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Spectroscopy of Mn2+ ions in CdCl2
Volume 205, number4,5
CHEMICAL PHYSICS LETTERS
16Aprill993
Spectroscopy of Mn2+ ions in CdC12 V. Ghiordanescu ‘, M. Vo& a and C. Pedrini b ’ Institute ofPhysics and Technology ofMaterials, Bucharest-Magurele, P.O. Box MG-7, Romania b Laboratoirede Physico-Chimie des Mat&au Luminescents, URA 442 CNRS, UniversithLyon I, 43 Boulevard du 11 Novembre 1918.69622 Villeurbanne Cedex, France
Received 4 December 1992;in final form 4 February 1993
An investigation of ultraviolet absorption and excitation spectra of Cd,_,Mngl, crystals is reported. At lower temperatures, T< 77 K, it is shown that a number of lines are associated with the manganese ions. No assignments are given for room temperature excitation spectra because the Mn2+ states are in the fundamental absorption edge.
1. Introduction There are several studies on the absorption of Mn2+ ions in MnC12 [l-3] and Cd,_,Mn,Clz [46] single crystals. Since Mn2+ belongs to the 3d5 electron configuration the observed absorption spcctra are weak and highly doped crystal and manganese compounds were used. The general features of the optical absorption spectra up to the 4E,(D) level have been satisfactorily explained via analyses of line positions, exchange mechanisms, phonon sidebands and spinorbit coupling. In spite of these studies there are still many aspects which are either not completely understood or have ambiguities of interpretation, especially in the ultraviolet (LJV) part of the optical spectrum. The aim of this work is to study the optical spectroscopy of Mn2+ in CdC12 and to obtain new information concerning the modification of the spectrum on lowering the manganese content. This research was focused on studying.only the UV Mr?+ lines using absorption and excitation techniques.
2. Experimental procedures The Cd,_Jvln,Cl, crystals were grown by the Bridgman technique with the addition of MnClz to CdC12. Two crystals were prepared with nominal 410
concentrations x1 ~0.06 (crystal 1) and x,=0.001 (crystal 2). The MnZf ions replace the Cd2+ ions and there is no segregation at these concentrations because cadmium chloride and manganese chloride are isomorphous and belong to the same space group D53d (R,,) 171. The absorption spectra were recorded using a Varian Cary 2300 spectrophotometer. The emission excitation spectra were measured using a grating Eu700 series Heath monochromator and UV light from a chopped 100 W Xenon lamp. The phosphorescence from Mn2+ was detected by an EMI 9558 QB photomultiplier tube and the time-resolved data were obtained by utilizing a boxcar averager model 162PAR.
3, Results The absorption spectra of Mn2+ ions in crystal 1 ( C!,) have well developed, often sharp, fine structure at 7 K (fig. 1). The absorption intensity of the band in the 30300 cm-’ region is weak and this may be the reason why it was not observed by Pappalardo [ 11. This deficiency caused a “shift” in the assignments of 4T1,(P), 4A2,(F), 4T,,(F), and 4T2,(F) to the blue. We associate the 30300 cm-’ absorption band to the 6A1,-+4T,,(P) transition. The next absorption in
0009-2614/93/S 06.00 @ 1993 Elsevier Science Publishers B.V. AU rights reserved.
Volume 205, number 4,5
CHEMICALPHYSICSLETTERS
u 36
I
I
37
30
Wavenumber
I 39x103
( cm” )
‘Eg(D)
E :g 4e i
3-
.g s 0
2-
B a l-
0 15
I
I
I
I
20
25
30
35
Wavenumber
Fig. 1. Absorption spectra of Cd&dn,&&
I 40x10'
( ad )
at T= 7 K. (a, Overall spectrum. (b) Detailed spectrum in the ‘A,(F)-‘T,,(F)
the 36500 cm- * region showsa large band and some discrete structure superimposedon it. As can be seen from fig. lb, there is a shoulder on each of the sharp lines indicating the presence of two components separated by about 55t 10 cm-‘. The tine structure of the absorption in the 38400 cm-’ region is dominated by the sharp intense line at 38454cm-‘. It was difficult to distinguish the peak position of another large band in the 40500 cm-’ region as it overlaps on the rising slope of the fundamental absorption edge of the Cd,,94Mn0.&12crystal (C, ), but it is definitely observed (fig. 2). Table 1 presents our assignments along with corresponding literature data on the subject. Figs. 3 and 4 show the room (RT) and liquid ni-
region.
