- Email: [email protected]
Energy transfer from Eu3+ → Ho3+ in calibo glass
1 JuIy 1978
CHEMICAL PHYSICS LEmRS
Volume 57, mnnber 1
ENERGY TRAN§FER FROM Eu-
IN CALIBO GLASS
+ I@;
Received28 !&rch l-978
Study of energytrander from opticallyexcited I%* + Ho3 + has been carriedout Zncaliio @ss Probabilitiesand effi dencies of energytrar&zr from Eu3* -+ Ho3* have been caIctdz&dfrom the life tin-~ am?emi&on irztensityof Eu3‘ + Has*_ At low acceptorc~ncentratiorz3dav-arksBneariywith Ca show&g m&rationof energyam0118dono% At hi& acceptor coocer%ations&a dependsIir:earlyon (Ca + Ca)‘, which is inconsistentwith Fong and Diestier’sthfzoryofdipoledipole mechanism of entransf%r.At low temperatureemissionintetity and life time increases,decrea&g probability (pda) md effkiency (lrr> suggestingat raom temperature the energy is transferred to the Latticeby donor Ioweringlife me aridintensity.At hi& temperature no emissionfrom higher levelsof the donor is obtainedsuggestingblurredenergy levelsin the glassym3rrk
I. Iimoduction
The emission spectra, intensity and decay plots of tlvalent rare earth ions are IargeIy affected by the presence of the same or other rare earth ions [t-4] _ This 3henomenon has long been know to exist due to en:rgy transfer from one ion to another. Several attempts [S-l O] have been made to find the mechanism of energy transfer process and different systems have been chosen for the study. It has been shown that the nature of the emitting levels of the acceptor as well as the environment surrounding the ions have mtxh influence on the transfer mechanism_ When a system containing randomly distributed doner and acceptor ions, where migration of excitation energy among the donors is absent, is optically excited the decay of donor emission as a funeti0n of time t is given by (1) r. is the donor decay time, s is the interaction parameter and is equal to 6 for dipole-dipole, 8 for dipoleqnadrupole and IO for qnad~~le-q~ad~pole interaction respectively, C, is the acceptor concentration
and CO is critical transfer concentration of acceptor which corresponds to critical transfer distance (Rz = 314 zCo) for which the energy transfer rate equals intrinsic decay rate. In order to fmd out the interaction mechanism the experimental data are tallied with those obtained from eq. (1). This is also coroborated by plotting Pa, versus 62 (C being the sum of the donor and acceptor concentrations ) [l I]_ Nakajawa and Shionoya [.5] have studied the energy transfer from Eu3+ + Ho3’- in Ca(PCk& glass and observed a d-q energy transfer mechanism The pose of the present paper is to fmd the mechanism of the energy transfer from Eu3* + Ho3+ in calibo glass and to give a quantitative analysis for the transfer probability and transfer efficiency. The transfer probabiity (&J and transfer efficientcy cm) were calculated using the formulae f6,10] P& = (f/Q)(@ qT’i
-
1) ,
- T&o _
(2)
7 and r. are the decay times of the donor in the presence and absence of the acceptor2. Experimental
* To whom zZI+&ecorrespondenceshoufdbe addressed: AX. A8aswalc/o ICI). K-talc
NainiT&263002, 50
India.
& Bros -RamsayRoad, Talli Tai,
Calibo glass containing CaO : LiO : B203
(AX.
Vohune 57, numb& 1
~EMICAJL PHYSICS LETTERS
Grade) in the ratio 20 : 10 : 70 by wt.% were prepared with Eu,O, (99.9%) and Ho,03 (99.9%) doped h-rit, density being 2.4 gm/cm3. The concentration of Eu3* was kept fmed and the concentration of Ho3+ was varied from 0.2% to 890. The mixed powder was melted in a platinum crucible for about 45 minutes at about 1OClO”C.To obtain glasses of fxed dimensions the melt was poured in a brass ring resting on an aluminium base. The sample was acted upon by 3650 A of Hg. The fluorescence spectra and decay times were plotted in the usual manner. The observations were made at room temperature, liquid air temperature and at 513 and 633 IS.
