- Email: [email protected]
Fluorescence spectra of some triply ionized rare earths in calibo and silicate glasses
Solid State Communications,
Vol. 93, No. 7, pp. 623-628, 1995 Elsevier Science Ltd Printed in Great Britain. 1X138-1098/95 $9.50 + .OO
oo?;s-1098(94)00690-3
FLUORESCENCE
SPECTRA OF SOME TRIPLY IONIZED IN CALIBO AND SILICATE GLASSES
RARE EARTHS
Brajesh Sharma, J. Vipin Prasad, S.B. Rai and D.K. Rai Laser and Spectroscopy Laboratory,
Physics D’epartment, Banaras Hindu University, Varanasi 221005, India
(Received 11 August 1994; in revised form 12 September 1994 by C.N. R. Rao)
The fluorescence spectra of Sm, Sm + Ho, Eu and Eu + Ho doped in calibo and silicate glasses at 300 and 77 K have been recorded in the visible region as excited by the different lines of an Ar+ laser. Stark corn onents of a few levels involved in the observed transitions in Sm 3P and Eu3+ are resolved. The change in the intensity of the spectral lines of Sm3+ and Eu3+ in the presence of Ho3+ in the same glass has also been studied. Keywords: acceptor.
Stark splitting, fluorescence, energy transfer, donor and
1. INTRODUCTION SUITABLE glasses doped with rare earth atoms are often used as materials for high power lasers [l, 21. The spectral features are mainly due to triply ionized rare earth atoms which exhibit a large number of sharp fluorescence lines, many of which under suitable conditions may act as lasers. The presence of other ions in the vicinity of these radiating ions may have considerable influence on their radiative properties. It is therefore, a matter of increasing interest to study the absorption and luminescence characteristics of various glasses doped with rare earth ions, either singly or in combination [3]. Early work [l-5], used high intensity lamps with rather large spectral bandwidths as the excitation source for fluorescence measurement which caused overlapping of Stark components. Also due to the relatively small spectral brightness of the exciting source the resulting fluorescence had very feeble intensity. Recently Buddhudu et al. [6, 71 have used tb: Ar+ (X = 488.0 nm) laser for fluorescence measurements on Eu’+ doped alkali heavy metal fluoride glasses getting five well defined peaks and ascribed these to 5D0 4 ‘F2, 5D, 4 5F5,3,2, ‘D2 + ‘Fg, Stark COmponents were not seen. In the present paper we have restudied the fluorescence spectra of Sm, Eu, Sm + Ho and Eu + Ho doped in calibo and in silicate glasses. The different lines of a 4W Art laser (linewidths N 0.01 nm) were used as exciting lines
and in some favourable cases the Stark components are resolved. 2. MATERIAL PREPARATION AND EXPERIMENTAL PROCEDURE The calibo glasses were formed by mixing (20%) CaO, (10%) Li203, and (70%) B2O3 (the compounds were obtained from BDH (India) with 99% purity), while for silicate glasses the materials used are Si02 (70%), Na20 (10%) and CaO (20%). The rare earth elements Eu3+ or Sm3+ were used in the metallic form (obtained from Aldrich with stated purity of 99.9%) and these were mixed in the proportion of 4% by weight in the glass mixture. For obtaining samples with Ho as an additional dopant Ho metallic powder was also mixed in the proportion of 3% by weight into the mixture already containing the other rare earth metal. The final mixture was melted in a platinum crucible at about 1000°C in an electric oven and the melt was poured into a suitable cast. The moulded glass was polished carefully to make it amenable to spectroscopic studies. The glass samples have the dimensions 5 mm x 5 mm x 2mm. The spectroscopic investigations were carried out at room temperature (- 300K) as well as at liquid N2 temperature (77K). For recording the fluorescence spectrum at 300K the sample was mounted on a blackened metallic stand and the fluorescence emission was recorded at 90“ to the incident beam. Suitable
623
TRIPLY IONIZED
624
RARE EARTHS IN CALIBO AND SILICATE
light stoppers were used to prevent the scattered direct light from reaching into the spectrometer. A doubled walled cell of glass was fabricated for recording the spectra at 77K. The diameter of the inner and the outer tubes are 2” and 5” respectively. The space between the two tubes was evacuated to lop3 torr and then sealed off to maintain vacuum. The cell was fitted with three windows two along the same axis for the laser beam and one in the perpendicular direction to receive the fluorescence. The glass was suspended in the inner cell with the help of a rod and screw arrangement so that its flat surface is perpendicular to the laser beam. 3. RESULTS AND ANALYSIS We have used the following different lines of the Ar+ laser viz. 514.5 nm, 501.7nm, 488.0nm, 476.5 nm, 465.8 nm and 457.9 nm. The output radiant power at all these wavelengths was adjusted to remain 20mW. The nature of the observed fluorescence is the same, though the yield is different for different exciting lines. The fluorescence yield in the case of Sm 31- doped glass (silicate or calibo) is 1