trogen temperature (LNT)excitation spectra (ES) for both C, and C2 crystals. The advantage of the emissionexcitationtechniqueis that a.relativelysmall crystal can be used no matter what the Mn2+ concentration is. The excitation band peaking in the 35100 cm-’ region might not be due to Mn2+ because its intensity cannot be correlated relative to other Mn2+ peaks. The experimental evidence indicated that in all cases energy transfer occurs between this impurity center and Mn2+ as the absorption in this band produces some green emission (decay time in the millisecond range) and red Mn*+ emission at around 660 nm (decay time again in the millisecondrange). Our crystals contain lead traces which could be the 411
Volume 205, number 4,5
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CdCl2:0.06Mn2t T-SK
- 341
I
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16Aprill993
CHEMICAL PHYSICS LETTERS
I
I
I
36
I
38
37
39
40
’ 1 41x10
WAVENUMBER(CM-1)
Fig. 2. High-energy side absorption spectrum of C&.9,Mn,,&12 atT=8K.
source of the observed unwanted excitation. Further
investigationsof this energy transfer are under way. 4. Discussion The observed UV absorption-excitation lines of
Mn*+ are from the 6A1,(S) ground state to upper quartet states. The transitions on “pure” states are forbidden by spin and parity rules but the mixture between Mn*+ states and “symmetricallyadapted” energy states in the lattice or spin-spin interaction of Mn2+ pairs are put forward to explain the observed absorption spectra. In the UV part of spectra the absorption due to Mn2* ions is hidden by the fundamental absorption edge of the Cd,_,Mn,Cl, system. On loweringthe temperature it is evident from fig. 1 and table 1 that considerable structure is present in the absorption at 7 K. For examplethe 4A2,band shows discrete structure due to the A,, totally symmetric vibration (234 cm-’ in the ground state for MnC12) which consists of a four-membered progression in 211f 10 cm-‘. The first peak at 36433 cm-’ is not detected for the low concentration C2 crystal so its intensity is due to the pairs. The relativelyintensebroad band between4A2g( F) and 4TIs(F), observed only in the high concentration crystal (C, ), can be ascribedto a double quantum transition 6A,,(S)+6A~,(S)+4T~,(G)+
Table 1 Energies (cm-‘) of UV absorption bands observed in Cd,_,Mn$lz wJ.94mm6a
MnCI,
‘)
[ 1] b,
Cdo.psMno.o,Cl* 151c’
29715 30370
W,,(P)1
30300
vTI,(P)
1
36432 36599 36641 36796 36852 37000 37065
[4AzSF)I
37700
[‘AI% -+TT,~,
36500
[~T,‘(P)l
1
38201 38308 38454 38523 38663 38886
[q,,(F)
1
38400
[.‘Az&F) 1
38800
I’At(F)+q,,(F)
40500
[q,(F)
1
40650
[‘.f,,(F)
1
41285
[qz,(F)
*) Absorption spectra at 6.7 K. b, Absorption spectra at 77 K. ‘) Excitation spectra at 77 K.
412
1
1
Volume 205, number 4,5
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34
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36
CHEMICALPHYSICSLETTERS
37
30
39
4I
16 April 1993
4 -103
WAVENUMBER(CM-1)
Fig. 3. The Wexcitationspectraofthe redfluorescence(1=660 nm)of~.s*Mno.osC1,atRT(---)andLNT(-).Alsoshown is the broad band (-,-.-) of the double transition. WAVENUMBERtCM-1)
4T,,(G).
Several reasons support
the above
assignment: ( 1) The band position is about 37700cm-', which is twice the energy of the 6A,JS)-+4T,,(G) transition. (2 ) The half width is about the same as the single electronic transition but the intensity is greater by a factor two (the oscillator strength is about 4 x lo-* for the single transition). ( 3 ) We did not observe it in the low concentration crystal (C,). Such transitions have been observed in MnClz [ 8 ] and in related compounds [ 9 ] but have not been recognized as a double transition. Other exampleswhere double excitations were observed should be mentioned: CSM~~.~~MQ,~&~~ [lo], Rb,MnCl., [ll] andNaCI:Mn [12]. As shown in figs. 3 and 4 there is a main RT excitation peak in the UV spectral region. This band shifts continuously to higher energies with decreasing temperature. We explain this behaviour by the temperature dependence of the position of the fundamental absorption edge. In fact, the intensity of the large UV excitation band at RT is not correlated with manganese concentration. Moreover, in this
Fig. 4. The UV excitation spectra of the red fluorescence.(1~660 nm) of C&.99Mn,,01C12 at RT (- - -) and LNT (-).
spectral region the lifetime occurs along a nonexponential line and we cannot explain the intensity of the RT excitation band only by the manganeseemission (which has exponential decay for any excitation band in the visible region). Clearly, further work on the decay kinetics analysis of the emission and energy transfer processesis needed before one can usefully attempt the interpretation of UV excitation spectra of the Cdl _,Mn,Q system.
