CARS, THE ONLY TOOL FOR THE DIAGNOSTICS OF REACTIVE MEDIA ?

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CARS, THE ONLY TOOL FOR THE DIAGNOSTICS OF REACTIVE MEDIA ?

B. Attal, M. Péalat, J. Taran

To cite this version:

B. Attal, M. Péalat, J. Taran. CARS, THE ONLY TOOL FOR THE DIAGNOSTICS OF REACTIVE MEDIA ?. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-287-C7-298.

�10.1051/jphyscol:1983726�. �jpa-00223283�

JOURNAL DE PHYSIQUE

Colloque C7, supplement au n O 1 l , Tome 44, novembre 1983 page C7-287

CARS, THE ONLY TOOL FOR THE DIAGNOSTICS OF REACTIVE MEDIA ?

B.

A t t a l ,

M .

P 6 a l a t and J.P. Taran

O f f i c e National d ' E t u d e s e t de Recherches Ae'rospatiales IO!IERAI,

L3.P. 7 2 ,

92322 Chiiti LLon Ceder, France

R b s d - La D i f f u s i o n Raman a n t i - S t o k e s Coh6rents (DRASC) e s t une t e c h n i q u e b i e n a d a p t b e

2(

l a mesure des t e m p e r a t u r e s e t c o n c e n t r a t i o n s molecu-

l a i r e s dans l e s m i l i e u x e n r e a c t i o n .

Les p r i n c i p e s g6nGraux s o n t e x p o s e s , a i n s i que l e s c a r a c t e r i s t i q u e s t e c h n i q u e s l e s p l u s i m p o r t a n t e s . Les exemples d l a p p l i c a t i o n l e s p l u s s i g n i f i c a t i f s s o n t d e c r i t s , de f a s o n L p r e s e n t e r l e n i v e a u ac- t u e l de performance

:

temps de mesure, r 6 s o l u t i o n s p a t i a l e , s e n s i - b i l i t e de d k t e c t i o n . La mesure de t e m p b r a t u r e i n s t a n t a n d e dans un moteur L p i s t o n , l l b t u d e des ~ r o d u i t s de p h o t o l y s e de H2C0 e t l a DRASC r b s o n n a n t e s u r C2 s o n t d i s c u t e s .

A b s t r a c t - Coherent a n t i - S t o k e s Raman s c a t t e r i n g (CARS) i s w e l l a d a p t e d t o t h e measurement o f t e m p e r a t u r e s and c o n c e n t r a t i o n s of m o l e c u l e s i n r e a c t i v e g a s e c u s media. The g e n e r a l p r i n c i p l e s a r e g i v e n a l o n g w i t h t h e major t e c h n i c a l advantages and r e q u i r e m e n t s . Characte-

r i s t i c a p p l i c a t i o n s a r e d e s c r i b e d i n o r d e r t o p r e s e n t s t a t e of t h e a r t performance l e v e l s

:

measurement t i m e , s p a t i a l r e s o l u t i o n , d e t e c t i o n s e n s i t i v i t y . I n s t a n t a n e o u s t e m p e r a t u r e measurements i n a p i s t o n e n g i n e , a n a l y s i s of p h o t o l y s i s products o f H CO and r e s -

onance-enhanced CARS of C a r e d i s c u s s e d .

2 2

The s u c c e s s o f o p t i c a l t e c h n i q u e s i n t h e d i a g n o s t i c s o f r e a c t i v e media stems from t h e i r n o n - i n t r u s i v e c h a r a c t e r . Some h a v e b e e n k n w n and used f o r a l o n g t i m e . I n p a r t i c u l a r , s p e c t r o s c o p y by a b s o r p t i o n o r e m i s s i o n i n t h e i n f r a - r e d , v i s i b l e and u l t r a - v i o l e t c a n be e x t r e m e l y s e n s i t i v e ; t h e s e t e c h n i q u e s have made p o s s i b l e t h e d e t e c t i o n o f numerous s p e c i e s and t h e measurement o f t e m p e r a t u r e s i n flames where s o l i d p r o b e s would d i s t u r b t h e r e a c t i n g f l o w f i e l d and g i v e e r r o n e o u s r e a d i n g s . Al- though t h e y a r e s t i l l w i d e l y i n u s e , t h e s e two methods g e n e r a l l y l a c k t h e s p a t i a l r e s o l u t i o n o f t e n r e q u i r e d i n thermometry and a n a l y t i c a l c h e m i s t r y .

The a d v e n t o f t h e l a s e r h a s a l l o w e d t h e development o f a new b r e e d of o p t i c a l methods b a s e d on t h e s c a t t e r i n g o f i n t e n s e , monochromatic l i g h t by t h e gaseous media and c a p a b l e of g i v i n g mm, and even s u b - m , s p a t i a l r e s o l u t i o n . F l u o r e s c e n c e and Spontaneous Raman s c a t t e r i n g h a v e b e e n t h e o b j e c t of an i n t e n s e r e s e a r c h a c t i v i t y i n t h e e a r l y s e v e n t i e s by t h e combus t i o n e n g i n e e r i n g c o r n u n i t y /1,2/. Today, Spontane- ous Raman S c a t t e r i n g i s v i r t u a l l y abandoned f o r l a c k o f d e t e c t i o n s e n s i t i v i t y , ex- c e p t i n a few l i m i t e d a p p l i c a t i o n s . F l u o r e s c e n c e h a s more a p p e a l b e c a u s e o f i t s much improved s e n s i t i v i t y and i s t h e o b j e c t o f d e d i c a t e d r e s e a r c h work by s e v e r a l groups 13 /.

S i n c e 1973, t h e s e two methods have been c h a l l e n g e d by c o h e r e n t Raman t e c h n i q u e s , and by Coherent a n t i - S t o k e s Raman S c a t t e r i n g (CARS) i n p a r t i c u l a r 141. CARS h a d been k n o b n s i n c e 1965 /S/ b u t h a d only been used f o r t h e s p e c t r o s c o p y o f some g a s e s , li- q u i d s and s o l i d s . Because of t h e i r h i g h l u m i n o s i t y and of t h e h i g h c o l l i m a t i o n of t h e s i g n a l beams t h e y produce, t h e s e c o h e r e n t Raman t e c h n i q u e s a r e e x t r e m e l y w e l l s u i t e d f o r t h e work on g a s e o u s media wherein s t r o n g luminous backgrounds a r e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983726

JOURNAL DE PHYSIQUE

~ e n e r a t e d . CARS i s t h e most p o p u l a r f o r s e v e r a l t e c h n i c a l r e a s o n s

:

- ^{i t} i s s i m p l e r and c h e a p e r t o u s e t h a n o t h e r t e c h n i q u e s l i k e RIKES and Raman g a i n s p e c t r o s c o p y ;

- ^{i t} l e n d s i t s e l f t o p r e c i s e c a l i b r a t i o n through t h e u s e o f a r a r e g a s r e f e r e n c e ;

- i t can o p e r a t e i n a m u l t i p l e x mode which a l l o w s i n s t a n t a n e o u s measurements o f tem- p e r a t u r e and s i m u l t a n e o u s d e t e c t i o n o f s e v e r a l s p e c i e s .

F i n a l l y , O p t o g a l v a n i c S p e c t r o s c o p y was a l s o p r o p o s e d r e c e n t l y f o r t h e i n v e s t i g a t i o n o f f l a m e s / 6 / . T h i s form o f s p e c t r o s c o p y i s v e r y s u c c e s s f u l w i t h t h e d e t e c t i o n o f t r a c e c o n s t i t u e n t s . S i n c e i t i s l a r g e l y d e b a t e d i n t h i s m e e t i n g , we s h a l l n o t d i s - c u s s i t h e r e .

