Laser site selective excitation of KY3F10-doped with samarium

Journal of Luminescence 85 (1999) 91}102

Laser site selective excitation of KY F -doped with samarium 3 10 J.-P.R. Wells*, A. Sugiyama1, T.P.J. Han, H.G. Gallagher Optical Materials Research Centre, Department of Physics and Applied Physics, University of Strathclyde, Glasgow, G1 1XN, Scotland, UK Received 1 March 1999; received in revised form 12 July 1999; accepted 12 July 1999

Abstract Laser selective excitation has identi"ed three Sm2` centres in KY3 F10 . Of these, two exhibit strong up-conversion #uorescence due to sequential absorption from the metastable 5D0 multiplet and high lying 4f55d states. The observation of Sm2` in KY3 F10 appears to provide further evidence for the presence of F~ vacancies as proposed by previous workers. The expected Sm3` C47 symmetry centre is also observed. A crystal-"eld analysis of the 46 measured electronic energy levels of this centre, yields parameters consistent with those of other trivalent rare-earth ions in this material. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser selective excitation; Sm2`; Sm3`; Up-conversion #uorescence; Crystal-"eld analysis

1. Introduction Crystals that o!er isovalent substitution for trivalent rare-earth ions have received much attention by spectroscopists and laser physicists. In part at least, this is motivated by the lack of complexity associated with the need for charge compensation. Of these, crystals of the KF}YF system are 3 a popular choice for their wide transparency, high optical damage threshold and rigid thermomechanical properties [1]. KY F is a cubic compound with space group 3 10 O5 (Fm3m). Trivalent rare-earth ions substitute ) for the Y3` ion and thus reside on a site of C 47 * Corresponding author. Current address: FOM institute for plasmaphysics &Rijnhuizen', FELIX free electron laser facility, Edisonbaan 14, P.O. Box 1207, 3430 BE Nieuwegein, Netherlands. Tel.: #31-30-609-6894; fax: #31-30-603-1204. 1 Permanent Address. Advanced Photon Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Nakagun, Ibaraki-ken 319-11, Japan. E-mail address: [email protected] (J.-P.R. Wells)

point group symmetry. An extremely comprehensive study of KY F : Eu3` [2}4] established 3 10 crystal-"eld parameters, analysed the intra-4f transition moments and used this information to study non-radiative decay within the higher 5D term multiplets. This early study demonstrated the importance of accounting for crystal-"eld Jmixing when analysing rare-earth ion spectra and quantitatively accounted for both the radiative and non-radiative relaxation processes of the lower lying multiplets of Eu3` in KY F . 3 10 We report the "rst laser selective excitation study of KY F : Sm known to the authors. This mate3 10 rial is interesting as both Sm2` and Sm3` are present in the crystal, despite substitution for trivalent yttrium. This appears to be attributable to signi"cant F~ vacancies within the host crystal lattice. Laser selective excitation has identi"ed three Sm2` sites in KY F , corresponding to distinct Sm2`3 10 F~(v) con"gurations. Excitation of the intra-4f, 7F P5D transitions yields up-conversion #u0 0 orescence (via sequential absorption) from the