I
Vol. 93, No. 7
maximum when the exciting wavelength is 476.5 nm. This is because the energy of 476.5nm (- 20986cm-‘) is very close to the energy of the 41, 1i2 level of Sm3+ (- 20 976 cm-‘) above its ground state. In the case of Eu3+ doped silicate and calibo glasses, 465.8 nm excitation gave the maximum fluorescence yield. This is because the energy of the ‘D2 level of Eu3+ from its ground state is close to the energy of the 465.8 nm radiation. Some of the observed fluorescence lines are resolved into components when the slit width is reduced and the intensity of the incident radiation (476.5nm for Sm3+ and 465.8nm for Eu3+) is increased. Emission lines from a Fe-Ne hollow cathode lamp were used for wavelength calibration. No additional fluorescent lines are observed when the sample is cooled to liquid N2 temperature. Though observed lines become slightly sharper. Assignment of the observed lines including the Stark splitted components were made on the basis of the known spectral lines of the triply ionized rare earth ions. The two different lattices used in our experiments yield somewhat different values for the energy of the ionic levels as reported. 4. DISCUSSION 4.1. Spectrum of Eu 3f
3-
3-
D-
D-
Wavelength
(nm) -
Fig. 1. Fluorescence lines and their assignment in the spectrum of Eu3+ and Eu3+ + Ho3+ doped calibo glass.
The fluorescence spectra of Eu3’ doped in calibo glass is shown in Fig. 1 A similar structure is observed for silicate glass also. The assignments of the observed lines along with their wavelength, intensity and FWHM is given in Table 1. The assignments of the observed fluorescence peaks show that the corresponding transitions are forbidden under the electric dipole selection rules though many of these are allowed under the magnetic dipole selection rules. In the fluorescence spectrum of Eu3+ doped glasses the transition assigned as ‘D,, + ‘F2 is seen to be more intense than that assigned to ‘DO + ‘Fl indicating that the former is an induced electric dipole transition while the latter is a magnetic dipole one. The transition 5D0 -+ ‘F1 shows three close lying components (see [8]), two of these are due to Stark splitting of the ‘F, level while the third component is due to the 5DI-7F3 transition. Similarly the transition ‘DO-‘F2 reported as a single line in calibo glass by Joshi and Joshi [8] and in heavy metal fluoride glass by Buddhudu and Bryant [7] is also seen to have three components. We see four lines in our spectrum when the host is a silicate glass. Two of these components are assigned as due to the Stark splitted components of the 7F2 level while the other
Vol. 93, No. 7
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE
625
Table I. Fluorescence lines and their assignment in the spectrum of Eu 3’ doped in calibo and silicate giass Observed Wavelength (nm)
Assignment
FWHM (cm-‘)
Relative intensity
5D0-‘F0 ‘D,-‘Fs ‘D,,-‘FI ‘Do-‘F,
74 147 264 145
1 1.90 1.74 1.29
‘Do-‘F2
245
2.20.
17276
Energy (cm- ’ )
Calibo glass 579.2 17 267 588.1 17004 591.8 16 898 596.7 16 759 612.6 16 324 Silicate glass 17286 578.5 587.6 17018 592.4 16 880 594.8 16812 611.7 16 348 615.9 16236 15291 654.0 687.2 14 552 703.5 14215
111
I I
7F2
128 224 257
0.07
147
0.10
I
t
‘Foe Fig. 2. Energy levels and observed in Eu3+ doped in calibo glass.
3692 3071 3005 2750 1962
0.15 0.33 0.40 0.39 1.:!8
1.oo
two are assigned to transitions from the ‘Fq level. The present spectrum also shows one intense and five weak peaks on the lower wavelength side Iof this transition. These lines are attributed to transitions from the ‘0, level to the various components of the ‘F level. The weak peaks are not seen in the calibo glass probably because of the rapid relaxation of the 5D, level of Eu3+ in calibo glass due to the larger magnitude of the phonon frequencies in calibo. The
7FA
%I
515 369 7FOt
0
Fig. 3. Energy levels and observed in Eu3+ doped in silicate glass.
spectra1 lines
intense peak is ascribed to the ‘Do -+ ‘F. transition and it is present in both the lattices. There are three distinct peaks in the spectrum of Eu3+ in silicate glass at 654, 687.2, 703 nm which are not seen in the spectrum of Eu 3’ doped calibo g lass. These are due to 5Do -+ 7F3 and 5Do + ‘Fq transitions. Joshi and Joshi [8] have reported only one peak in this region. An energy level diagram for Eu3+ levels in the two lattices are shown in Figs 2 and 3.
