Vibronic transitions of Tm in various lattices

8/12/2019 Vibronic transitions of Tm in various lattices

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JOURNAL OFLUMI NES ENEISEVIER Journal of Luminescence 69 (1996) l- I5

Vibronic transitions of Tm3+ in various latticesA. Ellens*, S. Schenker, A. Meijerink, G. Blasse

Debye Institute Utrecht University P. 0. Box 80 000 NL 3508 TA Utrecht The NetherlandsReceived 17 November 1995;revised and accepted 31 March 1996

AbstractVibronic transition probabilities have been derived for Tm 3+ in LiYFc YOCl and Na=,La(WO&. Determination of

the vibronic transition probabilities for Tm 3+ is not easy, but, as far as they can be determined, it is shown that they havethe same order of magnitude as those for Pr3+ and are significantly larger than those of Gd3+. These results give anindication that the electron-phonon coupling strength is large in the beginning and at the end of the trivalent lanthanideion series and small in the centre. To explain the variation in vibronic transition probabilities for Pr3+, Gd3 and Tm3 +,the position of the opposite parity states, the lanthanide contraction and the shielding of the 4 f electrons are considered.Keywords: electron-phonon coupling; vibronic transitions; transition probabilities

1. IntroductionThe interaction of the 4f electrons of lanthanide

ions with their surroundings (calledelectron-phonon or ion-phonon coupling) is weak;however, important phenomena like multi-phononrelaxation, vibronic transitions and line-widthbroadening as a function of temperature depend onit. Knowledge of this electron-phonon interactionis not only interesting from a fundamental point ofview, but it also has several applications. For ap-plications, in which a high quantum efficiency isrequired, usually weakly interacting systems arepreferable to prevent losses by nonradiative decay.On the other hand, in some cases multi-phononrelaxation is useful. For example, to obtain laseraction a fast decay of the upper excited state toa lower lying metastable state is desired [l, 21.

*Corresponding author. Present address: Institut fiir Anorganische Chemie, Univer-sitat Bern, FreiestraBe 3, CH-3000 Bern 9, Switzerland.

One of the research topics in our group is astudy on the parameters that influence the elec-tron-phonon coupling strength. This has beendone by studying multi-phonon relaxation and vib-ronic transitions of the ions Pr3, Sm2+, Eu3+,Eu +, Gd3 + in various lattices like LiYF,, LaF,,YOCl and La203 [2-lo]. Two effects were deducedfrom these measurements on vibronic transitions:

(1) the vibronic transition probabilities increasewith increasing covalency,

(2) the vibronic transition probabilities are muchsmaller for Gd3+ than for Pr3+.These effects were explained in terms of covalency,polarizability, position of the opposite parity states,lanthanide contraction and shielding of the 4felectrons.Systematic research on vibronic transition proba-bilities (Avib) was done for the early lanthanidePr3+ and the middle lanthanides Eu3+ and Gd3+.In view of this, it seemed interesting to studya heavy lanthanide ion in detail. Tm3+(4f 12) has

0022-2313/96/$15.00 c 1996 Elsevier Science B.V. All rights reservedPII SOO22-2313(96)00036-l


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A. Ellens et al. 1 Journal of Luminescence 69 (1996) I-15

40000

- 30000VQ 20000c12w 10000

3pJ, I,D*G3F* 33H;

---~~~~3H53Fl 3H6

Fig. 1. Schematic energy level scheme of Tm3+.

been chosen, because it is with respect to Gd3(4f7),just the mirror image of Pr3+(4f2). An energy levelscheme of Tm3+ is shown in Fig. 1. Taking intoaccount that the 4f orbitals at the end of the seriesare even more contracted [11,12] and that the4f- 5d states are high in energy [13], one mightthink that the vibronic transition probabilities forthe heavy lanthanides will be small or even smallerthan those for Gd3+.

However, in an article by Hellwege, published in1941 [14], he states that the electron-phononcoupling is strong in the beginning of the series,weak in the centre, and strong at the end of theseries again. Hellwege derived this conclusion froma study on the luminescence intensity, the linewidth, mean crystal-field splitting of the terms andthe intensity of the vibronic sidebands of trivalentlanthanide ions in REz(S0J3.8H20.

Later, Krupke found similar results and reportedthat the vibronic transitions are strong in the begin-ning and at the end of the series (Pr3 + and Tm3 +),but weak in the centre (Eu3+) [15,16]. In anotherpaper, however, a small vibronic intensity is re-ported for Tm3+ in GdOCl [17].These conflicting ideas and results on the elec-tron-phonon coupling strength for the trivalentlanthanide ions form a good reason to study inmore detail the vibronic transitions of Tm3 inseveral compounds. It has been shown before thatthe best way to compare the intensity of vibroniclines of lanthanide ions is to calculate vibronictransition probabilities. In none of the reports onvibronic transitions of heavy lanthanide ions suchcalculations have been done. This papers describes

the search for vibronic transitions of Tm3+ inLiYF_+,YOCl and Na,La(WO,),, and also evalu-ates the vibronic transition probabilities derivedfrom the spectra and compares these with those ofPr3+ and Gd3+.The reason why we have chosen these three latti-ces is that we wanted to compare an ionic system(LiYFJ and a covalent system (YOCl). The advan-tage of Na,La(WO,), is that it has high energeticvibrations which makes it easier to separate vib-ronic lines from the zero-phonon lines, which isa problem for the two other systems. Also previousmeasurements on Pr3 and Gd3 + were performedin these lattices [3, 4, 18, 191.