0022-2313/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 1 5 0 - 7

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higher lying 5D multiplets. Laser excitation and J #uorescence spectroscopy of the single Sm3` centre present in KY F identi"es 46 levels of this centre. 3 10 This energy level structure can be well accounted for by a single-electron crystal-"eld analysis which assumes C symmetry. 47 2. Experimental The KY F : Sm crystals were grown by the 3 10 Bridgman}Stockbarger technique, using a custom built, two-zone resistance heating furnace. YF and 3 KF were mixed together in essentially stoichometric amounts, taking into account the small amounts (0.1 mol%) of SmF to be added. The mixed charge 3 was then placed in a graphite crucible which was itself put into the furnace vaccum chamber. This was then evacuated to 10~6 mbar so as to be free of oxygen. Ultimately, a slight positive pressure of puri"ed argon gas was employed as the growth atmosphere to minimise evaporative losses. The crystals were lowered at a rate of 0.8 mm/h through the temperature gradient provided by the resistance coils of the furnace. After translating 10 cm, the crystals were cooled over a period of 24 h. The as-grown crystal boules were up to 5 cm long and of an excellent optical quality with a uniform yellowish colouration due to the 4f6P4f55d transitions of Sm2`. Absorption spectra were recorded with an AVIV associates 14DS double beam spectrometer with crystals cooled to a base temperature of 10 K using a Leybold cryogenic refridgeration unit. Laser excitation and #uorescence spectra were recorded using a 5W Coherent Innova 70 argon ion laser to optically pump a Spectra-Physics 375B dye laser. DCM dye was used to excite the 7F P5D and 0 0 5D transitions of Sm2`. The sample was cooled by 1 a CTI-cryogenics model 22C cryogenic refridgerator and temperature variability was maintained by a Palm Beach Cryogenics temperature controller. Fluorescence was dispersed by a SPEX 500M single monochromator with the light detected using a thermoelectrically cooled Hamamatsu R9249 photomultiplier. Fluorescence lifetimes were measured using a Laser Science Inc. VSL-337 nitrogen laser pumped dye laser. The transient was averaged

on a Stanford Research Systems model SR430 multichannel averager. The integrated transients were least-squares "tted to single exponential decays on a constant background for the #uorescence decay times.

3. Optical absorption and laser selective excitation of Sm2` Divalent samarium is isoelectronic with Eu3` and thus the 4f6 con"guration is appropriate. This con"guration has 3003 non-degenerate energy levels, of which the levels of the 5D and 7F terms are most J J readily accessible by dye laser excitation and #uorescence. As the energy levels of the 4f55d con"guration are comparatively low lying these must also be considered. The 4f55d levels arise from the interaction of the 5d electron with the 4f5 core [5]. 3.1. Optical absorption Fig. 1 shows optical absorption for a 0.8 mm thick slice of KY F : 0.1%Sm cooled to 10 K. No 3 10 signi"cant absorption features are apparent below

Fig. 1. 10 K optical absorption of KY F : Sm. 3 10

J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

20 000 cm~. Above 20 000 cm~1, a series of comparatively broad features are observed. Due to the linewidth of these transitions, it would be reasonable to suggest that these features are not associated with the Sm3` content of the crystal. Instead, they are the 4f6(7F )P4f55d intercon0 "gurational transitions of Sm2` ions. As these transitions are parity allowed by the electric dipole mechanism, they are strong and because they absorb in the blue, they give the crystal a yellowish tinge. The observed spectra are a superposition of the splittings of the 4f5(6H ) and 5d1 states. Under the J in#uence of a tetragonal crystal-"eld, the 5d1 con"guration splits into 3 singlet states, 2B , 2A and 1' 1' 2B , and the doublet 2E [6]. This gives rise to 2' ' considerable complexity before we even consider that there is more than one crystal-"eld potential for the Sm2` ions to reside in (Section 2). Thus, it proves non-trivial to assign gaps in the observed absorption spectrum based on the spin}orbit splittings on the 4f5 con"guration ground term, 6H. 3.2. Laser site selective excitation and yuorescence Laser site selective excitation is a powerful tool which exploits the intensity and monochromicity of tunable, coherent laser radiation. Under DCM dye laser excitation of the 14 500}16 200 cm~1 region (detecting #uorescence in zero order of di!raction with appropriate "ltering of the exciting laser light) two distinct sets of peaks attributable to intra-4f6 transitions can be observed. These are assigned as the 7F P5D and 5D transitions. 0 0 1 Fig. 2 presents laser selective excitation spectra selectively monitoring the #uorescence of three distinct Sm2` centres. Fig. 2(b) and (c) show the recorded spectra for monitoring at frequencies of 14 455 and 14 458 cm~1, respectively. For these centres, the 5D state is located at 14 731 and 0 14 693 cm~1 as appropriate. In each case, only one of the 5D states could be located. For resonant 1 excitation of either the 14 455 or 14 458 cm~1 transition, the crystal is seen to glow bright turquoise, as a consequence of sequential absorption processes. An additional centre is also present. This centre, whose excitation spectrum is shown in Fig. 2(d) (for monitoring #uorescence at 14 428 cm~1) has low lying 4f55d bands which are

