Absorption and laser excited fluorescence of Ho3+:LaF3

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Pramina, Vol. 10, No. 5, May 1978, pp. 467--475, © printed in India.

Absorption and laser excited fluorescence of He s+ : LaF3

B A N S I L A L a n d D R A M A C H A N D R A R A O

Department of Physics, Indian Institute of Technology, Kanpur 208 016 MS received 16 August 1977; revised 6 March 1978

Abstract. The absorption and fluorescence spectra of He3+: LaF3, in the wavelength region from 4000 .~. to 7500 A have been reinvestigated incorporating polarization features, for the first time. The fluorescence spectrum, recorded using a spectre- photometer assembled in the laboratory, was excited with a He-Ne laser and an At+

laser. The present study enabled the interpretation of the data in terms of the Cz v site symmetry of He 3+ and most of the Stark components of the observed states have been classified under the irreducible representations of the C2v point group.

Keywords. He ~+ :LaF3; polarized absorption and fluorescence; symmetry classification.

1. Introduction

The SLJ states o f the lowest configuration o f H e 8+, 4 f 1°, have been fairly well established b y various studies made from holmium salts and H e a+ as impurity in various matrices. T h e h o l m i u m salts studied so far are: He2 (SO4)a (Gobrecht 1938;

Mechan a n d N u t t i n g 1939), HoCI a, HozO ~ (Khale 1956; Singh 1957) Ho2(C2HsSO4) 3 • 9HzO (Spedding a n d Rothwell 1968) and H o F e O z (Walling and White 1974). Dieke and Pandey (1964) studied the absorption and fluorescence spectra o f HoS+: LaCI 3 in detail, while R a j n a k a n d K r u p k e (1968) gave a comprehensive theoretical interpretation o f H e 3+ : LaC13 spectra. The other crystals containing H e ~+ that are studied so f a r a r e : CaF2 (Merz and Parshan 1967), Y G a G , Y I G (Johnson et al 1969), CaWO 4 ( W o r t m a n a n d Sanders 1970), YPO 4 (Becker 1971), Y A l e 3 (Weber a n d Matsinger 1972) LiTmF~ (Wortman et al 1973) and very recently in tellurite, calibo and phosphate glasses (Reisfeld and H o r m a d a l y 1976). Caspers et al (1970) have extensively studied H e 3+ : L a F 3 b o t h in absorption as well as in fluorescence at l'5°K and have established the complete energy level scheme up to 26,000 c m -1.

They also calculated the intermediate coupling free ion energy levels with a n rms deviation o f a b o u t 29 cm -1. However, there is no study so far o n the polarization features o f the H e a+ : L a F z spectra or on the site s y m m e t r y o f H e 3÷ in L a F 3.

In the present w o r k the absorption and fluorescence spectra o f H e a+ : L a F 3 h a v e been reinvestigated incorporating the polarization features. The polarized absorption spectrum in the region 4000-7000 A, p h o t o g r a p h e d at 80°K and the laser excited polarized fluorescence spectra also recorded at 80 ° K, are reported for the first time in this work. The polarization features o f the observed spectra have been interpreted in terms o f Czv site s y m m e t r y o f H e 3+ and most o f the Stark components o f the states, observed in a b s o r p t i o n and fluorescence have been classified according to the irreducible representations o f this symmetry group.

467

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468

Bansi Lal and D Ramachandra Rao

2. Ho 3+ site symmetry and selection rules

The crystal structure of LaF s has not been established uniquely. Different experimental studies have indicated different crystal structures for this crystal. For example, the E P R measurements made on a number o f rare-earth ions doped in L a F z (Baker and Rubins 1969) are consistent with a hexagonal and hexamolecular unit cell belonging to C 3 space group. These results are further substantiated by neutron

6v

scattering studies (Collete and Zelwer 1966). However, two more symmetric 3 and

D4d

have been proposed on the basis of x-ray measurements structures viz.

D6h

(Mansman 1964). The Roman spectra (Bauman and Porto 1967) of LaF 8 could be interpreted assuming D4dStructure but only small deviations from

Drh

3 structure were observed. The polarization features of the optical spectra of Pr 8+ : L a F s (Wong

et al

1963) and Eu 3÷ : LaF 3 (Kumar

et al

1977) could be explained on the

3 3

basis of

Drh

structure, indicating that

Drh

structure is not far from reality. The present study is also compatible with

D3h

structure.

