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Simultaneous Multiple Species Detection In A Flame Using Laser-inducedFluorescence

Westblom, U; Aldén, Marcus

Published in:Optical Society of America. Journal B: Optical Physics

DOI:10.1364/AO.28.002592

Published: 1989-01-01

Link to publication

Citation for published version (APA):Westblom, U., & Aldén, M. (1989). Simultaneous Multiple Species Detection In A Flame Using Laser-inducedFluorescence. Optical Society of America. Journal B: Optical Physics, 28(13), 2592-2599. DOI:10.1364/AO.28.002592

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Simultaneous multiple species detection in a flame usinglaser-induced fluorescence

Ulf Westblom and Marcus Ald6n

An approach for simultaneous detection of NO, OH and 0 in flames using laser-induced fluorescence ispresented. The technique is based on spectral coincidences using a Nd:YAG-based laser system producing afrequency-doubled and frequency-mixed laser beam at 287 and 226 nm, respectively. The possibility ofmaking spatially resolved measurements using a diode-array detector was also investigated. The use of thistechnique for studying potential laser-induced disturbances was also demonstrated.

1. Introduction

The environmental situation and our growingawareness of limited energy resources have stronglyencouraged research in the field of fundamental com-bustion processes during the last several years. Be-sides the use of more advanced computers for predic-tions and modeling of combustion phenomena, theintroduction of different laser techniques for nonin-trusive measurements with high temporal and spatialresolution has greatly improved the possibility for adeeper understanding of different combustion relatedphenomena, e.g., chemical kinetics, turbulent flows,ignition phenomena, and mechanisms for formation ofair pollutants. Of numerous laser techniques thathave emerged since the invention of the laser morethan 30 years ago, three techniques have proven to beof special importance for diagnostics of combustionprocesses: Raman scattering (RS), coherent anti-Stokes Raman scattering (CARS), and laser-inducedfluorescence (LIF).

The three techniques are complementary ratherthan competitive since Raman scattering is mostlyused for measurements of major species and tempera-ture in clean laboratory flames, whereas CARS is bestused for temperature measurements in a highly parti-cle-laden environment. The laser-induced fluores-cence technique has the great advantage of being able

The authors are with Lund Institute of Technology, CombustionCenter, P.O. Box 118, S-221 00 Lund, Sweden.

Received 19 July 1988.0003-6935/89/132592-08$02.00/0.© 1989 Optical Society of America.

to measure low number densities of molecules, radicalsand atoms preferable in laboratory flames. For differ-ent applications of these techniques for studies of com-bustion processes see, e.g., Refs. 1-3.

In the LIF technique a molecule, radical, or atom isexcited from the ground state to an excited electronicstate. After a short time period wavelength-specificfluorescence radiation is emitted when the species re-turns to its ground state. This technique is thus spe-cies specific in both the excitation and fluorescencewavelength, which reduces the possibility for spectralinterferences, at least for small molecules. LIF hasalready been used for detection of flame radicals ab-sorbing in the UV and visible region, e.g., CH, OH,CNand C2.4-6 With the introduction of high-power solid-state lasers, the technique was also extended to includemeasurements of flame species absorbing in the VUVspectral region, e.g., 0, H, and CO,7-9 by using multi-photon processes.

Of particular importance with the LIF technique isthe possibility of making simultaneous multiple-pointmeasurements, so-called imaging measurements.This technique was first demonstrated on OH radicalsalong a line using a diode-array detector, 1 0 shortlyfollowed by 2-D measurements utilizing a diode-ma-trix detector.11 12 The imaging technique has alsobeen developed for temperature measurements, 1 3 1 4 3-D species detection,15 and, recently, flow-velocity mea-surements with high (10 ns) temporal resolution.16

Traditionally LIF is a single-species detection tech-nique; however, there have been some multiple-speciesLIF experiments reported. NH and OH were mea-sured using a Kr+ laser,17 and the same group also usedan Ar+ laser for simultaneous measurements of CH,CN and NCO.1 8 Using two independent laser systems,simultaneous spatially resolved measurements of C2

2592 APPLIED OPTICS / Vol. 28, No. 13 / 1 July 1989

and OH have also been reported. 19 By combining one-and two-photon processes, detection of NO2 and NOusing a laser wavelength at 452 nm was demonstrat-ed.20 It has recently also been demonstrated how sev-eral flame species could be detected using laser-in-duced fluorescence with overlapping resonances.21 Inthis experiment, OH, NH, CH, and CN were simulta-neously excited by the use of a laser wavelength at312.22 nm. It was also reported how nonlaser-excitedspecies could be indirectly excited through collisionalenergy transfer processes. 21

