- Email: [email protected]
Anti-Stokes emission from Gd3+ in BaGdF5
Volume 193, number 5
CHEMICAL PHYSICS LETTERS
5 June 1992
Anti-Stokes emission from Gd3+ in BaGdF, J. Sytsma
and G. Blasse
Debye Research Institute, University of Utrecht. P.O. Box 80000, 3508 TA Utrecht, The Netherlands
Received 16 March I992
We report anti-Stokes emission from Gd 3+ in BaGdF,. After excitation into the 6I,,2 level, emission is observed from the 6D9,2level and after excitation into the 6P7,2 level emission is observed from the 61,,* level. The responsible mechanism involves energy migration and photon addition at Gd’+ ions with low 4f6Sd levels.
1. Introduction The optical transitions of the Gd3+ ion occur in the near-ultraviolet. After lasers in this region became available, interest in the spectroscopy of this ion increased. These lasers yield resonant excitation of the Gd3+ levels, concentrated in time and space. This allows determination of the Judd-Ofelt parameters [ 11, fluorescence line narrowing [ 21 and siteselective spectroscopy [ 3 1, but may also give rise to resonant multi-photon processes such as excited-state absorption, upconversion and photon addition by energy transfer. Such processes have been extensively investigated for other rare earth ions, Tb3+ [4],Tm3+ [5],Nd3+ [6] andEr3+ [7],butseldom for Gd3+ [ 81 #I. In this paper, we study the spectroscopy of singlecrystalline BaGdF, with fluorite structure. AntiStokes emission is observed from the 6D,,z and the 6I,,2 levels of Gd3+. Remarkably enough, the antiStokes intensity is linear with laser intensity. After a qualitative analysis of the results a model is derived. 2. Experimental A single crystal of BaGdFs was kindly provided by Correspondence to: G. Blasse, Debye Research Institute, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands. tl’ We do not consider two-photon excitation of Gd3+ here, because this technique does not imply single photons to be resonant with the Gd3+ levels.
342
0009-2614/92/$
Dr. L.H. Brixner (E.I. du Pont Nemours, Wilmington, USA). The crystal was grown by slow cooling of the melt. The sample is approximately 5 x 5 x 1 mm and is optically transparent. According to X-ray diffraction analysis the crystal has the fluorite structure. The Ba2+ and Gd3+ ions occupy the cation sites and the excess charge of the Gd3+ ion is compensated by interstitial F- [ 9- 13 1. For the interpretation of the optical measurements it is important to realize that this implies a strong disorder in the crystal structure. The optical equipment has been described [ 141.
3. Results At liquid-helium temperature and the excitation energy tuned to 35962 cm-‘, i.e. in the 8S,,,+61,region, luminescence is observed around 32000 and 35780 cm-’ (fig. 1). The former, containing approximately 85% of the total emission, corresponds to the 6P,,2+8S,,2, and the latter to the 61,,2-+8S,,2 transition. Both emission lines show a shoulder on the low-energy side, the shoulder for the 6P,,2-+ 8S7,2 transition being more pronounced than for the 61,,2+8S,,2 transition. At room temperature the (‘PSI 2 level is populated thermally and also shows emission (fig. 2). This spectrum is in good agreement with the emission spectrum under X-ray excitation #2. No #’ This spectrum was kindly put at our disposal by Dr. L.H. Brixner. 05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
CHEMICAL PHYSICS LETTERS
Volume 193, number 5
5 June 1992
‘I 112-->‘S
31000
31500
Energy
32000
32500
35550
7/Z
35650
Energy
---)
35750
35850
---a
Fig. 1. Emission of the 6I,,2, 6P,,2+8S,,2 transitions of Gd3+ in BaGdF, at 4.2 K. Excitation is at 35962 cm-‘.