The emphasis w i l l b e on CARS and i t s a p p l i c a t i o n s , p a r t i c u l a r l y t h o s e a p p l i c a t i o n s which a r e t h e most r e p r e s e n t a t i v e and w i l l e n a b l e t h e r e a d e r t o make u s e f u l coln- p a r i s o n s w i t h o p t o g a l v a n i c t e c h n i q u e s .

A

b r i e f o v e r v i e w o f t h e t h e o r y and p r a c t i c a l a s p e c t s of CARS w i l l be g i v e n a s a n i n t r o d u c t i o n .

I - THEORETICAL CONSIDERATIONS

CARS i s o b s e r v e d i n m o l e c u l a r g a s e s u s i n g two p a r a l l e l monochromatic l a s e r beams o f p l a n e waves, w i t h a n g u l a r f r e q u e n c i e s

wl

a n d w 2 r e s p e c t i v e l y , w i t h -

,w

If t h e s e l a s e r beams h a v e a f r e q u e n c y d i f f e r e n c e - _-u2 e q u a l t o t h e v i i r a t i o n f r e -

2

-

q u e n c y u o f a Raman a c t i v e v i b r a t i o n a l mode of t h e g a s m o l e c u l e s , a new i n t e n s e beam o f T i g h t i s g e n e r a t e d i n t h e medium. T h i s beam i s a monochromatic beam o f p l a n e waves s u p e r p o s e d t o t h e two l a s e r beams ( F i g . 1). I t s f r e q u e n c y i s w

=wl

+

^(W

-

c3

1. I t s g e n e r a t i o n i s t h e r e s u l t o f t h e s c a t t e r i n g o f t h e incoming q a s e r be- i y t h e g a s m o l e c u l e s , t h e v i b r a t i o n o f which i s b e i n g d r i v e n s y n c h r o n o u s l y and c o h e r e n t -

2

l y by t h e s e beams. S i n c e t h e s i g n a l a p p e a r s on t h e h i g h f r e q u e n c y s i d e o f t h e

e x c i t i n g l a s e r beams ( o r pump beams), i . e . on t h e s i d e l a b e l l e d a n t i - S t o k e s s i d e by t h e s p e c t r o s c o p i s t s , and s i n c e i t i s o n l y o b s e r v a b l e i f t h e m o l e c u l a r v i b r a t i o n s a r e Raman a c t i v e , t h i s p a r t i c u l a r g e n e r a t i o n mechanism h a s been c a l l e d C o h e r e n t a n t i - S t o k e s Raman S c a t t e r i n g . Note t h a t a s i g n a l beam i s a l s o g e n e r a t e d a t t h e s y m m e t r i c f r e q u e n c y

2w2

- w l by t h e same c o u p l i n g mechanism, b u t t h i s s i g n a l g e n e r a l l y l i e s i n a s p e c t r a l domain where i t s d e t e c t i o n i s d i f f i c u l t and i t i s s e l d o m u s e d f o r t h a t r e a s o n . The f r e q u e n c i e s o f a l l t h e s e beams a r e d e p i c t e d i n F i g .

2 .

I V L gas with vibrational resonance

Fig. 1

-

CARS and coherent Stokes Rarnan Scattering

Fig. 2

-

Spectrum of the beams emerging from the sample

'Ihe e x p r e s s i o n o f t h e s i g n a l f l u x a t w 3

=

2 w l -

w 2

i s g i v e n by / 7 /

:

I3 --- 1.9 10-'= W: 1 ~ ) ~ l;l2 2 '

where I 2

12, I ₃ a r e t h e power f l u x e s ( i n W/cm

)

a t f r e q u e n c i e s w ,

^W

*,

^W,,

r e s p e c t ~ v e l y , and

z

i s t h e t h i c k n e s s o f t h e gas sample ( i n cm). d e r e s p o n s e o f t h e g a s e x c i t e d by t h e beams i s r e p r e s e n t e d by t h e n o n l i n e a r o p t i c a l s u s c e p t i b i l i t y x , whose e x p r e s s i o n i s g i v e n below. The l a s e r beams a l s o can b e f o c u s e d a t a p o i n t . The s i g n a l beam i s t h e n g e n e r a t e d i n t h e f o c a l r e g i o n f o r t h e most p a r t , and i t a p p e a r s i n t h e same cone a n g l e a s t h e pump beams. The power o f t h i s s i g n a l beam i n Watts i s g i v e n by

:

We assume i n t h i s e x p r e s s i o n t h a t t h e pump beams h a v e powers P a t - and e same d i a m e i e r .

f d 2 ,

r e s p e c t i v e l y , t h a t t h e y a r e d i f f r a c t i o n - l i m i t e d and h a v e hand ^P2

One n o t i c e s t h a t t h i s e x p r e s s i o n i s i n d e p e n d e n t o f t h e f o c a l l e n g t h o f t h e l e n s and o f t h e beam d i a m e t e r . One a l s o n o t i c e s t h a t t h e s i g n a l i s p r o p o r t i o n a l t o P

h e n c e t h e i n t e r e s t o f u s i n g h i g h power p u l s e d s o u r c e s . 1 P2' The p u r p o s e o f a l l CARS e x p e r i m e n t s i s t o r e c o r d t h e v a r i a t i o n o f P , i . e . t h e v a r i a t i o n o f s u s c e p t i b i l i t y , I f one assumes t h a t t g e r e i s o n l y one v i b r a t i o n a l r e s o n a n c e u

v

where ( d ~ / d Q ) i s t h e s p o n t a n e o u s Raman c r o s s s e c t i o n (cm / s r ) , b t h e d i f f e r e n c e i n

L

p o p u l a t i o n p r o b a b i l i t y between t h e Qwer and u p p e r v i b r a t i o n a l s t a t e s , N t h e number of m o l e c u l e s p e r u n i t volume ( i n cm

)

a n d y t h e damping c o n s t a n t .

- The f i r s t p a r t o f X i s t h e Raman r e s o n a n t p a r t , which d e s c r i b e s t h e r e s p o n s e o f t h e medium when

^W

-

w

i s s c a n n i n g a c r o s s t h e v i b r a t i o n a l r e s o n a n c e . I t i s t h e most i m p o r t a n t p a r k o f t g e s u s c e p t i b i l i t y , t h e p a r t t h a n k s t o which i t i s p o s s i b l e t o conduct a c h e m i c a l a n a l y s i s o f t h e medium and t o measure t h e t e m p e r a t u r e . G e n e r a l l y , t h e medium h a s s e v e r a l v i b r a t i o n a l r e s o n a n c e s , each a s s o c i a t e d w i t h d i s t i n c t r o v i b r a t i o n a l s t a t e s

;

t h i s Raman r e s o n a n t p a r t t h e n i s made up o f a summation o v e r t h e r e s o n a n c e s i n q u e s t i o n , e a c h o f whichhas a w e i g h t p r o p o r t i o n a l t o t h e c o r r e s p o n d i n g d .

-.Xn i s t h e non r e s o n a n t s u s c e p t i b i l i t y . I t c o n t a i n s t h e c o n t r i b u t i o n s o f remote

v l b r a f i o n a l r e s o n a n c e s and t h o s e a s s o c i a t e d w i t h t h e n o n l i n e a r d i s t o r t i o n o f t h e

e l e c t r o n c l o u d o f t h e m o l e c u l e s c a u s e d by t h e i n t e n s e l a s e r f i e 1 d s . X i s r e a l

and p r o p o r t i o n a l t o t h e number d e n s i t y o f t h e m o l e c u l e s

; i t

i s a slow"fy v a r y i n g

f u n c t i o n o f w l

- u 2 ;

i t i s g e n e r a l l y n e g l i g i b l e , b u t t h e r e a r e s i t u a t i o n s f o r

which i t s complex ~ n t e r f e r e n c e w i t h t h e Raman r e s o n a n t p a r t is- t r o u b l e s o m e ,

e s p e c i a l l y when one t r i e s t o d e t e c t t r a c e s i n a m i x t u r e .

