Luminescence and energy levels of Sm2+ in alkali halides

Journal of Luminescence 6 (1973) 270—284. © North-Holland Publishing Company

LUMINESCENCE AND ENERGY LEVELS OF Sm2~ IN ALKALI HALIDES M. GUZZI and G. BALDINI * Istituto di Fisica, Università degli Studi and Gruppo Nazionale Struttura della Materia, C.N.R., Via Celoria 16, Milano, ItaI~ Received 20 January 1973 Photoluminescence spectra and their lifetimes of Sm2~diluted in NaCl, NaBr, Nal, KI, RbI and RbBr at temperatures 15—300°K were studied in order to extend previous results. It is confirmed that an excited level labelled E lies in the proximity of the f6 (5D 0) state and that the energy of E is strongly dependent upon the host. This level is shown to give rise to luminescence 55d configin uration all theofcrystals Sm2t The studied E level here,shifts with its theenergy exception from of above RbBr, 5Dand is assigned to the 4f 0 inj(Cl, KBr, RbCl and RbBr to below in the other crystals and as a consequence the luminescence spectra are essentially of two kinds with lifetimes either of the order of 102 sec or 10~6sec, respectively. We have determined the energy of the E level in all crystals; an interpretation of the several emission lines is also suggested.

1. Introduction 2+ It is known from several investigations on the optical properties of Sm diluted in ionic crystals that sharp emission lines associated with f6 f6 transitions are found in the luminescence spectra which also show structured phonon sidebands. The sequence of electron energy levels of the Sm2+ free ion begins with the 7F multiplet derived from the f6 configuration which gives also 5D multiplets at approximately 18000 cm~above 7F [I].4f55d Dipole transitions are allowed 7F 7K states of0the configuration at the energy from the 0 ground statecm1. to theAbove the 7K states we find the ~I states. In of approximately 24000 hexagonal matrices of the LaCl family is found that a partially negative ion vacancy ac2~ion [2,3] 3with the itresult of making allowed transicompanies Sm tions of thethe type 0 -÷ 0, 1 -÷ 0 and 0 -÷ 1 for the total angular momentum because of the low symmetry of the Sm2+ environment; in the free ion the above transitions are not allowed, The decay times for these transitions in the hexagonal lattices are of the order of 10_2_10_3 sec [2]. Many studies have also been made on the optical properties of Sm2~in alkaline earth halides (see, e.g. refs. [4]—[12]) —~

*

Also: Universit’a degli Studi, Sassari, Italy. 270

M. Guzzi, G. Baldini, Luminescence and energy levels

271

and the data are found to be consistent with those of the hexagonal hosts. However, according to Feofilov and Kaplyanskii [5], the excited state of Sm2’~in CaF 5D 6) and should belong to some other configuration such 2 is5ddifferent 0 (f as 4f5 or 4f5 6sfrom and is expected to lie below 5D 0 because of crystal field effects. It was also found from2~[10—12]that, calculations andafter fromrelaxation, the application of external fields on the excited state belongs CaF2 doped with Sm to the 4f55d configuration with symmetry F 1 or F12 [12]. Furthermore low temperature lifetime measurements on the luminescence confirm the above conclusions and yield time constants of the order of 10_2_l0_3 sec in BaF2 [5, 9], SrF2 [4—9],SrCl2 [6] and BaC1F [8], whereas CaF2 yields 10—6 sec [5, 8, 9]. A number of papers have halides also been aimed atfrom the investigation of of theKarapetyan optical 2~in alkali beginning the early works properties of Sm et al. [13] and Kaplyanskii and Feofilov [14]. Bron and Heller [15], in accordance with previous authors, have recognized the role of the 4f6 -÷ 4f55d transitions in absorption and that of the 4f6 (5D 6 (7 F~)in emission adding also that the -÷ 4f observed transitions arise from the 0) complex Sm2+ positive ion vacancy which has symmetry C2v (110). Although, according to Fong and co-workers, the centres responsible for the emission in KC1: Sm might be different [16], however Bradbury and Wong [17—19]have confirmed the C2v symmetry of the centre by examining Zeeman and lifetime data referring to the emission. The properties of Sm2+ in alkali halides have also been studied by us, by means of measurements on the luminescence and its lifetime which were interpreted in terms of a 5D 0 excited state responsible foremission the large[20, temperature dependence of intensity and 5D 7F~ 21]. lifetime of the -+ Here we report0new data on some crystals of alkali halides that were not investigated previously. It has been found that the Sm2~excited state which is responsible for the observed luminescence, may be either the 5 D 6) (e.g. RbBr) 0 (f dependence or a level of another configuration (e.g. K!); in the latter case a strong of its energy upon the host matrix is also found. New data are reported on the phonon sidebands accompanying the electronic transitions; the electron—phonon coupling is also found dependent upon the host matrices. 2. Experimental results Single crystals of alkali chlorides and bromides have been grown in our laboratory by means of the Kyropoulos method in a hydrogen atmosphere which reduces the SmCl 3 dopant to SmCl2 and makes it soluble in the molten salts. The iodide crystals, instead, have been grown with the Bridgman technique by adding to the melt SmCl3 and aluminium powder which also acts so as to reduce the valence of Sm [13]. No new structure was found in the optical spectra of the latter crystals, which 2~incould the be attributed tobeen the presence metal concentration crystals has not measuredofbut we impurities. know that The 0.3 mole % of SmClof Sm 3 was added

