Measurements of the Collisionally Quenched Lifetime of CO in Hydrocarbon Flames
S A R A A G R U P * a n d M A R C U S A L D E N
Department of Combustion Physics, Lund Institute of Technology, P.O. Box 118, S-221 00 Lund, Sweden
Time-resolved laser-induced fluorescence (LIF) from CO molecules in hydrocarbon flames was studied. Collisional quenching constants were evaluated on the basis of the exponential decays. Effective lifetime in a methane/oxygen flame was observed to vary between 250 and 400 ps depending on the position within the flame, and from 400 to 600 ps in the non-sooty parts of an ethylene/air flame. Fluorescence, constituting simultaneous spatially and temporally resolved decays, was also regis- tered from various sections along a laser beam that probed different parts of the flame. Spectral recordings revealed not only the expected CO peaks but also, in the ethylene flame, laser-induced emission from C2 Swan bands and from polyaromatic hydrocarbon (PAH) emission that affected the fluorescence time decay in the sooty part of the flame.
Index Headings: Laser-induced fluorescence; Flame radicals; Collisional quenching rate; CO.
INTRODUCTION
Combustion provides a complex and challenging en- vironment. Measuring the concentration of the various species involved and their temporal evolution is an im- portant way to understand such a process. A diagnostic problem in terms of measurement techniques is that a physical probe may disturb the temperature and gas flow of the probe volume in a significant way; there may also be difficulties in sustaining the high temperature and in registering the rapid events with sufficient temporal res- olution. Pulsed laser techniques are non-disturbing in the sense that no physical probe is employed. Such techniques also have inherent advantages such as high temporal and spatial resolution and represent suitable experimental tools.
Radicals that are highly reactive fragments of molecules appear during the combustion process, governing the de- velopment to a great extent. Concentrations of these key species are important for the validation of different com- bustion models. One of the most sensitive laser tech- niques, laser-induced fluorescence (LIF), is commonly used for such measurements since the concentration is often at a parts-per-million (ppm) level. For further ap- plications of LIF in combustion diagnostics, see for ex- ample, Refs. 1 and 2.
A simple two-level model is illustrative of the relative importance of the coefficients involved. In such a model of an atom or a molecule, the populations Ni and N2 are those at the ground and upper levels, respectively. If the species is excited by a laser beam, the upper-level pop- ulation, to which the LIF signal is proportional, changes with time, before the end of the laser pulse, as
Received 31 January 1994; accepted 26 May 1994. * Author to whom correspondence should be sent.
N i B I N2(t) = { 1 - e -(2BI+A+Q)t} (1) 2 B I + A + Q
where A is the rate of spontaneous emission; B, the Ein- stein coefficient for absorption and stimulated emission; L the intensity of the laser pulse, and Q, the rate of non- radiative collisional deexcitation. Q is termed the quench- ing rate. It is the only parameter on the right-hand side of the equation which is unknown.
The single largest problem with the fluorescence tech- nique is the high rate of collisional deexcitation, which leads to a quenching of the fluorescence emission. In a flame at atmospheric pressure the quenching rate Q is typically 100 to 1000 times the spontaneous emission rate A. The effective lifetime and the fluorescence yield are both decreased by the same factor. The collisional quenching varies nondeterminedly with the surrounding species concentration, temperature, and pressure, all of which vary with position in the flame. If the quenching is unknown and varies with position in the flame, then not even relative concentration results can be achieved.
A number of techniques have been developed to avoid the problem of quenching. One approach to achieving quantified LIF results is to use saturated fluorescence; 3 another is to employ photoionization-controlled loss spectroscopy (PICLS). 4 In both techniques, the idea of inducing the dominance of processes other than colli- sional quenching is applied. Another technique used for avoiding quenching corrections, predissociative LIF, 5 has as its major disadvantage the characteristic of providing a considerably lower signal intensity than, for example, saturated fluorescence. All these techniques attempt to avoid the problem of an unknown collisional rate entirely, rather than gauging Q.