C7-290 JOURNAL DE PHYSIQUE

Of e x c e p t i o n a l i n t e r e s t i s t h e s i t u a t i o n f o r which t h e o p t i c a l f r e q u e n c i e s o f i n t e r e s t

(U

, w ~ , w ~ ) a r e c l o s e t o o r a t r e s o n a n c e w i t h a b s o r p t i o n l i n e s o f t h e m o l e c u l e s u n a e r s t u d y ( r e s o n a n c e CARS). The Raman c r o s s s e c t i o n d G / d R t h e n under- goes a l a r g e enhancement. Consequently, t h e Raman r e s o n a n t p a r t of t h e s u s c e p t i - b i l i t y i s l a r g e r , and t h e d e t e c t i o n s e n s i t i v i t y can b e improved o r d e r s o f magni- t u d e . The e x p r e s s i o n g i v i n g t h e s u s c e p t i b i l i t y i s v e r y complex and t h e s t r u c t u r e o f t h e s p e c t r u m becomes i n t r i c a t e 171. Although v e r y few groups h a v e t a c k l e d t h e problem o f r e s o n a n c e CARS i n g a s e s

/ 8 - 1 4 / ,

t h i s i s one of t h e most e x c i t i n g a r e a s of r e s e a r c h t o d a y . An example of r e s o n a n c e CARS i n C2 w i l l b e p r e s e n t e d i n t h i s communication.

I I - PRACTICAL CONS1 DERATIONS

I t i s s u f f i c i e n t t o s c a n

W

w h i l e k e e p i n g u ~ ~ f i x e d , and t o measure P3, i n o r d e r t o conduct t h e s p e c t r a l a n a l y s ~ s o f t h e g a s . From t h i s a n a l y s i s , one e x t r a c t s e x a c t l y t h e sarne i n f o r m a t i o 3 a s in16pontaneous Raman s c a t t e r i n g , b u t t h e g a i n i n l u m i n o s i t y i s c o n s i d e r a b l e

(10

t o 10 i f comparable l a s e r s a r e used) and t h e r e j e c t i o n of s t r a y l i g h t i s f a r more e f f i c i e n t b e c a u s e of t h e e x c e l l e n t d i v e r g e n c e o f t h e s i g n a l beam. From t h e s p e c t r a , one c a n e x t r a c t t h e f o l l o w i n g i n f o r m a t i o n

:

- t h e c o m p o s i t i o n o f t h e gas m i x t u r e ( s i n c e each s p e c i e s h a s i t s own c h a r a c t e r i s t i c s e t o f v i b r a t i o n a l f r e q u e n c i e s and can b e i d e n t i f i e d from i t s CARS s p e c t r a l s i g n a - t u r e ) ,

- t h e number d e n s i t y o f t h e s p e c i e s ( s i n c e t h e s i g n a l i s p r o p o r t i o n a l t o N 2 , N can be deduced from t h e measurement of P3),

- t h e t e m p e r a t u r e ( b e c a u s e t h e s i g n a l a l s o depends on A , i . e . on t h e Boltzmann p o p u l a t i o n f a c t o r s ) . More p r e c i s e l y , one t a k e s a d v a n t a g e o f t h e f a c t t h a t t h e v i b r a t i o n a l frequency weakly depends on t h e r o t a t i o n a l s t a t e , and t h a t i t de- c r e a s e s w i t h h i g h e r r o t a t i o n a l s t a t e s . The l a t t e r becomirrymore p o p u l a t e d when t h e t e m p e r a t u r e i n c r e a s e s , t h e s p e c t r a l w i d t h i n c r e a s e s .

Two methods a r e a t o u r d i s p o s a l i n o r d e r t o conduct t h e r e q u i r e d s p e c t r a l a n a l y s i s

:

s p e c t r a l s c a n n i n g and m u l t i p l e x r e c o r d i n g ( F i g . 3 ) . The f i r s t one u s e s two mono- c h r o m a t i c l a s e r s

;

i t h a s an e x c e l l e n t s i g n a l t o n o i s e r a t i o , b u t i t i s a p p l i c a b l e only t o s t a b l e media. h e s e c o n d one i s w e l l a d a p t e d t o t h e s t u d y of f l u c t u a t i n g media

;

i t i s implemented u s i n g a l a s e r c o v e r i n g a wide r a n g e o f f r e q u e n c i e s w _2' which a l l o w s one t o s i m u l t a n e o u s l y e x c i t e a l l r o t a t i o n a l components and t o g e n e r a t e

the e n t i r e s p e c t r u m u n d e r s t u d y w i t h a s i n g l e p u l s e . The d e t e c t i o n t h e n i s done w i t h a h i g h l y d i s p e r s i v e s p e c t r o g r a p h and w i t h a m u l t i c h a n n e l d e t e c t o r such a s a TV camera 1151.

Fig. 3

-

Recording of spectra in CARS:

a) scanning; bl multiplex

l b o remarkable examples of t e m p e r a t u r e and c o n c e n t r a t i o n measurements a r e p r e s e n t e d

i n f i g u r e s 4 and 5.

Figure 4 gives t h e v a r i a t i o n of the CARS s i g n a l c o l l e c t e d on t h e main v i b r a t i o n a l l i n e

~ ( 1 )

w h i l e changing the H c o n c e n t r a t i o n i n a i r a t room temperature 1 4 1 . The q u a d r a t i c dependence vs concenzration i s c l e a r l y demonstrated down t o the very low value of 100 ppm, below which the non resonant background of N2 and

0

molecules

becomes predominant.

2

H p CONCENTRATION ( p p m )

Fig. 4

-

CARS signal on 0111 H2 line in air vs mole fraction

Fig. 5

-

Normalizedspectrum of N2 calculed for different temperaturm assuming a resolution of 1 c 6

'

F i g u r e 5 gives a s e t of t h e o r e t i c a l s p e c t r a of N2 c a l c u l a t e d f o r s e v e r a l values of

the temperature. I n t h i s s i m u l a t i o n , one has assumed t h a t t h e instrument ( i . e . t h e

l a s e r sources t o g e t h e r with t h e d e t e c t i o n system) has a s p e c t r a l r e s o l u t i o n of

1.2cm-1.The f i n e r o t a t i o n a l s t r u c t u r e i s p a r t i a l l y b l u r r e d as a r e s u l t of t h a t low

r e s o l u t i o n , b u t t h e envelope d i s t o r t i o n which r e s u l t s from t h e change i n Boltzmann

f a c t o r s when t h e temperature i s changed i s c l e a r l y demonstrated, and i t can be used

f o r s e n s i n g temperature with good accuracy. This p r o p e r t y i s g e n e r a l and can b e used

with o t h e r s p e c i e s l i k e HI 02, e t c . . Let us a l s o n o t e t h a t a temperature measurement

g e n e r a l l y i s r e q u i r e d p r i o r o any c o n c e n t r a t i o n measurement.

