- Email: [email protected]
O2 flames by resonance ionization
T w e n t y - S e c o n d S y m p o s i u m ( I n t e r n a t i o n a l ) on C o m b u s t i o n / T h e
Combustion Institute, 1988/pp.
1421-1432
P R O F I L E S O F H C O A N D C H 3 IN CH4/O9~ A N D C~H4/O 2 F L A M E S BY R E S O N A N C E
IONIZATION
TERRILL A. COOL, JEFFREY S. BERNSTEIN, XIAO-MEI SONG AND PETER M. GOODWIN School of Applied and Engineering Physics Cornell University, Ithaca, New York 14853
Measurements are presented of the relative density profiles of formyl and methyl radicals in stoichiometric methane/oxygen and ethylene/oxygen flames which exemplify recent progress in the use of resonance-enhanced multiphoton ionization (REMPI) spectroscopy for flame diagnostics. The advantages and present limitations of this new laser-based technique are discussed. The high sensitivity of the REMPI method for the detection of HCO presents an opportunity for better measurements of flame profiles and of the rate constants for consumption of this important intermediate in hydrocarbon combustion.
Introduction Progress has been made in the implementation of resonance-enhanced muhiphoton ionization (REMPI) as a laser-based diagnostic method for monitoring the concentrations of combustion species at the part-per-million level. 1 14 The REMPI approach has a sensitivity rivaling that of the wello established laser induced fluorescence (LIF) method. Both techniques possess the sensitivity for in situ monitoring of trace concentrations of reaction intermediates, a capability not found in two other popular laser-based diagnostics: Raman scattering and coherent anti-Stokes Raman scattering (CARS). The REMPI and LIF methods are complementary since the REMPI approach is well suited for the detection of some weakly fluorescing species which cannot be easily monitored by LIF. Both techniques offer excellent spatial resolution and minimal perturbation of the combustion environment. Even though REMPI spectroscopy has provided a remarkable body of new spectroscopic data on previously unobserved electronic states of many molecules, the widespread application of REMPI for flame diagnostics is presently limited by a scarcity of spectroscopic data for Rydberg states of flame radicals. The CH315-17 and HCO 1s,19 radicals are examples of reaction intermediates of prime importance in hydrocarbon combustion for which the recent identification of new electronic states with REMPI spectroscopy now permits flame diagnostics to be performed with this sensitive and unique laserbased method. The growing list of species that have been detected in flames by REMPI includes: C, wA4 LI 4,6,9 t~ 5,9 /'ILl 14 p ¢ ~ 8 /~ 14 ~.r/~ 1,2 D¢~, 3 lj¢~/~, 13 ii, u, ~,~11, bu, ~-/2, ~-4x~, l~J, llbU,
C20, s CH3, l°'lz and C4H6.7,H The value of REMPI for the ultrasensitive detection of chlorinated hydrocarbons has been demonstrated, z° The REMPI
spectroscopy of transient radicals was recently reviewed. 21 The REMPI approach to flame species detection and monitoring, in its simplest form, consists of the collection of electrons on a thin (-
1421
1422
LAMINAR FLAMES: FLAME STRUCTURE
mechanisms built into models of hydrocarbon combustion. The REMPI method provides a useful laser-based alternative to conventional flame sampling and mass spectrometric techniques which have provided much of the experimental justification for existing flame models. The principal advantages of this new approach include a very high sensitivity, a potential for direct in situ density measurements with negligible perturbation of the flame environment, high spatial resolution, and a capacity for discrimination between isomers such as CH30 and CHzOH. z2'z3 These same advantages are possessed by LIF, the laser-based method of choice for strongly fluorescing species, but CH3 and HCO are examples of radicals which have not been detected in flames with the LIF technique. At this early stage of development there are also some disadvantages of the REMPI technique that need investigation. Present measurements provide only relative densities; the capability for direct, quantitative measurements of absolute species densities, exemplified for NO at 300 K, z4 has yet to be demonstrated for in situ flame measurements. Studies of the possible effects of catalytic reactions at the probe surface, of laser-induced resonant and nonresonant background ionization, of electron attachment by electronegative flame species, and of local flame perturbations introduced by the probe are urgently needed.
Experimental Apparatus A fiat flame burner with a porous stainless steel head (McKenna Products) with a 6 cm diameter flame zone and surrounding argon shroud flow was operated with premixed CH4/O2/Ar and C2H4/O2/ Ar stoichiometric mixtures at pressures ranging from 18 to 30 Torr. The gas flow rates were regulated with calibrated mass flow controllers (Tylan FC-280). The burner assembly was mounted on a micrometer-driven stage to permit precise vertical translation of the burner within a 15 cm diameter stainless steel housing. The burner head could thus be raised or lowered so that the position of intersection of a horizontal laser beam with the flame zone could be varied. The housing was equipped with optical ports and a feed-through port for the ionization probe, as is illustrated in Fig. 1. The platinum ionization probe used for most of this work, illustrated in inset A of Fig. 1, has been fully described elsewhere. 24 For the present studies the burner head itself was the grounded cathode surface; the "cathode" wire of the probe was removed from the circuit by connecting it to the anode potential. This arrangement gave probe bias characteristics with a better defined "plateau" for saturated charge collection in the present studies.
The ionization probe was angled at 50° to the vertical flow axis to facilitate positioning of the probe within the flow. Inset B of Fig. 1 illustrates a simpler probe geometry, used in the latter stages of this work, which also was used with the grounded burner head as the cathode. REMPI probe voltage pulses measured across a 150 kl~ leak resistor (connected between the anode and the positive bias voltage source)z4 were monitored by a boxcar (SRS250) with a gate, 1 Its wide, situated near the pulse peak. A frequency doubled Quanta-Ray PDL-2 tunable dye laser, pumped with a Quanta-Ray DCR-3 NdYAG laser, was used for most of the REMPI studies of CH3. This system provided 5 mJ pulses of 7 ns duration in the 333 nm wavelength region when operated with a DCM/LDS dye mixture. 1° Useful preliminary results for CH3 were also obtained with 5 ns pulses of 0.5-1.0 mJ energy near 333 nm supplied with an excimer(XeC1)-pumped frequencydoubled tunable dye laser [Lambda Physik EMG i01 and FL 2002 system] operated with DCM dye. REMPI detection of HCO was accomplished with the excimer-pumped dye laser system operated with PBBO dye to produce 3-5 mJ, 5 ns pulses near 397 nm. Both lasers were operated at a 10 Hz repetition rate. Data points taken at a fixed laser wavelength for determination of the probe bias characteristics and the spatial profiles of REMPI ionization were always an average of 100 laser shots. The laser beams were focused directly within the flame zone with single piano-convex lenses, The excimerpumped dye laser gave estimated focal spot areas of 6 × 10-5 cm 2 and 4 x 10-a cm 2, respectively, with the use of 25 cm and 70 cm focal length lenses. The frequency doubled Nd-YAG pumped dye laser was used with 25 cm and 40 cm focal length lenses to provide estimated focal spot areas of 5 × 10-5 cm2 and 1 × 10-4 cm 2, respectively. Laser induced fluorescence, collected at right angles to the laser beam by an 11 cm focus pianoconvex lens, was focused through a 1.5 mm square aperture onto the photocathode of an RCA 1P28 photomultiplier tube. A narrow band interference filter was used to isolate fluorescence from the (v',v") = (1,1) vibronic band of the OH A2~ ÷ ~--X2II system in response to laser scanning of the (1,0) vibronic band. The PMT output was amplified with a Tektronix 7904 oscilloscope and sampled with the SRS-250 boxcar. Individual data points of LIF profiles taken at a fixed laser wavelength are averages of 100 laser shots.
