HAL Id: jpa-00223282
https://hal.archives-ouvertes.fr/jpa-00223282
Submitted on 1 Jan 1983
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
RESONANT MULTIPHOTON OPTOGALVANIC SPECTROSCOPY OF RADICALS IN FLAMES
J. Goldsmith
To cite this version:
J. Goldsmith. RESONANT MULTIPHOTON OPTOGALVANIC SPECTROSCOPY OF RAD- ICALS IN FLAMES. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-277-C7-285.
�10.1051/jphyscol:1983725�. �jpa-00223282�
JOURNAL DE PHYSIQUE
Colloque C7, supplement au n o l l , Tome 44, novembre 1983 page C7-277
RESONANT MULTIPHOTON OPTOGALVANIC SPECTROSCOPY OF RADICALS I N FLAMES
J.E.M. G o l d s m i t h
Sandia National Laboratories, Livermore, CA 94550, U.S.A.
Resume
-
Nous decrivons l ' u t i l i s a t i o n de l a spectroscopie optogalvanique reson- nantea
p l u s i e u r s photons pour l ' o b s e r v a t i o n de radicaux dans un m i l i e u en combustion. L ' a c c e n t e s t mis s u r l a d e t e c t i o n d'hydrogene e t d ' o x y - gene atomiques dans l e s flammes.A b s t r a c t
-
We.describe t h e use of resonant m u l t i p h o t o n optogalvanic spectroscopy f o r observing r a d i c a l s i n combustion environments, emphasizing t h e d e t e c t i o n o f atomic hydrogen and oxygen i n flames.INTRODUCTION 1.
O p t i c a l techniques have been used f o r i n - s i t u d e t e c t i o n o f most o f t h e important species i n combustion research (1). These methods, w i t h r e c e n t emphasis on laser-based techniques, have t h e important p r o p e r t y o f being non-perturbing under many conditions, as w e l l as p r o v i d i n g h i g h s e n s i t i v i t y w i t h e x c e l l e n t s p a t i a l and temporal r e s o l u t i o n . However, the hydrogen atom ( r a d i c a l ) , which p l a y s a c r i t i c a l r o l e i n almost a l l combustion processes, has been conspicuously absent from t h e l i s t o f observed species, and t h e oxygen atom has been added t o t h e l i s t o n l y v e r y r e c e n t l y (2-4). This paper describes t h e d e t e c t i o n o f atomic hydrogen (5) and oxygen ( 6 ) i n atmospheric-pressure flames using resonant m u l t i p h o t o n optogalvanic spectroscopy. D e t e c t i o n o f NO and (we b e l i e v e ) OH i s a l s o b r i e f l y discussed.
Resonant e x c i t a t i o n o r d e t e c t i o n o f hydrogen o r oxygen atoms r e q u i r e s r a d i a t i o n i n t h e vacuum-ultraviolet r e g i o n o f t h e spectrum, and thus i s u s u a l l y incompatible w i t h combustion environments. The resonant m u l t i p h o t o n optogalvanic d e t e c t i o n scheme described i n t h i s paper avoids these d i f f i c u l t i e s by e x c i t i n g t h e atoms w i t h resonant two-photon absorption using u l t r a v i o l e t r a d i a t i o n . The e x c i t e d atoms are subsequently i o n i z e d by absorbing one more photon, and detected e l e c t r i - c a l l y w i t h biased probes mounted i n o r near t h e flame. Throughout t h i s paper, the term optogalvanic e f f e c t w i l l be used i n a very general sense, r e f e r r i n g t o changes i n t h e e l e c t r i c a l p r o p e r t i e s o f a flame because o f photoioniz.ation, as w e l l as c o l l i s i o n a l i o n i z a t i o n from o p t i c a l l y populated states.
EXPERIMENTAL APPARATUS 11-
The two-photon e x c i t a t i o n was accomplished w i t h a s i n g l e l a s e r beam a t 226 nm f o r atomic oxygen, and w i t h two l a s e r beams a t 266 nm and 224 nm f o r atomic hydrogen ( t h e e x c i t a t i o n schemes are discussed i n more d e t a i l i n f o l l o w i n g sections).