C7-292

JOURNAL DE PHYSIQUE

These two examples c l e a r l y e s t a b l i s h t h e p o t e n t i a l o f CARS i n a n a l y t i c a l c h e m i s t r y and t e m p e r a t u r e measurement. The f i r s t example a l s o shows t h e a d v e r s e r o l e p l a y e d by t h e non r e s o n a n t s u s c e p t i b i l i t y o f t h e d i l u e n t which p r e c l u d e s t h e measurement o f t h e c o n c e n t r a t i o n s o f v e r y d i l u t e s p e c i e s . As a m a t t e r o f f a c t , t h e r e a l d e t e c - t i o n s e n s i t i v i t y o f CARS i s i n t h e r a n g e o f 1% t o 1 0 ppm depending on t h e s p e c i e s , on t h e thermodynamic c o n d i t i o n s , and on t h e e x p e r i m e n t a l a r r a n g e m e n t . I n s c a n n i n g CARS, i t i s f r e q u e n t l y advantageous t o u s e a t e c h n i q u e o f non r e s o n a n t background s u p p r e s s i o n b a s e d on t h e d i f f e r e n c e i n t h e t e n s o r p r o p e r t i e s o f t h e Raman-resonant and non r e s o n a n t p a r t s o f t h e s u s c e p t i b i l i t y . F o r t h a t , t h e pump beams a r e a p p l i e d w i t h d i s t i n c t s t a t e s o f p o l a r i z a t i o n ( g e n e r a l l y t h e s e a r e l i n e a r w i t h a 60" a n g l e between them), and one p a r t i c u l a r p o l a r i z a t i o n component o f t h e s i g n a l h a s t o b e s e l e c t e d / 1 6 , 1 7 / .

Using t h i s t e c h n i q u e c a u s e s a r e d u c t i o n o f t h e s l g n a l

;

i n s p i t e o f t h e s u p p r e s s i o n o f t h e background, t h e r e g e n e r a l l y f o l l o w s a l o s s i n d e t e c t i v i t y o f m u l t i p l e x s p e c t r o s c o p y b e c a u s e t h e s i g n a l s p e c t r a l d e n s i t y is weak and t h e m u l t i c h a n n e l d e t e c t o r s a r e n o i s y

/ 4 , 1 8 , 1 9 / .

The l a s t i m p o r t a n t f e a t u r e o f CARS i s i t s s p a t i a l r e s o l u t i o n . The l a t t e r i s o f t h e o r d e r o f 1 cm i f one u s e s c o l l i n e a r beans a t t h e d i f f r a c t i o n limit which a r e f o c u s e d a t t h e p o i n t o f i n t e r e s t under a n f number o f 60. F o r some a p p l i c a t i o n s , p a r t i c u l a r - ly i n t u r b u l e n t combustion, t h i s i s i n s u f f i c i e n t . A b e t t e r r e s o l u t i o n i s o b t a i n e d w i t h a crossed-beam geometry c a l l e d BOXCARS / 2 0 / . I n t h i s c o n f i g u r a t i o n , a n a d d i t i o n a l beam o f f r e q u e n c y ~ ~ i s f o c u s e d a t t h e same p o i n t , t h u s g e n e r a t i n g a new s i g n a l beam i n i t s d i r e c t i o n ( F i g . 6 ) . The l a t t e r i s g e n e r a t e d from t h e volume common t o a l l pump b e a m . The s p a t i a l r e s o l u t i o n depends on t h e a n g l e between t h e beams and i s o f t h e o r d e r o f 1 mm.

1

a1 .a2 (I - -

fa) ^U3

^detection

Fig. 6 - Beam arrangement in CARS a1 collinear beams ; 61 BOXCA RS

I11 - CARS ANALYSIS OF REACTIVE MEDIA

A l l t h e measurements r e p o r t e d i n t h i s s e c t i o n , e x c e p t t h e l a s t one, h a v e b e e n ob- t a i n e d w i t h a CARS i n s t r u m e n t t h e d e s c r i p t i o n o f which can b e found i n / 1 7 / . l h i s i n s t r u m e n t i s v e r y v e r s a t i l e and can r e c o r d m u l t i p l e x o r s c a n n i n g s p e c t r a , w i t h BOXCARS o r c o l l i n e a r beams, and w i t h o r w i t h o u t background s u p p r e s s i o n . The l a s t r e s u l t s a r e resonance-enhanced CARS s p e c t r a o f C2 which w e r e r e c o r d e d u s i n g two f lash-pumped dye l a s e r s .

Temperature measurements

The f i r s t s u c c e s s f u l CARS measurements i n a p i s t o n e n g i n e were made s e v e r a l y e a r s a g o

/ 2 1 / .

These measurements a r e made v e r y d e l i c a t e by s e v e r a l t e c h n i c a l o b s t a c l e s s u c h a s s y n c h r o n i z a t i o n o f t h e l a s e r p u l s e s w i t h e n g i n e c r a n k r o t a t i o n ,

window f o u l i n g by l u b r i c a t i n g o i l , c y c l e t o c y c l e f l u c t u a t i o n s i n flame f r o n t deve-

lopment which i n t e r d i c t u s e o f s c a n n i n g CARS, and beam s t e e r i n g c a u s e d by t h e s t r o n g

d e n s i t y g r a d i e n t s . I n s p i t e o f t h e s e d i f f i c u l t i e s , s e v e r a l groups h a v e r e p o r t e d

s u c c e s s f u l a t t e m p t s /21-23/. S i m i l a r measurements were t a k e n a t

ONERA

r e c e n t l y / 2 4 / .

The e n g i n e was a f o u r - s t r o k e , i s o o c t a n e - f i r e d s i n g l e c y l i n d e r e n g i n e d r i v e n by a n

e l e c t r i c motor. The problem o f o p t i c a l a c c e s s was s o l v e d by p a s s i n g t h e beams

through 1 . 5 mm d i a m e t e r h o l e s i n t h e c y l i n d e r w a l l s . These h o l e s n a t u r a l l y c a u s e a s m a l l p r e s s u r e d r o p which remains a c c e p t a b l e a t t h e h i g h rpm. Note t h a t , i f n e c e s - s a r y , one h a s t h e p o s s i b i l i t y t o u s e m e c h a n i c a l s h u t t e r s i n o r d e r t o s t o p t h e l e a k u n t i l t h e t i m e o f t h e o p t i c a l measurement. The CARS pump beams w e r e i n a BOXCARS c o n f i g u r a t i o n which gave a b o u t 2 m s p a t i a l r e s o l u t i o n w i t h a 5 0 cm f o c a l l e n g t h f o c u s i n g l e n s .

F i g u r e 7 i s a t y p i c a l m u l t i p l e x CARS spect_rym of N2 r e c o r d e d a t a c r a n k a n g l e o f 30°. The s p e c t r a l r e s o l u t i o n i s a b o u t 1 cm . The r e s u l t o f a n u m e r i c a l s i m u l a t i o n assuming a t e m p e r a t u r e o f 2 . 1 5 0 K and a p r e s s u r e o f 15 a t m o s p h e r e s i s shown f o r comparison. The p r e s s u r e was o b t a i n e d from a p r e s s u r e gauge. The t e m p e r a t u r e i s o b t a i n e d w i t h a n a c c u r a c y o f a b o u t 5 0 K ( o n s p e c t r a l i k e t h a t o f F i g . 7) u s i n g e i t h e r v i s u a l f i t s o r a n automated l e a s t s q u a r e s r o u t i n e .

Crank angle pos~tron 30'

..----v

'Jl -'J2 Fig. 7

-

Single shot CARS spectrum i n piston engine and numerical simulation

The t e m p e r a t u r e o f t h e g a s e s through t h e f u l l c y c l e i s shown i n F i g . 8. About 20 measurements were made a t e a c h p o s i t i o n . T h e i r a v e r a g e and s t a n d a r d d e v i a t i o n a r e g i v e n . The s t a n d a r d d e v i a t i o n r e f l e c t s combined measurement u n c e r t a i n t i e s and r e a l t e m p e r a t u r e f l u c t u a t i o n s . A t 15 and 30°, t e m p e r a t u r e s measured a r e i n a r a n g e o f 600 t o 2 2 0 0

K

b e c a u s e o f f l u c t u a t i o n s i n flame f r o n t development from s h o t t o s h o t . F o r t h e s e two p o s i t i o n s , o n l y t h e a v e r a g e s a r e g i v e n .

