OPTICAL MEASUREMENTS AND THE PROPERTIES OF GLASSES

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OPTICAL MEASUREMENTS AND THE

PROPERTIES OF GLASSES

R. Silbey

To cite this version:

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JOURNAL

DE

PHYSIQUE

Colloque C7, suppl6ment au

nOIO,

Tome 46, octobre 1985 page C7-323

OPTICAL MEASUREMENTS AND THE PROPERTIES OF GLASSES

R. S i l b e y

Massachusetts Institute of Technology, Cambridge, M A 02139, U. S. A.

Abstract

-

I n t h i s paper, we review recent experimental and t h e o r e t i c a l work on t h e o p t i c a l p r o p e r t i e s o f molecules embedded i n g l a s s y hosts. We note t h a t d i f f e r e n t t h e o r e t i c a l models can y i e l d agreement w i t h experiment, and t h a t t h e p r o p e r t i e s of t h e glass are not e a s i l y accessible by these methods.

I n t h i s t a l k , we w i l l discuss recent experimental [ l - 5 1 and t h e o r e t i c a l work

[ 6 ] on o p t i c a l p r o p e r t i e s o f molecules doped i n glasses. An o p t i c a l t r a n s i t i o n o f a guest molecule i n a glassy host w i l l be broadened inhomogeneously due t o t h e wide d i s t r i b u t i o n o f s t a t i c s i t e s i n t h e host. Each s i t e w i l l l e a d t o a d i f f e r e n t ground s t a t e energy as w e l l as a d i f f e r e n t e x c i t e d s t a t e energy. This leads t o a d i s t r i b u t i o n o f e x c i t a t i o n energies which i s a c o n v o l u t i o n o f t h e e x c i t e d and ground s t a t e d i s t r i b u t i o n s . Although t h i s d i s t r i b u t i o n i s c e r t a i n l y a f u n c t i o n o f t h e p r o p e r t i e s o f t h e glass, t h e l a c k o f knowledge o f host-guest i n t e r a c t i o n s makes i t impossible a t present t o l e a r n about t h e glass, except q u a l i t a t i v e l y , from t h e inhomogeneous l i n e shape.

Homogeneous l i n e shapes o f o p t i c a l t r a n s i t i o n s i n a glass have been s t u d i e d f o r t h e l a s t t e n years by fluorescence l i n e narrowing techniques, h o l e burning and photon echo experiments [I]. Because t h e homogeneous l i n e measures t h e dynamic f l u c t u a t i o n s o f t h e guest host i n t e r a c t i o n , i t i s hoped t h a t such measurements w i l l l e a d t o a b e t t e r understanding o f dynamic p r o p e r t i e s o f glasses. Other measurements, such as s p e c i f i c heat and thermal c o n d u c t i v i t y , a l s o g i v e us i n f o r m a t i o n ,on such p r o p e r t i e s , and care should be taken so t h a t a d e s c r i p t i o n c o n s i s t e n t w i t h a l l p r o p e r t i e s i s found.

A

good example o f such consistency i s f o r t h e photochemical hole burning of f r e e base porphin i n alkane c r y s t a l l i n e matrices [2]. Here t h e molecule can undergo an i n t r a m o l e c u l a r rearrangement upon e x c i t a t i o n which burns a hole i n t h e o r i g i n a l s p e c t r a l l i n e . The temperature dependence o f t h e excess hole w i d t h i s described n i c e l y by i n v o k i n g dephasing by a l i b r a t i o n a l mode. This l i b r a t i o n a l frequency i s a l s o seen i n t h e o p t i c a l spectrum. From these measurements, a model based on decay o f t h e l i b r a t i o n a l p o p u l a t i o n by a one phonon process can be fit.

When we examine t h e l i n e widths ( i n glasses) o f i n o r g a n i c ions o r organic molecules, we f i n d t h a t t h e excess w i d t h s vary as Tn w i t h ltn<2.2 over a wide range o f temperature. A number o f t h e o r i e s have been presented t o e x p l a i n t h i s behavior on t h e basis o f t h e p r o p e r t i e s o f t h e glass. A l l t h e t h e o r i e s use t h e i d e a o f two l e v e l systems (TLS) which a r e invoked t o e x p l a i n t h e low temperature

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C7-324 J O U R N A L D E PHYSIQUE

heat c a p a c i t y and thermal c o n d u c t i v i t y o f a glass. Depending on c e r t a i n assump- t i o n s , t h e t h e o r i e s can describe t h e data.

I n t h i s t a l k , we w i l l examine a number o f t h e o r i e s and experiments and a t - tempt t o formulate a c o n s i s t e n t d e s c r i p t i o n o f hole burning and photon echo measurements. I n p a r t i c u l a r we w i l l focus on t h e experiments o f t h e groups o f Volker [2], Wiersma [3], Fayer [4], and BEssler [5]. Although a l l o f these i n v o l v e organic molecules embedded i n glasses, we w i l l compare these w i t h i n o r g a n i c systems.

I n a d d i t i o n , we w i l l examine some aspects o f t h e k i n e t i c s o f hole burning and ask whether these can be explained by c u r r e n t models.

References

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[3] L. MolenkampTnTD. W i e r s m z J. CheCPhys. ( i n press). [4] C. Walsh and M. Fayer, J. Luminescence ( j n press). [5] H. B k s l e r ( p r i v a t e communication).

[6] S. Lyo and R. Orbach, Phys. Rev. B22, 4223 (1980); S. Lyo, Phys. Rev. L e t t . 48, 688 (1982); S. Lyo and R. Orb=, Phys. Rev. 829, 2300 (1984); P.