- Email: [email protected]
Crystal field splitting of highly excited electronic states of the 4fn–1 5d electronic configuration of trivalent rare earth ions in wide band gap crystals
Crystal Engineering 5 (2002) 203–208 www.elsevier.com/locate/cryseng
Crystal field splitting of highly excited electronic states of the 4fn–1 5d electronic configuration of trivalent rare earth ions in wide band gap crystals A.C. Cefalas a,∗, S. Kobe b, Z. Kollia a, E. Sarantopoulou a a b
National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens, 11635, Greece Department of Nanostructured Materials, Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Abstract The energy position and the spacing of the levels of the 4fn–15d electronic configuration of the trivalent rare earth ion dopands in wide band gap fluoride crystal, depend on the symmetry and the type of the host matrix. Crystal field splitting of the 4f25d electronic configuration of the Nd3+ ions in SrF2 crystals have been observed in the VUV region of the spectrum. The absorption bands were due to the interconfigurational 4f3→4f25d dipole allowed transitions between the ground state with 4f3 electronic configuration of the Nd3+ ions and the Stark components of the 4f25d electronic configuration. The VUV spectra can be interpreted by applying the crystal field model, and taking into consideration both that lanthanide contraction of the 4fn–15d electronic configuration of the rare earth ions is taking place, and that the contribution of the positively charged ions in the total electric field, is effectively decreased with increasing numbers of the electrons in the 4fn electronic configuration due to effective charge shielding. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum ultraviolet; 157 nm; Rare earth ions; Stark splitting
1. Introduction The absorption and the excitation spectroscopic characteristics of the trivalent rare earth (RE) ions in the VUV spectral region, activated in wide band gap fluorine
∗
Corresponding author. Tel.: +30-210-7273839/40; fax: +30-210-7273842. E-mail address: [email protected] (A.C. Cefalas).
1463-0184/02/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1463-0184(02)00030-8
204
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
dielectric crystals, are due to the transitions between the levels of the 4fn electronic configuration of the trivalent RE ion and the levels of the 4fn–15d electronic configuration, where a 4f electron is promoted to a 5d localized level [1]. The 4fn↔4fn - 15d electronic transitions are characterized by strong Frank-Condon factors with broadband absorption and emission spectra in the VUV and UV. On the contrary, the intraconfigurational 4fn↔4fn transitions are parity forbidden. They are forced by the crystal field configuration mixing and they appear to be weak and sharp. The interest of investigating the 4fn–15d electronic configurations of the trivalent RE ions in different wide band gap dielectric crystal hosts is based on a variety of applications in the vacuum ultraviolet (VUV) region of the spectrum, [2]. Among the RE trivalent ions, the Nd3+ is the most interesting one since it has two electrons in the 4f electronic configuration and only one electron in the 5d electronic configuration. Thus the Nd3+ ions could be used as the starting point for the development of any theoretical models describing the fundamental interactions in RE ions with more than two electrons in the 4fn electronic configuration, and in the presence of strong crystal field. In an attempt to interpret qualitatively the interconfigurational interactions in wide band gap fluoride dielectric crystals by avoiding complex ab initio calculations, we have investigated the VUV absorption and emission spectra of Nd3+ ions in different crystal hosts such as LaF3, LiYF4, LiCaAlF6, SrF2. The Nd3+ ions were excited to the 4f25d electronic configuration with LaF3 crystals with an F2 molecular laser at 157 nm. The interconfigurational transitions mainly originate from the edge of the levels of the 4f25d electronic configuration and thus strong phonon-electron interaction between the vibrational modes of the crystal host ligands and the 4f25d electronic configuration of the Nd3+ ions is taking place. A simple physical interpretation of the spectra was based on the assumption that the “lanthanide contraction” in RE ions (the extent of the 4fn orbital is reduced as we move to the right in the series) is applied for the 4fn–15d electronic configurations as well as for the 4fn ones. The experimental results suggest that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ions.