272

M. Guzzi, G. Baldini, Luminescence and energy levels

to the melts in all cases. In order to dissolve aggregates which may form in the doped crystals, these were quenched from ‘-‘600°Cto room temperature. The emission spectra and their lifetime were examined with a 50 cm Jarrel—Ash monochromator, equipped with a 1200 lines/mm grating blazed at 5000 A. The emission was excited with a 200 W high pressure mercury arc (OSRAM HBO) and detected with a cooled photomultiplier (P~iilips150 CVP) followed by a phase sensitive detector (Brookdeal Model 411). The lifetime measurements were performed on the apparatus previously described [201which could be operated in the range from 3 X 10—6 to 10—1 sec by counting single photons. 2.1. Emission spectra At room temperature the luminescence of Sm2~consists of a broad band which peaks from 7500 to 8200 A depending upon the host lattices and, when the temperature is lowered, this band becomes very structured in most cases. 2.1.1. RbBr. The emission of Sm2~at 15°K(fig. 1) is very similar to that observed in KC1, KBr and RbCI [15]. Sharp lines are found and they are assigned to 5D 6)transitions. The 5D 7F 0 (4f~)—~’F~(4f 0 0 line, whose linewidth is 0.74 cm~is2’followed by a vibrational sideband (fig. 2) resulting The fromHuang—Rhys the coupling of optic electron with the crystal phonons [21—23]. the Sm factor, S = 0.22 at 15°Kis smaller than in the previously studied halides [21, 24]. The spectrum of fig. 1 shows that this crystal gives rise to more lines than reported ‘~

-+

ENERGY

(Cm-’)

14.500 1.0.’

14.000

‘

RbBr: Sm~ 15°K I—

~~

6900

I

I

7000 7100 WAVELENGTH

7200

7300

(A)

Fig. 1. High-energy portion of the emission spectrum of RbBr: Sm2~at 15°K. The energies of the lines indicated by an arrow are reported in table 1. The intensities have not been corrected for the spectral response of the apparatus.

5D~+~Fj 14525.2

14264.1 14234.1 14207.8 13763.7 13739.7 13734.6 13708.9 13675.4 13022.0 13020.0 13017.9 13015.2 13005.4 12219.0 12217.6 12211.8

J

0

1

2

3

4

RbBr

14302 14239

14557 14508 14496

5Do~7Fj

Rb!

