MICRO-WAVE DETECTION OF LASER ENHANCED IONIZATION OF METALS IN FLAMES

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MICRO-WAVE DETECTION OF LASER ENHANCED IONIZATION OF METALS IN FLAMES

T. Berglind, L. Casparsson

To cite this version:

T. Berglind, L. Casparsson. MICRO-WAVE DETECTION OF LASER ENHANCED IONIZA- TION OF METALS IN FLAMES. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-329-C7-334.

�10.1051/jphyscol:1983730�. �jpa-00223287�

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JOURNAL DE PHYSIQUE

Colloque C7, supplCrnent au n o l 1 7 Tome 44, novernbre 1983 page C7-329

MICRO-WAVE DETECTION OF LASER ENHANCED I O N I Z A T I O N OF METALS I N FLAMES

T . Berglind and L. C a s p a r s s o n

Departmart of Physics, Chalmers University of Technology, Goteborg, Sweden

Ri.sum6

-

^Danset article, l a d s t e c t i o n h l ' a i d e de micro-ondes des signaux

=sation a u g m n t e h l ' a i d e d'un l a s e r est present&. W p r i n c i p a l avan- tage d e l ' u t i l i s a t i o n d e s micro-ondes par rapport 5 une detection en courant continu est que le champ produit par ces ondes p u t &re maintenu h si bas niveau que l a cin6tique de l a f l a m e n ' e s t pas perturb&.

A b s t r a c t

-

I n t h i s paper micro-wave d e t e c t i o n o f l a s e r enhanced i o n i z a t i o n s i g n a l s i s r e p o r t e d . The b a s i c advantage o f u s i n g micro-waves i n s t e a d o f u s i n g DC d e t e c t i o n i s t h a t t h e micro-wave f i e l d can b e k e p t a t such low l e v e l s t h a t t h e k i n e t i c s o f t h e f l a m e i s l e f t u n d i s t u r b e d .

I n t r o d u c t i o n

I n 1976 a g r o u p a t NBS, Washington, D.C., developed a new l a s e r t e c h n i q u e f o r de- t e c t i o n o f s m a l l t r a c e s o f atoms and molecules i n flames. I n s t e a d o f u s i n g t h e f l u o r e s c e n c e l i g h t , t h a t atoms and m o l e c u l e s e m i t when e x c i t e d , t h e i o n i z a t i o n enhancement was d e t e c t e d /l/. A DC f i e l d was a p p l i e d t o t h e f l a m e and t h e l a s e r induced i o n i z a t i o n enhancement was measured as a c u r r e n t i n c r e a s e t h r o u g h t h e f l a m e . T h i s method proved t o b e v e r y p o w e r f u l . D e n s i t i e s o f m e t a l atoms down t o t h e o r d e r o f 106 atoms/cm3 have been d e t e c t e d .

However, when s t u d y i n g t h e f l a m e one has t o t a k e i n t o c o n s i d e r a t i o n t h a t t h e presence o f a DC f i e l d m i g h t change t h e k i n e t i c s , s i n c e t h e charges t h a t a r e c r e a t e d i n t h e f l a m e by thermal processes a r e e x t r a c t e d f r o m t h e flame.

I n t h i s paper we p r e s e n t a d i f f e r e n t p o s s i b i l i t y t o d e t e c t l a s e r enhanced i o n i - z a t i o n ( L E I ) w i t h o u t removing t h e charges from t h e flame. T h i s i s done by d e t e c - t i n q t h e a t t e n u a t i o n o f a weak micro-wave f i e l d t r a v e r s i n g t h e flame.

L a s e r enhanced i o n i z a t i o n

The process o f LE1 can b e d e s c r i b e d i n t h e f o l l o w i n g way:

1. Laser e x c i t a t i o n o f t h e t r a c e atom o r m o l e c u l e X X t h v -p Xexc

g r

2. Subsequent c o l l i s i o n a l i o n i z a t i o n : Xexc+M + X + + ~ - + M

where M i s a flame molecule. h,,

Since t h e r e e x i s t s of t h e o r d e r e

~

more ~ 0 1 1 i s i o n p a r t n e r s w i t h s u f f i c i e n t energy t o i o n i z e t h e e x c i t e d atoms o r molecule, Xexc, than c o l l i s i o n p a r t n e r s w i t h s u f f i c i e n t energy t o i o n i z e t h e atoms o r m o l e c u l e s i n t h e ground s t a t e , Xgr,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983730

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C7-330 JOURNAI. DE PHYSIQUE

t h e l a s e r e x c i t a t i o n leads t o a s h a r p i n c r e a s e o f charges i n t h e f l a m e .

