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Energy levels of HoBr63−
CHEMICAL PHYSICS LETTERS
Volume 132, number 2
ENERGY LEVELS
5 December 1986
OF HoBr,3-
Peter A. TANNER’ Department
of Chemistry, Btrkbeck College, Malet Street, London WCIE 7HX, UK
Received 8 September 1986 The excitation, electronic absorption and luminescence spectra of cubic CQN~HOBI~ have been recorded at temperatures down to that of liquid helium. The detailed spectral analyses enable comparisons to be made of the crystal-field splittings of Russell-Saunders terms with those in C~NakIoCle. Under intense 647.1 mn laser excitation, luminescence is observed in the neat material ln the spectral region between 17800 and 21750 cm-l.
1. Introduction From a rationalisation [ 11 of the electronic spectra of HoCli- we have been able to assign the energy levels below 24000 cm-l for the Ho3+ ion in octahedral symmetry. The luminescence and absorption spectra of this system are more complex than for other crystals containing Ho3+ at a tripositive cation site since the electronic transitions are largely vibronic in character. This allows a greater degree of crosschecking for the accuracy of the derived energy level scheme which is more reliable than in other studies of Ho3+ (in a lower symmetry environment) where a larger number of crystal-field components are assigned on the basis of agreement with energy level calculations. The aim of this study is to provide energy level data so that a comparison of the crystal-field parameters may be made for the chloro- and bromosubstituted hohnium(II1) cation. The analysis of the electronic spectra of HoBrz- is analogous to that for the chloro-species, for which extensive tabulations have been given. In view of this and the large amount of data, we do not present the results of this study in detail. We note that in some cases several alternative assignments were made for spectral features of Ho@ but the different electronic energy levels and vibrational force field in HoBri- permit the major inten’ Present address: Department of Analytical and Biological Chemistry, Kingston Polytechnic, Kingston upon Thames, Surrey KTl2EE, UK.
116
sity source to be identified. The electronic spectra of Cs2NaHoBrg recorded in this study are of poorer quality than those for Cs2NaHoCle. This is true for several other bromoelpasolites that have been investigated and not only reflects the difficulty in preparing pure samples but is also due to the smaller spacing between energy levels and the congestion of vibronic structure in the spectra of the bromo-substituted anion. Structural data are available for a large number of hexabromoelpasolites [2].
2. Experimental The preparation of hexabromoelpasolites has been described [3]. 457 mu and 488 nm excited luminescence spectra, absorption spectra and xenon lamp excitation spectra were recorded using the apparatus at Birkbeck College. Cs2NaHoBrg is not excited by 632.8 nm He-Ne laser radiation and additional experiments utilised the 647.1 nm Kr+ line and the laser Raman system at Imperial College. The excitation spectra are similar to, but more clearly resolved than the absorption spectra except that the relative intensities of different groups of bands are changed. In particular, the structure of 5S2, SF, + ‘Ia is strong in the 514 + ‘Ia but weak in the SF, + 51a excitation spectrum as expected from ion pair interactions [4]. The infrared and electronic absorption spectra of the Cs2NaYBre crystal, employed as the host for Ho3+, showed a large number of bands not due to transi0 009-26 14/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume132,number2
CHEMICALPHYSICSLETTERS
5 December1986
3. Results and discussion
3.1. Absorption and excitation spectra
20:o
wareerberh~cw~~
2d.2
20.2
20:4
Fig. 1.457 nm excited 15 K luminescence. spectrumof HoBri- between 20500 an3d_19!%0 cm-’ in (a) C~NtioBr6 and (b) Cs$!aY& :HoBr,j .