2. Experimental The experimental apparatus mainly consists of the F2 pulsed fast discharge molecular laser that emits at 157.6 nm [3], the vacuum chamber where the crystal samples were placed and the detection electronics. The laser head delivers 12 mJ per pulse, at 3 atm working pressure of its gases and the pulse width was 12 ns (FWHM). The absorption spectrum in the VUV and UV was recorded using a hydrogen lamp operating in a longitudinal stabilized discharge mode. The high stability of the discharge, results in good signal-to-noise ratio (better than 2000). The optical path of the exciting and the fluorescent light was under vacuum at 10-5 mbar background pressure, using vacuum lines from stainless steel 316. The single crystal samples were optically polished disks. Their diameter was 5 mm and their thickness was varied from 0.5 to 1 mm. The concentration of the Nd3+ ions was 0.5 at. %.
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
205
Table 1 Crystal field splitting of the terms of the Nd3+ ions of the 4f2 5d electronic configuration at different site symmetries. The number in the parenthesis, in the expected transitions column, represents the number of non-degenerate states 4fn configuration
4fn-15d configuration
Number Ground 4fn → 4fn-1 Lowest of levels state 5d level transitions (gas)
Dipole allowed transitions ⌬S=0, ⌬L=0±1, ⌬L=⌬J
Excited terms from ground state
Site Symmetry
C4h H7/2 (2) (2)[(⌫6+⌫7)+⌫6+⌫7] 4 I9/2 2( ⌫6+⌫7)+ ⌫6 4 K11/2 2( ⌫6+⌫7)+ ⌫6+2⌫7 4
C3U H7/2 (2) (2)[⌫4,⌫4, (⌫4+⌫5)+⌫6] 4 I9/2 2[(⌫4+⌫5+⌫6)], ⌫4 4 K11/2 2(⌫4+⌫5+⌫6), ⌫4 , ⌫4 4
41
4
I9/2
107
4
I9/2
O H7/2 (2) (2)[⌫6+⌫7+⌫8] 4 I9/2 ⌫6+2⌫8 4 K11/2 2⌫8+⌫7+⌫6 4
Expected Observed transitions transitions
11(19)
11 [8]
13 (25)
20[9]
4(13)
9 This work
3. Results and discussion The vacuum absorption and emission spectra of the trivalent RE ions in the wide band gap fluoride crystals are due to the interconfigurational 4fn–15d↔4fn transitions of the RE ions, however it is difficult to interpret complex spectra arising from interconfigurational transitions because spectral terms cannot be constructed in the presence of a strong crystal field. For assigning spectroscopic terms to the 4fn–15d electronic configuration, the 4fn–1 and the 5d electronic configurations should have the same transformation properties under rotations, and the usual rules of the summation of the angular momentum could be applied. For the d electron, this is possible provided that the extent of the orbital is small in comparison to the dimension of the crystal constants, and the positive charge is well screened by the inner electrons belonging to different configurations. In this case, overlapping with neighboring ligand orbitals is avoided and the spherical symmetry of the electric field around the origin of the d electron is partially restored. Based on the previous arguments, a qualitative interpretation of the 4fn–15d↔4fn interconfigurational transitions of tri-
206
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
Fig. 1.
VUV transmission spectrum of the SrF2: Nd3+ crystal from 110 to 125 nm.
Fig. 2.
VUV absorption spectrum of SrF2: Nd3+ crystal from 120 to 200 nm.
valent RE ions in the wide band gap dielectric crystals was made by T. Szczurek and M. Schlesinger [4], by constructing first spectral terms (addition of angular momentum) and then allowing for crystal field splitting. Therefore, the 5d and the 4fn–1 electronic configurations might well have the same transformation properties under the three-dimensional rotational group, Table 1, and the VUV absorption spectra can be interpreted taking into consideration that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ion [5]. Therefore, the effect of the lanthanide contraction of the 4fn electronic configuration, is taking place for the 4fn–15d electronic configuration of the Nd3+ ions [6,7].