T h i s i n c r e a s e o f charges i s d e t e c t e d i n o r d i n a r y L E 1 spectroscopy by a p p l y i n g a DC f i e l d across t h e r e g i o n of i n t e r a c t i o n . The l a s e r e x c i t a t i o n i s f o l l o w e d by a c u r r e n t i n c r e a s e t h r o u g h t h e flame, which can be e a s i l y measured as a v o l t a g e i n c r e a s e across a r e s i s t o r . As mentioned e a r l i e r t h i s DC f i e l d removes t h e charges from t h e flame and m i g h t i n t h i s way change t h e k i n e t i c s o f combustion. There a r e however a l t e r n a t i v e s t o a DC f i e l d f o r d e t e c t i o n of t h e l a s e r induced charge i n - crease.

Mi cro-wave a b s o r p t i o n

When a micro-wave f i e l d i s a p p l i e d t o t h e flame, t h e charges i n t h e f l a m e a r e a c c e l e r a t e d . The charges w i l I t h e n r a d i a t e as d i p o l e s . I f a c o l l i s i o n d i s t u r b s t h e o s c i l l a t i o n , r a d i a t i o n energy i s c o n v e r t e d i n t o h e a t . I n t h i s way a f r a c t i o n o f t h e micro-wave f i e l d i s absorbed.

B e l c h e r and Sugden / 2 / showed t h a t t h e a t t e n u a t i o n can be w r i t t e n : C.2ne n 2 V

B= ---

2

3

mc, w + v

where 6 i s the a t t e n u a t i o n i n dB/cm, e i s t h e e l e c t r o n charge, n t h e number o f charges p e r u n i t volume, m t h e mass o f t h e charged i o n o r e l e c t r o n , V i s t h e c o l l i s i o n frequency f o r t h e charge a n d w i s t h e frequency o f t h e micro-wave f i e l d . From t h i s e q u a t i o n i t can be seen t h a t e l e c t r o n s w i l l c o n t r i b u t e -100 t i m e s more t o t h e a t t e n u a t i o n than i o n s s i n c e B-Jfi. I

I t i s a l s o i n t e r e s t i n g t o n o t e t h a t t h e a t t e n u a t i o n decreases when W i s r a i s e d above v. T h i s imposes a upper l i m i t on t h e micro-wave f r e q u e n c y .

The l o w e r l i m i t f o r t h e micro-wave f r e q u e n c y i s g i v e n by t h e f a c t t h a t t h e s p a t i a l r e s o l u t i o n i s o f t h e same o r d e r as t h e wavelength of t h e micro-wave f i e l d .

f l a m e

m icro-wave micro-wave

reciever transducer

\

sc:ut i o n l‘'\+am l a s e r \.

F i g . 1

-

The e x p e r i m e n t a l set-up.

.'.

signal f 1

s c o p e

- m 1

^t^{r i g}

i

p u l s e d d y e l a s e r

W

.

e

b o x c a r C recorder

-

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Experimental arrangement

The b u r n e r used i n t h e s e experiments was an a c e t y l e n e / a i r b u r n e r f r o m a n o r d i n a r y a t o m i c a b s o r p t i o n s p e c t r o m e t e r . The l e n g t h o f t h e b u r n e r was 10 cm and t h e w i d t h o f t h e i n t e r a c t i o n r e g i o n -1 cm. A w a t e r - s o l u t i o n c o n t a i n i n g sodium was a s p i r e d i n t o t h e b u r n e r a t a f l o w r a t e o f -5 m l / m i n u t e .

A pulsed dye l a s e r (Chromatix CMX-4) was used as l i g h t source. The p u l s e energy was -1 mJ, t h e p u l s e d u r a t i o n -1 us and t h e r e p e t i t i o n f r e q u e n c y was 25 Hz. The beam d i a m e t e r was -3 mm. The beam was d i r e c t e d i n t o t h e f l a m e -2 cm above t h e b u r n e r head.

The micro-wave t r a n s d u c e r was o f Gunn-diode t y p e ( P h i l i p s PM 7015 X) and gave

-

10 mW microwave r a d i a t i o n a t 10 GHz. A d i o d e (IN23B) was used as r e c e i v e r . The e x p e r i m e n t a l c o n f i g u r a t i o n i s shown i n f i g u r e 1 above.

R e s u l t s and d i s c u s s i o n

The micro-wave a b s o r p t i o n s i g n a l as a f u n c t i o n o f t i m e i s shown i n f i g u r e 2 .

Signal arb units

0 1 time

F i g . 2

-

Micro-wave a b s o r p t i o n s i g n a l as a f u n c t i o n o f t i m e .

I n t h i s experiment 10 ppm Na was a s p i r e d i n t o t h e b u r n e r and t h e l a s e r p u l s e energy was -2 mJ. T h i s gave r i s e t o -1010 electrons/cm3. The r i s e t i m e corresponds t o t h e

r i s e t i m e o f t h e l a s e r . The f a l l t i m e i s much l o n g e r , p r o b a b l y due t o t h e l o n g r e c o m b i n a t i o n t i m e f o r t h e e l e c t r o n s .