The energy levels of Ho3+ in Cs2NaHoBr6 located by excitation and absorption spectroscopy are listed in table 1. With the exception of the 5 I7 f 518 mag neticdipole-allowed transition the assignments are deduced from vibronic analyses, analogous to those for Cs2NaHoC&, The maxima for 71u moiety mode vibrations are observed near 77,87 cm-l (S7 TO and LO) and 170,175,192 cm-l (S6 ZB, TO and LO). Slo is observed near 55 cm-l. Besides the S, , Sg and ZB acoustic modes at lower energy and,the S8 lattice mode near 154 cm-l these vibrations account for nearly all of the intensity in the vibronic sidebands of electronic transitions. Very weak progressions on the vibronic origins in the Sl (CQ) moiety mode are detected in the 5F5 + 518 luminescence transition, the derived stretching mode wavenumber being 174 cm-l. The value of 209 cm-l was deduced from a band attributed to (3Po)I’l + Sl + S, + rl(3H4) in Cs2NaYBr6 : PrBri- but this very weak feature may be spurious [5]. 3.2. Luminescence spectra
tions in octahedral symmetry. Under 457 nm excitation, the 5F3 + 518, 516 and 51, + 518 luminescence of the sample of Cs,NaYBr, : HoBrz- was similar to but more clearly resolved than the corresponding transitions in Cs2NaHoBrg (see fig. 1). A large number of additional bands, due to emission from Ho3+ at noncentrosymmetric defect sites, were observed in the former material under 488 mn excitation, and also under 457 nm excitation in the regions where the 5S2 + 517, 518 and SF, + 518 transitions were expected. In samples of Cs2NaHoBr6 prepared in this study, very weak bands were observed in the luminescenee spectrum (below 12600 cm-l) due to the 3H4 + 3H6 transition of TmBri- [3]. However a more careful analysis shows that the weak features in the 20K spectrum between 17900 and 18350 cm-l are due to intrinsic emission and not to the 4S3/2 + 411512transition of ErBrz- as previously stated [3]. It is interesting that luminescence from traces of Er3+ is observed in Cs2NaHoC16 [l] but riot in Cs2NaHoBr6.
In the visible spectral region at or above liquid nitrogen temperature the SF, + 518 transitionis strongly observed in Cs2NaHoBr6 under 457 run excitation and emission from 5F3, although very weak, is stronger than in th&chloro-substituted compound. For the cross-relaxation 5F3-5F5, the energy gap is nearly the same in Cs2NaHoCk and Cs2NaHoBr6 but more closely matches the 518-517 separation in the former compound. The energy levels of Cs2NaHoBrg deduced from luminescence are listed in table 1 and selected spectra are shown in figs. l-3. Fromananalysisofthe517,515,514,5F5,5S2, 5F3 + 518 transitions all of the crystal-field components of the 518 electronic ground state are reliably located. The 517 levels were determined from the MD structure of the 517 + 518 transition (fig. 2) and the largely vibronic SF5 + 51, transition. The 516 levels were assigned from analyses of 5S2, 5F3, SF5 + 516. The highest energy bands of 5F3 + 516 are sharp, medium intensity and coincident with the inferred locations of the r2 + ar3, r5 origins. Only r2 -f 117
CHEMICAL PHYSICS LETTERS
Volume 132, number 2
5 December 1986
Table 1 Energy levels of HoBr$ - in Cb NaHoBrg Term
Crystal-field level
Energy (cm-‘) from absorption and excitation measurements
in C%NaHoBra from luminescence measurements
0 9 34 -
‘I7
ar4 ar5 r2
brs
-
r3 br4
-
r3
ar5
r2 br5 r4 rl %
br4
-
rl r4 r3
-
rs ar4
h r3
br4 %
-
ar4
r5 r3
%
15096*5
15334 15371 15488 15505
r4
rl 5F3
r2
rs r4
118
Cg NaHoCb a)
0 8 3152 176 210 238 238
0 8 36 176 210 238 238
0 10 39 199 239 268 268
5099&S
10
0 3 93 106 125 152
5182 5199 5213 5240 8608 8613 8636 8682 8704 (8722) b) 11181 11206*3 11239 11252+2 13224 13231 13237 (13416) 15334 15372k2 15502
83 100 114 141 0 5 28 74 (l& 0 25*3 58 71i2 0 I
(1;:) 0 38 154 170
0 8 47 85 106 124 0 32 59 15 0 9 15 217 0 43 174 193 0 21
I -
-
0 14 19 53
20404 20527 (20578)
20404 20520 20578
rs r3
1
CszNaHoBy
18346 (18372)
r3
r5 3F4
CrystaI-field splitting (cm-‘)
18364+2 -
0 119*4 174
0 132 189
CHEMICAL PHYSICS LETTERS
Volume 132, number 2
5 December 1986
Table 1 (continued) Term
Crystal-field level
Energy (cm-i) from absorption and excitation measurements
5F*
rS r3
3Ka
aPs ar3 brs ar4
Pl bP4 br3
SGe
rl r4 ar5
P2 bP5 r3
a) From ref. [ 11.