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
207
Fig. 3. Laser induced fluorescence spectrum of Nd3+ ions in different crystal hosts.
The absorption spectra of the LiYF4: Nd3+ and the LiCaAlF6: Nd3+ crystals in the VUV from 120 to 200 nm have been recorded previously [8,9]. The peaks were assigned to the strong dipole allowed transitions between the 4I9 / 2 ground level of the 4f3 electronic configuration and the Stark components of the levels of the 4f25d electronic configuration, taking into consideration that partial restoration of spherical symmetry of the 4fn5d electronic configuration is taking place, in agreement with the experimental results. Table 1. The transmission spectrum of the SrF2 : Nd3+ crystal from 113 to 122 nm is shown in Fig. 1. The peaks from 122 to 119 nm probably correspond to excitonic transitions between the valence and the conduction band of the host lattice. Considering that the cut-off wavelength of the LiF window (which is placed in front of the hydrogen lamp and the solar blind photomultiplier) is at 110 nm, the absorption spectrum between 113 and 120 nm indicates only the wavelength position of the corresponding atomic transitions. From the absorption spectrum, Fig. 2, nine main stark components of the 4f25d electronic configuration have been observed. The edge of the levels of the 4f25d electronic configuration was placed at 183.4 nm. The Laser Induced Fluorescence (LIF) spectra at 157 nm for the LiCaAlF6: Nd3+, LiYF4: Nd3+, and LaF3: Nd3+, crystal is indicated in Fig. 3. In the case of SrF2: Nd3+ crystal, the emission was absorbed within the crystal volume. For all cases the LIF emission originated from the edge of the band.
208
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
4. Conclusions Crystal field splitting of the 4f25d electronic configuration and LIF of the Nd3+ ions were observed in different dielectric crystal hosts. The VUV spectra were interpreted on the basis that the “lanthanide contraction” is taking place for the 4f2 5d electronic configuration of the Nd3+ ions in various dielectric wide band gap crystal hosts. The experimental results suggest that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ions.
Acknowledgements Part of this work was financed by the IST project. IST-2000-30143 157 CRISPIES.
References [1] K.H. Yang, J.A. Deluka, Appl. Phys. Lett V29 (1976) 499. [2] E. Sarantopoulou, A.C. Cefalas, VUV Laser Spectroscopy of Trivalent Rare Earth Ions in Wide Band Gap Fluoride Crystals, in: Ultraviolet Spectroscopy and UV Lasers, 30, Marcel and Dekker, New York, 2002, pp. 281–336. [3] C. Skordoulis, E. Sarantopoulou, S.M. Spyrou, A.C. Cefalas, J. Mod. Op. 37 (1990) 501. [4] T. Szczurek, M. Schlesinger, Vacuum ultraviolet absorption spectra of CaF2:RE3+ crystals. Proceedings of the Rare Earth Spectroscopy Symposium 85, (1985), 309-330. Edited by World Scientific, Singapore. [5] E. Sarantopoulou, Z. Kollia, A.C. Cefalas, V.V. Semashko, R.Yu. Abdulsabirov, A.K. Naumov, S.L. Korableva, T. Szczurek, S. Kobe, P.J. McGuiness, Opt. Commun. 208 (2002) 345. [6] S. Nicolas, M. Laroche, S. Girard, R. Moncorge, Y. Guyot, M.F. Joubert, E. Descroix, A.G. Petrosyan, J.Physics: Condensed Matter 11 (1999) 7937. [7] J. Becker, J.Y. Gesland, M.Yu. Kirikova, J.C. Krupa, V.N. Makhov, M. Runne, M. Queffelec, T.V. Uvarova, G. Zimmerer, J. Alloys Compd. 275 (1998) 205. [8] Z. Kollia, E. Sarantopoulou, A.C. Cefalas, A.K. Naumov, V.V. Semashko, R.Y. Abdulsabirov, S.L. Korableva, Opt. Commun 149 (1998) 386. [9] E. Sarantopoulou, Z. Kollia, A.C. Cefalas, Opt. Commun. 177 (2000) 377.