111 931 355 506
c,
spectral lines
1
1
1
660
600
560
Wawhgth
(nm)
_
Fig. 4. Stark components observed in different transitions in Sm3+ doped calibo glass.
626
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE
Vol. 93, No. 7
4.2. Spectrum of Sm3+ The fluorescence spectra of Sm3+ in calibo (see Fig. 4) and silicate glasses are quite distinct. Two intense peaks are seen at 599.3 nm and 611.5 nm for Sm3+ in calibo glass. The peak at 599.3 nm shows five (Stark) components in the calibo host while only three (Stark) components are seen in the silicate glass with low intensities. No line corresponding to the 611.5 nm peak seen in Sm doped calibo glass is seen for Sm3+ doped silicate host. In contrast to this dissimilarity, other spectral features seen in the fluorescence of Sm3’ in the two glasses are quite similar (though the number of split components for any line may differ in the two cases). Thus, the spectral peak observed at 563.5 nm and ascribed to the transition 4G5,2 --+ 6H512 appears in both spectra showing three Stark components in the silicate glass and four Stark components in the calibo glass. Another peak observed near 570 nm is ascribed to 4G7,2 + 6H9,2 transition. An intense peak seen at 647.4nm in the fluorescence spectrum of Sm ‘+ in calibo glass shows two Stark components due to splitting of the 6Hg,2 level but the same 4G5,2 + 6H,l, transition is seen to split in the case of the silicate glass into five components. The assignments of the fluorescent lines in the two lattices are given in Table 2. The Stark splitting in energy levels of Sm3+ in the calibo glass is shown in Fig. 5. We have also observed Stark splitting in Sm3+ doped in silicate glass. A similar energy level diagram
Table 2. Fluorescence
- 200& to+ -18912 -lb924
(91) (92)
-1~
(Aa)
-17949
(A21
-17659 (Al)
2592 (x9)
I
2505(U)
lines and their assignment in
Sm3+ and Sm3’ + Ho doped in calibo and silicate glass Observed peaks
Assignment
FWHM cm-’
Relative intensity
Wavelength Energy nm cm-’ Calibo glass 563.5 17 746 569.0 17750 599.3 16 686 611.5* 16 353
647.4*
15 447
Silicate glass 564.6 17 710 570.7 17 52 1
600.4 651.2 707.4
16655 15 356 14 135
4G,,2-bH9/2 166 4G,,2-6Hg,2 215 4G5/2-6H,/2 183 4F3,2-6Hg/2 233 4G5,2-6H9/2 136
1.21 0.68 2.06
1.oo 0.09
85 (2,) 47 (Z2)
0
4G5,2-6HS,2 123 4G7/2-6H9/2 397 4G5/2-6H7,2 140 4G5,2-6H9,2 154 4Gs,2-6H,,/2 330
* Not observed in the presence of Ho.
(Zll
2.04
1.oo 3.14 2.17 0.17 Fig. 5. Stark splitting in different energy levels and observed transition in Sm3’ doned in calibo glass.