Experimental Results REMPI Spectra for HCO and CH3: The variation in the electron charge collected on the ionization probe was recorded for short scans
1423
PROFILES OF H C O AND CH3
Insulator
B
fat r~ v ~ ,o8o o. 31
FlowAxis------... IonizationProbe~
/
/
~
~
~
_P atnum
IJ
~..,,j-PlatinurnAnode
~
~'LaserFocus
+~Laser Focus
I
~....JVertical Translation Stage
FIG. 1. Schematic diagram of the b u r n e r housing assembly. Two different ionization probe configurations A and B were used; the probes were inserted, with axes inclined at 50 ° to the vertical flow axis, through an access port in the stainless steel flame housing. Probe A is described in reference 26. Probe B consisted of a platinum wire anode of 0.5 m m diameter enclosed within a 1.5 mm O.D. ceramic insulator. A 7.5 mm length of the anode wire, terminated in a 0.8 mm diameter beaded tip, was exposed to the flame and aligned vertically with the flow direction. The grounded b u r n e r head was the cathode for both probe configurations. of the dye laser wavelength near 397 nm for HCO and near 333 nm for CH3, respectively, to yield the REMPI spectra of Figs. 2 and 3. These are (2 + 1) REMPI spectra that arise from two-photon resonant excitations of Rydberg states followed by single-photon ionization. The relative amplitude of the ionization signal (electron charge) is plotted against the energy of the two-photon resonant excitation step. The prominent feature which appears at 50328 cm -1 (a dye laser wavelength of 397.4 nm) in the HCO spectrum of Fig. 2 is a P-branch head 2z of the (K',K") = (3,1) subband of the (060) ~-- (000) 3pzlI(A") ~ :~2II(A') vibronie transition of HCO. 18,19 Measurements of flame profiles of HCO were performed with the dye laser tuned to the peak of this P-branch head. The Q-branch head of the 3pZA'~ *-:~2A~ band of CH3, shown in Fig. 3 at a 59,970 em -1 two-photon energy corresponding to a dye laser wavelength of 333.5 nm, was convenient for flame profile measurements of CH3, as shown by previous investigators. 1°A2
Density Profiles of HCO and CH3: Measurements of spatial profiles of the relative densities of flame species require that the ioniza-
~60 5 ~50 ~, ~ 120 ~ 90 ._~ t~ 60 g =~ .~N 30 050240
50300 50360 50420 50480 50540 50600 2 PhotonEnergy(era-1)
FIG. 2. The REMPI spectrum of HCO near 397 nm. The (K',K") = (3,1) and (1,1) subbands of the (0,6,0) *-- (0,0,0) 3p21](A ") ~-- :~2II(A') vibronic transition 1sJ9 are illustrated. This spectrum was taken for the stoichiometric CH4/Oz/Ar flame. Excimer pumped dye laser pulses of 2.5 mJ energy and 5 ns duration with a focal spot area of 4 × l0 4 cm 2 were used. The ionization signal, with the probe (configuration A of Fig. 1) at 120 V, is plotted as a function of the two-photon resonant excitation energy.
1424
LAMINAR FLAMES: FLAME STRUCTURE
36
24 35
b~
g
12
g o
,59820
59880
59940 2 Photon
60000
80060
60120
60180
transition near 322.4 nm. The probe bias curve exhibited a well defined plateau as shown in the inset of Fig. 5. A probe bias voltage of 140 V was selected for the probe response measurements. The data of Fig, 5 are the electron charge collected by the probe as a function of the distance between the burner head and the laser focus. The focus was fixed at a position within 0.5 mm of the anode surface while the burner-probe separation was varied. As it happens, the data of Fig. 5 show that the probe response is nearly independent of the separation of the probe and burner head when the laser focus is kept at a fixed distance (---0.5 mm) from the probe
Energy (cm -t) 2
FIG. 3. The REMPI spectrum of CH3 near 333 mn showing the Q-branch head of the 3p2A~ ~ XZAg band, for the stoichiometric C2H4/Oz/Ar flame, taken with a Nd-YAG pumped dye laser energy of 5 mJ focused to 5 × 10 -2 cm 2. A 150 V probe (configuration A of Fig. 1) bias was used. tion probe be operated at a bias voltage corresponding to saturated collection of the REMPI electrons associated with the species of interest. 24 The appropriate voltage is determined from observations of the variation in ionization signal with probe voltage. These variations in probe response (for the probe of inset A of Fig. 1) with probe voltage are graphed in Figs. 4a and 4b for CH3 ionized at 333.5 nm and HCO ionized at 397.4 nm, respectively. Both curves exhibit a well defined plateau region of nearly constant probe response, where saturated electron collection occurs, followed by a "proportional counting" region where the ionization signal increases quasi-linearly with probe voltage. 24 The probe bias curves of Fig. 4 were found to be independent of burner-probe separation (see below) and were the same for both methane and ethylene flames. The question arises in the use of the ionization probe for density profile measurements as to the possible variation in probe response which may result from variation in the electric field distribution as the separation is changed between the positively charged probe anode and the grounded burner head. Such a variation would be of purely electrostatic origin determined exclusively by the probe-burner geometry and the separation of the laser focus from the probe, assuming that ionization levels within the flame are insuflqcient to significantly influence the electric field distribution, z6 The probe (configuration A of Fig. 1) response as a function of burner-probe separation was determined with a series of measurements in which the cold burner housing was filled with 100 Torr of air containing 200 ppm of added NO. The laser was tuned for (2 + 1) REMPI ionization of NO via a P-branch head of the FZA ~-- XZlI (0,0) vibronic
(3 2
0 C
2
.N_
i
L
b
i
A
80
120
160
200
240
I
I
I
i 160
h 200
i 240
,
b5 c ~ O
i
40
L
280
320
i 280
320
(b) HCO .=,, 5
,o2 5
2 0
i 40
i0 8
i 120
Bios Voltoge (volts) FIC, 4. Operating characteristics of the ionization probe (configuration A of Fig. 1) for the REMPI detection of (a) CHa and (b) HCO in the methane flame. The ionization probe signal (collected electron charge) is plotted as a function of the probe bias voltage. Saturated collection of REMPI electrons from CH3 (a) occurs on the plateau region extending from about 70-190 V. The plateau for the detection of REMPI electrons from HCO (b) extends from about 80-150 V. Similar operating characteristics were observed for HCO and CH3 detection in the ethylene flame. The data for (a) CH3 were taken at a burner-probe separation of 1.8 mm with a Nd-YAG pumped dye laser energy of 5 mJ and a focal area of 5 × 10 -5 cm2; the data for (b) HCO were taken at a burner-probe separation of 1.6 mm with an excimer-pumped dye laser energy of 6 mJ and a focal area of 4 × 10-4 cm z.
PROFILES OF H C O AND CH3 1.2
1.0 N
,8', p•
•
o e e , e
0.8
e .
•
1@
0.6
o ~04
0.4
c o
0.2
0.0
0
~ 160 240 Bios~ltoge(volts) *
I
4
8
12
Distonce
320 i
16 •bore
20
24
Burner
(ram)
28
32
Fic. 5. Ionization signals (collected REMPI electrons) for various separations between the burner head and the laser focus. These data were taken with probe configuration A of Fig. 1 at a constant probe bias voltage of 140 V chosen to lie on the plateau for saturated electron collection: see the probe bias curve shown in the inset. These data are for the 322.4 nm REMPI ionization of NO in a room temperature mixture of 200 ppm NO in air introduced within the cold burner housing; the frequency-doubled Nd-YAG pumped dye laser was operated with a fixed laser pulse energy of 1.8 mJ focused to an area of 5 x 10 -s cm 2. The invariance of the ionization signal with changes in the separation of the movable burner head from the fixed positions of the laser focus and the ionization probe indicates that corrections to the probe response arising from changes in the electric field distribution between the grounded burner head and the ionization probe anode are negligible. anode. It was also established that the shape of the probe bias curve (inset of Fig. 5) was independent of burner-probe separation. The results of Fig, 5 indicate that species density profiles can be inferred from measurements of ionization signals with negligible corrections for inherent changes in probe response with burner-probe separation. There may well be, however, important factors associated with the flame environment itself to consider in relating probe signals to species densities. Until questions are fully addressed concerning the possible effects of catalytic reactions at the probe surface, of laserinduced resonant and nonresonant background ionization, of electron attachment by electronegative species, and of local flame perturbations introduced by the probe, the quantitative accuracy of species profiles inferred from REMPI measurements must rema/n in doubt.
LIF Spectra of OH: Relative density profiles for OH were obtained from fluorescence excitation spectra for comparison
1425
with the HCO and CH3 profiles determined with the REMPI method. Observations were made of fluorescence from the (v',v") = (1,1) vibronic band of the A2~ + ~-- XZlI system, passed by a filter centered at 313 nm with a 12 nm F W H M bandwidth, while the frequency-doubled dye laser was scanned over a region from 281 to 284 nm of the (1,0) vibronic band. Spatial profiles within the methane flame were obtained for the fluorescence excited by absorption on several rotational transitions of the OH (1,0) vibronic band. Data for a 30 Torr CH4/O2/Ar flame with flow rates (slm) of 0.75/1,5/0.6 (cold flow velocity = 46 cm/s) are plotted in Fig. 6. The profiles for the individual transitions have been norm a l i z e d to the s a m e scale, As e x p e c t e d , no systematic differences between the profiles for differing rotational states occur in the nearly isothermal postflame region where the largest densities of OH are found.
Species Profiles for the CH4/O,~/Ar Flame: The relative variations with height above the burner head of the densities of CH3, HCO, and OH, inferred from the REMPI and LIF measurements, are presented in Fig. 7 for the flame conditions of Fig. 6. Each profile is normalized to the maximum species density since the present measurements do not yield absolute densities. Careful alignment and focusing of the laser beam enabled useful data to be obtained with both techniques to within about 0.3 mm of the burner head without t.2
~ 1.0
o~ ~ 0.8 H ~ 0.6 c~ ~ 0.4
!
o!
0
I. o R,(4) Q(9) + Q~(2) Q,C5
c~ z 0.2 o 0.0
o ~) . Q,(3)
10 Distonce
20 •boy•
30
40
50
E3urner ( r a m )
FIG. 6. Profiles of OH fluorescence intensity as a function of the separation of the laser focus from the burner head for several rotational lines of the (v',v") = (1,0) vibronic band of the OH A ~ + ~-- X2II system. The frequency-doubled excimer-pumped dye laser was operated with coumarin 540 dye; the laser pulse energy near 281 nm was 0.5 mJ; the 5 ns laser pulses were focused to an area of 6 × 10 n cm
z.