Crystal field splitting of highly excited electronic states of the 4fn–1 5d electronic configuration of trivalent rare earth ions in wide band gap crystals A.C. Cefalas a,∗, S. Kobe b, Z. Kollia a, E. Sarantopoulou a a b
National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens, 11635, Greece Department of Nanostructured Materials, Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Abstract The energy position and the spacing of the levels of the 4fn–15d electronic configuration of the trivalent rare earth ion dopands in wide band gap fluoride crystal, depend on the symmetry and the type of the host matrix. Crystal field splitting of the 4f25d electronic configuration of the Nd3+ ions in SrF2 crystals have been observed in the VUV region of the spectrum. The absorption bands were due to the interconfigurational 4f3→4f25d dipole allowed transitions between the ground state with 4f3 electronic configuration of the Nd3+ ions and the Stark components of the 4f25d electronic configuration. The VUV spectra can be interpreted by applying the crystal field model, and taking into consideration both that lanthanide contraction of the 4fn–15d electronic configuration of the rare earth ions is taking place, and that the contribution of the positively charged ions in the total electric field, is effectively decreased with increasing numbers of the electrons in the 4fn electronic configuration due to effective charge shielding. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum ultraviolet; 157 nm; Rare earth ions; Stark splitting
1. Introduction The absorption and the excitation spectroscopic characteristics of the trivalent rare earth (RE) ions in the VUV spectral region, activated in wide band gap fluorine
∗
Corresponding author. Tel.: +30-210-7273839/40; fax: +30-210-7273842. E-mail address: [email protected] (A.C. Cefalas).
1463-0184/02/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1463-0184(02)00030-8
204
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
dielectric crystals, are due to the transitions between the levels of the 4fn electronic configuration of the trivalent RE ion and the levels of the 4fn–15d electronic configuration, where a 4f electron is promoted to a 5d localized level [1]. The 4fn↔4fn - 15d electronic transitions are characterized by strong Frank-Condon factors with broadband absorption and emission spectra in the VUV and UV. On the contrary, the intraconfigurational 4fn↔4fn transitions are parity forbidden. They are forced by the crystal field configuration mixing and they appear to be weak and sharp. The interest of investigating the 4fn–15d electronic configurations of the trivalent RE ions in different wide band gap dielectric crystal hosts is based on a variety of applications in the vacuum ultraviolet (VUV) region of the spectrum, [2]. Among the RE trivalent ions, the Nd3+ is the most interesting one since it has two electrons in the 4f electronic configuration and only one electron in the 5d electronic configuration. Thus the Nd3+ ions could be used as the starting point for the development of any theoretical models describing the fundamental interactions in RE ions with more than two electrons in the 4fn electronic configuration, and in the presence of strong crystal field. In an attempt to interpret qualitatively the interconfigurational interactions in wide band gap fluoride dielectric crystals by avoiding complex ab initio calculations, we have investigated the VUV absorption and emission spectra of Nd3+ ions in different crystal hosts such as LaF3, LiYF4, LiCaAlF6, SrF2. The Nd3+ ions were excited to the 4f25d electronic configuration with LaF3 crystals with an F2 molecular laser at 157 nm. The interconfigurational transitions mainly originate from the edge of the levels of the 4f25d electronic configuration and thus strong phonon-electron interaction between the vibrational modes of the crystal host ligands and the 4f25d electronic configuration of the Nd3+ ions is taking place. A simple physical interpretation of the spectra was based on the assumption that the “lanthanide contraction” in RE ions (the extent of the 4fn orbital is reduced as we move to the right in the series) is applied for the 4fn–15d electronic configurations as well as for the 4fn ones. The experimental results suggest that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ions.
