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Vibrational structure in optical spectra of the Ca4GdO(BO3)3 (GdCOB) single crystal doped with Re3+ (Eu, Tb, Yb)
Journal of Molecular Structure 614 (2002) 195–201 www.elsevier.com/locate/molstruc
Vibrational structure in optical spectra of the Ca4GdO(BO3)3 (GdCOB) single crystal doped with Re3þ (Eu, Tb, Yb) G. Dominiak-Dzika,*, W. Ryba-Romanowskia, S. Goła˛ba, M. Babab, A. Paja˛czkowskac a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Oko´lna Street, 50-950 Wroclaw, Poland b Department of Electrical and Electronic Engineering, Iwate University, Morioka 020, Japan c Institute of Electronic Materials Technology, 133 Wo´lczyn´ska Street, 01-919 Warsaw, Poland Received 11 October 2001; revised 21 January 2002; accepted 21 January 2002
Abstract The optical spectra of compounds containing trivalent rare earth (Re3þ) ions often show vibronic features. The intensity of vibronic transitions is determined by the strength of the electron – phonon (ion– lattice) coupling and depends strongly on the rare earth and the host lattice. In this paper we report on electron – phonon coupling in the GdCOB system doped with Eu3þ, Tb3þ and Yb3þ ions. According to general theory intense vibronic lines are to be expected for electronic transitions with DJ ¼ 0; ^ 2 and these transitions were mainly considered in this paper for both Eu3þ and Tb3þ ions. In the case of Yb3þ ions, a strong vibronic sideband belongs to the 2F7/2 ! 2F5/2 transition with DJ ¼ 1 although it seems to be in contradiction to the general selection rule. The vibronic features observed in absorption, excitation and emission spectra were analysed on the basis of the host lattice modes obtained from Raman and the far infrared spectra of the matrix. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Vibronic spectroscopy; Optical spectra; Rare earth ions; Ca4GdO(BO3)3
1. Introduction Optical spectra of rare earth ions are characterised by sharp lines due to intraconfigurational 4f – 4f transitions. Nevertheless, they show also weak features that belong to coupling of the 4fn electrons with the host lattice. This electron –phonon coupling, also called ion – phonon or ion – lattice coupling, manifests itself in several ways: viz. vibronic transitions, line broadening, multiphonon relaxation
and phonon-assisted energy transfer processes. This phenomenon has been a subject of study for several decades not only from a fundamental point of view but also from an applied one. In the application of rare earth ions in, for example, laser materials, fibre amplifies for telecommunication or luminescent materials, electron – phonon coupling play an important role. The oscillator strength Pn of the vibronic transition can be approximated by the expression [1,2]: Pn , nðg þ naR23 Þ2
* Corresponding author. Tel.: þ48-71-343-5021; fax: þ 48-71344-1029. E-mail address: [email protected] (G. Dominiak-Dzik).
ðJð1; 2ÞÞ2 kJllU 2 llJ 0 l2 x
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 4 7 - 8
1 k0llT 1 llpl2 ð1Þ 2J þ 1
196
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Fig. 1. The 7F0 – 5D0 absorption spectrum at 5 K (dotted line in (a)) and excitation spectra of GdCOB:Eu3þ monitoring 5D0 – 7F0 emission of Eu3þ (a) and 5D2 – 7F0 emission of Eu3þ. (b), both at 9 K. For excitation spectra lcollection ¼ 610 nm: The zero-phonon lines are on the left-hand side.
where n is its frequency, n the number of ligands around Ln3þ ion, g and a the charge and the polarisability of the ligand, R is the Ln-ligand distance, J(1,2) describes the first multipolar term of the opposite-parity mixing [3], J and J 0 are the total quantum numbers of the initial and final electronic states. The kJllU2llJ 0 l2 is the reduced matrix element, linking the initial and final electronic states. The reduced matrix element k0llT 1llpl2 describes the transition from the initial l0l to the final lpl vibronic state. The expression (1) combines electronic and vibronic transitions. In this paper, we deal with vibronic transitions observed in optical spectra of Eu3þ, Tb3þ and Yb3þ ions in gadolinium calcium oxoborate Ca4GdO(BO3)3 crystal, which exhibits good nonlinear properties and efficient second harmonic generation (SHG). The intensities of vibronic transitions of considered ions indicate that the strength of the electron – phonon coupling is different for each ion. In order to separate
electronic lines from vibronics, an analysis of the absorption, excitation and emission spectra by using the infrared and Raman data [4] was undertaken.