The results show that it is difficult to calculatevibronic transition probabilities for Tm3+, mainlydue to the overlap of vibronic lines with zero-phonon lines. Nevertheless, it is concluded that thevibronic transition probabilities for transitions onTm3+ are higher than those for Gd3+ and compa-rable to those for Pr3+ in the same host lattice.2. Experimental

Powdered samples of YOCl and Na,La(WO&,containing 1 mol% Tm3+ were prepared usingmethods described in Refs. [18] and [20], respec-tively. For the preparation of YOCl doped withthulium the rare-earth oxides were dissolved inconcentrated HCl. This solution was heated untildryness is achieved and then the sample(YCl3xH,O :Tm3 ) was heated for about 2 h in airat 500C. A second firing was carried out at 600Cfor about 2 h in an N2 atmosphere. TheNa5La(W0,), sample was prepared by mixingNa2C03, W03, La203 and Tmz03 and a sub-sequent heating for 6 h in air at 630C. In the caseof LiYF,:Tm a crystal was grown using theBridgman method. The crystal growth melt con-tained 1 mol% Tm3+. The crystal was transparent.For the measurements a polished piece of about2 mm thick was used.

With X-ray powder diffraction the samples werefound to be single phase; with diffuse reflectancespectroscopy the optical purity of the samples waschecked.

YOCl has the layered PbFCl structure. Thespace group is P4/nmm, with a Cbv site symmetry


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A. Ellens et al. 1 Joumul o Luminescence 69 (1996) l-15 3

for the Y3+ ion [21]. LiYF4 has the inversescheelite structure, and the space group is 14Ja.The site symmetry for the lanthanide ion (Ln3+) isS4 [22]. The compound Na,La(WO,), has a struc-ture related to the scheelite structure and the spacegroup in this case is also 14,/a. The site symmetryfor the Ln3+ ion is S4 [23, 241.

The low resolution excitation and emissionmeasurements were performed with a SPEXDM3000F spectrofluorometer with 0.22 m SPEX1680 double mononchromators and a 450W Xe-lamp. The samples were mounted in anOxford LF205 liquid helium flow cryostat. Thespectral resolution of this apparatus is at best12 cm-. Detection is with a Hamamatsu R928photomultiplier.High-resolution measurements were performedwith an excimer-laser-pumped dye laser for excita-tion spectra around 360 nm. The excimer laser set-up consists of a Lambda Physik LPD02 tunabledye laser which is pumped by a Lambda PhysikLPXlOO excimer (XeCl) laser. DMQ (LC3590) dyewas used to excite in the ID1 level. The typical linewidth of the dye laser output is 0.18 cm-. Thesample is cooled in an Oxford Instruments liquidhelium bath cryostat. Measurements were per-formed at 4.2K. Emission spectra were recordedwith a 1 m focal length Spex 1704 monochromatorwith a resolution of about 0.2 cm-. The blazewavelength of the 1200 lines/nm grating is at600 nm. The monochromator is equipped with anRCA C31034 photomultiplier tube. Decaymeasurements were measured with a Tektronix2430 sample oscilloscope which was triggered bythe laser pulse.

3. ResultsThis section is divided into two parts (Sections

3.1 and 3.2). Section 3.1 is again divided accordingto the three lattices LiYF4, YOCl andNa,La(WO& These three subsections have thefollowing structure. First an overall emission spec-trum of Tm3 m the particular lattice is shown andis commented upon. For three different transitionsof Tm3+ vibronics are found and shown: theD2e3H6 transition, the G4e3Hs transition and

the D2 * F4 transition. Section 3.2 describes thedetermination of the vibronic transition probabilities.

The R values mentioned in this paper indicatethe relative intensity of the vibronic sideband andare defined as the ratio of the integrated vibronicintensity and the integrated zero-phonon line(s) in-tensities for a particular transition between twomultiplets.3.1. Excitation and emission spectra3.1.1. LiYF,: Tm33.1.1.1. General. Fig. 2 shows a low resolutionemission spectrum between 12 000 and23000cm- at 4.2 K for LiYF4:Tm3+. Excitationis in the D, level. This spectrum is not correctedfor the photomultiplier sensitivity: correction willenhance the region beyond 14000 cm- . The finallevels, after emission from the D2 level, are given inthe figure. The assignment of the peaks is in agree-ment with literature [25]. The asterisk indicates theG4 3 3H6 emission. This emission is quite strongalthough excitation is in the D2 level. The relativeintensity of this emission increases with the concen-tration of Tm3+and decreases with rising tempe-rature. Also in the region of the D2 emissions to3H4 and 3F3, emissions belonging to the G, levelare found. Multi-phonon relaxation from D2 to

12000 14000 16000 18000 20000 22000 24000Energy (cm-)

Fig. 2. Low-resolution emission spectrum of LiYF4:Tm + at4.2 K. Excitation is in the D, level. The final levels after emis-sion from the Da level are given in the figure. The asteriskindicates the G, + 3H6 transition. The symbol indicates the3H4 = H, transition.


