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Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr3+ and Ho3+ ions
Optical Materials 34 (2012) 803–806
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr3+ and Ho3+ ions L. Alyabyeva a, V. Burkov a, O. Lysenko a,⇑, B. Mill b a b
Moscow Institute of Physics and Technology, 9 Institutskii per., Dolgoprudny, Moscow Oblast 141701, Russia Moscow State University, 1 Leninskie Gory, Moscow 119991, Russia
a r t i c l e
i n f o
Article history: Received 14 June 2011 Received in revised form 8 November 2011 Accepted 8 November 2011 Available online 9 December 2011 Keywords: Circular-dichroism Crystal LaBGeO5
a b s t r a c t Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr3+ and Ho3+ ions were investigated at 8 K in the range 300–900 nm. Electronic transitions of these ions, which substitute La3+ ions in positions with a local symmetry C1 are observed in the spectra. All transitions are active in both absorption and circular-dichroism spectra. Values of the dipole strengths Dom, rotation strengths Rom and anisotropy factors g are calculated for some well-separated bands. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Circular-dichroism (CD) spectra induced by a crystal field give information regarding parameters of electronic transitions of doped atoms in gyrotropic crystals [1,2]. In papers published by Prof. Frederick Richardson (short review presented in [3]) CD spectra of Na3[La(OCOCH2OCH2OCO)3] 2NaClO4 6H2O crystals (space group R32) doped with rare-earth ions were investigated. The doped lanthanide ions substitute La atoms and occupy positions with a local symmetry D3. Circular-dichroism spectra of double borate crystals (space group R32) doped with europium and neodymium were investigated in [4,5]. In these crystals rare-earth ions substitute yttrium ions and occupy positions in the lattice with a local symmetry C3 [6]. In axial absorption and CD spectra of the crystals mentioned above only transitions to the E states are active. In LaBGeO5 crystal lanthanum ions are situated in the center of the polyhedron with the coordination number 9 and occupy general positions in the lattice with the local symmetry C1 [6,7]. In this case in the axial absorption and CD spectra of the crystals doped with rare-earth ions, which are integrated in La positions the transitions to all Stark components can be active. In the present paper CD method was applied for investigation of gyrotropy of the doped Pr3+ and Ho3+ ions in the lattice of LaBGeO5 crystal. The absorption and luminescence of LaBGeO5:Pr3+ crystal were investigated in [8] for the purpose of studying of its potential use as a laser material.
LaBGeO5 crystals with a structure of stillwellite CeBSiO5 (space group P31, Z = 3) and doped with Pr3+ and Ho3+ ions were grown by the Czochralski method. For measurements, z-cuts of crystal with the thickness 1.5 mm were used. A concentration of rare-earth ions is 1.43 1020 cm–3 for praseodymium and 1.02 1020 cm–3 for holmium. The absorption spectra were recorded in the range 300– 900 nm on a Hitachi-330 spectrophotometer and circular-dichroism spectra were measured on a Mark-3S (Jobin–Yvon) dichrograph. A cryostat ‘‘CTI Cryogenics’’ was used for cooling of the samples.
⇑ Corresponding author. E-mail address: [email protected] (O. Lysenko). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.11.009
3. Results and discussion The absorption and CD spectra of LaBGeO5 crystals doped with Pr3+ and Ho3+ at 8 K are shown on Figs. 1 and 2. The bands in spectra are slightly spread due to an effect of the apparatus function of the used equipment. However this spread does not affect the characteristic parameters of the bands, which are investigated in our work. All transitions of the doped ions are active in CD spectra and appear as both positive and negative bands. The crystal cooling down to 8 K allowed freeze the high-energy Stark components of the ground level. That is why the main contribution in absorption and circular-dichroism spectra is given by transitions from the lowest Stark component of the ground level. The Stark decomposition of the bands at 8 K is clearly shown. It allowed calculate characteristic parameters of the bands – the dipole strengths and rotation strengths for transitions from the lowest Stark component of the ground level to other Stark components of excited levels.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
Fig. 1. Absorption and CD spectra of LaBGeO5:Pr3+ crystal in the range of transitions: 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2.