Tunable beams a t 224 nm (226 nm) were produced by pumping a Rhodamine 66 dye l a s e r a t 567 nm (573 nm) w i t h t h e 532-nrn o u t p u t o f a frequency-doubled Nd:YAG laser, frequency-doubling t h e dye l a s e r , and f i n a l l y frequency-mixing t h e doubled dye beam w i t h the remaining Nd:YAG 1.06-pm i n f r a r e d output. For hydrogen e x c i t a t i o n , an a d d i t i o n a l l a s e r beam a t 266 nm was simultaneously produced by frequency-doubling t h e 532-nm beam from t h e same Nd:YAG laser, w i t h t h e remaining 532-nm beam used t o pump t h e dye l a s e r as described above.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983725
C7-278 JOURNAL DE PHYSIQUE
The apparatus used t o implement t h i s scheme f o r hydrogen d e t e c t i o n i s shown i n Fig. 1. (The change t o oxygen d e t e c t i o n i s accomplished simply by t u n i n g t h e dye l a s e r t o 573 nm t o produce a UV beam a t 226 nm, w i t h o u t having t o change dyes o r doubling and m i x i n g c r y s t a l s ; t h e 266-nm beam i s n o t used.) The Molectron MY34-20 Nd:YAG l a s e r was operated w i t h an i n t r a c a v i t y etalon, produc'ng a 266-nm beam w i t h
1
up t o 50 mJ/pulse and a bandwidth o f about 0.001 nm (0.2 cm- ). The 532-nm o u t p u t o f t h e Nd:YAG l a s e r pumped a Quanta-Ray POL-1 dye l a s e r u s i n g Rhodamine 6G dye. The dye l a s e r could b scanned over l a r g e wavelength ranges w i t h a nominal bandwidth of
f
0.01 nm (0.25 cm- ) by removing t h e i n t r a c a v i t y e t a l o n and mechanically scanning t h e g r a t i n g angle. Narrowband o p e r a t i o n o f t h e dye l a s e r w i t h i n t r a c a v i t y e t a l o n (nominal bandwidth o f 0.002 nm, o r 0.05 cm-l) r e q u i r e d pressure scanning t h e dye l a s e r o s c i l l a t o r , l i m i t i n g t h e scan range t o 0.3 nm w i t h n i t r o g e n as t h e scan gas.
The dye l a s e r o u t p u t was frequency-doubled and mixed w i t h t h e 1.06-km o u t p u t of the Nd:YAG l a s e r by a Quanta-Ray WEX-1 wavelength extender. The r e s u l t i n g 224-nm beam was separated from t h e t h r e e c o l i n e a r longer-wavelength beams by a s e r i e s o f four P e l l i n - B r o c a prisms arranged t o e l i m i n a t e displacement o f t h e beam as t h e dye l a s e r was scan ed, t y p i c a l l y y i e l d i n g -0.5-mJ 224-nm pulses w i t h approximately 0.003-nm (0.5-cm-
P
) bandwidth when t h e dyf l a s e r was operated w i t h o u t i n t r a c a v i t y etalon, and approximately 0.0005-nm (0. l-cm- ) bandwidth w i t h i n t r a c a v i t y etalon. The one o r two UV beams were focused i n t o t h e flame, t y p i c a l l y w i t h a 35-cm f o c a l l e n g t h lens.The d e l a y l i n e i n t h e 266-nm beam p a t h was adjusted so the two UV pulses a r r i v e d a t the flame simultaneously.
Nd. YAG SYSTEM
ENERGY METER
F i g .
-
Apparatus used f o r resonant-
m u l t i p h o t o n optogalvanic d e t e c t i o n o f atomic hydrogen and oxygen i n flames.Unless s t a t e d otherwise, t h e measurements described i n t h e f o l l o w i n g s e c t i o n s were made w i t h premixed s t o i c h i o m e t r i c flames generated by a water-cooled porous- p l u g f l a t - f l a m e burner. Argon was o f t e n used as a d i l u e n t t o avoid formation of NO.
The laser-produced i o n i z a t i o n s i g n a l was c o l l e c t e d by s t a i n l e s s - s t e e l e l e c t r o d e s i n s e r t e d i n t o t h e flame, t y p i c a l l y biased a t 500-1000 V r e l a t i v e t o each other, and the r e s u l t i n g s i g n a l processed by f a s t a m p l i f i e r s b e f o r e being d i g i t i z e d and stored i n a computer.