2. Experimental details The Czochralski method was used to obtain the Ca4GdO(BO3)3 single crystals doped with Re3þ ions. The growth technique was described elsewhere [5,6]. The nominal concentrations of active ions were 4 at.% for Eu3þ, 5 at.% for Tb3þ and 9 at.% for Yb3þ with respect to Gd3þ site. The structural description of the Ca4GdO(BO3)3 crystal is given in Ref. [7]. Optical absorption was recorded by using Cary 2300 spectrophotometer that operates from 0.185 to 3.1 mm. The spectral resolution was 0.2 nm in UV – Vis and 0.8 nm in IR region. The absorption
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197
Fig. 2. Emission bands associated with the 5D0 – 7FJ electronic transitions of Eu3þ in the GdCOB crystal (a). The vibronic part of the 5D0 – 7F2 transition (b). The 7F2 crystal field components are marked on the pictures. The Raman (RS) and infrared (IR) spectra of the Ca4GdO(BO3)3 crystal are presented for comparison. T ¼ 5 K: lexc ¼ 524 nm:
spectra were acquired at 5 K with unpolarized light. Emission was carried out at 5 K. The samples were excited by argon laser or by Surelite Optical Parametric Oscillator (OPO) pumped by a third harmonic of Nd-YAG laser. The bandwidth of the OPO was about 50 cm21 in 700 – 400 nm spectral range. The spectra were analysed with a Zeiss model GDM 1000 grating monochromator (set to a spectral bandwidth of 2 cm21) and detected by a cooled photomultiplier. The SRS 250 boxcar integrator averaged a resulting signal. Excitation spectra were recorded with the apparatus consisting of the 150 W Xenon lamp, two Nikon G 250 grating monochromators and the Daiken UV 202CL closed loop refrigerating system. The measurements were carried out at 9 K monitoring selected visible emission. A continuous flow helium cryostat, Oxford model CF 1204, equipped with a temperature controller was used for low temperature measurements.
3. Results and discussion 3.1. The vibronic features in electronic transitions of Eu3þ In the case of Eu3þ the 7F0 ! 5D0 and 7F0 ! 5D2 absorption transitions are very suitable to study vibronic transitions since the spectral regions on the shorter wavelength-side are completely free from electronic transitions. Additionally, since the 5D0 level is non-degenerate ðJ ¼ 0Þ; the 7F0 ! 5D0 should consist of one electronic line only, at 5 K. This is an ideal situation to observed vibronic lines. The 7F0 ! 5D0 absorption spectrum and excitation spectra of GdCOB:Eu3þ emissions (Fig. 1) as well the 5 D0 ! 7FJ emissions (Fig. 2) have been used to monitor the electron – phonon coupling in the GdCOB:Eu3þ system. A detailed absorption at 5 K and the 7F0 ! 5D0 excitation spectrum monitoring the 5 D0 ! 7F2 emission of GdCOB:Eu3þ at 9K, is shown in Fig. 1(a). The spectral positions of the zero-phonon
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Table 1 Relative positions of the vibronic lines associated with the 7F0 – 5D0 (Eu3þ), 2F7/2 – 2F5/2 (Yb3þ) and the 5D4 – 7F6 (Tb3þ) transitions in GdCOB at 5K Relative position of vibronic lines 3þ
3þ
Eu
Tb
63 100 127 141 199 219 246 286 317 351 374
63 100 126
239 274 315 347 380 461 575
Infrared (IR) and Raman (R) vibrational frequencies of GdCOB [4] 3þ
Yb
102
259 283 306
461
603 642 942
897 935 958
99 (R) 140 (R) 200 (R) 215 (IR) 243 (IR), 245 (R) 285 (IR) 305 (IR) 338 (IR) 382 (IR), 375 (R) 468 (IR), 464 (R) 579 (R) 599 (R), 600 (IR) 636 (IR), 635 (R) 945 (R)
Vibrational frequencies of the lattice are given for comparison. All values are in cm21
lines been have been shifted to 0 cm21 for each presented spectrum. At low temperature the 7 F0 ! 5D0 spectrum consists of one electronic line at 17 290 cm21 (0-phonon position) and the vibronic part stretching up to 17 970 cm21 (up to 700 cm21 in shifted spectrum). The separation of vibronic lines is rather poor. Broad and intense vibronic bands are observed in 200– 450 spectral range and at 600 cm21. Up to 200 cm21 a weak electron – phonon coupling appears in the spectrum. In Table 1 the energies of the coupling phonons are listed and compared with the vibrational frequencies of the Ca4Gd(BO3)3 lattice. The agreement is seen to be good. The 7F0 ! 5D2 region is more complicated to study the vibronic lines because this transition consists of a number of electronic lines due to J ¼ 2 for the final level. The excitation spectrum of europium emission at 9 K, presented in Fig. 1(b), consists of three crystal field components at 21 430, 21 470 and 21 590 cm21 (marked by arrows). A broad vibronic band is located in 380 –530 cm21 energy range from the zero-phonon line and can be assigned to T0 (Ca) translatory lattice modes. Its intensity and separation is very low, so the exact