Vol. 93, No. 7
TRIPLY IONIZED
627
RARE EARTHS IN CALIBO AND SILICATE
Table 3. Stark components observed in different transitions in Sm” in calibo and silicate glass 4G~,2-6H5/2 (cm-‘) Calibo 17 811 17773 17872 18 000
glass (A,-Zz) (A,-23)
(/t-Z,) (A’-Z2)
Silicate glass 17 961 (,4-Z,) 17775 (Al-z’) 17755 (/t-Z*)
4G/z-6H~/z (cm-‘)
4G,/H9/z (cm-‘)
4F3,z-6H9/z (cm-‘)
4G//Hgjz (cm-‘)
16854 16807 16689 16 902 16 830
15494 15 444 (&X4) 15 534 (A3-X4) 15 464 (A’-XS)
16 127 16 689 (&-X2) 15 586 (B, -X3) 16445 @,-X4) 16 353
17 833 17691 17512 17 746 17 664
::5 615 (4-X,) :I5 429 (&X4) .[6 248 .I6 248 I5 393 (&Xs)
16 627 @,-Xl)
17 519 (C,-Xl)
(Al-Y,) (Al-Y*) (A,-Y4) (A*- Y,) (A*- Y3)
16 641 (A’- Y,) 16 833 (As-Z,)
can be drawn for Sm3+ doped in silicate glass. The transition between the Stark components of the energy levels of Sm ‘+ in the two lattices are given in Table 3. 5. NON-RADIATIVE
ENERGY TRANSFER
Reisfeld [9, lo] and Boureet and Fang [I l] have interpreted the change in the intensity of the fluorescent lines of two rare earth ions doped in the same host lattice as being due to non-radiative energy transfer from the excited ion (donor) to the unexcited ion (acceptor). Glass samples containing Eu’+ and Sm” along with Ho’+ in calibo glass have also been prepared and their fluorescence spectrum has been recorded. In all the cases the presence of Ho”+ (see Fig. 1) is seen to lower the peak intensity of the isolated Eu’+ (Sm’+) 1.mes. It is surmised that in the case of Sm 3f it is the 41,5/2 level which is initially populated by the laser pulse, from which the energy levels 4G3/2 and 4Gs,2 are populated. In the case of Eu’+, the ‘D2 level is the one which is initially populated and in turn populates the fluorescence levels 5D, and 500 non-radiatively. The energy levels in Ho3+ with energy close to the 41,l/2 level of Sm3+ and the ‘D2 level of Eu’+ are the ‘Kg, ‘F2, 5F3 and ‘F4 levels. Out of these four levels of Ho’+, three namely ‘Kg, 5F2 and ‘F4 can decay only non-radiatively while ‘F3 decays radiatively to the ground state of Ho’+ (Dieke [12]). We conclude that in glasses containing both Sm and Ho the “I,, 12level of Sm’+ populated by the laser (Xexci, 476.5 nm) transfer their excitation energy to the above mentioned energy levels of Ho’+. In the case of glasses containing Eu + Ho excited at (Xex,-i,- 465.8nm). the laser excited Eu’+ (in level ‘D2) atoms lose their
(C,-X1) (Cl-X,) (C,-X,) (C,-X3) (C,-X4)
energy and result in the excitation of ‘Kg level of Ho’+. Non-radiative energy transfer efficiency is given by nr =
1 -
(7d/TdO)
=
1 -
(Id/ldO).
Td(TdO) are the donor, excitation decay times in the presence (absence) of the acceptor while Id(rds) are the corresponding fluorescence intensities. The energy transfer efficiency corresponding to 579.2, 591.8 and 612.6nm lies in Eu+ Ho are 0.7143, 0.7123 and 0.7127 respectively. Similarly, for 599.3 and 663.5 nm lines of Sm + Ho, this value is 0.3902 and 0.5305 respectively. Khandpal and Tripathi [ 131have studied the spectra of these ions in calibo glass at very low concentrations of Ho and have concluded that the energy transfer is due to dipole-dipole interaction. Our results are also consistent with this mechanism even though the concentration of Ho is higher.
Acknowledgements - We express our thanks to Prof. B.S. Tyagi of Department of Ceramic Engineering for help in preparation of the glasses. The financial support from UGC and DST are also acknowledged. REFERENCES 1. 2. 3. 4. 5.
R. Reisfeld & C.K. Jorgensen, Laser and Excited States of Rare Earth. Springer, Berlin, Heidelberg, New York (1977). A.C. John 8c L.G. Deshazer, IEEE J. Quantum Electron (USA) 3, 97 (1973). R. Reisfeld & Y. Eckstein, J. Chem. Phys. (USA) 63,400l (1975). R.J. Ralph, B.L. Clyde, J.W. Marvin & F.R. Charles, J. Appl. Phys. (USA) 47, 2020 (1976). V.N. Rai, L.B. Tewari, S.N. Thakur & D.K. Rai, Pramana (India) 19, 579 (1982).
628 6. 7. 8. 9.
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE R. Harinath, S. Buddhudu, F.J. Bryant & Luo Xi, Solid State Commun. 74, 1147 (1990). S. Buddhudu & F.J. Bryant, Spectrochemica Acta #A, 1381 (1988). J. Joshi & J.C. Joshi, Ind. J. Phys. 6OB, 217 (1986). R. Reisfeld, Structure and Bonding 22, 123 (1975).
10. 11. 12. 13.
Vol. 93, No. 7
R. Reisfeld, Structure and Bonding 13, 53 (1973). J.C. Boureet & F.K. Fang, J. Chem. Phys. 60, 34 (1974). G.H. Dieke, Spectra and Energy Level of Rare Earth Ions in Crystals. Interscience Publishers, New York (1968). H.G. Khandpal & H.B. Tripathi, Solid State Commun. 40,673 (1981).