Fig. 2. Absorption and CD spectra of LaBGeO5:Ho3+ crystal in the range of transitions: 5I8 ? 5F1, 5I8 ? 5G6, 5I8 ? 3K8, 5I8 ? 5F2, 5I8 ? 5F4, 5I8 ? 5S2 and 5I8 ? 5F5.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
Table 1 Wavelengths, frequencies, values of the dipole strengths Dom, rotation strengths Rom and anisotropy factors g of the investigated transitions of Pr3+ and Ho3+ ions between the lowest Stark components of ground state and Stark components of the excited states, n is the number of observed band within a single transition. Ion Pr
3+
Transition
n
k (nm)
m (cm1)
Dom 1044 (D2)
Rom 1044, D lB
g 104
3
3
434.5 435.5 468.5 469.5 470.5 479 572 579 582 593 596 444 445 445.5 446.5 448 460.7 463.6 464.5 467 474.5 475.5 479 527.7 530 531 533 535 630 635 636.5 637 638
23,015 22,962 21,345 21,299 21,254 20,877 17,483 17,271 17,182 16,863 16,779 22,523 22,472 22,447 22,396 22,321 21,706 21,570 21,529 21,413 21,075 21,030 20,877 18,950 18,868 18,832 18,761 18,692 15,873 15,748 15,711 15,699 15,674
1642 650 390 3400 2806 3610 473 1086 934 1436 1034 1974 570 400 537 484 682 – – – – – – 730 740 590 – – 698 1090 105 1130 250
93
3
1 2 1 2 3 1 1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2 3 1 2 3 4 5 1 2 3 4 5
5.2
3
7.3
45
31 15.8 0.75 0.81 0.5 – 29 – – – 5.6 14.6 11 5.6 53 1.5 2.5 1.9 13.9 5.5
352 1369 28 35 14
H4 ? P2 DJ = 2 H4 ? P1 = 3
3
H4 ? 3P0 DJ = 4 H4 ? 1D2 = 2
3
Ho3+
5
I8 ? 5F1 DJ = 7
5
I8 ? 5K8 DJ = 0
5
I8 ? 5F2 DJ = 6
5
I8 ? 5F4 DJ = 4
5
I8 ? 5F5 DJ = 3
3.4 3.4 7.5 4.3 2.5 9.8
602
474 877
780 170
440 162 976 291
Note: D – Debye (1018 CGS units), lB – Bohr magneton (1020 CGS units).
LaBGeO5 crystal is a ferroelectric with phase transition from low-temperature polar phase P31 to high-temperature phase P3121 at 530 °C [9]. In [10] one showed the possibility of monodomainization and rotation of polarization in the domain structure LaBGeO5 under rather high pressure. In case of LaBGeO5:Pr3+ crystal the measurements were performed with a sample without monodomainization (after growth) as well as the same sample after monodomainization. The results of circular-dichroism measurements coincided with a precision of 0.5%. Thus, these data are consistent with the conclusions obtained in [11], according to them the rotation of polarization of ferroelectric crystals with phase transition 32 ? 3 should not change the sign of gyrotropic characteristics of the crystal. Thus, measurements of CD spectra of the doped LaBGeO5 crystals do not depend on the sign of the ferroelectric domains. 3.1. Absorption and CD spectra of LaBGeO5:Pr3+ Some intensive bands of the Pr3+ ion are presented in the infrared range of spectrum (1000–5000 nm) [12], where CD measurements are complicated. In the range 420–600 nm at T = 8 K, several bands are presented as well, their correspondence to the relevant transitions is indicated in Table 1 and Fig. 1. The ground state of praseodymium 3H4 in the crystal field of symmetry C1 splits into 9 Stark sublevels and the excited levels 3P2, 1I6,1 3P1, 3 P0 and 1D2 in the range 420–600 nm split into 5, 13, 3, 1 and 5 sublevels respectively [13]. The bands selected for determination of absorption and chiral characteristics are a manifestation of transi1
3
1
The values of Dom and Rom for transition H4 ? I6 were not calculated because this transition is forbidden by spin, has low intensity and split into 13 bands that are not resolved in the spectra obtained in our devices.