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4 A. Ellens et al. / J ournal of Lumnescence 69 1996) I -15

Table 1(U2))2, (UC4) and (UC6) matrix elements for Tm3+ fortransitions starting or ending at the Dz or rGL levelsTransition (U(*))* (U@))* (U@)) Wave length Energy

(nm) (cm-)rDs o rGq 0.1925 0.1713 0 1400 7143Dz 9 3FZ 0.0637 0.3059 0.0000 776 12888Dz * 3F3 0.1604 0.0677 0.0000 741 135021D2~3H., 0.1285 0.0121 0.2267 651 15350Ds o 3Hs 0.0000 0.0011 0.0188 507 19725Dz * F, 0.5598 0.0941 0.0233 450 22203Dz o 3H6 0.0000 0.3074 0.093 360 28061G4 o 3F, 0.0057 0.0708 0.0408 1620 6179Gz, 0 3Fz 0.0010 0.0705 0.2995 1470 6793tG4 * 3Hd 0.1569 0.0036 0.3620 1157 8641G4 0 3H5 0.0729 0.0051 0.5380 768 13016Gz, o 3F4 0.0034 0.0201 0.0752 645 15494Gq * 3H6 0.0494 0.0762 0.0131 468 21352

G4 is improbable in view of the large energy gap( +. 6400 cm- ) with a maximum phonon energy of560 cm- in the LiYF4 lattice [2, 26, 271. Also,a large radiative decay rate of D2 to G4 can beexcluded on the basis of the small UC)matrix ele-ments for this transition (see Table 1). The mostprobable mechanism for feeding the G4 level iscross-relaxation. This is confirmed by the tempe-rature dependence of the relative intensity of theG4 emission.

The sharp line at 12577 cm- (indicated witha ) is, in view of the long decay time (some3000 us), a 3H, * 3H, emission. The decay has aninitial rise of some 80 us, comparable to the decaytime of the D2 level (86.7 us), indicating that the3H4 level is directly fed by radiative transition fromthe Dz level or via cross-relaxation. This meansthat quantum cutting (or photo cascade emission)seems to play a role, as has been observed earlierfor Tm3+ [28]. The emissions found at about17300 and 18300cm- are due to the presence ofDy3 + and Ho3+ impurities, respectively.3.1.1.2. The Dp3H, transition. In the low-resolu-tion excitation spectrum in the D2e3H6 region(Fig. 3), a sharp zero-phonon line is found at28205 cm- and intense sidebands at higher en-ergy. One should expect to see two zero-phonon

10- 8

28000 28200 28400 28600 28800 29000Energy (cm-)

Fig. 3. Low-resolution excitation spectrum of the D2e3H6region of LiYF4:Tm3 at 4.2 K. The DZ + 3F4 emission ismonitored.

Table 2Positions of vibronic transitions of Tm3+ in LiYF, compared tothose of Raman and Infrared vibrations of LiYF, [29,30]. Onlythe vibronic data from Fig. 3 have been included in this table.Raman(cm-)

155179199248264

329425

Infrared(cm- )

143173195224252283303424

Vibronic positionsfor Tm3 + (cm- )

84120

172229

342432

lines, but even in high-resolution spectrum onlyone distinct zero-phonon line is found. The secondzero-phonon might be found in the sideband. Thisstrong sideband was not described in literature[25]. The position of the peaks within the sideband,as calculated from the zero-phonon line, roughlyagrees with the vibronic data on LiYF4:Pr3+ [4]and LiYF4: Gd3 + [3] and the IR and Raman dataon LiYF4 [29,30], see also Table 2. In view of this,


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A. Ellens et al. / Journal of Luminescence 69 (1996) l-15 5

020800 21300 21800 22300 22800

Energy (cm-)Fig. 4. Low-resolution excitation spectrum of the G4t3H6region of LiYF,:Tm at 4.2 K. The G, * 3F, emission ismonitored.

the sidebands are ascribed to vibronics. TheR value for this transition is 1.7.

The decay time of the Dz level, fitted to a one-exponential, is 86.7 us. This value for the D, decaytime is not influenced by possible cross-relaxationeffects since the fitting of the decay is performed inthe tail of the decay curve. In the tail, only decay ofisolated Tm3+ ions (that cannot relax via cross-relaxation) is measured.3.1.1.3. The Gqc=3H6 ransition. In Fig. 4 a low-resolution excitation spectrum of the G4e3H6region of LiY F4 :Tm3 + at 4.2 K is shown. A vib-ronic sideband at 308 cm- from the most intensezero-phonon line is found. This vibrational mode ismost likely the E, Raman vibration, which has alsoa good coupling with the transitions studied forPr3+ and Gd3+ [3,4]. Except for the pronouncedvibronic at 308 cm- , further assignment of thevibronic sideband is impossible because the vib-ronic region, belonging to the strongest zero-phonon line (at 21603 cm-), is obscured byvibronic contributions of the other crystal-fieldcomponents. The small peaks at 22 198 and 22 302cm-, respectively, are either due to the presence ofa second phase or due to electronic noise. TheR value for the G4t3Hs transition is 0.006. Forcalculation of this value the vibronic region bet-ween 21700 and 22 100 cm- has been taken. It isclear that in this way the R value is underestimated.

10 v, / 1 y _

oL____L, ,__A21000 21500 22000 22500 23000

Energy (cm-i)Fig. 5. Low-resolution emission spectrum of the D, * Feregion of LiYF,:Tm at 4.2 K. Excitation is in the ID*level.

The Gq decay time, fitted to a one-exponential,is 570 us. There is an initial build up ( N 90 us)when the excitation is in the D2 level, and whenthe excitation is in the G4 level itself, there isa slight deviation from one-exponential behaviourin the beginning.3.1.1.4. The Dzj3F4 transition. In Fig. 5 a low-resolution emission spectrum in the iDz * F, re-gion of LiYF,:Tm at 4.2 K is shown. Excitationis in the D2 level. Of all the transitions found in thevisible part of the spectrum, the ID, * 3F4transition has the largest U matrix element (seealso Table 1) [28,3 11. Theory predicts intense vib-ronic transitions for transitions with largeU matrix elements (see also Section 4) [8,9]. Alsoin this case the vibronic sideband is obscured by thepresence of several crystal-field components andthe presence of vibronic contributions of othercrystal-field components. As far as the vibronicscan be assigned, the position of the vibronics agreesfairly well with those found in literature (Table 2).The vibronic peak at 21750 cm- will belong tothe lower energetic zero-phonon line at22065 cm- i. In that case the energy of this vib-ronic, relative to its zero-phonon line is 302 cm- .This vibronic line was also found in the spectrum ofthe G4e3H6 transition (Fig. 4). The R value forthe Dz * 3F4 transition is 0.05. This value is ob-tained from the ratio of the intensity of the vibronic