Vol. 93, No. 7, pp. 623-628, 1995 Elsevier Science Ltd Printed in Great Britain. 1X138-1098/95 $9.50 + .OO
oo?;s-1098(94)00690-3
FLUORESCENCE
SPECTRA OF SOME TRIPLY IONIZED IN CALIBO AND SILICATE GLASSES
RARE EARTHS
Brajesh Sharma, J. Vipin Prasad, S.B. Rai and D.K. Rai Laser and Spectroscopy Laboratory,
Physics D’epartment, Banaras Hindu University, Varanasi 221005, India
(Received 11 August 1994; in revised form 12 September 1994 by C.N. R. Rao)
The fluorescence spectra of Sm, Sm + Ho, Eu and Eu + Ho doped in calibo and silicate glasses at 300 and 77 K have been recorded in the visible region as excited by the different lines of an Ar+ laser. Stark corn onents of a few levels involved in the observed transitions in Sm 3P and Eu3+ are resolved. The change in the intensity of the spectral lines of Sm3+ and Eu3+ in the presence of Ho3+ in the same glass has also been studied. Keywords: acceptor.
Stark splitting, fluorescence, energy transfer, donor and
1. INTRODUCTION SUITABLE glasses doped with rare earth atoms are often used as materials for high power lasers [l, 21. The spectral features are mainly due to triply ionized rare earth atoms which exhibit a large number of sharp fluorescence lines, many of which under suitable conditions may act as lasers. The presence of other ions in the vicinity of these radiating ions may have considerable influence on their radiative properties. It is therefore, a matter of increasing interest to study the absorption and luminescence characteristics of various glasses doped with rare earth ions, either singly or in combination [3]. Early work [l-5], used high intensity lamps with rather large spectral bandwidths as the excitation source for fluorescence measurement which caused overlapping of Stark components. Also due to the relatively small spectral brightness of the exciting source the resulting fluorescence had very feeble intensity. Recently Buddhudu et al. [6, 71 have used tb: Ar+ (X = 488.0 nm) laser for fluorescence measurements on Eu’+ doped alkali heavy metal fluoride glasses getting five well defined peaks and ascribed these to 5D0 4 ‘F2, 5D, 4 5F5,3,2, ‘D2 + ‘Fg, Stark COmponents were not seen. In the present paper we have restudied the fluorescence spectra of Sm, Eu, Sm + Ho and Eu + Ho doped in calibo and in silicate glasses. The different lines of a 4W Art laser (linewidths N 0.01 nm) were used as exciting lines
and in some favourable cases the Stark components are resolved. 2. MATERIAL PREPARATION AND EXPERIMENTAL PROCEDURE The calibo glasses were formed by mixing (20%) CaO, (10%) Li203, and (70%) B2O3 (the compounds were obtained from BDH (India) with 99% purity), while for silicate glasses the materials used are Si02 (70%), Na20 (10%) and CaO (20%). The rare earth elements Eu3+ or Sm3+ were used in the metallic form (obtained from Aldrich with stated purity of 99.9%) and these were mixed in the proportion of 4% by weight in the glass mixture. For obtaining samples with Ho as an additional dopant Ho metallic powder was also mixed in the proportion of 3% by weight into the mixture already containing the other rare earth metal. The final mixture was melted in a platinum crucible at about 1000°C in an electric oven and the melt was poured into a suitable cast. The moulded glass was polished carefully to make it amenable to spectroscopic studies. The glass samples have the dimensions 5 mm x 5 mm x 2mm. The spectroscopic investigations were carried out at room temperature (- 300K) as well as at liquid N2 temperature (77K). For recording the fluorescence spectrum at 300K the sample was mounted on a blackened metallic stand and the fluorescence emission was recorded at 90“ to the incident beam. Suitable
623
TRIPLY IONIZED
624
RARE EARTHS IN CALIBO AND SILICATE
light stoppers were used to prevent the scattered direct light from reaching into the spectrometer. A doubled walled cell of glass was fabricated for recording the spectra at 77K. The diameter of the inner and the outer tubes are 2” and 5” respectively. The space between the two tubes was evacuated to lop3 torr and then sealed off to maintain vacuum. The cell was fitted with three windows two along the same axis for the laser beam and one in the perpendicular direction to receive the fluorescence. The glass was suspended in the inner cell with the help of a rod and screw arrangement so that its flat surface is perpendicular to the laser beam. 3. RESULTS AND ANALYSIS We have used the following different lines of the Ar+ laser viz. 514.5 nm, 501.7nm, 488.0nm, 476.5 nm, 465.8 nm and 457.9 nm. The output radiant power at all these wavelengths was adjusted to remain 20mW. The nature of the observed fluorescence is the same, though the yield is different for different exciting lines. The fluorescence yield in the case of Sm 31- doped glass (silicate or calibo) is 1
