A NOVEL TECHNIQUE FOR THE ENHANCEMENT OF PHOTOACOUSTIC SIGNAL

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A NOVEL TECHNIQUE FOR THE ENHANCEMENT OF PHOTOACOUSTIC SIGNAL

T. Somasundaram, P. Ganguly

To cite this version:

T. Somasundaram, P. Ganguly. A NOVEL TECHNIQUE FOR THE ENHANCEMENT OF PHOTOACOUSTIC SIGNAL. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-239-C6-242.

�10.1051/jphyscol:1983637�. �jpa-00223196�

A NOVEL TECHNIQUE FOR THE ENHANCEMENT OF PHOTOACOUSTIC SIGNAL*

T. Somasundaram and P . Ganguly

S o l i d S t a t e and StructuraZ Chemistry U n i t , JTi?dian I n s t i t u t e of Science, Rangazore 12, India

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I n t h e c o u r s e o f o u r i n v e s t i g a t i o n s on t h e t e m p e r a t u r e dependence o f p h o t o a c o u s t i c s p e c t r a from s o l i d s we have o b s e r v e d i n s e v e r a l c a s e s a n i n c r e a s e i n t h e s i g n a l w i t h i n c r e a s i n g t e m p e r a t u r e r e a c h i n g sometimes a maximum c o r r c s p o n d i n g t o t h e b o i l - i n g p o i n t o f w a t e r . \Je have a l s o o b s e r v e d a s l i g h t enhancement oT t h e PA s i g n a l on t h e a d s o r p t i o n o f g a s e s . On a n a l y s i s o f o u r r e s u l t s we have been a b l e t o show a s a consequence t h a t t h e p h o t o a c o u s t i c s i g n a l c o u l d b e enhanced s e v e r a l t i m e s by t h e i n t r o d u c t i o n o f a v o l a t i l e l i q u i d i n t o t h e c e l l , t h e v a p o u r s o f which presumably a r e a d s o r b e d on t h e s u r f a c e o f t h e s a m p l e .

The c e l l and t h e s p e c t r o p h o t o n e t e r u s e d f o r o u r s t u d i e s h a v e been d e s c r i b e d e a r l i e r 1 . A s c h e m a t i c drawing o f t h e c e l l s used f o r room t e m p e r a t u r e and v a r i a b l e t e m p e r a t u r e s t u d i e s a r e shovm i n F i g . l a and l b , r e s p e c t i v e l y . The sample h o l d e r

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*communication No. 208 from t h e S o l i d S t a t e and S t r u c t u r a l C h e m i s t r y C n i t .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983637

C6-240 JOURNAL DE PHYSIQUE

( F i g . l c ) is s u c h t h a t t h e s a m p l e i s p l a c e d i n a n aluminium cup (8mm d i a , 0 . 8 mm d e p t h ) which i n t u r n i s p l a c e d on t o p o f a non-absorbing s o l i d ( s u c h a s s i l i c a - g e l o r T i 0 2 ) i n a c a v i t y i n s i d e t h e s a m p l e - h o l d e r . The image o f t h e beam i s i n c i d e n t o n l y on t h e samplc. A l i q u i d i s i n t r o d u c e d i n t h e c e l l by s o a k i n g t h e non-absorbing s o l i d w i t h t h e d e s i r e d l i q u i d w i t h t h e h e l p o i a m i c r o s y r i n g e . A GR 9602 e l e c t r e t micro-

phone was used f o r t h e s t u d y . The i l l u m i n a t i n g s o u r c e was a 200 N t u n g s t e n - h a l o g e n lamp.

I n Fig. 2 we show t h e n o r m a l i s e d PA s p e c t r a of As2S g l a s s i n t h e p r e s e n c e o f a i r a s w e l l a s d i e t h y l e t h c r i n t h e c e l l . The i n t e n s i t y o? t h e s i g n a l i s found t o b e enhan-

ced s e v e r a l f o l d i n t h e p r e s e n c e o f e t h e r . I t i s however, found t h a t t h e enhancement i s h i g h e r f o r l o w e r a b s o r p t i o n c o c f f i c i e n t ( F i g . 2 b ) . T h i s dependence i s g e n e r a l and