12611

13272 13245

13750

E—*7Fj

Rbl

12563

13228 13194

13700

E-#7Fj

KI

12341 12323 12287

13119.9 13102.9

13670.0 13645.0 13622.5 13600.0 13523.6

14293.8 14273.9

7Fj 0~ 14522.2

5D

NaCl

11817.2 11807.9 11796.9

12553.7 12530.7 12507.7 12497.3 12481.0

13216.1 13170.0 13 150.6

13763.7 13743.4 13691.5

14070.6

E+7Fj

NaCl 7F~ 0+ 14530.6 14515.8

5D

NaBr

11728.8 11721.9

12486.7 12467.9 12447.7 12428.8

13145.3 13115.8 13105.3 13067.8

13636.8 13622.9

E-±7Fj

NaBr

11848 11808 11794 11772

12527 12516 12488 12463

12933

E47FJ

Nal

Table 1 2~multiplets. The uncertainty in energy is ± 0.7 cm’ when the decimal digit is given; in the Energies of the prominent emission lines of the Sm other cases is ± 5 cm1. For a complete explanation of the table, see text.

274

M Guzzi, G. Baldini, Luminescence and energy levels

6900 10•

6950

‘

RbRr: Sm

F~

~~J\~=lSOK

0

50

100 150 ENERGY (Cm-’) 5D 7F 2 + at 15°K. Fig.. 2. Vibrational side-band of 0 -~ 0 transition of RbBr: Sm

for KC1 and KBr [15]. Table 1 gives the energy of the prominent lines in the ~D~ 7F~multiplets, those indicated by an arrow in fig. 1. —~

2.1.2. NaG!. The emission spectrum of NaCl is given in fig. 3. The spectrum results from a superposition of sharp lines and broad bands. Some of the observed lines lie at energies which correspond to those of the 5D 7Fj transitions found in 0 —~

ENERGY

14.000 1.0

(cm-’)

13.000

12.000

I

I

NaCI: Sm~

15 °K I—

0.5

7000

7500 8000 8500 WAVELENGTH Fig. 3 Emission spectrum of NaCl: Sm2 + at 15°K. The intensities have not been corrected for the spectral response of the apparatus.

(A)

M Guzzi, G. Baldini, Luminescence and energy levels

275

other crystals (table 1). The 5D0 7F01 lines, those at higher energies, are weak compared with other lines in the spectrum; extra lines also appear. 2~in this matrix, as given in fig. 4, differs from the 2.1.3. NaBr. Thesince, emission of Sm on an intense background, there appear groups the above data superimposed of strong lines which are repeated with a separation of 122 cm~.The energy of the lines of the first (highest energy) multiplet for each series is reported in table 1. In fig. 4 we observe also at an energy corresponding to the 5D 7F~~ transition 0 -~ (“‘ 14 500 cm~)two weak lines. —~

ENERGY (cm-’) 13.000

14 .000 1.0

-‘

Cl, I-.

I

I 44444

.12.000 I

4

4

4

4

4

NaBr: Sm~~ 15°K

7000

7500

8000

8500 0

WAVELENGTH (A)

Fig. 4. Emission spectrum of NaBr: Sm2+ at 15 ° K. The intensities have not been corrected fQr the spectral response of the apparatus.

2.1.4. Na!, KIandRbI. In these matrices the Sm2~ion gives a broad emission which is made of a few bands. Some repeated structures appear, which are phonon replicas. As an example of the iodides, in fig. 5 we show the spectrum of Nal: Sm2~at 15 °K. The energy separation between adjacent replicas is found to be 90 cm~(Na!), 79 cm~(K!) and 81 cm’ (RbI). In the case of Rb! we have found that at lS°K the emission spectrum shows broad bands with structures similar to those of Na! and also some weak lines at energies between 14 200 and 14600 cm~(table 1) corresponding to the 5D 7F 0 -÷ 0,1 transitions. 2.2. Lifetime of the emission 2~luminescence in R~Br(fig. 6) The lifetime and its thermal dependence of Sm

276

M Guzzi, G. Baldini, Luminescence and energy levels

ENERGY 73.000

14.000 1.0 ++

:1~

(cm-’)

12.000

I

I 444ê444

~

7000

7500 8000 8500 2+ WAVELENGTH at 15 °K.The intensities have not been corrected for Fig. 5. Emission spectrum of Na! : Sm the spectral response of the apparatus.