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6 A. Ellens et al. / J ournal of Lumnescence 69 1996) I -15

lines between 21600 and 22000 cm- and the zero-phonon lines. The real R value will be somewhatlarger because vibronics in between the zero-phonon lines cannot be taken into account.3.1.2. YOGI: Tm3+3.1.2.1. General. The second system we havestudied is YOCl doped with Tm3+. In Fig. 6 theemission spectrum upon Dz excitation ofYOCl: Tm3 + at 4.2 K is shown. Just as forLiYF4:Tm3+, there is a remarkable contributionof Gd emissions (indicated with an asterisk). TherG4 + 3H6 contribution is quite large and at lowerenergies (around 15 100 cm- ) also the rG4 9 3F4transition is found. In the case of LiYF,:Tm3 inthis region also contributions from the D2 * 3H4transition are found; for YOCl this is not the case.The weak emissions that can be found around12800cm- are assigned to the Gd j3HStransition. Assignment of the spectral lines is basedon decay time measurements. It is clear that theD2 emission spectrum is dominated by theD2 emission to the 3F, level: no other Dz emis-sions are found.

For the 1Gq~3Hg transition the same tempe-rature and concentration dependence is found as itwas for LiYF4 (see above). Fig. 6 shows the emis-sion of the sample with 1 mol % Tm3+; in the caseof lOmol% Tm3+ the contribution of theG,=s~H, emission is as large as that of theDa + 3F4 emission. Most likely, cross-relaxationis responsible for the D, * G4 relaxation.3.1.2.2. The D2-=H, transition. In Fig. 7 a low-resolution excitation spectrum in the D2e3H6region of YOCl:Tm3+ at 4.2 K is shown, whilemonitoring the ID? =E- F4 emission. Four crystal-field components are found (at 27949, 28 217,28 257 and 28 297 cm-), as one expects forC& symmetry [32]. The peak at 28 353 cm-, at96 cm- distance from the strongest zero-phononline at 28 257 cm-i, might be a vibronic transition.This vibrational Raman mode couples also strong-ly in the case of Gd3 + in YOCl[18,19]. The broadfeatures beyond 28 297 cm- and the unstructuredband between 28 600 and 29000 cm- are mostlikely a summation of vibronic contributions of

70n 3F,

a

64 *

2 *II II0 1I

12000 14000 16000 18000 20000 22000 24000Energy (cm-)

Fig. 6. Low-resolution emission spectrum of YOCI:Tm3+ at4.2 K. Excitation is in the Dz level. The D, emission is mainlyto the 3F, level. The asterisks indicate the G4 emissions.

1086

27500 27800 28100 28400 28700 29000Energy (cm-)

Fig. 7. Low-resolution excitation spectrum of the D2e3H,region of YOCI:Tm at 4.2 K. The D2 =r 3F4 emission ismonitored. The inset shows a high resolution spectrum ofthe region between the two lowest energetic crystal-fieldcomponents.

the different zero-phonon lines. Unfortunately,unambiguous assignment in this region is thereforeimpossible, except for the one at 96 cm- from thestrongest zero-phonon line.It is clear that the vibronic intensity for thistransition is rather large, as was also the case forthis transition in LiYF4:Tm3+. This rather largevibronic intensity is also found in the region bet-ween the zero-phonon lines at 27949 and28 217 cm- (see inset in Fig. 7). Here only vib-ronics up to N 230 cm-, belonging to thezero-phonon line at 27 949 cm- , are found. The


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Table 3Positions of vibronic transitions of Tm3+ in YOCl compared tothose of Raman and Infrared vibrations in YOCl[18,19]. Onlythe vibronic data from Fig. 7 have been included in this tableRaman(cm-)65

Infrared(cm- )

Vibronic positionsfor Tm3+ (cm-)50101

120 123132

153181 192

206220 221248376400525560

position of these vibronics agree with those foundin literature (see Table 3) [18, 193. An estimate ofthe R value for the Dzt3H6 transition is ratherdifficult because of the overlap of the vibronics andthe zero-phonon lines. This does not apply for theregion between 27949 and 28217 cm-. TheR value for the vibronics in this region (belongingto the zero-phonon line at 27949 cm- ) is N 0.2.When one takes into account that the vibronicregion normally extends to some 550 cm- andthat in this case the vibronic sideband can only bedetected up to N 230 cm- , the real R value isestimated to be about 0.5.