3. Results and d&t&on In fig. 1 the fluorescence spectra of Eu3+ 1 wt.% and Eu3+ wi*h 1 wt.% and 4 wt.% of Ho3+ are shown. The Intensity of the donor (Eu3*) with respect to various concentrations of the acceptor (HOG*) has been plotted in fig. 2. It can be seen that with increasing concentrations of Ho3* the intensity of the Eu3* emission decreases. However, the decrease for different intensities being differential. The most intense
1 July 1978
r
200
Fig. 2. Intensityof Eu3* versusC,.
em&ion of Errs+ (SD0 + 7F2) is relatiely affected most by the presence of Ilo3+ showing a rapid transfer of energy from Et@ + Host_ In fig. 3, the emission decay plots for the 5Do --f 7k2 level of Eu3* have been plotted for various concentrations of Ho3*. The emission decay curves deviate from the exponential nature with increasing concentration of Ho3+ showing multipolar mechanism
I
570
610 A(nm)
650
Fig. 1. (A) Emissionspectraof Eu3* (1 wt_%). (B) Emission spectraof Eu3+ (1 wL%) + Ho3* (1 wt%). (C) Emissionspectra of Eu3* (1 wt.%) + Ho3+ (4 wt.%).
I
2
t (ms)
3
6
Fig. 3. Decay plots of Eu3+_(A) 1 wt.% Eu3*_(B) 1 wt.% Eu3* + 1 wt.% Ho3”. (C) 1 wt% Eu3’ i- 2 wt.% Ho3+_@) lwt.%Eu3++3wt_%Ho 3+. Q 1 wt_%Eu3’ + 4 wt% Ho3*.
51
CHEMICAL
Volume 57, number 1
PHYSICS
/
I
0.16-
of energy transfer_ The linear dependence of Pda versus L”, at low acceptor (fig. 4) concentration shovs a migration of energy i.e. the energy of one donor hops to the same species of Eu3+ and tiuaily sinks to acceptor. At high acceptor concentrations the acceptor ions are in abundance and hence the energy will relax directly. To find the mechanism we tallied experimental data with theoreticahy obtained ones (fig. 3, where the solid lines are the experimental observations and circles the values calculated from eq. (1) for s = 5). The critical transfer distance has been calculated and comes out to be = 15.2 A. Keeping the value of s = 6 in eq. (1) fits the data best within experimental errors suggesting a dipoIe-dipole interaction mecharr&ri of energy transfer. This is however different from the mechanism propsed by Nakajawa and Shionoya I.51_ The calculated vahres of P’ and VT are presented in tabie 1. Table 1 Cahk&?d
52
LE-ITERS
1 July 1978
At liquid air temperature (80 K) the life time becomes 3 ms. Energy transfer from-Eu3’ + Ho3* decreases at this temperature. This is evident by comparing the calculated VaheS Of Ph and VT at room temperature and at 80 K, presented in table 1, the increased life time of Eu3+ at low temperature however indicates that energy is dissipated to lattice from Eu3+ at room temperature. At high temperature, it is well known that higher levels are populated by Boltzmanur’s distribution law and with the expectation that emission from other itveIs SD1 and SD2 of Eu3+ may be obtained, we rnrde observations at 240°C and 360% However, no such emission was obtained but only reduced life time and intensity. So it seems that in the glassy matrix due randomness of their structure the energy levels of doped rare earth ions broaden inhomogeneously and iattice vibrations also increase at higher temperatures. Thus the blurred out energy levels of rare earth ions in a glassy matrix at high temperature due to thermal vibration make a broad path for energy transfer. No exchange type of energy transfer occurs because the donor-acceptor distance is relatively large compared to 3-4 A. The dependence of Pb on C* i.e. on the squared sum of the donor and acceptor concentrations has been interpreted by Fong and Diestler [9] as the statistical probability that two acceptor ions in the neighbourhood of one donor ion participate in the energy transfer (fig. 5).