LAMINAR FLAMES: FLAME STRUCTURE
1426
,o N = i
nm; a two-photon energy of 60,120 cm -1) to show that no important background contributions to the CH3 ionization signal (less than 2%) were present anywhere in the flame. A comparable check could not be carried out in the vicinity of the 397.4 nm HCO wavelength because the vibronic subbands of the 3p2II(A") ~-- X2FI(A') transitions of HCO ls'19 overlap to such an extent that it is probably impossible to separate background contributions from the REMPI spectra of HCO.
~H,/~co OH
0.8
2 '~ 0.4 ~
0.2 0.0
-
1
2
3
Distance
4
5
6
7
8
o b o v e Burner ( m m )
FIG. 7. Normalized species density profiles for the stoichiometric CH~/Oz/Ar flame inferred from the laser-based REMPI and LIF methods. The data have been normalized by the maximum value for each species; the absolute values of these maximum densities are not determined by the present measurements. The OH density profile shown is a least squares fit to the data points of Fig. 6. For CH3, the Nd-YAG pumped dye laser with 6.5 mJ pulses focused to 1 × 10 4 cm 2 was used; the probe voltage (configuration A of Fig. 1) was 150 V. For HCO, the excimer-pumped dye laser with 6 mJ pulses focused to 4 × 1O-4 cm 2 was used; the probe (configuration A of Fig. 1) voltage was 140 V. significant scattering of the laser by the burner. The burner was mounted on a micrometer stage which could be positioned with a precision of 0.05 mm. Each data point represents an average of 100 laser pulses. The HCO profile of Fig. 7 is seen to peak much later in the flame than the CH3 profile. The maximum HCO density occurs just below the lower boundary of the visible flame zone. Previous studies of premixed stoichiometric methane/oxygen flames at comparable pressures 27 indicate that the flame temperature varies rapidly in the preheat zone between the burner head and the luminous flame zone, reaching a temperature of about 1300-1500 K near the HCO density peak. The ionization signal at the 397.4 nm HCO excitation wavelength does not quite vanish above the visible flame zone; a residual signal, attributable to background ionization, of about 2% of the maximum HCO ionization signal is present above the luminous zone. 13 No residual signal above the visible flame zone was observed at the 333.5 nm CH3 excitation wavelength. The possible influence of nonresonant background ionization in flame regions with appreciable HCO and CH3 concentrations was also considered. For CH3, it was possible to select a nearby laser wavelength giving negligible CH3 ionization (332.7
A Comparison of HCO and CH3 Profiles for CH4/Oe and C~H4/02 Flames: A series of measurements was made in which the dye laser was tuned to monitor either the HCO or CH3 radical and the probe was given the appropriate bias voltage for saturated collection of REMPI electrons. REMPI ionization profiles were recorded for both CH4/Oz and C2H4/Oz flames with the dye laser and probe parameters fixed for CH3 detection (Fig. 8). The measurements were then repeated in the two flames with the laser and probe parameters appropriate for the HCO radical (Fig. 9). Care was taken to ensure that measurements for the two flames were performed in identical fashion so that the profiles in both flames could be compared for each radical. For these comparisons the flames were operated at lower pressures and higher flow velocities than for the data of Figs. 6 and 7 to ensure that most of the flame zone was well removed from the burner surface. (Examination of the CH3 profile of Fig. 7 suggests that the flame front may reside within the burner surface for the methane flame conditions of Figs. 6 and 7. 28) A flame pressure of 1.2
:•
1.0
o
CH4/02
c~ 0.8 '2
0.6 0.4
:~
0.2
(O 0.0 0
1
2
3 Distance
4
5
6
7
8
9
10
above Burner (ram)
FIG. 8. A comparison of the CH3 radical profiles obtained for stoichiometric C H d O J A r and C2H~/ O J A r flames. The frequency-doubled Nd-YAG pumped dye laser with 5.5 mJ pulse energy focused to 5 × 10 -5 cm 2 was used; the probe (configuration B of Fig. 1) was operated at 190 V.
PROFILES OF HCO AND CH3
t.2 .~ 1.0
. . Z. '. ~.
, o CH4,/02 .,
0.8 ::.., "~ 0.6 -~ 0.4 0 0.2 0.0
1
2
3 4 5 6 7 8 Distonce above Burner (rnm)
9
10
FIG. 9. A comparison of the HCO radical profiles obtained for stoichiometric CH4/OJAr and CzH4/ OJAr flames, An excimer pumped dye laser was used with a pulse energy of 3 mJ focused to 4 × 10-4 cmZ; the probe (configuration B of Fig. 1) voltage was 150 V. 18 Torr and flow rates (slm) of 0.75/1.5/3.5 (cold flow velocity = 156 cm/s) and 0.37/1.11/3.0 (cold flow velocity = 122 cm/s) were used, respectively, for the CH4/O2/Ar and C2H4/O2/Ar flames of Figs. 8 and 9. These flow conditions give equal (per carbon) fuel flow rates to facilitate comparisons of CH3 and HCO densities between the methane and ethylene flames. The CH3 radical profiles for the two flames are compared in Fig. 8. The CH3 density is highest in the methane flame. The maximum for the ethylene flame is reached 1.9 mm from the burner head, while the maximum CH 3 density for the methane flame is not reached until 2.9 mm from the burner head. Figure 9 illustrates the formyl radical profiles for the two flames. The two HCO profiles are similar in shape and magnitude. Like the CH3 profiles, the maximum HCO density is reached closest to the burner for the ethylene flame. The lower edge of the visible flame zone is located about 1.4 mm above the HCO peak, at about 5.2 mm and 5.5 mm, respectively, for the ethylene and methane flames. The HCO profiles peak much further from the burner than the corresponding CH3 profiles; the separations between the HCO and CH3 peaks are about 1.2 mm and 2.0 mm for the methane and ethylene flames, respectively. Some features of the profiles of Figs. 8 and 9 are in qualitative agreement with profiles previously obtained by molecular beam mass spectrometry for low pressure, premixed, stoichiometric methane/ oxygen and ethylene/oxygen flames. 27'29'3° Peeters and Vinckier29 observed that the CH3 profile peaked closer to the burner for the ethylene flame than for the methane flame; the peak CH3 density was high-
1427
est for the methane flame. The profiles for CH3 and HCO measured by Biordi and coworkersz7 for the methane/oxygen flame indicate that the CH3 profile peaks before the HCO profile. Discrepancies also exist between the present results and previous work. The ratio of peak CH 3 densities of the methane to the ethylene flame observed by Peeters and Vinckier, 29 adjusted to a common (per carbon) fuel flow rate, is about twice that of Fig. 8. The CH3 and HCO profiles of Biordi and coworkers27 peaked within the luminous flame zone, while our results exhibit peaks nearer the burner surface. The CH3 and HCO profiles of Figs. 7-9 were reproducible under varied experimental conditions which included changes in the probe bias voltage, in the laser pulse energy, in the laser spot size, and in the separation between the laser focus and the probe anode surface. The results were insensitive to the probe voltage, provided the selected voltages were adequate to ensure saturated charge collection; somewhat greater voltages gave comparable results. Laser pulse energies and laser spot sizes were selected to provide good sigual-to-noise (SNR) ratios for the spectral features of interest, but significantly different values also gave similar ionization profiles. In some experiments the separation between the laser focus and the probe anode was increased from the usual values of -<0.5 mm to as large as 2 mm with no apparent effect on the measured radical profiles to within 0.5 mm of the burner surface. We interpret this result to mean that local perturbations to the flame immediately in front of the 0.8 mm diameter probe anode have no important effect on the measured CH3 and HCO profiles.
OH Profiles: Profiles of OH density are presented in Fig. 10 for the methane and ethylene flames of Figs. 8 and 9. The OH profiles were inferred by monitoring the fluorescence excited by laser absorption on the Qt(5) transition of the (v'Y') = (1,0) vibronic band in the manner used for the data of Fig. 6. The laser energy was fixed at 2 mJ for the LIF measurements in both flames. The data for each flame of Fig. 10 have been normalized relative to the OH densities observed 10 mm above the burner surface. Within experimental error, the OH LIF signals for both flames were equal at this distance; the profiles appear to be identical for these flames.
Discussion
Several aspects, already mentioned, of the detection of REMPI electrons with ionization probes need further study at the present state of development of REMPI techniques for flame diagnostics.