2. Experimental The experimental apparatus mainly consists of the F2 pulsed fast discharge molecular laser that emits at 157.6 nm [3], the vacuum chamber where the crystal samples were placed and the detection electronics. The laser head delivers 12 mJ per pulse, at 3 atm working pressure of its gases and the pulse width was 12 ns (FWHM). The absorption spectrum in the VUV and UV was recorded using a hydrogen lamp operating in a longitudinal stabilized discharge mode. The high stability of the discharge, results in good signal-to-noise ratio (better than 2000). The optical path of the exciting and the fluorescent light was under vacuum at 10-5 mbar background pressure, using vacuum lines from stainless steel 316. The single crystal samples were optically polished disks. Their diameter was 5 mm and their thickness was varied from 0.5 to 1 mm. The concentration of the Nd3+ ions was 0.5 at. %.
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
205
Table 1 Crystal field splitting of the terms of the Nd3+ ions of the 4f2 5d electronic configuration at different site symmetries. The number in the parenthesis, in the expected transitions column, represents the number of non-degenerate states 4fn configuration
4fn-15d configuration
Number Ground 4fn → 4fn-1 Lowest of levels state 5d level transitions (gas)
Dipole allowed transitions ⌬S=0, ⌬L=0±1, ⌬L=⌬J
Excited terms from ground state
Site Symmetry
C4h H7/2 (2) (2)[(⌫6+⌫7)+⌫6+⌫7] 4 I9/2 2( ⌫6+⌫7)+ ⌫6 4 K11/2 2( ⌫6+⌫7)+ ⌫6+2⌫7 4
C3U H7/2 (2) (2)[⌫4,⌫4, (⌫4+⌫5)+⌫6] 4 I9/2 2[(⌫4+⌫5+⌫6)], ⌫4 4 K11/2 2(⌫4+⌫5+⌫6), ⌫4 , ⌫4 4
41
4
I9/2
107
4
I9/2
O H7/2 (2) (2)[⌫6+⌫7+⌫8] 4 I9/2 ⌫6+2⌫8 4 K11/2 2⌫8+⌫7+⌫6 4
Expected Observed transitions transitions
11(19)
11 [8]
13 (25)
20[9]
4(13)
9 This work
3. Results and discussion The vacuum absorption and emission spectra of the trivalent RE ions in the wide band gap fluoride crystals are due to the interconfigurational 4fn–15d↔4fn transitions of the RE ions, however it is difficult to interpret complex spectra arising from interconfigurational transitions because spectral terms cannot be constructed in the presence of a strong crystal field. For assigning spectroscopic terms to the 4fn–15d electronic configuration, the 4fn–1 and the 5d electronic configurations should have the same transformation properties under rotations, and the usual rules of the summation of the angular momentum could be applied. For the d electron, this is possible provided that the extent of the orbital is small in comparison to the dimension of the crystal constants, and the positive charge is well screened by the inner electrons belonging to different configurations. In this case, overlapping with neighboring ligand orbitals is avoided and the spherical symmetry of the electric field around the origin of the d electron is partially restored. Based on the previous arguments, a qualitative interpretation of the 4fn–15d↔4fn interconfigurational transitions of tri-
206
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
Fig. 1.
VUV transmission spectrum of the SrF2: Nd3+ crystal from 110 to 125 nm.
Fig. 2.
VUV absorption spectrum of SrF2: Nd3+ crystal from 120 to 200 nm.
valent RE ions in the wide band gap dielectric crystals was made by T. Szczurek and M. Schlesinger [4], by constructing first spectral terms (addition of angular momentum) and then allowing for crystal field splitting. Therefore, the 5d and the 4fn–1 electronic configurations might well have the same transformation properties under the three-dimensional rotational group, Table 1, and the VUV absorption spectra can be interpreted taking into consideration that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ion [5]. Therefore, the effect of the lanthanide contraction of the 4fn electronic configuration, is taking place for the 4fn–15d electronic configuration of the Nd3+ ions [6,7].
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
207
Fig. 3. Laser induced fluorescence spectrum of Nd3+ ions in different crystal hosts.