values of efficiency and probability of energy transfer Eu~+ + Ho3* (with concentration of donor EI? -
Acceptor concentration (wt.%)
Eu3+ life time (ms)
80K
300 K
Efficiency
513 K
633 K
80K
PdaX 300 K
80K
0.0
3.0
2.6
2.2
2.1
-
-
0.2
2.8
ii_6 1.0 1.6 20 4.0 6.0
2.7 2.4 2.1 X.8 1.2 0.7
8.3
2.4 2.2 2.0 1.8 1.4 0.8 O-4
ZO 1.9 1.6 1.5 1.1 0.6 0.35
0.5
1.9 1.8 1.5 1.3 1.0 0.5 0.25
0.07 0.10 0.20 0.30 O-40 0.60 0.74
0.2
0.08 0.16 0.29 0.31 0.46 0.70 0.85
0.024 0.040 0.083 0.143 0.220 0.500 0.920
0.15
0.10
0.80
0.93
1.330
1 wt.% futed)
lo3 300 K 0.032 0.070 0.115 0.171 0.330 0.800 2-100
4.600
CHEMICAL
Volume 57, number 1
PHYSICS
LETTERS
1 July 1978
References
l.Or
D.L. Dexter, J. Chem. Phys. 21<1953) 836. hl. Inokuti and F. Hirayama, J. Chem. Phys. 43 (1965) 1978. 131 L-G. van Uitert, J_ Chem. Phya 44 (1966) 3514. 141 L.G. van Uizert, in: Proceedings of the International Conference on I umiuescence, Budapest, 1966, eb G_ Saigeti (Akadami Kiado. Budapest, 1968) p_ 1588. 15: E. Nakajawa and S. Shionoya, J. Chem. Phys. 47 (1966) 3211. I61 R Reisfeld, Structure Bonding 13 (1973) 53_ 171 J.C. Bourcet, J.P. Denis and J-k Loris, Proceedings of the 10th Rare Earths Conference, Carefree, Arizona (1973) p_ 960. ISJ R. Reisfeld, E. Greenberg, R.A. Velapoldi and B. Bamet, I. Chem. Phys. 56 (1972) 1698. PI FK Fong and D.J. DiestIer, J. Chem. Phys. 56 (1972) 2875. [lOI R Reisfeld, Structure Bonding 22 (1975) 123.
fll
PI
~~:
_:
/ 4
8
12
16
20
24
5’
Fig_ .5_P&
veias
C*.
Achowkdgement Two of us (XC. Kandpal and A.K. Agarwal) are grateful to C.S.I.R., New Delhi for financial assistance.
53
CHEMICAL PHYSICS LEmRS
Volume 57, mnnber 1
ENERGY TRAN§FER FROM Eu-
IN CALIBO GLASS
+ I@;
Received28 !&rch l-978
Study of energytrander from opticallyexcited I%* + Ho3 + has been carriedout Zncaliio @ss Probabilitiesand effi dencies of energytrar&zr from Eu3* -+ Ho3* have been caIctdz&dfrom the life tin-~ am?emi&on irztensityof Eu3‘ + Has*_ At low acceptorc~ncentratiorz3dav-arksBneariywith Ca show&g m&rationof energyam0118dono% At hi& acceptor coocer%ations&a dependsIir:earlyon (Ca + Ca)‘, which is inconsistentwith Fong and Diestier’sthfzoryofdipoledipole mechanism of entransf%r.At low temperatureemissionintetity and life time increases,decrea&g probability (pda) md effkiency (lrr> suggestingat raom temperature the energy is transferred to the Latticeby donor Ioweringlife me aridintensity.At hi& temperature no emissionfrom higher levelsof the donor is obtainedsuggestingblurredenergy levelsin the glassym3rrk
I. Iimoduction
The emission spectra, intensity and decay plots of tlvalent rare earth ions are IargeIy affected by the presence of the same or other rare earth ions [t-4] _ This 3henomenon has long been know to exist due to en:rgy transfer from one ion to another. Several attempts [S-l O] have been made to find the mechanism of energy transfer process and different systems have been chosen for the study. It has been shown that the nature of the emitting levels of the acceptor as well as the environment surrounding the ions have mtxh influence on the transfer mechanism_ When a system containing randomly distributed doner and acceptor ions, where migration of excitation energy among the donors is absent, is optically excited the decay of donor emission as a funeti0n of time t is given by (1) r. is the donor decay time, s is the interaction parameter and is equal to 6 for dipole-dipole, 8 for dipoleqnadrupole and IO for qnad~~le-q~ad~pole interaction respectively, C, is the acceptor concentration
and CO is critical transfer concentration of acceptor which corresponds to critical transfer distance (Rz = 314 zCo) for which the energy transfer rate equals intrinsic decay rate. In order to fmd out the interaction mechanism the experimental data are tallied with those obtained from eq. (1). This is also coroborated by plotting Pa, versus 62 (C being the sum of the donor and acceptor concentrations ) [l I]_ Nakajawa and Shionoya [.5] have studied the energy transfer from Eu3+ + Ho3’- in Ca(PCk& glass and observed a d-q energy transfer mechanism The pose of the present paper is to fmd the mechanism of the energy transfer from Eu3* + Ho3+ in calibo glass and to give a quantitative analysis for the transfer probability and transfer efficiency. The transfer probabiity (&J and transfer efficientcy cm) were calculated using the formulae f6,10] P& = (f/Q)(@ qT’i
-
1) ,
- T&o _
(2)
7 and r. are the decay times of the donor in the presence and absence of the acceptor2. Experimental
* To whom zZI+&ecorrespondenceshoufdbe addressed: AX. A8aswalc/o ICI). K-talc
NainiT&263002, 50
India.