Crank angle

0 I

0 180 360 540 720

Fig. 8 - Temperature in the combustion chamber of a single cylinder, 4 stroke gasoline-fired engine type Bernard 61 7: The average of 2 0 consmutive measurements is plotred; the bars represent the standard deviation in temperature fluctuations. The standard deviation is not given at 75 and 3 0 ', where temperature fluctuates between 6 0 0 and 2200 K because of engine cycle non reproducibility. Spatial resolution: 1 mm.

C7-294

JOURNAIL

DE

PHYSIQUE

S i m i l a r measurements have been made s u c c e s s f u l l y under h i g h l y s o o t i n g c o n d i t i o n s b u r n i n g t o l u e n e i n s t e a d of t h e i s o o c t a n e .

The p i s t o n e n g i n e example i s t y p i c a l of some o f the d i f f i c u l t t u r b u l e n t combustion e x p e r i m e n t s which can b e u n d e r t a k e n w i t h CARS. Good s p a t i a l r e s o l u t i o n , l i m i t e d r e q u i r e m e n t s f o r o p t i c a l a c c e s s and i n s t a n t a n e o u s measurement c a p a b i l i t y a r e h e r e r e m a r k a b l e a s s e t s . However, t h e d e t e c t i v i t y remains l i m i t e d . I t i s t y p i c a l l y 1% i n t h e s e m u l t i p l e x e x p e r i m e n t s . Much b e t t e r d e t e c t i o n s e n s i t i v i t i e s c a n b e a c h i e v e d u s i n g s c a n n i n g CARS.

Study of v i b r a t i o n a l l y e x c i t e d

H

produced i n H2C0 d i s s o c i a t i o n 2

A t room t e m p e r a t u r e , s c a n n i n g CARS can o f f e r e x c e l l e n t d e t e c t i o n s e n s i t i v i t i e s on s p e c i e s l i k e

H 2

o r CO2 R e c e n t l y , H2 d e n s i t i e s o f 10'' - ^{1 0} l 2 c m 3 on v i b r a t i o n a l s t a t e v

=

3 have been d e t e c t e d i n a d i s c h a r g e through 40 m T o r r H / 2 5 / . Such s e n s i t i v i t i e s , combined w i t h t h e p o s s i b i l i t y o f q u i c k s y n c h r o n i z a $ i o n o f t h e measu- rements w i t h f a s t e v e n t s , make CARS v e r y a t t r a c t i v e f o r p h o t o c h e m i s t r y e x p e r i m e n t s

;

t h e p h o t o l y s i s o f C H and 0 was s t u d i e d r e c e n t l y

/ 2 6 , 2 7 / .

Also o f c o n s i d e r a b l e i n t e r e s t i s t h e f o r k a l i e h y d e z h o t o d i s s o c i a t i o n .

As

a m a t t e r of f a c t , t h e UV p h o t o l y s i s of

H CO Aslone of-tpe

most a c t i v e l y s t u d i e d phenomena i n p h o t o c h e m i s t r y . E x c i t a t i o n o f $he A A2 - ^X A t r a n s i t i o n between 330 and 355 nm y i e l d s H + CO. About 65% o f t h e d i s s o c i a t i o n e n e r g y goes i n t o 1 p r o d u c t t r a n s l a t i o n /283. F u r t h e r m o r e , t h e prompt CO c a r r i e s l i t t l e v i b r a t i o n a l e n e r g y b u t h a s some r o t a t i o n a l e x c i t a t i o n w i t h J

=

26 - 6 3 / 2 9 / . I t was s p e c u l a t e d t h a t prompt H2 would a p p e a r w i t h fewer t h a n 4 q u a n t a o f v i b r a t i o n and J ( 15. CARS can b e used t o v e r i f y t h e s e a s s u m p t i o n s . Although t h e d e t e c t i o n s e n s i t i v i t y on p u r e H i s e x c e l l e n t , i t i s reduced i n t h i s i n s t a n c e b e c a u s e t h e t r a n s l a t i o n a l tempera- t u r e i s h i g h ( v i z 2

=

30 000 K) and b e c a u s e t h e p r e s e n c e o f

H

CO adds a l a r g e non r e s o n a n t CARS s u s c e p t i b i l i t y which swamps t h e weaker H2 l i n e s . I t i s t h e n n e c e s s a r y 2 t o add He a s a b u f f e r a t a p r e s s u r e s u f f i c i e n t t o c o o l t h e t r a n s l a t i o n and t h e r o t a t i o n , b u t n o t t h e v i b r a t i o n / 3 0 / .

The p h o t o l y s i s was performed u s i n g a frequency-doubled YAG-pumped dye l a s e r d e l i v e r - i n g 1 0 d n e a r 339 nmLlwith a p u l s e w i d t h o f 1 0 ns and a bandwidth a d j u s t a b l e a t e i t h e r 1 . 5 o r 0.15 cm . The CARS s p e c t r o s c o p y was performed w i t h t h e ONERA i n s t r u - ment i n t h e s c a n n i n g mode w i t h non r e s o n a n t background c a n c e l l a t i o n . The beams e m i t t e d by t h e s e two i n s t r u m e n t s were i n t r o d u c e d through s e p a r a t e windows and c r o s - s e d a t a s m a l l a n g l e i n t h e t e s t v e s s e l

; t h e l a t t e r was f i l l e d w i t h 1 . 3 mbar o f

H CO and 130 mbar of He. Two d e l a y s of 0 . 1 and l p s were t r i e d between t h e

UV

and c ~ R S p u l s e s . The U Y wfs tuned c l o s e t o t h e c e n t e r of a group o f l i n e s o f o r t h o H CO

a t 339 nm i n t h e 2 -4 band / 3 1 / . 2

Fig. 9

-

CARS spectrum of Hz formed in the H2C0 photolysis showing the absence of the para form

The n u c l e a r s p i n o r i e n t a t i o n o f t h e

H

f o m d was t h e n s t u d i e d . The H 2 s p e c t r u m of F i g . 9 was o b t a i n e d a f t e r an e s t i m a t e 3 150 J o f UV r a d i a t i o n had been a b s o r b e d by a p a r t i c u l a r f i l l of 1 3 . 3 mbar of H2C0 i n 150 mbar o f He b u f f e r . The W was tuned t o be c o i n c i d e n t w i t h t h e r~ ( 7 ) 0 a b s o r p t i o n l i n e and had t h e narrow bandwidth. From

5 ¹

t h e s p e s r a l i n t e n s i t y , we can c a l c u l a t e t h e H p a r t i a l p r e s s u r e and a r r i v e a t

28.7 10 mbar. Only t h e o r t h o form i s d e t e c t e i , which confirms c o n s e r v a t i o n of HH

n u c l e a r s p i n o r i e n t a t i o n d u r i n g t h e d i s s o c i a t i o n . F o r t h e s e e x p e r i m e n t a l c o n d i t i o n s ,

- ³

we e s t i m a t e o u r d e t e c t i o n s e n s i t i v i t y t o b e o f t h e o r d e r o f 0.44 1 0 mbar f o r p a r a H 2

;

we deduce t h a t l e s s t h a n 4% o f n a t u r a l abundance was g e n e r a t e d r t h a t s p e c i e s . Knowing t h e c e l l volume ( 2 6 l ) , we a l s o c a l c u l a t e t h a t 2.10 F9 o r t h o H 2 m o l e c u l e s were c r e a t e d and t h a t t h e d i s s o c i a t i o n y i e l d i s a b o u t 10%.

