Gd
6P
5
365
8S
observed: (i) a change in the intrinsic
7/2 —* 7/2 fluorescence decay with a sharp decrease at a short time after the laser pulse, and a long time behavior which
1/(2w~) equal to the 6P half value of the intrinsic fluorescence decay time of 7/2 [1]. However in the case of the high pumping density only the first part of the
correlates well with the decay at low pumping intensity [5]; (ii) a drastic increase of the 6G initial intensity (at 8S 6 17/2 = 0) _98 forS the intrinsic anti-Stokes 7/2 — 7/2 and 7/2 fluorescences; (iii) a non-exponential behavior in the long tail of the anti-Stokes fluorescence decay. The above observations give some evidence for the
fluorescence decays are reproduced by a numerical solution assuming an initial condition nA(O) * 0. The trans3 s~ fer coefficient WT is found to be — 2 x 10_14 cm at 4.4 K. The long component of the decays can be due to a thermal 6P back transfer from the traps (like for the intrinsic 7/2 8 S7/2 fluorescence decay in ref. [1]). In contrast the fit of the decay profile of the — 2420 A fluorescence with eq. (1) results in unphysical parame-
up-conversion energy transfer process which competes with an excited state absorption mechanism at high pumping regime [3,6]. Fig. 3 shows the fluorescence decays recorded3.at 4.4 K under an excitation density of — 1016 ions/cm
3. Discussion The existence of UV anti-Stokes fluorescence, the non-exponential behavior of the initial intrinsic decay under high pumping powerpart can of be the understood in terms of an APTE model [7,8]. Considering this model with the assumption that the different anti-Stokes fluorescences are not connected between them, we write the rate equations as follows: (after the laser pulse)
ters. 6GIn summary, the two anti-Stokes fluorescences (havior. 7/2 ~TheS~/.2and —, 8 S7/2) exhibit origin of 617/2 the fluorescence aroundsimilar 2420 Abeis
not yet clearly determined. This fluorescence could be due to the 1S 3 ± which could _~3H ~transition the Pr of a very low be present in0 our sample as an ofimpurity concentration. The other possibility could be that this fluorescence results from a coupling of the 6P 7/2 This level 3 to a level of an impurity or a color center. of Gd last assumption is supported by the fact that the sample emits a fluorescence which changes from the red (Eu3~ impurity emission) to the yellow when the laser power increases.
dn 0/dt dnA/dt
—wSnS—wTnS, =
0.5wTn~ —
WA/lA,
(1)
where n5 and ~A stand for the populations of the excited states Stokes and anti-Stokes, respectively. W~ and WA are the probabilities of the intrinsic and antiStokes fluorescences, respectively. WT is the APTE transfer coefficient. Taking advantage of our measurement of w~ 6G = 2.7 ms (at 4.4 K) and an estimation of WA for the 7/2 and 61 levels on the rise time of the fluorescence decays of the ‘anti-Stokes’ traps (at 2048.1 A and 2798.3 A in fig. 1), the system (1) is used to reproduce the difference decays registered. Analytical (low pumping regime: w1.n~— 0) or numerical solution [3] of eq. (1) reproduces fairly well the UV anti-Stokes fluorescence decay curve with the expected rise time (i/WA = 187 ~ss)and a time constant
References [1] R. Mahiou, J. Metin, M.T. Fournier, J.C. Cousseins and B. Jacquier, J. Lumin. 43 (1989) 51. [2] R. Mahiou, iC. Cousseins and MT. Fournier, J. Chim. Phys. 85 (1988) 769. [3] R. Mahiou, B. Jacquier and C. Madej, J. Chem. Phys. 89 (1988) 5931. [4) J. Metin, These de doctorat d’etat, Universite ClermontFerrand 11(1987). [5] R. Mahiou, These de doctorat d’etat, Universite ClermontFerrand 11(1987).
[6] J.C. Gâcon, B. Bouattou, iL. Jouvray and B. Jacquier, J. de Phys. 47 (1986) 279. [7] F. Auzel, Proc. IEEE 61(1973) 758. [8] P.C. Diggle, J.A. Gehring and R.M. Macfarlane, Solid State commun. 18 (1976) 391.