1 31000
I 31500
Energy
32000
32500
33000
ticm-‘J ---a
Fig. 2. Emission of the 6P,+8S,,2 transition of Gd3+ in BaGdF, at 290 K. Excitation is at 35962 cm-‘.
crystal field components can be resolved in the roomtemperature emission. Even at low excitation power of the laser, the decay curves of both emissions are non-exponential. When increasing the laser intensity, still tuned at 35962 cm-‘, an anti-Stokes emission is observed around 39450 cm-’ which is shown in fig. 3. This is assigned to the 6D9/2+‘S7/2 transition according to the well-known energy level scheme of Gd3+. The intensity ratio of the 6D9/2+8S7/2 and the 6I7,2+ 8S7,2emission is x 0.01. The intensity of this emission varies linearly with the laser intensity, as shown in fig. 4. The decay curve of the 6D9/2+8S7/2 transition shows a clear build up. Raising the temperature shortens the lifetimes of the emission drastically. Up to 80 K the positions of
the maxima of all emission lines shift to higher energies. Above 80 K these positions remain the same. same. After excitation at room temperature into the 6P7,2 level, anti-Stokes emission is observed from the 617,2-+8S7,2 transition but not from the 6D9,2+ 8S7,2 transition. The intensity ratio of the 6I7,2+ 8S7,2and the 6P7,2+8S7,2 emission is only 1Om3.The intensity of the anti-Stokes 617,2+8S7,2 emission is also linear with the laser power (fig. 4). At 4.2 K this anti-Stokes emission is not observable. Finally, we note that for the diluted system BaLac,99Gd,,0,F, under similar conditions no antiStokes emission was observed. The decay curves are exponential, yielding decay times of 8.5 and 4 ms for the 6P7,2-+8S7,2 and 617,2+8S7,2 transitions, respectively.
4. Discussion The emission lines (fig. 1) have large widths. This is ascribed to excessive inhomogeneous broadening due to the disorder in the crystal lattice. The spectra also show certain subsets of Gd3+ ions. This is especially clear in the 617,2emission in fig. 1. The emission lines have side bands on the low-energy side, the one accompanying the 617,2-‘8S7,2 transition being the weakest. These bands can be ascribed partly to vibronic transitions [ 15 1, but extend down to x 800 cm- ’ below the maximum of the main emission. This is much further than vi343
Volume 193, number 5
38500
CHEMICAL PHYSICS LETTERS
39000
Bnergy
39500
hi’~
40000
35550
5 June 1992
35675
Energy
- -a
35800
km-‘)
35925
36050
---)
Fig. 3. The 6D,,, (left) and 6I,,z (right)+ 8S,,z transitions as anti-Stokes emissions in BaGdF,. The former spectrum is measured at 4.2 K, the latter at room temperature. The excitation energy is indicated in both figures.
0.00 0.2s 0.50 0.7s 1.00 1.2s
Laser
power
0.00 0.25 030
Laser
0.75 1.00 1.25
power
Fig. 4. Power dependence of the anti-Stokes emission intensity observed in BaGdF,. Left: 6D9,2+8S,,2 at 4.2 K, right: 6I,,2+ 8S7,2at room temperature. The laser power is given relative to the maximum laser intensity ( x 10 mJ/pulse).
bronic transitions are expected to extend for a fluoride, but also their intensity is too high for a vibronic transition in a fluoride [ 15 1. Therefore, these side bands contain emission from deep traps, most probably Gd3+ ions near an oxygen ion [ 161. The width of the side bands indicates that there is also a considerable inhomogeneous broadening among these traps. The shift of the positions of the emission maxima with temperature, and the strong temperature dependence of the decay curves, indicate that energy migration occurs over all the three excited levels (6D9,2, 6I,,2 and 6P7,~ ). These processes will not be 344
analyzed quantitatively here. As shown below, their occurrence is a prerequisite for the anti-Stokes emission. After excitation into the 6I,,2 level, emission is observed from the 6D9,2 level and after excitation into the 6P,,2 level emission is observed from the 6I,,2 level. In both cases the energy difference between the emission and the excitation energies is approximately 3700 cm-‘. These anti-Stokes emissions can therefore not be due to thermal population (exp( -hAY/k,T)= lo-*). The build up of the 6D9,2 emission after the laser pulse shows that the second excitation step is not due to a photon from the laser pulse. Combining this with the fact that for the diluted system, BaLao.9PGdo.o,F,, the 6D,,z, 617,2-+8S,,2 transitions were not observed as antiStokes emission, and with the fact that in BaGdF, energy migration occurs over the levels in which excitation took place, we conclude that the multi-photon process involves photon addition by energy transfer [ 17 1. An excited Gd3+ ion transfers its energy to a nearby Gd 3+ ion which is already in an excited state. The latter ion reaches an intermediate state which is higher in energy than the 6D and 61 levels. After relaxation to these levels, radiative relaxation to the ground state takes place. The energy migration is therefore a prerequisite for the occurrence of the anti-Stokes emission. After excitation at x 32000 cm- I the starting level is the 6P,,, level. The energy position of the intermediate state then has to be at x 64000 cm- ‘. After
