Laser-Induced Fluorescence Spectroscopy in Flames - ACS Publications

3 Laser-Induced Fluorescence Spectroscopy

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 31, 2014 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch003

in Flames JOHN W. DAILY Department of Mechanical Engineering, University of California, Berkeley, CA 94720

The purpose of this paper is to review the use of laser induced fluorescence spectroscopy (LIFS) for studying combustion processes. The study of such processes imposes severe con straints on diagnostic instrumentation. High velocities and temperatures are common, as well as turbulent inhomogeneities, and there is a need to make space and time resolved species concentration and temperature measurements. The development of LIFS has reached the point where it is capable of making significant contributions to experimental combustion studies. Fluorescence is spontaneous radiation that arises because of the stimulation of an atomic or molecular system to energies higher than equilibrium. This is illustrated in Figure 1 for a simple two-level atom. The atom is excited by absorption of a photon of energy hυ. If the fluorescence is observed at 90° to a collimated excitation source, then a very small focal volume may be defined resulting in fine spatial resolution. The fluorescence power an optical system will collect is

P

F

=

h

V

Wl*

( 1 )

where V i s the e f f e c t i v e f o c a l volume, ^ the s o l i d angle of the c o l l e c t i o n o p t i c s , and A21 i s the E i n s t e i n c o e f f i c i e n t f o r spontaneous emission, the p r o b a b i l i t y of decay i n any d i r e c t i o n . The fluorescence s i g n a l can be used i n a number of ways. Most simply i t provides a measure of the population of the excited s t a t e or states through Equation 1. In a d d i t i o n , i f a r e l a t i o n s h i p can be found between the number density of a l l the quantum states under e x c i t a t i o n c o n d i t i o n s , then the t o t a l number density of the species can be deduced. Unfortunately, c o l l i s i o n a l decay process can cause r e d i s t r i b u t i o n of population from the excited l e v e l , complicating i n t e r p r e t a t i o n . c

c

0-8412-0570-1/80/47-134-061 $ 1 0 . 0 0 / 0 ©

1980 American Chemical Society

In Laser Probes for Combustion Chemistry; Crosley, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


62

LASER

PROBES FOR

COMBUSTION

CHEMISTRY

By e x a m i n i n g t h e e x c i t a t i o n s p e c t r u m o f a m o l e c u l a r s p e c i e s one c a n deduce a g r o u n d s t a t e B o l t z m a n n t e m p e r a t u r e . A l s o , as w i l l be d i s c u s s e d b e l o w , i f one c a n p r e d i c t t h e p o p u l a t i o n d i s t r i b u t i o n i n t h e atom o r m o l e c u l e u n d e r e x c i t a t i o n c o n d i t i o n s , t h e n one c a n u s e t h e o b s e r v e d f l u o r e s c e n c e s p e c t r u m t o r e c o v e r t h e gas temperature. F i n a l l y , e n e r g y t r a n s f e r and c h e m i c a l p r o c e s s e s c a n be s t u d i e d by o b s e r v i n g t h e t r a n s i e n t and s t e a d y s t a t e r e s p o n s e of a molecular system to l a s e r e x c i t a t i o n . L a s e r s a r e u s e d as an e x c i t a t i o n s o u r c e f o r t h r e e r e a s o n s . Because the l a s e r output i s coherent i t o f f e r s s p e c i a l a d v a n t a g e s i n d i r e c t i o n a l i t y and f o c u s i n g . T u n a b l e l a s e r s a l l o w the p o s s i b i l i t y of examining s e v e r a l s p e c i e s . Finally, l a s e r s p r o v i d e s i g n i f i c a n t l y h i g h e r power l e v e l s t h a n conventional l i g h t sources. I n t h e f o l l o w i n g we c o n s i d e r t h e n a t u r e o f L I F S i n more detail. The t h e o r e t i c a l f o u n d a t i o n s o f l a s e r e x c i t a t i o n and f l u o r e s c e n c e a r e o u t l i n e d and s u c h i s s u e s as d e t e c t a b i l i t y and dynamic r a n g e a r e d i s c u s s e d . F i n a l l y t h e s t a t u s o f L I F S i s summ a r i z e d and a p r o g n o s i s f o r f u t u r e d e v e l o p m e n t g i v e n .