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Fig. 2 - a ) Normal.ised PA s p e c t r a o f As2S3 g l a s s i n p r e s e n c e o f e t h e r a n d a i r . b ) R a t i o o f I c t h e r / I a i r f o r v a r i o u s w a v e l e n g t h s .

c ) Phase dependence o f t h e s i g n a l a s a f u n c t i o n o f w a v e l e n g t h .

i s found i n s e v e r a l c a s e s . T h e r e i s a l s o a marked change i n t h e p h a s e dependence o f t h e s i g n a l ( F i g . 2 c ) . The enhancement i s found t o b e i n d e p e n d e n t o f t h e power o f t h e i r r a d i a t i n g s o u r c e , n a t u r e o f t h e s a m p l c h o l d e r ( t e f l o n o r aluminium) s l i t w i d t h , o r t h e geometry o f t h e microphone w i t h r e s p e c t t o t h e sample. T h e r e i s a dependence on t h e l e n g t h , l g , o f t h e g a s p h a s e above t h e s a m p l e , t h e enhancement b e i n g l a r g e r f o r s m a l l e r l g .

I n Fig. 3a we show t h e changes o f t h e i n t e n s i t y o f t h e PA s i g n a l (Iliq) a t 600 nm f o r Cr203 i n t h e p r e s e n c e o f v a r i o u s l i q u i d s i n t h e c e l l . A l i n e a r

r e l a t i o n i s o b s e r v e d between Iliq and t h e v a p o u r p r e s s u r e , p , of t h e l i q u i d . I n a c c o r d a n c e w i t h t h e above r e s u l t t h e PA s i g n a l i n t e n s i t y from l i q u i d s a l s o i n c r e a s e s w i t h t e m p e r a t u r e b e i n g p r o p o r t i o n a l t o t h e v a p o u r p r e s s u r e o f t h e l i q u i d a t e a c h t e m p e r a t u r e . P i g . 3b shows t h e r e s u l t s o b t a i n e d d u r i n g c o o l i n g from a .001 M aqueous s o l u t i o n o f m c t h y l e n e b l u e t h e w a v e l e n g t h b e i n g 660 nm. The enhancement t h a t i s o b s e r v e d t h e r e f o r e s e c n s t o b e l i n e r a l y r e l a t e d t o t h e vapour p r e s s u r e of t h e l i q u i d .

I n F i g . 4 t h e enhancement o b s e r v e d from c a r b o n b l a c k a t v a r i o u s c h o p p i n g f r e q u e n c i e s (250

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t o a w-- dependence seems t o i n d i c a t e t h a t f o r h i g h chopping f r e q u e n c i e s t h e flow of h e a t i n t o t h e g a s p h a s e by t h e u s u a l RG mechanism i s l e s s i m p o r t a n t t h a n t h e e f f e c t s due t o t h e h e a t i n g o f t h e a d s o r b e d l a y e r on t h e sample.

F i g . 3 - a ) P l o t o f I v s P l i q 4=CH30H, S=CIICl

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f o r v a r i o u s l i q u i d s : l = H 0 , 2=CH CN, 3=n-Hexane,

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b ) P l o t of I v s P ( v a p o u r p r e s s u r e a t v a r i o u s t e m p e r a t u r e s f o r a 0 . 0 0 1 >I a q u e o u s s g + u t i o n o f Elethylene b l u e .

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where p ( i n mm Hg) i s t h e v a p o u r p r e s s u r e o f t h e l i q u i d , C ( B , U ) i s a c o n s t a n t f o r a p a r t i c u l a r o p t i c a l a b s o r p t i o n

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The enhancement o f t h e p h o t o a c o u s t i c s i g n a l o v e r t h a t n o r m a l l y o b s e r v e d i n a i r ( a d e q u a t e l y d e s c r i b e d by t h c KG t h e o r y 2 ) c o u l d b e a t t r i b u t e d t o t h e p h y s i c a l l y a d s o r b e d l a y e r ( p r o b a b l y a m u l t i l a y e r ) on t h e s u r f a c e o f t h e sample. P e r i o d i c h e a t f l o w t o t h e s u r f a c e of t h e s o l i d c o u l d c a u s e a p e r i o d i c e v a p o r a t i o n and c o n d e n s a t i o n o f a p a r t o f t h e p h y s i c a l l y a d s o r b e d l a y e r . S i n c e t h e p h y s i c a l l y a d s o r b e d l a y e r i s