If one could assume that the major surrounding species in the flame dominated the quenching rate, and that the concentration of each of these could be measured by some other technique (e.g., absorption or Raman spectroscopy), and finally that the bimolecular collisional cross sections between these species and the radical were all known at the relevant spans of pressure and temperature (although this is normally not the case), then the quenching constant could be calculated. 6
LIF can be used to experimentally measure and eval- uate the quenching coefficient, if the time resolution of both the laser and the detection instrument is high enough. Stepowski and Cottereau 7 have indicated that if the du- ration 7- of the laser pulse is such that ( 2 B I + A + Q)r << 1, then Eq. 1 is nearly equal to
N2(r) ~ N i B I r (2)
and is thus not dependent on quenching, but only on
known parameters. The exponential fluorescence decay that occurs following the laser pulse has the decay con- stant 1/(Q + A). At atmospheric pressure, A << Q and can thus be neglected. Therefore, by measurement of the time decay and evaluation of the exponential decay con- stant, the collisional quenching rate Q in a specific probe volume can be determined, allowing quantified concen- tration results to be obtained. In Ref. 7 this was achieved with the use of a nanosecond-pulsed laser to probe a subatmospheric flame. At atmospheric pressure the ex- perimental equipment needs to allow for a temporal res- olution of a few picoseconds, which can be achieved by using a ps-pulsed laser and a streak camera for detection purposes. This approach has been employed in experi- ments probing OH, s-~° NO, II and CN. 12 Later CH was probed in subatmospheric pressure flames by use of nano- second resolution.'3 In Ref. 14 the two-photon excitation of H in a subatmospheric pressure flame, with the use of a 50-ps pulsed laser is reported. A fast photomultiplier for the detection of the H fluorescence emission and a deconvolution procedure were used for the evaluation of the time decay. It should be noted that Eq. 2 is valid only for an isolated two-level system and is not applicable in reality where more levels are needed to account for quenching to levels other than the ground state or for ionization. Even when ionization is present, the fluores-
45O
Fla. I B.
500 550 600
Wovetength (rim)
The CO emission from a methane flame.
cence decay allows the quenching to be measured, which is the major result here.
In this paper we report on measurement of the colli- sional quenching rates of the CO naturally present in hydrocarbon flames. CO in the ground state was excited by two photons at 230 nm, and detection of the emission occurring between 451 and 608 nm was made. This is an excitation and detection scheme which we used earlier to measure effective lifetime in a CO/air diffusion flame? 5 Fluorescence decay of CO in methane and ethylene flames of different stoichiometries was registered. Spatially re- solved effective lifetimes from different zones of the flame were also recorded along a traversing laser beam. This allowed the trend for r according to position in the flame to be determined. A spectrum from a fuel-rich ethylene flame revealed not only CO emission but also, in the non- sooty region, a small emission from the Swan band in C2. In the sooty part of the flame, emission from polyaro- matic hydrocarbons (PAHs) was registered as well. C2 emission was too low to have an effect on decay. However, PAH emission interfered with the CO emission in the fluorescence signal from the sooty regions.
SPECTROSCOPY
Since the main absorption transitions of CO are found in the VUV spectral region, multiphoton excitation pro- cesses can most readily be used when fluorescence is laser induced in a flame environment. The laser spectroscopic technique we employed for the detection of CO is the two-photon excitation at 230 nm of ground X iE+ state molecules to the B ~+ state. The resulting fluorescence from the transition of the B state to the A qYI state that occurred between 451 and 608 nm was registered. Figures 1A and 1B present the energy-level diagram and the cor- responding spectrum from the CO molecules in a meth- ane flame. The rotational constants of the X and the B states are almost equal in magnitude, yielding a spectrally narrow Q branch. At room temperature the population is such that the Q branch is almost completely covered by the laser bandwidth, giving an overlap function of 6%, whereas at flame temperature in a methane/oxygen flame, 3000 K, there is a significant population of higher rota- tional levels extending upwards to about J = 80, the over- lap being reduced then by a factor of three.