In this paper we present experiments investigatingthe potential for simultaneous detection of NO, 0, andOH. NO and 0 are excited around 226 nm, wherethese species have a one- and two-photon resonance,respectively. Since this wavelength was produced us-ing a Nd:YAG-based laser system through frequencydoubling of the dye-laser beam followed by frequencymixing with the residual 1.06 4m, a laser beam around287 nm was also produced. Consequently, there is apossibility that this beam could be used for excitationof OH radicals, which have absorption bands in thisspectral region. Investigations were made to deter-mine the extent to which the spectral coincidenceswere large enough for simultaneous detection and howthe laser linewidth could influence these effects. Ex-periments were also performed on imaging measure-ments, where the potential for recording single pulseimages of NO, 0, and OH in an H2 /N2 0 flame wereinvestigated. Finally, experiments were performed todetermine whether potential laser-induced distur-bances could be investigated using this technique.

II. Relevant Spectroscopy

Before starting the experiments, the spectroscopyrelevant to simultaneous excitation of the three spe-cies, OH, NO, and 0, was examined. The energy-leveldiagrams illustrating the relevant transitions in thesespecies are shown in Fig. 1.

Oxygen atoms were first detected in a cell by Bischelet al.2 2 The two-photon atomic oxygen transitionaround 226 nm is between two 3 P states and can there-fore result in nine different fine-structure transitions.Two-photon dipole selection rules for the case w = 2and 7r polarization in both beams, however, excludesthe 0-1 and the 1-0 transitions resulting in seven al-lowed transitions. 2 3 These seven lines are usually notresolved since the energy splittings in the excited state,0.54 cm-1 and 0.16 cm-1 , respectively, are too small tobe resolved under the combined effect of Doppler andcollisional broadening and the broadening due to thefinite bandwidth of the laser. In the ground state, theenergy splittings are 158 cm-' and 226 cm-', respec-tively, resulting in a spectrum consisting of three re-solved peaks of which the strongest (No. 1) consists ofthree unresolved fine-structure components and thefollowing two (No. 2 and No. 3) each consists of twounresolved components. From the excited state theatom can decay either to the 3 3S1 state giving fluores-cence at 845 nm, or by collisional energy transfer popu-late the 3 5 P1 , 2,3 state which then decays to the 3 5S2

I

Fig 1.

0I Energy

3 p. 5P

nm

Energ

2Z.

226 nr

NOly

n

2

0

OH

V1

2

0

I 226 nmn X 4 2

Energy-level diagram illustrating the adequate transitionsfor simultaneous excitation of 0, NO, and OH.

state giving fluorescence at 777 nm. The fluorescenceemitted when the atom decays to its ground state is inthe VUV and can thus not be used for flame detectionof this species because of strong absorption in thiswavelength region from other flame species.

For NO and OH, the situation is quite different.Not only are there different electronic states but also alarge manifold of vibrational and rotational levels.The rotational transitions for these species that havethe potential of overlapping with the two-photon oxy-gen lines are those that belong to the A 22+ - X 2H, VI =0 - = 0, transition, the so-called y band system at227 nm for NO, and to the A 21+ -X 2H, V' = 1 -V = 0band system at 281 nm for OH. The wavelengths, 227nm for NO and 281 nm for OH, refers to the position oftheir respective bandheads. Expressed in terms of thedye-laser fundamental beam they become 577 nm and562 nm, respectively. Since the NO and OH bands aredegraded in opposite directions, NO to the blue andOH to the red, there is a region around 574 nm corre-sponding to the atomic oxygen resonances in the UVregion where they overlap. This is especially true in aflame where the high temperature moves the rotation-al population distribution towards higher rotationalquantum numbers. Since the spectral density of rota-tional transitions is much higher in NO than in OH(BNO = 1.7 cm 1, BOH = 18.9 cm-1 ) it can be expectedthat finding a spectral coincidence between 0 and NOwill pose a smaller problem than finding one for 0 andOH.

