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Clays and Clay Minerals,1973, Vol. 21, pp. 363-368. Pergamon Press. Printed in Great Britain

T H E I N F L U E N C E O F T H E C H R I S T I A N S E N E F F E C T

O N I .R . S PE C T R A O F P O W D E R S

R. PROSTInstitut National de la Recherche Agronomique, Route de Saint-Cyr, 78, Versailles, France

(Received 7 December1972)

Abstract--The Christiansen effect appears in the i.r. spectrum of powders embedded in a solid, liquidor air matrix as an apparently anomalous transmittance (Christiansen peak) of the incident elec-tromagnetic radiations. The peak appears at wavelengths for which the refractive index of the sampleand the refractive index of the matrix are equal (Christiansen wavelength: Ach,).

On account of the great variation of the sample refractive index in the immediate neighborhood ofthe absorption bands (anomalous dispersion curve), one often observes the occurrence of a transmit-tance peak or of a band deformation in this spectral range. A change in the position of this transmissionpeak with the value of the matrix refractive index is indicative of the Christiansen effect.

The equality of the refractive indices of the sample and of the matrix for AChr has been used todetermine some points of the anomalous dispersion curve in the neighborhood of the hydroxyl stretch-ing band (3678 era-t).

Spectral distortions caused by the Christiansen effect can be reduced by preparing the sample insuch a manner that the width at half-maximum (Av~2) of the Christiansen peak is several times greaterthan this of the absorption band itself. Clarke's theoretical formula, which gives an estimation of Avl~2,has been qualitatively verified and thus gives an appropriate guide in the choice of the parameterswhich one can optimize during the sample preparation. One can reduce the Christiansen effect spectralmodifications, without running the risk of modifying the sample itself, particularly by overly severegrinding.

INTRODUCTIONI.R. SPECTROSCOPY o f powders is possible by sev-eral techniques (Lecomte, 1958; Farmer et a l . ,1968). The-preferred tec hnique dep ends not only onthe size and shape of particles and the extinctioncoefficient of the sample, but also on the charac-teristic to be measured (for example, the dichroiccharacter of the bands). These various procedurescan be grouped as follows: deposition on a disctransparent to electromagnetic radiation; self sup-porting films; and e mbeddin g the powde r in a solidor liquid transparent to radiation. To decrease lossof light for reaso ns other th an absorptio n, the parti-cle size should be reduced to a value smaller than

the wavelength of the radiation. Also, the embed-ding medium should have a refractive index nearthat of the sample in the desired spectral range.

When oriented deposits are used to study thedichroic character of the i.r. bands, the second con-dition cannot always be satisfied. In this case, thematrix is air with a refractiv e index of one. Becaus eof anomalous dispersion (i.e. the change of the re-fractive index cff the sample near the absorptionbands, see Fig. l) the sample and matrix refractiveindices are equal only for certain values of thewavelengt h (A). Between these values of A, varia-

tion of the refractive index can induce great varia-tions in optical properties of the sample, which re-sult in an increase of light lost other tha n by absorp-tion. Some aspects of the variability of powdertransmittance with )t have been studied previously(Henry, 1948). In particular, the Christ iansen effectis the increase of the crystalline powder trans mit-tance at those wavelengths where the sample andmatrix refractive indices are equal.

The purpos e of this paper is to study the changes,with respect to A, in transm ittance of powders em-bedded in various matrices, absorption being neg-lected. The essential features of the Christianseneffect will be reviewed and its existence demon-

strated in the case of powdered talc. The influenceof the Christiansen effect on the profile, intensityand position of the i.r. absorption bands willthen be discussed. The paper will conclude withprecautions to be taken in sample preparation andthe quantitative and qualitative interpretation ofspectra.