Volume 193, number 5
CHEMICAL PHYSICS LETTERS
into the 6I,,2 level this position will be a little higher, viz. at = 72000 cm-‘. The nature of the intermediate state is not immediately clear. It seems unlikely that there are 4p states at exactly twice the energy of the +?,,* as well as of the 6P,,2 levels. Moreover, 4p states as the intermediate state are not able to account for the observed linear relation between the anti-Stokes intensity and the laser power. This relation suggests a single rate-determining step. Opposite-parity states for Gd3+ are expected to be at even higher energies. In CaF,: Gd3+ the oppositeparity state (4f65d) has been reported to have its onset at about 77500 cm-’ [ 181. The laser intensity dependence of the anti-Stokes emission intensity suggests that the second step is an allowed optical transition, i.e. from the 6I,,2 or 6P,,2 to one of the lowest levels of the opposite-parity 4f65d configuration. Due to the variation of the nephelauxetic effect and the crystal field strength of the Gd3+ ions, it is proposed that part of the Gd3+ ions have their lowest 4f65d component at low energy, viz. 72000 cm-’ for 6I,,2 excitation and 64000 cm- ’ for 6P,,2 excitation. The latter part is, of course, considerably smaller than the former part, explaining the higher anti-Stokes emission intensity for 61,, z excitation. Energy migration is necessary to reach these Gd3+ ions. At first sight it might be thought that the Gd3+ ions next to oxygen impurities are the centers from which the anti-Stokes emission originates, since they certainly have low-lying 4f65d components. However, in that case the emission should be at a few hundreds of wavenumbers lower energy than expected for the fluoride. This is not the case. Using data for LaF,: Gd3+ [ 191 the 6D9,2 emission of BaGdF, is expected to center round 39500 cm-‘. This is what is observed. For a Gd3+-02- center the emission is expected to be centered below 39000 cm-‘. Actually, a relatively low-lying 4f65d level might be expected for a cluster of Gd3+ ions. Such clusters are known to occur in compositions like BaGdF, [ 12,13 1. In this context we note that the lowest level of the excited 5d configuration of Ce3+ shifts some 3000 cm-’ to lower energy if the Ce3+ concentration in BaF, increases from 0.2 to 1.O mol% [ 201. Finally, we note that Hiilsa has reported a similar anti-Stokes emission for Eu3+ in LaOCl [ 2 11. The rate-determining step in that case is the 7F0-+5D0
excitation
5 June 1992
transition, the second one the allowed ‘D,-+chargetransfer transition. Anti-Stokes emission occuts from the higher sDJ levels. In conclusion, we have observed anti-Stokes emission from Gd3+ ions in BaGdF, by energy-migration-assisted photon addition.
Acknowledgement
We thank Dr. L.H. Brixner for providing us with the crystal and Dr. A. Meijerink for clarifying discussions. The investigations were partly supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Foundation for Advancement of Pure Research (NWO) and the Netherlands Foundation for Technical Research (STW).
References [ I] J. Sytsma, G.F. Imbusch and G. Blasse, J. Chem. Phys. 9 1 (1989) 1456; J. Chem. Phys. 92 (1990) 3249 (E). [ 2 ] J.W.M. Verweij, G.F. Imbusch and G. Blasse, J. Phys. Chem. Solids 50 (1989) 813. [ 31 J. Sytsma, S.J. Kroes, G. Blasse and N.M. Khaidukov, J. Phys. Condensed Matter 3 ( 199 1) 8959. [4] R. Raue, K. Nieuwesteeg and W. Busselt, J. Luminescence 48/49(1991)485. [ 51 M. Dulick, G.E. Faulkner, N.J. Cockroft and D.C. Nguyen, J. Luminescence 48/49 (1991) 517. [6] N. Pelletier-Allard and R. Pelletier, J. Luminescence 48/49 (1991) 867. [7] A.J. Silversmith, W. Lenth and R.M. Macfarlane, Appl. Phys. Letters 51 (1987) 1977. [8] C. Linares, P. Jung, G. Boulon and F. Gaume, J. LessCommon Metals 93 ( 1983) 89; R. Mahiou, B. Jacquier and C. Madej, J. Chem. Phys. 89 (1988) 5931; R. Mahiou, J. Metin and J.C. Cousseins, J. Luminescence 45 (1990) 363. [ 91 E.G. Ippolitov, L.S. Garashina and A.G. Maklachkov, Inorg. Mater. 3 (1967) 59. [lo] W. Bollmann, Phys. Stat. Sol. (a) 18 (1973) 313. [ 111 H.W. den Hartog, P. Dorenbos and W.J. Postma, Phys. Rev. B 34 (1986) 7496. [ 121 K.E.P. Wapenaar, J.L. van Koesveld and J. Schoonman, Solid State Ionics 2 ( 1981) 145. [ 13 ] J. Schoonman, in: Fast ion transport in solids, eds. P. Vashita, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam, 1979).