ATOMIC HYDROGEN
111- --p
Hydrogen concentrations i n flames have p r e v i o u s l y been determined by two general schemes ( 7 ) . Gas samples obtained from flames by probe sampling techniques have been analyzed f o r hydrogen content, b u t these methods s u f f e r f r o m p e r t u r b a t i o n s
i n t h e flame caused by i n s e r t i o n o f t h e probe. (The optogalvanic d e t e c t i o n scheme described here does r e q u i r e e l e c t r i c a l probes i n or near t h e flame, b u t t h e y can be located w e l l away from t h e sampling volume defined by t h e l a s e r beams.) Hydrogen c o n c e n t r a t i o n can be i n f e r r e d from measurements o f other species i n t h e flame, b u t t h i s r e q u i r e s d e t a i l e d modeling o f t h e flame chemistry. These l i m i t a t i o n s and t h e importance o f t h e hydrogen r a d i c a l i n combustion g i v e v e r y s t r o n g m o t i v a t i o n f o r t h e
development of a direct optical technique for measuring hydrogen concentration in flames.
Optical detection of atomic hydrogen in any environment is made difficult by the large 10-eV gap between the IS ground state and the n=2 excited state (near- degenerate 2S1/2,
2Pand 2P3/2 levels), as shown in Fig. 2. Raman scattering techniques are ~mpra%;al because of the lack of suitable structure in the ground electronic state. Atomic resonance absorption spectroscopy is suitable for low pressure environments, and has been used in shock tube studies
(8),but has many undesirable features for use in combustion environments, not the least of which is the required use of radiation at 122 nm in the vacuum ultraviolet. Previous to the work described in this paper, multiphoton excitation followed by fluorescence (9,lO) or optogalvanic (10-13) detection of atomic hydrogen had been demonstrated only in flow cells or discharges. Very recently, two-photon-excited fluorescence detection of atomic hydrogen has also been demonstrated in flames (14).
*IONIZATION LIMIT (13.6 eV) ...
A, or A,
.
V
Fig. 2 - Two-photon resonant multiphoton ionization
X*
-
224 nmexcitation scheme used for optogalvanic detection of
(5.64 e ~ )
atomic hydrogen in flames. The dashed line at 5.5-eV
energy represents a virtual, not a real, intermediate
0 - n - l
level.
The multiphoton optogalvanic detection scheme used for atomic hydrogen is shown in Fig. 2 (11). Excitation from the n=l ground state to the n=2 excited state requires simultaneous absorption of one photon from each of the 224-nm and 266-nm laser beams. One photon from either beam then has sufficient energy to photoionize the n=2 state. This method provides greater spatial resolution than single- wavelength excitation using one beam at 243 nm, since it can be used in a crossed- beam configuration, and the relatively higher power beam produced at 266 nm provides greater two-photon excitation and ionization rates. Furthermore, since the two- photon excitation rate is proportional to the product of the two beam intensities, while the photoionization rate is roughly proportional to their sum, it is possible to control the excitation and ionization rates independently. The beam intensities can thus be adjusted to limit depletion of the hydrogen ground state to avoid laser-induced chemistry effects, while still permitting nearly complete photo- ionization of the excited state to minimize quenching corrections.
The quality of the spectra obtainable with this method is illustrated in Fig. 3. This spectrum, plotted as a function of the dye laser wavelength (upconverted to a scan from 223.6-224.0 nm), was recorded at the base of a
hydrogen/oxygen flame with argon shroud flow, averaging 20 laser shots per displayed data point without any form of normalization. The dashed curve visible in a few places is a least-squares fit of the experimental spectrum to a Lorentzian profile.
The fit indicates that homogeneous (Lorentzian) contributions to the linewidth
'dominated inhomogeneous (Gaussian) contributions (primarily laser l inewidth and the
indicated Doppler width calculated at the flame temperature). The homogeneous
contribution is from a decrease in the lifetime of the 2s state because of rapid
photoionization, as well as from pressure broadening.
JOURNAIL DE PHYSIQUE
-
3
&
DOPPLER2
t m
5 -
2
P0 )
z 0
5
W F i 3-
M u l t i p h o t o n optogalvanic spectrumz -
Q 68612 60b42 66b72 68i02 5&32 o f atomic hydrogen recorded i n a premixed
DYE LASER WAVELENGTH (nm) hydrogen/oxygen flame.
F i g u r e 4 shows two wavelength scans recorded under i d e n t i c a l c o n d i t i o n s several mm above t h e base o f a hydrogen/oxygen/argon flame w i t h argon shroud flow, w i t h and w i t h o u t t h e 266-nm beam present. The dashed curve represents a n o n l i n e a r l e a s t - squares f i t o f a V o i g t p r o f i l e t o t h e atomic hydrogen s i g n a l : t h e r e s u l t i n g
parameter A=0.83*(Avc/AvD)=0.9 i n d i c a t e s r o u g h l y equal c o n t r i b u t i o n s o f Gaussian and L o r e n t z i a n c h a r a c t e r ~ s t i c s to t h i s lineshape. We b e l i e v e t h a t t h e o t h e r s t r u c t u r e present i n both scans i s from two-photon resonant e x c i t a t i o n o f t h e h y d r o x y l r a d i c a l OH i n t h e C+X band, a l s o f o l l o w e d by single-photon i o n i z a t i o n .