The decay time of the D2 level, fitted to anone-exponential, amounts to 12 ps.3.1.2.3. The Gp3H6 transition. In Fig. 8, an exci-tation spectrum of the G,=z=~H~ region ofYOCl: Tm3 at 4.2 K is shown. The increasingsignal at lower energy is due to scattered excitationlight because the resonant lGq 3 3H6 emission at20 47 1 cm- is monitored. Vibronic transitions arefound at distances of 423 and 547 cm- from thecrystal-field component at 21730 cm-. The posi-tions agree fairly well with those measured beforefor Gd3+ and Pr3 in YOCl [18,19]. Vibronicsbelonging to the strongest zero-phonon line (at21409 cm- ) cannot be observed clearly due to the

5z

o L___ Y-f v.L2i----;--;----;-. 120800 21300 21800 22300 22800

Energy (cm-)Fig. 8. Excitation spectrum of the G4t3H, region ofYOCl:Tm3+ at 4.2 K. Due to the fact that the resonantCd* 3Hb emission is monitored, the baseline rises at lowenergy.

presence of zero-phonon lines in its vibronic region.An R value for this transition can be estimated butis inaccurate because only a part of the vibronicspectrum can be distinguished from the zero-phonon lines. The R value is approximately 0.01.For this R value only the vibronic region between21900 and 22 400 cm- has been taken into account.The decay time of the G4 level, fitted to a one-exponential, is 180 ps.3.1.2.4. The D2 - 3F, transition. Finally, theD2 q 3F4 transition of Tm3+ in YOCl is shown inFig. 9. Here the observation of vibronic lines is alsocomplicated by the presence of different zero-phonon lines. Vibronics are found at about43 l-473 cm- from the most intense line at21978 cm- and at 254, 436 and 557 cm- fromthe zero-phonon line at 21654 cm- . The positionof the vibronics agrees quite well with those foundin literature (see also Table 3). The R value, in thiscase the ratio of the vibronic area between 21000and 21600 cm- and the zero-phonon lines, isabout 0.03.3.1.3. Na,La(WO& Tm3+3.1.3.1. General. In Fig. 10 the emission spectrumbetween 12000 and 23 000 cm- of Tm3 + in


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8 A. Ellens et al. 1 Journal of Luminescence 69 (1996) I-15

&

20500 21000 21500 22000 22500Energy (cm-)

Fig. 9. Emission spectrum of the D, q3F4 region ofYOC1:Tm3+ at 4.2 K. Excitation is in the ID2 level.

101 ,2 8J I 3F4ic

6 x 20 3H5-9 4-Z2d

12000 14000 16000 18000 20000 22000 24000Energy (cm-)

Fig. 10. Low-resolution emission spectrum ofNa,La(W0,),:Tm3+ at 4.2 K. Excitation is in the ID2 level. Thefinal levels after emission from the ID2 level are given in the figure.The symbol indicates the 3H4 * 3H6 emission. The emissionat about 16 100 cm-r is due to an impurity, most likely Eu3+.

Na5La(W0& at 4.2 K is shown. Excitation is in 3.1.3.2. The Dp-H, transition. Fig. 11 showsthe D, level (27988 cm-). In comparison with a low-resolution excitation spectrum of thethe two former overall spectra it is clear that the Dze3H6 region, while monitoring the D2 * 3F4contribution of the lG4 =S 3Hs emission is much transition at (22 158 cm-). On the higher energysmaller: in fact it is nearly absent. This is due to the side of the zero-phonon line at 27 980 cm- broadlow Tm3+ concentration, viz. 0.1 mol%: the low structures are found which can be ascribed to vib-concentration prevents cross-relaxation. Although ronic transitions. At a distance of about 405 andcross-relaxation does not occur, one might expect 772 cm- from the line at 27 980 cm- , the pres-that multi-phonon relaxation will play a role to ence of vibronics is clear. They belong to thefeed G4 level because this host lattice has relatively bending (v2 and vq) and stretching (vr and v3) vibra-high energetic lattice vibrations (fiw,,, is some tions of the tungstate group, respectively [20, 331.900 cm-) [20,33]. But since multi-phonon relax- In the calculation of the R value for this transition

10

$ 3:1;-.1027500 27800 28100 28400 28700 29000

Energy (cm-)Fig. 11. Low-resolution excitation spectrum of the D,t3H,region of NasLa(W0,),:Tm3+ at 4.2 K. The D2 = 3F, emis-sion is monitored.

ation becomes very improbable when more thanfive phonons are necessary to bridge a gap non-radiatively [27], multi-phonon relaxation is ex-cluded as a pathway to feed the lG4 level even forNa,La(WO,), :Tm3 +.

Around 12 700 cm- (indicated with a sym-bol), emissions of the 3H4 =S He transition (maincontribution) and the D1 => 3Fz transition (minorcontribution) are found. This means that also inthis case quantum cutting occurs. The emissionsfound at some 16 100 cm- cannot be assigned toa Tm3+ emission. Probably it is an emission of anundesired impurity, most likely Eu3+. The broad-band in the spectrum is due to tungstate emission.From the picture it can also be concluded that theID2 emission spectrum is completely dominated bythe D, * 3F4 transition.


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A. Ellens et al. / Journal of Luminescence 69 1996) l-15 9

20800 21300 21800 22300 22800

Energy (cm-)Fig. 12. Low-resolution excitation spectrum of the G4t3H,region of Na,La(W0,),:Tm3 at 4.2 K. The G.+ + 3F4 emis-sion is monitored.

only the bend and stretch vibrations, which arefound between 28 200 and 29 000 cm- , are in-cluded, as this was also done for Pr3+ in this lattice[ 153. In that case the R value is 0.034. It is clear thatwith this procedure the R value is underestimated:the real value for the whole vibronic sideband forthis transition might be some two times larger.