energetic positions of individual vibronic lines are difficult to determine. Up to 250 cm21 from the zerophonon line, the vibronic frequencies are well camouflaged by electronic transition. Unexpected a weak intensity of this vibronic band indicates a weak ion –lattice coupling which seems to be in contradiction to the general selection rule DJ ¼ 2: It is not possible to test this electronic transition in emission spectrum since the 5D2 states relaxes according to classical quenching cascade: 5D2 ! 5D1 ! 5D0. This process is favoured by lattice phonons of the highest frequency (, 1300 cm21) which are available in this matrix. So, the 5D0 state is the only emitting level in the GdCOB:Eu system. The 5D0 ! 7FJ emission of Eu3þ ions, recorded at 5 K, is shown in Fig. 2(a). The band associated with the 5D0 ! 7F2 transition contributes to the emission spectrum and exhibits vibronic structure. Fig. 2(b) presents this band in detailed, together with the Raman (RS) and infrared (IR) spectra. Spectrum was shifted so the zero-phonon line peaks at 0 cm21. In crystal field with Cs symmetry, the 7F2 multiplet is split into five components. Only strong electronic lines at 15 860, 16 372 and 16 440 cm21 (zero-phonon line)
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199
Fig. 3. Excitation spectrum of GdCOB:Tb3þ monitoring 5D4 – 7F6 emission of Tb3þ at 9 K (a). For excitation spectrum lcollection . 550 nm. The 5 D4 ! 7F6 emission of Tb3þ in GdCOB obtained at 5 K under excitation lexc ¼ 355 nm (b). For both spectra zero-phonon lines are indicated by 0 energy. The Raman (RS–dotted line) and infrared (IR–dash-dotted line) absorptions of the Ca4GdO(BO3)3 crystal are presented for comparison.
have been unambiguously assumed as the crystal field components of the 7F2 level (see Fig. 2). The lattice phonons clearly couple to different Stark levels and weak electronic lines are hard to distinguish from vibronic transitions involved. A comparison with the Raman and infrared spectra identifies peaks at 86 and 537 cm21 from zero-phonon line as electronic transitions with energy 16 354 and 15 903 cm21, respectively. Strong phonon modes in 100 –500 cm21 energy range are nearly resonant to external lattice vibrations and the Ln– O modes (, 150 cm21) of the GdCOB lattice and dominate the sideband structure. The overlap of some vibrational transitions with Eu3þ electronic transitions leads to broadening of spectral feature in the 0 – 150 and 520 –650 cm21 energy range. Unquestionable conclusion comes in respect to intensity of vibronic rare-earth transitions with DJ ¼ 2: the electron – phonon coupling is stronger for the 5D0 – 7F2 vibronic emission than for the
7
F0 – 5D2 vibronic excitation. It should be realised that these two transitions are not equal. Eq. (1) shows that they should be different in electronic part, at least. First, the matrix element kJllU2llJ 0 l2 is different for the two transitions involved 0.0033 for 5D0 – 7F2 [8] and 0.0008 for 7F0 – 5D2 [9]. Second, the value of n is different too: 16 440 cm 21 for 5D 0 – 7F2 and 21 420 cm21 for 7F0 – 5D2 vibronics. Third, the opposite-parity mixing J(1,2) can be different for initial electronic states and significantly affect electron– phonon coupling. 3.2. The vibronic transitions in optical spectra of Tb3þ:GdCOB The GdCOB:Tb system is more complex since the high multiplicity (2J þ 1) for most of terbium multiplets involved, gives rise to a high number of Stark levels. Additionally, in crystal field with Cs symmetry assumed for Ln3þ in Ca4GdO(BO3)3, the degeneracy
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Fig. 4. Vibronic part of the 2F7/2 – 2F5/2 absorption (solid line) and emission (dotted line) spectrum at 5 K. The electronic transitions are marked by arrows.In the inset, absorption spectrum is presented together with the Raman (RS) an dinfrared (IR) spectra.