tions 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2 (Fig. 1, Table 1) [8,13]. 3.2. Absorption and CD spectra of LaBGeO5:Ho3+ The ground state of holmium 5I8 in the crystal field of symmetry C1 splits into 17 Stark sublevels and the excited states 5F5, 5S2, 5F4, 5 F2, 5K8, 5G6, 5F1, which give the bands in the range 400–650 nm split into 11, 5, 9, 5, 17, 13 and 3 sublevels, respectively [12]. The bands observed in absorption and CD spectra of LaBGeO5:Ho3+ crystal are a manifestation of transitions 5I8 ? 5F1, 5I8 ? 5G6, 5I8 ? 3K8, 5 I8 ? 5F2, 5I8 ? 5F4, 5I8 ? 5S2 and 5I8 ? 5F5 (Fig. 2, Table 1) [12,13]. These transitions in the spectra were chosen for analysis of absorption and chiral characteristics. Absorption and CD spectra allowed calculate the values of dipole strengths Dom, rotation strengths Rom and anisotropy factors g, which are commonly used in analysis of spectroscopic characteristics of optically active compounds [14]:
Dom ¼
Rom ¼
g¼
3ð2303Þhcn
Z
2
N4p2 kmax b
3ð2303Þhc N16p2 kmax b
4n Rom b Dom
Z
edk
ð1Þ
Dedk
ð2Þ
ð3Þ
Here, e is the decimal molar extinction coefficient, De is the decimal molar dichroism coefficient, n is the refractive index, b = (n2 + 2)/3 is the Lorentz factor. The calculated values of Dom, Rom, and g are given in Table 1.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
3.3. Discussion In absorption spectra of rare-earth ions f–f transitions are parity-forbidden in the electric-dipole approximation [13]. Crystal field effect in the case of symmetry C1 leads to splitting of the levels as well as mixing of the f–f states, so all transitions become allowed by symmetry in the magnetic-dipole and electric-dipole approximations. An effect of odd components of the crystal field leads to mixing of f- and d- states and the intensity borrowing from the allowed 4f–4f 5d transitions. The borrowed component of the electric dipole defines the intensity of the bands in absorption. The intensity of the bands in circular-dichroism spectra depends on the borrowed moment as well as the relative orientation of the electric and magnetic moments. Transitions 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2 of the free Pr3+ ion are allowed in the electric-quadrupole approximation, transition 3H4 ? 3P1 is forbidden in this approximation, and all transitions are forbidden in the electric-dipole and magnetic-dipole approximations [12]. In absorption spectra of LaBGeO5:Pr3+ crystal, intensity of allowed transitions 3H4 ? 3P2 and 3H4 ? 3P0 almost coincides with the intensity of the forbidden transition 3H4 ? 3P1, another allowed transition 3H4 ? 1D2 has a dramatically less value of the dipole strength. Values of the rotation strength for the investigated transitions are changed in a smaller scale (Table 1). For some transitions, which differ in intensity by one order in the absorption spectra, the rotation strengths have approximately the same values. Transitions 5I8 ? 5G6, 5I8 ? 5F2, 5I8 ? 5F4 and 5I8 ? 5S2 of the free Ho3+ ion are allowed in the electric-quadrupole approximation, transition 5I8 ? 3K8 is allowed in the magnetic-dipole approximation, transitions 5I8 ? 5F1 and 5I8 ? 5F5 are forbidden in these approximations. In absorption spectra of LaBGeO5:Ho3+ all transitions have approximately the same intensity, except transition 5 I8 ? 5S2. This quadrupole transition has a much lower intensity. One can see from the table that the dipole strengths of transitions of the Ho3+ ion are generally less than the dipole strengths of transitions of the Pr3+ ion. At the same time the rotation strengths of transitions of these ions have quite comparable values. This is also confirmed by anisotropy factors relationship. For LaBGeO5:Pr3+ crystal, values of the anisotropy factors of transitions 3 H4 ? 3P2 and 3H4 ? 3P0 allowed in the electric-quadrupole approximation are almost equal to that of transition 3H4 ? 3P1, which is forbidden in this approximation. On the other side, in spectra of LaBGeO5:Ho3+ crystal, anisotropy of transition 5 I8 ? 5F2 allowed in quadrupole approximation dramatically less than that of the forbidden transition 5I8 ? 5F5. In general, the value of anisotropy factor weakly depends on the contribution of electric-quadrupole mode in the intensity of the bands in the absorption and circular-dichroism spectra, and is determined by