111. Experimental

The experimental setup used in the multiple-speciesLIF measurements is shown in Fig. 2 A Nd:YAG laser(Quantel YG 581-10) producing about 400 mJ in thefrequency-doubled output at 532 nm was used to pumpa dye laser (Quantel PDL-50) operating with a dyemixture of Rhodamine 590 and Rhodamine 610 toyield a maximum output of around 100 mJ at 574 nm in10 ns pulses. The dye-laser beam was thereafter firstfrequency doubled to 287 nm, then frequency mixed to226 nm with the residual IR output from the YAG laser

1 July 1989 / Vol. 28, No. 13 / APPLIED OPTICS 2593

using KDP crystals for both processes. The resultingpowers at 287 nm and at 226 nm were both about 5 mJ.(However, when the doubled beam was used withoutadditional frequency mixing the output power in thedoubled beam was about 15 mJ.) The laser powerfocused into the probe volume was measured by aScientech (38-0101) power meter and could be set by aNewport Research continuously variable attenuator(model 935-10) to any value between 0 and 3 mJ with-out changing the beam properties and/or steering thebeam. The frequency doubling and mixing crystalswere mounted in a tracking system (Quantel UVT)enabling well-controlled wavelength scans of both thefrequency-doubled and mixed-UV beams, retaining atleast 80% of maximum laser power over the entire scanrange, which was about 50 A. The bandwidth of theYAG fundamental at 1.06 gm was 0.8 cm-' during theexperiment. The dye-laser bandwidth could be either0.4 cm- 1 or 0.08 cm-' depending on whether a narrowbandwidth package, NBP, including a prism beamexpander, was installed in the dye-laser oscillator ornot. The experimental recordings were made usingthe dye laser in both modes in order to evaluate theeffect of a change in laser line width. After the fre-quency-doubling and mixing crystals a Pellin Brocaprism was used to spatially separate the different wa-velengths. The UV beams were then sent counterpro-pagating through the measurement region, focused byf = 1000 mm lenses mounted on x - y translators,enabling exact positioning of the foci relative to eachother. The relatively long focal lengths used weremotivated by the fact that it is possible to produceadditional atomic oxygen from vibrationally excitedmolecular oxygen even at low laser power densities,(0.2 mJ, f = 1000 mm). This phenomena has beeninvestigated by Goldsmith. 2 4

A Nikon quartz lens, f = 105 mm f/14.5, collected thelaser-induced fluorescence and focused it either ontothe entrance slit (200 Am) of a monochromator, mount-ed parallel with the laser beam, or, in the imagingmode, through suitable filters directly onto the photo-cathode of a diode-array detector (Tracor Northernmodel TN-6144, 700 channels). The monochromator(Jarrell Ash) was equipped with three interchangablegratings of 150,600, and 2400 lines/mm, which yieldeddispersions of 24, 6, and 1.5 nm/mm, respectively. Aphotomultiplier tube (RCA 31034) connected to a box-car averager (PARC model 4402) was used as detectorfor the monochromator. In the imaging mode, thediode-array detector was used for recording the spa-tially resolved fluorescence profiles of the differentspecies across the flame. The monochromator, PMtube, and boxcar averager were used to record theexcitation spectra. The laser-induced fluorescencespectra were also recorded spectrally resolved usingthe spectrometer equipped with the diode-array detec-tor. Since the lense used is well corrected for chromat-ic aberrations there was no need for refocusing whenchanging the experimental setup from detecting fluo-rescence in the IR region from oxygen to recordingfluorescence in the UV from OH and NO. The quality

W L Yd W td O-, : aPrisma L G B PB: Pellin Brocca PrismL:LensF: Flame

Spectro- -:: BS Flow BS: Beam splitter

meter Meter PM: PhotomultiplierTubeDA: Diode Array

PM . D.A. N0 Ht X-Y:XY-Translator

Multi-|Boxcar Channel P

Analyzer

Fig. 2. Experimental setup used in the multiple species LIF experi-ments.

of the lens also made it possible to resolve spatialfeatures of the intensity distribution across the flamewith low image distortion. Thus, correct images couldbe achieved without applying the correction proceduredescribed in Ref. 25. Correct focusing alignment wasaccomplished by replacing the flame with a thin (25,um) electrically heated wire and then adjusting thefocal distances to minimize the halfwidth of the wireimage on the detector. The imaging ratio was 1:1.