& Bros -RamsayRoad, Talli Tai,
Calibo glass containing CaO : LiO : B203
(AX.
Vohune 57, numb& 1
~EMICAJL PHYSICS LETTERS
Grade) in the ratio 20 : 10 : 70 by wt.% were prepared with Eu,O, (99.9%) and Ho,03 (99.9%) doped h-rit, density being 2.4 gm/cm3. The concentration of Eu3* was kept fmed and the concentration of Ho3+ was varied from 0.2% to 890. The mixed powder was melted in a platinum crucible for about 45 minutes at about 1OClO”C.To obtain glasses of fxed dimensions the melt was poured in a brass ring resting on an aluminium base. The sample was acted upon by 3650 A of Hg. The fluorescence spectra and decay times were plotted in the usual manner. The observations were made at room temperature, liquid air temperature and at 513 and 633 IS.
3. Results and d&t&on In fig. 1 the fluorescence spectra of Eu3+ 1 wt.% and Eu3+ wi*h 1 wt.% and 4 wt.% of Ho3+ are shown. The Intensity of the donor (Eu3*) with respect to various concentrations of the acceptor (HOG*) has been plotted in fig. 2. It can be seen that with increasing concentrations of Ho3* the intensity of the Eu3* emission decreases. However, the decrease for different intensities being differential. The most intense
1 July 1978
r
200
Fig. 2. Intensityof Eu3* versusC,.
em&ion of Errs+ (SD0 + 7F2) is relatiely affected most by the presence of Ilo3+ showing a rapid transfer of energy from Et@ + Host_ In fig. 3, the emission decay plots for the 5Do --f 7k2 level of Eu3* have been plotted for various concentrations of Ho3*. The emission decay curves deviate from the exponential nature with increasing concentration of Ho3+ showing multipolar mechanism
I
570
610 A(nm)
650
Fig. 1. (A) Emissionspectraof Eu3* (1 wt_%). (B) Emission spectraof Eu3+ (1 wL%) + Ho3* (1 wt%). (C) Emissionspectra of Eu3* (1 wt.%) + Ho3+ (4 wt.%).
I
2
t (ms)
3
6
Fig. 3. Decay plots of Eu3+_(A) 1 wt.% Eu3*_(B) 1 wt.% Eu3* + 1 wt.% Ho3”. (C) 1 wt% Eu3’ i- 2 wt.% Ho3+_@) lwt.%Eu3++3wt_%Ho 3+. Q 1 wt_%Eu3’ + 4 wt% Ho3*.