The

H

v i b r a t i o n a l d i s t r i b u t i o n , N(v), was s u b s e q u e n t l y s t u d i e d by m o n i t o r i n g t h e Q(1) Z i p e i n t e n s i t i e s u n d e r t h e nominal c o n d i t i o n s d e s c r i b e d above and w i t h t h e

1 . 5 cm bandwidth. V i b r a t i o n a l s t a t e s v = 1 t o 4 were measured. Because o f a n a m b i g u i t y a s s o c i a t e d w i t h t h e f a c t t h a t CARS measures t h e a b s o l u t e v a l u e o f t h e p o p u l a t i o n d i f f e r e n c e between v i b r a t i o n a l s t a t e s , we have two p o s s i b i l i t i e s f o r N ( 1 ) . The l a r g e r i s t h e most p r o b a h l e one. Note t h a t t h e a m b i g u i t y c o u l d be

l i f t e d by n o t c a n c e l l i n g t h e background t o t a l l y and d e d u c i n g t h e s i g n of t h e popu- l a t i o n d i f f e r e n c e from t h e l i n e asymmetry. The r e s u l t s a r e p r e s e n t e d i n F i g . 1 0 f o r t h e two d e l a y s o f 0 . 1 and 1p

S .

The a c c u r a c y o f t h e measurements i s o f t h e o r d e r of

+ 30%. The r e s u l t s show t h a t t h e p r e f e r r e d d i s t r i b u t i o n i s p e a k e d o n t h e l o w e r v ' s .

- The liq5 a s s g c i a t e d w i t h v

=

4 c o u l d n o t b e d e t e c t e d , h e n c e a n u p p e r l i m i t o f 0 . 8 ~ 1 0 cm can b e e s t i m a t e d f o r t h e p o p u l a t i o n . The p o p u l a t i o n s a p p e a r t o b e

s l i g h t l y h i g h e r w i t h t h e l o n g e r d e l a y . T h i s r e s u l t i s n o t t r u l y s i g n i f i c a n t g i v e n o u r measurement a c c u r a c y , b u t c o u l d a l s o b e e x p l a i n e d i n p a r t by t h e s l o w e r r e - l a x a t i o n o f h i g h J s t a t e s

;

t h i s e f f e c t would b e more pronounced f o r low v ' s , where h i z h e r r o t a t i o n a l e n e r g i e s a r e e x p e c t e d .

3 0 ~ ~ ~ v ~ ~ ~ ~ w c m " l

Fig. 10

-

Number density of vibrationalstates of H2C0 from 0/1/ line intensifies assuming Boltzmann equilibrium at 300 K for translation and rotation. :O. Ips delay; X : lys delay. Results in circles obtained if NIII

<

N12I.

Th7ge e - q e r i m e n t s were c o n d u c t e d w i t h a d e t e c t i o n s e n s i t i v i t y i n t h e r a n g e o f 1 0 cm d e p e n d i n g on o r t h o o r p a r a n a t u r e , on v i b r a t i o n a l quantum number v ( i t improves a s l / ( v + l ) ) and on dye l a s e r e f f i c i e n c y . The s p a t i a l r e s o l u t i o n was 8 cm.

S l i g h t l y b e t t e r d e t e c t i o n s e n s i t i v i t i e s would h a v e b e e n d e m o n s t r a t e d i f background s u p p r e s s i o n h a d n o t b e e n n e c e s s a r y .

With t h e i n t r o d u c t i o n o f b e t t e r componepp, mqre p o w e r f u l l a s e r s and photon c o u n t i n g , one c a n e x p e c t t h a t a d e t e c t i v i t y o f 10 cm w i l l b e o b t a i n e d on H a t room

t e m p e r a t u r e o r t h a t t h e d e t e c t i v i t y w i l l b e m a i n t a i n e d w h i l e t h e s p a $ i a l r e s o l u t i o n i s improved t o a b o u t 1 mm.

To t r y and make p r o g r e s s beyond t h e s e f i g u r e s would be e x t r e m e l y c o s t l y w i t h conven- t i o n a l CARS. However t h e u s e o f r e s o n a n c e enhanced CARS may a l l o w some f u r t h e r g a i n s . R e c e n t l y , r e s o n a n c e CARS was u s e d t o d e t e c t C2 i n a d i s c h a r g e and i n a flame 1321.

Resonance-enhanced CARS i n C 2

The C r a d i c a l h a s a v e r y b r j g h t e m i s 3 i o n s p e c t r u m i n t h e v i s i b l e r a n g e which i s known a s t h e Swan s y s tem 2

I d

V g a U U ] and which c a n b e u s e d f o r resonance- enhanced CARS s t u d i e s . Resonant CARS l i n e s of C2 produced by U.V. p h o t o l y s i s o f b e n z e n e h a v e a l r e a d y b e e n o b s e r v e d by N i b l e r e t a 1 1 3 3 1 b u t c o u l d n o t b e a s s i g n e d unambiguously. We have completed a CARS s t u d y of C u n d e r w e l l c o n t r o l l e d c o n d i t i o n s and f u l l y i n t e r p r e t e d t h e s p e c t r u m . Two d i f f e r e n t s o u r c e s of C were used f o r t h a t

2

s t u d y

:

a s t a b l e microwave d i s c h a r g e i n a f l o w i n g m i x t u r e of 33 a c e t y l e n e i n helium

( P t o t L 4 0 mb), and a n o x y a c e t y l e n e f l a m e .

The r e s o n a n t CARS s p e c t r o m e t e r i s composed o f two t u n a b l e flash-pumped dye l a s e r s

1171. The " l a s e r " and "Stokes" beams a r e f o c u s e d i n t o t h e d i s c h a r g e o r i n t h e i n n e r

cone o f t h e flame. The a n t i - S t o k e s s i g n a l i s f i l t e r e d by a monochromator and de-

t e c t e d by a p h o t o m u l t i p l i e r . The l a s e r f r e q u e n c y y was t u n e d i n t o t h e s p e c t r a l

C7-296

JOURNAL DE PHYSIQUE

range which was expected t o g i v e n e a r l y t r i p l e resonance enhancement of t h e CARS s i g n a l

1321.

The s e a r c h f o r t h a t t r i p l e resonance c o n d i t i o n i s done by u s i n g p r e c i s e s p e c t r o s c o p i c c o n s t a n t s . These d a t a were not a v a i l a b l e from e a r l i e r work and an i n t e r f e r o m e t r i c s t u d y of t h e Swan emission spectrum h a s been c a r r i e d o u t i n col- l a b o r a t i o n with P . Luc and C. Amiot a t CNRS.

ResonantCARS s p e c t r a of C2 a r e p r e s e n t e d i n Figs 11 ( d i s c h a r g e ) and 12 ( f l a m e ) . I n t h e s e experiments, t h e f r e uency -P w l was k e p t c l o s e t o t h e ~ ~ ( 0 - 0 ) 1 9 l i n e of t h e green band a t 19494.18 cm . The Stokes frequency

W

was tuned through t h e (0-1) band.

t

^'"l

1

Fig.

1 7 -

Resonance CARS spectrum of

C2

in a microwave discharge with computer simulation.

The positions of the various single resonances are marked in the theorical plot with bars of unequal length to distinguish the J 18,

19

and 20contributions.