The absorption spectra of the LiYF4: Nd3+ and the LiCaAlF6: Nd3+ crystals in the VUV from 120 to 200 nm have been recorded previously [8,9]. The peaks were assigned to the strong dipole allowed transitions between the 4I9 / 2 ground level of the 4f3 electronic configuration and the Stark components of the levels of the 4f25d electronic configuration, taking into consideration that partial restoration of spherical symmetry of the 4fn5d electronic configuration is taking place, in agreement with the experimental results. Table 1. The transmission spectrum of the SrF2 : Nd3+ crystal from 113 to 122 nm is shown in Fig. 1. The peaks from 122 to 119 nm probably correspond to excitonic transitions between the valence and the conduction band of the host lattice. Considering that the cut-off wavelength of the LiF window (which is placed in front of the hydrogen lamp and the solar blind photomultiplier) is at 110 nm, the absorption spectrum between 113 and 120 nm indicates only the wavelength position of the corresponding atomic transitions. From the absorption spectrum, Fig. 2, nine main stark components of the 4f25d electronic configuration have been observed. The edge of the levels of the 4f25d electronic configuration was placed at 183.4 nm. The Laser Induced Fluorescence (LIF) spectra at 157 nm for the LiCaAlF6: Nd3+, LiYF4: Nd3+, and LaF3: Nd3+, crystal is indicated in Fig. 3. In the case of SrF2: Nd3+ crystal, the emission was absorbed within the crystal volume. For all cases the LIF emission originated from the edge of the band.
208
A.C. Cefalas et al. / Crystal Engineering 5 (2002) 203–208
4. Conclusions Crystal field splitting of the 4f25d electronic configuration and LIF of the Nd3+ ions were observed in different dielectric crystal hosts. The VUV spectra were interpreted on the basis that the “lanthanide contraction” is taking place for the 4f2 5d electronic configuration of the Nd3+ ions in various dielectric wide band gap crystal hosts. The experimental results suggest that partial restoration of the spherical symmetry of the electric field around the extension of the d orbital is taking place for the Nd3+ ions.
Acknowledgements Part of this work was financed by the IST project. IST-2000-30143 157 CRISPIES.
References [1] K.H. Yang, J.A. Deluka, Appl. Phys. Lett V29 (1976) 499. [2] E. Sarantopoulou, A.C. Cefalas, VUV Laser Spectroscopy of Trivalent Rare Earth Ions in Wide Band Gap Fluoride Crystals, in: Ultraviolet Spectroscopy and UV Lasers, 30, Marcel and Dekker, New York, 2002, pp. 281–336. [3] C. Skordoulis, E. Sarantopoulou, S.M. Spyrou, A.C. Cefalas, J. Mod. Op. 37 (1990) 501. [4] T. Szczurek, M. Schlesinger, Vacuum ultraviolet absorption spectra of CaF2:RE3+ crystals. Proceedings of the Rare Earth Spectroscopy Symposium 85, (1985), 309-330. Edited by World Scientific, Singapore. [5] E. Sarantopoulou, Z. Kollia, A.C. Cefalas, V.V. Semashko, R.Yu. Abdulsabirov, A.K. Naumov, S.L. Korableva, T. Szczurek, S. Kobe, P.J. McGuiness, Opt. Commun. 208 (2002) 345. [6] S. Nicolas, M. Laroche, S. Girard, R. Moncorge, Y. Guyot, M.F. Joubert, E. Descroix, A.G. Petrosyan, J.Physics: Condensed Matter 11 (1999) 7937. [7] J. Becker, J.Y. Gesland, M.Yu. Kirikova, J.C. Krupa, V.N. Makhov, M. Runne, M. Queffelec, T.V. Uvarova, G. Zimmerer, J. Alloys Compd. 275 (1998) 205. [8] Z. Kollia, E. Sarantopoulou, A.C. Cefalas, A.K. Naumov, V.V. Semashko, R.Y. Abdulsabirov, S.L. Korableva, Opt. Commun 149 (1998) 386. [9] E. Sarantopoulou, Z. Kollia, A.C. Cefalas, Opt. Commun. 177 (2000) 377.