5 1
H Doppler widthThe i d e n t i f i c a t i o n o f t h i s s t r u c t u r e as b e i n g from OH i s supported by t h e s e r i e s o f scans p l o t t e d i n Fig. 5 a t t h e i n d i c a t e d h e i g h t s above t h e base o f a hydrogen/oxygen flame w i t h argon shroud flow, u s i n g a v e r y s l i g h t l y curved burner t o make i t p o s s i b l e f o r t h e l a s e r beams t o c r o s s extremely c l o s e t o t h e burner surface (0.0 m). The v e r t i c a l scales o f t h e p l o t s were expanded w i t h t h e i n d i c a t e d gains so t h e amplitude o f t h e hydrogen s i g n a l s appear i d e n t i c a l ( t h e hydrogen concentra- t i o n f a l l s o f f r a p i d l y above t h e burner surface). The amplitude o f t h e o t h e r s t r u c t u r e r o u g h l y t r a c k s t h e amplitude o f t h e hydrogen s i g n a l , suggesting t h a t i t i s from another r a d i c a l d i r e c t l y t i e d i n t o t h e chemistry o f t h e hydrogen/oxygen flame.
( I t s amplitude i s r e l a t i v e l y s m a l l e r compared t o t h e hydrogen s i g n a l near t h e cooled burper surface s i n c e atomic hydrogen d i f f u s e s more r a p i d l y t o t h e s u r f a c e than o t h e r flame species.) OH appears t o be t h e o n l y r a d i c a l w i t h a p p r o p r i a t e e l e c t r o n i c t r a n s i t i o n s t h a t would be p r e s e n t i n s u f f i c i e n t q u a n t i t y i n a hydrogen/oxygen flame t o produce mu1,tiphoton optogalvanic s i g n a l s o f t h i s s t r e n g t h .
12 1
(I)
Q z 'Z '5 z
N i t r i c oxide (NO), produced by flames c o n t a i n i n g nitrogenous species (e.g.
a i r ) , a l s o has s t r o n g s p e c t r a l f e a t u r e s i n t h e UV. F i g u r e 6 d i s p l a y s NO s p e c t r a recorded i n a room-temperature i o n i z a t i o n c e l l c o n t a i n i n g 1 t o r r o f pure NO ( t o p )
I 224 nrn only
Fig. 4
-
M u l t i p h o t o n optogalvanic s i g n a l s recorded i n a premixed hydrogen/oxygen/Q 686.12 66b42 68b72 68j.02 d . 3 2
DYE LASER WAVELENGTH (nm) argon flame. The c e n t r a l peak i n t h e t o p scan i s t h e atomic hydrogen resonance.
1 . 3 mm
0.9 m m
c? U) 0.6 mm
z
0.3 mm
M z
-
F i g .-
Multiphoton i o n i z a t i o n spectra, m u l t i p l i e d by t h e i n d i c a t e d g a i n f a c t o r s , recorded a t t h esee,42 see,72 56702 i n d i c a t e d h e i g h t s above t h e base of a premixed
DYE LASER WAVELENGTH (nm) hydrogenloxygen flame.
and a hydrogen/air flame (bottom), recorded w i t h o u t t h e 266-nm beam present. This p o r t i o n o f t h e NO spectrum i s displayed because t h e e x c i t a t i o n wavelength range o f 223.6-224.0 nm i s i d e n t i c a l t o t h a t produced by the 566.12-567.32 nm dye l a s e r scan range p l o t t e d i n previous f i g u r e s . These s p e c t r a l f e a t u r e s are v e r y s t r o n g because t h e y are produced by single-photon e x c i t a t i o n o f NO i n the A+X band, f o l l o w e d by single-photon p h o t o i o n i z a t i o n , as p r e v i o u s l y observed i n a flame i n t h e s p e c t r a l range 270-317 nm (15). This f i g u r e i n d i c a t e s t h a t t h e v i b r a t i o n a l - r o t a t i o n a l
s t r u c t u r e o f NO may be very u s e f u l f o r flame thermometry, as suggested i n a study o f NO i n flames u s i n g two-photon resonant, four-photon i o n i z a t i o n d e t e c t i o n (16).