93

observed from 15 950 cm~1 upwards. Due to the low lying 4f55d states of this centre, any up-conversion of the optical excitation will decay non-radiatively to the lowest lying metastable state, in this case the 4f6(5D ) state, and no up-converted #u0 orescence will be observed. The 4f6(5D ) multiplet 0 of this centre is at an energy of 14 677 cm~1. 3.2.1. The A centre The most intense excitation features of Sm2` are at 14 731 and 16 084 cm~1. These are transitions to the 5D and 5D multiplet of the same centre 0 1 which we arbitrarily label &A'. For excitation of either of these transitions, a system of 16 #uorescence transitions is recorded as emanating from the 5D state, as shown in Fig. 3(p}t). Fig. 3(a}o) 0 shown up-converted #uorescence spectra obtained for excitation of the 7F P5D transition. The 0 0 observed #uorescence emanates from the 5D , 5D 3 2 and 5D multiplets and con"rms the 7F energy 1 J eigenstates inferred from the 5D #uorescence (see 0 Table 1). The up-conversion process is attributed to sequential absorption from 5D as no other popu0 lated states are available to act as an intermediate level. This is analogous to that observed for Eu3` in the alkaline earth #uorides [7]. From this #uorescence, it is noted that the full degeneracy of the 7F multiplets does not appear to J have been removed as less than 2J#1 transitions are observed. This would appear to indicate that this centre has at least axial symmetry, although no obvious selection rules are operational. 3.2.2. The B centre The #uorescence transitions detected when exciting the 14 693 cm~1, 7F P5D transition of the B 0 0 centre are "ve times less intense than those of the A centre. This most probably re#ects the relative population of Sm2` ions in each centre. As with the A centre, up-conversion #uorescence is observed for excitation of the metastable 5D state (Fig. 4). 0 In this case, no emission could be detected from the 5D multiplet to the limits of the detection sensitiv3 ity and excitation power available in this study. The highest frequency transition is observed at 18 018 cm~1 (assigned as the 5D (C )P7F (Z ) 2 3 0 1 transition). An explanation for this maybe that the lowest states of the 4f55d excited con"guration lie

94

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Fig. 2. 10 K, laser excitation spectra of the 5D and 5D multiplets for KY F : Sm2`. (a) Monitoring #uorescence detecting with the 0 1 3 10 spectrometer in zero order of di!raction, (b) the A centre, monitoring at 14 455 cm~1, (c) the B centre, monitoring #uorescence at 14 458 cm~1 and (d) the C centre, monitoring #uorescence at 14 428 cm~1.

below the 5D3 multiplet due to a greater crystal"eld splitting of the 4f55d con"guration. As a consequence of the greater electron}phonon coupling for electrons in the 5d con"guration, excitation into these states will relax to 5D via e$cient non2 radiative decay processes. It is noticeable that the 7F crystal-"eld splittings of the B centre are larger J

than the A centre splittings which seems to corroborate this explanation. 3.2.3. The C centre An additional centre is observable for which one cannot be selective when the frequency of the exciting laser is above 16 000 cm~1. The emission

J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

95

Fig. 3. 10 K, #uorescence transitions of the A centre, 5D to the (a) 7F , (b) 7F , (c) 7F , (d) 7F and (e) 7F multiplets. (f}j) As above 3 0 1 2 3 4 except emanating from 5D , (k}o) as above except emanating from 5D , (p}t) as above except emanating from 5D . The notation of 2 1 0 a number is used to indicate the terminal state of the transition, whilst an underline is used to denote a transitions from the "rst excited state of a multiplet and d is used to denote a transition from the second excited state of a multiplet.