Theoretical

Considerations

As d i s c u s s e d a b o v e , a L I F S s i g n a l i s p r o p o r t i o n a l t o t h e e x c i t e d s t a t e number d e n s i t y o f t h e s p e c i e s b e i n g e x c i t e d . T h i s i n f o r m a t i o n i s n o t i t s e l f n o r m a l l y u s e f u l . What i s d e s i r e d i s a measure o f t h e t o t a l p o p u l a t i o n , o r the temperature. Often one s e e k s t h e p o p u l a t i o n o f i n d i v i d u a l g r o u n d s t a t e s . To be a b l e t o r e l a t e t h e o b s e r v e d s i g n a l t o v a r i a b l e s o f i n t e r e s t one must be a b l e t o d e s c r i b e t h e d y n a m i c s o f t h e e x c i t a t i o n p r o c e s s .

molecules

The R a t e E q u a t i o n s . As i l l u s t r a t e d i n F i g u r e 1, a r e e x c i t e d by p h o t o n a b s o r p t i o n i n t h e p r o c e s s

N

+

k

hv

JELl*

N

£

, I

> k

(2)

where i s t h e E i n s t e i n B c o e f f i c i e n t f o r a b s o r p t i o n and P i s t h e s p e c t r a l e n e r g y d e n s i t y due t o l a s e r e x c i t a t i o n . L i k e w i s e , m o l e c u l e s c a n be d e - e x c i t e d by t h e i n d u c e d e m i s s i o n process v

B

N

£

+ hv

i_P ^>

N

k

, I > k

and by s p o n t a n e o u s e m i s s i o n t o l o w e r hi

N

£

— U N

+ hv,

(3)

levels

I > 1 = 1,2,3


(4)

3.

Fluorescence

DAILY

63

Spectroscopy in Flames

I n a d d i t i o n , c o l l i s i o n s c a n cause b o t h e x c i t a t i o n and de-excitation i n the process Q


i i Nj + M -^=+ N

+ M ,

±

i + j.

(5)

C o l l i s i o n a l d e - e x c i t a t i o n i s c a l l e d quenching because i t competes w i t h s p o n t a n e o u s e m i s s i o n , and i f s i g n i f i c a n t t h e f l u o r e s c e n t s i g n a l w i l l b e r e d u c e d , o r quenched. C h e m i c a l d e c a y can a l s o b e i m p o r t a n t i n some c i r c u m s t a n c e s . T a k i n g i n t o a c c o u n t t h e s e p r o c e s s e s one may w r i t e r a t e e q u a t i o n s ( D a i l y , 1) f o r t h e i n d i v i d u a l e n e r g y l e v e l s . F o r a s i m p l e t w o - l e v e l s y s t e m one h a s dN "3T

^12

=

(Q

"

+

B

21

+

N

l

p

12 v>

A

+

21

B

N

l p

N

(

21 v> 2>

6

a

)

and N

The

=

TOT

+

N

2

(

•

6

b

)

steady s t a t e s o l u t i o n t o t h i s system i s (using t h e B

:

d e t a i l e d b a l a n c e r e l a t i o n g^B-^ ~ ^2 21^

„ 2

+

[

_

B

^12

[Q

1 2

+ Q

12%

T + (B

+

2 1

] N

+ B

1 2

2 1

. )p ] v

The f i r s t t e r m r e p r e s e n t s t h e e q u i l i b r i u m e x c i t e d s t a t e number d e n s i t y N * , when p = 0 . G e n e r a l l y N » N * and Q « Q so that 2

2

2

V

1 2

2 1

N

It

*

[Q

2 1

A

+

i s conventional

P

so t h a t

2

v

E

(Q

12

+

N

+

(B

A

=

2 +

B

)p ]

2 1

v

N

T

) / ( B

12

+

B

21>

(

•

t o define a "saturation

^21 * 2 1

(noting g ^ B ^

1

1 1

energy

8

)

density

( 9 )

B

§2 21^


LASER

64

Ci +

PROBES FOR

CHEMISTRY

(10)

/g )

g l

COMBUSTION

s

2

D

+ D

T h e r e a r e two l i m i t s o f i n t e r e s t . Low i n t e n s i t y l i m i t . I n t h i s c a s e E q u a t i o n 10 becomes