Fig. 72 -

C2

spectrum in the flame. The collisional width (half width at half maximum) is taken as 0.25 cm- ' for all lines in the simulation

The s p e c t r a l p r o p e r t i e s of t h e CARS s u s c e p t i b i l i t y t e n s o r a r e r e f l e c t e d i n t h e s e s p e c t r a . Because of t h e high v i b r a t i o n a l temperature, t h e s u s c e p t i b i l i t y i s mainly c o n t r i b u t e d by 4 terms, one of which i s p r o p o r t i o n a l t o t h e v

=

0 l e v e l p o p u l a t i o n of t e a 3 n u s t a t e , t h e o t h e r t h r e e e ng p r o p o r t i o n a l t o t h e v

=

1 l e v e l p o p u l a t i o n

c 09

(p ,, i n r e f . / 7 / ) .

' h 0

of t h e s e terms a r e Dopp J e r - f r e e

;

two of them c o n t a l n t h e v i b r a t i o n frequency of t h e molecule i n t h e d l l g s t a t e .

Thgas t r o n g e s t f e a t u r e i n Fig. n ' a 11 i s a laser-enhanced Raman resonance of t h e type [Q. ( 1 - O), Rna

( 0

- O), Pi ( 1 - 0 ) ] J c o n t r i b u t e d by t h e J

=

18, 19, 20 l e v e l s .

~ h 6 t r i p l e t s p l i t t i n g i s not r e s o l v e d ( w b a = 1611.74

;

1611.72 and 1611.70 r e s p e c t -

i v e l y ) . The weaker li e a t t h e l e f t i s due t o thg c o n t r i b u t i o n s of %pnresonances

w h i p a r e labelAp2 [ Z a ( 1 - 0 ) . R';' ( 0 - 01, P" a ( l - 0 ) I18 and [ol ^{( l} - 01,

P; ( l - l ) , p1 ( l - ^{0 )}

¹²⁰

i n o u r n o t a t i o n . 20th major and minor c o n t r i b u t i o n s

a r e taken i n t o account i n t h e c a l c u l a t i o n as d e p i c t e d by t h e s h o r t b a r s on t h e

t h e o r e t i c a l p l o t s . F i n a l l y t h e s m a l l ~ h o u l d e r on t h e l e f t of t h g , z a i n l i n e i s of double e l e c t r o n i c resonance n a t u r e [q2a ( 1 - 01, ( 0 - O), P2 ( 1 - ^0)119.

S e v e r a l s p e c t r a were ob.tained i n t h e flame a t v a r i o u s h e i g h t s above the b u r n e r out- l e t f o r

W, =

19494.08 cm-'. The l i n e i n t e n s i t y i s about 10 times lower than i n t h e discharge b e c use the l i n e w i d t h s a r e much l a r g e r a t atmospheric p r e s s u r e

(

1 ~ 0 . 2 1 cmdf. On t h e o t h e r hand, t h e C2 formation was known t o b e confined t o t h e i n n e r cone of t h e flame. S i n c e t h e cone 1s i e s s than 1 mm i n diameter, t h e a c t i v e volume of t h e flame i s much s h o r t e r than i n "the d i s c h a r g e . These reasons e x p l a i n

t h e lower i n t e n s i t y of t h e flame p r o f i l e . The computer f i t given f a r a temperature of about 2500 K i s considered as s a t i s f a c t o r y .

I t was p o s s i b l e t o c a l c u l a t e a rough e s t i m a t e of t h e C d e n s i t y

; t h i s c a l c u l a t i o n

i s based on our p a s t r e s u l t s on I 2 o b t a i n e d under s i m i 3 a r p r e s s u r e c o n d i t i o n s /17/.

A c a l i b r a t i o n was p o s s i b l e because t h e number d e n s i t y i n i o d i n e was p r e c i s e l y known.

The comparison i s p o s s i b l e because t h e s p e c t r o s c o p i c c o n s t a n t s and t r a n s i t i o n moments of

I2

and C 134-371 a r e known and t h e r o v i b r a t i o n a l l e v e l populations probed i n t h two experiments a r e accounte

2

f o r . We thus a r r i v e a t a number d e n s i t y of a b o u t 5 10' cm-3 i n t h e d i s c h a r g and 1 0 f l cm-3 i n t h e flame. The d e t e c t i o n s e n s i - t i v i t y a t low p r e s s u r e i s about 10' cme3 given our n o i s e l e v e l and experimental c o n d i t i o n s . However, t h e e x c e p t i o n a l g a i n o b t a i n e d i n C2 i s mainly due t o the l a r g e t r a n s i t i o n moment of t h e Swan system. Unfortunatly t h e s i t u a t i o n i s n o t s o f a v o r a b l e f o r o t h e r r a d i c a l s such as OH and NO, f o r which lower d e t e c t i o n l i m i t s a r e expected from the kmwn s p e c t r o s c o p i c d a t a .

I V - CONCLUSION

CARS i s t h e most popular, and one of t h e b e s t of t h e modern n o n i n t r u s i v e d i a g n o s t i c s t o o l s f o r r e a c t i v e media. I t s f i e l d i s extremely v a s t , thanks t o s e v e r a l o u t s t a n d i n g p r o p e r t i e s

: s p a t i a l r e s o l u t i o n , a c q u i s i t i o n time, immunity t o s t r a y l i g h t and t o

i o n i z a t i o n . I t i s r e l a t i v e l y simple t o use

; a l l s p e c i e s o f - i n t e r e s t a r e covered by

tuning the U 2 l a s e r over a l i m i t e d s p e c t r a l range (4000 cm

) ;

f i n a l l y , one was a b l e t o develop a commercial u n i t . The d e t e c t i o n s e n s i t i v i t y remains i t s weak p o i n t . I n mixtures, i t i s not b e t t e r than 1% i n t h e m u l t i p l e x mode i n flames. I n some c a s e s , scanning can be used g i v i n g a f a c t o r of 10 improvement

;

f o r s e v e r a l l g e c i e s ':6i4nd-S02 , pure and a t room temperature), d e t e c t i v i t i e s i n t h e range of 10 -

cm can be o r have been demonstrated. S i m i l a r d e t e c t i v i t i e s a r e p o s s i b l e with o t h e r s p e c i e s a t lower temperatures ( e . g . i n beams).

Resonant CARS i s a v a r i a n t which takes advantage of e l e c t r o n i c resonance enhancement of t h e s u s c e p t i b i l i t y . I t h a s a p o t e n t i a l f o r 2 o r 3 o r d e r s of magnitude improvement a t a g r e a t c o s t i n experimental complexity and i n d a t a processing, g i v i n g s e n s i - t i v i t y comparable t o t h o s e of f l u o r e s c e n c e s c a t t e r i n g .

-

However, t h e use of CARS i s r e s t r i c t e d t o t h e molecular s p e c i e s , although a few atoms (Na, 0, F f o r &S-tance)can be

cr

have been s t u d i e d . Monoatomic s p e c i e s a r e n o t e a s i l y handled by CARS because of t h e extreme wavelength coverage needed f o r t h e two l a s e r s .

For atomic s p e c i e s , t to galvanic. Spectroscopy appears s u p e r i o r as f a r as d e t e c t i o n s e n s i t i v i t y i s concerned.

But t h a t technique i s n o t q u i t e as s u c c e s s f u l with molecules. The d e t e c t i o n i s l i m i t e d t o a s i n g l e channel which makes

it

d e l i c a t e t o t a c k l e t u r b u l e n t flames.

F i n a l l y , t h e e l e c t r i c f i e l d a p p l i e d may have a s i g n i f i c a n t i n f l u e n c e upon t h e

chemistry.

JOURNAL DE

PHYSIQUE

REFERENCES

/ I / P r o j e c t SQUID Workshop o n " L a s e r Raman D i a g n o s t i c s " , e d . by M. Lapp a n d C.M.

Penney, Plenum, New York, London 1974.