The potential of this scheme for CO diagnostics was first described by Loge et al. ~6 The first flame detection and one-dimensional visualization of this species were made by Ald6n et al. 17 Two-dimensional imaging of CO using a multiple-pass arrangement was demonstrated by Seitzmann et al. TM
The natural lifetime of the B state, iE+, v = 0, is 22 ns. ~9 The two-photon cross section for this transition has been measured to be 1.10 -35 m4/W. 2° Stimulated emis- sion from the B to the A state may also occur after ex- citation. This process has been shown to compete with spontaneous emission. It complicates the linear relation between the LIF signal and the concentration. 21 Multiple scattering of the spontaneous emission from the B to the X state already occurs at low pressures (< 1 mbar), and the lifetime of the B state then approaches the inverse of the A-factor between the B and the A state. No predis- sociation has been observed at room temperature. 22 It has
APPLIED SPECTROSCOPY 1119
been shown that the ionization rate can be of considerable magnitude; Bergstr6m et al., using ionizing photons of 230 nm, measured the cross section to be 2- 10 -21 m2; 2° Tjossem and Smyth, also employing 230 nm, estimated the ionization cross section to be at least 7.10 -21 m2, 22 whereas Rottke and Zacharias reported it to be 5-10 T M
m 2 with u' = 1.23 The impact of ionization on our mea- surements is considered in a later section.
E X P E R I M E N T A L
The experimental arrangement is shown in Fig. 2; it is described in detail in Ref. 15. Radiation of the required wavelength is achieved in several steps. A Nd:YAG laser (Quantel YG572-C) produces 35-ps-long 10-Hz pulses through passive and active mode-locking. The funda- mental IR beam, frequency doubled to 532 nm, is used to pump a tunable dye laser (Continuum PTL10) having a short cavity oscillator. With the use of Rhodamine 590 in the oscillator and Kiton Red in the two amplifier cells, 3-mJ pulses of 586.9 nm are achieved. This yellow beam has a linewidth of 1.5 A FWHM and a pulse-to-pulse wavelength jitter of + 1.5 A. The light is frequency dou- bled in a KDP crystal and then mixed with the funda- mental of the Nd:YAG laser in a second KDP crystal to produce 10-ps-long pulses at 230 nm with a pulse energy of 50-80 #J.
In the experiment, the beam was focused by an f = 1 m quartz lens that produced a several-millimeter-long focus, with a beam waist of about 0.6 mm. The focal length chosen allowed the beam to traverse the width of the flame with almost constant laser intensity per cross section area, this being necessary for the spatially resolved measurements.
The fuel and oxidant were mixed before entering the 300-mm-long burner tube. This arrangement provided a laminar, premixed flame at the rectangular orifice, the dimensions of which were 5 × 18 mm or, with use of an inset, 2 × 18 mm. The inset, used except during some of the spatially resolved measurements, permitted the flame to be more stable at low stochiometric mixtures. Such a burner was chosen since it provided a reaction zone that was ridge-shaped and was thus separate from the burner head. This characteristic allowed measure- ments through the reaction zone to be made, something which would not have been possible with a one-dimen- sional burner at atmospheric pressure. For a burner of that type, the reaction zone would have been too close to the burner head to be reached by a laser beam focus. In the present arrangement, when the laser beam was led along the long side of the orifice, it probed the flame at a constant distance from the reaction zone. This pattern allowed the signal to be integrated along the beam, in- creasing the total fluorescence. In contrast, when the laser beam was made to traverse the short side of the burner orifice, different parts of the flame perpendicular to the reaction zone could be probed. The resulting signal could be used for spatially resolved measurements.