Crystal-field splitting (cm-‘)
in CszNaHoBre from luminescence measurements
20954 (21046)
CszNaHoCle a)
CszNtioBy
-
(9;)
-
-
-
-
-
0 105 -
21223 21260 21316 (21350) (21354)
-
(13;) (94) (38) (4) (0)
132 89 48 4 0
(21767) (21808) (21866) (22042) (22132) (22160)
-
(4:) (99) (275) (365) (393)
(4:) 96 265 3.53 380
b) Tentative or inaccurate values are in parentheses.
ar5 is observed in Cs2NaHoClg so that the highest energy band is either due to a defect site or to a distortion of the chromophore from octahedral symmetry. Two features are observed with similar relative intensity and at these wavenumbers in Cs2NaYBr6 : HoBri- under 457 mn excitation but a further’two
lsl0
I
wavenumber/io3cm~1
15L
wavenumber/ioJsm~l
Fig. 2.488 nm excited 5I7 + s Ia luminescence spectrum of C$NaHoBre at (a) 300 K and (b) 85 K.
126
Fig. 3. (a) 5Fs + 5Ia 15 K luminescenceand 25 K excitation spectra of CQNaHoBra.
(b) ‘Fs + ‘Ia
119
Volume 132, number 2
CHEMICAL PHYSICS LETTERS
bands due to Ho3+ at defect sites are also observed to higher energy under 488 mn excitation. The only other observation of additional bands near the expected locations of electronic origins in the luminescence of Cs2NaHoBr6 occurs for the (5F5)aI’4 --f al?,, aP4(‘18) transitions (fig. 3a). For the nexthighest Russell-Saunders term of Cs2NaHoBr6, the lowest level is assigned from the 20 K 515 --, 518 spectrum and the other three levels are assigned from hot bands in the 85 K spectrum. In the 20 K 5F3 -f 515 spectrum the strongest band (at 9201 cm-l) corresponds to the I’2 -+ I’5 MD origin, confirming the location of (515)I’5. The P2 -, $4 origin is inferred from the vibronic analysis to be at 22 cm-l to higher energy. The analysis Of the 514 k 518 transition, observed between 13200 and 12800 cm-l in CS2NaHOBr6 at 20 K, is similar to that of the chloro-compound. In addition to transitions from the lowest (rl) level, those from r3 terminating at vibronic levels of bI’, , bI’, and from r, to vibronic levels of brd, d’, , brg, br, (51s) permit the location of the p14)r3, r, levels. Several bands remain unassigned in the analysis and are due to the overlapping (5S2) r3 * (517) ar,, ar5+S10, S7, S6 and r3 + br4 t S, transitions, these features being most prominent in the luminescence of Cs2NaGdCle : HOC@ in this region. Studies of the 5F5, 5F3, 5S2 + 518 transitions at various temperatures give energies for crystal-field levels of the excited terms in agreement with absorption spectroscopy.