the values of the electric-dipole moment, magnetic-dipole moment, and their mutual orientation. The selection rules for changing the projection of angular momentum DMJ [15] is determined by specific symmetry of the object. Since the local symmetry of the doped ions in our crystals is C1, all the transitions between the Stark components of ground and excited states are allowed by symmetry. The results show that values of Dom, Rom and g for transitions to different Stark components within a multiplet differ dramatically – more than one order of magnitude. 4. Conclusion According to the theory of optical activity, electronic transition is active in the circular-dichroism spectrum, if it is allowed in the electric-dipole and magnetic-dipole approximations. A mutual orientation of the electric-dipole moment and magnetic-dipole moment does not depend on the local symmetry of the crystal field, and plays an important role in the induction of chiral gyrotropic crystal field on the electronic transitions of the doped ion. In the case of rare-earth ions f–f transitions in the crystal field of symmetry C1 split and mix. In the framework of symmetry C1, all transitions are allowed in the magnetic-dipole approximation. In turn, an appearance of the electric-dipole moment is due to the existence of an effective mechanism of intensity borrowing from the parity-allowed transitions in d-states. The mechanism of this borrowing and its source depend on the symmetry of the crystal lattice, the symmetry of the doped ion position, and the intensity of vibronic interactions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
V.A. Kizel, V.I. Burkov, Gyrotropy of Crystals, Nauka, 1980. V.I. Burkov, Inorg. Mater. 30 (1994) 12. F.S. Richardson, J. Less-Common Met. 149 (1989) 151. V.I. Burkov, V.A. Kizel, N.I. Leonuk, N.M. Sitnikov, Cryst. Rep. 27 (1982) 196. V.I. Burkov, V.A. Kizel, N.I. Leonuk, N.M. Sitnikov, Cryst. Rep. 29 (1984) 101. V.P. Samygina, E.A. Genkina, B.A. Maximov, N.I. Leonuk, Cryst. Rep. 38 (1993) 61. A.A. Kaminskii, B.V. Mill, E.L. Belokoneva, A.B. Butashin, Proc. USSR Acad. Sci., Ser. Inorg. Mater. 26 (1990) 1105. A.A. Kaminskii, A.B. Butashin, B.V. Mill, V.D. Shigorin, Proc. USSR Acad. Sci. Ser. Inorg. Mater. 28 (1992) 150. S.U. Stefanovich, B.V. Mill, A.B. Butashin, Cryst. Rep. 37 (1992) 965. E.V. Milov, B.A. Strukov, V.N. Milov, Ferroelectrics 269 (2002) 15. L.A. Shuvalov, N.R. Ivanov, Cryst. Rep. 9 (1964) 363. P. Görlich, H. Karras, G. Kötitz, R. Lehmann, Phys. Status Solidi 5 (1964) 437– 461 (6 (1964) 277–318; 8 (1965) 385–429). B. Wybourne, Spectroscopic Properties of Rare Earths, John Wiley & Sons, Inc., New York, 1965. J.A. Schellman, Chem. Rev. 75 (1975) 323. F.S. Richardson, Inorg. Chem. 19 (1980) 2806.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr3+ and Ho3+ ions L. Alyabyeva a, V. Burkov a, O. Lysenko a,⇑, B. Mill b a b
Moscow Institute of Physics and Technology, 9 Institutskii per., Dolgoprudny, Moscow Oblast 141701, Russia Moscow State University, 1 Leninskie Gory, Moscow 119991, Russia
a r t i c l e
i n f o
Article history: Received 14 June 2011 Received in revised form 8 November 2011 Accepted 8 November 2011 Available online 9 December 2011 Keywords: Circular-dichroism Crystal LaBGeO5
a b s t r a c t Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr3+ and Ho3+ ions were investigated at 8 K in the range 300–900 nm. Electronic transitions of these ions, which substitute La3+ ions in positions with a local symmetry C1 are observed in the spectra. All transitions are active in both absorption and circular-dichroism spectra. Values of the dipole strengths Dom, rotation strengths Rom and anisotropy factors g are calculated for some well-separated bands. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Circular-dichroism (CD) spectra induced by a crystal field give information regarding parameters of electronic transitions of doped atoms in gyrotropic crystals [1,2]. In papers published by Prof. Frederick Richardson (short review presented in [3]) CD spectra of Na3[La(OCOCH2OCH2OCO)3] 2NaClO4 6H2O crystals (space group R32) doped with rare-earth ions were investigated. The doped lanthanide ions substitute La atoms and occupy positions with a local symmetry D3. Circular-dichroism spectra of double borate crystals (space group R32) doped with europium and neodymium were investigated in [4,5]. In these crystals rare-earth ions substitute yttrium ions and occupy positions in the lattice with a local symmetry C3 [6]. In axial absorption and CD spectra of the crystals mentioned above only transitions to the E states are active. In LaBGeO5 crystal lanthanum ions are situated in the center of the polyhedron with the coordination number 9 and occupy general positions in the lattice with the local symmetry C1 [6,7]. In this case in the axial absorption and CD spectra of the crystals doped with rare-earth ions, which are integrated in La positions the transitions to all Stark components can be active. In the present paper CD method was applied for investigation of gyrotropy of the doped Pr3+ and Ho3+ ions in the lattice of LaBGeO5 crystal. The absorption and luminescence of LaBGeO5:Pr3+ crystal were investigated in [8] for the purpose of studying of its potential use as a laser material.