is lifted and any J level splits completely into Stark components depending on the J-value. Therefore, this is not very suitable situation to observed vibronic lines in optical spectra. For the GdCOB:Tb system, the excitation spectrum of the 5D4 ! 7F6 fluorescence in spectral region of the 7 F6 ! 5D4 absorption (Fig. 3a) and the 5D4 ! 7F6 emission (Fig. 3b) have been considered from the vibronic and electronic features point of view. Because the initial and final levels are degenerate the investigations were carried out at low temperature, since the lowest Stark components are the only ones significantly populated. The 7F6 ! 5D4 excitation spectrum consists of three electronic lines at 20 656 (zero-phonon line), 20 708 and 20 820 cm21 and a vibronic side band spreading up to 800 cm21 from the zero-phonon line. The vibronic lines can be assigned to coupling with IRand the Raman active modes Up to 470 cm21 the vibrational components of the spectrum arise from GdO, Tb-O and external lattice vibrations. The n4(E0 )
bending vibrations of the (BO3)23 group appear at 575 and 642 cm21. The 5D4 ! 7F6 emission was compared with the far infrared (IR) and the Raman (RS) spectra to draw a distinction between vibronic transitions and weak electronic ones. Arrows (Fig. 3b) mark the positions of electronic lines being the Stark components of the 7 F6 ground state. The most of vibronic frequencies agree with those obtained from the vibronic side band of the 7F6 ! 5D4 transition. Extra vibronic lines, observed in 46 – 150 cm21 energy range, have been assigned to Tb– O vibrations whereas, a broad band at , 950 cm21 response to n1(A1) stretching vibrations of the (BO3)23 anions. The energy positions of vibronic lines associated with the 5D4 $ 7F6 emission of Tb3þ in GdCOB are listed in Table 1. It is worth noting that strong n3(E0 ) stretching vibrations of the (BO3)23 groups, observed in IR spectrum at 1216 and 1256 cm21, are not present in low temperature terbium emission spectra.
G. Dominiak-Dzik et al. / Journal of Molecular Structure 614 (2002) 195–201
3.3. The vibronic transitions in optical spectra of Yb3þ:GdCOB The 4f13 configuration of Yb3þ ions has only two multiplets: the 2F7/2 ground state and the 2F5/2 excited state. In Cs symmetry they split into four and three Stark components, respectively. Although the electronic structure of this ion is simple, the optical spectra are rather complex, in connection with vibronic. Fig. 4 presents the 2F7/2 $ 2F5/2 transitions in absorption and emission, at 5 K. Strong vibronic lines are observed in presented optical spectra. Only the 2F7/2 $ 2F5/2 strong lines at 10 246 cm21 can be easily identified as the zero-phonon electronic transition. Remaining electronic transitions, that intensities are low, are hart to distinguish from vibronic transitions involved. A comparison of the infrared and Raman spectra indicates that peaks at 10 260, 10 650 and 11 089 cm21 are the Stark components of the 2F5/ 2 excited level whereas, maxima at 9670, 9743, 9834 and 9898 cm21 present the electronic structure of the 2 F7/2 ground state. The main contribution to the phonon sidebands (up to 10 750 cm21 in absorption spectrum) comes from the coupling of ytterbium electronic states with Gd – O, Yb – O local modes and also with certain external lattice modes. A broad vibrational band centered at 11 180 cm21 is due to the interaction with vibrations of the BO33 anion groups. The overlap of some vibrational transitions with Yb3þ electronic ones leads to broadening of the spectral feature in the 9400– 10 200 cm21 range in emission and at , 10 600 and , 11 100 cm21 in absorption. Relative positions of the vibronic lines accompanying ytterbium optical spectra are given in Table 1. Line broadening of vibronic bands makes frequency analysis less precise.
tron– phonon coupling is observed in optical spectra of both Eu3þ, Tb3þ and Yb3þ ions. In this lattice, the electron – phonon coupling manifests mainly as vibronic sideband and line broadening of transitions. External vibrations of the GdCOB lattice play important role in ion –phonon interactions. According to crystal structure six oxygens coordinate the Gd3þ ions. Two of them have no bond to boron and form the shortest Gd – O bonds. It seems that these two bonds are particularly sensitive to change of surrounding. The substitution of Gd3þ by Eu3þ, Tb3þ and Yb3þ probably changes covalency of these bonds, contributing to enhance of ion – phonon interactions. The vibronic coupling is much stronger in Tb3þ than in Eu3þ. This fact is mainly attributed to the larger opposite-parity mixing J(1,2) because the 4f7 5d1 configuration of Tb3þ ions is located in 210 – 290 nm spectral range (our data).
Acknowledgment The Committee for Scientific Research supported this work under grant no. 8T 11B 007 16.