363
ANTI-STOKES FLUORESCENCE OF Gd~IN K2GdF5 R. MAHIOU, J. METIN and J.C. COUSSEINS Laboratoire de Chimie des Solides (URA 444), Université Blaise Pascal (Clerinont-Ferrand) et ENSCCF, 63177 Aubiere Cedex, France
ofW the of K 2GdF5 whenwethe is absorbed into the 6PWe report on the observation3~. ithintrinsic the aid anti-Stokes of a doublefluorescence frequency pulsed dye laser, arelaser ablelight to carefully analyse excitation 7/2 first and excited emission state spectra of Gd as well as the decay time at very low temperatures. Three anti-Stokes fluorescences appear efficiently. Two are assigned to the 6G 8S 7/2 617/2 —~ 7/2 transitions6P while the fluorescence observed around 2420 A could be due to an emitting level of an impurity or to the coupling of the 7/2 state with an impurity or a color center. The high excitation density effect on the intrinsic decay as well as the decay of the anti-Stokes fluorescences are well described by a model involving an Photon Addition by Energy Transfer process.
1. Introduction
a!u
In an earlier paper, we reported the
/\
observation of
the UV Stokes fluorescence of the one-dimensional compound: K2GdF5 [1]. 8S One photon absorption was used to characterize the 7/2 ~6 P7/2 energy transitions. Measurements at low excitation densityand (N0the 1014 3) show that the intrinsic imexcited ions/cm purities induced traps fluorescence dynamics are well described in terms of diffusion-limited energy transfer. The non-exponential (T < 20 K) intrinsic fluorescence decay consists of a short time component (;) corresponding to diffusion and a long time component (T
/ /
a
~
//
‘N
\ ~
~G7,2
\
as 7/2
4.4K
-—-TRS
~:
:~:‘:~~ ‘
2048
—‘.~—-
U
2049
I
1)
coming from thermal back transfer (;= 2.7 ms and = 6.9 ms at 4.4 K). In the following we will give a brief description of the experimental results obtained in high pumping regime. The anti-Stokes fluorescence spectra of K2GdF5 will be discussed.
4AK b
~TRS
~
2422
2423
2424
2. Experimental 1u
2.1. Emission
a
The UV excitation in one of the stark components of the 6P 712 multiplet (— 3120 A) [1] produces fluorescences around 2047 A, 2420 A and 2798 A. The emis-
61
7/2
a~
7/2
44K
I
-—-TRS d•Iay 0.5 ~
C
sion spectra at 4.4 K are reported in fig. 1. By comparison with an absorption spectrum, the fluorescence lines at 2047.2 A and 2798.1 A are attributed to the resonant 8
6
8
9at.
50 is
6
transitions S7/2 — G712 and S7/2 —~ 17/2, respectively. The lines on the low energy side of the resonant transition 3~trap have emission the already same origin reported of impurity in refs. [1—3] induced for Gd 6P the 7/2 level for various compounds. However the 0022-2313/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
2797.5 2798.5 1 Fig. 1. Anti-Stokes fluorescences when absorbing the laser light 6P into the spectra integrated 7/2 stateand at the 4.4 dot—dashed K. The solidcurves curve represent the
time resolved spectra.
364
R. Mahiou et at.
/ Anti-Stokes fluorescence of Gd3
fluorescence around 2420 A does not correspond to8Sany 3~.In addition the 6D absorbing level of Gd 9/2 —* 7/2
+
in K,GdF 5
a~i A1
A2
2520 A) fluorescence observed at low temperature in GdC13 [3] and RbGd3F10 [41compounds is not observed in K2GdF5 studied here. 2.2. Excitation
A3
is ~
(—
712
712 -
a
The excitation spectra at 4.4 K of the UV anti-Stokes fluorescences arising from ‘7/2 and 2420 A (fig. 2(a)) correlates fairly well with the absorption spectrum 8S 6P of the 6G 7/2 — 7/2 transition. Excitation spectrum of the 7/2 8 S7/2 transition registered in the 8S same conditions exhibits the general features of the 7/2 _~6 P7/2 excitation spectrum except for the linewidth and the intensity of the peaks (fig. 2(b)). This spectrum is identical to that obtained under two (red) photon excitation conditions at the same temperature [5].
3115
3120
I
A
a.u
4
as
6~ 112
712 4.4 K
2.3. Fluorescence decay
At 4.4 3), K, when under thelow laserpumping excitation condition stands at(N0 3123.7 — 10~ A ions/cm (line A 4 on fig. 2), the fluorescence decay 8Sof the emission line centered at 2798.1 A (617/2 —* 7/2) exhibits an initial rise time (i~ — 187 ~s) followed by an exponential (T1 — 6G 1.3 ms). A8Ssimilar behavior is observed for the intrinsic 7/2 —* 7/2 line at 2047.2 A(~ 122 ps). On the other hand, the fluorescence decay of the 2421.5 A emission line is nonexponential (r1 3.7 ms) and does not exhibit any rise time. When increasing the 3), three effects are pumping power (N0 > iO~ions/cm
b
~1
~2
3115 3120 A 6P Fig. 2. Excitation8Sspectra in the 7/2 multiplet at 4.4. K of: (a) the 617/2 —96G7/2 and 8S — 2420 A fluorescence, (b) the 7/2 - 7/2 fluorescence.