spectrum obtained exciting the sample at a laser frequency of 16 150 cm~1 is shown in Fig. 5. The intensity of this #uorescence is over 100 times less than that of the A centre. No up-conversion #uorescence is apparent for this centre, under 14 677 cm~1 excitation of the 7F P5D transition. The broad 0 0 bands observed in Fig. 2(d) are thus assigned as the 4f 6P4f 55d transitions of this centre. 3.3. Discussion The observation of Sm2` in KY F is in itself 3 10 surprising as considerations of ionic size alone would indicate the samarium should substitute for Y3`. Indeed, this is the case in LiYF : Sm where 4

only the trivalent species is present [8]. It is possible to explain this phenomenon from the results of Ref. [9] where measurements of the electricial conductivity for KY F are shown to reach values 3 10 which are typical of ionic conductors. The authors attribute this to the presence of F~ vacancies. Evidence for such vacancies is also available from Raman scattering [10]. Thus, the three centres observed could be attributed to di!erent arrangements of the charge compensating F~ vacancies around the Sm2`, which itself occupies an Y3` site as required by considerations of ionic size. An additional possibility is that the samarium substitutes for the larger monovalent cation K` although any determination of this is not possible in the current

96

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Table 1 Crystal-"eld levels of the 7F and 5D multiplets of the A and J J B centres in KY F : Sm2`. The state energies are in wave 3 10 numbers (as measured in air, $1) Multiplet

State

A

B

7F 0

Z 1

0

0

7F 1

Y 1 Y 2

274 293

232 275

7F 2

X 1 X 2 X 3 X 4

794 802 827 844

750 876 891 *

7F 3

W 1 W 2 W 3 W 4 W 5

1472 1477 1483 1501 1518

1458 1498 1521 1526 1576

7F 4

V 1 V 2 V 3 V 4 V 5 V 6 V 7

2184 2234 2255 2302 2314 2317 2344

2234 2275 2305 2327 2339 2351 *

5D 0

A

1

14731

14693

5D 1

B 1 B 2

16084 16094

16025 16076

5D 2

C 1 C 2 C 3

18037 18046 *

17990 18002 18018

5D 3

D 1 D 2 D 3

20359 20369 20372

* * *

work. Considering the three Sm2 centres, the C centre is excited whenever the laser light is greater than 16 000 cm~1 due to the low lying broad 4f55d absorption bands of this centre. However, the population of this centre is relatively low. The two dominant Sm2` centres, appear to have near axial symmetry, as judged from the crystal"eld splitting of the 7F multiplets. This is altoJ gether reasonable assuming substitution on an Y3

site as this provides a C symmetry electrostatic 47 potential and it is possible that an F~ vacancy is only a slight perturbation upon this. It is apparent from these splitttings that the B centre has a greater &non-centrosymmetric' crystal-"eld strength (see the relative 7F and 5D splittings in Table 1) and 1 1 hence greater electric-dipole moment. This is veri"ed by the 5D lifetimes for these centres which 0 have been measured to be 11.9$0.1 ms for the A centre and 4.33$0.1 ms for the B centre. Schematic diagrams for the energy levels of these three centres are shown in Fig. 6. It is clear that a full understanding of the defect distribution for divalent samarium in KY F re3 10 quires signi"cant additional studies. In particular, it would be useful to study the angular dependence of the electronic Zeeman splittings of the transitions attributable to these centres. Determination of the centre symmetries would clearly be of signi"cant assistance in the assignment of their exact con"gurations, which we are unable to achieve with the data available in this work. 4. Spectroscopy of Sm3` in KY3 F10 The 4f5 con"guration, appropriate for trivalent samarium, consists of 1001 two-fold degenerate states. In #uoride and oxide host crystals, all #uorescence is observed from the 4G multiplet 5@2 whose barycentre is near 18 000 cm~1. The lowlying energy levels are those of the 6H and J 6F terms, whose states are considerably admixed J by crystal-"eld J-mixing. As the Sm3` substitutes isovalently for the Y3` in KY F , the point group symmetry of 3 10 the Sm3` centre is C . Thus, the Sm3` wave 47 functions transform as one of the c or c 6 7 irreducible representations of the C double 47 group. The complete labelling scheme of the Sm3` crystal-"eld levels includes the parent 2S`1L mulJ tiplet, and arbitrary alphabetical label following Dieke [11] and, where possible, the irreducible representation of the Sm3` ion wave function. In practice, the wave function symmetries have been assigned from preliminary crystal-"eld calculations using Eu3` calculations [2] and are only listed in Table 2.