34.5
Volume 193, number 5
CHEMICAL PHYSICS LETTERS
[ 141J. Sytsma, G.F. lmbusch and G. Blasse, J. Phys. Condensed Matter 2 (1990) 5171. [ 151 J. Sytsma, W. van Schaik and G. Blasse, J. Phys. Chem. Solids 52 (1991) 419. [ 161 L.H. Brixner and G. Blasse, European J. Solid State Inorg. Chem. 27 (1990) 581. [ 171 F. Auzel, Compt. Rend. Acad. Sci. (Paris) 262 (1966) 1016; in Advances in non-radiative processes, ed. B. DiBartolo (Plenum Press, New York, 199 1) p. 135.
346
5 June 1992
[ 18 ] T. Szczurek and M. Schlesinger, in: Rare earth spectroscopy, eds. B. Jezowska-Trezbiatowska, J. Legendziewicz and W. Strek (World Scientific, Singapore, 1985) p. 325. [ 191 W.T. Carnall, H. Crosswhite and H.M. Crosswhite, in: Energy Level Structure and Transition Probabilities of the Trivalent Lanthanides in LaF,, Argonne National Laboratory Report ( 1977). [20] R. Visser, P. Dorenbos, C.W.E. van Eijk, R.W. Hollander and P. Schotanus, IEEE Trans. Nucl. Science 38 ( 1991) 178. [ 2 1 ] J. Holsa, Chem. Phys. Letters 112 ( 1984) 246.
CHEMICAL PHYSICS LETTERS
5 June 1992
Anti-Stokes emission from Gd3+ in BaGdF, J. Sytsma
and G. Blasse
Debye Research Institute, University of Utrecht. P.O. Box 80000, 3508 TA Utrecht, The Netherlands
Received 16 March I992
We report anti-Stokes emission from Gd 3+ in BaGdF,. After excitation into the 6I,,2 level, emission is observed from the 6D9,2level and after excitation into the 6P7,2 level emission is observed from the 61,,* level. The responsible mechanism involves energy migration and photon addition at Gd’+ ions with low 4f6Sd levels.
1. Introduction The optical transitions of the Gd3+ ion occur in the near-ultraviolet. After lasers in this region became available, interest in the spectroscopy of this ion increased. These lasers yield resonant excitation of the Gd3+ levels, concentrated in time and space. This allows determination of the Judd-Ofelt parameters [ 11, fluorescence line narrowing [ 21 and siteselective spectroscopy [ 3 1, but may also give rise to resonant multi-photon processes such as excited-state absorption, upconversion and photon addition by energy transfer. Such processes have been extensively investigated for other rare earth ions, Tb3+ [4],Tm3+ [5],Nd3+ [6] andEr3+ [7],butseldom for Gd3+ [ 81 #I. In this paper, we study the spectroscopy of singlecrystalline BaGdF, with fluorite structure. AntiStokes emission is observed from the 6D,,z and the 6I,,2 levels of Gd3+. Remarkably enough, the antiStokes intensity is linear with laser intensity. After a qualitative analysis of the results a model is derived. 2. Experimental A single crystal of BaGdFs was kindly provided by Correspondence to: G. Blasse, Debye Research Institute, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands. tl’ We do not consider two-photon excitation of Gd3+ here, because this technique does not imply single photons to be resonant with the Gd3+ levels.