- r
Z 3
a t
m
5
3
BU7
z 0
5
EZ Fig. 6
-
Multiphoton i o n i z a t i o n spectra o fg i23.8 213.7 223.8 2213.9 2 i 4 . 0 NO recorded i n a c e l l ( t o p ) and a EXCITATION WAVELENGTH (nm) hydrogen/air flame (bottom).
The e f f e c t o f these s t r o n g resonances on hydrogen spectra recorded i n flames w i t h a i r as an oxidant i s demonstrated i n Fig. 7. This f i g u r e shows a s e r i e s of scans ov.er a s l i g h t l y l a r g e r wavelength i n t e r v a l recorded i n a s t o i c h i o m e t r i c hydrogen/air flame produced by a s l o t burner, w i t h t h e l a s e r beams focused a t t h e i n d i c a t e d h e i g h t s above t h e burner. As t h e flame gases proceed upward from t h e burner, t h e atomic hydrogen peak around t h e v e r t i c a l dashed l i n e disappears because o f r a d i c a l recombination a t t h e same t i m e t h a t the NO c o n c e n t r a t i o n increases, i n c o n t r a s t t o t h e behavior o f t h e r e l a t i v e amplitudes o f the s i g n a l s i n Fig. 5.
Optogalvanic s i g n a l s i n flames can be produced by c o l l i s i o n a l i o n i z a t i o n o r p h o t o i o n i z a t i o n from e x c i t e d states. I n t h e case o f atomic hydrogen, f o r a 266 nm, 1 mJ, 5 n ec l a s e r p u l s e focused t o f8spo$ s i z e o f 200 pm, t h e peak i n t e n s i t y of
3
0.2 GW/cm combined w i t h t h e 7 . 5 ~ 1 0 - cm p h o t o i o n i z a t i o n c r o S sec i o n o f t h e hydrogen 2s l e v e l (11) y i e l d s a p h o t o i o n i z a t i o n r a t e o f l . l x 1 0 3 secmf, comparable t o t h e g a s - k i n e t i c c o l l i s i o n r a t e i n t h e flame. Since r e l a t i v e l y few c o l l i s i o n s would
C7-282 JOURNAL DE PHYSIQUE
be a b l e t o supply the 3.4 eV necessary t o i o n i z e t h e 2s l e v e l , i t i s e v i d e n t t h a t p h o t o i o n i z a t i o n i s t h e dominant mechanism i n t h e c u r r e n t experiment. These estimates a l s o i n d i c a t e t h a t i t may be p o s s i b l e t o approach 100% i o n i z a t i o n e f f i c i e n c y from t h e 2s l e v e l , thus m i n i m i z i n g t h e need f o r c o r r e c t i o n s due t o c o l l i s i o n a l quenching.
I V . --- ATOMIC OXYGEN
-
U)Measurement o f atomic oxygen concentrations i n flames i s complicated by the same f a c t o r s t h a t make d e t e c t i o n o f hydrogen atoms so d i f f i c u l t . As i n t h e case o f hydrogen, oxygen c o n c e n t r a t i o n s i n flames have a l s o been measured p r i m a r i l y w i t h sampling techniques, and i n f e r r e d from measurements o f other species i n flames (7).
Conventional o p t i c a l d e t e c t i o n i s again d i f f i c u l t because o f t h e 10-eV gap between t h e ground s t a t e and t h e f i r s t equal-spin e x c i t e d s t a t e (see Fig. 8). I n t r a c a v i t y absorption spectroscopy (17) and two-photon laser-induced fluorescence d e t e c t i o n (18) o f atomic oxygen have been demonstrated i n low-pressure c e l l s , and t h e l a t t e r has r e c e n t l y been extended t o flame s t u d i e s ( 4 ) . Atomic oxygen has a l s o been detected i n flames u s i n g spontaneous Raman s c a t t e r i n g ( 2 ) and oherent anti-Stokes
5
Raman s c a t t e r i n g (3) w i t h i n t h e f i n e s t r u c t u r e o f t h e ground 2
P
s t a t e .HEIGHT ABOVE BURNER: 3.5 m m
t -
IONIZATION LIMIT ...
l
. . Fig. 7
-
M u l t i p h o t o n i o n i z a t i o n s p e c t r a recorded a t5 8 5 . 5 5 @ 8 58e.5 587 5e7.5 568 t h e i n d i c a t e d h e i g h t s a b o v e t h e b a s e o f a s l o t b u r n e r
DYE LASER WAVELENGTH (nm) r u n n i n g a premixed hydrogen/air flame.
z
E
e c
6 - (5.5 eV)t!