I
Vol. 93, No. 7
maximum when the exciting wavelength is 476.5 nm. This is because the energy of 476.5nm (- 20986cm-‘) is very close to the energy of the 41, 1i2 level of Sm3+ (- 20 976 cm-‘) above its ground state. In the case of Eu3+ doped silicate and calibo glasses, 465.8 nm excitation gave the maximum fluorescence yield. This is because the energy of the ‘D2 level of Eu3+ from its ground state is close to the energy of the 465.8 nm radiation. Some of the observed fluorescence lines are resolved into components when the slit width is reduced and the intensity of the incident radiation (476.5nm for Sm3+ and 465.8nm for Eu3+) is increased. Emission lines from a Fe-Ne hollow cathode lamp were used for wavelength calibration. No additional fluorescent lines are observed when the sample is cooled to liquid N2 temperature. Though observed lines become slightly sharper. Assignment of the observed lines including the Stark splitted components were made on the basis of the known spectral lines of the triply ionized rare earth ions. The two different lattices used in our experiments yield somewhat different values for the energy of the ionic levels as reported. 4. DISCUSSION 4.1. Spectrum of Eu 3f
3-
3-
D-
D-
Wavelength
(nm) -
Fig. 1. Fluorescence lines and their assignment in the spectrum of Eu3+ and Eu3+ + Ho3+ doped calibo glass.
The fluorescence spectra of Eu3’ doped in calibo glass is shown in Fig. 1 A similar structure is observed for silicate glass also. The assignments of the observed lines along with their wavelength, intensity and FWHM is given in Table 1. The assignments of the observed fluorescence peaks show that the corresponding transitions are forbidden under the electric dipole selection rules though many of these are allowed under the magnetic dipole selection rules. In the fluorescence spectrum of Eu3+ doped glasses the transition assigned as ‘D,, + ‘F2 is seen to be more intense than that assigned to ‘DO + ‘Fl indicating that the former is an induced electric dipole transition while the latter is a magnetic dipole one. The transition 5D0 -+ ‘F1 shows three close lying components (see [8]), two of these are due to Stark splitting of the ‘F, level while the third component is due to the 5DI-7F3 transition. Similarly the transition ‘DO-‘F2 reported as a single line in calibo glass by Joshi and Joshi [8] and in heavy metal fluoride glass by Buddhudu and Bryant [7] is also seen to have three components. We see four lines in our spectrum when the host is a silicate glass. Two of these components are assigned as due to the Stark splitted components of the 7F2 level while the other
Vol. 93, No. 7
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE
625
Table I. Fluorescence lines and their assignment in the spectrum of Eu 3’ doped in calibo and silicate giass Observed Wavelength (nm)
Assignment
FWHM (cm-‘)
Relative intensity
5D0-‘F0 ‘D,-‘Fs ‘D,,-‘FI ‘Do-‘F,
74 147 264 145
1 1.90 1.74 1.29
‘Do-‘F2
245
2.20.
17276
Energy (cm- ’ )
Calibo glass 579.2 17 267 588.1 17004 591.8 16 898 596.7 16 759 612.6 16 324 Silicate glass 17286 578.5 587.6 17018 592.4 16 880 594.8 16812 611.7 16 348 615.9 16236 15291 654.0 687.2 14 552 703.5 14215
111
I I
7F2
128 224 257
0.07
147
0.10
I
t
‘Foe Fig. 2. Energy levels and observed in Eu3+ doped in calibo glass.
3692 3071 3005 2750 1962
0.15 0.33 0.40 0.39 1.:!8
1.oo
two are assigned to transitions from the ‘Fq level. The present spectrum also shows one intense and five weak peaks on the lower wavelength side Iof this transition. These lines are attributed to transitions from the ‘0, level to the various components of the ‘F level. The weak peaks are not seen in the calibo glass probably because of the rapid relaxation of the 5D, level of Eu3+ in calibo glass due to the larger magnitude of the phonon frequencies in calibo. The
7FA
%I
515 369 7FOt
0
Fig. 3. Energy levels and observed in Eu3+ doped in silicate glass.
spectra1 lines
intense peak is ascribed to the ‘Do -+ ‘F. transition and it is present in both the lattices. There are three distinct peaks in the spectrum of Eu3+ in silicate glass at 654, 687.2, 703 nm which are not seen in the spectrum of Eu 3’ doped calibo g lass. These are due to 5Do -+ 7F3 and 5Do + ‘Fq transitions. Joshi and Joshi [8] have reported only one peak in this region. An energy level diagram for Eu3+ levels in the two lattices are shown in Figs 2 and 3.