The site symmetry of La ions is C2o in

D6h

3 space symmetry o f LaF s and the unit cell is hexamolecular. The six La ions lie on two planes (La planes) forming equi- lateral triangles which are rotated by 60 ° with respect to each other with a separation of

c[2

between the p!anes. The crystal z-axis or c-axis is perpendicular to ionic c-axis. To derive the selection rules a co-ordinate system

XYZ

in which X and Z lie in a plane perpendicular to c-axis (Y-axis coinciding with c-axis) is used. The horizontal plane being a symmetry plane the X-axis and the Z-axis are indistinguish- able. This degeneracy of X and Z-axes leads to ambiguity in the classification of various states in terms of C2v irreducible representations. Specifically, it is not always possible to distinguish between A 1 and B 1 or between A~ and B~ (A1, A2, B1 and B l are the irreducible representations of C2v point group).

The seleetion rules for electric dipole transitions are given in table 1. The o-transition refer to the electric vector perpendicular to c-axis (Y-axis) of the crystal and the ~r-transition refer to the electric vector parallel to the c-axis.

3. Experimental details

A single crystal of LaF a containing Ho 3+ as impurity purchased from the Optovac Co.

(USA), was kindly loaned to us by Professor H P Broida of the University of Cali- fornia at Santa Barbara. The crystal (10 mm × 7 m m × 4 mm) is of very good optical quality and according to the manufacturer's specifications, the holmium concentration in the crystal is 0"5 Yo and the c-axis is parallel to 4 mm side. This c-axis has been further checked by viewing the crystal under a polarization microscope. From the present fluorescence data, it is found that the L a F a crystal contains an unknown

but small quantity (<0"1 per cent) of praseodymium also.

The absorption spectrum in the region 4000-7000 A at 80 ° K has been photographed on a 3"4 m Jarrell-Ash plane grating (30,000 LPI) spectrograph at a dispersion of 2"5 A/mm. A 750 watt tungsten (projector) lamp provided the con- tinuum. The plates were measured on a Carl-Zeiss Model B Abbe eomparator.

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Absorption and laser excited fluorescence

6324.1A

0

6421,5,~

6 4 i 2 . 8 ~ (5404.1A

o

638(9.4 A

O

6410.7,~

6 3 9 9 - 8 A

o

63(90.7 ,~ (a)

469

6 3 7 5

.(9

^AO

6360.0,~,

~'6352-9 A

o

5359.1

A

O

5 3 7 4

.(9

5 3 6 0 . 7

"5349.5

5 3 4 1 . 4 5 3 3 7 - 2

5 3 6 1 . 4 A o

5 3 s 6 1

5 3 4 9 - 5 A

O

5336" 8 A

O

(b)

-5325.6

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470 Bansi Lal and D Ramachandra Rao

4 5 0 3 , ] o

A"

4488.8 J 42 8z9 ,,

4 4 7 3 ; 7 A

o

4 4 6 1 . 4 ~

4 449.2/~

o

4 4 4 1 4 A

4 5 0 4 . 7

A 0

4 4 9 7 . 3 A

0

4 4 8 5 . 8 A

0

4 4 7 4 . 6 A

0

4 4 6 5 . 7 A

o

4 4 5 5 - 2 A

o

4 4 4 9 . 3 A

o

4 4 4 0 . 6 J

(~

4 1 5 3 . 2

~ 5 4 . 8

A

o

4 1 4 5 ' 4 1 4 6 . 2 A

o

(d),

( O

4123. ,..5 ,Z 2 3 - 7 A

Figure 1. Ho 3+ in LaF3: Polarized absorption at 80°K, (a) 6400 A. group Co) 5305/x group (c) 4480 A (d) 4135

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Absorption and laser excited fluorescence

471 A He-Ne laser and an Ar ÷ laser (Spectra Physics USA, Model No. 165-03) were used to excite the fluorescence. The He-Ne laser has been fabricated in the laboratory. It consists of a 2m long plasma tube (4 mm dia) terminated at Brewster's windows. A dielectric coated spherical mirror (radius of curvature 2 m) and a dielectric coated half prism at Brewster's angle, form the laser cavity. A 5 kV, 100 mA unregulated power supply is used for exciting the plasma tube.

The He-Ne laser excited fluorescence spectrum has been photographed on a 3-prism glass spectrograph (Carl-Zeiss) fitted with 27 cm camera. The Ar ÷ laser excited fluorescence has been recorded using a spectrophotometer assembled in the laboratory (Rao

et al

1976). The dispersion on the recorder chart paper o f the spectrophotometer is variable between 5 A/ram to 1"0 A / m m and the wavelengths measured, are accurate to :t:1"5/~. A demountable cold finger liquid nitrogen dewar was used for low temperature work. The minimum temperature to which the crystal mounted in the dewar, could be cooled is about 80 ° K. A Glan-Thomson polarizer was used to study the polarization features of the absorption and fluorescence spectra.