The decay time of the D2 emission is 8 ps.3.1.3.3. The G,z=~H, transition. In Fig. 12 an exci-tation spectrum in the G4t3H6 region is shown.At a distance of about 230,430,732 and 85 1 cm- of the zero-phonon line at 21436 cm-, vibronictransitions are found. Around 230 cm- the inter-nal (local) modes are found, at - 430 cm- thebending (v2 and vq), and in the regions at about 732and 851 cm- the stretching vibrations (vl and v3)are found. Unfortunately, the low energetic vib-ronics belonging to the zero-phonon lines at about21 150 cm- will be found in the electronic regionof the zero-phonon lines at about 21450 cm-. TheR value for this transition is calculated to be 0.018.There is no doubt that this value is underestimated:it might be about two times larger, because onlythe vibronic sidebands between 21600 and22400 cm- can be taken into account.The decay time of the G, emission is 146 ps.3.1.3.4. The D, =+ F, transition. In Fig. 13 the lowresolution emission spectrum of the ID2 *F,

r

, , x,5y)4) 1ii

-r-7. ~-7

20500 21000 21500 22000 22500

Energy (cm-)Fig. 13. Low-resolution emission spectrum of the D2 * F4region of Na5La(W0&:Tm3+ at 4.2 K. Excitation is in theID2 level.

transition is shown. In this figure also vibronics arefound. The energy distances to the zero-phononlines are given in the picture. The R value, cal-culated for this transition, is 0.03. This value will besomewhat underestimated because the low ener-getic vibronic lines (internal vibrations) belongingto the zero-phonon line(s) at about 22 150 cm- cannot be distinguished from the electronic lines.

3.2. Transition probabilities3.2.1. Calculation of the transition probabilities

The procedure that has been used to calculatethe (vibronic) transition probabilities has been out-lined in detail in Ref. [4]. The vibronic transitionprobability of a transition (J 3 J) is calculatedusing the radiative decay time of level J and thetotal emission spectrum. The procedure is as fol-lows: the decay time of the emission of J has to bemeasured. From the total emission spectrum oflevel J it has to be derived which part of the totalemission belongs to the transition studied (branch-ing ratio). From these two data it can be derivedwhich part of the total (radiative) transition prob-ability can be found in the transition. Then, usingthe R value, the vibronic transition probability ofthat transition can be calculated. This method canbe used in excitation and emission. Importantpoints which one has to consider in this method arethe amount of multi-phonon relaxation and that


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10 A. Ellens et al. / Journal of Luminescence 69 (1996) I-15the entire emission region belonging to emissionfrom level J has to be measured.

Multi-phonon relaxation is not a serious prob-lem for the transitions of Tm3+ that were investi-gated. The latter point, however, can be a problem,especially for the transitions involving theG4 level, where a large part of the emission issituated in the IR: three of the six transitions of theiG4 level are found in the IR. Since our photomul-tiplier tube is not sensitive to IR light, we cannotcalculate the branching ratios of the G, emissions,and hence we cannot calculate vibronic transitionprobabilities for transitions involving the G4 level.This problem manifests itself to a lesser extent alsofor transitions from the DZ level, where theD, + iG4 transition is found in the IR. Fortunately,this is no problem since, as has been concluded above(in Sections 3.1.1 and 3.1.3), the branching ratio forthe ID2 =E=G4 transition is negligibly small.3.2.2. The vibronic transition probabilities for Trnin UYF,, YOCl and Na5La(W04),

The procedure for the calculation of the vibronictransition probabilities has been outlined in Sec-tion 3.2.1 and Ref. [4]. Besides the decay times andthe R values, the branching ratios for the differentemissions (of level J) have to be determined. Tocalculate these branching ratios the integrated lineintensities (corrected for the background signal) ofthe transitions that are involved are taken. Forthese calculations the spectra are corrected for thewavelength dependence of the detector response.

The branching ratios for the iDz emissions ofTm3+ in the three different lattices are shown inTable 4. Determination of the branching ratios isnot easy for the D2 emission of Tm3+ and theerror in the values in Table 4 can in some cases beas large as 50%. The main reason for this largeerror is the uncertainty in the contribution of theID2 * 3Hs transition upon ID2 e3H6 excitation.Scattering of excitation light makes it difficult toobserve emission lines at the wavelengths close tothe excitation wavelength. Two methods were usedto estimate the contribution of the ID2 =z.3Hsemission to the total iDz emission:

On the high resolution (laser) set-up the scatteredlaserlight can be separated from the ID, + 3H6emission with a 1 m monochromator.

Table 4Branching ratios of the D, emission of Tm3+ in LiYF,, YOGIand Na,La(WO,),Transition ID, emission ID, emission ID, emission of

of LiYF,:Tm3+ of YOCI:Tm3+ Na,La(WO,),:Tm3+Dz=.lG, -D,=+3F, 1 1 2D,=-3F3 22 7D,=-3H, 19 2D,a3HD/F:

4 1 145 95 48

D,=s~H, 9 3 40

The D2 emission spectrum was measured afterexcitation in the I6 level. All Dz emission linescan be observed now, but the overlap withI6 emission lines makes it difficult to determinethe branching ratios accurately.

3.2.2.1. LiYF,: Tm+. The two methods mentionedabove were applied to estimate the relative inten-sity of the D2 =E- H6 emission lines: a value of(9 + 5)% was found. This value is different from theone reported by Dulick et al. for Tm3 in LiYF,,which is 30% [25]. Also for other transitions, thebranching ratios for the D2 emissions of Tm3+ inLiYF4 reported by Dulick et al. are different fromthe values in table 4. The origin of these differencesis not clear. Dulick et al. did not show theoverall emission spectra and did not indicate howor if they corrected for the instrumental response(photomultiplier sensitivity and efficiency of themononchromator). In view of the even larger un-certainties in the R values, the uncertainty in thebranching ratios does not constitute the main prob-lem in the determination of the vibronic transitionprobabilities on Tm3 +.