(ID (1 +

T h i s may

g /g ) 1

2

be r e w r i t t e n a s N,

(B

( B

1 2

12

/A

/ A

2 1

21

)(A

) Y p

)/(Q

2 1

2 1 +

A

2 1

)p N v

T

(12)

N

V T

so t h a t t h e c o l l e c t e d f l u o r e s c e n c e power becomes

^ " S

B

n

V

Y p

12 c c

(13)

N

V TOT

Y i s c a l l e d t h e f l u o r e s c e n c e y i e l d , and f o r c o m b u s t i o n c o n d i t i o n s i s t y p i c a l l y 10"^-10"^; t h a t i s , q u e n c h i n g i s s i g n i f i c a n t . G i v e n k n o w l e d g e o f t h e a t o m i c p a r a m e t e r s B-^ d A i and t h e c o l l i s i o n a l r a t e s , one c a n d i r e c t l y r e l a t e t h e o b s e r v e d N t o t h e t o t a l number d e n s i t y o f t h e s p e c i e s . If calibration i s p o s s i b l e , o n l y t h e t e m p e r a t u r e dependence o f t h e y i e l d , Y, need be known. The d i f f i c u l t y w i t h E q u a t i o n 13 i s t h a t u n d e r c o n d i t i o n s o f t u r b u l e n t c o m b u s t i o n t h e t e m p e r a t u r e and c o m p o s i t i o n , and t h u s Y, may v a r y i n an unknown manner. For c e r t a i n s p e c i a l cases, going to the h i g h i n t e n s i t y l i m i t p r o v i d e s a remarkably simple s o l u t i o n to the quenching problem.. High i n t e n s i t y l i m i t ( s a t u r a t i o n ) . Consider the r a t e e q u a t i o n f o r l e v e l 2 o f o u r s i m p l e atom, E q u a t i o n 6a. I n t h e l i m i t of l a r g e energy d e n s i t y , t h i s e q u a t i o n reduces t o a n

2

2

2

N

2

= (B

1 2

/B

2 1

)

N

which because o f d e t a i l e d b a l a n c i n g N

2

»(g /g ) 2

1

N

(14)

l

(g

B 1

= 1 2

8

B

2 21^

b

e

c

o

m

e

1


s

(15)

Fluorescence

DAILY

3.

Spectroscopy

in

Flames

65

This remarkable r e s u l t s t a t e s that i f the l a s e r i n t e n s i t y i s high enough, then and N w i l l occur i n a f i x e d , known ratio. Equation 10 a l s o reduces i n t h i s l i m i t to 2

1 + g /g


x

(16)

TOT

2

Atomic Systems. Many atomic species may be modeled as t h r e e - l e v e l systems. Figure 2 i l l u s t r a t e s the energy l e v e l diagram f o r sodium. Other a l k a l i and a l k a l i n e metals behave i n a s i m i l a r manner. For e x c i t a t i o n from l e v e l 1 to 2 the steady s t a t e r a t e equations become Q

( Q

12

+

B

N

+

13 1 p

) N

12 v

Q

l

2 3 2 * %2 N

%2

+

+

+

+

Q1

A

3

N

V

(Q

3=

32 +

21

+

A

Q

N

31> +

23

A

(17a)

3> +

21

B

p

N

21 v> 2 ' (17b)

and N I f one

= N

TOT

1

+ N

2

+ N

(17c)

3

defines (N /N ) 3

*

2

=

(Q

+

1 3

Q

2

3

)/(Q

3

+ Q

2

3

+ A

1

3

+

2

A

3

1

)

as a q u a s i - e q u i l i b r i u m population r a t i o , then one may

(18) show that

N N 2

[1 +

g ; L

/g

3

(19)

s

+ N /N )*]

2

2

p

+ p

)

