SELECTIVE KINETIC SPECTROSCOPY OF QUENCHING AND MIGRATION OF OPTICAL EXCITATIONS IN DISORDERED SOLIDS

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SELECTIVE KINETIC SPECTROSCOPY OF

QUENCHING AND MIGRATION OF OPTICAL

EXCITATIONS IN DISORDERED SOLIDS

T . T . Basiev

To cite this version:

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SELECTIVE KINETIC SPECTROSCOPY OF QUENCHING AND MIGRATION OF OPTICAL

EXCITATIONS IN DISORDERED SOLIDS

T.T. Basiev

I n s t i t u t e of Genera2 Physics, Academy of Sciences of t h e U.S.S.R., VaviZov s t r . 38, Moscow, U.S. S.R.

Abstract

-

The report is based on the kinetic and spectral-luminescence da-

ta studies of crystals and glasses doped with rare-earth ions.

The recent data are discussed concerning the superiority of the high-multipo- le interactions; attaining ordered arrangement of impurities in glasses by means of optical techniques; revealing a diffusional stage of the donor-donor migration kinetics in a stochastic ensemble of the impurity centers.

I

-

INTRODUCTION

This paper will deal with the analysis of two fundamental questions of the energy transfer in a stochastic ensemble of centers, which is the basis of all more complicated degradation processes. The questions:quenching energy transfer without migration and migration transfer without quenching.

They may be investigated in two levels: macro- and microlevel.

I. Macrolevel includes: determination of the kind of the decay curve of the donor excitation due to quenching transfer and energy migration; selection of various time stage; determination of average decay rates ( W ) on each stage.

11. Microlevel includes: determination of microcharacteristics of the ion-ion elementary interactions, such as multipolarity (

s

),microefficiency (

c

DA'

CDD

!;

elucidation of microfeatures of the spacial impurity center distribution In disor-

dering medium ( R ).

As an experimental method we have used a method of the short

8

-pulse laser

excitation with registration of damping kinetics and luminescence spectra with high temporal resolution in the dynamic range/{/.

Silidate and phosphate lasses oped with sm3+, ~ d ~ + , E U ~ + , yb3' ions have se-

lected as model b'ects. 3&nd Ndd+ are characterized strong selfquenching of the

luminescence. EuBtJand Yb have no selfquenching.

I1

-

DIRECT QUENCHING ENERGY TRANSFER/Z/

The forbiddenness of the optical transitio 4 excludes in practice

the influence of the migration intaraction (Sm

a

"5/2;m3"~: whereas the concen- tration quenching is quite strongly observed for it ( Fig. I 1. The analyses of the different stages of the nonexponential luminescence-quenching kinetics is conveni- ent to do by choice linearization scale, such as 'it has shown on Fig.1. The quenching kinetics can be divided into two time domains ( Fig.Ib ) .

The slope tan 'fl of these plots ( Fig.Ib ) yields the exponent of the parame-

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C7-160

JOURNAL

DE

PHYSIQUE

ter t, making it possible by the same token to distinguish between ordefy: ( expo- I

nential) quenching ( t ) and disordered (nonexponential) quenching ( t ) and to estimate the multipolarity of the latter.

The final sf7ge of the kinetics ( t>t ) has well known Forster type

I(t)=exp(-(WFt) ). The slope tan (4 of the plots of Fig.Ib takes on at t

>

tl values 0,39; 0,36; 0,41

,

which are close to f p val e 0,375=3/8, thus pointing to a dipole-quadrupole ( 5 ~ 8 ) mechanism of the Sm - SmY+ interaction. The first (t<t ), exponential (orderd) stage of the kinetics I(t)=exp(-W t) carries direct informat%-

on on the most important parameters of the spatial arr!ngement of the impurity ions,

namely on the shortest-approach distance of . .

loo t 7 ps the perticles ( R ).

From the slopgs of the plots on Fig.Ia and F i g . 1 ~ we can determine the average quenching rates on the first ( W ) and second ( W _F) sta-

ae of kinetics. The exEerimental1~ obta~ned

values of the macroparameters W ; WF

,

which

increases linearly, enable us t! find the va-

lue of the microefficiency of the donor-acceptor quenching interaction ( C ) and the near- est - approach distan s?? (

R

)?A8~oA(~m

-

.).5

t , s

=W /(4/33r (1-3/S)n) 17~A2J)nm /ms.

10-4

10-z

R ( ~ m - s~)=(?c n / ~ ) =5,6

I.,

The large vgfue

W

(Sm - Sm)=5,6

A

for I ,

I I Li-La-Sm-phosphate gl!ss which is close to the

comparable value for several high-concentrati- on crystal phosphates (R =5,2

-

6,5 A) and cor- relates fairly well withm

R

(Nd

-

Nd)=4,6-5,4A

for Li-La-Nd-phosphate lase! glass is one of

the most important causes the existence of ano- maliously weak quenching of rare-earth ions in this glass.