The trigger signal to the streak camera needs to have a temporal jitter that is small in comparison with the time decay. This requirement seemed best achieved by use of a fast diode with an amplifier that was irradiated by an IR pulse from the Nd:YAG laser. The trigger pulse must
Nd:YAG laser
diode trigger
CCD I streak
camera
~'~ lens
dye laser doubling & mixing unit
FIG. 2.
lenses mirrors / B I ,
lens [ ]
burner Pellin-Broc
The experimental arrangement.
come in advance of the fluorescence signal temporally, which is why the laser beam was delayed 7-8 m before the flame was probed.
The fluorescence along the laser beam was collected and made parallel by a camera lens ( f = 50 mm, f /1.2) . The image of the fluorescence emission was rotated by two aluminum mirrors in order to enter the vertical slit of the streak camera. The light was focused onto the streak camera entrance slit by a camera lens identical to the first one. The output of the streak camera (Delli Delti Dellis- trique III) is a phosphor screen on which the horizontal axis represents temporal evolution and the vertical axis indicates the fluorescence along the beam, i.e., the spatial resolution. The phosphor screen was imaged onto a charge- coupled device (CCD) detector (EG&G 1430-P) by a relay lens consisting of two camera lenses similar to those al- ready referred to. The signal intensities from the 576 x 384 pixels of the CCD detector were binned in the vertical direction when the detectivity of the instrument needed to be enhanced. The data were made DOS-compatible and were evaluated by a lifetime evaluation program. 24
Power Dependence. For a two-photon process, the fluo- rescence signal intensity in a log-log plot ideally depends quadratically on the laser intensity. A deviation from linear dependence indicates that additional deexcitation processes are strong. With high laser intensity in the focus there is a risk of photochemical processes. A lowering of the linear dependence indicates either saturation of the fluorescence or the presence of ionization. It may also be an indication of stimulated emission between the B and A states, which was not observed, however, during the present experiments. Photochemical creation of CO would result in a power dependence of three or more. We mea- sured signal strength while varying the pulse energy be- tween 18 and 50 uJ in a methane/oxygen flame near the reaction zone. Through linear regression in a log-log plot, a coefficient of 1.6 _+ 0.3 was obtained. The results are shown in Fig. 3. Because of the low signal intensity within
1120 Volume 48, Number 9, 1994
3 , 5
2 . 8
FiG. 3.
/ / , /
/
/ a / /
' C / Y /
/ /
f
/ I / / I b I
3 3.2 3,4 3.6 3 . 8 4 ~,~
I l l ( p u l s e enerl~r/n~roJoqlle)
The laser power dependence of LIF from CO molecules in a methane/oxygen flame.
the flame, the power dependence was also measured at atmospheric pressure in cold flowing CO. At a laser pulse energy of less than 50 #J, which was the usual pulse energy used during the experiments, the power dependence was measured as being 1.8 ± 0.2. With increasing pulse en- ergy, power dependence decreased.
The overlap function between the laser bandwidth and the Q branch at the temperature of the oxygen/methane flame (viz., 3000 K) is only 2%. Thus, most of the laser energy is wasted during excitation, making saturation of the two-photon transition difficult. However, an addi- tional photon ionizes, without any constraint on the wavelength within the laser line profile. For comparison, the two-photon rate for our experiment was 2.44.109 s -1, and the ionization rate 5.82.10 ~1 s-~. :° Therefore, ion- ization probably decreases fluorescence strength more ef- fectively in the case of low-line profile overlap than of high. The degree of saturation in our experiment is com- mented upon in a later section.
Methane Flame Measurements. Prior to the lifetime determination, spectral analysis was performed in order to check undesirable interferences with the CO peaks shown in Fig 1. In a sooty flame, there is some risk of creating C:, the peaks of which are very much intermin- gled with those of CO. However, no C2 emission was detected in the methane flame.