1428
.~
LAMINAR FLAMES: FLAME STRUCTURE
o
1.0 •
•
o
o
o
o o
0.8 o 0.6
~o0 0.4
o
CH4/02
•
C2H4/0 2
8 8
0.2
0.0 1
2
3 Distonce
4
5
above
6 Burner
7
8
9
10
(ram)
FIG. 10. Profiles of normalized OH density (OH fluorescence intensity) as a function of the separation of the laser focus from the burner head with the laser tuned for absorption on the Q~(5) rotational line of the (v',v") = (1,0) vibronic band of the OH A2~+ <---X2II system. The frequency-doubled Nd-YAG pumped dye laser was operated with rhodamine 590 dye; the laser pulse energy near 281 nm was 2.0 mJ; the 5 ns laser pulses were focused to an area of 6 × 10-5 cmz. The methane and ethylene flame conditions were the same as for the data of Figs. 8 and 9 (see text).
with a laser pulse energy of 2.5 mJ at 397.4 nm; for CH3, with a pulse energy of 5 mJ at 333.5 nm, the SNR exceeded 103 at the density peaks. The reproducibility and good precision of these data may warrant comparisons of the qualitative features of the density profiles with existing flame models, even in the absence of absolute density calibrations. The significant lag in HCO formation with respect to CH3 formation for the two flames has been mentioned. The flames exhibit a second kinetic similarity; comparable amounts of HCO are present in the methane and ethylene flames with equal (per carbon) fuel flow rates. HCO is produced in the methane flame via CH3 and H2CO i n t e r m e d i a t e s through the welldocumented39 reactions CH3 + O ~ H2CO + H
(1)
and H2CO + R ~ HCO + RH, where R = O, H, and OH,
(2)
following CH3 production via H-atom abstraction from methane by the reactions CH4 + R ~ CH3 + RH, where R
If, as seems likely, these questions can be satisfactorily resolved, the two primary remaining limitations to the use of this method will be the lack of spectroscopic data for Rydberg states of combustion radicals and the need for accurate calibration procedures which yield quantitative measurements of absolute species densities. The need for absolute density data is illustrated by a dilemma faced by flame modelers in the assignment of rate constants and mechanisms for the description of the fate of HCO radicals in hydrocarbon combustion.31-33 Sensitivity analyses show that reactions consuming HCO have strikingly large effects on computed species profiles, rivaled in importance only by the well-studied reaction between CO and OH. 31 Absolute concentration measurements of HCO with conventional flame sampling and mass spectrometry have proved to be notoriously difficult;30 "34 - 36 available estimates of HCO densities in methane flames appear inconsistent with present estimates of HCO consumption rates. 31 33 This need for more reliable rate constants for reactions with HCO and better measurements of HCO flame profiles may be met with further development of the quantitative capabilities of REMPI spectroscopy applied to HCO. The excellent spatial resolution and high sensitivity of the REMPI method are exemplified by data for HCO and CH3 presented here. The SNR at the peaks of the density profiles for HCO exceeded 20
= O, H, and OH.
(3)
Thus for the methane flame Rxns. (1)-(3) provide an efficient route to HCO formation via intermediate formaldehyde; since HCO is formed with the consumption of CH3, the peaking of the HCO profile follows the decay of the CH3 profile in agreement with the data of Figs. 8 and 9. The kinetic similarities between the ethylene and methane flames suggest a comparably efficient mechanism for HCO formation in the ethylene flame. The product channels for the oxidation of C2H4 by O and OH have been much discussed. 37-44 The kinetics of the formation of CH3 and HCO in the ethylene flame is complicated by the low flame pressure, which lies within the falloff region for adduct stabilization, and by the wide variation of temperature in the preheat zone between the burner head and the luminous flame zone. Methyl radicals may be produced in the ethylene flame by the reactions 394°,45,46 C2H4 + OH ~ CH3 + HeCO
(4)
C.2H4 + O ~ CH3 + HCO
(5)
C2H4 + O ~ C2H30 + H
(6)
and
or
1429
PROFILES OF HCO AND CH3 followed by C2H30 ~ CH3 + CO
(7)
C2H30 + O ~ CH3 + CO2.
(8)
and
The branching ratio between Kxns. (5) and (6) has been controversial. Buss, et al. 37 found no evidence for the CH3 + HCO product channel of Rxn. (5) under single collision conditions. Other studies indicate that Rxn. (5), which requires an intersystem crossing between triplet and singlet energy surfaces, 37 does indeed occur under typical flame conditions. 39,40 The equal per carbon atom production of HCO for the methane and ethylene flames would be consistent with strong contributions from reactions (4) or (5) followed by the formaldehyde route of Rxns. (1) and (2). Figures 8 and 9 exhibit little evidence tbr the direct formation of HCO coincident with the formation of CH3 as a result of Rxn. (5) in the ethylene flame. The delayed peaking of HCO would be more consistent with reaction (4) which is known to occur at low temperatures 4z and has been included in recent ethylene flame models. 45'46 Several studies, 43 most recently those of Tully and coworkers, 44 have shown, however, that the addition reaction C2H4 + OH --~ C2H4OH*
(9)
followed by adduct stabilization C2H4OH* + M ~ C2H4OH + M
(10)
dominates other OH reaction channels [including reaction (4)] for temperatures below 600-700 K and pressures above a few Torr. At higher temperatures the H-atom abstraction C2H 4 + OH --~ C2H3 + H20
(11)
reaction is expected to predominate. 43-46 The studies of Baldwin and Walker4v and of Gutman and coworkers 48 indicate that the subsequent reaction of vinyl radicals with 02. C2H3 + 02 ~ H2CO + HCO
(12)
may be quite important under the present flame conditions. Reactions (11), (12) and (2) provide an efficient mechanism for HCO formation in the ethylene flame consistent with the equal per carbon atom formation of HCO observed for the methane and ethylene flames. The kinetics of CH3 and HCO formation in the preheat zone of the low pressure stoichiometric
ethylene flame is thus apparently more cvmplex than for the comparable methane flame. A comparison of the CH3 profiles of Fig. 8 and the HCO profiles of Fig. 9 with flame model calculations, for the present flame conditions, would be informative concerning the above uncertainties in the mechanisms of HCO production and consumption for the two flames. An effort is in progress in our laboratory to place the REMPI density measurements of HCO on an absolute scale with a calibration based on a measurement of laser absorption on the HCO A2II(A") ~ XzII(A') band system. Acknowledgments
We wish to thank John E. M. Goldsmith for providing the design for the fiat flame burner facility. Support was provided by the U.S. Army Research Office under Contract DAAL-87-K-0066 and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Grant DE-FG02-86-ER13508.
REFERENCES 1. MALLARD, W. G., MILLER, J. H., AND SMYTH, K. C.: J. Chem. Phys. 76, 3483 (1982). 2. ROCKNEY, B. H., COOL, T. A., AND GRANT, E. R.: Chem. Phys. Lett. 87, 141 (1982). 3. SMrrH, K. C. AND MALLARD, W. G.: J. Chem. Phys. 77, 1779 (1982). 4. GOLDSMITH,J. E. M.: Opt. Lett. 7, 437 (1982). 5. GOLDSMITH,J. E. M.: J. Chem. Phys. 78, 1610 (1983). 6. TJOSSEM,P. J. H. AND COOL, T. A.: Chem. Phys. Lett. 100, 479 (1983). 7. MALLARD,W. G., MILLER, J. H. AND SMYTH, K. C.: J. Chem. Phys. 79, 5900 (1983). 8. TJOSSEM, P. J, H. AND COOL, T. A.: Twentieth Symposium (International) on Combustion, p. 1321, The Combustion Institute, 1985. 9. GOLDSMITH, J. E. M.: Twentieth Symposium (International) on Combustion, p. 1331. The Combustion Institute, 1985. 10. SMYTH, K. C. AND TAYLOR, P. H.: Chem. Phys. Lett. 122, 518 (1985). 11. TAYLOR, P. H., MALLARD, W. G. AND SMYTH, K, C.: J. Chem. Phys. 84, 1053 (1986). 12. MEIER, U. AND KOHSE-HOINGHAUS, K.: Chem. Phys. Lett. 142, 498 (1987). 13. BERNSTEIN, J. S., SONG, X.-M. AND COOL, T. A,: Chem. Phys. Lett. 145, 188 (1988). 14. TJOSSEM, P. J. H. AND SMYTn, K. C.: Chem. Phys. Lett. 144, 51 (1988). 15. DEGIUSEPPE, T. G., HUDGENS, J. W. AND LIN, M. C.: J. Phys. Chem. 86, 36 (1982); Chem.
1430
16. 17.
18. 19.
20.