Since the emphasis of the present work was to inves-tigate spectral coincidences rather than to do quanti-tative measurements, a standard welding torch, oper-ating at atmospheric pressure, was used as a burner.Hydrogen and nitrous oxide were used as fuel andoxidant, respectively, during most of the work. Acety-lene/air and hydrogen/air flames were also investigat-ed but all measurements reported here are from theH 2/N2 0 flame. The gas flows were controlled by elec-tronic mass-flowmeters and the flame was kept fuellean in all measurements, with an equivalence ratio ofabout 0.6.

IV. Spectroscopic Measurements

In Fig. 3 (a-c) excitation spectra of 0, NO, and OHare shown where, since the linewidth at 1.06 m was - afactor of ten larger than that for the dye laser (0.8 cm 1

compared to 0.08 cm-1 ), the resolution in the OH re-cording is given by the line width of the dye laser and inthe NO and 0 recordings by the Nd:YAG laser. Thewavelength of the dye-laser beam was scanned over thesame wavelength region for all three spectra, and theywere recorded with one laser pulse per point. Duringthese experiments, only one beam was present at atime in the probing volume to avoid potential crosseffects (see below). The 0 spectrum, shown in Fig.3(a), is two-photon induced, and shows the three re-solved fine structure components of the 23P2, 1,o-33P1,2,0 transition. In Fig. 3(b) the NO spectrum withthe A 22+ - X 2fI, V' = 0 to v" = 0, transitions in the yband at around 226 nm is shown. The OH spectrum,with the A 2 - X 2 fl, v' = to v" = 1 transitionsaround 287 nm, is shown in Fig. 3(c). Both thesespectra are one-photon induced.

2594 APPLIED OPTICS / Vol. 28, No. 13 / 1 July 1989

:i)

4-p

0-0

C

(D)00

225.0 225.5

2'lP - P"..

1

2 AP - 3 P.

226.0

0 Flame

a

2'P. - 3.1i.13

111 11-., 10A lT wL ]tLhtUtd td U .j.# ,

226.5 227.0 227.5

OH Flame

v"=O-v'=1

C

286.0 287.0 288.0 289.0

Excitation wavelength [nm]Fig. 3. Excitation spectra of (a) 0, (b) NO, and (c) OH recorded

from a H 2 /N2 0 flame.

To make a detailed investigation of possible spectralcoincidences, the wavelength scale of the NO, OH, andO excitation spectra were overlapped and expandedaround each of the three oxygen lines as illustrated inFig. 4 (a-c). The solid lines represent the 0 profiles,the dashed, NO, and the dotted, OH. As can be seen inFigs. 4 (a) and 4 (b), which show the wavelength regionaround the two strongest oxygen lines, respectively,there is an overlap between NO and 0 and between NOand OH, but not between 0 and OH. Of course, anoverlap can be achieved by making the laser linewidthsufficiently large but then the spectral power densityhas to be substantially sacrificed resulting in loss ofsignal, especially from oxygen atoms. (The effect ofthis can be seen in the recordings made in the imagingmode, which are shown both with the dye laser operat-

- 0 "hAW......... YWW#4MM572.8 A

b 2

Z%~~~~~~~~'

at257n,() h 3I3 , trnito at 22. nd() h

C

C ~

O 574.1A

0

Dye k4ser fundamental (n m)Fig. 4. Expanded wavelength scale of the excitation spectra of OHand NO around the three peaks of 0 (a) the 23P2 - 33P,,2,0 transitionat 225.7 nmn, (b) the 23p1 - 33p1, 2 transition at 226.1 am, and (c) the3 Po - 33P2,0 transition at 226.2 nm. [The wavelength scale is differ-ent in Figs. (a), (b) and (c)]: Solid lines = 0; dotted lines = OH;

dashed lines = NO.

ed broadband and narrowband as discussed below.)Consequently, since the distance to the nearest OHline is several times the laser linewidth both for thefirst and for the second 0 line, these transitions do notseem to be attractive alternatives for simultaneousexcitation of both NO, OH and 0. In Fig. 4 (c) thewavelength scale is further expanded around the thirdoxygen line and the spectrum is recorded with the dyelaser operated in the broadband mode. As can be seen

1 July 1989 / Vol. 28, No. 13 / APPLIED OPTICS 2595

^ ^ P ^ A ^ _ _ _ _ . . _ _ . _ b

Afilbwj.'i

FluorescenceIntensity 0

NO

226 287 310 777 845 [nm]

Fluorescence wavelength

Fig. 5. Laser-induced fluorescence spectrum when exciting NO,OH, and 0 with a dye-laser fundamental wavelength at 574.65 -nm.

there is here an overlap in wavelength for the threespecies which is easily achievable with standardNd:YAG-pumped dye lasers.