(A)

Table 2 De-excitation parameters of the D~excited state as obtained by fitting the experimental data with eq. (1) (part (b)) and eq. (2) (part (a)).

a

b

rR(msec)

rJ(~.ssec)

~.E(cm1)

KC1* KBr* RbCl*

11.5 10.5 10.7

±

1.0 1.0 1.0

1.14 1.29 1.26

± ± ±

0.10 0.14 0.07

130.1 105.0 257.9

±

± ±

± ±

2.5 2.5 3.0

RbBr

9.5

±

0.5

4.65

±

0.4

211.0

±

4

1.31 4.60 1.08

±

0.07 0.4 0.07

133.1 89.2 276.0

± ± ±

1.4 1.2 3.5

221.5

±

2.7

KC1* KBr* RbC1* RbBr

*

———

———

± ±

See ref. [20].

is consistent with that of KC1, KBr and RbC1 [20]. Within the experimental uncertainty, it has been found that at 15 °Kthe lifetime of the 5D 7F 0 -+ 0 transition 5D 7F~~ coincides with that of the other transitions 0 -÷ a0s already observed in some alkali halides1 line [19, of 20].RbBr: In table 2 we reportwith the de-excitation parameters for Sm2~ together those of previously studied the 14 525.2 crystals [20]. cm— The other crystals which are discussed here show, at low temperature, a short lifetime, ~ 3 psec, and a long decay time of the order of 100 psec. In table 3 we report the lifetimes for the latter crystals.

M. Guzzi, G. Baldini, Luminescence and energy levels

15

20

50

I

70’

I

277

700 T(’K) I

-

t(T)

-70’

~

0.7

0~O5

7/~ ~

0

5D 7F 2~in RbBr. Solid curves Fig. 6. Oscillator strength and lifetime of the have been obtained employing eqs. (1) and (2), respectively, 0 -÷ 0 transition and theofparameters Sm reported in table 2. Table 3 Lifetimes of Sm2~emission in sodium-halides and alkali-iodides at 15 °K. A (A)

~x(A)

r

2~

7700

28

1 (~.ssec) ~3

r2 (Msec) 292±74

NaCl:Sm NaBr:Sm2~

7700

28

~3

649±50

NaI:Sm2+

8200

28

~3

379±121

Kl:Sm2~

7900

28

~3

262

±16

RbI:Sm2~

7700

28

~3

83

±17

278

M Guzzi, G. Baldini, Luminescence and energy levels

3. Discussion According to our previous analysis of the Sm2~emission in alkali halides [20] it appears that the RbBr data can be readily interpreted in terms of a level, labelled E (fig. 7) lying slightly above the 5D 2~which is 0 level. created by the absorbed light relaxes to theThe 5Dexcited state of Sm 0 level and the presence of the nearby E level causes a strong thermal dependence of both the zero phonon line intensity and its lifetime. It was verified that the line strength follows the expression [20]: (1)

f0(T)=f0(0)~—, where the lifetime

L~ R

r is given

by: (2)

+±exp(_~E/kfl. T~

By fitting the experimental data for RbBr with the above expressions, as it was done for KC1, KBr and RbCI [20], we have obtained the parameters of table 2. Although this scheme accounts for the emission spectrum and the lifetime of RbBr, it is necessary, on the other hand, to extend this model, since the other crystals examined here display emission with short decay at all temperntures and different spectra. From table 2 we see that level E shifts considerably 5D when changing the host lattice (RbCl, RbBr, KC1, 0 level, as can be 5D KBr) 7F to the other side of the deduced from the energies of 0 0 tninsitions (table 1). This observation led us to the conclusion thatground the E level to a configuration 5D different from the 6 which gives both the state belongs 7F f 0 and 55d the configuration excited state can 0. be viewed as According [25], the of levels of the 4f resulting fromtotheLoh interaction the 4f5 electrons with the Sd electron in symmetry C 2,,,, when considering crystal field effect and spin—orbit interaction. In C2v sym—~

~0,,

2~5D Fig. 7. Decay scheme for the Sm not in scale).