(N /N )

where

s

Q

+ Q

2 1

2

-

3

(Q

+ A

3 2

3

2

3

2

(20) B

12

[ 1

+

g

l

/ g

2

+

(v) t h e n o r m a l i z e d a b s o r p t i o n l i n e s h a p e p a r a m e t e r , e t h e e l e c t r o n c h a r g e , £Q t h e p e r m i t t i v i t y o f f r e e s p a c e , M t h e e l e c t r o n mass, c t h e s p e e d o f l i g h t , a n d f j ^ t h e o s c i l l a t o r strength for the transition. The i m p o r t a n c e o f t r a p p i n g m u s t , o f c o u r s e , b e a s s e s s e d f o r each experiment and t e s t s p e c i e s c o n s i d e r e d . I f one a d o p t s a c r i t e r i a f o r t h e maximum a b s o r p t i o n , a n u p p e r l i m i t i s p l a c e d on t h e number d e n s i t y o f t h e a b s o r b i n g e n e r g y l e v e l w h i c h c a n be allowed. We c a n r o u g h l y e s t i m a t e t h i s b y a s s u m i n g a n o s c i l l a t o r s t r e n g t h o f u n i t y f o r a t o m i c t r a n s i t i o n s , a n d o f 10"~ f o r molecular t r a n s i t i o n s . T h i s l e a d s t o l i n e c e n t e r ground s t a t e absorption c o e f f i c i e n t s of the order of o t ~ 10" N(m" )m"^ and o t ~ 1 0 " N(m"" )m" f o r a 2000°K a t m o s p h e r i c pressure flame. F o r a 1-m p a t h l e n g t h and a n o p t i c a l d e p t h o f u n i t y , t h i s c o r r e s p o n d s t o a n u p p e r l i m i t i n m o l e f r a c t i o n o f a b o u t 0.01 PPM and 10 PPM f o r atoms and m o l e c u l e s , r e s p e c t i v e l y . Of c o u r s e , f o r a b s o r p t i o n t h a t o r i g i n a t e s i n h i g h e r energy l e v e l s , both t h e o s c i l l a t o r s t r e n g t h and t h e number d e n s i t i e s d r o p r a p i d l y . F i g u r e 7 i l l u s t r a t e s t h e t r a p p i n g e f f e c t f o r sodium ( 6 ) . The measurements w e r e made a c r o s s t h e t o p o f a f l a t f l a m e b u r n e r , and a s c a n be s e e n , t r a p p i n g i s s i g n i f i c a n t f o r m o l e f r a c t i o n s l a r g e r t h a n a b o u t 0.15 PPM. e

3

a t O T n

1 9

3

1

m o l

N e a r R e s o n a n t R a y l e i g h S c a t t e r i n g . One p o t e n t i a l method f o r o v e r c o m i n g t h e p r o b l e m o f r a d i a t i v e t r a p p i n g t h a t a p p e a r s t o w o r k w e l l f o r atoms i s n e a r r e s o n a n t R a y l e i g h s c a t t e r i n g ( 7 ) . I f a n atom i s e x c i t e d n e a r a r e s o n a n t l i n e , p a r t o f t h e l i g h t i s s c a t t e r e d a s enhanced R a y l e i g h s c a t t e r i n g . I f t h e atom b e i n g e x c i t e d a l s o u n d e r g o e s c o l l i s i o n s t h e n t h e p o s s i b i l i t y e x i s t s t h a t a s e c o n d component o f l i g h t w i l l b e emitted a t the resonant frequency. This process i s c a l l e d



76

LASER

3 o.o ' ^

-6

1

-4

1

1

1

-2

0

1

2

PROBES FOR COMBUSTION

1

4

1

'

6

8

i 10

12

PATH LENGTH FOR TRAPPING , mm

Figure 7.

Radiative trapping of sodium in a methane-air flame


CHEMISTRY


3.