J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

97

Fig. 4. 10 K, #uorescence transitions of the B centre, 5D to the (a) 7F , (b) 7F , (c) 7F , (d) 7F and (e) 7F multiplets, 5D to the 2 0 1 2 3 4 1 (f) 7F , (g) 7F , (h) 7F and (i) 7F multiplets. (j}n) As for the 5D transitions except emanating from 5D . 0 1 2 3 2 0

4.1. Laser selective excitation and yuorescence Although the 6H P4G transitions could 5@2 5@2 not be observed by absorption detected in transmission, they can be observed employing the vastly enhanced sensitivity of laser selective excitation (or #uorescence detected absorption) as shown in Fig. 7. For monitoring #uorescence at 16 775 cm~1, an isolated, sharp transition at 17 883 cm~1 is identi"ed as the transition to the lowest electronic state of the 4G multiplet. The second electronic state is 5@2 observed at 17 973 cm~1 and is accompanied by broader features. As a consequence of the fact that only a single site is available for occupancy by a trivalent RE3` ion in KY F , additional centres 3 10

are not expected and at 0.1 mol% pair centres are improbable. Thus, we reasonably assign the structural features observed close to the Z PA 1 2 transition as due to phonon sideband absorption. The highest-energy state of 4G remains un5@2 located. Crystal-"eld calculations predict the position of this state to be close to 18 190 cm~1, however no likely candidates are apparent. Fig. 8 shows #uorescence to the 6H and 6F J J multiplets. At 10 K, all #uorescence is observed to emanate from the 4G (A ) state at 17 883 cm~1. 5@2 1 Transitions to the 6H , 6H , 6H , 6H and 5@2 7@2 9@2 11@2 6H multiplets are very clear with all of the 13@2 expected J#1 levels observed except two. As in 2 most materials, in KY F : Sm3` the 6H , 6F 3 10 15@2 1@2

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J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

and 6F multiplets are superimposed and the 3@2 crystal-"eld states of these multiplets are strongly admixed through crystal-"eld J-mixing. Thus, we denote these states by the label S. Not all of the expected transitions to these states could be observed as many of these transitions are broad and overlapping. Transitions to the 6F multiplet are 7@2 observed amongst strong transitions attributed to Yb3` impurities. The source of the impurities has been traced to the SmF used in the start materials 3 for the growth of the crystals and these impurities have also been identi"ed in LiYF : Sm3` crystals 4 grown in our laboratories [8]. No transitions to the 6F multiplet could be observed. We attribute 11@2 this to weak oscillator strengths of the transitions in question and the limited sensitivity of the germanium detector used in this work. The 4G lifetime has been measured under 5@2 pulsed laser excitation. This is observed to be a single exponential at all times with a decay constant of 4.86$0.10 ms. This value is of a comparable magnitude with the measured value of 10.4 ms for the C symmetry centers in Sm3`-doped 47 #uorite [12].

# + Pkp # + ¹it . (1) k i k/2,4,6 i/2,3,4,6,7,8 In this Hamiltonian, the "rst two interactions represent the electrostatic and spin}orbit interactions which are of the largest magnitude. Whilst the

Fig. 5. 10 K, #uorescence spectrum recorded exciting the 7F P4f55d transitions of the C centre. 0

Fig. 6. Schematic energy level diagram for the A, B and C centres in KY F : Sm2`. 3 10

4.2. Crystal xeld analysis of the C centre spectra 4v The crystal-"eld calculations performed in this work, employ the &f-Shell Empiricial' crystal-"eld "tting routines of Dr Mike Reid of the University of Canterbury, NZ. Due to the fact that the f5 con"guration consists of 1001 two-fold degenerate electronic states, we truncate this to a basis set consisting of the 30 lowest free-ion multiplets in order to obtain a realistic approximation to the entire con"guration. The free-ion Hamiltonian has been parametrised as H " + Fkf #+ fl ) s #a¸(¸#1) &.*. k i i k/2,4,6 i #bG(G )#cG(R )# + Mhm 2 7 h h/2,4