7F 0 —°

0 transition in RbCl, RbBr, KC1 and KBr (energies

M Guzzi, G. Baldini, Luminescence and energy levels

279

metry, due to crystal field effects, the d level splits into S levels with a separation of the order of —~10~ cm~ [26]; the spin—orbit interaction is generally much smaller than that due to the crystal field. The 4f5 level is split by spin—orbit interaction into several states and the lowest is 6H 5. which separated next, 6H 3 cm 1 [2]. The level 6H} is spfit by theiscrystal fieldfrom into the three levels whfch 1 byare doubly i0 degenerate because 6f their spin. Their separation is of the order of 10—100 cm1 [2]. If we assume that the lowest state of the 4f55d configuration is constructed from the lowest of the five d states and from 6H 1 we therefore expect 12 states with a separation of 10—100 cm~between adjacent levels, i.e. that corresponding to the crystal field effect. The energy of the group of lines is determined by the shift arising from the crystal field effect on the d level and therefore it is expected to depend strongly on the host matrix. From group theoretical analysis it is found that the 12 sublevels so obtained correspond to the four irreducible representations A1, A2, B1 and5B2 of C2voneach three times.we note d states the taken emission process In order to examine the role of the f that, when this configuration lies above 5D 0(KBr, KC1, RbBr and RbCl), emission 5D lines are observed only from 0. Assuming a Boltzmannn distribution for the 5D population ratio of 0 and E,5D according to their energy separation given in table 2, we find that at 77 °K,n(E)/n( 0) varies from 10—1 in KBr to 10—2 in RbC1. A transition from E to ~F0 should then 5D 7F be seen only if allowed in electric dipole approximation as found for 0 0 in C2v. We are forced to co:~ludethat E -÷ F0 is forbidden line corresponding to this energy (14617RbCl) cm~in 1 insince KC1, no14736 cm~inRbBr and 14779 cm~in is found KBr, 14638 cm in the Sm2~spectra at 77 °K.We suggest that the E level be attributed to the irreducible representation A 7F 2 of C2,,, which is not connected to 0 (A1) by an electric dipole. Electric 7F dipole transitions from a level of A2 symmetry should occur to the three levels 1 with5D symmetry 7F A2, B1 and B2 and are expected to fall in the phonon sideband of the 0 0 transition. This result, together with the estimated population ratio (lO_1_lO_2) might5 dmake it difficultmust to detect these lines. configuration lead to a depenThen, thatthe E lattice belongsconstant to the f of the host crystal. Kaplyanskii and dence the of itsassumption energy upon Feofilov, when studying the absorption spectra of Sm2~in alkali halides, were unable to interpret them in terms of crystal field splitting of the d electron [14], and assumed that a covalent bonding may be established between the d electron of Sm2~and those of the neighbouring atoms, together with a local lattice distortion. From table 2 we find that E lowers when the radius of the halide ion increases in both the K and Rb halide sequences. When going from RbCl to KC1 or from RbBr to KBr, E lowers about 100 cm—1 with respect to 5D 0 which is separated from E by about the same energy. It may then occur that in the Na halides 5D or in the alkali iodides, the E level moves below 0. Let us examine then the spec2+ according to the above suggestion. tra of Sm ,