111 931 355 506
c,
spectral lines
1
1
1
660
600
560
Wawhgth
(nm)
_
Fig. 4. Stark components observed in different transitions in Sm3+ doped calibo glass.
626
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE
Vol. 93, No. 7
4.2. Spectrum of Sm3+ The fluorescence spectra of Sm3+ in calibo (see Fig. 4) and silicate glasses are quite distinct. Two intense peaks are seen at 599.3 nm and 611.5 nm for Sm3+ in calibo glass. The peak at 599.3 nm shows five (Stark) components in the calibo host while only three (Stark) components are seen in the silicate glass with low intensities. No line corresponding to the 611.5 nm peak seen in Sm doped calibo glass is seen for Sm3+ doped silicate host. In contrast to this dissimilarity, other spectral features seen in the fluorescence of Sm3’ in the two glasses are quite similar (though the number of split components for any line may differ in the two cases). Thus, the spectral peak observed at 563.5 nm and ascribed to the transition 4G5,2 --+ 6H512 appears in both spectra showing three Stark components in the silicate glass and four Stark components in the calibo glass. Another peak observed near 570 nm is ascribed to 4G7,2 + 6H9,2 transition. An intense peak seen at 647.4nm in the fluorescence spectrum of Sm ‘+ in calibo glass shows two Stark components due to splitting of the 6Hg,2 level but the same 4G5,2 + 6H,l, transition is seen to split in the case of the silicate glass into five components. The assignments of the fluorescent lines in the two lattices are given in Table 2. The Stark splitting in energy levels of Sm3+ in the calibo glass is shown in Fig. 5. We have also observed Stark splitting in Sm3+ doped in silicate glass. A similar energy level diagram
Table 2. Fluorescence
- 200& to+ -18912 -lb924
(91) (92)
-1~
(Aa)
-17949
(A21
-17659 (Al)
2592 (x9)
I
2505(U)
lines and their assignment in
Sm3+ and Sm3’ + Ho doped in calibo and silicate glass Observed peaks
Assignment
FWHM cm-’
Relative intensity
Wavelength Energy nm cm-’ Calibo glass 563.5 17 746 569.0 17750 599.3 16 686 611.5* 16 353
647.4*
15 447
Silicate glass 564.6 17 710 570.7 17 52 1
600.4 651.2 707.4
16655 15 356 14 135
4G,,2-bH9/2 166 4G,,2-6Hg,2 215 4G5/2-6H,/2 183 4F3,2-6Hg/2 233 4G5,2-6H9/2 136
1.21 0.68 2.06
1.oo 0.09
85 (2,) 47 (Z2)
0
4G5,2-6HS,2 123 4G7/2-6H9/2 397 4G5/2-6H7,2 140 4G5,2-6H9,2 154 4Gs,2-6H,,/2 330
* Not observed in the presence of Ho.
(Zll
2.04
1.oo 3.14 2.17 0.17 Fig. 5. Stark splitting in different energy levels and observed transition in Sm3’ doned in calibo glass.