5 December 1986
ms) agrees with that calculated from the decay of 5F3 and 5F5. The lifetime of 515 was 11-16 ms between 300 and 85 K. The decay of luminescence in Cs,NaYBr, : HoBri- is considerably longer - 1 .O (3.3), 66(102) and 4.4(14) ms being measured for the lifetimes of the 5F3, 515 and 5F5 levels at 300 (85) K. Decay measurements were also carried out for samples of Cs2ZrBr6 : HoB$ but no emission from Ho3+ at a site of octahedral symmetry was identified in this material at the range of concentrations studied. Under intense 647.1 run Kr+ laser excitation near 120 K (i.e. into the (5F5)I’5 + S, vibronic origin) upconversion was observed involving luminescence from the 5F,, 5S2 and 5F, terms in Cs2NaHoBr6. The absence of concentration quenching in the luminescence from these terms is attributed to the excitation of Ho3+ neighbours from the electronic ground state. A further group of bands was observed between 21300 and 2 1750 cm-l but the assignment to the vibronic sideband of (5G6) rl, r, + 518 requires further investigation.
Acknowledgement I would like to thank Professor CD. Flint for allowing me to use the apparatus at Birkbeck College and Mr. N. Campbell for assistance in recording the 647.1 nm excited luminescence spectrum at Imperial College.
3.3. Luminescence decay measurements and upconversion
References The decay lifetimes for levels in Cs2NaHoBr6 were measured to be shorter than in Cs,NaHoC16, due to the poorer‘quality of the crystals. The long-term (e-l) lifetime of 5F5 was between 0.6 and 1.l ms from room temperature to 30 K. The 20 K lifetime of 5F, was 0.08 ms and the measured risetime of 5F5 (near 0.2
120
[l] P.A.Tanner, to be published. [2] G. Meyer, Progr. Solid State Chem. 14 (1982) 141. [3] P.A. Tanner, J. Chem. Sot. Faraday Trans. II 81 (1985) 1285. [4] P.A. Tanner, Chem. Phys. Letters 126 (1986) 137. [5] P.A. Tanner, Mol. Phys. 57 (1986) 697.
Volume 132, number 2
ENERGY LEVELS
5 December 1986
OF HoBr,3-
Peter A. TANNER’ Department
of Chemistry, Btrkbeck College, Malet Street, London WCIE 7HX, UK
Received 8 September 1986 The excitation, electronic absorption and luminescence spectra of cubic CQN~HOBI~ have been recorded at temperatures down to that of liquid helium. The detailed spectral analyses enable comparisons to be made of the crystal-field splittings of Russell-Saunders terms with those in C~NakIoCle. Under intense 647.1 mn laser excitation, luminescence is observed in the neat material ln the spectral region between 17800 and 21750 cm-l.
1. Introduction From a rationalisation [ 11 of the electronic spectra of HoCli- we have been able to assign the energy levels below 24000 cm-l for the Ho3+ ion in octahedral symmetry. The luminescence and absorption spectra of this system are more complex than for other crystals containing Ho3+ at a tripositive cation site since the electronic transitions are largely vibronic in character. This allows a greater degree of crosschecking for the accuracy of the derived energy level scheme which is more reliable than in other studies of Ho3+ (in a lower symmetry environment) where a larger number of crystal-field components are assigned on the basis of agreement with energy level calculations. The aim of this study is to provide energy level data so that a comparison of the crystal-field parameters may be made for the chloro- and bromosubstituted hohnium(II1) cation. The analysis of the electronic spectra of HoBrz- is analogous to that for the chloro-species, for which extensive tabulations have been given. In view of this and the large amount of data, we do not present the results of this study in detail. We note that in some cases several alternative assignments were made for spectral features of Ho@ but the different electronic energy levels and vibrational force field in HoBri- permit the major inten’ Present address: Department of Analytical and Biological Chemistry, Kingston Polytechnic, Kingston upon Thames, Surrey KTl2EE, UK.