/ 2 / " L a s e r P r o b e s f o r Combustion C h e m i s t r y " , e d . by D.R. C r o s l e y , ACS Symposium S e r i e s v o l . 1 3 4 , American Chemical S o c i e t y , W a s h i n g t o n D.C. ( 1 9 8 0 ) .

/ 3 / S e e , f o r i n s t a n c e , t h e two s e s s i o n s on f l u o r e s c e n c e s c a t t e r i n g i n r e f . 2.

/ 4 / P.R. R g g n i e r a n d J . P . T a r a n , Appl. Phys. L e t t e r s 23, 2 4 0 ( 1 9 7 3 ) . 1 5 1 P.D. Maker a n d R.W. T e r h u n e , Phys. Rev. 137, A 8 0 1 ( 1 9 6 5 ) .

/ 6 / C.P. A u s s c h i t t , G.C. B j o r k l u n d , a n d R.R. Freeman, Appl. P h y s . L e t t e r s 33, 8 5 1 (1978)

;

P.K. S c h e n c k , W.G. M a l l a r d , J.C. T r a v i s , and K.C. S m i t h , J . Chem. p h y s . 69, 5147 ( 1 9 7 8 ) .

-

/ 7 / S. D r u e t a n d J . P . T a r a n , P r o g r . Quant. E l e c t . 7, 1 ( 1 9 8 1 ) .

/8/ B. A t t a l , O.O., Schnepp, a n d J . P . E . T a r a n , O p t . Commun, 77 ( 1 9 7 8 ) . / 9 / D.F. G u t h a l s , K.P. G r o s s , a n d J.W., N i b l e r , J . Chem. P h y s . 70, 2393 ( 1 9 7 9 ) . / 1 0 / K.P., G r o s s , D.B. G u t h a l s , and J.W., N i b l e r , J . Chen. Phys. 2, 4673 ( 1 9 7 9 ) . / 1 1 / M.E., N c I l w a i n , and J . C . Hindman, J . Chem. P h y s .

z 1 6 8

( 1 9 8 0 ) .

/ 1 2 / B. A t t a l , K. P e a l a t , and J . P . E . T a r a n , J . Energy 4 , 1 3 5 ( 1 9 8 0 ) .

/ l 3 1 A. E e c h a n n ,

H.

F i e t z , P. B a i e r l , and

W .

K i e f e r , Chem. P h y s . L e t t . 8 6 , 4 0 ( 1 9 8 2 ) . 1 1 4 1 ~ . ~ t r a l , D.D6barre, K. I f t i l l e r - D e t h l e f s , and J . P . T a r a n , Revue P h y s . Appl. 18

39 ( 1 9 8 3 ) .

/ 1 5 / W.B. Roh, P.W. S c h r e i b e r , a n d J . P . T a r a n , Appl. Phys. L e t t e r s 2 9 , 1974 ( 1 9 7 6 ) . / 1 6 / L.A. Rahn, L . J . Zych, a n d P.L. M a t t e r n , O p t i c s Corn. 30, 249 ( - g 7 9 ) .

1 1 7 1 B . A t t a 1 , M. P d a l a t , a n d J . P . T a r a n , J . Energy 6 , 135 (1980).

1181 A.C. E c k b r e t h a n d R . J . H a l l , Cornbust. S c i e n c e a n d Techn. 2, 175 ( 1 9 8 1 ) .

/ 1 9 / R. Bbdu6, P. G a s t e b o i s , R. B a i l l y ,

M.

P e a l a t and J . P . T a r a n , "CARS Measurements i n a S i m u l a t e d Turbomachine Combus t o r " , ( t o be p u b l i s h e d ) .

1 2 0 1 A.C. E c k b r e t h , Appl. P h y s . L e t t e r s 2, 4 2 1 ( 1 9 7 8 ) .

/ 2 1 / I . A . S t e n h o u s e , D.R. W i l l i a m s , J . B . Cole a n d M.D. Swords, Appl. O p t i c s 18,

3819 ( 1 9 7 9 ) .

/22/ D. K l i c k , K.A. Marko, and L. Rimai, Appl. O p t i c s g, 1178 ( 1 9 8 1 ) .

/ 2 3 / L.A. Rahn, S.C. J o h n s t o n , R.L. Farrow, a n d P.L. M a t t e r n , i n " T e m p e r a t u r e , i t s Measurement a n d C o n t r o l i n S c i e n c e a n d I n d u s t r y " , Vo1.5, American I n s t i t u t e o f P h y s i c s , New York ( 1 9 8 2 ) .

/ 2 4 / M. P d a l a t , J . J . M a r i e , P. Bouchardy a n d J . P . T a r a n , " O p t i c a l Access f o r CARS D i a g n o s t i c s i n P i s t o n E n g i n e s " , ( t o b e p u b l i s h e d ) .

/ 2 5 / M . P & a l a t , J . P . T a r a n , J. T a i l l e t , M. B a c a l and A.M. B r u n e t e a u , J. Appl. P h y s . , 52, 2687 ( 1 9 8 1 )

;

- M. B a c a l , M. P d a l a t and J . P . T a r a n , 1 6 t h I n t e r n a t i o n a l C o n f e r e n c e o n Phenomena i n I o n i z e d G a s e s , D \ i s s e l d o r f , 1983, ( t o b e p u b l i s h e d ) .

/ 2 6 / W.M. T o l l e s , J . W . N i b l e r , J.R.

MC

Donald a n d A.B. H a r v e y , Appl. S p e c t r o s c . 2,

2 5 3 ( 1 9 7 7 ) .

1 2 7 1 J . J . V a l e n t i n i , D.S. Moore a n d D.S. Bomse, Chem. P h y s . L e t t e r s 83, 217 ( 1 9 8 1 ) . / 2 8 / P. Ho, D. Bamford, R.J. B u s s , Y.T. Lee a n d C.B. Moore, J . Chem. P h y s . , 76,

3630 ( 1 9 8 2 ) .

/ 2 9 / P . Ho a n d A.V. S m i t h , Chem. P h y s . L e t t e r s , 90, 407 ( 1 9 8 2 ) .

/ 3 0 /

M.

P d a l a t , D. D e b a r r e , J . J . M a r i e , J . P . T a r a n , A. Tramer a n d C.B. Moore, "CARS S t u d y o f V i b r a t i o n a l l y E x c i t e d H p P r o d u c e d i n Formaldehyde D i s s o c i a t i o n "

(Chem. Phys. L e t t e r s , t o b e published) .

1 3 1 1 D.A. Ramsey, Can. J . P h y s . , 57, 1224 ( 1 9 7 9 ) .

/ 3 2 / B . A t t a 1 , D. D e b a r r e ,

K .

M t i l l e r - D e t h l e f s , a n d J . P . T a r a n , Revue P h y s . Appl. 18,

39 ( 1 9 8 3 ) .

/ 3 3 / K.P. G r o s s , D.M. G u t h a l s a n d J . W . N i b l e r , J. Chem. P h y s . , 70, 4 6 7 3 ( 1 9 7 9 ) . / 3 4 / L.L. Danylewych a n d R.W. N i c h o l l s , P r o c . R. Soc. London,, S e r .

A Z , 197 ( 1 9 7 4 ) .

1 3 5 1 C. Amiot, J . C h a u v i l l e , and J . P . N a i l l a r d , J . Ztol. S p e c t r o s c . 75 19 ( 1 9 7 9 ) . 1 3 6 1 T e l l i n g h u i s e n , J . J . , J . Chem. P h y s . 5 8 , 2 8 2 1 ( 1 9 7 3 ) .

/ 3 7 / Yee, K.K., P r i v a t e communication t o C. Bordd.