Fig. 8-
Two-photon resonant m u l t i p h o t o n i o n i z a t i o n e x c i t a t i o n scheme used f o r optogalvanic d e t e c t i o n o f atomic oxygen i n flames. The dashed l i n e a t 5.5-eV energy represents a v i r t u a l , n o t a r e a l , i n t e r m e d i a t e l e v e l .The m u l t i p h o t o n optogalvanic d e t e c t i o n scheme used f o r atomic oxygen i s shown i n Fig. 8. The l a r g e r s e p a r a t i o n between t h e 2P and 3P s t a t e s (compared t o t h e
hydrogen 1s and 2s s t a t e s ) made i t d i f f i c u l t t o use t h e Nd:YAG l a s e r f o u r t h harmonic as one o f t h e wavelengths f o r two-photon e x c i t a t i o n , since t h i s would r e q u i r e a tunable beam a t 196 nm. The single-wavelength scheme shown w i t h 226-nm r a d i a t i o n was used instead, w i t h some l o s s o f s p a t i a l r e s o l u t i o n and e x c i t a t i o n and i o n i z a t i o n r a t e s . Very f o r t u i t o u s l y , t h i s scheme o n l y r e q u i r e d changing t h e 567-nm dye l a s e r wavelength used f o r t h e hydrogen d e t e c t i o n t o 573 nm, w i t h o u t t h e need f o r changing any dyes o r doubling/mixing c r y s t a l s .
F i g u r e 9 shows s p e c t r a recorded w h i l e scanning t h e l a s e r across t h e 2P-3P t r a n s i t i o n i n t h e i n d i c a t e d flames. Each o f t h e 500 p o i n t s i n t h e scans were averaged from 20 consecutive l a s e r shots w i t h o u t any form o f normalization. Tne t h r e e peaks i n t h e top spectrum a r e ( f r o m 1 f t t o r i g h t ) t r a n s i t i o n s J=1, and J=O f i n e - s t r u c t u r e l e v e l s o f t h e 2 P s t a t e t o t h e unresolved
9
fr m t h e J=2, 3 P s t a t e .9
The a d d i t i o n a l s t r u c t u r e i n t h e bottom spectrum i s again from single-photon
resonant, two-photon i o n i z a t i o n o f NO produced i n t h e flame when a i r i s used as the oxidant.
-
Resonant multiphoton optogalvanic0 2 2 6 6 226.7 226.9 226 1
LASER WAVELENGTH (nm) recorded i n t h e i n d i c a t e d flames.
DISCUSSION
v. - -
Several important f e a t u r e s o f resonant m u l t i p h o t o n optogalvanic d e t e c t i o n i n flames are worth emphasizing, many i n common w i t h o t h e r m u l t i p h o t o n techniques.
Atoms o r molecules w i t h dipole-allowed t r a n s i t i o n s i n the vacuum u l t r a v i o l e t can be probed w i t h v i s i b l e o r u l t r a v i o l e t r a d i a t i o n , g r e a t l y s i m p l i f y i n g d i f f i c u l t i e s w i t h l a s e r sources, windows, atmospheric absorption, etc. The l a s e r beams used f o r the e x c i t a t i o n are n o t s t r o n g l y absorbed by t h e species being studied, so r e l a t i v e l y dense media can be probed w i t h o u t any d i f f i c u l t i e s w i t h o p t i c a l depth or resonance trapping. Two-photon e x c i t a t i o n w i t h two d i s t i n c t wavelengths provides e x c e l l e n t s p a t i a l r e s o l u t i o n , determined by t h e common f o c a l volume o f t h e two beams.
( E x c i t a t i o n w i t h a s i n g l e wavelength s t i l l provides some s p a t i a l r e s o l u t i o n along t h e beam, since the e x c i t a t i o n r a t e i s p r o p o r t i o n a l t o the square o f t h e l a s e r i n t e n s i t y , and occurs p r i m a r i l y i n t h e f o c a l volume o f t h e beam.) A temporal r e s o l u t i o n o f -5 ns i s a consequence o f t h e use o f a Q-switched Nd:YAG l a s e r .