342
0009-2614/92/$
Dr. L.H. Brixner (E.I. du Pont Nemours, Wilmington, USA). The crystal was grown by slow cooling of the melt. The sample is approximately 5 x 5 x 1 mm and is optically transparent. According to X-ray diffraction analysis the crystal has the fluorite structure. The Ba2+ and Gd3+ ions occupy the cation sites and the excess charge of the Gd3+ ion is compensated by interstitial F- [ 9- 13 1. For the interpretation of the optical measurements it is important to realize that this implies a strong disorder in the crystal structure. The optical equipment has been described [ 141.
3. Results At liquid-helium temperature and the excitation energy tuned to 35962 cm-‘, i.e. in the 8S,,,+61,region, luminescence is observed around 32000 and 35780 cm-’ (fig. 1). The former, containing approximately 85% of the total emission, corresponds to the 6P,,2+8S,,2, and the latter to the 61,,2-+8S,,2 transition. Both emission lines show a shoulder on the low-energy side, the shoulder for the 6P,,2-+ 8S7,2 transition being more pronounced than for the 61,,2+8S,,2 transition. At room temperature the (‘PSI 2 level is populated thermally and also shows emission (fig. 2). This spectrum is in good agreement with the emission spectrum under X-ray excitation #2. No #’ This spectrum was kindly put at our disposal by Dr. L.H. Brixner. 05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
CHEMICAL PHYSICS LETTERS
Volume 193, number 5
5 June 1992
‘I 112-->‘S
31000
31500
Energy
32000
32500
35550
7/Z
35650
Energy
---)
35750
35850
---a
Fig. 1. Emission of the 6I,,2, 6P,,2+8S,,2 transitions of Gd3+ in BaGdF, at 4.2 K. Excitation is at 35962 cm-‘.
1 31000
I 31500
Energy
32000
32500
33000
ticm-‘J ---a
Fig. 2. Emission of the 6P,+8S,,2 transition of Gd3+ in BaGdF, at 290 K. Excitation is at 35962 cm-‘.
crystal field components can be resolved in the roomtemperature emission. Even at low excitation power of the laser, the decay curves of both emissions are non-exponential. When increasing the laser intensity, still tuned at 35962 cm-‘, an anti-Stokes emission is observed around 39450 cm-’ which is shown in fig. 3. This is assigned to the 6D9/2+‘S7/2 transition according to the well-known energy level scheme of Gd3+. The intensity ratio of the 6D9/2+8S7/2 and the 6I7,2+ 8S7,2emission is x 0.01. The intensity of this emission varies linearly with the laser intensity, as shown in fig. 4. The decay curve of the 6D9/2+8S7/2 transition shows a clear build up. Raising the temperature shortens the lifetimes of the emission drastically. Up to 80 K the positions of
the maxima of all emission lines shift to higher energies. Above 80 K these positions remain the same. same. After excitation at room temperature into the 6P7,2 level, anti-Stokes emission is observed from the 617,2-+8S7,2 transition but not from the 6D9,2+ 8S7,2 transition. The intensity ratio of the 6I7,2+ 8S7,2and the 6P7,2+8S7,2 emission is only 1Om3.The intensity of the anti-Stokes 617,2+8S7,2 emission is also linear with the laser power (fig. 4). At 4.2 K this anti-Stokes emission is not observable. Finally, we note that for the diluted system BaLac,99Gd,,0,F, under similar conditions no antiStokes emission was observed. The decay curves are exponential, yielding decay times of 8.5 and 4 ms for the 6P7,2-+8S7,2 and 617,2+8S7,2 transitions, respectively.
4. Discussion The emission lines (fig. 1) have large widths. This is ascribed to excessive inhomogeneous broadening due to the disorder in the crystal lattice. The spectra also show certain subsets of Gd3+ ions. This is especially clear in the 617,2emission in fig. 1. The emission lines have side bands on the low-energy side, the one accompanying the 617,2-‘8S7,2 transition being the weakest. These bands can be ascribed partly to vibronic transitions [ 15 1, but extend down to x 800 cm- ’ below the maximum of the main emission. This is much further than vi343
Volume 193, number 5
38500
CHEMICAL PHYSICS LETTERS
39000
Bnergy
39500
hi’~
40000
35550
5 June 1992
35675
Energy
- -a
35800
km-‘)
35925
36050
---)
Fig. 3. The 6D,,, (left) and 6I,,z (right)+ 8S,,z transitions as anti-Stokes emissions in BaGdF,. The former spectrum is measured at 4.2 K, the latter at room temperature. The excitation energy is indicated in both figures.
