Continuous-Wave Intracavity Dye Laser Spectroscopy: Dependence of

39 Continuous-Wave Intracavity Dye Laser Spectroscopy: Dependence of Enhancement on Pumping Power

Downloaded by CORNELL UNIV on September 21, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch039

STEPHEN J. HARRIS Physical Chemistry Department, General Motors Research Laboratories, Warren, MI 48090

I n t r a c a v i t y dye l a s e r spectroscopy (IDLS) can be a powerful technique f o r d e t e c t i n g t r a c e s p e c i e s important i n combustion. The technique i s based on the phenomenal s e n s i t i v i t y o f a l a s e r to small o p t i c a l l o s s e s w i t h i n the l a s e r c a v i t y . Since molecu l a r a b s o r p t i o n s represent wavelength-dependent o p t i c a l l o s s e s , the technique allows d e t e c t i o n o f minute q u a n t i t i e s o f f r e e r a d i c a l s by p l a c i n g them i n s i d e the l a s e r c a v i t y and monitoring t h e i r e f f e c t on the s p e c t r a l output o f the l a s e r . IDLS was d i s c o v e r e d n e a r l y a decade ago (1,2), and, a l though there have been many demonstrations o f the technique (3-6) and s e v e r a l t h e o r i e s proposed to e x p l a i n i t (4.7-10) there are few r e p o r t s ( 1 1 J 2 ) o f the technique a c t u a l l y being used to gain new chemical i n f o r m a t i o n . Hardly any work has been reported (13,14) i n q u a n t i f y i n g the experimental parameters a f f e c t i n g IDLS. The present work r e p r e s e n t s the f i r s t quan t i t a t i v e comparisons between theory and experiment f o r cw IDLS. ..The experimental arrangement i s b a s i c a l l y s i m i l a r to t h a t of Hansch e t a l . ( 4 ) . A Spectra Physics A r l a s e r o p e r a t i n g a t 514.5 nm pumps a Rhodamine 6G dye l a s e r tuned with a biréfrin gent f i l t e r . The l i n e w i d t h i s 25 t o 30 GHz, and the wavelength i s tuned between 585.0 nm and 585.2 nm. The output m i r r o r has a 1 meter r a d i u s o f c u r v a t u r e and a r e f l e c t i v i t y o f 98% at 585.0 nm. The dye l a s e r c a v i t y i s 74 cm l o n g , and the l a s e r i s always run TEMoo ( t h i s sometimes n e c e s s i t a t e s the use o f an i n t r a c a v i t y aperture). Ip i s degassed and then d i s t i l l e d i n t o a p r e v i o u s l y evac uated 23 cm long quartz c e l l with wedged (1.5°) a n t i - r e f l e c t i o n coated windows epoxied on the ends. The c e l l i s mounted i n the l a s e r c a v i t y on X-Y-Z t r a n s l a t i o n s t a g e s , and the I2 i s f r o z e n i n t o the sidearm by d i p p i n g i t i n a c o l d bath. The dye l a s i n g t h r e s h o l d i s measured, and the A r l a s e r i s then s e t to the d e s i r e d power. For one s e t o f experiments, t h r e s h o l d pump power i s near (±11%) 550 mW, w h i l e f o r a second s e t the t h r e s h o l d i s near 790 mW. +

+

0-8412-0570-l/80/47-134-451$05.00/0 ©

1980 A m e r i c a n Chemical

Society

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

LASER PROBES FOR COMBUSTION


452

CHEMISTRY

The dye l a s e r i r r a d i a t e s an e x t e r n a l c e l l which contains 40 Pa (0.3 t o r r ) Ip vapor. A 1P28 p h o t o m u l t i p l i e r whose f a c e i s covered by a 610 nm long pass f i l t e r measures the 12 f l u o r e s cence, while a photodiode monitors a r e f l e c t e d spot o f the dye l a s e r . The r a t i o o f f l u o r e s c e n c e t o dye l a s e r power i s d i s played on a s t r i p c h a r t r e c o r d e r . The sidearm temperature i s g r a d u a l l y (1-2 hours) r a i s e d from about 210 Κ o r 220 K, where f l u o r e s c e n c e i n the e x t e r n a l c e l l i s s t r o n g , t o whatever tem perature i s r e q u i r e d t o reduce the s i g n a l by about 60%. The e x t r a c a v i t y a b s o r p t i o n c o e f f i c i e n t o f I2 a t the l a s e r wavelength i s measured by d e t e c t i n g l a s e r power with a thermo p i l e before and a f t e r an I2 c e l l . I n t r a c a v i t y enhancement, r e l a t i v e t o conventional s i n g l e pass a b s o r p t i o n spectroscopy, i s due t o mode competition and t o t h r e s h o l d e f f e c t s . A simple c a l c u l a t i o n o f the l a t t e r f o r a s i n g l e mode l a s e r , s t a r t i n g with α

gives , 0 ) _ d In I _ % / L

m

where I i s the dye l a s e r i n t e n s i t y , I i s the s a t u r a t i o n i n t e n s i t y , a and L a r e the unsaturated s i n g l e pass g a i n and l o s s , and ξ ' i s the enhancement o f a s i n g l e mode l a s e r with an absorber dL i n s i d e the c a v i t y . Equation (1) says, b a s i c a l l y , t h a t the dye l a s e r output becomes very s e n s i t i v e t o a d d i t i o n a l i n t r a c a v i t y l o s s when the l a s e r i s run near t h r e s h o l d (gain « l o s s ) . The e f f e c t o f mode competition, which i s g e n e r a l l y the dominant e f f e c t , i s more s u b t l e s i n c e , t o a f i r s t approximation, a cw dye l a s e r has o n l y one mode. Theories o f IDLS which account f o r mode competition have been put forward by Hansch, Schlawlow, and Toschek (HST) and by Brunner and Paul (BP). HST s t a r t with a r e a l i s t i c s e t o f l a s e r r a t e equations but use s u b s t a n t i a l approximations t o s o l v e them. BP use an approximate and very e m p i r i c a l s e t o f r a t e equations which they s o l v e a n a l y t i c a l l y . Each theory y i e l d s a p r e d i c t i o n f o r the dependence o f enhance ment on pumping power Ρ r e l a t i v e t o the t h r e s h o l d pumping power 0