Several methods can be used t o overcome i n t e r f e r e n c e s t h a t may mask t h e desired species-selected i o n i z a t i o n signal. Except f o r t h e r e s t r i c t i o n t h a t t h e sum o f the two photon energies equals t h e two-photon resonance, t h e two wavelengths can be chosen f r e e l y t o avoid i n t e r f e r e n c e s from other species. For some probe configura- t i o n s , i t may prove p o s s i b l e t o use t h e time dependence o f t h e optogalvanic s i g n a l t o p r o v i d e a t i m e - o f - f l i g h t mass spectrometer t o d i s c r i m i n a t e against i n t e r f e r e n c e s (19); t h i s method has perhaps the most promise f o r atomic hydrogen, which has a v e r y h i g h m o b i l i t y because o f i t s l i g h t mass. Perhaps t h e most severe form o f i n t e r f e r - ence i s when a molecule i s i o n i z e d a l l along one o f t h e l a s e r beams, (e.g. f o r OH o r NO as described i n t h e sections above). For a crossed-beam c o n f i g u r a t i o n such as
JOURNAL DE PHYSIQUE
t h a t used f o r atomic hydrogen, a segmented c o l l e c t i o n e l e c t r o d e can be used t o d i s c r i m i n a t e against t h i s form o f background. We have used a three-segment e l e c - t r o d e w i t h a l l t h r e e segments biased t o t h e same voltage, b u t w i t h the s i g n a l c o l l e c t e d o n l y from t h e s h o r t center segment placed between two longer segments.
The hydrogen spectra shown i n Fig. 10 were recorded under i d e n t i c a l c o n d i t i o n s i n a hydrogen/oxygen/argon flame, b u t w i t h the s i g n a l c o l l e c t e d o n l y from t h e center e l e c t r o d e i n t h e t o p scan, and c o l l e c t e d from a l l t h r e e segments ( a c t i n g as a s i n g l e e l e c t r o d e ) i n t h e bottom scan; t h e d i s c r i m i n a t i o n against background i o n i z a t i o n w i t h t h i s method i s e v i d e n t i n the reduced n o i s e and wavelength-independent background.
t Z
3 SEGMENTED
>
,X
S
t- -
m
5
a
ELECTRODEz P
V) Z
0 5
g - I
,
Fig. 10-
Resonant m u l t i p h o t o n optogalvanicQ 688.12 688.42 68872 68702 68732 spectra o f atomic hydrogen recorded w i t h
DYE LASER WAVELENGTH (nm) segmented and s o l i d electrodes.
The above sections have o n l y discussed spectra recorded i n premixed hydrogen flames. We have a l s o detected atomic hydrogen and oxygen, as w e l l as NO and OH (assuming our i d e n t i f i c a t i o n i s c o r r e c t ) , i n methane flames and d i f f u s i o n flames.
This technique i s by no means l i m i t e d t o these atoms and molecules. Atoms such as N and C, which have been observed by two-photon e x c i t a t i o n and fluorescence d e t e c t i o n (20), are a l s o prime candidates f o r multiphoton optogalvanic detection.
The h i g h s i g n a l - c o l l e c t i o n e f f i c i e n c y p o s s i b l e w i t h i o n i z a t i o n d e t e c t i o n has made m u l t i p h o t o n i o n i z a t i o n spectroscopy i n general an extremely s e n s i t i v e t o o l (21). The atomic hydrogen and oxygen d e t e c t i o n l i m i t s were n o t d i r e c t l y measured i n these p r e l i m i n a r y experiments, b u t t h e strong s i g n a l s obtained i n post-flame gases a t 1500 K i n d i c a t e a s e n s i t i v i t y o f a few p a r t s per m i l l i o n f o r some flames.
S i m i l a r s e n s i t i v i t y i s estimated f o r atomic hydrogen u s i n g two-photon-excited fluorescence d e t e c t i o n (14). For atomic oxygen, our estimated s e n s i t i v i t y i s s u b s t a n t i a l l y g r e a t e r than t h a t estimated f o r t h e Raman techniques (2,3) o r two-photon e x c i t a t i o n f o l l o w e d by fluorescence d e t e c t i o n (4).
The author wishes t o thank L. A. Rahn, R. L. Farrow, F. P. T u l l y , P. L.
Mattern, and R. E. Palmer f o r many h e l p f u l discussions. This work was supported by t h e U.S. Department o f Energy.
V I I . REFERENCES
1. For a recent review o f many o f these techniques, see t h e f i r s t s i x a r t i c l e s i n Opt. Engin. 20, 493-545 (1981).
2. C. J. Oasch G d J. H. Bechtel, "Spontaneous Raman s c a t t e r i n g by ground-state oxygen atoms," Opt. L e t t .
5,
36 (1981).3. R. E. Teets and J. H. Bechtel, "Coherent anti-Stokes Raman spectra o f oxygen atoms i n flames," Opt. L e t t .