1. 7/2 4
8
12
mi
7/2
3). The dotted line represents the experimental
data while the solid line represents a fit Fig. 3. Decays of the intrinsic anti-Stokes fluorescence at 4.4. K to (N0the— data 1016 derived ions/cmfrom eq. (1) in the text.
3 + in K,GdF
R. Mahiou et at. / Anti-Stokes fluorescence of Gd
6P
5
365
8S
observed: (i) a change in the intrinsic
7/2 —* 7/2 fluorescence decay with a sharp decrease at a short time after the laser pulse, and a long time behavior which
1/(2w~) equal to the 6P half value of the intrinsic fluorescence decay time of 7/2 [1]. However in the case of the high pumping density only the first part of the
correlates well with the decay at low pumping intensity [5]; (ii) a drastic increase of the 6G initial intensity (at 8S 6 17/2 = 0) _98 forS the intrinsic anti-Stokes 7/2 — 7/2 and 7/2 fluorescences; (iii) a non-exponential behavior in the long tail of the anti-Stokes fluorescence decay. The above observations give some evidence for the
fluorescence decays are reproduced by a numerical solution assuming an initial condition nA(O) * 0. The trans3 s~ fer coefficient WT is found to be — 2 x 10_14 cm at 4.4 K. The long component of the decays can be due to a thermal 6P back transfer from the traps (like for the intrinsic 7/2 8 S7/2 fluorescence decay in ref. [1]). In contrast the fit of the decay profile of the — 2420 A fluorescence with eq. (1) results in unphysical parame-
up-conversion energy transfer process which competes with an excited state absorption mechanism at high pumping regime [3,6]. Fig. 3 shows the fluorescence decays recorded3.at 4.4 K under an excitation density of — 1016 ions/cm
3. Discussion The existence of UV anti-Stokes fluorescence, the non-exponential behavior of the initial intrinsic decay under high pumping powerpart can of be the understood in terms of an APTE model [7,8]. Considering this model with the assumption that the different anti-Stokes fluorescences are not connected between them, we write the rate equations as follows: (after the laser pulse)
ters. 6GIn summary, the two anti-Stokes fluorescences (havior. 7/2 ~TheS~/.2and —, 8 S7/2) exhibit origin of 617/2 the fluorescence aroundsimilar 2420 Abeis
not yet clearly determined. This fluorescence could be due to the 1S 3 ± which could _~3H ~transition the Pr of a very low be present in0 our sample as an ofimpurity concentration. The other possibility could be that this fluorescence results from a coupling of the 6P 7/2 This level 3 to a level of an impurity or a color center. of Gd last assumption is supported by the fact that the sample emits a fluorescence which changes from the red (Eu3~ impurity emission) to the yellow when the laser power increases.
dn 0/dt dnA/dt
—wSnS—wTnS, =
0.5wTn~ —
WA/lA,
(1)
where n5 and ~A stand for the populations of the excited states Stokes and anti-Stokes, respectively. W~ and WA are the probabilities of the intrinsic and antiStokes fluorescences, respectively. WT is the APTE transfer coefficient. Taking advantage of our measurement of w~ 6G = 2.7 ms (at 4.4 K) and an estimation of WA for the 7/2 and 61 levels on the rise time of the fluorescence decays of the ‘anti-Stokes’ traps (at 2048.1 A and 2798.3 A in fig. 1), the system (1) is used to reproduce the difference decays registered. Analytical (low pumping regime: w1.n~— 0) or numerical solution [3] of eq. (1) reproduces fairly well the UV anti-Stokes fluorescence decay curve with the expected rise time (i/WA = 187 ~ss)and a time constant
References [1] R. Mahiou, J. Metin, M.T. Fournier, J.C. Cousseins and B. Jacquier, J. Lumin. 43 (1989) 51. [2] R. Mahiou, iC. Cousseins and MT. Fournier, J. Chim. Phys. 85 (1988) 769. [3] R. Mahiou, B. Jacquier and C. Madej, J. Chem. Phys. 89 (1988) 5931. [4) J. Metin, These de doctorat d’etat, Universite ClermontFerrand 11(1987). [5] R. Mahiou, These de doctorat d’etat, Universite ClermontFerrand 11(1987).
[6] J.C. Gâcon, B. Bouattou, iL. Jouvray and B. Jacquier, J. de Phys. 47 (1986) 279. [7] F. Auzel, Proc. IEEE 61(1973) 758. [8] P.C. Diggle, J.A. Gehring and R.M. Macfarlane, Solid State commun. 18 (1976) 391.