Vol. 93, No. 7
TRIPLY IONIZED
627
RARE EARTHS IN CALIBO AND SILICATE
Table 3. Stark components observed in different transitions in Sm” in calibo and silicate glass 4G~,2-6H5/2 (cm-‘) Calibo 17 811 17773 17872 18 000
glass (A,-Zz) (A,-23)
(/t-Z,) (A’-Z2)
Silicate glass 17 961 (,4-Z,) 17775 (Al-z’) 17755 (/t-Z*)
4G/z-6H~/z (cm-‘)
4G,/H9/z (cm-‘)
4F3,z-6H9/z (cm-‘)
4G//Hgjz (cm-‘)
16854 16807 16689 16 902 16 830
15494 15 444 (&X4) 15 534 (A3-X4) 15 464 (A’-XS)
16 127 16 689 (&-X2) 15 586 (B, -X3) 16445 @,-X4) 16 353
17 833 17691 17512 17 746 17 664
::5 615 (4-X,) :I5 429 (&X4) .[6 248 .I6 248 I5 393 (&Xs)
16 627 @,-Xl)
17 519 (C,-Xl)
(Al-Y,) (Al-Y*) (A,-Y4) (A*- Y,) (A*- Y3)
16 641 (A’- Y,) 16 833 (As-Z,)
can be drawn for Sm3+ doped in silicate glass. The transition between the Stark components of the energy levels of Sm ‘+ in the two lattices are given in Table 3. 5. NON-RADIATIVE
ENERGY TRANSFER
Reisfeld [9, lo] and Boureet and Fang [I l] have interpreted the change in the intensity of the fluorescent lines of two rare earth ions doped in the same host lattice as being due to non-radiative energy transfer from the excited ion (donor) to the unexcited ion (acceptor). Glass samples containing Eu’+ and Sm” along with Ho’+ in calibo glass have also been prepared and their fluorescence spectrum has been recorded. In all the cases the presence of Ho”+ (see Fig. 1) is seen to lower the peak intensity of the isolated Eu’+ (Sm’+) 1.mes. It is surmised that in the case of Sm 3f it is the 41,5/2 level which is initially populated by the laser pulse, from which the energy levels 4G3/2 and 4Gs,2 are populated. In the case of Eu’+, the ‘D2 level is the one which is initially populated and in turn populates the fluorescence levels 5D, and 500 non-radiatively. The energy levels in Ho3+ with energy close to the 41,l/2 level of Sm3+ and the ‘D2 level of Eu’+ are the ‘Kg, ‘F2, 5F3 and ‘F4 levels. Out of these four levels of Ho’+, three namely ‘Kg, 5F2 and ‘F4 can decay only non-radiatively while ‘F3 decays radiatively to the ground state of Ho’+ (Dieke [12]). We conclude that in glasses containing both Sm and Ho the “I,, 12level of Sm’+ populated by the laser (Xexci, 476.5 nm) transfer their excitation energy to the above mentioned energy levels of Ho’+. In the case of glasses containing Eu + Ho excited at (Xex,-i,- 465.8nm). the laser excited Eu’+ (in level ‘D2) atoms lose their
(C,-X1) (Cl-X,) (C,-X,) (C,-X3) (C,-X4)
energy and result in the excitation of ‘Kg level of Ho’+. Non-radiative energy transfer efficiency is given by nr =
1 -
(7d/TdO)
=
1 -
(Id/ldO).
Td(TdO) are the donor, excitation decay times in the presence (absence) of the acceptor while Id(rds) are the corresponding fluorescence intensities. The energy transfer efficiency corresponding to 579.2, 591.8 and 612.6nm lies in Eu+ Ho are 0.7143, 0.7123 and 0.7127 respectively. Similarly, for 599.3 and 663.5 nm lines of Sm + Ho, this value is 0.3902 and 0.5305 respectively. Khandpal and Tripathi [ 131have studied the spectra of these ions in calibo glass at very low concentrations of Ho and have concluded that the energy transfer is due to dipole-dipole interaction. Our results are also consistent with this mechanism even though the concentration of Ho is higher.
Acknowledgements - We express our thanks to Prof. B.S. Tyagi of Department of Ceramic Engineering for help in preparation of the glasses. The financial support from UGC and DST are also acknowledged. REFERENCES 1. 2. 3. 4. 5.
R. Reisfeld & C.K. Jorgensen, Laser and Excited States of Rare Earth. Springer, Berlin, Heidelberg, New York (1977). A.C. John 8c L.G. Deshazer, IEEE J. Quantum Electron (USA) 3, 97 (1973). R. Reisfeld & Y. Eckstein, J. Chem. Phys. (USA) 63,400l (1975). R.J. Ralph, B.L. Clyde, J.W. Marvin & F.R. Charles, J. Appl. Phys. (USA) 47, 2020 (1976). V.N. Rai, L.B. Tewari, S.N. Thakur & D.K. Rai, Pramana (India) 19, 579 (1982).
628 6. 7. 8. 9.
TRIPLY IONIZED RARE EARTHS IN CALIBO AND SILICATE R. Harinath, S. Buddhudu, F.J. Bryant & Luo Xi, Solid State Commun. 74, 1147 (1990). S. Buddhudu & F.J. Bryant, Spectrochemica Acta #A, 1381 (1988). J. Joshi & J.C. Joshi, Ind. J. Phys. 6OB, 217 (1986). R. Reisfeld, Structure and Bonding 22, 123 (1975).
10. 11. 12. 13.
Vol. 93, No. 7
R. Reisfeld, Structure and Bonding 13, 53 (1973). J.C. Boureet & F.K. Fang, J. Chem. Phys. 60, 34 (1974). G.H. Dieke, Spectra and Energy Level of Rare Earth Ions in Crystals. Interscience Publishers, New York (1968). H.G. Khandpal & H.B. Tripathi, Solid State Commun. 40,673 (1981).