Tuning the dye laser to the peak of the third oxygenline and resolving the emitted fluorescence throughthe spectrometer using the grating with the smallestdispersion, 240 A/mm, we detected and recorded theradiation of the emission spectra with the diode-arraydetector as shown in Fig. 5. Since the spectrographcould not cover the entire wavelength range between226 nm and 845 nm, the UV and IR fluorescence wasrecorded in sequence changing only the grating anglebetween the recordings.

V. Imaging Measurements

After the spectroscopic work described above wasfinished, the spectrograph was removed and the fluo-rescence was focused directly onto the diode-array de-tector through suitably chosen filters. In the case ofNO an interference filter transmitting radiationaround 260 nm (v' = 0 - = 3) was used, whereas forOH an interference filter transmitting around 310 nm(v' = 0-v" = and v' = 1 - v" = 1) was chosen. Foroxygen, a RG 9 3 mm Schott filter transmitting over90% between 750 and 880 nm, was used. In Fig. 6 thespatially resolved fluorescence images of 0, NO andOH are shown using the dye laser both in the narrowbandwidth mode (left column) and in the broad band-width mode (right column). For these recordings thedye laser was tuned to the strongest transition of oxy-gen. As was concluded above, the possibility for de-tection of OH radicals was limited here. The background seen in the OH recordings is caused mainly byflame luminescence and background radiation notsuppressed by the gating (1 s gate) of the diode-arraydetector. The recordings acquired when the laser wastuned to the second oxygen peak are not shown sincethe results were similar to those shown in Fig. 6. How-ever, if only NO and 0 are to be simultaneously detect-ed, this is the oxygen transition having the largestoverlap with NO.

In the recordings taken when the laser was tuned tothe third oxygen peak, as shown in Fig. 7, where anoverlap between all three species was found, it was seenthat going from narrowband operation of the dye laserto broadband operation increased the OH fluorescenceintensity by a factor of 10, while reducing the oxygenintensity by a factor of 3. The images show the spatial

(I)

C4l)(-C:

0(I,ci)

0Ei

0 220

25mm

0 630

25mm

25mm

O=0.4 cm- 1 25mm a= 0.08 cm_ 25mm

Fig. 6. Spatially resolved fluorescence distributions of NO, OH,and 0 with the laser tuned to the first oxygen peak. The distribu-tion in the left column was recorded with the dye laser broadband(0.4 cm-') whereas the right column was recorded with a narrowband dye laser (0.08 cm-'). The numbers shown in the figure

indicate the number of counts at the peak.

intensity profile measured 5 mm above the burnerhead and through the reaction zone. For OH and NOthere are two peaks indicating the position of the reac-tion zone and a dip in the middle where the fuel andoxidant are still unaffected. The nonzero concentra-tion level in this area is mainly due to diffusion. Foroxygen, the image is somewhat different; there is a firstwell-defined flame front, but also a second peak indi-cating a secondary reaction zone. The power levelsused for the imaging recordings exceed those given inRef. 24 for reliable quantitative measurements. How-ever, the validity of the relative shape of the profileswas checked by comparing with recordings acquiredwith a laser power that was reduced by a factor of 10,and they were found to be almost identical. All re-cordings taken in the imaging mode were averagedover nine laser shots. They are not corrected for ab-sorption which affects the symmetry of the profiles.As described in Ref. 26 this effect can be used forabsolute concentration measurements when there is areliable detected amount of absorption, which is nor-mally a few percent when using pulsed lasers.

Investigations were also made to determine whetherthe technique described above could be used for mea-surements of laser-induced disturbances. We mea-

2596 APPLIED OPTICS / Vol. 28, No. 13 / 1 July 1989

25mm

0 220

25mm

NO 9400

OH 180

a= 0.08 cm1 25mm

Fig. 7. Spatially resolved fluorescence distributions of NO, OH,and 0 when the laser was tuned to the third oxygen peak. Thedistribution in the left column was recorded with the dye laserbroadband (0.4 cm-') whereas the right column was recorded with anarrow band dye laser (0.08 cm-'). The numbers shown in the

figure indicate the number of counts at the peak.