LaBGeO5 crystals with a structure of stillwellite CeBSiO5 (space group P31, Z = 3) and doped with Pr3+ and Ho3+ ions were grown by the Czochralski method. For measurements, z-cuts of crystal with the thickness 1.5 mm were used. A concentration of rare-earth ions is 1.43 1020 cm–3 for praseodymium and 1.02 1020 cm–3 for holmium. The absorption spectra were recorded in the range 300– 900 nm on a Hitachi-330 spectrophotometer and circular-dichroism spectra were measured on a Mark-3S (Jobin–Yvon) dichrograph. A cryostat ‘‘CTI Cryogenics’’ was used for cooling of the samples.
⇑ Corresponding author. E-mail address: [email protected] (O. Lysenko). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.11.009
3. Results and discussion The absorption and CD spectra of LaBGeO5 crystals doped with Pr3+ and Ho3+ at 8 K are shown on Figs. 1 and 2. The bands in spectra are slightly spread due to an effect of the apparatus function of the used equipment. However this spread does not affect the characteristic parameters of the bands, which are investigated in our work. All transitions of the doped ions are active in CD spectra and appear as both positive and negative bands. The crystal cooling down to 8 K allowed freeze the high-energy Stark components of the ground level. That is why the main contribution in absorption and circular-dichroism spectra is given by transitions from the lowest Stark component of the ground level. The Stark decomposition of the bands at 8 K is clearly shown. It allowed calculate characteristic parameters of the bands – the dipole strengths and rotation strengths for transitions from the lowest Stark component of the ground level to other Stark components of excited levels.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
Fig. 1. Absorption and CD spectra of LaBGeO5:Pr3+ crystal in the range of transitions: 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2.
Fig. 2. Absorption and CD spectra of LaBGeO5:Ho3+ crystal in the range of transitions: 5I8 ? 5F1, 5I8 ? 5G6, 5I8 ? 3K8, 5I8 ? 5F2, 5I8 ? 5F4, 5I8 ? 5S2 and 5I8 ? 5F5.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
Table 1 Wavelengths, frequencies, values of the dipole strengths Dom, rotation strengths Rom and anisotropy factors g of the investigated transitions of Pr3+ and Ho3+ ions between the lowest Stark components of ground state and Stark components of the excited states, n is the number of observed band within a single transition. Ion Pr
3+
Transition
n
k (nm)
m (cm1)
Dom 1044 (D2)
Rom 1044, D lB
g 104
3
3
434.5 435.5 468.5 469.5 470.5 479 572 579 582 593 596 444 445 445.5 446.5 448 460.7 463.6 464.5 467 474.5 475.5 479 527.7 530 531 533 535 630 635 636.5 637 638
23,015 22,962 21,345 21,299 21,254 20,877 17,483 17,271 17,182 16,863 16,779 22,523 22,472 22,447 22,396 22,321 21,706 21,570 21,529 21,413 21,075 21,030 20,877 18,950 18,868 18,832 18,761 18,692 15,873 15,748 15,711 15,699 15,674
1642 650 390 3400 2806 3610 473 1086 934 1436 1034 1974 570 400 537 484 682 – – – – – – 730 740 590 – – 698 1090 105 1130 250
93
3
1 2 1 2 3 1 1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2 3 1 2 3 4 5 1 2 3 4 5
5.2
3
7.3
45
31 15.8 0.75 0.81 0.5 – 29 – – – 5.6 14.6 11 5.6 53 1.5 2.5 1.9 13.9 5.5
352 1369 28 35 14
H4 ? P2 DJ = 2 H4 ? P1 = 3
3
H4 ? 3P0 DJ = 4 H4 ? 1D2 = 2
3
Ho3+
5
I8 ? 5F1 DJ = 7
5
I8 ? 5K8 DJ = 0
5
I8 ? 5F2 DJ = 6
5
I8 ? 5F4 DJ = 4
5
I8 ? 5F5 DJ = 3
3.4 3.4 7.5 4.3 2.5 9.8
602
474 877
780 170
440 162 976 291
Note: D – Debye (1018 CGS units), lB – Bohr magneton (1020 CGS units).