‘~

-~

—*

280

M. Guzzi, G. Baldini, Luminescence and energy levels

3.1 3.1.1. RbBr. The data are consistent with those of KBr, RbC1 and KC1 [20] with the exception of extra lines (fig. 1) which might arise from a less effective quenching in this crystal. The presence of lines due to aggregates and the effect of the quenching on the number of lines in the spectra have been studied by Heist et al. [16]. It is more likely, however, that impurities such as potassium might be responsible for these extra lines. The level E is found 211 cm~above 5D0 (table 2). The decay 1extra line is 4.6 msec and the E—5D time of the 14512 cm~ separation energy 1, thus indicating that the E level0 does not belong to in this case is z~sE = 155 cm any f6 configuration. 3.1.2. NaG!. Several lines are observed in this crystal and among them, some correspond with their energies to the 5D 7F~transitions as found in other crys0 we find the 5D tals (see fig. 3 and table 1). Among these 7F 0 -÷ 0 zero phonon line 5D 7F and its phonon sideband and 5Dtwo of the7F 0 -÷ 1 lines. A third line is missing since it may correspond to 0 (A1)-+ 1 (A2) which5Dshould7F~ be >2 very since areweak not easily it is caused by aa strong magnetic dipole [15]. The together transitions -÷ assigned since background appears with 0narrow extra lines. Groups of lines are also found whose energy and relative separation indicate transitions to the F~levels from a level E ‘-‘450 cm1 below 5D 0. The line 14070.6 cm~ 7F should correspond to E (A2) 0 (A1) forbidden in electric dipole and allowed in magnetic dipole. It should be noted that this is the only crystal where we have ob2~ served linedue attributed -÷ ~F0. state In table in NaCla as to eitherto theE excited 5D I we have classified the lines of Sm 0 or E. Because of the many lines found in the spectra, this classification may be regarded partly as tentative and further studies should be made. We may addbythat our assignment 7F~multiplets at most 100 cm~1.implies deviations from the sequence of the 3.1.3. NaBr. The spectrum is similar to that of NaC1 and consists of a doublet at the energy of 5D 7F 0 01 transition plus fourmultiple groups of lines. The energy separation and this suggests phonon processes which arise in each group is 122 cm from the coupling of the optic electron with a localized phonon. The relative strength of the lines‘n/’O in each group can be fitted with the theory of the linear electron phonon coupling which gives, at 0°K[27]: -+

-~

-+

(3)

where n is the number of phonons associated with the transition and S is the Huang— Rhys parameter. It is found that a good fitting occurs for S = 1. We can then measure the energies of the purely electronic transitions which correspond to n = 0 and are reported in table 1. It is also found that the spacings of the lines are those of the

M Guzzi, G. Baldini, Luminescence and energy levels

281

7F~multiplets but their energies do not correspond to those of the 5D 7FJ 0 and this transitions. We propose that these lines arise from the E 7F~transitions locates E at 550 cm~below 5D 0. This assignment is also supported by the ab7F sence of the E 0 transition. 7F~ It should be pointed out thatfrequency the vibrational lines falls in the phonon gap offrequency[28]. associated with the -÷ NaBr Furthermore thisE frequency is consistent with absorption data reported by Bron and Wagner [29] thereby giving new support to the assignment of E to the f5d configuration. —~

-*

-‘

—~

3.1.4. Na!, K! and RbI. The weak structures which appear superimposed on the broad emissions in these crystals (see e.g. fig. 5) show again a repetitive pattern with a separation energy, between adjacent lines, which is consistent with that measured in absorption by Bron and Wagner [29]. As found for NaBr, the repeated structures are thought to be associated with phonons localized at the impurity centre. The strength of the lines is described by expression (3) with an S factor which is “-‘3 for Nal, “—‘4 for K1 and larger for RbI. This allows the exact determination of the purely electronic transitions which are given in table 1. For RbI the structures are weak and we take the first line observed as the zero-phonon process. The E 7F 1 lines in Na! and K! do not show a clear repetitive pattern and therefore we have reported in table I and fig. 8 only7F~ the transitions first (highest energy) line. The which is equivalent zero phonon processes are assigned to the E -÷ to saying that E lies below 5D 0 by “-‘450 cm~ in Rb!, “-‘500 cm~in KI and “—‘1200 cm’ in Na!. In RbI the isolated lines at higher energies are interpreted 5D 5D 7F as 0 ‘1 F0,1 transitions. The presence of a few extra lines around 0 0 can be justified in the same way as for RbBr. Transitions involving the other multiplets Fj>2 are not observed since they should occur where the emission is stronger. From the absorption [15] and emission data it is seen that the emission occurs at energies “—‘1000 cm~,or more, below the lowest energy absorption. This can 7F be ascribed either to selection rules (e.g. E to 0 forbidden) as suggested above, 2~impurity. At present little evidence or also to lattice around the Sm since, e.g. in CaF can be given as torelaxation the role of the relaxation, 2 the d states do not relax significantly [9]. In the alkali halides the effect of lattice relaxation in the excited state has not been studied to our knowledge; we therefore suggest that absorption spectra at different temperatures should be examined in order to get 2~. further lowestofexcited state of Smof Sm2~in the different crysSomeinformation remarks on on thethe lifetime the luminescence tals are in order. The lifetime for RbBr is found to agree with the previously measured lifetime of Sm2~in KC1, KBr and RbCl (see also table 2) ~20]as one would expect from the close similarity of the emission spectra. A different behaviour is found in the other crystals (table 3) which all show a lifetime r <3 Jlsec. We can correlate this small value of the lifetime with the excited state E belonging to the f5d configuration. We recall that the f5d f6 lines have larger transition rates than the f6 f6 lines, as found from absorption data. Also the lifetimes of the -+