sured to see if the shape of the recorded images wasdependent on whether they were recorded with bothbeams overlapping in time (and space) or recordedsequentially and added on the computer. This wasdone for different combinations of excitation wave-lengths, on resonance with (r.w.) (NO, OH, 0), or r.w.(NO, OH) and off r.w. 0, on r.w. NO and of r.w. (OH,0), on r.w. (NO, 0) and off r.w. OH, and detecting thefluorescence at 260 nm, 310 m, and in the IR. Duringthis part of the experiment the focusing lenses werechanged to f = 20 cm for the laser beam at 226 nm andto f = 15 cm for the beam at 287 nm to enhance possiblelaser-induced effects. Figure 8 shows the only casewhere a difference was found. For these recordingsthe dye-laser wavelength was tuned to the third oxygentransition where there is an overlap with NO and OH.Figure 8 (a) shows the spatial fluorescence distributionfrom OH, detected at 310 nm (v' = 0 - v" = 0 and v' = 1- v" = 1), across the flame, recorded with only thefrequency-doubled beam at 287 nm present. Figure 8(b) shows the corresponding fluorescence from NO,also detected at 310 nm (v' = 0 - v" = 6) and recordedwith only the frequency-mixed beam at 226 nm

.D

Cca)Ca)cCa)

a)0)

d

25 mm

Fig. 8. Spatially resolved distributions as recorded through aninterference filter at 310 nm. (a) With only the laser at 287 nmpresent, indicating the OH distribution. (b) With only the laser at226 nm present, indicating the NO distribution. (c) The sum of thedistributions in (a) and (b). (d) The distribution recorded experi-mentally with both laser beams present, indicating the presence of

laser-induced disturbances.

present. If the profiles in Figs. 8 (a) and (b) are added,the result, shown in Fig. 8 (c), is expected to look likesurements in principle require knowledge about thisparameter. However, since the quenching is a func-tion of pressure, temperature, and species compositionin the probing point, a measurement of this quantity isa formidable task. One promising technique to getaround the problem with quenching is to saturate thetransition. This technique has proved to work verywell for several flame species, e.g., OH.1 6 ,28

During the course of this work we made preliminaryexperiments on the potential for saturation of the NOtransition. The laser was tuned to the R1(22) transi-tion in the NO (0 - 0) -y band and focused in the flamewith a f = 20-cm lens. It could then be seen that thefluorescence intensity curve as a function of laser in-tensity showed an appreciable amount of saturationbehavior attributed to saturation, and thus in princi-ple would permit absolute concentration measure-ments to be performed. Consequently, there will be apossibility for simultaneous concentration measure-ments of NO and OH by saturating the transitions.Further work in absolute concentration measurementsis currently in progress in our laboratory. However, itshould be noted that the power density required tosaturate a transition in NO is apparently higher thanwhat should be used for quantitative atomic oxygenmeasurements, without any laser-induced distur-bances. In the case of making absolute number densi-ty measurements of oxygen atoms another approachhas to be used: e.g., the calibration approach as pro-posed by Kohse-Hoinghaus et al.29

In this work three species could be excited simulta-neously. However, as has been pointed out,30 thisnumber could actually be extended by another species,since at flame temperature, in the spectral region of

1 July 1989 / Vol. 28, No. 13 / APPLIED OPTICS 2597

0 80

25mm

NO 12200

c

.25 m

25mm

OH 2100

a=0.4 Cmn 25mm

interest, it is also possible to excite 02 in the Schu-mann-Runge bands. There are spectral coincidencesbetween 02 transitions and all three oxygen transi-tions,3 1 which thus might permit detection of four spe-cies, e.g., NO, OH, 0, and 02, using only two single-laser pulses from one laser shot.

As indicated above there is always a probability ofintroducing errors in the measurements, especiallywhen using high laser power in the UV spectral region.Thus, there have been several indications of such ef-fects, e.g., C2 formation in CARS3 2 and fluorescence' 9

measurements, appearance of artificial oxygen atomsignals in room air33 and also an artificially increased Hand OH signal intensity when exicting hydrogen atomsat 205 nm.3 4 Obviously great care has to be taken inorder not to introduce errors in the measurements.These artifacts can not be unveiled only by going offresonance since the probed species can still be createdalthough not detected. One technique which can beused to facilitate a correct measurement is a pump-probe technique, as described in Ref. 24, where a sec-ond laser system was used to probe the effect of toohigh laser power in measurements of 0 atoms in aflame. This technique was also used in Ref. 34.