References [1] [2] [3] [4]
[5] [6] [7]
4. Conclusion Our results on GdCOB:Ln3þ indicate that elec-
201
[8] [9]
B.R. Judd, Phys. Scr. 21 (1980) 543. J. Dexport-Ghys, F. Auzel, J. Chem. Phys. 80 (1984) 4003. B.R. Judd, Phys. Rev. 127 (1962) 750. G. Dominiak-Dzik, W. Ryba-Romanowski, S. Goła˛b, L. Macalik, L. Hanuza, A. Paja˛czkowska, J. Mol. Struct. 555 (2000) 213. A. Aka, A. Kahn-Harari, D. Vivien, J.M. Benitez, F. Salin, J. Godard, Eur. J. Solid State 33 (1996) 727. H. Furuya, M. Yoshimura, T. Kobayashi, K. Murase, Y. Mori, T. Sasaki, J. Crystal Growth 198/199 (1999) 560. A.B. Ilyukhin, F.B. Dzhurinskii, Russ. J. Inorg. Chem. 36 (1993) 847. P. Babu, C.K. Jayasankar, Physica B 279 (2000) 262. W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4450.
Vibrational structure in optical spectra of the Ca4GdO(BO3)3 (GdCOB) single crystal doped with Re3þ (Eu, Tb, Yb) G. Dominiak-Dzika,*, W. Ryba-Romanowskia, S. Goła˛ba, M. Babab, A. Paja˛czkowskac a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Oko´lna Street, 50-950 Wroclaw, Poland b Department of Electrical and Electronic Engineering, Iwate University, Morioka 020, Japan c Institute of Electronic Materials Technology, 133 Wo´lczyn´ska Street, 01-919 Warsaw, Poland Received 11 October 2001; revised 21 January 2002; accepted 21 January 2002
Abstract The optical spectra of compounds containing trivalent rare earth (Re3þ) ions often show vibronic features. The intensity of vibronic transitions is determined by the strength of the electron – phonon (ion– lattice) coupling and depends strongly on the rare earth and the host lattice. In this paper we report on electron – phonon coupling in the GdCOB system doped with Eu3þ, Tb3þ and Yb3þ ions. According to general theory intense vibronic lines are to be expected for electronic transitions with DJ ¼ 0; ^ 2 and these transitions were mainly considered in this paper for both Eu3þ and Tb3þ ions. In the case of Yb3þ ions, a strong vibronic sideband belongs to the 2F7/2 ! 2F5/2 transition with DJ ¼ 1 although it seems to be in contradiction to the general selection rule. The vibronic features observed in absorption, excitation and emission spectra were analysed on the basis of the host lattice modes obtained from Raman and the far infrared spectra of the matrix. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Vibronic spectroscopy; Optical spectra; Rare earth ions; Ca4GdO(BO3)3
1. Introduction Optical spectra of rare earth ions are characterised by sharp lines due to intraconfigurational 4f – 4f transitions. Nevertheless, they show also weak features that belong to coupling of the 4fn electrons with the host lattice. This electron –phonon coupling, also called ion – phonon or ion – lattice coupling, manifests itself in several ways: viz. vibronic transitions, line broadening, multiphonon relaxation
and phonon-assisted energy transfer processes. This phenomenon has been a subject of study for several decades not only from a fundamental point of view but also from an applied one. In the application of rare earth ions in, for example, laser materials, fibre amplifies for telecommunication or luminescent materials, electron – phonon coupling play an important role. The oscillator strength Pn of the vibronic transition can be approximated by the expression [1,2]: Pn , nðg þ naR23 Þ2
* Corresponding author. Tel.: þ48-71-343-5021; fax: þ 48-71344-1029. E-mail address: [email protected] (G. Dominiak-Dzik).
ðJð1; 2ÞÞ2 kJllU 2 llJ 0 l2 x
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 4 7 - 8
1 k0llT 1 llpl2 ð1Þ 2J þ 1
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Fig. 1. The 7F0 – 5D0 absorption spectrum at 5 K (dotted line in (a)) and excitation spectra of GdCOB:Eu3þ monitoring 5D0 – 7F0 emission of Eu3þ (a) and 5D2 – 7F0 emission of Eu3þ. (b), both at 9 K. For excitation spectra lcollection ¼ 610 nm: The zero-phonon lines are on the left-hand side.
where n is its frequency, n the number of ligands around Ln3þ ion, g and a the charge and the polarisability of the ligand, R is the Ln-ligand distance, J(1,2) describes the first multipolar term of the opposite-parity mixing [3], J and J 0 are the total quantum numbers of the initial and final electronic states. The kJllU2llJ 0 l2 is the reduced matrix element, linking the initial and final electronic states. The reduced matrix element k0llT 1llpl2 describes the transition from the initial l0l to the final lpl vibronic state. The expression (1) combines electronic and vibronic transitions. In this paper, we deal with vibronic transitions observed in optical spectra of Eu3þ, Tb3þ and Yb3þ ions in gadolinium calcium oxoborate Ca4GdO(BO3)3 crystal, which exhibits good nonlinear properties and efficient second harmonic generation (SHG). The intensities of vibronic transitions of considered ions indicate that the strength of the electron – phonon coupling is different for each ion. In order to separate
electronic lines from vibronics, an analysis of the absorption, excitation and emission spectra by using the infrared and Raman data [4] was undertaken.