DAILY

Fluorescence

Spectroscopy

in

Flames

77

collisional redistribution. The i m p o r t a n c e o f t h e s e p r o c e s s e s i s t h a t t h e R a y l e i g h component i s o u t s i d e t h e a b s o r p t i o n l i n e and c a n n o t be t r a p p e d . F i g u r e 8 shows a t y p i c a l f l u o r e s c e n c e s p e c t r u m f o r n e a r r e s o n a n t e x c i t a t i o n o f s o d i u m . The l a r g e peak a r i s e s f r o m c o l l i s i o n a l r e d i s t r i b u t i o n , t h e o t h e r i s t h e enhanced R a y l e i g h component. The r e l a t i v e i n t e n s i t i e s a g r e e w i t h t h e t h e o r y o f M o l l o w ( 8 ) . M o l l o w ' s t h e o r y c a n a l s o be u s e d t o p r e d i c t t h e r a t i o of the R a y l e i g h s i g n a l to the resonant f l u o r e s c e n c e s i g n a l one w o u l d o b t a i n i f t h e r e w e r e no t r a p p i n g . T h i s r a t i o i s shown i n F i g u r e 9 w h i c h i l l u s t r a t e s t h a t t h e R a y l e i g h component c a n be q u i t e l a r g e e v e n a t d e t u n i n g s s e v e r a l A n g s t r o m s from l i n e c e n t e r . F i g u r e 10 i l l u s t r a t e s t h e e f f e c t i n s o d i u m , s h o w i n g t h a t t h e R a y l e i g h component was n o t t r a p p e d . Applications Use o f S a t u r a t i o n . B e c a u s e o f t h e p o t e n t i a l f o r s i m p l i f i c a t i o n o f t h e p o p u l a t i o n b a l a n c e e q u a t i o n s , much r e c e n t w o r k has c o n c e n t r a t e d on s t u d y i n g s a t u r a t i o n phenomena. First p r o p o s e d by P i e p m e i e r ( 9 ) , and e l a b o r a t e d on by D a i l y ( 1 0 ) , s a t u r a t i o n i n atomic s p e c i e s can l e a d to complete e l i m i n a t i o n o f t h e n e e d t o know any c o l l i s i o n a l r a t e s , and i n m o l e c u l a r s p e c i e s may p r o v i d e s u b s t a n t i a l s i m p l i f i c a t i o n o f t h e b a l a n c e equation a n a l y s i s . The a p p r o a c h t o s a t u r a t i o n i n s o d i u m has b e e n s t u d i e d i n d e t a i l , w i t h s e v e r a l e a r l y w o r k e r s r e p o r t i n g anomalous r e s u l t s . Such r e s u l t s seem t o be e x p l a i n e d by t a k i n g a c c o u n t o f t h e l a s e r beam i n t e n s i t y d i s t r i b u t i o n ( 1 1 , j3, 1 2 ) . I n c o n t r o l l e d measurem e n t s , v a n C a l c a r , e t a l . (13) and B l a c k b u r n (14) h a v e demons t r a t e d s a t u r a t i o n o f s o d i u m i n f l a m e s u n d e r p u l s e d and CW l a s e r operations respectively. S a t u r a t i o n i n m o l e c u l a r s p e c i e s i s more d i f f i c u l t due t o syphoning of p o p u l a t i o n to other l e v e l s . Thus h i g h e r l a s e r powers a r e r e q u i r e d . B a r o n a v s k i and M c D o n a l d (15) h a v e s t u d i e d t h e a p p r o a c h t o s a t u r a t i o n o f C£ and s u g g e s t e d means t o u s e t h e s a t u r a t i o n curve to e x t r a c t c o l l i s i o n a l r a t e information. E c k b r e t h , e t a l . (16) h a v e s t u d i e d s a t u r a t i o n i n CH and CN and v e r i f i e d t h a t under s a t u r a t i o n c o n d i t i o n s reasonable estimates o f m o l e c u l a r number d e n s i t y c a n be o b t a i n e d . C u r r e n t l y i t a p p e a r s t h a t t h e r e a r e no d i f f i c u l t i e s i n s a t u r a t i n g a t o m i c s p e c i e s , w h i l e m o l e c u l a r s p e c i e s may be s a t u r a t e d w i t h s u f f i c i e n t l a s e r power. T h e r e a r e some d i f f i c u l t i e s a s s o c i a t e d w i t h s a t u r a t i o n . Because of c h e m i s t r y , the q u a s i - e q u i l i b r i u m p o p u l a t i o n o f a s p e c i e s may change s u b s t a n t i a l l y when e x c i t e d . See, f o r e x a m p l e , D a i l y and Chan ( 7 ) , and M u l l e r , et a l . (17).



LASER PROBES FOR COMBUSTION

Figure 8.

CHEMISTRY

Fluorescence spectrum resulting from near-resonant excitation

DETUNING, A

Figure 9.

Ratio of Rayleigh to resonance fluorescence signal (Q is the Rabi frequency; at O = 1 A the radiation density is p = 5 X 10~ J/m ), v

17

3



3.