I11 - SPECTRAL-SPATIAL MIGRATION OF THE OPTI- CAL EXITATION / I , 3.51

L _{tl= 40 ps}

Nonstationary selective laser-luminescent

0,04 0,08 t3/8, .3/8 spectroscopy allows to investiqate the most

complicated and interesting probems of a con- densed state. I mean collective interactions in nonuniform group of impurity particles at their optical excitation.

Further on we dwell on collective interactions which do not lead to the optical excitation decay, but only cause their migration in the coordinate and frequency space. A direct investigation of the processes of spectral-

.

space migration turned out to be available on-

ly in non-uniform media due to application of frequency-selective laser excitation.

At the low temperatures T=4,2 K , when a nongomogeneous broadening exceeds by many or-

Fig.1. Kinetics nf quenching lumi- ders the homogeneous one, the phonon partici-

nescence from 4~ 2!)veimgt d:LL:- pation in attaining the resonance conditions

rent densities n ?bm ) is decisive and has typical manifestation in

in Li-La-Sm- hosphate gla

28.

the spectral migration. A low-temperature for-

1

-

2,5. 1;':4 2

-

9,2.10

,

biddenness of transitions with .phonon absorp-

3 - 2,3.10 tion determines an irr.eversible Stokes-type

energy migration

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sive long-wave or red shift of the latter with the time delay increasing. Such spectra dynamics emphasizes a non-resonance character of the considered irreversible Stockes energy migration with the emmission of an acoustic phonon.

Direct measurement of t k decay curve of the narrow component of the spectrum allowed to identify rel3ably a root character of decay. It pointed to a dipole-dipole interaction for Yb + in glass (S=6). The selection frequency variation from

the long-wave to the short-wave wings leads to a sharp increasing of the mean migration rate due to growing number of the "acceptor" centers (

gA

<

\IL).

Such 2ependences were observed for ~dl' ions having unlike Yb

+,

a more complicated multi-stark structure of nonuniformly broadened bands.

It is assumed to employ the established dependence with the aim of optical control of the rate of low-temperature energy migration.

A

detailed investigation of the form of non-stati-

onary spectra of nonhomogeneously broadened Stokes base made it possible to obtain the frequency dependence

of the efficiency

gf

elementary phonon-assisted inter- 1 , ~ ms

actions between Yb lons.

We have found that the spectrum shape of the Stoc- kes base proportional to the center distribution over transition energy and a root of frequency dependence of elementary interaction microefficiency C (Yb-Yb). As a result, an interesting dependence was %taied of

the efficiency of elementary interactions versus ener- _-₁

gy mismatch, which corresponds to the emitted phonon

energy (Fig.3). This dependence obtained is close to quadratic. This show the main role of acoustic phonon in the processes of the low temperature spectral migrations.

The sample temperature increasing stimulates anti-Stockes acts energy transfer and makes the problem of interaction between the non-uniform centers spectrally uniform. The spectral analysis of migration is in this case considerably simplified,al- lowing to describe the process by means of unique for all centers microparameter

CDD without energy mismatch dependence. However, a correct account of the energy

transfer reversibility considerably complicates averaging over the space for a stochastic ensemble of centers.

The analysis has shown that in according with Huber's pair model /

4

/ the ki-

netics of the resonance peak decay I (t) is well described the expression similar

N. to those for the kinetics of irreversible

transfer, except for the fact that an acco- 10 '

unt of reversibility in the Huber's pair model makes t h ~ l ~ ~ ) ~ ~ of average rate diffe-

rent WH=WF.2

.

But it is correct only

?

for the first stage, not for the long tail \

where the pair model is not availaible.

We have measured the time-resolved lumi-

'

nescence spectra and the kine ics of the re-

_f+

_. ^o

son2yce-geak decay for the Eu Ions (n= un

=10 cm ) in Na-B-Y-silicate391ass at

T=

1

-

=300 K see Fig.4) and &r

Y3

lons (n -

1.1. l ~ ~ ~ c m - ~ ; n -2,9.10 cm ) in Eia-AiIphos- phate glass at T i 8 0 K (see Fig.5).

The luminescence spectra consist of two components, namely, a narrow resonance peak responsible for the luminescence of the ini- tially excited centers,

I

(t) and a wide non-

N

homogeneously broadened base. You can see, that we have now not only stockes but also

antistockes energy migration into short or 10-1

Fi9.3

0

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C7-162 JOURNAL DE PHYSIQUE

F i g . 4 b shows t h e k i n e t i c s two s t a g e s e x p o n e n t i a l ( o r d e r e d t < t - 6 0 p s ) and nonex- p o n e n t i a l ( d i s o r d e r e d t > t l ) k i n e t i c o f m i g r a t i o n I ( t ) f o r t h e E U ' ~ + i o n s i n Na-8- - Y - s i l i c a t e g l a s s . The s l o p e

t31P

of t h e p l o t a t p > t p o i n t s t o q u a d r u p o l - q u a d r u - p o l (S=10) mechanism o f t h e Eu - Eu + i n t e r a c t i o ~ .