4. Absorption spectrum

The absorption spectrum photographed at 80 ° K in the range 4000-7000 A consists o f four groups of lines around 4135A, 4480A, 5305A and 6400A (figure 1). All the absorption lines are completely polarized. By comparison with the absorption data reported by Caspers

et al

(1970) all the lines in 4135A and

5305A

groups are easily identified as due to electric dipole transitions

5Is(Z)--SG~(J )

and

sI8(Z)--nF4(E),

nSz(E°), respectively. When such comparison is made for 6400A group, it is found that though most of the lines of this group are also due to electric dipole transitions

5Is(Z)--SFs(D),

six additional lines observed on the higher energy side of the main absorption lines could not be assigned. These additional lines are tentatively assigned to be from vibronic transitions. Most of the lines of the remaining absorption group around 4480~k are identified as due to electric dipole transition

5Is(Z)--SGe(I ).

How- ever, it is not possible to assign any transitions to some ten lines observed on the low energy side of the

5Is(Z)--SGe(I)

absorption group.

5. Fluorescence spectrum

The fluorescence spectrum has been recorded at 80°K in the wavelength region 4500- 7600/k, using 6328/k laser line of the He-Ne laser and various laser lines of the Ar ÷ laser as excitations. The fluorescence excited by 6328]~ laser line is identified to correspond to the electric dipole transition

5Fs(D)--51s(Z )

while the states

5F3(F), 5F4(E ), ~S2(E °)

and

~F~(D)

are found to fluoresce when excited by Ar+ laser. The various transitions (figure 2) involved in fluorescence are:

4, % ) - - 5Isand

P.~2

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472 Bansi Lal and D Ramachandra Rao

Out of these transitions, the transitions 5F8--518 and (~F 4, ~S~)--~I7 have been observed for the first time in fluorescence while in the remaining transitions, more number of lines have been observed in the present study than reported by Caspers et al (1970).

Most of the fluorescence lines are completely polarized and the rest of the partially polarized lines could be explained as due to overlapping transitions. The two fluorescence groups observed for the first time in the present study, are shown in figure 3.

Among the various wavelengths of Ar + laser used for exciting the fluorescence, 4658 A. laser line coincides with the transition from the lowest Stark component of the

x 'P03C m -1 2 O

1 0 - ~ - / 3

- )F,

5S2

5F~

5 I 4 5 I 5 516

, 5 I 7

m %

. F : E

0 - C - B - A

. y i i

I I

I I ' _ Z

o ~ o ~ o < o<1:

o o o o o

Figure 2. Partial energy level diagram o f H o 3+ : LaFa showing the observed fluore- scence transitions

ground state to aK 8 state. The excitations due to other wavelengths could be phonon assisted processes. However, from the present studies it is not possible to pinpoint the actual processes responsible for exciting the ions.

6. Classification of the Stark components

The transitions corresponding to the various lines observed in the absorption spectrum are identified from the absorption data given by Caspers et al (1970). Further, the Stark components are classified under the irreducible representations of C2v point group using the polarization features of the absorption spectrum and the selection rules given in table 1. Also, the analysis of the fluorescence spectrum has been used to confirm the assignment made for some of the Stark components from the absorption, data, while it could suggest new assignments where absorption data were insufficient in the classification of the Stark components. The latter is true, particularly, for the Table 1. Selection rules for electric dipole transitions at rare-earth ion site in LaFs

C2v At BI At B~

A 1 6 r G - - ~ r

n I { 7 a "~T - -

A . - - 7r ~

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Absorption and laser excited fluorescence

J

7r - polorlzed

I I I l l I I I I I I I

o~0

(a)

473

II s t

(b)

~ z e d w

! I III ,,~v ~ .oO~ ~- ~ ~ ' 0 . .

~,~ ~ ~-'~ 8 ~_'2

Figure 3. At+ laser excited fluorescence of Ho a+ : LaFs at 800K (a) 4800 A group

(b) 7450 A

g r o u p .

ground state, because its higher Stark components cannot be observed in absorption.

To classify the various Stark components, the following procedure is followed. As the absorption is observed only from the lowest three components, Z 1, Zs, Z3 o f the ground

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474 Bansi Lal and D Ramachandra Rao

state, s/s, o n l y these along with the involved s t a r k c o m p o n e n t s o f excited states arc classified with the help o f the absorption data. I f the fluorescence included a n y states where a b s o r p t i o n d a t a are analysed, the analysis o f these fluorescence groups is considered first either to confirm the assignments m a d e or to m a k e assignment for the unclassified S t a r k components. Thus, thc higher Stark c o m p o n e n t s o f s i s, are classified with the help o f fluorescence t e r m i n a t i n g in them. T h e states n o t observed in a b s o r p t i o n b u t in fluorescence only are analysed at the end.