Time-resolved fluorescence was recorded for different ,I, > 2, in methane/oxygen flames at atmospheric pressure (~ is the ratio between the fuel and the oxidant giving only CO2 and HzO as combusion products). Normally 100 scans were added. The effective lifetime close to the reaction zone was 250 ± 50 ps; it increased with increas- ing distance to the reaction zone to 400 ± 60 ps, near the outer reaction zone. In the flame produced by this burner, it was difficult to determine exactly where the laser focus was situated. When the fuel/oxygen ratio was changed, the shape of the reaction zone changed too, so that the probe position in the flame could be estimated only through visual inspection. It was possible to over- come the problem of following the trend of the effective lifetime in relation to distance from the reaction zone by turning the burner by 90 ° and resolving the signals arriv- ing from different points along the laser beam in a way similar to that recently reported for OH measurement. 25 Lifetime decays at different positions in the flame could be registered simultaneously and with better spatial res- olution than has been possible in earlier measurements.
Fxo. 4. The flame as seen by looking at the short side of the burner. Different heights in the flame, marked with a, b, and e, are traversed. The sooty region of the flame is indicated.
Even with the reduction in fluorescence intensity, which reduces precision, such recordings are valuable since the signal does not exhibit the sensitivity to unintentional changes from one recording to the next (such as those due to fluctuations of the flame and jitter of the laser wave- length) that was found in earlier measurements. Figure 4 displays where in the flame the spatially resolved quench- ing rates shown in Figs. 5A-5C were registered--from above the reaction zone and just below the sooty region. The stochiometry employed (~ = 9.0) was chosen for good signal quality and for stability of the flame. A clear trend between the different regions is observable, in contrast to what it was possible to obtain in earlier experiments. Figures 5A, 5B, and 5C display the evaluated lifetimes at increasing height within the flame extending on up to the sooty region. The decay constant is higher just above the edge of the ridge-shaped reaction zone. The variation that was found in the previous results is now connected to a position within the flame in order to provide better precision and accuracy. The signal intensity decreased from Figs. 5A to 5C by a factor of two. The variation observed within the flame is stronger than the alteration connected with the stochiometry. We could not observe any trend for change o f t when • was increased from 2.0 to 5.0, within the limits of our accuracy and precision.
Lifetime decay in a non-sooty methane/air flame, • = 2.2, was registered just above the reaction zone. For • < 2, the flame was difficult to stabilize, the fluorescence signal also being unreliably low. The effective lifetime was 600 ± 200 ps. Since the signal was weak, however, we pursued no systematic investigations for lower ,I~ val- ues.
Ethylene Flame Measurements. Lifetime measure- ments in ethylene flames were also preceded by spectral recordings. The spectrum from a fuel-rich ethylene flame, probed in the non-sooty region of a sooty flame (Fig. 6A),
APPLIED SPECTROSCOPY 1121
410
390
370
• 350
• 310
:290"
270
250
-3.00
I I I t I I
-2,00 -1,00 0.00 1.00 2,00 3.00
D i s t a n c e a l o n g the laser beam (mm)
390
370
350 g
=
310
290
250
-3.00
I I I I i
- 2 . ~ -1.00 0 . ~ 1.00 2,00 3.00
D i s t a n c e a l o n g the laser b e a m (ram)
390 ¸
370
350
• 310
2 9 0
270
2 ~
-3.00
FIG. 5. taneously across a methane/oxygen flame for three different heights as indicated by a, b, and e, respectively, in Fig. 4. The precision of the points are _+ 10%. The burner center line is located at 0.00.