C6-242 JOURNAL DE PHYSIQUE

l i q u i d - l i k e , t h e e x t e n t o f e v a p o r a t i o n would b e p r o p o r t i o n a l t o dp/dT o f t h e l i q u i d a t t h e ambient t e m p e r a t u r e (and h e n c e e s s e n t i a l l y t o t h e v a p o u r p r e s s u r e o f t h e l i q u i d ) when t h e s u r f a c e t e m p c r a t u r e i s i n c r e a s e d by a s m a l l amount dT. The d e s o r h e d vapour c o u l d c a u s c a momentary ( n o n - e q u i l i b r i u m ) i n c r e a s e i n t h e p r e s s u r e w i t h i n t h e c e l l t h u s s e t t i n g up a n a c o u s t i c wavc. l-loreover c o l l i s i o n a l h e a t t r a n s f e r from t h e h o t d e s o r b c d l i q u i d molecules t o t h e a i r m o l c c u l c s c o u l d a l s o l e a d t o a t h e r m a l wave.

The problem t h e n becomes one o f s o l v i n g c o u p l e d e q u a t i o n s f o r p r e s s u r e and tempera- t u r e i n t h e v a p o u r p h a s e . E s s e n t i a l l y s i m i l a r e q u a t i o n s were s o l v e d by McDonald a n d 1 d e t s e l 3 f o r t h e i r "mechanical" p i s t o n model. The model used by u s , which we s h a l l c a l l t h e " a d s o r b e d p i s t o n " modcll i s 5hcn c x p c c t e d t o g i v e t h e same dependence a s t h o s e o b s e r v e d by McDonald and W e t s e l namely a h i g h e r enhancement f o r low 6 and h i g h w . IJe n o t e t h a t t h e "adsorbed" p i s t o n e f f e c t would b e much pronounced t h a n t h e

"mechanical" p f s t o n e f f e c t a s t h e e f f e c t i v e t h e r m a l e x p a n s i o n c o e f f i c i e n t o f t h e a d s o r b c d l a y e r on d e s o r p t i o n i s e x p e c t c d t o be much l a r g e r t h a n t h e b u l k t h e r m a l e x p a n s i o n c o e f f i c i e n t invoked by McDonald and w e t s e l 3 i n t h e i r "mechanical" p i s t o n e f f e c t .

The enhanccrnent o f t h e p h o t o a c o u s t i c s i g n a l f r o m condensed m a t e r i a l s due t o t h e p r e s e n c e of l i q u i d v a p o u r s i s q u i t e u n i v e r s a l and i s applicable n o t o n l y t o s o l i d s b u t a l s o t o b i o l o g i c a l s y s t e m s s u c h a s g e l s , membranes, l e a v e s e t c . , o r t o t h i n f i l m s and e v e n t o l i q u i d s . The t e c h n i q u e s h o u l d t h e r e f o r e h e r a t h e r u s e f u l i n t h e p r a c t i c e of p h o t o a c o u s t i c s p e c t r o s c o p y . A consequence o f t h i s work i s t h a t i t may b e neces- s a r y t o reexamine t h e i n t e r p r e t a t i o n s of t h e p h a s e and f r e q u e n c y dependence o f photo- a c o u s t i c s i g n a l e s p e c i a l l y where s u f f i c i e n t c a r e h a s n o t b e e n t a k e n t o remove

p h y s i c a l l y a d s o r b e d g a s e s o r f o r s t u d i e s i n v o l v i n g l i q u i d s and b i o l o g i c a l m a t e r i a l s . Acknowledgment - We acknowledge v a l u a b l e d i s c u s s i o n s w i t h and encouragement from P r o f e s s o r C.N.R. Rao.

R e f e r e n c e s :

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(1) Ganguly, P. a n d Rao, C . N . R . , P r o c . I n d . Acad. S c i . (Chem. S c i . ) ,

90

( 2 ) Rosencwaig, A. and Gcrsho, A . , J. Appl. P h y s . ,

47

( 3 ) XcDonald, F.A.M. a n d W e t s e l , G.C., J. Appl. Phys. 49 (1978) 2313.