5. Concludingremarks The emission of Mn’+ in Cd, _,MnJ& gives the opportunity to compare the absorption spectra of the concentrated crystals with the same for lower manganese concentration. Such analysis reveals the coupling effect on the manganeseUV optical spectra. In our case the large absorption in the 37700 cm-' spectral region can be assigned to the 6A,,(S)+ 413
Volume 205, number 4,s
CHEMICAL PHYSICS LETTERS
6A,,(S)+“T,,(G)+4T1,(G) double transition. Energytransfer processeswith the manganeseions as acceptors take place by absorption in the fimdamental absorption edge and/or other impurity states. More experimental data are needed to explain the RT excitation spectra.
References [ 1] R. Pappalardo, J. Chem. Phys. 31 (1959) 1050. 121 M. Re& and Y. Farge, J. Phys. (Paris) 37 ( 1976) 627. [ 31 I. Pollini, G. Spinolo and G. Bendek, Phys. Rev. B 22 ( 1980) 6369.
414
16 April 1993
[ 41 Ath. Trutia, V. Ghiordanescu and M. Vod& Phys. Stat. Sol. b70(1975)K19. [ 51B. Ghosh and RK Mukhetjce, Phys. Stat. Sol. b 102 ( 1980) K89. [6] P.J. McCarthy and H.U. Gtidel, Inorg. Chem. 25 (1986)
838. [ 7 ] R.W.G. Wyckoff, Crystal structures, Vol. 1,2nd Ed. (WileyInterscience, New York, 1965) p. 270. [B]L.L.LohrJr., J.Chem.Phys.45 (1966) 3611. [9] D.H. Goode, J. Chem. Phys. 43 (1965) 2830. [lo] P.J. McCarthy and H.U. Gildel, Inorg. Chem. 23 (1984) 880. [ 1 I ] B. Ghosh and R.K. Mukheijee, Phys. Stat. Sol. b 106 (1981) 699. [ 12 ] F. Rodriguez, M. Moreno, F. Jaque and F.J. Lopez, J. Chem. Phys. 78 (1983) 73.
CHEMICAL PHYSICS LETTERS
16Aprill993
Spectroscopy of Mn2+ ions in CdC12 V. Ghiordanescu ‘, M. Vo& a and C. Pedrini b ’ Institute ofPhysics and Technology ofMaterials, Bucharest-Magurele, P.O. Box MG-7, Romania b Laboratoirede Physico-Chimie des Mat&au Luminescents, URA 442 CNRS, UniversithLyon I, 43 Boulevard du 11 Novembre 1918.69622 Villeurbanne Cedex, France
Received 4 December 1992;in final form 4 February 1993
An investigation of ultraviolet absorption and excitation spectra of Cd,_,Mngl, crystals is reported. At lower temperatures, T< 77 K, it is shown that a number of lines are associated with the manganese ions. No assignments are given for room temperature excitation spectra because the Mn2+ states are in the fundamental absorption edge.
1. Introduction There are several studies on the absorption of Mn2+ ions in MnC12 [l-3] and Cd,_,Mn,Clz [46] single crystals. Since Mn2+ belongs to the 3d5 electron configuration the observed absorption spcctra are weak and highly doped crystal and manganese compounds were used. The general features of the optical absorption spectra up to the 4E,(D) level have been satisfactorily explained via analyses of line positions, exchange mechanisms, phonon sidebands and spinorbit coupling. In spite of these studies there are still many aspects which are either not completely understood or have ambiguities of interpretation, especially in the ultraviolet (LJV) part of the optical spectrum. The aim of this work is to study the optical spectroscopy of Mn2+ in CdC12 and to obtain new information concerning the modification of the spectrum on lowering the manganese content. This research was focused on studying.only the UV Mr?+ lines using absorption and excitation techniques.
2. Experimental procedures The Cd,_Jvln,Cl, crystals were grown by the Bridgman technique with the addition of MnClz to CdC12. Two crystals were prepared with nominal 410
concentrations x1 ~0.06 (crystal 1) and x,=0.001 (crystal 2). The MnZf ions replace the Cd2+ ions and there is no segregation at these concentrations because cadmium chloride and manganese chloride are isomorphous and belong to the same space group D53d (R,,) 171. The absorption spectra were recorded using a Varian Cary 2300 spectrophotometer. The emission excitation spectra were measured using a grating Eu700 series Heath monochromator and UV light from a chopped 100 W Xenon lamp. The phosphorescence from Mn2+ was detected by an EMI 9558 QB photomultiplier tube and the time-resolved data were obtained by utilizing a boxcar averager model 162PAR.