116
sity source to be identified. The electronic spectra of Cs2NaHoBrg recorded in this study are of poorer quality than those for Cs2NaHoCle. This is true for several other bromoelpasolites that have been investigated and not only reflects the difficulty in preparing pure samples but is also due to the smaller spacing between energy levels and the congestion of vibronic structure in the spectra of the bromo-substituted anion. Structural data are available for a large number of hexabromoelpasolites [2].
2. Experimental The preparation of hexabromoelpasolites has been described [3]. 457 mu and 488 nm excited luminescence spectra, absorption spectra and xenon lamp excitation spectra were recorded using the apparatus at Birkbeck College. Cs2NaHoBrg is not excited by 632.8 nm He-Ne laser radiation and additional experiments utilised the 647.1 nm Kr+ line and the laser Raman system at Imperial College. The excitation spectra are similar to, but more clearly resolved than the absorption spectra except that the relative intensities of different groups of bands are changed. In particular, the structure of 5S2, SF, + ‘Ia is strong in the 514 + ‘Ia but weak in the SF, + 51a excitation spectrum as expected from ion pair interactions [4]. The infrared and electronic absorption spectra of the Cs2NaYBre crystal, employed as the host for Ho3+, showed a large number of bands not due to transi0 009-26 14/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume132,number2
CHEMICALPHYSICSLETTERS
5 December1986
3. Results and discussion
3.1. Absorption and excitation spectra
20:o
wareerberh~cw~~
2d.2
20.2
20:4
Fig. 1.457 nm excited 15 K luminescence. spectrumof HoBri- between 20500 an3d_19!%0 cm-’ in (a) C~NtioBr6 and (b) Cs$!aY& :HoBr,j .
The energy levels of Ho3+ in Cs2NaHoBr6 located by excitation and absorption spectroscopy are listed in table 1. With the exception of the 5 I7 f 518 mag neticdipole-allowed transition the assignments are deduced from vibronic analyses, analogous to those for Cs2NaHoC&, The maxima for 71u moiety mode vibrations are observed near 77,87 cm-l (S7 TO and LO) and 170,175,192 cm-l (S6 ZB, TO and LO). Slo is observed near 55 cm-l. Besides the S, , Sg and ZB acoustic modes at lower energy and,the S8 lattice mode near 154 cm-l these vibrations account for nearly all of the intensity in the vibronic sidebands of electronic transitions. Very weak progressions on the vibronic origins in the Sl (CQ) moiety mode are detected in the 5F5 + 518 luminescence transition, the derived stretching mode wavenumber being 174 cm-l. The value of 209 cm-l was deduced from a band attributed to (3Po)I’l + Sl + S, + rl(3H4) in Cs2NaYBr6 : PrBri- but this very weak feature may be spurious [5]. 3.2. Luminescence spectra
tions in octahedral symmetry. Under 457 nm excitation, the 5F3 + 518, 516 and 51, + 518 luminescence of the sample of Cs,NaYBr, : HoBrz- was similar to but more clearly resolved than the corresponding transitions in Cs2NaHoBrg (see fig. 1). A large number of additional bands, due to emission from Ho3+ at noncentrosymmetric defect sites, were observed in the former material under 488 mn excitation, and also under 457 nm excitation in the regions where the 5S2 + 517, 518 and SF, + 518 transitions were expected. In samples of Cs2NaHoBr6 prepared in this study, very weak bands were observed in the luminescenee spectrum (below 12600 cm-l) due to the 3H4 + 3H6 transition of TmBri- [3]. However a more careful analysis shows that the weak features in the 20K spectrum between 17900 and 18350 cm-l are due to intrinsic emission and not to the 4S3/2 + 411512transition of ErBrz- as previously stated [3]. It is interesting that luminescence from traces of Er3+ is observed in Cs2NaHoC16 [l] but riot in Cs2NaHoBr6.