From the branching ratio (45%), the decay time(86.7 us) and the R value (0.05) for theDz * 3F,transition, the vibronic transition prob-ability for this transition can be calculated: it is- 250 s- . With the branching ratio of 9% and the

R value of 1.7 the vibronic transitions probabilityfor the D2e3Hs transitions is calculated to be650 s-l. As menioned before, the vibronictransition probability belonging to the G4e3H6


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A. Ellens et al. 1 Journal of Luminescence 69 (1996) I-15 11

transition cannot be determined because thebranching ratios could not be calculated.3.2.2.2. YOCl: Tm3+. The iDa G. 3H6 contributionto the total Da emission of Tm3+ in YOCl wasestimated from the high-resolution set-up only: it isabout 3%. The vibronic transition probability be-longing to the 1D2-=3H6 transition, calculatedfrom the branchng ratio (3%) the decay time (12 ns)and the R value (0.5) is about 850 s-i, but it mightbe about two times larger because of the largeuncertainty in the branching ratio. The vibronictransition probability of the iDz * 3F4 transition,calculated from the branching ratio (95%), the decaytime (12 us) and the R value (0.03) is about 2300 s- .3.2.2.3. NaJ,Qz(WO,),: Tm3+. The last lattice in thisseries for which the vibronic transition probabilit-ies are to be calculated is NaSLa(W0,),:Tm3+.For NaSLa(WO& :Tm +, the conribution fromthe Dz * 3H6 transition seems to be quite large,according to the high-resolution measurements: theD2 s 3H6 branching ratio is 40%. An attempt toexcite in the I6 level did not result in D2 emission,so that we could not check this value. Therefore, itwas tried to excite (in low resolution) in a highenergetic vibration of the D2 level and observe theDz emission. For Na5La(W0&, the problem ofa resonant emission might be less, because it hasrelatively high energetic vibrations and thus, byexcitation in a high energetic vibronic line, there isa considerable difference in excitation and emissionwavelengths. The branching ratio calculated from thisexperiment was in the same order of magnitude as the40% derived from the high-resolution experiment.

From the decay time (8 us), the branching ratio(40%) and the R value (0.034) the vibronictransition probability for the Dz e3H6 transitioncan be calculated: Avib is 1100 s- . CalculatingAvibfor the Dz * 3F4 transition from the branch-ing ratio (48%), the decay time (8 ns) and theR value (0.03), the value for Avib s found to be some2000ss.4. Theory on vihronic transitions

Before discussing the results on the vibronictransition probabilities, a short summary of the

theory on vibronic transitions is given [3-~5,8]. Thevibronic transition probability is a summation ofcontributions of the M process and the A process:AVib=AVib (M process) + AVib (A process). (1)M process vibronics are also known as vibration-ally induced forced electric dipole transitions. Thevibronic transition probability arises from the ad-mixture of the opposite parity states into the 4fstates by coupling with assymmetric vibrations.Theoretical work on the transition probabilitiespredicts that:Avib M process)x x v(U2) (7y2(2J + l)(Avib.St AVib.D). (2)In this formula x is the local field correction term,IJc2) is the matrix element as tabulated by Carnall,[31] and T (l) is the matrix element linking theinitial and final vibrational levels. According to thisformula (2) one expects strong vibronics fortransitions with a large U matrix element anda good coupling with IR active vibrations for cen-trosymmetric systems. In formula (2), Ayib,St is thestatic contribution and Avib,D is the dynamic contri-bution. It is assumed that the importance of thedynamic contribution increases with increasingcovalency [ 31.

For the static contribution the following equa-tion applies:

Here R is the ion-ligand distance, g and c( are thecharge and polarizability of the ligand, N the co-ordination number and Ec1.2Jdescribes the admix-ing of the opposite parity states. Ec1,2j scales withthe l/AE(,,,,, where AE,,,,, is the difference inenergy between the 4f states and the oppositeparity states. The lower the position of the oppositeparity states, the better the admixing of oppositeparity states in the 4f levels and the stronger thevibronic transitions will be. The formula for thedynamic contribution is


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2 A. Ellens et al. /Journal of Luminescence 69 (1996) I-15

Here, o2 accounts for the screening of the 4f elec-trons by the multipole field of the 5s and 5p elec-trons; (r2)4f is the average electron-nucleus dis-tance.

For f-f transitions of trivalent lanthanide ions,usually only the M process is assumed to contrib-ute, although there are reports that show a signifi-cant contribution from vibronic contributionsinduced by the A process [3,4]. These A processvibronic transitions are due to an offset between theground state and the excited state parabolae in theconfigurational coordinate diagram (i.e.Franck-Condon phonon replicas). The formula forthe A process is the following:

e ssiAvib (A-process) CCAzP -. i (5)A,, is the zero-phonon line transition probability,S is the Huang-Rhys factor and i gives the numberof phonons involved. This forula states that thelarger the zero-phonon line transition probability,the larger the vibronic transition probability. Thiszero-phonon line transition probability (A,,) de-pends on the U (2)*Uc4and U@)matrix elements. Italso shows that for small S, Avib is proportional tothe Huang-Rhys factor, which is a measure of theoffset. Vibronics due to the A process can be eitherinfrared active or Raman active vibrations.

5. Discussion5.1. Vibronic transition probabilities of Tm3

It is clear that the search for the vibronictransitions of Tm3+ is not an easy one. The mainproblem is that this ion has no ideal transitions tostudy the vibronics. In the case of Pr3+, the3P0-+3H4 transition can be studied. In that case,due to J = 0 for the 3P,, level, in the excitationspectra only one crystal-field component and itsvibronic sideband are found at 4.2 K. For Gd3 + thesame applies, when in emission the 6P7,2 * sS,,Ztransition is studied. Due to the fact that the8S7,2 level is orbitally nondegenerate, at 4.2 K onlyone zero-phonon line and its vibronic sideband arefound. This makes the assignment of the vibronics

and the calculation of the R value for thetransitions of these two ions much easier than forTm3+. The only level of Tm3+ that has one crystal-field component, the 3P,, level, does not satisfy: it isfound between the numerous crystal-field compo-nents of the 16 level. That means that whatevertransition for Tm3+ is studied, the vibronic regionwill always be obscured by the presence of (other)zero-phonon lines.