2. Experimental details The Czochralski method was used to obtain the Ca4GdO(BO3)3 single crystals doped with Re3þ ions. The growth technique was described elsewhere [5,6]. The nominal concentrations of active ions were 4 at.% for Eu3þ, 5 at.% for Tb3þ and 9 at.% for Yb3þ with respect to Gd3þ site. The structural description of the Ca4GdO(BO3)3 crystal is given in Ref. [7]. Optical absorption was recorded by using Cary 2300 spectrophotometer that operates from 0.185 to 3.1 mm. The spectral resolution was 0.2 nm in UV – Vis and 0.8 nm in IR region. The absorption
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197
Fig. 2. Emission bands associated with the 5D0 – 7FJ electronic transitions of Eu3þ in the GdCOB crystal (a). The vibronic part of the 5D0 – 7F2 transition (b). The 7F2 crystal field components are marked on the pictures. The Raman (RS) and infrared (IR) spectra of the Ca4GdO(BO3)3 crystal are presented for comparison. T ¼ 5 K: lexc ¼ 524 nm:
spectra were acquired at 5 K with unpolarized light. Emission was carried out at 5 K. The samples were excited by argon laser or by Surelite Optical Parametric Oscillator (OPO) pumped by a third harmonic of Nd-YAG laser. The bandwidth of the OPO was about 50 cm21 in 700 – 400 nm spectral range. The spectra were analysed with a Zeiss model GDM 1000 grating monochromator (set to a spectral bandwidth of 2 cm21) and detected by a cooled photomultiplier. The SRS 250 boxcar integrator averaged a resulting signal. Excitation spectra were recorded with the apparatus consisting of the 150 W Xenon lamp, two Nikon G 250 grating monochromators and the Daiken UV 202CL closed loop refrigerating system. The measurements were carried out at 9 K monitoring selected visible emission. A continuous flow helium cryostat, Oxford model CF 1204, equipped with a temperature controller was used for low temperature measurements.
3. Results and discussion 3.1. The vibronic features in electronic transitions of Eu3þ In the case of Eu3þ the 7F0 ! 5D0 and 7F0 ! 5D2 absorption transitions are very suitable to study vibronic transitions since the spectral regions on the shorter wavelength-side are completely free from electronic transitions. Additionally, since the 5D0 level is non-degenerate ðJ ¼ 0Þ; the 7F0 ! 5D0 should consist of one electronic line only, at 5 K. This is an ideal situation to observed vibronic lines. The 7F0 ! 5D0 absorption spectrum and excitation spectra of GdCOB:Eu3þ emissions (Fig. 1) as well the 5 D0 ! 7FJ emissions (Fig. 2) have been used to monitor the electron – phonon coupling in the GdCOB:Eu3þ system. A detailed absorption at 5 K and the 7F0 ! 5D0 excitation spectrum monitoring the 5 D0 ! 7F2 emission of GdCOB:Eu3þ at 9K, is shown in Fig. 1(a). The spectral positions of the zero-phonon
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Table 1 Relative positions of the vibronic lines associated with the 7F0 – 5D0 (Eu3þ), 2F7/2 – 2F5/2 (Yb3þ) and the 5D4 – 7F6 (Tb3þ) transitions in GdCOB at 5K Relative position of vibronic lines 3þ
3þ
Eu
Tb
63 100 127 141 199 219 246 286 317 351 374
63 100 126
239 274 315 347 380 461 575
Infrared (IR) and Raman (R) vibrational frequencies of GdCOB [4] 3þ
Yb
102
259 283 306
461
603 642 942
897 935 958
99 (R) 140 (R) 200 (R) 215 (IR) 243 (IR), 245 (R) 285 (IR) 305 (IR) 338 (IR) 382 (IR), 375 (R) 468 (IR), 464 (R) 579 (R) 599 (R), 600 (IR) 636 (IR), 635 (R) 945 (R)
Vibrational frequencies of the lattice are given for comparison. All values are in cm21
lines been have been shifted to 0 cm21 for each presented spectrum. At low temperature the 7 F0 ! 5D0 spectrum consists of one electronic line at 17 290 cm21 (0-phonon position) and the vibronic part stretching up to 17 970 cm21 (up to 700 cm21 in shifted spectrum). The separation of vibronic lines is rather poor. Broad and intense vibronic bands are observed in 200– 450 spectral range and at 600 cm21. Up to 200 cm21 a weak electron – phonon coupling appears in the spectrum. In Table 1 the energies of the coupling phonons are listed and compared with the vibrational frequencies of the Ca4Gd(BO3)3 lattice. The agreement is seen to be good. The 7F0 ! 5D2 region is more complicated to study the vibronic lines because this transition consists of a number of electronic lines due to J ¼ 2 for the final level. The excitation spectrum of europium emission at 9 K, presented in Fig. 1(b), consists of three crystal field components at 21 430, 21 470 and 21 590 cm21 (marked by arrows). A broad vibronic band is located in 380 –530 cm21 energy range from the zero-phonon line and can be assigned to T0 (Ca) translatory lattice modes. Its intensity and separation is very low, so the exact