Q

E x p e r i m e n t a l l y , f l u o r e s c e n c e i s measured as a f u n c t i o n o f 12 pressure. Since IDLS i s an a b s o r p t i o n technique, and s i n c e f l u o r e s c e n c e i s p r o p o r t i o n a l t o the l i g h t " t r a n s m i t t e d " by the i n t r a c a v i t y c e l l a t Ip wavelengths, i t makes sense t o p l o t the logarithm o f the f l u o r e s c e n c e a g a i n s t pressure. We f i n d a l i n e a r r e l a t i o n s h i p , and t h e s l o p e i s then the i n t r a c a v i t y a b s o r p t i o n c o e f f i c i e n t , ε · . Enhancement i s d e f i n e d e x p e r i mentally as ξ = ε · ^ ε χ ΐ » where e t i s the conventional s i n g l e pass a b s o r p t i o n c o e f f i c i e n t . The r e s u l t s a r e compared with p r e d i c t i o n s o f HST and BP i n Figures 1 and 2, r e s p e c t i v e l y . In Ί

η

η

θ

η ΐ

e x


39.

Continuous-Ware

HARRIS

Intracavity Dye

Laser

Spectroscopy

453

12,000 r 10,000 c φ Ε ω υ

8,000 6,000


CO

-C

c

LU

4,000

•ο

2,000 1.0

2.0

p/pTh

3.0

4.0

Ρ—Ρ T h Figure 1. A comparison of the theory of,HST ( ) with the data. It is assumed that there are 50 longitudinal modes. A threshold of 790 mW; (O), a thresh old of 550 mW (15).

12,000 10,000j 8000

Φ

6000

ι5 4000

2000| 0

Figure 2.

1

4 p/pTh

A comparison of the data with the theory of BP ( 550 mW. M = 50; ν = .050 ± .01 (15).

). Threshold is


454

LASER PROBES FOR COMBUSTION CHEMISTRY

judging these comparisons, i t must be kept i n mind that the theory of HST has no f r e e parameters, while that of BP has a f r e e e m p i r i c a l parameter whose p h y s i c a l s i g n i f i c a n c e i s at best unclear. However, i t i s c l e a r that HST s p r e d i c t i o n of a l a r g e enhancement a t high power ( l i m (P-*») ξ ~ 10 ) i s not c o n s i s t e n t with these data. Much more work, both t h e o r e t i c a l and experimental, needs to be done f o r IDLS to become a well c h a r a c t e r i z e d technique. Numerical s o l u t i o n s to r e a l i s t i c l a s e r r a t e equations, f o r example, as well as measurement of enhancement as a f u n c t i o n of various l a b o r a t o r y parameters w i l l i n c r e a s e IDLS' usefulness as an a n a l y t i c a l technique. 1


3

Abstract Intracavity absorption by I vapor has been studied for a cw dye laser. The sensitivity enhancement varies from 10 at pump powers near threshold (550 mW and 790 mW) to about 500 at the highest pump powers (near 5 watts). The results can be inter preted quantitatively in terms of a previously proposed theory. 2

4

Literature Cited 1. Pakhomycheva, L. Α.; Sviridenkov, Ε. Α.; Suchkov, A. F.; Titova, L. V.; Churilov, S. S., JETP Letters, 1970, 12, 43. 2. Peterson, N. C.; Kurylo, M. J.; Braun, W.; Bass, A. M.; Keller, R. Α., J. Opt. Soc. Am., 1971, 61, 746. 3. Thrash, R. J.; Weyssenhoff, H.; Shirk, J. S., J. Chem. Phys., 1971, 55, 4659. 4. Hansch, T. W.; Schlawlow, A. L.; Toschek, P. E., IEEE J. Quantum Electron, 1972, QE-6, 802. 5. Schroder, H.; Neusser, H. J.; Schlag, E. W., Opt. Commun., 1975, 14, 395. 6. Atkinson, G. H.; Lavfer, A. H.; Kurylo, M. J., J. Chem. Phys., 1973, 59, 350. 7. Keller, R. Α.; Zalewski, E. F.; Peterson, N. C., J. Opt. Soc. Am., 1972, 62, 319. 8. Brunner, W.; Paul, H., Opt. Commun., 1974, 12, 252. 9. Holt, H. K., Phys. Rev., 1976, A 14, 1901.


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Dye Laser Spectroscopy

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10. Tohma, K., J. Appl. Phys., 1976, 47, 1422. 11. Bray, R. G.; Henke, W.; Liv, S. K.; Reddy, Κ. V.; Berry, M. J., Chem. Phys. Letters, 1977, 47, 213. 12. Reilly, J. P.; Clark, J. H.; Moore, C. B.; Pimentel, G. C., J. Chem. Phys., 1978, 69, 4381.


13. Keller, R. Α.; Simmons, J. D.; Jennings, D. Α., J. Opt. Soc. Am., 1973, 63, 1552. 14. Childs, W. J.; Fred, M. S.; Goodman, L. S., Appl. Opt., 1974, 13, 2297. 15. Harris, Stephen J., J. Chem. Phys., 1979,71, 4001. Received February 1, 1980.


Continuous-Wave Intracavity Dye Laser Spectroscopy: Dependence of

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