DAILY

Fluorescence

Spectroscopy

PATH LENGTH OF FLAME BETWEEN COLLECTION OPTICS a SCATTERING SCATTERING F O C A L VOLUME, mm

in

Flames

Figure 10. Comparison of Rayleigh and fluorescence trapping in sodium



80

LASER PROBES FOR COMBUSTION CHEMISTRY

E x c i t a t i o n Dynamics. The response of atomic and molecular systems to e x c i t i n g r a d i a t i o n has long been of i n t e r e s t and work has been going on to understand such phenomena f o r over one hundred years (18). Recent work has involved the use of l a s e r s and modern d e t e c t i o n systems to observe and measure i n d i v i d u a l r a d i a t i v e and c o l l i s i o n a l r a t e s . The choice of s u i t a b l e species i s d i c t a t e d to a l a r g e extent by the a v a i l a b i l i t y of r a t e data, and although a great deal of work has been done, l i t t l e has been d i r e c t e d at the problems of a p p l y i n g LIFS to the study of the turbulent combustion e n v i r o n ment. Chan and D a i l y (3) and Chan (19) have studied OH dynamics i n atmospheric flames and found u s e f u l the low pressure data of Lengel and Crosley (20). Lucht and Laurendeau (2) have naalyzed OH n u m e r i c a l l y . Stepowski and Cottereau (21) have used pulsed f l u o r e s c e n c e (22) to measure decay r a t e s i n lower pressure flames and t h e i r cross s e c t i o n data i s of d i r e c t i n t e r e s t to higher pressure combustion a p p l i c a t i o n s . Little other work has appeared although p h y s i c a l chemists are i n c r e a s i n g l y becoming i n t e r e s t e d i n p r o v i d i n g a p p r o p r i a t e data. Concentration Measurements. The use of LIFS to measure atomic s p e c i e s concentrations i n flames has been demonstrated repeatedly i n a n a l y t i c a l a p p l i c a t i o n s and the f i e l d i s w e l l reviewed by Winefordner and E l s e r (23) and Winefordner (24). For atomic species the s a t u r a t i o n approach appears to be most f r u i t f u l , although care must be taken to avoid chemical e f f e c t s . D a i l y and Chan (7) have measured sodium concentrations i n flames u s i n g saturated LIFS with a pulsed l a s e r source and compared the r e s u l t s with a b s o r p t i o n measurements. Smith, et a l . (25) and Blackburn, et a l . (14) have done the same under CW l a s e r excitation. Molecular measurements i n flames have been made of C£ by Baronavski and McDonald (15) and of CH and CN by Eckbreth, et a l . (16). Chan and D a i l y (3) have worked with OH and Chan (19) has done more extensive measurements i n OH. 1

Temperature Measurements. There are a number of techniques f o r measuring temperature u s i n g LIFS which show promise. The f i r s t , c a l l e d two-line f l u o r e s c e n c e by Omenetto, et a l . (26) , i n v o l v e s seeding the flow with an a p p r o p r i a t e atomic s p e c i e s , such as indium or t h a l l i u m , which has two e x c i t e d e l e c t r o n i c s t a t e s , one of which i s c l o s e to the ground s t a t e . The seed i s s e l e c t i v e l y and s e q u e n t i a l l y pumped with a l i g h t source at two wavelengths and the non-resonant f l u o r e s c e n c e i s observed i n each case. The r a t i o of the two f l u o r e s c e n c e s i g n a l s i s r e l a t e d to the temperature. The method was f i r s t demonstrated by Haraguchi, et a l . (27), who measured temperatures i n a v a r i e t y of flames and whose work has been extended by



3.