MATERIALS AND M E T H O D STwo size fractions of talc were used. The first,

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3 6 4

~ k C h r X C h r X c . , x

Fig. 1 . Schematic representat ion of the re la t ive var ia t ionsof the an omalous d ispers ion curve of sample in the neigh-borhood of an absorpt ion band (solid l ine) and o f the d is -persio n curve o f the m atrix (dotted line). The three figures

correspond to matr ices with d ifferent d ispers ion curves .

o b t a i n e d b y t h e p e l l e t t e c h n i q u e w i t h K B r, C s B r ,a n d C s I i n a 1 : 5 0 w e i g h t r a t i o . A t h i n o r i e n t e d d e -p o s i t o n a C aF 2 w i n d o w , p r e p a r e d b y e v a p o r a t i o no f a t a lc s u s p e n s i o n i n C C L , w a s u s e d t o o b t a i n t h et a l c s p e c t r u m i n a n a i r m a t r i x . T h e t r a n s m i s s i o np e a k ( C h r i s t ia n s e n p e a k ) i n c r e a s e s i n i n t e n s i t y w i tht h e a n g l e b e t w e e n t h e n o r m a l t o t h e w i n d o w a n dt h e i . r. b e a m d i r e c t io n . C h a n g e o f t h i s a n g l e p e r m i t so b s e r v a t i o n o f t h e d i c h r o i c c h a r a c t e r o f t h e a b s o r p -t i o n b a n d d u e t o t a l c h y d r o x y l g r o u p s . T h e s p e c -t r u m m a r k e d " a i r " i n F i g . 2 c o r r e s p o n d s t o a n a n g l eo f 4 5 ~ S p e c t r a o f t a l c s u s p e n d e d i n d i f f e r e n t so l -v e n t s w e r e o b t a i n e d i n a l i q u i d c e l l o f v a r i a b l e

K B r

a67~,/

3 8 p 0 3 ~ 0 0 r

C H B ~ , C s,3 6 7 8 , 5 3 6 7 8 . 5

38 ,00 36 ,00 cm "4 33p 0 360 0cn 4I" 36j15 T 3 6 7 0

3d6.5H e C s ,B r

3 6 7 9, t " 1C I ~ 3 ~ 0 0 3 6 10 0 C 1

3681,538,00 36,00

38 . 00 3677 . 5 36 , 00

3676.5

A i r

cm -1 3 8 0 0 3600cm-1

Fig. 2. I .R. spectra of talc pow der (grain size

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366 R. PROST

PRACTICAL CONSEQUENCESOF THE CHRISTIANSEN EFFECT

T h e m o s t v i s i b le m a n i f e s t a t i o n s o f t h e C h r i s t ia n -s e n e f f e c t a r e t h e c h a n g e o f t h e a b s o r p t i o n b a n dp r o f i le a n d a s i g n i f ic a n t s h i ft o f i t s m a x i m u m f r e -q u e n c y. I n t h e t a l c c a s e , t h e s h i f t is s m a l l b e c a u s et h e b a n d i s n a r r o w. T h e q u e s t i o n o f b a n d p r o f i l ec o m e s u p o n l y i f w e t r y t o r e s o l v e th e c o m p o n e n t so f m o r e c o m p l e x a b s o r p t i o n f e a t u r e s . I n f a c t, i t i st h e b a s e l i n e r a t h e r t h a n t h e a b s o r p t i o n b a n d w h i c hi s a lt e r e d . T h e b a s e l i n e i f f o r m e d b y t r a n s m i s s i o np e a k s w h i c h a p p e a r a t e a c h v a l u e o f A e q u a l t o) [ C h r .T h u s , w h e n t h e m a t r i x d i s p e r s i o n c u r v e i n t e r s e c t st h e a n o m a l o u s d i s p e r s i o n c u r v e o f t h e s a m p l e a tt w o d i f f er e n t f r e q u e n c i e s t h e r e a r e t w o C h r i s t ia n -s e n p e a k s . T h e c h a r a c t e r i s t i c s o f t h e s e p e a k s a r es i g n i f ic a n t , b u t d u e t o t h e d i f fi c u l t y o f e x a c t d e t e r -m i n a t i o n w e c a n o n l y i n d i c a t e t h e d i r e c t i o n o fc h a n g e o f e a c h . O n e p r i n c i p a l l y d i s c u s s e s t h e w i d t ha t h a l f - m a x i m u m o n t h e b a s i s o f t h e C l a r k e ' s (1 96 8)f o r m u l a .