LAMINAR FLAMES: FLAME STRUCTURE
Phys. Lett. 82, 267 (1981; J. C h e m . Phys. 76, 3338 (1982). DANON, J., ZACHARIAS, Iq[., ROTFKE, H. AND WELGE, K. H.: J. Chem. Phys. 76, 2399 (1982). HUDGENS, J. W., DIGuISEPPE, T. G. AND LIN, M. C.: j, C h e m . Phys. 79, 571 (1983). TJOSSEM, P. J. H., GOODWIN, P. M. AND COOL, T. A.: J. C h e m . Phys. 84, 5334 (1986). TJOSSEM, P. J. H., COOL, T. A., WEBB, D. A. AND GRANT, E. R.: J. C h e m . Phys. 88, 617 (1988). ROHLFING, E, A. AND CHANDLER, D. W.: "'Laser
Spectroscopy of Jet-Cooled Chlorinated Aromatic Hydrocarbons, "" Proceedings of the 1986 International Laser Science Conference, (1986), in press. 21. H(JDCENS, J. W.: Advances in Multi-photon Processes and Spectroscopy (S. H. LIN, Ed.), p. xxx, World Scientific, 1988, in press. 22. DULCEY, C. S. AND HUDGENS, J. E.: J. C h e m . Phys. 84, 5262 (1986); J. Phys. Chem. 87, 2296 (1983). 23. LONG, G. R., JOHNSON, R. D, AND HUDGENS, J. W.: J. Phys. Chem. 90, 4901 (1986). 24. CooL, T, A.: Appl, Optics 23, 1559 (1984). 25. The K" = 1 vibronic level of the ground state of H C O is subject to a large asymmetry splitring. l~ The P-branch head o b s e r v e d at 50328 cm -1 arises from transitions to the lower d levels of the K-doubled pairs; a second head located near 50290 em ~, for transitions to the u p p e r c levels, does not appear prominently in the spectra of Fig. 2. 26. HAVRILLA, G. J., SCHENCK, P. K., TRAVIS, J. C. AND TURK, G. C.: Anal. C h e m . 56, 186 (1984). 27. BIORDI, J. C.: Prog. E n e r g y Combust, Sci. 3, 151 (1977); BIORDI, J. C., LAZZARA, C. P. AND PAPP, J. F.: Fifteenth Symposium (International) on Combustion, p. 917, The C o m b u s tion Institute, 1975; BIORDI, J. C., LAZZARA, C. P. AND PAPP, J. F.: Fourteenth Symposium (International) on Combustion, p. 367, The Combustion Institute, 1973. 28. W e are i n d e b t e d to one of the anonymous reviewers of this paper for pointing out this possibility to us. 29. PEETERS, J. AND VINCKIER, C.: Fifteenth Symposium (International) on Combustion, p. 969, The Combustion Institute, 1975.
30. HENNESSY, R. J., PEACOCK, S. J. AND SMITH, D, B.: Comb. F l a m e 58, 73 (1984). 31. OLSSON, J. O. AND ANDERSSON, L. L.: Comb. Flame 67, 99 (1987). 32. COFFEE, T. P.: Comb. Flame 55, 161 (1984). 33. BECHTEL, J. H., BLINT, R. J., DASCH, C. J. AND WEINBERGER, D. A.: C o m b . F l a m e 42, 197 (1981). 34. PEETERS, J. AND MAHNEN, G.: Fourteenth Symposium (International) on Combustion, p. 133, The Combustion Institute, 1973. 35. HARVEY, R. AND MACCOLL, A.: Seventeenth
Symposium (International) 36,
37.
38.
39.
40. 41. 42.
on Combustion, p.
857, The Combustion Institute, 1979. VANDOOREN, J., OLDENHOVE DE GUERTECH1N, L. AND VAN TIGGELEN,, P. J.: Comb. Flame 64, 127 (1986). Buss, R. J., BASEMAN, R. J., GUOZHONG, H. AND LEE, Y. T.: J. Photochem. 17, 389 (1981). CLEMO, A. R., DUNCAN, G. L. AND GraCE, R.: J. Chem. Soc., Faraday Trans. 2, 78, 1231 (1982). WARNATZ, J.: Combustion Chemistry (W. C. GARDINER, JR., Ed.), Chpt. 5, Springer-Verlag, 1984. HUCKNALL, D. J.: Chemistry of Hydrocarbon Combustion, C h a p m a n and Hall, 1985. MILLER, J. A. AND F1SK, G. A.: C h e m . & Engr, News 65, 22, Aug. 31, 1987. BARTELS, n . , HOYERMANN, K. AND SIEVERT, R.:
Nineteenth Symposium (International) on Combustion, p. 61, Tile Combustion Institute, 1983. 43. LIU, A.-D., MULAC, W. A. AND JONAH, C. D.: Int. J. C h e m . Kinet. 19, 25 (1987), and references therein, 44, TULLY, F. P. : Chem. Phys. Lett. 96, 148 (1983); Chem. Phys. Lett. 143, 510 (1988). 45. WESTBROOK, C. K., DRYER, F. L. AND SCHUG,
K. F.: Nineteenth Symposium (International) on Combustion, p. 153, The C o m b u s t i o n Institute, 1983. 46. WESTBROOK, C. K., DRYER, F. L. AND SCHUG, K. P.: Comb. Flame 52, 299 (1983). 47. BALDWIN, R. R., AND WALKER, R. W.: Eigh-
teenth Symposium (International) on Combustion, p. 819, The Combustion Institute, 1980. 48. SLAGLE, I. R,, PARK, J.-Y,, HEAVEN, M. C., AND GUTMAN, D.: J. Am. C h e m . Soc. 106, 4356 (1984).
PROFILES OF HCO AND CH3
1431
COMMENTS K. Kuhse-Hfinghous, DFVLR, Fed. Rep. of Germany. Do you have any direct evidence in the relative CH3 profiles in the CH4 and the CzFI4 flames, that your collection efficiency does not depend in any way on the chemical composition, even if you are free from perturbations of the flame by either the probe or the laser?
Author's Reply. The chemical composition of the flame may indeed have an influence on electron collection efficiency if negative ions are formed by electron attachment to eleetronegative species. The presence of negative ion formation by electron attachment may be revealed by observation of the temporal profile of the REMPI electron signal) We saw no indications of negative ion formation in the flame preheat zone where the CH, and HCO profiles were measured.
REFERENCE 1. COOL, T. A.: Appl. Optics 23, 1559 (1984).
K. C. Smyth, National Bureau of Standards, USA. Hydrocarbon/oxygen flames exhibit high ionization levels, which can significantly influence the electric field distribution when making multiphoton ionization measurements. Thus, the detection sensitivity may vary as a function of electrode separation and distort relative profile data. To check this, it is necessary to make flame measurements in which ionization profiles are compared against another detection method, such as laser-induced fluorescence of mass spectrometry. The NO molecule would seem to be a good candidate for such studies.
Author's Reply. Variations in detection sensitivity caused by changes in the electric field distribution as a function of electrode separation must indeed be considered when interpreting REMPI profile measurements of species concentrations. Electric field variations of geometrical origin arising as the electrode separation is changed must be distinguished from variations caused by flame ionization. For the ionization probes we have used, the influence on detection sensitivity caused by changes in the electric field geometry is slight. This was determined through measurements of the probe response as a function of probe-burner separation when the cold burner housing was uniformly filled with air containing a trace (<0.1%) of NO. We have not yet investigated changes in probe response under actual flame conditions. NO is indeed a good candidate for studies in which REMPI flame profiles
are compared with LIF or mass spectrometric profiles. Another possibility would be the use of a chemically inert calibration species whose flame density profile could be calculated from measurem e n t s of the t e m p e r a t u r e profile, e.g. (3 + 1) REMPI measurements on N2 near 287 nm t. This latter approach would not be subject to uncertainties inherent in the LIF or mass spectrometric procedures.
REFERENCE 1. PRATT, S. T., DEHMER, P. M., and DEHMER, J. L.: J. Chem. Phys., 81, 3444 (1984).
D. R. Crosley, SRI International, USA. I assume you measure electrons, not the heavy ions. Why, then, do the signals vary differently with bias voltage, etc., between CH:3 and HCO? How does the electron remember where it came from? How does this affect your ability to calibrate the signals, using NO, at different points in the flame? Author's Reply. The small differences in the probe bias curves (the variation in ionization signal with probe voltage) between CH3 and HCO may be caused by the tendency for the effective volume from which electrons are drawn to increase with increases in probe voltage. Since the focal volume associated with laser-induced ionization decreases as the number of photons per molecule required for multiphoton ionization increases, the probe bias curves are sensitive to the dependence of the ionization on laser intensity. High bias voltages favor the collection of electrons produced by ionization processes which have a low order dependence on laser intensity; at low probe bias voltages, higher order processes (including nonresonant background ionization) are favored. Changes in lsaer intensity distributions near the probe will also cause changes in the probe bias curves. Markedly different focal geometries and spatial mode patterns existed between the CH3 and HCO cases; moreover, background ionization processes of high order in laser intensity will make different contributions at the differing laser intensities and wavelengths for the two cases. In practice the plateau regions of both probe bias curves overlap the range from about 80 to 150 volts so that a single probe voltage chosen within this range will serve for both CH3 and HCO. We anticipate no problems with REMPI measurements made for a calibration species, e.g. NO, provided that care is taken to maintain an appropriate probe voltage, determined for each species of in-
1432
LAMINAR FLAMES: FLAME STRUCTURE
terest with laser parameters appropriate for optimal detection of that species.
D. J. Seery, United Technologies Research Center, USA. Are your experimental conditions (stoiU. Meier, DFVLR, Fed. Rep. of Germany. Did you observe differences in the qualitative shapes of the CH3 profiles in the flame for the two different types of ion probes? This could be possible since for the type that uses the burner housing as ground electrode, the field and hence the detection efficiency may change as the burner is moved with respect to the anode.