LaBGeO5 crystal is a ferroelectric with phase transition from low-temperature polar phase P31 to high-temperature phase P3121 at 530 °C [9]. In [10] one showed the possibility of monodomainization and rotation of polarization in the domain structure LaBGeO5 under rather high pressure. In case of LaBGeO5:Pr3+ crystal the measurements were performed with a sample without monodomainization (after growth) as well as the same sample after monodomainization. The results of circular-dichroism measurements coincided with a precision of 0.5%. Thus, these data are consistent with the conclusions obtained in [11], according to them the rotation of polarization of ferroelectric crystals with phase transition 32 ? 3 should not change the sign of gyrotropic characteristics of the crystal. Thus, measurements of CD spectra of the doped LaBGeO5 crystals do not depend on the sign of the ferroelectric domains. 3.1. Absorption and CD spectra of LaBGeO5:Pr3+ Some intensive bands of the Pr3+ ion are presented in the infrared range of spectrum (1000–5000 nm) [12], where CD measurements are complicated. In the range 420–600 nm at T = 8 K, several bands are presented as well, their correspondence to the relevant transitions is indicated in Table 1 and Fig. 1. The ground state of praseodymium 3H4 in the crystal field of symmetry C1 splits into 9 Stark sublevels and the excited levels 3P2, 1I6,1 3P1, 3 P0 and 1D2 in the range 420–600 nm split into 5, 13, 3, 1 and 5 sublevels respectively [13]. The bands selected for determination of absorption and chiral characteristics are a manifestation of transi1
3
1
The values of Dom and Rom for transition H4 ? I6 were not calculated because this transition is forbidden by spin, has low intensity and split into 13 bands that are not resolved in the spectra obtained in our devices.
tions 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2 (Fig. 1, Table 1) [8,13]. 3.2. Absorption and CD spectra of LaBGeO5:Ho3+ The ground state of holmium 5I8 in the crystal field of symmetry C1 splits into 17 Stark sublevels and the excited states 5F5, 5S2, 5F4, 5 F2, 5K8, 5G6, 5F1, which give the bands in the range 400–650 nm split into 11, 5, 9, 5, 17, 13 and 3 sublevels, respectively [12]. The bands observed in absorption and CD spectra of LaBGeO5:Ho3+ crystal are a manifestation of transitions 5I8 ? 5F1, 5I8 ? 5G6, 5I8 ? 3K8, 5 I8 ? 5F2, 5I8 ? 5F4, 5I8 ? 5S2 and 5I8 ? 5F5 (Fig. 2, Table 1) [12,13]. These transitions in the spectra were chosen for analysis of absorption and chiral characteristics. Absorption and CD spectra allowed calculate the values of dipole strengths Dom, rotation strengths Rom and anisotropy factors g, which are commonly used in analysis of spectroscopic characteristics of optically active compounds [14]:
Dom ¼
Rom ¼
g¼
3ð2303Þhcn
Z
2
N4p2 kmax b
3ð2303Þhc N16p2 kmax b
4n Rom b Dom
Z
edk
ð1Þ
Dedk
ð2Þ
ð3Þ
Here, e is the decimal molar extinction coefficient, De is the decimal molar dichroism coefficient, n is the refractive index, b = (n2 + 2)/3 is the Lorentz factor. The calculated values of Dom, Rom, and g are given in Table 1.