3, Results The absorption spectra of Mn2+ ions in crystal 1 ( C!,) have well developed, often sharp, fine structure at 7 K (fig. 1). The absorption intensity of the band in the 30300 cm-’ region is weak and this may be the reason why it was not observed by Pappalardo [ 11. This deficiency caused a “shift” in the assignments of 4T1,(P), 4A2,(F), 4T,,(F), and 4T2,(F) to the blue. We associate the 30300 cm-’ absorption band to the 6A1,-+4T,,(P) transition. The next absorption in
0009-2614/93/S 06.00 @ 1993 Elsevier Science Publishers B.V. AU rights reserved.
Volume 205, number 4,5
CHEMICALPHYSICSLETTERS
u 36
I
I
37
30
Wavenumber
I 39x103
( cm” )
‘Eg(D)
E :g 4e i
3-
.g s 0
2-
B a l-
0 15
I
I
I
I
20
25
30
35
Wavenumber
Fig. 1. Absorption spectra of Cd&dn,&&
I 40x10'
( ad )
at T= 7 K. (a, Overall spectrum. (b) Detailed spectrum in the ‘A,(F)-‘T,,(F)
the 36500 cm- * region showsa large band and some discrete structure superimposedon it. As can be seen from fig. lb, there is a shoulder on each of the sharp lines indicating the presence of two components separated by about 55t 10 cm-‘. The tine structure of the absorption in the 38400 cm-’ region is dominated by the sharp intense line at 38454cm-‘. It was difficult to distinguish the peak position of another large band in the 40500 cm-’ region as it overlaps on the rising slope of the fundamental absorption edge of the Cd,,94Mn0.&12crystal (C, ), but it is definitely observed (fig. 2). Table 1 presents our assignments along with corresponding literature data on the subject. Figs. 3 and 4 show the room (RT) and liquid ni-
region.
trogen temperature (LNT)excitation spectra (ES) for both C, and C2 crystals. The advantage of the emissionexcitationtechniqueis that a.relativelysmall crystal can be used no matter what the Mn2+ concentration is. The excitation band peaking in the 35100 cm-’ region might not be due to Mn2+ because its intensity cannot be correlated relative to other Mn2+ peaks. The experimental evidence indicated that in all cases energy transfer occurs between this impurity center and Mn2+ as the absorption in this band produces some green emission (decay time in the millisecond range) and red Mn*+ emission at around 660 nm (decay time again in the millisecondrange). Our crystals contain lead traces which could be the 411
Volume 205, number 4,5
:4-
CdCl2:0.06Mn2t T-SK
- 341
I
35
16Aprill993
CHEMICAL PHYSICS LETTERS
I
I
I
36
I
38
37
39
40
’ 1 41x10
WAVENUMBER(CM-1)
Fig. 2. High-energy side absorption spectrum of C&.9,Mn,,&12 atT=8K.
source of the observed unwanted excitation. Further
investigationsof this energy transfer are under way. 4. Discussion The observed UV absorption-excitation lines of
Mn*+ are from the 6A1,(S) ground state to upper quartet states. The transitions on “pure” states are forbidden by spin and parity rules but the mixture between Mn*+ states and “symmetricallyadapted” energy states in the lattice or spin-spin interaction of Mn2+ pairs are put forward to explain the observed absorption spectra. In the UV part of spectra the absorption due to Mn2* ions is hidden by the fundamental absorption edge of the Cd,_,Mn,Cl, system. On loweringthe temperature it is evident from fig. 1 and table 1 that considerable structure is present in the absorption at 7 K. For examplethe 4A2,band shows discrete structure due to the A,, totally symmetric vibration (234 cm-’ in the ground state for MnC12) which consists of a four-membered progression in 211f 10 cm-‘. The first peak at 36433 cm-’ is not detected for the low concentration C2 crystal so its intensity is due to the pairs. The relativelyintensebroad band between4A2g( F) and 4TIs(F), observed only in the high concentration crystal (C, ), can be ascribedto a double quantum transition 6A,,(S)+6A~,(S)+4T~,(G)+
Table 1 Energies (cm-‘) of UV absorption bands observed in Cd,_,Mn$lz wJ.94mm6a
MnCI,
‘)
[ 1] b,
Cdo.psMno.o,Cl* 151c’
29715 30370
W,,(P)1
30300
vTI,(P)
1
36432 36599 36641 36796 36852 37000 37065
[4AzSF)I
37700
[‘AI% -+TT,~,
36500
[~T,‘(P)l
1
38201 38308 38454 38523 38663 38886
[q,,(F)
1
38400
[.‘Az&F) 1
38800
I’At(F)+q,,(F)
40500
[q,(F)
1
40650
[‘.f,,(F)
1
41285
[qz,(F)
*) Absorption spectra at 6.7 K. b, Absorption spectra at 77 K. ‘) Excitation spectra at 77 K.