0.00 0.2s 0.50 0.7s 1.00 1.2s
Laser
power
0.00 0.25 030
Laser
0.75 1.00 1.25
power
Fig. 4. Power dependence of the anti-Stokes emission intensity observed in BaGdF,. Left: 6D9,2+8S,,2 at 4.2 K, right: 6I,,2+ 8S7,2at room temperature. The laser power is given relative to the maximum laser intensity ( x 10 mJ/pulse).
bronic transitions are expected to extend for a fluoride, but also their intensity is too high for a vibronic transition in a fluoride [ 15 1. Therefore, these side bands contain emission from deep traps, most probably Gd3+ ions near an oxygen ion [ 161. The width of the side bands indicates that there is also a considerable inhomogeneous broadening among these traps. The shift of the positions of the emission maxima with temperature, and the strong temperature dependence of the decay curves, indicate that energy migration occurs over all the three excited levels (6D9,2, 6I,,2 and 6P7,~ ). These processes will not be 344
analyzed quantitatively here. As shown below, their occurrence is a prerequisite for the anti-Stokes emission. After excitation into the 6I,,2 level, emission is observed from the 6D9,2 level and after excitation into the 6P,,2 level emission is observed from the 6I,,2 level. In both cases the energy difference between the emission and the excitation energies is approximately 3700 cm-‘. These anti-Stokes emissions can therefore not be due to thermal population (exp( -hAY/k,T)= lo-*). The build up of the 6D9,2 emission after the laser pulse shows that the second excitation step is not due to a photon from the laser pulse. Combining this with the fact that for the diluted system, BaLao.9PGdo.o,F,, the 6D,,z, 617,2-+8S,,2 transitions were not observed as antiStokes emission, and with the fact that in BaGdF, energy migration occurs over the levels in which excitation took place, we conclude that the multi-photon process involves photon addition by energy transfer [ 17 1. An excited Gd3+ ion transfers its energy to a nearby Gd 3+ ion which is already in an excited state. The latter ion reaches an intermediate state which is higher in energy than the 6D and 61 levels. After relaxation to these levels, radiative relaxation to the ground state takes place. The energy migration is therefore a prerequisite for the occurrence of the anti-Stokes emission. After excitation at x 32000 cm- I the starting level is the 6P,,, level. The energy position of the intermediate state then has to be at x 64000 cm- ‘. After
Volume 193, number 5
CHEMICAL PHYSICS LETTERS
into the 6I,,2 level this position will be a little higher, viz. at = 72000 cm-‘. The nature of the intermediate state is not immediately clear. It seems unlikely that there are 4p states at exactly twice the energy of the +?,,* as well as of the 6P,,2 levels. Moreover, 4p states as the intermediate state are not able to account for the observed linear relation between the anti-Stokes intensity and the laser power. This relation suggests a single rate-determining step. Opposite-parity states for Gd3+ are expected to be at even higher energies. In CaF,: Gd3+ the oppositeparity state (4f65d) has been reported to have its onset at about 77500 cm-’ [ 181. The laser intensity dependence of the anti-Stokes emission intensity suggests that the second step is an allowed optical transition, i.e. from the 6I,,2 or 6P,,2 to one of the lowest levels of the opposite-parity 4f65d configuration. Due to the variation of the nephelauxetic effect and the crystal field strength of the Gd3+ ions, it is proposed that part of the Gd3+ ions have their lowest 4f65d component at low energy, viz. 72000 cm-’ for 6I,,2 excitation and 64000 cm- ’ for 6P,,2 excitation. The latter part is, of course, considerably smaller than the former part, explaining the higher anti-Stokes emission intensity for 61,, z excitation. Energy migration is necessary to reach these Gd3+ ions. At first sight it might be thought that the Gd3+ ions next to oxygen impurities are the centers from which the anti-Stokes emission originates, since they certainly have low-lying 4f65d components. However, in that case the emission should be at a few hundreds of wavenumbers lower energy than expected for the fluoride. This is not the case. Using data for LaF,: Gd3+ [ 191 the 6D9,2 emission of BaGdF, is expected to center round 39500 cm-‘. This is what is observed. For a Gd3+-02- center the emission is expected to be centered below 39000 cm-‘. Actually, a relatively low-lying 4f65d level might be expected for a cluster of Gd3+ ions. Such clusters are known to occur in compositions like BaGdF, [ 12,13 1. In this context we note that the lowest level of the excited 5d configuration of Ce3+ shifts some 3000 cm-’ to lower energy if the Ce3+ concentration in BaF, increases from 0.2 to 1.O mol% [ 201. Finally, we note that Hiilsa has reported a similar anti-Stokes emission for Eu3+ in LaOCl [ 2 11. The rate-determining step in that case is the 7F0-+5D0
excitation
5 June 1992
transition, the second one the allowed ‘D,-+chargetransfer transition. Anti-Stokes emission occuts from the higher sDJ levels. In conclusion, we have observed anti-Stokes emission from Gd3+ ions in BaGdF, by energy-migration-assisted photon addition.