Author's Reply. We have observed no qualitative differences in the CH3 and HCO profiles for the two different types of ion probes. Measurements of the detection eflqciency as a function of probe-burner separation were performed for both probes, Both probes had an essentially constant response, indep e n d e n t of p r o b e - b u r n e r separation. For t h e s e measurements the cold burner housing, filled with an air/NO mixture as described in the paper, were performed with a fixed separation between the laser focus and the probe surface of less than 0.5 ram.
chiometry, pressure, and temperature) close enough to those of previous workers (e.g., Biordi et al. or Hennessey et al.) to permit detailed comparisons?
Author's Reply. Comparisons of the CH3 and HCO profiles with the results of some previous workers is discussed in the paper. We were unaware of the results of Hennessy, et al. l which were still in press when this paper was prepared. The shape and position of the CH3 profile for the stoichiometric CH4/ O J A r flame at 18 Torr presented here is in excellent agreement with that given by Hennessy et al. for a similar 20 Torr flame.
REFERENCE
1. HENNESS't', R. J., ROBINSON, C., AND SMITH, D. B.: Twenty-first Symposium (International) on Combustion, p. 761, The Combustion Institute, 1988.
Combustion Institute, 1988/pp.
1421-1432
P R O F I L E S O F H C O A N D C H 3 IN CH4/O9~ A N D C~H4/O 2 F L A M E S BY R E S O N A N C E
IONIZATION
TERRILL A. COOL, JEFFREY S. BERNSTEIN, XIAO-MEI SONG AND PETER M. GOODWIN School of Applied and Engineering Physics Cornell University, Ithaca, New York 14853
Measurements are presented of the relative density profiles of formyl and methyl radicals in stoichiometric methane/oxygen and ethylene/oxygen flames which exemplify recent progress in the use of resonance-enhanced multiphoton ionization (REMPI) spectroscopy for flame diagnostics. The advantages and present limitations of this new laser-based technique are discussed. The high sensitivity of the REMPI method for the detection of HCO presents an opportunity for better measurements of flame profiles and of the rate constants for consumption of this important intermediate in hydrocarbon combustion.
Introduction Progress has been made in the implementation of resonance-enhanced muhiphoton ionization (REMPI) as a laser-based diagnostic method for monitoring the concentrations of combustion species at the part-per-million level. 1 14 The REMPI approach has a sensitivity rivaling that of the wello established laser induced fluorescence (LIF) method. Both techniques possess the sensitivity for in situ monitoring of trace concentrations of reaction intermediates, a capability not found in two other popular laser-based diagnostics: Raman scattering and coherent anti-Stokes Raman scattering (CARS). The REMPI and LIF methods are complementary since the REMPI approach is well suited for the detection of some weakly fluorescing species which cannot be easily monitored by LIF. Both techniques offer excellent spatial resolution and minimal perturbation of the combustion environment. Even though REMPI spectroscopy has provided a remarkable body of new spectroscopic data on previously unobserved electronic states of many molecules, the widespread application of REMPI for flame diagnostics is presently limited by a scarcity of spectroscopic data for Rydberg states of flame radicals. The CH315-17 and HCO 1s,19 radicals are examples of reaction intermediates of prime importance in hydrocarbon combustion for which the recent identification of new electronic states with REMPI spectroscopy now permits flame diagnostics to be performed with this sensitive and unique laserbased method. The growing list of species that have been detected in flames by REMPI includes: C, wA4 LI 4,6,9 t~ 5,9 /'ILl 14 p ¢ ~ 8 /~ 14 ~.r/~ 1,2 D¢~, 3 lj¢~/~, 13 ii, u, ~,~11, bu, ~-/2, ~-4x~, l~J, llbU,
C20, s CH3, l°'lz and C4H6.7,H The value of REMPI for the ultrasensitive detection of chlorinated hydrocarbons has been demonstrated, z° The REMPI
spectroscopy of transient radicals was recently reviewed. 21 The REMPI approach to flame species detection and monitoring, in its simplest form, consists of the collection of electrons on a thin (-
1421
1422
LAMINAR FLAMES: FLAME STRUCTURE
mechanisms built into models of hydrocarbon combustion. The REMPI method provides a useful laser-based alternative to conventional flame sampling and mass spectrometric techniques which have provided much of the experimental justification for existing flame models. The principal advantages of this new approach include a very high sensitivity, a potential for direct in situ density measurements with negligible perturbation of the flame environment, high spatial resolution, and a capacity for discrimination between isomers such as CH30 and CHzOH. z2'z3 These same advantages are possessed by LIF, the laser-based method of choice for strongly fluorescing species, but CH3 and HCO are examples of radicals which have not been detected in flames with the LIF technique. At this early stage of development there are also some disadvantages of the REMPI technique that need investigation. Present measurements provide only relative densities; the capability for direct, quantitative measurements of absolute species densities, exemplified for NO at 300 K, z4 has yet to be demonstrated for in situ flame measurements. Studies of the possible effects of catalytic reactions at the probe surface, of laser-induced resonant and nonresonant background ionization, of electron attachment by electronegative flame species, and of local flame perturbations introduced by the probe are urgently needed.
Experimental Apparatus A fiat flame burner with a porous stainless steel head (McKenna Products) with a 6 cm diameter flame zone and surrounding argon shroud flow was operated with premixed CH4/O2/Ar and C2H4/O2/ Ar stoichiometric mixtures at pressures ranging from 18 to 30 Torr. The gas flow rates were regulated with calibrated mass flow controllers (Tylan FC-280). The burner assembly was mounted on a micrometer-driven stage to permit precise vertical translation of the burner within a 15 cm diameter stainless steel housing. The burner head could thus be raised or lowered so that the position of intersection of a horizontal laser beam with the flame zone could be varied. The housing was equipped with optical ports and a feed-through port for the ionization probe, as is illustrated in Fig. 1. The platinum ionization probe used for most of this work, illustrated in inset A of Fig. 1, has been fully described elsewhere. 24 For the present studies the burner head itself was the grounded cathode surface; the "cathode" wire of the probe was removed from the circuit by connecting it to the anode potential. This arrangement gave probe bias characteristics with a better defined "plateau" for saturated charge collection in the present studies.
The ionization probe was angled at 50° to the vertical flow axis to facilitate positioning of the probe within the flow. Inset B of Fig. 1 illustrates a simpler probe geometry, used in the latter stages of this work, which also was used with the grounded burner head as the cathode. REMPI probe voltage pulses measured across a 150 kl~ leak resistor (connected between the anode and the positive bias voltage source)z4 were monitored by a boxcar (SRS250) with a gate, 1 Its wide, situated near the pulse peak. A frequency doubled Quanta-Ray PDL-2 tunable dye laser, pumped with a Quanta-Ray DCR-3 NdYAG laser, was used for most of the REMPI studies of CH3. This system provided 5 mJ pulses of 7 ns duration in the 333 nm wavelength region when operated with a DCM/LDS dye mixture. 1° Useful preliminary results for CH3 were also obtained with 5 ns pulses of 0.5-1.0 mJ energy near 333 nm supplied with an excimer(XeC1)-pumped frequencydoubled tunable dye laser [Lambda Physik EMG i01 and FL 2002 system] operated with DCM dye. REMPI detection of HCO was accomplished with the excimer-pumped dye laser system operated with PBBO dye to produce 3-5 mJ, 5 ns pulses near 397 nm. Both lasers were operated at a 10 Hz repetition rate. Data points taken at a fixed laser wavelength for determination of the probe bias characteristics and the spatial profiles of REMPI ionization were always an average of 100 laser shots. The laser beams were focused directly within the flame zone with single piano-convex lenses, The excimerpumped dye laser gave estimated focal spot areas of 6 × 10-5 cm 2 and 4 x 10-a cm 2, respectively, with the use of 25 cm and 70 cm focal length lenses. The frequency doubled Nd-YAG pumped dye laser was used with 25 cm and 40 cm focal length lenses to provide estimated focal spot areas of 5 × 10-5 cm2 and 1 × 10-4 cm 2, respectively. Laser induced fluorescence, collected at right angles to the laser beam by an 11 cm focus pianoconvex lens, was focused through a 1.5 mm square aperture onto the photocathode of an RCA 1P28 photomultiplier tube. A narrow band interference filter was used to isolate fluorescence from the (v',v") = (1,1) vibronic band of the OH A2~ ÷ ~--X2II system in response to laser scanning of the (1,0) vibronic band. The PMT output was amplified with a Tektronix 7904 oscilloscope and sampled with the SRS-250 boxcar. Individual data points of LIF profiles taken at a fixed laser wavelength are averages of 100 laser shots.