Since a two-laser experiment requires a rather com-plex experimental setup, an alternative complement tothis technique seems desirable. With the techniquedescribed in this paper it has been shown that undercertain circumstances, it is possible to make a pump-probe experiment utilizing only a one-laser system.

We believe that the technique based on simulta-neous excitation through spectral coincidences can bethe profile acquired with both beams overlapping inspace and time, which is shown in Fig.8 (d). However,as can be seen when comparing Figs.8 (c) and (d), thereis an extra contribution in the recording acquired withboth beams present. This effect was present bothwhen the laser was tuned on and when tuned off reso-nance with oxygen. It was not possible however, totune the wavelength off resonance with NO since thelaser bandwith was not narrow enough. The origin ofthe extra contribution to the inner peaks in Fig. 8 (d)has not yet been determined. The fact that it is onlyobserved when both laser beams are present wouldseem to indicate that some form of photochemicalperturbation in the flame is being induced by one ofthe laser beams (most likely the more energetic pho-tons in the 226 nm laser beam), and then being probedby the other laser beam. This effect is currently beinginvestigated in our laboratory.

VI. Discussion

As has been demonstrated in this paper it is possible tosimultaneously detect NO, OH and 0 in a flame, usingspectral concidences. Also imaging recordings ofthese species are clearly possible.

In the work presented here the images were recordedsequentially by changing different filters by hand.However, by using a multispectral imaging device, e.g.,as described in Ref. 27, simultaneously imaging re-cordings also seem quite feasible. The imaged record-

ings shown in this paper were all obtained by averagingover several laser pulses, while the spectra were record-ed with one laser pulse per point. From the signalintensities during the spectral measurements, i.e., thespectra shown in Fig. 3, it was evident that the speciescould be detected simultaneously even using a singlelaser pulse. In the imaging experiment a trade offbetween large linewidth (to assure spectral coinci-dence) and high spectral brightness has to be made inorder to make simultaneous single-shot recordings ofall species. The spectral brightness is of special im-portance in the two-photon experiment in measure-ment of oxygen atoms. It is, however, clear that withthe linewidth used in our experiments, out of NO, OHand 0, any combination of two species, except 0 andOH, can easily be detected spatially resolved using asingle laser pulse. The reason for this is simply thatthe NO resonances are so close that, with the laserlinewidth used in this experiment and in the spectralregion of interest, there is always a probability forexcitation of NO as can be seen in Fig. 4 (a-c).

In this work no attempt was made to make quantita-tive measurements. There are two main problemswith converting fluorescence distributions (like thosein Figs. 6 and 7) to absolute number densities. First,the spectral coincidence for simultaneous excitation ofall species is limited to certain rotational quantumnumbers. Therefore, for NO the Q,(6) and Pi(14)(which are unresolved) and P2 (10) for OH, one cannotchoose a rotational transition which is almost insensi-tive for temperature changes in the temperature inter-val of interest to avoid temperature effects in the spe-cies distributions. In our case this means that, e.g., forOH one would expect about a factor of 2 in populationdifference when going from 1000 K to 2000 K.

Second, since excited species suffer from quenching,i.e., nonradiating transitions, quantitative LIF mea-used to determine when not to use too high a laserpower, although the technique can only indicate thepresence of a perturbation and not the absence of sucheffects. It is also clear despite the fact that tempera-ture and quenching effects limit the possibility forabsolute number density determinations, that thetechnique could be of great value in showing trendsand especially for detection of changes, for example,when introducing additives in a flame.

The authors gratefully acknowledge the helpful contri-bution from C. Lofstrom and P. Cederbalk in the earlystage of this work. Helpful advice and suggestions tothe manuscript from J. E. M. Goldsmith, Sandia Na-tional Laboratories, Livermore, are also appreciated.We also acknowledge constant support from S. Svan-berg. This work was financially supported by theSwedish Board for Technical Developments, and theSwedish National Energy Administration.

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1. M. Lapp and C. M. Penny, Eds., Laser Raman Gas Diagnostics,(Plenum, Now York, 1974).

2598 APPLIED OPTICS / Vol. 28, No. 13 / 1 July 1989

2. D. R. Crosley, Ed. Laser Probes for Combustion Chemistry, ACSSymposium Series 134 (American Chemical Society, Washing-ton, DC, 1980).

3. A. C. Eckbreth, Laser Diagnostics for Combustion Tempera-ture and Species, (Abacus, Cambridge MA, 1987).

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