I ! I I I
-2.00 -1.00 0 .00 1 .00 2.00 3.00
D i s t a n c e alol~g the laser b e a m ( into)
(A, B, and C) The effective lifetimes of CO measured simul-
exhibited not only stronger CO emission but the C 2 Swan band; A 3IIg-X 311, was also visible. This emission has been observed previously in rich ethylene flames by use of nanosecond laser p u l s e s . 16 The spectrum registered in the sooty part of the flame (Fig. 6B) manifests spectral interferences in the form both of C2 emission and of a broad, unstructured background, which is likely to have arisen from polyaromatic hydrocarbons. The C~ is prob- ably created by the photodissociation of soot particles and of PAHs. 26,27 When the fluorescence decays were re- corded in the nonsooty region of the ethylene flames, the C2 emission could not be spectrally filtered from the CO emission, since this would have resulted in a fluorescence intensity that was too low. Nevertheless, since the total C2 signal strength in this spectrum was much less than the CO emission and since the time-resolved signal fol- lowed a single exponential decay, we considered this a minor source of error.
1200
i N | -
0 . . . .
4O0 dSO Sea 55O 6O0 65O 700 75O
w ~ e n g e , (rim)
FIG. 6A. Spectrally resolved emission from the non-sooty part of a sooty ethylene/air flame. The CO lines only are shown in Fig. 1B for comparison. The C2 lines are those around 438 and 473 nm.
The effective lifetimes found in the ethylene/air flame did not vary much as ,I, was changed successively from 2 to 5. In the non-sooty flame and the non-sooty part of the sooty flame, where the signal was not disturbed by any interference emission, effective lifetimes were found to vary from 400 ± 60 ps near the inner reaction zone to 600 ± 75 ps with increasing distance from the reaction zone, following a trend similar to that obtained for the methane flame. In the sooty part of the flame, where we believe there is PAH emission, the signal strength was about the same, but the lifetime of the fitted exponential decay increased to 950 ± 200 ps. Higher up in the sooty region the signal strength decreased considerably. For this reason, proper evaluation of the signal there was difficult, and we could not distinguish contributions from the two different decays as expected from the spectral measure- ments. Peripheral to the CO resonance line, the signal strength decreased by a factor of two, and the lifetime (likely of PAHs) was found to be 1300 ± 500 ps.
DISCUSSION
For proper assessment of the accuracy and precision of our results, it seemed advisable not only to estimate and add together all the possible errors involved but also to test the total procedure of detection and evaluation through measuring a species of known lifetime. The linearity of the time sweep of the streak camera was tested with the use of a train of equidistant pulses. After perfforming the
1000
f ~
0 _ F r . . z ~
350 400 450 500 550 600 650 700 750
w a v e l e n g l t ~ (rim)
FIG.. 6B. Emission spectrum from the sooty region of an ethylene/air flame. The CO emission is strong on the broad, unstructured PAH background, and the C2 emission is barely visible.
1122 Volume 48, Number 9, 1994
time calibration, we evaluated the lifetime of a saturated solution of DCM in water and ethanol. The lifetime of this solution had previously been gauged, through use of a continuous ps laser and single photon counting, a meth- od for which the accuracy and the precision are better than 1%,2s to be 990 ps. The same excitation and detection wavelength, respectively, were employed on both occa- sions. The analysis considered both the experiments in which the laser probed parallel to and those perpendicular to the reaction zone.
The precision in the present experiment is limited by several sources of errors. One is the wavelength jitter, which is inherent in a picosecond laser of this type. If the wavelength jitters out of the resonance of CO, only back- ground is registered. This factor does not alter the lifetime but adds to the background noise. It is important in the evaluations here that the background is correct. The tem- poral jitter of the trigger signal to the streak camera adds to the effective width of the apparatus function, which should be kept well below the estimated lifetime. In our case, it restrains the boundaries of evaluation and again may deteriorate the signal-to noise ratio.