Experimental Results REMPI Spectra for HCO and CH3: The variation in the electron charge collected on the ionization probe was recorded for short scans
1423
PROFILES OF H C O AND CH3
Insulator
B
fat r~ v ~ ,o8o o. 31
FlowAxis------... IonizationProbe~
/
/
~
~
~
_P atnum
IJ
~..,,j-PlatinurnAnode
~
~'LaserFocus
+~Laser Focus
I
~....JVertical Translation Stage
FIG. 1. Schematic diagram of the b u r n e r housing assembly. Two different ionization probe configurations A and B were used; the probes were inserted, with axes inclined at 50 ° to the vertical flow axis, through an access port in the stainless steel flame housing. Probe A is described in reference 26. Probe B consisted of a platinum wire anode of 0.5 m m diameter enclosed within a 1.5 mm O.D. ceramic insulator. A 7.5 mm length of the anode wire, terminated in a 0.8 mm diameter beaded tip, was exposed to the flame and aligned vertically with the flow direction. The grounded b u r n e r head was the cathode for both probe configurations. of the dye laser wavelength near 397 nm for HCO and near 333 nm for CH3, respectively, to yield the REMPI spectra of Figs. 2 and 3. These are (2 + 1) REMPI spectra that arise from two-photon resonant excitations of Rydberg states followed by single-photon ionization. The relative amplitude of the ionization signal (electron charge) is plotted against the energy of the two-photon resonant excitation step. The prominent feature which appears at 50328 cm -1 (a dye laser wavelength of 397.4 nm) in the HCO spectrum of Fig. 2 is a P-branch head 2z of the (K',K") = (3,1) subband of the (060) ~-- (000) 3pzlI(A") ~ :~2II(A') vibronie transition of HCO. 18,19 Measurements of flame profiles of HCO were performed with the dye laser tuned to the peak of this P-branch head. The Q-branch head of the 3pZA'~ *-:~2A~ band of CH3, shown in Fig. 3 at a 59,970 em -1 two-photon energy corresponding to a dye laser wavelength of 333.5 nm, was convenient for flame profile measurements of CH3, as shown by previous investigators. 1°A2
Density Profiles of HCO and CH3: Measurements of spatial profiles of the relative densities of flame species require that the ioniza-
~60 5 ~50 ~, ~ 120 ~ 90 ._~ t~ 60 g =~ .~N 30 050240
50300 50360 50420 50480 50540 50600 2 PhotonEnergy(era-1)
FIG. 2. The REMPI spectrum of HCO near 397 nm. The (K',K") = (3,1) and (1,1) subbands of the (0,6,0) *-- (0,0,0) 3p21](A ") ~-- :~2II(A') vibronic transition 1sJ9 are illustrated. This spectrum was taken for the stoichiometric CH4/Oz/Ar flame. Excimer pumped dye laser pulses of 2.5 mJ energy and 5 ns duration with a focal spot area of 4 × l0 4 cm 2 were used. The ionization signal, with the probe (configuration A of Fig. 1) at 120 V, is plotted as a function of the two-photon resonant excitation energy.
1424
LAMINAR FLAMES: FLAME STRUCTURE
36
24 35
b~
g
12
g o
,59820
59880
59940 2 Photon
60000
80060
60120
60180
transition near 322.4 nm. The probe bias curve exhibited a well defined plateau as shown in the inset of Fig. 5. A probe bias voltage of 140 V was selected for the probe response measurements. The data of Fig, 5 are the electron charge collected by the probe as a function of the distance between the burner head and the laser focus. The focus was fixed at a position within 0.5 mm of the anode surface while the burner-probe separation was varied. As it happens, the data of Fig. 5 show that the probe response is nearly independent of the separation of the probe and burner head when the laser focus is kept at a fixed distance (---0.5 mm) from the probe
Energy (cm -t) 2
FIG. 3. The REMPI spectrum of CH3 near 333 mn showing the Q-branch head of the 3p2A~ ~ XZAg band, for the stoichiometric C2H4/Oz/Ar flame, taken with a Nd-YAG pumped dye laser energy of 5 mJ focused to 5 × 10 -2 cm 2. A 150 V probe (configuration A of Fig. 1) bias was used. tion probe be operated at a bias voltage corresponding to saturated collection of the REMPI electrons associated with the species of interest. 24 The appropriate voltage is determined from observations of the variation in ionization signal with probe voltage. These variations in probe response (for the probe of inset A of Fig. 1) with probe voltage are graphed in Figs. 4a and 4b for CH3 ionized at 333.5 nm and HCO ionized at 397.4 nm, respectively. Both curves exhibit a well defined plateau region of nearly constant probe response, where saturated electron collection occurs, followed by a "proportional counting" region where the ionization signal increases quasi-linearly with probe voltage. 24 The probe bias curves of Fig. 4 were found to be independent of burner-probe separation (see below) and were the same for both methane and ethylene flames. The question arises in the use of the ionization probe for density profile measurements as to the possible variation in probe response which may result from variation in the electric field distribution as the separation is changed between the positively charged probe anode and the grounded burner head. Such a variation would be of purely electrostatic origin determined exclusively by the probe-burner geometry and the separation of the laser focus from the probe, assuming that ionization levels within the flame are insuflqcient to significantly influence the electric field distribution, z6 The probe (configuration A of Fig. 1) response as a function of burner-probe separation was determined with a series of measurements in which the cold burner housing was filled with 100 Torr of air containing 200 ppm of added NO. The laser was tuned for (2 + 1) REMPI ionization of NO via a P-branch head of the FZA ~-- XZlI (0,0) vibronic
(3 2
0 C
2
.N_
i
L
b
i
A
80
120
160
200
240
I
I
I
i 160
h 200
i 240
,
b5 c ~ O
i
40
L
280
320
i 280
320
(b) HCO .=,, 5
,o2 5
2 0
i 40
i0 8
i 120
Bios Voltoge (volts) FIC, 4. Operating characteristics of the ionization probe (configuration A of Fig. 1) for the REMPI detection of (a) CHa and (b) HCO in the methane flame. The ionization probe signal (collected electron charge) is plotted as a function of the probe bias voltage. Saturated collection of REMPI electrons from CH3 (a) occurs on the plateau region extending from about 70-190 V. The plateau for the detection of REMPI electrons from HCO (b) extends from about 80-150 V. Similar operating characteristics were observed for HCO and CH3 detection in the ethylene flame. The data for (a) CH3 were taken at a burner-probe separation of 1.8 mm with a Nd-YAG pumped dye laser energy of 5 mJ and a focal area of 5 × 10 -5 cm2; the data for (b) HCO were taken at a burner-probe separation of 1.6 mm with an excimer-pumped dye laser energy of 6 mJ and a focal area of 4 × 10-4 cm z.
PROFILES OF H C O AND CH3 1.2
1.0 N
,8', p•
•
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0.8
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•
1@
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~ 160 240 Bios~ltoge(volts) *
I
4
8
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320 i
16 •bore
20
24
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(ram)
28
32
Fic. 5. Ionization signals (collected REMPI electrons) for various separations between the burner head and the laser focus. These data were taken with probe configuration A of Fig. 1 at a constant probe bias voltage of 140 V chosen to lie on the plateau for saturated electron collection: see the probe bias curve shown in the inset. These data are for the 322.4 nm REMPI ionization of NO in a room temperature mixture of 200 ppm NO in air introduced within the cold burner housing; the frequency-doubled Nd-YAG pumped dye laser was operated with a fixed laser pulse energy of 1.8 mJ focused to an area of 5 x 10 -s cm 2. The invariance of the ionization signal with changes in the separation of the movable burner head from the fixed positions of the laser focus and the ionization probe indicates that corrections to the probe response arising from changes in the electric field distribution between the grounded burner head and the ionization probe anode are negligible. anode. It was also established that the shape of the probe bias curve (inset of Fig. 5) was independent of burner-probe separation. The results of Fig, 5 indicate that species density profiles can be inferred from measurements of ionization signals with negligible corrections for inherent changes in probe response with burner-probe separation. There may well be, however, important factors associated with the flame environment itself to consider in relating probe signals to species densities. Until questions are fully addressed concerning the possible effects of catalytic reactions at the probe surface, of laserinduced resonant and nonresonant background ionization, of electron attachment by electronegative species, and of local flame perturbations introduced by the probe, the quantitative accuracy of species profiles inferred from REMPI measurements must rema/n in doubt.
LIF Spectra of OH: Relative density profiles for OH were obtained from fluorescence excitation spectra for comparison
1425
with the HCO and CH3 profiles determined with the REMPI method. Observations were made of fluorescence from the (v',v") = (1,1) vibronic band of the A2~ + ~-- XZlI system, passed by a filter centered at 313 nm with a 12 nm F W H M bandwidth, while the frequency-doubled dye laser was scanned over a region from 281 to 284 nm of the (1,0) vibronic band. Spatial profiles within the methane flame were obtained for the fluorescence excited by absorption on several rotational transitions of the OH (1,0) vibronic band. Data for a 30 Torr CH4/O2/Ar flame with flow rates (slm) of 0.75/1,5/0.6 (cold flow velocity = 46 cm/s) are plotted in Fig. 6. The profiles for the individual transitions have been norm a l i z e d to the s a m e scale, As e x p e c t e d , no systematic differences between the profiles for differing rotational states occur in the nearly isothermal postflame region where the largest densities of OH are found.