DAILY

Fluorescence

Spectroscopy

in

Flames

81

Bradshaw, et a l . (28). In t h e i r experiments, a continuum l i g h t source was used although they have s i n c e used pulsed l a s e r sources. We have a l s o performed some p r e l i m i n a r y experiments (29). The r e s u l t of these experiments show the n e c e s s i t y of using l a s e r e x c i t a t i o n sources i f there i s to be adequate s i g n a l noise to perform measurements i n turbulent flows. We are c u r r e n t l y assembling a CW l a s e r system f o r two-line f l u o r e s c e n c e i n our l a b o r a t o r y . The second promising method i s the use of the spectrum of a diatomic or l a r g e r molecule. As discussed i n S e c t i o n II-C, i f one can d e s c r i b e a c c u r a t e l y the population d i s t r i b u t i o n f o r the molecule under e x c i t a t i o n c o n d i t i o n s , then the temperature can be extracted from the measured spectrum. The d i f f i c u l t y l i e s i n capturing the spectrum i n a s u f f i c i e n t l y short time p e r i o d . T h i s can be accomplished through the use of a m u l t i p l e detector array, or O p t i c a l M u l t i c h a n n e l Analyzer such as i s manufactured by P r i n c e t o n Applied Research Co. (30). There i s another approach which can be used i n s u i t a b l e circumstances. Developed by Kowalik and Kruger (31), i t i n v o l v e s measuring the population of an e x c i t e d atomic s t a t e by LIFS. I f the ground s t a t e population i s known to be uniform i n the flow f i e l d , then information about temperature can be i n f e r r e d . They have used the method to measure e l e c t r o n number d e n s i t y i n MHD plasma flows. Summary and Conclusions We have examined the nature of LIFS i n some d e t a i l . The response of an atomic or molecular system i s described i n terms of appropriate r a t e (or balance) equations whose i n d i v i d u a l terms represent the r a t e at which i n d i v i d u a l quantum s t a t e s are populated and depopulated by r a d i a t i v e and c o l l i s i o n a l processes. Given the response of a system to l a s e r e x c i t a t i o n , one may use the r a t e equations to recover information about t o t a l number d e n s i t y , temperature and c o l l i s i o n parameters. The d e t e c t a b i l i t y l i m i t f o r any given measurement i s defined i n terms of measurement u n c e r t a i n t y and f o r LIFS can be q u i t e small. This l i m i t , however, can be a f f e c t e d by i n t e r f e r e n c e s of v a r i o u s kinds and care must be taken i n instrument design to avoid d i f f i c u l t i e s . The dynamic range f o r LIFS i s g e n e r a l l y c o n t r o l l e d by r a d i a t i v e trapping e f f e c t s . The phenomena of s a t u r a t i o n has a l s o been examined and s a t i s f a c t o r i l y described. I t has a l s o been shown that LIFS i s s u i t a b l e f o r studying e x c i t a t i o n and c o l l i s i o n dynamics, and f o r measuring species concentrations and temperatures. LIFS i s now ready to begin being s e r i o u s l y a p p l i e d to turbulent flows. For some s p e c i e s , s u f f i c i e n t information already e x i s t s to o b t a i n q u a n t i t a t i v e r e s u l t s of d i r e c t a p p l i c a b i l i t y , although a major e f f o r t to c o l l e c t and c o l l a t e c o l l i s i o n data must continue. R e l i a b l e equipment i s a v a i l a b l e


LASER PROBES FOR COMBUSTION


82

CHEMISTRY

which can be used to b u i l d measuring systems that i n t e r f a c e to conventional analog or d i g i t a l data processing systems. There are s e v e r a l areas i n which f u t u r e development w i l l concentrate. Laser systems which provide more power and f l e x i b i l i t y than current systems are needed. Higher power frequency doubled CW l a s e r s and high rep r a t e pulsed systems would both be u s e f u l . O p t i c a l multichannel analyzers that are f a s t e r reading and easy to i n t e r f a c e would be e s p e c i a l l y u s e f u l f o r r a p i d s p e c t r a r e c o r d i n g and i n t e r p r e t a t i o n . Methods f o r i n c r e a s i n g instrument dynamic range without s a c r i f i c i n g d e t e c t a b i l i t y l i m i t s w i l l be u s e f u l i n studying r a d i c a l s p e c i e s . I t seems i n e v i t a b l e that LIFS w i l l s t a r t to be used by more and more researchers. Combined with a technique such as coherent Anti-Stokes Raman S c a t t e r i n g (Eckbreth, et a l . , 16), which i s best s u i t e d f o r measuring major species concentrations, a common l a s e r and d e t e c t i o n system provide a wide range of measurement p o s s i b i l i t i e s . Acknowledgment The work reported that was performed by the Authors was supported by A i r Force O f f i c e o f S c i e n t i f i c Research Grant No. 77-3357.