Acknowledgement
We thank Dr. L.H. Brixner for providing us with the crystal and Dr. A. Meijerink for clarifying discussions. The investigations were partly supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Foundation for Advancement of Pure Research (NWO) and the Netherlands Foundation for Technical Research (STW).
References [ I] J. Sytsma, G.F. Imbusch and G. Blasse, J. Chem. Phys. 9 1 (1989) 1456; J. Chem. Phys. 92 (1990) 3249 (E). [ 2 ] J.W.M. Verweij, G.F. Imbusch and G. Blasse, J. Phys. Chem. Solids 50 (1989) 813. [ 31 J. Sytsma, S.J. Kroes, G. Blasse and N.M. Khaidukov, J. Phys. Condensed Matter 3 ( 199 1) 8959. [4] R. Raue, K. Nieuwesteeg and W. Busselt, J. Luminescence 48/49(1991)485. [ 51 M. Dulick, G.E. Faulkner, N.J. Cockroft and D.C. Nguyen, J. Luminescence 48/49 (1991) 517. [6] N. Pelletier-Allard and R. Pelletier, J. Luminescence 48/49 (1991) 867. [7] A.J. Silversmith, W. Lenth and R.M. Macfarlane, Appl. Phys. Letters 51 (1987) 1977. [8] C. Linares, P. Jung, G. Boulon and F. Gaume, J. LessCommon Metals 93 ( 1983) 89; R. Mahiou, B. Jacquier and C. Madej, J. Chem. Phys. 89 (1988) 5931; R. Mahiou, J. Metin and J.C. Cousseins, J. Luminescence 45 (1990) 363. [ 91 E.G. Ippolitov, L.S. Garashina and A.G. Maklachkov, Inorg. Mater. 3 (1967) 59. [lo] W. Bollmann, Phys. Stat. Sol. (a) 18 (1973) 313. [ 111 H.W. den Hartog, P. Dorenbos and W.J. Postma, Phys. Rev. B 34 (1986) 7496. [ 121 K.E.P. Wapenaar, J.L. van Koesveld and J. Schoonman, Solid State Ionics 2 ( 1981) 145. [ 13 ] J. Schoonman, in: Fast ion transport in solids, eds. P. Vashita, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam, 1979).
34.5
Volume 193, number 5
CHEMICAL PHYSICS LETTERS
[ 141J. Sytsma, G.F. lmbusch and G. Blasse, J. Phys. Condensed Matter 2 (1990) 5171. [ 151 J. Sytsma, W. van Schaik and G. Blasse, J. Phys. Chem. Solids 52 (1991) 419. [ 161 L.H. Brixner and G. Blasse, European J. Solid State Inorg. Chem. 27 (1990) 581. [ 171 F. Auzel, Compt. Rend. Acad. Sci. (Paris) 262 (1966) 1016; in Advances in non-radiative processes, ed. B. DiBartolo (Plenum Press, New York, 199 1) p. 135.
346
5 June 1992
[ 18 ] T. Szczurek and M. Schlesinger, in: Rare earth spectroscopy, eds. B. Jezowska-Trezbiatowska, J. Legendziewicz and W. Strek (World Scientific, Singapore, 1985) p. 325. [ 191 W.T. Carnall, H. Crosswhite and H.M. Crosswhite, in: Energy Level Structure and Transition Probabilities of the Trivalent Lanthanides in LaF,, Argonne National Laboratory Report ( 1977). [20] R. Visser, P. Dorenbos, C.W.E. van Eijk, R.W. Hollander and P. Schotanus, IEEE Trans. Nucl. Science 38 ( 1991) 178. [ 2 1 ] J. Holsa, Chem. Phys. Letters 112 ( 1984) 246.