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L. Alyabyeva et al. / Optical Materials 34 (2012) 803–806
3.3. Discussion In absorption spectra of rare-earth ions f–f transitions are parity-forbidden in the electric-dipole approximation [13]. Crystal field effect in the case of symmetry C1 leads to splitting of the levels as well as mixing of the f–f states, so all transitions become allowed by symmetry in the magnetic-dipole and electric-dipole approximations. An effect of odd components of the crystal field leads to mixing of f- and d- states and the intensity borrowing from the allowed 4f–4f 5d transitions. The borrowed component of the electric dipole defines the intensity of the bands in absorption. The intensity of the bands in circular-dichroism spectra depends on the borrowed moment as well as the relative orientation of the electric and magnetic moments. Transitions 3H4 ? 3P2, 3H4 ? 3P1, 3H4 ? 3P0 and 3H4 ? 1D2 of the free Pr3+ ion are allowed in the electric-quadrupole approximation, transition 3H4 ? 3P1 is forbidden in this approximation, and all transitions are forbidden in the electric-dipole and magnetic-dipole approximations [12]. In absorption spectra of LaBGeO5:Pr3+ crystal, intensity of allowed transitions 3H4 ? 3P2 and 3H4 ? 3P0 almost coincides with the intensity of the forbidden transition 3H4 ? 3P1, another allowed transition 3H4 ? 1D2 has a dramatically less value of the dipole strength. Values of the rotation strength for the investigated transitions are changed in a smaller scale (Table 1). For some transitions, which differ in intensity by one order in the absorption spectra, the rotation strengths have approximately the same values. Transitions 5I8 ? 5G6, 5I8 ? 5F2, 5I8 ? 5F4 and 5I8 ? 5S2 of the free Ho3+ ion are allowed in the electric-quadrupole approximation, transition 5I8 ? 3K8 is allowed in the magnetic-dipole approximation, transitions 5I8 ? 5F1 and 5I8 ? 5F5 are forbidden in these approximations. In absorption spectra of LaBGeO5:Ho3+ all transitions have approximately the same intensity, except transition 5 I8 ? 5S2. This quadrupole transition has a much lower intensity. One can see from the table that the dipole strengths of transitions of the Ho3+ ion are generally less than the dipole strengths of transitions of the Pr3+ ion. At the same time the rotation strengths of transitions of these ions have quite comparable values. This is also confirmed by anisotropy factors relationship. For LaBGeO5:Pr3+ crystal, values of the anisotropy factors of transitions 3 H4 ? 3P2 and 3H4 ? 3P0 allowed in the electric-quadrupole approximation are almost equal to that of transition 3H4 ? 3P1, which is forbidden in this approximation. On the other side, in spectra of LaBGeO5:Ho3+ crystal, anisotropy of transition 5 I8 ? 5F2 allowed in quadrupole approximation dramatically less than that of the forbidden transition 5I8 ? 5F5. In general, the value of anisotropy factor weakly depends on the contribution of electric-quadrupole mode in the intensity of the bands in the absorption and circular-dichroism spectra, and is determined by
the values of the electric-dipole moment, magnetic-dipole moment, and their mutual orientation. The selection rules for changing the projection of angular momentum DMJ [15] is determined by specific symmetry of the object. Since the local symmetry of the doped ions in our crystals is C1, all the transitions between the Stark components of ground and excited states are allowed by symmetry. The results show that values of Dom, Rom and g for transitions to different Stark components within a multiplet differ dramatically – more than one order of magnitude. 4. Conclusion According to the theory of optical activity, electronic transition is active in the circular-dichroism spectrum, if it is allowed in the electric-dipole and magnetic-dipole approximations. A mutual orientation of the electric-dipole moment and magnetic-dipole moment does not depend on the local symmetry of the crystal field, and plays an important role in the induction of chiral gyrotropic crystal field on the electronic transitions of the doped ion. In the case of rare-earth ions f–f transitions in the crystal field of symmetry C1 split and mix. In the framework of symmetry C1, all transitions are allowed in the magnetic-dipole approximation. In turn, an appearance of the electric-dipole moment is due to the existence of an effective mechanism of intensity borrowing from the parity-allowed transitions in d-states. The mechanism of this borrowing and its source depend on the symmetry of the crystal lattice, the symmetry of the doped ion position, and the intensity of vibronic interactions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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