T h e s p e c t r a o f F i g . 2 s h o w t h a t t h e C h r i s t i a n s e np e a k b e c o m e s n a r r o w e r a s A Ch, a p p r o a c h e s t h e a b -s o r p t i o n m a x i m u m . C o m p a r i s o n o f t h e s p e c t r a o ft a l c in K B r a n d t a lc d e p o s i t e d o n a C a F 2 w i n d o w i l-l u s t r a t e s t h is p o i n t . I t is a c o n s e q u e n c e o f t h e A n 'v a r i a t i o n w h i c h i s m u c h g r e a t e r w h e n t h e m a t r i x i sa i r t h a n w h e n i t i s K B r. T h i s f a c t h a s b e e n p o i n t e do u t p r e v i o u s l y b y s e v e r a l a u t h o r s ( D e n m a r k a n d

C a d y, 1 9 3 5 ; B a r n e s a n d B o n n e r, 1 9 3 6 ; J e r a m e c ,1958).

A n o t h e r p a r a m e t e r p r e v i o u s l y c o n s i d e r e d b yo t h e r w o r k e r s i s t h e s i z e o f th e p a r t i c l e s ( a ) . S u c h ac o n s i d e r a t i o n i s e s s e n t i a l fo r t h e r e a l i z a t io n o f g o o dC h r i s t i a n s e n f i l t e r s ( D e n m a r k a n d C a d y, 1 9 3 5 ) .J e r a m e c ( 1 9 5 8 ) o b t a i n e d a g o o d C h r i s t i a n s e n f i l t e rw i t h p a r t i c l e s w h i c h h a d a s i z e e q u a l t o t e n t i m e sAChr. N e v e r t h e l e s s , t h e o r y p r e d i c t s t h a t t h e C h r i s t -i a n s e n e f f e c t e x is t s e v e n w h e n t h e r e l a t i o n s h i p b e -tw e en pa r t i c l e s i ze a nd A Ch, i s no t fu l f i l l ed , and , inp a r t i c u l a r , e v e n w h e n t h e p a r t i c l e s i z e b e c o m e s

s m a l l e r t h a n A ch r. I n o r d e r t o c o n f i r m t h i s p r e d i c t i o nw e p r e p a r e d p e l l e t s o f t a l c p o w d e r w i t h p a r t i c l e s< 2 ~ m , i n K B r, C s B r a n d C s I . F i g u r e 4 g i v e s a b -s o r b a n c e s p e c t r a .

A d i s t i n c t d i s s y m m e t r y o f t h e a b s o r p t i o n b a n d i so b s e r v e d i n e v e r y c a s e . N e v e r t h e l e s s , A ch , i s d if fi -c u l t to l o c a t e b e c a u s e o f t h e i n c r e a s e o f A r m o f t h eC h r i s t i a n s e n p e a k . T h i s c h a n g e o fA / J I /2 w i t h p a r t i -c l e s i z e i s n o t n e w. D e n m a r k a n d C a d y ( 1 93 5) h a v es t u d i e d t h e e f f ec t in a s y s t e m a t i c w a y a n d s h o w as i g n i f ic a n t i n c r e a s e o f Av, 2 w i t h a d e c r e a s e o f p a r t i -c l e s iz e . H o w e v e r , t h e s e a u t h o r s u s e d l a r g e r p a r t i -c l e s ( 17 0 t x m ) t h a n t h o s e e x a m i n e d i n t h i s s t u d y.

3 8 0 0 3 6 0 0 c m ~ 3 8 0 0 3600cm 4 3 8 0 0 3 6 0 0 c r r rl

o b s o r b a n c e

0 . 8 IC ~ . I C s B r "

0 .6

0 .4

Fig. 4. I .R. spectra of talc powder (grain size

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Christiansen effect on i.r. spectra of powders 367

1 8 0 0 1 4 0 0 c m -1

\ \1 8 0 0 1 4 0 0 c m -1

Fig. 5. I.R. spectra of talc powder deposits of increasingthickness (L) on CaF2 window (dia. 35 ram). Total depositweight is noted on each curve. The experiment was madewith particles of two different sizes:

a--

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368 R. PROST

Ve d d e r , W. ( 19 64 ) Co r r e l a t i o n s b e t w e e n i n f r a r e d s p e c - Ve l b e r , B . ( 19 67 ) T h e m e a s u r e m e n t o f i n f r a r e d d i s p e r s i o nt r u m a n d c h e m i c a l c o m p o s i t i o n o f m i c a :Am. Mineral . i n s o l id s b y a m e t h o d b a s e d o n s m a l l p a r t i c le s c a t t e r i n g :49 , 736-768 . Appl. Optics.6 , 925-928 .