Literature Cited 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Daily, J. W. Applied Optics, 1977a, 16, 2322. Lucht, R. P.; Laurendeau, N. M. Applied Optics, 1979, 18, 856. Chan, C.; Daily, J. W. "Measurement of OH Quenching Cross -Sections in Flames using Laser Induced Fluorescence Spectroscopy," Western States Section/Combustion Institute Paper No. 79-20. Eckbreth, A. C. "Laser Raman Thermometry Experiments in Simulated Combustor Environments"; AIAA Paper No. 76-27, 1976. Daily, J. W. Applied Optics, 1978a, 17, 1610. Daily, J. W.; Chan, C. Combustion and Flame, 1978, 33, 47. Chan, C.; Daily, J. W. J. Quant. Spectrosc. Radiat. Transfer, 1979, 21, 527. Mollow, B. R. Phys. Rev., 1977, A15, 1023. Piepmeir, E. H. Spectroschem. Acta., 1972, 27B, 431. Daily, J. W. Applied Optics, 1977b, 16, 568. Rodrigo, A. B.; Measures, R. M. J. Quant. Elec., 1973, QE-9, 972. Daily, J. W. Applied Optics, 1978b, 17, 225. van Calcar, R. A.; van de Ven, M. J. M.; van Vitert, B. K.; Biewenga, K. J.; Hollander, Tj.; Alkemade, C. Th. J. J. Quant. Spectrosc. Radiat. Transfer, 19 , 21, 11.



3.

DAILY

Fluorescence

Spectroscopy

in Flames

83

14. Blackburn, M. B.; Mermet, J. M.; Boutitier, G. D.; Winefordner, J. D. Applied Optics, 1979, 18, 1804. 15. Baronavski, A. P.; McDonald, J. R. Applied Optics, 1977, 16, 1897. 16. Eckbreth, A. C.; Bonczyk, P. A.; Shirley, J. A. "Investigation of Saturated Laser Fluorescence and CARS Spectroscopic Techniques for Combustion Diagnostics"; EPA Report 600/7-78-104, 1978. 17. Muller, C. Hl; Schofield, K.; Steinberg, M.; Broida, H. P. "Sulphur Chemistry in Flames"; 17th Symposium on Combustion, Leeds, England, 20-25 August, 1978. 18. Kirchoff, G.; Bunson, R. Pogg. Ann., 1860, 110, 161. 19. Chan, C. "Measurement of OH in Flames using Laser Induced Fluorescence Spectroscopy"; Ph.D. thesis, Department of Mechanical Engineering, University of California, Berkeley, 1979. 20. Lengel, R. K.; Crosley, D. R. J. Chem. Phys., 1977, 67, 2085. 21. Stepowski, D.; Cottereau, M. J. Applied Optics, 1979, 18, 354. 22. Daily, J. W. Applied Optics, 1976, 15, 955. 23. Winefordner, J. D.; Elser, R. C. Anal. Chem., 1971, 43, 24A. 24. Winefordner, J. D. Chemtech, 1975, 128, February. 25. Smith, B.; Winefordner, J. D.; Omenetto, N. J. Appl. Phys., 1977, 48, 2676. 26. Omenetto, N.; Benetti, P.; Rossi, G. Spectrochem. Acta, 1972, 27B, 453. 27. Haraguchi, H.; Smith, B.; Weeks, S.; Johnson, D. J.; Winefordner, J. D. Applied Spectroscopy, 1977, 31, 156. 28. Bradshaw, J.; Bower, J.; Weeks, J.; Johnson, D. J.; Winefordner, J. D. "Application of the Two Line Fluorescence Technique to the Temporal Measurement of Small Volume Flame Temperature"; 10th Material Research Symposium on Characterization of High Temperature Vapors and Gases, NBS, Gaithersburg, Maryland, 18-22 September, 1978. 29. Pitz, R. W.; Daily, J. D. "Measurement of Temperature in a Premixed Methane-Air Flame by Two-Line Atomic Fluorescence"; Western States Section/The Combustion Institute Spring Meeting, 1977. 30. Princeton Applied Research. "OMA Vidicon Detectors"; PAR: Princeton, N.J., 1978. 31. Kowalik, R. M.; Kruger, C. H. "Experiments Concerning Inhomogeneity in Combustion MHD Generators"; 18th Symposium on the Engineering Aspects of MHD, Butte, Montana, 1979. RECEIVED

March 14, 1980.


Laser-Induced Fluorescence Spectroscopy in Flames - ACS Publications

Recommend Documents