In the visible spectral region at or above liquid nitrogen temperature the SF, + 518 transitionis strongly observed in Cs2NaHoBr6 under 457 run excitation and emission from 5F3, although very weak, is stronger than in th&chloro-substituted compound. For the cross-relaxation 5F3-5F5, the energy gap is nearly the same in Cs2NaHoCk and Cs2NaHoBr6 but more closely matches the 518-517 separation in the former compound. The energy levels of Cs2NaHoBrg deduced from luminescence are listed in table 1 and selected spectra are shown in figs. l-3. Fromananalysisofthe517,515,514,5F5,5S2, 5F3 + 518 transitions all of the crystal-field components of the 518 electronic ground state are reliably located. The 517 levels were determined from the MD structure of the 517 + 518 transition (fig. 2) and the largely vibronic SF5 + 51, transition. The 516 levels were assigned from analyses of 5S2, 5F3, SF5 + 516. The highest energy bands of 5F3 + 516 are sharp, medium intensity and coincident with the inferred locations of the r2 + ar3, r5 origins. Only r2 -f 117
CHEMICAL PHYSICS LETTERS
Volume 132, number 2
5 December 1986
Table 1 Energy levels of HoBr$ - in Cb NaHoBrg Term
Crystal-field level
Energy (cm-‘) from absorption and excitation measurements
in C%NaHoBra from luminescence measurements
0 9 34 -
‘I7
ar4 ar5 r2
brs
-
r3 br4
-
r3
ar5
r2 br5 r4 rl %
br4
-
rl r4 r3
-
rs ar4
h r3
br4 %
-
ar4
r5 r3
%
15096*5
15334 15371 15488 15505
r4
rl 5F3
r2
rs r4
118
Cg NaHoCb a)
0 8 3152 176 210 238 238
0 8 36 176 210 238 238
0 10 39 199 239 268 268
5099&S
10
0 3 93 106 125 152
5182 5199 5213 5240 8608 8613 8636 8682 8704 (8722) b) 11181 11206*3 11239 11252+2 13224 13231 13237 (13416) 15334 15372k2 15502
83 100 114 141 0 5 28 74 (l& 0 25*3 58 71i2 0 I
(1;:) 0 38 154 170
0 8 47 85 106 124 0 32 59 15 0 9 15 217 0 43 174 193 0 21
I -
-
0 14 19 53
20404 20527 (20578)
20404 20520 20578
rs r3
1
CszNaHoBy
18346 (18372)
r3
r5 3F4
CrystaI-field splitting (cm-‘)
18364+2 -
0 119*4 174
0 132 189
CHEMICAL PHYSICS LETTERS
Volume 132, number 2
5 December 1986
Table 1 (continued) Term
Crystal-field level
Energy (cm-i) from absorption and excitation measurements
5F*
rS r3
3Ka
aPs ar3 brs ar4
Pl bP4 br3
SGe
rl r4 ar5
P2 bP5 r3
a) From ref. [ 11.
Crystal-field splitting (cm-‘)
in CszNaHoBre from luminescence measurements
20954 (21046)
CszNaHoCle a)
CszNtioBy
-
(9;)
-
-
-
-
-
0 105 -
21223 21260 21316 (21350) (21354)
-
(13;) (94) (38) (4) (0)
132 89 48 4 0
(21767) (21808) (21866) (22042) (22132) (22160)
-
(4:) (99) (275) (365) (393)
(4:) 96 265 3.53 380
b) Tentative or inaccurate values are in parentheses.
ar5 is observed in Cs2NaHoClg so that the highest energy band is either due to a defect site or to a distortion of the chromophore from octahedral symmetry. Two features are observed with similar relative intensity and at these wavenumbers in Cs2NaYBr6 : HoBri- under 457 mn excitation but a further’two
lsl0
I
wavenumber/io3cm~1
15L
wavenumber/ioJsm~l
Fig. 2.488 nm excited 5I7 + s Ia luminescence spectrum of C$NaHoBre at (a) 300 K and (b) 85 K.