A compilation of the results on the vibronictransitions of Tm3+ in the three lattices is given inTable 5. For the G4e3H6 transition of Tm3+, thevibronic transition probabilities cannot be cal-culated (see Section 3.2.1). That leaves theDZe3H6 and D, = 3F4 transitions. In fact, thesetwo transitions are two interesting candidates tostudy because one would expect, according to the-ory, weak (or no) vibronic sidebands for the formerand strong vibronic sidebands for the latter, sincethe ( U2)J2matrix elements are 0.0 and 0.5598, res-pectively. On the other hand, the selection rule thatUc2) must be nonzero to find vibronic transitionshas been deduced from the theory on vibronictransitions of the centrosymmetric REClG3 - systemof elpasolites. Previous studies have proved thatthis selection rule does not apply [4-6, lo] and thathigher-order perturbations have to be included inthe theory. Also this study shows clearly that theU2 0 selection rule does not apply.

From table 5 we can conclude that there is anincrease in vibronic transition probability whengoing from the ionic lattice LiYF4 to the morecovalent systems YOCl and NaSLa(W04)4. Thedifference between YOCl and Na5La(W0J4 is notlarge. This increase in Avib with increasingcovalency of the host lattice was explained beforefor Gd3+ and Pr3+[3,4,9]. Both the M process andthe A process can explain this behaviour.

In the case of M process, the increase in Avibwithincreasing covalency can be explained if one as-sumes that the U O selection rule is lifted. Thefactors in Eqs. 3 and 4 that vary significantly withvarying covalency are the polarizability (a), theadmixture of the opposite parity states (Ec1,2,)andthe screening factor (cr2). These three factors doenhance the vibronic transition probability withincreasing covalency. The cooperative influence ofthese three factors could explain the variation in


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14 A. Ellens et al. / Journal of Luminescence 69 (1996) I- 15

and heavy lanthanide ions and a weak coupling inthe centre of the series. We do not have a quantitat-ive explanation for this trend, but with the threefactors mentioned above the trend can be explainedqualitatively. The lanthanide contraction will in-crease from the light to the heavy lanthanides andwill cause the vibronic coupling strength to de-crease from light to heavy lanthanides. The increaseof the vibronic coupling beyond Gd3+ (4f7) mightbe explained by the screening factor g2. Calcu-lations have shown that the screening factor be-comes smaller for the heavier lanthanides. Thiseffect influences the electron-phonon couplingstrength in a way opposite to the contraction of the4f orbitals. The combination of these two effectscan explain the trend.In addition, the variation of the position of thelowest energetic opposite parity states can causethe vibronic transition probabilities for lanthanideswith low-lying opposite parity states to be some-what higher than those expected and for lanthanideions with high energetic opposite parity states to besomewhat smaller than the trend. The weakelectron-phonon coupling for lanthanide ionsin the centre of the series is confirmed byvibronic transition probabilities that can bedetermined from Refs. [9,37] for Eu3+ (4f ). Thevibronic transition probabilities for the5D0 + 7Fz transition of Eu3+ in LiEuF, andNa5Gd(W04)4: Eu3+ are about 5 and 40 s-l, res-pectively. These are of the same order of magnitudeas those of Gd3+, and significantly smaller thanthose of Pr3+ and Tm3+.

It is clear from this paper and other work [3-51that the calculation of the vibronic transition prob-abilities for Gd3+ is relatively easy, for Pr3+ itbecomes more difficult, and for Tm3+ it is hard.For lanthanide ions with a more complex energylevel scheme it will be even more difficult to calcu-late vibronic transition probabilities. Therefore,to make a systematic comparison of the elec-tron-phonon coupling strength it is recommen-dable to use other methods. We have tried andmade a start with a more systematic research to theelectron-phonon interaction strengths of trivalentlanthanide ions from linewidth measurements[38]. This seems to indicate the same trend aspresented by Hellwege and as derived from the

vibronic transition probabilities of Pr3+, Gd3+ andTm3+.

6. ConclusionsFrom the measurements on the vibronic

transitions of Tm3+ it can be concluded that thevibronic transition probability is enhanced bycovalency, as it is also the case for Pr3+ and Gd3.The vibronic transition probabilities of Tm3 +seems to be of comparable magnitude as those ofPr3+, but significantly larger than those of Gd3+.Therefore, there is an indication that the elec-tron-phonon coupling strength is large in the be-ginning and the end of the series and small in thecentre. This trend might be explained with thefollowing parameters: position of the opposite par-ity states, lanthanide contraction and shielding ofthe 4f electrons.

Vibronic transition probabilities are not easy todetermine for Tm3+ and many other trivalent lan-thanide ions. Therefore, using vibronic transitionprobabilities as a measure of electron-phononcoupling strength is not an easy way to comparethe electron-phonon coupling strength for differenttrivalent lanthanide ions.

AcknowledgementsWe are grateful to Mr. G.J. Dirksen for growing

the LiYF,:Tm crystal and to Mr. D. Reindersfor doing introductory measurements. The inves-tigations were supported by the Netherlands Foun-dation for Chemical Research (SON) with financialaid from the Netherlands Organization for Scien-tific Research (NWO).

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Documents

Vibronic transitions of Tm in various lattices