R r s u m r- L ' e f f e t Ch r i s t i a n s e n s e m a n i f e s t e s u r le s p e c t r e d ' a b s o r p t i o n i n f r a r o u g e d e s p o u d r e s m r l a n -g r e s h u n e m a t r i c e q u i p e u t ~ t r e u n s o l id e , u n l iq u i d e o u l 'a i r , p a r u n e t r a n s m i s s i o n p l u s i m p o r t a n t e( p ic d e Ch r i s t i a n s e n ) d e s r a d i a t i o n s 6 1 e c t r o m a g n r t i q u e s i n c i d e n t e s . Ce t e f f et a p p a r a i t a u x l o n g u e u r sd ' o n d e p o u r l e s q u e l l e s l' i n d i c e d e r r c h a n t i l l o n e t l ' i n d i c e d e l a m a t r i c e s o n t 6 g a ux ( l o n g u e u r d ' o n d e d eCh r is t ianse n : hCh~).

A c a u s e d e l a t r b s g r a n d e v a r i a t i o n d e l ' i n d i c e d e r r f r a c t i o n d e l ' r c h a n t i l l o n a u v o i s i n a g e i m m r d i a td e s b a n d e s d ' a b s o r p t i o n ( c o u rb e d e d i s p e rs i o n a n o m a l e ) o n n o t e f r r q u e m e n t l a p r r s e n c e , d a n s c e t t er rg i o n d u s p e c t r e , d ' u n p i c d e t r a n s m i s s i o n o u d ' u n e d r f o r m a t i o n d e l a b a n d e . Si la p o s it i o n d e c e p icd e t r a n s m i s s i o n c h a n g e a v e c l a v a l e u r d e l ' i n d i c e d e r r f r a c t i o n d e l a m a t r i c e i l s ' a g i t a l o r s d ' e f f e tCh r i s t i a n s e n .

L ' rga l i t6 des ind ices d e r r f r a c t ion de l ' r cha n t i l lo n e t de la ma t r ice pou r ~-Chr a 6 t6 u t i l i s r e po urd r t e r m i n e r q u e l q u e s p o i n t s d e l a c o u r b e d e d i s p e r s i o n a n o r m a l e d u t a l c a u v o i s in a g e d e l a b a n d e d ev i b r a t i o n d e v a l e n c e d e s h y d r o x y l e s ( 3 6 7 8 c m - l ) .

O n a n a l y s e e n s u i t e l e s p e r t u r b a t i o n a p p o r t r e s p a r l 'e f f et Ch r i s t i a n s e n s u r l e p ro f il , l a p o s i t i o n e tl ' i n te n s i t 6 d e s b a n d e s d ' a b s o r p t i o n . U n m o y e n d e r O d u i re c e s i n c o n v r n i e n t s c o n s i s t e 5 . p r r p a r e r l ' r c h -an t i / ion de te l le sor te que la la rg eur ~ . mi-h au teu r (A 1, ~ ) du p ic de Ch r is t ian sen so i t p lus ieur s lo i s sup r r-i e u r e h c e ll e d e l a b a n d e d ' a b s o r p t i o n e l l e - m r m e . L a f o r m u l e t h r o r i q u e d e C l a r k e q u i d o n n e u n e

e s t i m a t i o n d e A v 89 e t d o n t o n n e p r o p o s e q u ' u n e v r r i f i c a ti o n q u a l i ta t i v e , c o n s t i t u e u n g u i d e a p p r o p r i 6d a n s l e c h o i x d e s g r a n d e u r s s u r l e s q u e l le s n o u s p o u v o n s a g i r l o t s d e l a p r r p a r a t i o n d e s 6 c h a n t i l lo n s .O n p e u t a i n s i r r d u i r e l e s i n c o n v r n i e n t s i n h r r e n t s h l ' e x i s t e n c e d e l' e f fe t Ch r i s t i a n s e n , s a n s c o u r i r l er i s q u e d ' a l t r r e r l ' r c h a n t i l l o n lu i - m ~ m e , e n p a r t i c u l ie r p a r u n b r o y a g e t r o p p o u s s r.