51
CHEMICAL
Volume 57, number 1
PHYSICS
/
I
0.16-
of energy transfer_ The linear dependence of Pda versus L”, at low acceptor (fig. 4) concentration shovs a migration of energy i.e. the energy of one donor hops to the same species of Eu3+ and tiuaily sinks to acceptor. At high acceptor concentrations the acceptor ions are in abundance and hence the energy will relax directly. To find the mechanism we tallied experimental data with theoreticahy obtained ones (fig. 3, where the solid lines are the experimental observations and circles the values calculated from eq. (1) for s = 5). The critical transfer distance has been calculated and comes out to be = 15.2 A. Keeping the value of s = 6 in eq. (1) fits the data best within experimental errors suggesting a dipoIe-dipole interaction mecharr&ri of energy transfer. This is however different from the mechanism propsed by Nakajawa and Shionoya I.51_ The calculated vahres of P’ and VT are presented in tabie 1. Table 1 Cahk&?d
52
LE-ITERS
1 July 1978
At liquid air temperature (80 K) the life time becomes 3 ms. Energy transfer from-Eu3’ + Ho3* decreases at this temperature. This is evident by comparing the calculated VaheS Of Ph and VT at room temperature and at 80 K, presented in table 1, the increased life time of Eu3+ at low temperature however indicates that energy is dissipated to lattice from Eu3+ at room temperature. At high temperature, it is well known that higher levels are populated by Boltzmanur’s distribution law and with the expectation that emission from other itveIs SD1 and SD2 of Eu3+ may be obtained, we rnrde observations at 240°C and 360% However, no such emission was obtained but only reduced life time and intensity. So it seems that in the glassy matrix due randomness of their structure the energy levels of doped rare earth ions broaden inhomogeneously and iattice vibrations also increase at higher temperatures. Thus the blurred out energy levels of rare earth ions in a glassy matrix at high temperature due to thermal vibration make a broad path for energy transfer. No exchange type of energy transfer occurs because the donor-acceptor distance is relatively large compared to 3-4 A. The dependence of Pb on C* i.e. on the squared sum of the donor and acceptor concentrations has been interpreted by Fong and Diestler [9] as the statistical probability that two acceptor ions in the neighbourhood of one donor ion participate in the energy transfer (fig. 5).
values of efficiency and probability of energy transfer Eu~+ + Ho3* (with concentration of donor EI? -
Acceptor concentration (wt.%)
Eu3+ life time (ms)
80K
300 K
Efficiency
513 K
633 K
80K
PdaX 300 K
80K
0.0
3.0
2.6
2.2
2.1
-
-
0.2
2.8
ii_6 1.0 1.6 20 4.0 6.0
2.7 2.4 2.1 X.8 1.2 0.7
8.3
2.4 2.2 2.0 1.8 1.4 0.8 O-4
ZO 1.9 1.6 1.5 1.1 0.6 0.35
0.5
1.9 1.8 1.5 1.3 1.0 0.5 0.25
0.07 0.10 0.20 0.30 O-40 0.60 0.74
0.2
0.08 0.16 0.29 0.31 0.46 0.70 0.85
0.024 0.040 0.083 0.143 0.220 0.500 0.920
0.15
0.10
0.80
0.93
1.330
1 wt.% futed)
lo3 300 K 0.032 0.070 0.115 0.171 0.330 0.800 2-100
4.600
CHEMICAL
Volume 57, number 1
PHYSICS
LETTERS
1 July 1978
References
l.Or
D.L. Dexter, J. Chem. Phys. 21<1953) 836. hl. Inokuti and F. Hirayama, J. Chem. Phys. 43 (1965) 1978. 131 L-G. van Uitert, J_ Chem. Phya 44 (1966) 3514. 141 L.G. van Uizert, in: Proceedings of the International Conference on I umiuescence, Budapest, 1966, eb G_ Saigeti (Akadami Kiado. Budapest, 1968) p_ 1588. 15: E. Nakajawa and S. Shionoya, J. Chem. Phys. 47 (1966) 3211. I61 R Reisfeld, Structure Bonding 13 (1973) 53_ 171 J.C. Bourcet, J.P. Denis and J-k Loris, Proceedings of the 10th Rare Earths Conference, Carefree, Arizona (1973) p_ 960. ISJ R. Reisfeld, E. Greenberg, R.A. Velapoldi and B. Bamet, I. Chem. Phys. 56 (1972) 1698. PI FK Fong and D.J. DiestIer, J. Chem. Phys. 56 (1972) 2875. [lOI R Reisfeld, Structure Bonding 22 (1975) 123.
fll
PI
~~:
_:
/ 4
8
12
16
20
24
5’
Fig_ .5_P&
veias
C*.
Achowkdgement Two of us (XC. Kandpal and A.K. Agarwal) are grateful to C.S.I.R., New Delhi for financial assistance.
53