5,
458 (1981).M. AldCn, H. Edner, P. GrafstrBm, and S. Svanberg, "Two-photon e x c i t a t i o n o f atomic oxygen i n a flame," Opt. Comun.
42,
244 (1982).J. E. M. Goldsmith, "Resonant m u l t i p h o t o n optogalvanic d e t e c t i o n o f atomic hydrogen i n flames," Opt. L e t t .
L,
437 (1982).J. E. M. Goldsmith, 'lResonant m u l t i p h o t o n optogalvanic d e t e c t i o n o f atomic oxygen i n flames," J. Chem. Phys.
78,
1610 (1983).For a d e s c r i p t i o n o f most o f these methods, see R. M. F r i s t r o m and A. A.
Westenberg, S t r u c t u r e (McGraw-Hi 11, New York, 1965), Chapter 10.
Th. Just. "Chemical k i n e t i c s s t u d i e d bv vacuum-UV spectroscopy i n shock tubes,"
i n s h o c k - ~ u b e s and Waves: proceedings- o f t h e ~ h i r t e e n t h 1nteFnationa1 S m osium Shock Tubes and Waves, C. E. Treanor and J. G. H a l l , eds.
S t a t e T ~ e w Y o r k ~ r e s s . - m b a c ~ . Y.. 1981).
T.
F-
W. Hznsch, S. A.lee,.^. aliens stein,
and.C. Wieman, "Doppler-free two-photon spectroscopy o f hydrogen 1s-ZS,' Phys. Rev. L e t t .34,
307 (1975).J. Bokor, R. R. Freeman, J. C. White, and R. H. Storz, "Two-photon e x c i t a t i o n o f t h e n=3 l e v e l i n H and D atoms," Phys. Rev. A
E,
612 (1981).G. C. Bjorklund, C. P. Ausschnitt, R. R. Freeman, and R. H. Storz, "Detection o f atomic hydrogen and deuterium by resonant three-photon i o n i z a t i o n , " Appl.
Phys. L e t t .
2,
54 (1978).C. P. Ausschnitt, G. C. ,Bjorklund, and R. R. Freeman, "Hydrogen plasma d i a g n o s t i c s by resonant multiphoton optogalvanic spectroscopy," Appl. Phys.
L e t t .
2,
851 (1978).G.' C. Bjorklund, R. R. Freeman, and R. H. Storz, " S e l e c t i v e e x c i t a t i o n o f Rydberg l e v e l s i n atomic hydrogen by t h r e e photon absorption," Opt. Connnun.
2,
47 11979).
R.
P.
~ u c h t , J. T. Salmon, G. B. King, D. W. Sweeney, and N. M. Laurendeau, ltTwo-~hoton-excited fluorescence measurement o f hydroqen atoms i n flames,' --
opt. i e t t . 8, 365 (1983).
W. G. M a l l a a . J. H. M i l l e r . and K. C. Smvth. "Resonantlv enhanced two-photon p h o t o i o n i z a t i o n o f NO i n anSatmospheric flame," J.
hem.-Phys.
76, 3 ~ ' ( 1 9 8 2 ) . B. H. Rockney, T. A. Cool, and E. R. Grant, "Detection o f nascent NO i n a methane/air flame by m u l t i p h o t o n i o n i z a t i o n , " Chem. Phys. L e t t .87,
141 (1982).S. J. H a r r i s and A. M. Weiner, "Detection o f atomic oxygen by i n t r a c a v i t y spectroscopy," Opt. L e t t . 6, 142 (1981).
W. K. Bischel, B. E. Perry, and D. R. Crosley, "Two-photon laser-induced fluorescence i n oxygen and n i t r o g e n atoms," Chem. Phys. L e t t .
82,
85 (1981);W. K. Bischel, B. E. Perry, and D. R. Crosley, "Detection o f fluorescence from 0 and N atoms induced by two-photon absorption," Appl. Opt.
21,
1419 (1982).W. G. M a l l a r d and K. C. Smyth, " M o b i l i t y measurements o f atomic ions i n flames using laser-enhanced i o n i z a t i o n , " Combust. Flame
5,
6 1 (1982).Reference 18 above, and references t h e r e i n .
G. S. Hurst, M. G. Payne, S. D. Kramer, and J. P. Young, "Resonance i o n i z a t i o n spectroscopy and one-atom detection," Rev. Mod. Phys.