Also limiting the accuracy of the final results are the nonlinear time sweep of the streak camera and the non- linear amplification of the signal. The average result for the calibration sample was 1110 ps, whereas the real value was 990 ps. It would have been preferable, of course, for the lifetime of the calibration sample to be closer to the measurement we obtained in the flame experiment in- stead of twice that value, since this would have meant that a smaller section of the streak camera phosphor screen was used during evaluation. Accuracy here was estimated to be ± 12% (or rather the real lifetimes can be assumed to have been up to 12% longer, not shorter) and the pre- cision to be ±10-15%, depending to a large degree on the strength of the fluorescence signal. For the spatially resolved measurements, the precision was better than it would have been if the decays had been achieved by moving the object. This outcome is due to the fact that the sources of error, such as flame fluctuations, gas flow pressure, and wavelength- and time-jitter, are then the same for all tracks, whereas if the spatial resolution were achieved by moving the burner, those parameters would change.
The self-quenching rate of CO was measured to be 5.9. l06 mbar -~ s-~. z° The quenching from other important flame species is unknown, but some have been estimated from hard-sphere collisional rates. 22 This approach yield- ed an upper-bound collisional quenching rate of 2.109 s -~ or an effective lifetime of 500 ps for CO in a 1500 K methane/oxygen/argon flame. In our flame there is Nz instead of argon, and the results may be compared to o u r s .
To obtain an indication of the degree of saturation and ionization in the focal volume at the end of the laser pulse, we modeled the system with the additional knowledge of the quenching rate acquired. The system is then repre- sented by the following elements: the ground level, the excited B state, the A state, the ionized population, and the population quenched out of the system. We have calculated the overlap integral between the laser wave- length profile and the molecular Q-branch at the relevant flame temperature and then treated the different states as
having only one level. The stimulated emission between the B and A states is not considered since this was not observed during the experiments.
If the laser pulse is assumed to have constant power while it lasts, the system of equations can be solved an- alytically. For the ionization with two-photon excitation, the cross sections from Ref. 20 were employed. These were measured at room temperature and with a laser of much smaller linewidth; both these circumstances were accounted for in the calculations. The analytical solution also allows us to compute absolute concentrations. An estimate of the concentration is reported in Re£ 29, and it yielded a value of 2% of CO in an ethylene/air flame, just above the reaction zone. For this experiment a sim- plified two-level model would have underestimated the concentration by approximately a factor of 1.6.
There may be partial saturation for those rotational lines which coincide with the center of the laser wave- length. However, with a laser beam diameter of 400 urn, the calculations indicated there to be no saturation. These calculations indicated that, at the end of the laser pulse, 1.7% of the original ground-state population has been excited and 1% of the original ground-state population has been ionized.
SUMMARY
In this investigation, time-resolved fluorescence decays of naturally present CO in methane and ethylene flames were measured. Excitation was achieved by two photons at 230 nm and with detection being made between 451 and 608 nm. The quenched lifetime in a methane/oxygen flame was found to increase from 250 ± 50 ps near the reaction zone to 400 ± 60 ps near the outer reaction zone. Probing perpendicularly to the reaction zone of the flame and registering the signal from the different regions in separated tracks yielded spatially and temporally resolved effective lifetimes. Since these had better potential for distinguishing trends within the flame than either alone, both methods were needed to provide a complete and correct picture. In an ethylene/air flame, the lifetime var- ied from 400 ± 60 to 600 ± 75 ps, following the same spatial pattern as that in the methane flame.
In a sooty ethylene flame, probed between the reaction zone and the sooty part of the flame, there was found to be C2 emission that interfered spectrally with the CO peaks. However, the total signal strength of the interfer- ence emission was so weak that it did not affect the life- time evaluation visibly. The fluorescence decay in the sooty regions of the ethylene flame, which is believed to arise from polyaromatic hydrocarbons, was registered. The strength of this emission was found to be lower than the CO emission by a factor of about two and the time decay to be approximately an order of magnitude larger than that of CO.
ACKNOWLEDGMENT
This work was financially supported by the Swedish Research Council for Engineering Sciences, TFR.
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APPLIED SPECTROSCOPY 1123
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