126
Fig. 3. (a) 5Fs + 5Ia 15 K luminescenceand 25 K excitation spectra of CQNaHoBra.
(b) ‘Fs + ‘Ia
119
Volume 132, number 2
CHEMICAL PHYSICS LETTERS
bands due to Ho3+ at defect sites are also observed to higher energy under 488 mn excitation. The only other observation of additional bands near the expected locations of electronic origins in the luminescence of Cs2NaHoBr6 occurs for the (5F5)aI’4 --f al?,, aP4(‘18) transitions (fig. 3a). For the nexthighest Russell-Saunders term of Cs2NaHoBr6, the lowest level is assigned from the 20 K 515 --, 518 spectrum and the other three levels are assigned from hot bands in the 85 K spectrum. In the 20 K 5F3 -f 515 spectrum the strongest band (at 9201 cm-l) corresponds to the I’2 -+ I’5 MD origin, confirming the location of (515)I’5. The P2 -, $4 origin is inferred from the vibronic analysis to be at 22 cm-l to higher energy. The analysis Of the 514 k 518 transition, observed between 13200 and 12800 cm-l in CS2NaHOBr6 at 20 K, is similar to that of the chloro-compound. In addition to transitions from the lowest (rl) level, those from r3 terminating at vibronic levels of bI’, , bI’, and from r, to vibronic levels of brd, d’, , brg, br, (51s) permit the location of the p14)r3, r, levels. Several bands remain unassigned in the analysis and are due to the overlapping (5S2) r3 * (517) ar,, ar5+S10, S7, S6 and r3 + br4 t S, transitions, these features being most prominent in the luminescence of Cs2NaGdCle : HOC@ in this region. Studies of the 5F5, 5F3, 5S2 + 518 transitions at various temperatures give energies for crystal-field levels of the excited terms in agreement with absorption spectroscopy.
5 December 1986
ms) agrees with that calculated from the decay of 5F3 and 5F5. The lifetime of 515 was 11-16 ms between 300 and 85 K. The decay of luminescence in Cs,NaYBr, : HoBri- is considerably longer - 1 .O (3.3), 66(102) and 4.4(14) ms being measured for the lifetimes of the 5F3, 515 and 5F5 levels at 300 (85) K. Decay measurements were also carried out for samples of Cs2ZrBr6 : HoB$ but no emission from Ho3+ at a site of octahedral symmetry was identified in this material at the range of concentrations studied. Under intense 647.1 run Kr+ laser excitation near 120 K (i.e. into the (5F5)I’5 + S, vibronic origin) upconversion was observed involving luminescence from the 5F,, 5S2 and 5F, terms in Cs2NaHoBr6. The absence of concentration quenching in the luminescence from these terms is attributed to the excitation of Ho3+ neighbours from the electronic ground state. A further group of bands was observed between 21300 and 2 1750 cm-l but the assignment to the vibronic sideband of (5G6) rl, r, + 518 requires further investigation.
Acknowledgement I would like to thank Professor CD. Flint for allowing me to use the apparatus at Birkbeck College and Mr. N. Campbell for assistance in recording the 647.1 nm excited luminescence spectrum at Imperial College.
3.3. Luminescence decay measurements and upconversion
References The decay lifetimes for levels in Cs2NaHoBr6 were measured to be shorter than in Cs,NaHoC16, due to the poorer‘quality of the crystals. The long-term (e-l) lifetime of 5F5 was between 0.6 and 1.l ms from room temperature to 30 K. The 20 K lifetime of 5F, was 0.08 ms and the measured risetime of 5F5 (near 0.2
120
[l] P.A.Tanner, to be published. [2] G. Meyer, Progr. Solid State Chem. 14 (1982) 141. [3] P.A. Tanner, J. Chem. Sot. Faraday Trans. II 81 (1985) 1285. [4] P.A. Tanner, Chem. Phys. Letters 126 (1986) 137. [5] P.A. Tanner, Mol. Phys. 57 (1986) 697.