—~

—i-

-+

—*

282

M. Guzzi, G. Baldini, Luminescence and energy levels

7000 RbBr

I

Rb!

III

7500

Ill

8000

~A)

Ull

8500

I

I I

I

K!

NaCI

I

NaBr

N

II

I

Na!

111111

I I~III

IL

iij

III

NI

II

III

1

~II

II

Iii

14.000

13.000 E(cm-’) 12.000 2+. The heavy lines correspond to 5D 7F~ Fig. 8. Energies of the observed transitions of Sm transitions whereas the light ones correspond to E 7F~transitions. It should be noted0 that —° the separations between the lines of the multiplets may be incorrect due to the small energy scale of the drawing.

-÷

emission, which are found very different in the case of the crystals of table 2 (5D 2~excited state. 0) andmay tableadd 3 (F), assignment the Sm We alsosupport that theour small lifetimes of of the the nature crystalof listed in table 3 agree with that found for Sm2~in CaF 2 [5, 8, 9]. As a final observation we report that in the luminescence of the latter crystals, another component of the luminescence was found, with lifetime ranging from 80 to 650 psec depending upon the host (table 3). The time integrated intensity of the long component is about an order of magnitude smaller than that of the main component. At present the explanation of this long emission is not clear. The above discussion can be summarized with the help of figs. 8 and 9 and table I. In fig. 8 we report5D the several lines observed in the luminescence and they are assigned either to the 0 or E excited level. The shift of the E level in different matrices is also evident in fig. 9, which shows the energy levels involved in the emission process for different host crystals. A more accurate comparison of the levels of fig. 9 and the energies of table 1 show that some of the transitions do not appear in all the crystals that were examined here and that the energies of the F~multiplets may vary by a few tens of cm~.However, these apparent inconsistencies may disappear when consideringJ—J’ mixing [15]. This mainly determines the intensities of the different lines in a multiplet and slightly affects the splittings; the splittings, on the other hand, are affected more severely by various

M. Guzzi, G. Baldini, Luminescence and energy levels

RbBr

Rb!

K!

NaCI

NaBr

283

Na!

E

2000

-

~i

1000

Fig. 9. Energy For NaCI only

-

.

—

___ ____

:

a!

_~

—

2+, as deduced from data reported in table 1 and fig. level scheme for Sm the levels associated with the 5D 7Fj transitions are reported.

8.

0 —~

shielding effects [15]. These effects can be reasonably expected to play their role in the crystals studied in this paper since the Sm2~levels involved in the transitions to the ground state 7F~change both their nature (E or 5D 0) and their energies in the different matrices. It is felt that only by detailed calculations could one achieve a closer consistency.

Acknowledgements Thanks are due to Miss Nilla Vannotti for having grown the crystals and for the considerable help in the measurements. We are grateful also to Prof. N. Terzi for useful comments and a critical reading of the manuscript.

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M. Guzzi, G. Baldini, Luminescence and energy levels

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