energetic positions of individual vibronic lines are difficult to determine. Up to 250 cm21 from the zerophonon line, the vibronic frequencies are well camouflaged by electronic transition. Unexpected a weak intensity of this vibronic band indicates a weak ion –lattice coupling which seems to be in contradiction to the general selection rule DJ ¼ 2: It is not possible to test this electronic transition in emission spectrum since the 5D2 states relaxes according to classical quenching cascade: 5D2 ! 5D1 ! 5D0. This process is favoured by lattice phonons of the highest frequency (, 1300 cm21) which are available in this matrix. So, the 5D0 state is the only emitting level in the GdCOB:Eu system. The 5D0 ! 7FJ emission of Eu3þ ions, recorded at 5 K, is shown in Fig. 2(a). The band associated with the 5D0 ! 7F2 transition contributes to the emission spectrum and exhibits vibronic structure. Fig. 2(b) presents this band in detailed, together with the Raman (RS) and infrared (IR) spectra. Spectrum was shifted so the zero-phonon line peaks at 0 cm21. In crystal field with Cs symmetry, the 7F2 multiplet is split into five components. Only strong electronic lines at 15 860, 16 372 and 16 440 cm21 (zero-phonon line)
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Fig. 3. Excitation spectrum of GdCOB:Tb3þ monitoring 5D4 – 7F6 emission of Tb3þ at 9 K (a). For excitation spectrum lcollection . 550 nm. The 5 D4 ! 7F6 emission of Tb3þ in GdCOB obtained at 5 K under excitation lexc ¼ 355 nm (b). For both spectra zero-phonon lines are indicated by 0 energy. The Raman (RS–dotted line) and infrared (IR–dash-dotted line) absorptions of the Ca4GdO(BO3)3 crystal are presented for comparison.
have been unambiguously assumed as the crystal field components of the 7F2 level (see Fig. 2). The lattice phonons clearly couple to different Stark levels and weak electronic lines are hard to distinguish from vibronic transitions involved. A comparison with the Raman and infrared spectra identifies peaks at 86 and 537 cm21 from zero-phonon line as electronic transitions with energy 16 354 and 15 903 cm21, respectively. Strong phonon modes in 100 –500 cm21 energy range are nearly resonant to external lattice vibrations and the Ln– O modes (, 150 cm21) of the GdCOB lattice and dominate the sideband structure. The overlap of some vibrational transitions with Eu3þ electronic transitions leads to broadening of spectral feature in the 0 – 150 and 520 –650 cm21 energy range. Unquestionable conclusion comes in respect to intensity of vibronic rare-earth transitions with DJ ¼ 2: the electron – phonon coupling is stronger for the 5D0 – 7F2 vibronic emission than for the
7
F0 – 5D2 vibronic excitation. It should be realised that these two transitions are not equal. Eq. (1) shows that they should be different in electronic part, at least. First, the matrix element kJllU2llJ 0 l2 is different for the two transitions involved 0.0033 for 5D0 – 7F2 [8] and 0.0008 for 7F0 – 5D2 [9]. Second, the value of n is different too: 16 440 cm 21 for 5D 0 – 7F2 and 21 420 cm21 for 7F0 – 5D2 vibronics. Third, the opposite-parity mixing J(1,2) can be different for initial electronic states and significantly affect electron– phonon coupling. 3.2. The vibronic transitions in optical spectra of Tb3þ:GdCOB The GdCOB:Tb system is more complex since the high multiplicity (2J þ 1) for most of terbium multiplets involved, gives rise to a high number of Stark levels. Additionally, in crystal field with Cs symmetry assumed for Ln3þ in Ca4GdO(BO3)3, the degeneracy
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Fig. 4. Vibronic part of the 2F7/2 – 2F5/2 absorption (solid line) and emission (dotted line) spectrum at 5 K. The electronic transitions are marked by arrows.In the inset, absorption spectrum is presented together with the Raman (RS) an dinfrared (IR) spectra.