412
1
1
Volume 205, number 4,5
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CHEMICALPHYSICSLETTERS
37
30
39
4I
16 April 1993
4 -103
WAVENUMBER(CM-1)
Fig. 3. The Wexcitationspectraofthe redfluorescence(1=660 nm)of~.s*Mno.osC1,atRT(---)andLNT(-).Alsoshown is the broad band (-,-.-) of the double transition. WAVENUMBERtCM-1)
4T,,(G).
Several reasons support
the above
assignment: ( 1) The band position is about 37700cm-', which is twice the energy of the 6A,JS)-+4T,,(G) transition. (2 ) The half width is about the same as the single electronic transition but the intensity is greater by a factor two (the oscillator strength is about 4 x lo-* for the single transition). ( 3 ) We did not observe it in the low concentration crystal (C,). Such transitions have been observed in MnClz [ 8 ] and in related compounds [ 9 ] but have not been recognized as a double transition. Other exampleswhere double excitations were observed should be mentioned: CSM~~.~~MQ,~&~~ [lo], Rb,MnCl., [ll] andNaCI:Mn [12]. As shown in figs. 3 and 4 there is a main RT excitation peak in the UV spectral region. This band shifts continuously to higher energies with decreasing temperature. We explain this behaviour by the temperature dependence of the position of the fundamental absorption edge. In fact, the intensity of the large UV excitation band at RT is not correlated with manganese concentration. Moreover, in this
Fig. 4. The UV excitation spectra of the red fluorescence.(1~660 nm) of C&.99Mn,,01C12 at RT (- - -) and LNT (-).
spectral region the lifetime occurs along a nonexponential line and we cannot explain the intensity of the RT excitation band only by the manganeseemission (which has exponential decay for any excitation band in the visible region). Clearly, further work on the decay kinetics analysis of the emission and energy transfer processesis needed before one can usefully attempt the interpretation of UV excitation spectra of the Cdl _,Mn,Q system.
5. Concludingremarks The emission of Mn’+ in Cd, _,MnJ& gives the opportunity to compare the absorption spectra of the concentrated crystals with the same for lower manganese concentration. Such analysis reveals the coupling effect on the manganeseUV optical spectra. In our case the large absorption in the 37700 cm-' spectral region can be assigned to the 6A,,(S)+ 413
Volume 205, number 4,s
CHEMICAL PHYSICS LETTERS
6A,,(S)+“T,,(G)+4T1,(G) double transition. Energytransfer processeswith the manganeseions as acceptors take place by absorption in the fimdamental absorption edge and/or other impurity states. More experimental data are needed to explain the RT excitation spectra.
References [ 1] R. Pappalardo, J. Chem. Phys. 31 (1959) 1050. 121 M. Re& and Y. Farge, J. Phys. (Paris) 37 ( 1976) 627. [ 31 I. Pollini, G. Spinolo and G. Bendek, Phys. Rev. B 22 ( 1980) 6369.
414
16 April 1993
[ 41 Ath. Trutia, V. Ghiordanescu and M. Vod& Phys. Stat. Sol. b70(1975)K19. [ 51B. Ghosh and RK Mukhetjce, Phys. Stat. Sol. b 102 ( 1980) K89. [6] P.J. McCarthy and H.U. Gtidel, Inorg. Chem. 25 (1986)
838. [ 7 ] R.W.G. Wyckoff, Crystal structures, Vol. 1,2nd Ed. (WileyInterscience, New York, 1965) p. 270. [B]L.L.LohrJr., J.Chem.Phys.45 (1966) 3611. [9] D.H. Goode, J. Chem. Phys. 43 (1965) 2830. [lo] P.J. McCarthy and H.U. Gildel, Inorg. Chem. 23 (1984) 880. [ 1 I ] B. Ghosh and R.K. Mukheijee, Phys. Stat. Sol. b 106 (1981) 699. [ 12 ] F. Rodriguez, M. Moreno, F. Jaque and F.J. Lopez, J. Chem. Phys. 78 (1983) 73.