Table 2. Classification of the Stark components of various states of Hog+: LaFs

SLJ Stark Species SLJ Stark Species

state component (under Csv) state component (under C2v)

eG~ J~ A~B~ ~Fe DI A1

J~ Bx D~ B1

Ja A2B~ Da A~Bz

J, A1 D4 A~Bz

Js A~B~ D5 A2B2

Je A~Be De A2B~

J~ A~Ba D, Bt

Js AzB~ De A2B2

J. AI D, AI

Jle B~ D~e B~

Jll B1 Dlx AzB~

5Oe Ix A2Bm 617 Yx - -

I2 A1 Y~ - -

Ia A2B2 Ya - -

I4 B1 Y4

]5 A2B~ Y5 A2B~

Ie A~B2 Ys - -

I, B1 Y7 AIBs

Is A1 Ya - -

I. B1 Ya A1B1

Ilo A1 YIo A1B1

In A1 Yu - -

Ils AsBi Y12 - -

Ixs A2B2 Yls

aFa FI A2Bs Y~, Y15 --

F2 A~B~

F8 A1B1 sI. Z, B~

F4 AsB2 Z2 A1BI

F5 A1B1 Z8 A1B1

F6 A2B2 Z4 B2

F, A1B~ Zs A2B2

Z6 A1Bt

Z, AIBi

Z. A1B~

Z. A~Ba

Zlo A1B~

Zu A~B~

Z~ A1Bx

Zla A2B~

Z~, A~Bt

Z~ A~B~

Zl~ A~B1

Z1, A~B2

5F,

5S I

E: A:B:

Ea AI

Ea A2Bz

E4 A1B1

E5 A2B~

Ee A2B~

E, A1B1

Ea A1

E9 A2B2

El ° A2B2

E2 ° A~

E3 ° A~

E~ ° A~B2

E5 ° B~

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A b s o r p t i o n a n d laser e x c i t e d f l u o r e s c e n c e 4 7 5 A s m e n t i o n e d e a r l i e r t h e f l u o r e s c e n c e s p e c t r u m c o n t a i n s b o t h c o m p l e t e l y a s w e l l a s p a r t i a l l y p o l a r i z e d lines. F o r t h e c l a s s i f i c a t i o n o f S t a r k c o m p o n e n t s t h e p o l a r i z a - t i o n p r o p e r t i e s o f c o m p l e t e l y p o l a r i z e d lines o n l y a r e u s e d .

A l l t h e S t a r k c o m p o n e n t s o f t h e states 5G 5, 5G 6, 5F3, b F 4, 5S 2, 5 F 5, a n d 518 a n d a f e w o f 517 h a v e b e e n c l a s s i f i e d i n t e r m s o f t h e i r r e d u c i b l e r e p r e s e n t a t i o n s o f C2v p o i n t g r o u p . T h e c l a s s i f i c a t i o n is p r e s e n t e d i n t a b l e 2.

7. Conclusions

T h e a s s u m p t i o n o f C2o s i t e s y m m e t r y f o r r a r e - e a r t h i o n s i n L a F s o n t h e w h o l e s e e m s t o b e f a i r l y v a l i d a s t h e p o l a r i z a t i o n f e a t u r e s o f a l m o s t o f a l l t h e lines i n a b s o r p t i o n a n d f l u o r e s c e n c e s p e c t r a o f H o 3+ : L a F 3 a r e s a t i s f a c t o r i l y i n t e r p r e t e d u n d e r C2o site s y m - m e t r y . H o w e v e r , f o r m o s t o f t h e cases, i t h a s n o t b e e n p o s s i b l e t o d i s t i n g u i s h b e t w e e n A 1 a n d B 1 o r b e t w e e n A9 a n d B 2 a n d t h i s a m b i g u i t y p e r s i s t s i n t h e p r e s e n t c l a s s i f i c a t i o n .

Acknowledgements

T h a n k s a r e d u e t o D r U V K u m a r f o r t h e d i s c u s s i o n s w e h a d d u r i n g t h e a n a l y s i s o f t h e d a t a . W e a r e g r a t e f u l t o P r o f . P V e n k a t e s w a r l u f o r h i s i n t e r e s t i n t h i s w o r k a s w e l l a s f o r p r o v i d i n g A r + l a s e r e x c i t e d f l u o r e s c e n c e s p e c t r u m r e c o r d e d a t S a n t a B a r b a r a , w h i c h s e r v e d a s a g u i d e l i n e t o o u r e x p e r i m e n t s .

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