The c r e a t e d e l e c t r o n s gave r i s e t o an a b s o r p t i o n o f -0.1 % o f t h e micro-wave f i e l d , i n t h i s case -2.5 ^p14o u t o f 2.5 mW. T h i s measurement g i v e s us t h e p o s s i b i l i t y t o e s t i m a t e t h e i n f l u e n c e on t h e k i n e t i c s due t o t h e micro-wave f i e l d .

The 10'' e l e c t r o n s absorb 2.5 pW o f r a d i a t i o n . Assuming t h a t t h e k i n e t i c energy t h e e l e c t r o n absorbs f r o m t h e micro-wave i e l d i s l o s t when i t c o l l i d e s w i t h a m o l e c u l e i n t h e flame, which happens .1Olf t i m e p e r se nd t k i n e t i c energy b u i l d - u p between c o l l i s i o n s i s -2.5 . 1 0 - ~

.

10-'0

.

10-FP ziO-99 J i e l e c t r o n o r

- 1 0 - ~ e V / e l e c t r o n which i s n e g l i g i b l e compared t o t h e thermal energy.

It i s a l s o p o s s i b l e t o g i v e an e s t i m a t e o f t h e o v e r a l l h e a t i n g o f t h e f ame due t o

3

t h e micro-wave f i e l d . Assume t h a t t h e f l a m e c o n t a i n s -10 e l e c t r o n s / c m

.

The f l a m e i s exposed t o t h e r a d i a t i o n -0.01 seconds. D u r i n g t h i s t i m e 0.025 UJ o f r a d i a t i o n

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C7-332 JOURNAL DE PHYSIQUE

i s c nverted i n t o h e a t . This i s spread out over ,1018 molecules. This gives .10-8

.

10-l8 J/molecule o r 1 0 - ~ el/molecule, which i s a l s o n e g l i g i b l e compared t o t h e thermal energy.

From t h e c o n s i d e r a t i o n s above we can conclude t h a t we operated a t s u f f i c i e n t l y low power of micro-wave r a d i a t i o n t h a t t h e k i n e t i c s of t h e flame were unchanged.

Figure 3 shows t h e s i g n a l f o r 10 p p Na when t h e l a s e r wavelength was scanned from -588-590 n m .

Fig. 3

-

Signal vs wavelength f o r 10 ppm Na

Figure 4 shows t h e signal f o r 0.1 ppm Na when t h e l a s e r was scanned around t h e 3sIl2

-

3 ~ 3 1 2 t r a n s i t i o n . 0.1 ppn seems t o be t h e d e t e c t i o n l i m i t f o r t h e system used.

This d e t e c t i o n l i m i t i s -1000 times higher than t h e d e t e c t i o n l i m i t f o r ordinary L E I .

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I

wavelength

F i g . 4 - S i g n a l vs wavelength f o r 0.1 ppm Na.

The main c o n t r i b u t o r t o t h e n o i s e i n t h e system was v a r i a t i o n s o f t h e micro-wave t r a n s d u c e r o u t p u t . T h i s can be reduced by a more c a r e f u l c h o i c e o f t r a n s d u c e r and by u s i n g an i n t e r f e r e n c e t e c h n i q u e where t h e micro-wave f i e l d i s s p l i t i n two p a r t s , one p a s s i n g o u t s i d e t h e i n t e r a c t i o n r e g i o n and one p a s s i n g through t h e i n t e r a c t i o n r e g i o n . F i g u r e 5 shows t h e s i g n a l as a f u n c t i o n o f Na c o n c e n t r a t i o n .

F i g . 5 - S i g n a l vs Na c o n c e n t r a t i o n .

Trace A: The waveguides p l a c e d where t h e l a s e r beams e n t e r s t h e f l a m e . Trace B: The waveguide p l a c e d where t h e l a s e r beam e x i t s t h e f l a m e .

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C7-334 JOURNAIL DE PHYSIQUE

F i g u r e 5 shows t h a t t h e upper l i m i t o f d e t e c t i o n i s h i g h e r f o r micro-wave d e t e c t i o n t h a n f o r o r d i n a r y LE1 where space charge e f f e c t s tend t o d e s t r o y t h e l i n e a r i t y a t l o w e r c o n c e n t r a t i o n s . For micro-wave d e t e c t i o n l i g h t a b s o r p t i o n seems t o g i v e t h e upper l i m i t o f d e t e c t i o n .

Conclusions

Micro-wave d e t e c t i o n o f LE1 s i g n a l s i s f e a s i b l e , though i t g i v e s -1000 times worse d e t e c t i o n l i m i t s than o r d i n a r y LEI. The advantage o f micro-wave d e t e c t i o n , when used f o r flame s t u d i e s , i s t h a t t h e micro-wave f i e l d does n o t change t h e k i n e t i c s o f combustion. Another advantage seems t o be t h a t t h e s i g n a l i s u n a f f e c t e d by space charge e f f e c t s .

References

1. Green e t al., J. o f t h e Am. Chem. Soc., v o l . 98, no 26, 1976, p 8517-8518.

2. H. B e l c h e r and T-M. Sugden, Proc. of Royal Soc. of London Sec. A, Vol. 201, 1950.