J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

99

Table 2 Experimental and calculated crystal-"eld levels for the 6H , 6F and 4G multiplets under C symmetry in KY F : Sm3`. The J J 5@2 47 3 10 experimental state energies are in wave numbers (as measured in air, $1) Multiplet

State

Symmetry

Calculated

Experimental

6H 5@2

Z 1 Z 2 Z 3

c 7 c 6 c 7

!7.6 229.4 275.9

0 219 285

6H 7@2

Y 1 Y 2 Y 3 Y 4

c 6 c 7 c 6 c 7

1099.6 1174.4 1279.1 1318.6

1108 1173 1278 1320

6H 9@2

X 1 X 2 X 3 X 4 X 5

c 6 c 6 c 7 c 6 c 7

2314.8 2347.4 2463.3 2488.4 2514.0

2320 2345 2472 2480 2508

6H 11@2

W 1 W 2 W 3 W 4 W 5 W 6

c 7 c 6 c 6 c 6 c 7 c 7

3602.7 3677.4 3785.5 3796.3 3816.7 3831.3

3605 3668 3782 3802 3812 3820

6H 13@2

V 1 V 2 V 3 V 4 V 5 V 6 V 7

c 7 c 7 c 6 c 7 c 6 c 7 c 6

4918.1 5118.7 5167.1 5195.0 5214.5 5237.7 5248.0

4916 5103 5173 5195 5229 * *

6H , 6F & 6F 15@2 1@2 3@2

S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 S 11

c 6 c 6 c 7 c 6 c 7 c 7 c 6 c 7 c 6 c 7 c 6

6216.3 6472.6 6568.9 6574.4 6661.4 6688.9 6695.3 6718.1 6722.9 6739.5 6756.1

6209 6456 6556 * 6658 6699 6710 6725 6729 6741 *

6F 5@2

Q 1 Q 2 Q 3

c 7 c 7 c 6

7206.1 7248.0 7261.1

7221 7240 7252

6F 7@2

R 1 R 2 R 3 R 3

c 7 c 7 c 6 c 6

8055.4 8086.5 8116.0 8143.6

8061 8107 8115 8127

6F 9@2

P 1 P 2

c 6 c 6

9242.8 9251.7

9235 9250

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Table 2 (continued) Multiplet

4G 5@2

State

Symmetry

Calculated

Experimental

P 3 P 4 P 5

c 7 c 7 c 6

9273.6 9288.4 9320.3

9278 9292 9323

A 1 A 2 A 3

c 7 c 7 c 6

17870.0 17986.7 18192.6

17883 17973 *

Table 3 Optimised free-ion C symmetry crystal-"eld parameters for 47 KY F : Sm3`. All quantities are in wave numbers except n, the 3 10 number of data points. The values in square brackets have not been varied. The additional columns gives optimised parameters for KY F : Eu3` [2] and KY F : Er3` [15] 3 10 3 10

Fig. 7. 10 K excitation spectrum of the Sm3`, 4G multiplet in 5@2 KY F . 3 10

remaining interactions are smaller, they nevertheless play an important role in an accurate description of the energy levels of rare-earth ions. These are the con"guration interactions (a, b, c), spin} spin and spin}other}orbit interactions, represented by the parameters Mh, the two body electrostatically correlated magnetic interactions, with parameters Pk and three particle con"guration interactions, ¹i. The only free-ion interactions have variable parameters in the "ts presented here, are the inter-electronic coulombic repulsion and spin}orbit interactions. The others are not varied, but rather are held constant at the values given in Ref. [13]. We "t the experimental crystal-"eld energy levels to a Hamiltonian which is appropriate for C sym47

Parameter

Sm3`

F2 F4 F6 a b c ¹2 ¹3 ¹4 ¹6 ¹7 ¹8 M505 P505 f B2 0 B4 0 B4 4 B6 0 B6 4 p n

78749 57785 39557.6 [20.16] [!566.9] [1500] [300] [36] [56] [!347] [373] [348] [2.60] [357.0] 1172 !600 !1388 410 596 127 10.3 46