T A ~

e x p e r i m e n t a l l y o b t a i n e d va- , - l u e s of t h e a y e r a g e m i g r a i t i o n r a t e s W = 5 , 3 . lo's-' 0,2ms 0,8rns 3ms 5ms and W,=2,1.10 s

,

e n a b l e u s t o f i n d t $ e yfllue o f _{I , D}

4

t h e m i c r o p a r a m e t e r s CDD(Eu-Eu)=3,8.10 nm / m s ; R (Eu-Eu)=4,7 A. We found t h ~ t a t time t < t 2 m i g r a t i o n k i n e t i c s I N ( t ) o f t h e Yb + i o n s i n g l a s s h a s a t y p i 5 a l r o o t form w i t h t h e a v e r a g e m i g r a t i o n r a t e Wrn

.

These r e s u l t s a r e i n a good a g r e e m e n t w i t h H u b e r ' s p a i r _0,5 mo e l a n 9 + i n d i c a t e a d i p o l e - d i p o l e c h a r a c t e r o f t h e 9+ Yb

-

Yb e l e m e n t a r y i n t e r a c t i o n s ( S = 6 ) . T h i s a l - lows o c a l c u l a t e t h e i r m i c r o e f f i c i e n c y CDD=

_k

=10nm / m s . P l o t t i n g t h e k i n e t i c s a s - l o g I ( t ) v e r s u s l o g t ( F i g . 6 a ) r e v e a l s a d e v i a t i o n from law /!l/ a t t > t 3 2 :

-

a r e p l a c e m e n t o f t h e power-law f u n c t i o n 0,434M@

.

576 580

,

nm Fig.4a

by t h e l i n e a r funpf)qn 3/2 (1ogWd+logt), which c o r r e s p o n d s t o a d i f f u s i v e e g r e s s k i n e t i c s , I ( t ) = ( W d t )

.

E q u a t i n g t h e d e r i v a t i v e s o f t h e 2 3 t w o l f u n c t i o n s , we f i n d t h e time m a r k i n g !he boundary between them t o b e t

-

S 2

'

W"

.

Using S = 6 , we t h e n f i n d t 2 = 9 ~ j

,

a s shown by t h e a r r o w s i n t h e ' ? ~ i u i e i . The o b s e r v a t i o n o f a d e c a y l a w e x i s t e n c e o f a d i f f u s i v e s t e p i n t h e m i g r a t i o n o f e x c i t a t i o n s t h r o - ugh a d i s o r d e r e d s y s t e m o f c e n t e r . T h i s r e s u l t j$2confirmed by t h e f a c t t h a t a p l o t o f t h e d e c a y k i n e t i c s G t ) i n t h e c o o r d i n a t e t - r e v e a l s a s t r a i g h t l i n e and by t h e f a c t t h a t t h i s s t r a i g h t l i n g p a s s e s t h r o u g h 3 y p o r i g i n ( F i g . 6 b ) . The s l o p e o f t h e l i n e a r p a r t o f t h i s f u n c t i o n i n t h e l i m i t t- - 0 , which c o r r e s p o n d s t o t h e d i f f u s i v e s t e p i n t h e e g r e s s k i n e t i c s i n t h e l i m i t t r o o , g i v e s u s d i f u s i v e - m i g r a t i o n ve- 1 0

l o 2

l o 3

4 -1

lo-*

vs

l o c i t i e s W ( n i = 2 , 5 . 1 0 3 s - I and W ( n ) = 2 . 1 0 s

,

I i n a p p r o x i d a t e a g r e e m e n t w i t h wd(n2) and W ( n ) . A c a l c u l a t i o n o f t h e d i ? f u & i o o e f f i - cPen$ Ddisfrom t h e e x p r e s s i o n W - 4 d n

WD

6 - d i z l l lEl f o ? t h e s e d e n s i t i e s n a n d n y i e l d s 0 , 9 . 1 0

-

cm /s. The r a t i o o f t A e s e va?ue, 4 , ' a p p r o -

-

,

x i m a t e l y e q u a l t o t h e v a l u e ( n /n )4t3=3,64, ₁₃_cn c o n f i r m i n o t h a t t h e d i f f u s i o n g o e f f i c i e n t i s 1 a n i r r a t i o n a l f u n c t t p g o f t h e c o n c e n t r a t i o n o f c e n t e r s : Ddi ~ n

.

Fiq.4b

The i n d e o e n a e n t o b t a i n i n a o f m i c r o e f f i c i - >

e n c e CDD(Yb-Yb) and d i f f u s i o n c o e f f i c i e n t Dd. o i v e s u s pos.$,gility t o f i n d o u t t h e n u m e r i c a l c o e f f i c i e n t d . = 1 , 8 f o r t h e express'?& D .= &C n

.

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/1/ T.T.Basiev et al. in Spect. kristallov (The Spectrum of Crystals), Nauka,

Leningrad, _{1983 p.57 ; 1978}

,

_{p 8 3 .}

/2/ A.G.Avanesov, T.T.Basiev et al. Sov. Phys. JETP 57(3), 1983, 596

/3/ O.K.Alimov, M.Kh.Ashurov, T.T.Basiev et al. Preprint P.N.Lebedev Physical Institute Moscow N 77, 1982, p.1; N160, 1983 p.1.