K u r z r e f e r a t - D e r Ch r i s t i a n s e n e f f e k t t r i t t i n I n f r a r o t s p e k t r e n v o n Pu l v e r n , d ie i n f e s t e r, f ti i ss i g er o d e rg a s f O r m i g er M a t r i x e i n g e b e t t e t s i n d, a l s e i n e o f f e n s i c h t li c h a n o m a l e D u r c h l i is s i g k e i t (Ch r i s t i a n s e n -p e a k ) d e r e i n f a l l e n d e n e l e k t r o m a g n e t i s c h e n S t r a h l u n g in E r s c h e i n u n g . D e r Pe a k t r i t t b e i We ll e nl ~ in g e na u f , f ii r w e l c h e d i e Br e c h u n g s i n d i z e s d e r P r o b e u n d d e r M a t r i x g l e ic h s i n d ( Ch r i s ti a n s e n - We l l e n l~ i n g e :Xch,).

I n fo l g e d e r g r o B en V e ~ n d e r u n g d e s B r e c h u n g s in d e x d e r P r o b e i n u n m i t t e l b a re r N a c h b a r s c h a f td e r A b s o r p t i o n s b a n d e ( a n o m a l e D i s p e r s i o n sk u r v e ) b e o b a c h t e t m a n o f t d as A u f t r e te n e i n e s D u r c h -I ~ i ss i gk e i ts m a x i m u m s o d e r e i n e r Ba n d e n d e f o r m a t i o n i n d i e se m Sp e k t r a l b e r e i c h . E i n We c h s e l i n d e rL a g e d i e s e s D u r c h l S .s s i g k e it s m a x i m u m s m i t d e r G r r Be d e s B r e c h u n g s i n d e x d e r M a t r i x i s t e i nA n z e i c h e n f i i r d e n Ch r i s t ia n s e n e f f e k t .

D i e G l e i c h h e i t d e r Br e c h u n g s i n d i z e s v o n P r o b e u n d M a t r i x f i.i r h ch r i s t b e n u t z t w o r d e n , u m e i n i g eP u n k t e d e r a n o m a l e n D i s p e r s i o n s k u r v e i n d e r N a c h b a r s c h a f t d e r H y d r o x y l v a l e n z s c h w i n g u n g s b a n d e( 3 6 7 8 c m -1 ) z u b e s t i m m e n .

D u r c h d e n C h r i s t i an s e n e f fe k t h e r v o r g e r u fe n e Ve r z e r r u n g e n d e s S p e k t r u m s k r n n e n v e r m i n d e r tw e r d e n , w e n n d i e P r o b e s o p r~ i p ar ie r t w i r d , d a b d ie Br e i t e d e s Ch r i s t i a n s e n - p e a k s i n h a l b e r H Sh ed e s M a x i m u m s ( Av l / 2 ) u m d a s M e h r f a c h e g r t i l Se r i s t a l s d i e d e r A b s o r p t i o n s b a n d e s e l b s t . C l a r k e ' st h e o r e t i s c h e Fo r m e l , d i e e i e n e n Sc h f i t z w e r t f o r A v l / 2 e rg i b t, w u r d e q u a l i t a ti v b e st ~ it ig t u n d s t e ll ts o m i t e i n g e e i g n e t e s H i l f s m i t t e l f i J r d i e Wa h l d e r Pa r a m e t e r d a r , d i e b e i d e r P r o b e n h e r s t e l l u n go p t i m i e r t w e r d e n k 6 n n e n . D i e Ve r ~ . n d e ru n g e n d e s Sp e k t r u m s d u r c h d e n Ch r i s t i a n s e n e f f e k t k i Sn n e nv e r m i n d e r t w e r d e n , o h n e d a b d i e G e f a h r e i n e r Ve r i in d e r u n g d e r P r o b e s e l b s t in s b e s o n d e r e d u r c hi ibe rm~iBig scha r fe s Mahlenbes teh t .PealoMe - - 3qbdpeKr Kp nc rn ar tc eu a npo~a:m eTc~ Ha ~tHqbpaKpacHoM cneKzpe nopotmcOB Br~e~apenm,~xn 3eMJIe, ~rH~KOCTa ric h MaTpHLte Bo3~ayxa O'~eaH~lHO a a .~ e aHOMaJlbHOrO nponycKaHHa (nHKIKpr~cTaan ceHa) na aa rot ue ~t 3.rleKTpOMaFHI4THO~ pa ~H ar t,n . F1 ,K HO~IB.qlteTC~I Ha ~ n , n e BO2"IHbl ~.qffKOTOpOII noi