Eu3`

Er3`

!551 !1360 345 394 234

!591 !1339 407 426 !10

metry. This is H "B2 C(2)#B4 C(4)#B6 C(6) #.&. 0 0 0 0 0 0 #B4 (C(4)#C(4) )#B6 (C(6)#C(6) ), (2) ~4 4 4 ~4 4 4 where all symbols are as de"ned in Ref. [14]. Forty six energy levels of the C centre in KY F : Sm3` 47 3 10 have been to nine free parameters excluding the average energy. Table 2 presents the comparision

J.-P.R. Wells et al. / Journal of Luminescence 85 (1999) 91}102

101

Fig. 8. 10 K, #uorescence spectrum of the (a) 6H , (b) 6H , (c) 6H , (d) 6H , (e) 6H , (f ) 6H , 6F and 6F , (g) 6F , 5@2 7@2 9@2 11@2 13@2 15@2 1@2 3@2 5@2 (h) 6F and (i) 6F multiplets of the C centre in KY F : Sm3`. * Notation indicates transitions of an unassigned impurity. 7@2 9@2 47 3 10

between the calculated and experimental energy levels. Good agreement is obtained with a very acceptable standard deviation of 10.3 cm~1. In Table 3, the optimized crystal-"eld parameters are given and compared with those of KY F : Eu3` 3 10 [2] and KY F : Er3` [15]. The three parameter 3 10 sets look comparable, although a general trend is not self evident.

5. Conclusions We have identi"ed three Sm2` centres in KY F : Sm using laser selective excitation and 3 10 #uorescence. Two of these centres show e$cient up-conversion #uorescence due to high lying 4f55d states and a metastable intermediate state available for the absorption of a second photon.

The observation of Sm2` in this material can be explained by the presence of F~ vacancies as proposed by previous workers. These vacancies act to charge compensate the Sm2` which occupy sites otherwise "lled by trivalent yttrium ions. Exact con"gurations of the Sm2` and F~ vacancies cannot be reasonably proposed at this point and a better understanding of the exact centre con"gurations would bene"t from studies of the angular variation of the Zeeman e!ect for #uorescence transitions of the respective centres. The ability to perform such studies is currently unavailable in the authors laboratory. We have also identi"ed 46 energy levels of the Sm3` centre for which C symmetry crystal-"eld 47 analyses provide an excellent account. The derived crystal-"eld parameters are in close agreement with those of other rare-earth ions in KY F . 3 10

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Acknowledgements This work was funded by the Engineering and Physical Sciences Research Council of the United Kingdom under research contract number GR/K/8802. The technicial assistance has been provided by D. Clark and R.G. Dawson.

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[6] S. Sugano, Y. Tanabe, H. Kamimura, &Multiplets of Transition-Metal Ions in Crystals', Academic Press, New York, 1970. [7] J.-P.R. Wells, R.J. Reeves, J. Lumin. 66 & 67 (1995) 219. [8] J.-P.R. Wells, M. Yamaga, T.P.J. Han, H.G. Gallagher, M. Honda, Phys. Rev. B, submitted for publication. [9] A.D. Toshmatov, F.L. Aukhadeev, D.N. Terpilovskiy, V.A. Dudkin, R.Sh. Zhdanov, Sh.I. Yagudin, Sov. Phys. Solid State 30 (1988) 61. [10] M. Mortier, J.Y. Gesland, M. Rousseau, M.A. Pimenta, L.O. Ladeira, J.C. Machado da Silva, G.A. Barbosa, J. Raman Spectros. 22 (1991) 393. [11] G.H. Dieke, Spectra and Energy Levels of Rare-Earth Ions in Crystals, Interscience, New York, 1968. [12] J.-P.R. Wells, R.J. Reeves, in preparation. [13] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, J. Chem. Phys. 90 (7) (1989) 3443. [14] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Interscience, New York, 1965. [15] E. Antic-Fidancev, M. Lemaitre-Blaise, P. Porcher, unpublished.