is lifted and any J level splits completely into Stark components depending on the J-value. Therefore, this is not very suitable situation to observed vibronic lines in optical spectra. For the GdCOB:Tb system, the excitation spectrum of the 5D4 ! 7F6 fluorescence in spectral region of the 7 F6 ! 5D4 absorption (Fig. 3a) and the 5D4 ! 7F6 emission (Fig. 3b) have been considered from the vibronic and electronic features point of view. Because the initial and final levels are degenerate the investigations were carried out at low temperature, since the lowest Stark components are the only ones significantly populated. The 7F6 ! 5D4 excitation spectrum consists of three electronic lines at 20 656 (zero-phonon line), 20 708 and 20 820 cm21 and a vibronic side band spreading up to 800 cm21 from the zero-phonon line. The vibronic lines can be assigned to coupling with IRand the Raman active modes Up to 470 cm21 the vibrational components of the spectrum arise from GdO, Tb-O and external lattice vibrations. The n4(E0 )
bending vibrations of the (BO3)23 group appear at 575 and 642 cm21. The 5D4 ! 7F6 emission was compared with the far infrared (IR) and the Raman (RS) spectra to draw a distinction between vibronic transitions and weak electronic ones. Arrows (Fig. 3b) mark the positions of electronic lines being the Stark components of the 7 F6 ground state. The most of vibronic frequencies agree with those obtained from the vibronic side band of the 7F6 ! 5D4 transition. Extra vibronic lines, observed in 46 – 150 cm21 energy range, have been assigned to Tb– O vibrations whereas, a broad band at , 950 cm21 response to n1(A1) stretching vibrations of the (BO3)23 anions. The energy positions of vibronic lines associated with the 5D4 $ 7F6 emission of Tb3þ in GdCOB are listed in Table 1. It is worth noting that strong n3(E0 ) stretching vibrations of the (BO3)23 groups, observed in IR spectrum at 1216 and 1256 cm21, are not present in low temperature terbium emission spectra.
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3.3. The vibronic transitions in optical spectra of Yb3þ:GdCOB The 4f13 configuration of Yb3þ ions has only two multiplets: the 2F7/2 ground state and the 2F5/2 excited state. In Cs symmetry they split into four and three Stark components, respectively. Although the electronic structure of this ion is simple, the optical spectra are rather complex, in connection with vibronic. Fig. 4 presents the 2F7/2 $ 2F5/2 transitions in absorption and emission, at 5 K. Strong vibronic lines are observed in presented optical spectra. Only the 2F7/2 $ 2F5/2 strong lines at 10 246 cm21 can be easily identified as the zero-phonon electronic transition. Remaining electronic transitions, that intensities are low, are hart to distinguish from vibronic transitions involved. A comparison of the infrared and Raman spectra indicates that peaks at 10 260, 10 650 and 11 089 cm21 are the Stark components of the 2F5/ 2 excited level whereas, maxima at 9670, 9743, 9834 and 9898 cm21 present the electronic structure of the 2 F7/2 ground state. The main contribution to the phonon sidebands (up to 10 750 cm21 in absorption spectrum) comes from the coupling of ytterbium electronic states with Gd – O, Yb – O local modes and also with certain external lattice modes. A broad vibrational band centered at 11 180 cm21 is due to the interaction with vibrations of the BO33 anion groups. The overlap of some vibrational transitions with Yb3þ electronic ones leads to broadening of the spectral feature in the 9400– 10 200 cm21 range in emission and at , 10 600 and , 11 100 cm21 in absorption. Relative positions of the vibronic lines accompanying ytterbium optical spectra are given in Table 1. Line broadening of vibronic bands makes frequency analysis less precise.
tron– phonon coupling is observed in optical spectra of both Eu3þ, Tb3þ and Yb3þ ions. In this lattice, the electron – phonon coupling manifests mainly as vibronic sideband and line broadening of transitions. External vibrations of the GdCOB lattice play important role in ion –phonon interactions. According to crystal structure six oxygens coordinate the Gd3þ ions. Two of them have no bond to boron and form the shortest Gd – O bonds. It seems that these two bonds are particularly sensitive to change of surrounding. The substitution of Gd3þ by Eu3þ, Tb3þ and Yb3þ probably changes covalency of these bonds, contributing to enhance of ion – phonon interactions. The vibronic coupling is much stronger in Tb3þ than in Eu3þ. This fact is mainly attributed to the larger opposite-parity mixing J(1,2) because the 4f7 5d1 configuration of Tb3þ ions is located in 210 – 290 nm spectral range (our data).
Acknowledgment The Committee for Scientific Research supported this work under grant no. 8T 11B 007 16.
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[5] [6] [7]
4. Conclusion Our results on GdCOB:Ln3þ indicate that elec-
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[8] [9]
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