Species Profiles for the CH4/O,~/Ar Flame: The relative variations with height above the burner head of the densities of CH3, HCO, and OH, inferred from the REMPI and LIF measurements, are presented in Fig. 7 for the flame conditions of Fig. 6. Each profile is normalized to the maximum species density since the present measurements do not yield absolute densities. Careful alignment and focusing of the laser beam enabled useful data to be obtained with both techniques to within about 0.3 mm of the burner head without t.2
~ 1.0
o~ ~ 0.8 H ~ 0.6 c~ ~ 0.4
!
o!
0
I. o R,(4) Q(9) + Q~(2) Q,C5
c~ z 0.2 o 0.0
o ~) . Q,(3)
10 Distonce
20 •boy•
30
40
50
E3urner ( r a m )
FIG. 6. Profiles of OH fluorescence intensity as a function of the separation of the laser focus from the burner head for several rotational lines of the (v',v") = (1,0) vibronic band of the OH A ~ + ~-- X2II system. The frequency-doubled excimer-pumped dye laser was operated with coumarin 540 dye; the laser pulse energy near 281 nm was 0.5 mJ; the 5 ns laser pulses were focused to an area of 6 × 10 n cm
z.
LAMINAR FLAMES: FLAME STRUCTURE
1426
,o N = i
nm; a two-photon energy of 60,120 cm -1) to show that no important background contributions to the CH3 ionization signal (less than 2%) were present anywhere in the flame. A comparable check could not be carried out in the vicinity of the 397.4 nm HCO wavelength because the vibronic subbands of the 3p2II(A") ~-- X2FI(A') transitions of HCO ls'19 overlap to such an extent that it is probably impossible to separate background contributions from the REMPI spectra of HCO.
~H,/~co OH
0.8
2 '~ 0.4 ~
0.2 0.0
-
1
2
3
Distance
4
5
6
7
8
o b o v e Burner ( m m )
FIG. 7. Normalized species density profiles for the stoichiometric CH~/Oz/Ar flame inferred from the laser-based REMPI and LIF methods. The data have been normalized by the maximum value for each species; the absolute values of these maximum densities are not determined by the present measurements. The OH density profile shown is a least squares fit to the data points of Fig. 6. For CH3, the Nd-YAG pumped dye laser with 6.5 mJ pulses focused to 1 × 10 4 cm 2 was used; the probe voltage (configuration A of Fig. 1) was 150 V. For HCO, the excimer-pumped dye laser with 6 mJ pulses focused to 4 × 1O-4 cm 2 was used; the probe (configuration A of Fig. 1) voltage was 140 V. significant scattering of the laser by the burner. The burner was mounted on a micrometer stage which could be positioned with a precision of 0.05 mm. Each data point represents an average of 100 laser pulses. The HCO profile of Fig. 7 is seen to peak much later in the flame than the CH3 profile. The maximum HCO density occurs just below the lower boundary of the visible flame zone. Previous studies of premixed stoichiometric methane/oxygen flames at comparable pressures 27 indicate that the flame temperature varies rapidly in the preheat zone between the burner head and the luminous flame zone, reaching a temperature of about 1300-1500 K near the HCO density peak. The ionization signal at the 397.4 nm HCO excitation wavelength does not quite vanish above the visible flame zone; a residual signal, attributable to background ionization, of about 2% of the maximum HCO ionization signal is present above the luminous zone. 13 No residual signal above the visible flame zone was observed at the 333.5 nm CH3 excitation wavelength. The possible influence of nonresonant background ionization in flame regions with appreciable HCO and CH3 concentrations was also considered. For CH3, it was possible to select a nearby laser wavelength giving negligible CH3 ionization (332.7
A Comparison of HCO and CH3 Profiles for CH4/Oe and C~H4/02 Flames: A series of measurements was made in which the dye laser was tuned to monitor either the HCO or CH3 radical and the probe was given the appropriate bias voltage for saturated collection of REMPI electrons. REMPI ionization profiles were recorded for both CH4/Oz and C2H4/Oz flames with the dye laser and probe parameters fixed for CH3 detection (Fig. 8). The measurements were then repeated in the two flames with the laser and probe parameters appropriate for the HCO radical (Fig. 9). Care was taken to ensure that measurements for the two flames were performed in identical fashion so that the profiles in both flames could be compared for each radical. For these comparisons the flames were operated at lower pressures and higher flow velocities than for the data of Figs. 6 and 7 to ensure that most of the flame zone was well removed from the burner surface. (Examination of the CH3 profile of Fig. 7 suggests that the flame front may reside within the burner surface for the methane flame conditions of Figs. 6 and 7. 28) A flame pressure of 1.2
:•
1.0
o
CH4/02
c~ 0.8 '2
0.6 0.4
:~
0.2
(O 0.0 0
1
2
3 Distance
4
5
6
7
8
9
10
above Burner (ram)
FIG. 8. A comparison of the CH3 radical profiles obtained for stoichiometric C H d O J A r and C2H~/ O J A r flames. The frequency-doubled Nd-YAG pumped dye laser with 5.5 mJ pulse energy focused to 5 × 10 -5 cm 2 was used; the probe (configuration B of Fig. 1) was operated at 190 V.
PROFILES OF HCO AND CH3
t.2 .~ 1.0
. . Z. '. ~.
, o CH4,/02 .,
0.8 ::.., "~ 0.6 -~ 0.4 0 0.2 0.0
1
2
3 4 5 6 7 8 Distonce above Burner (rnm)
9
10
FIG. 9. A comparison of the HCO radical profiles obtained for stoichiometric CH4/OJAr and CzH4/ OJAr flames, An excimer pumped dye laser was used with a pulse energy of 3 mJ focused to 4 × 10-4 cmZ; the probe (configuration B of Fig. 1) voltage was 150 V. 18 Torr and flow rates (slm) of 0.75/1.5/3.5 (cold flow velocity = 156 cm/s) and 0.37/1.11/3.0 (cold flow velocity = 122 cm/s) were used, respectively, for the CH4/O2/Ar and C2H4/O2/Ar flames of Figs. 8 and 9. These flow conditions give equal (per carbon) fuel flow rates to facilitate comparisons of CH3 and HCO densities between the methane and ethylene flames. The CH3 radical profiles for the two flames are compared in Fig. 8. The CH3 density is highest in the methane flame. The maximum for the ethylene flame is reached 1.9 mm from the burner head, while the maximum CH 3 density for the methane flame is not reached until 2.9 mm from the burner head. Figure 9 illustrates the formyl radical profiles for the two flames. The two HCO profiles are similar in shape and magnitude. Like the CH3 profiles, the maximum HCO density is reached closest to the burner for the ethylene flame. The lower edge of the visible flame zone is located about 1.4 mm above the HCO peak, at about 5.2 mm and 5.5 mm, respectively, for the ethylene and methane flames. The HCO profiles peak much further from the burner than the corresponding CH3 profiles; the separations between the HCO and CH3 peaks are about 1.2 mm and 2.0 mm for the methane and ethylene flames, respectively. Some features of the profiles of Figs. 8 and 9 are in qualitative agreement with profiles previously obtained by molecular beam mass spectrometry for low pressure, premixed, stoichiometric methane/ oxygen and ethylene/oxygen flames. 27'29'3° Peeters and Vinckier29 observed that the CH3 profile peaked closer to the burner for the ethylene flame than for the methane flame; the peak CH3 density was high-
1427
est for the methane flame. The profiles for CH3 and HCO measured by Biordi and coworkersz7 for the methane/oxygen flame indicate that the CH3 profile peaks before the HCO profile. Discrepancies also exist between the present results and previous work. The ratio of peak CH 3 densities of the methane to the ethylene flame observed by Peeters and Vinckier, 29 adjusted to a common (per carbon) fuel flow rate, is about twice that of Fig. 8. The CH3 and HCO profiles of Biordi and coworkers27 peaked within the luminous flame zone, while our results exhibit peaks nearer the burner surface. The CH3 and HCO profiles of Figs. 7-9 were reproducible under varied experimental conditions which included changes in the probe bias voltage, in the laser pulse energy, in the laser spot size, and in the separation between the laser focus and the probe anode surface. The results were insensitive to the probe voltage, provided the selected voltages were adequate to ensure saturated charge collection; somewhat greater voltages gave comparable results. Laser pulse energies and laser spot sizes were selected to provide good sigual-to-noise (SNR) ratios for the spectral features of interest, but significantly different values also gave similar ionization profiles. In some experiments the separation between the laser focus and the probe anode was increased from the usual values of -<0.5 mm to as large as 2 mm with no apparent effect on the measured radical profiles to within 0.5 mm of the burner surface. We interpret this result to mean that local perturbations to the flame immediately in front of the 0.8 mm diameter probe anode have no important effect on the measured CH3 and HCO profiles.
OH Profiles: Profiles of OH density are presented in Fig. 10 for the methane and ethylene flames of Figs. 8 and 9. The OH profiles were inferred by monitoring the fluorescence excited by laser absorption on the Qt(5) transition of the (v'Y') = (1,0) vibronic band in the manner used for the data of Fig. 6. The laser energy was fixed at 2 mJ for the LIF measurements in both flames. The data for each flame of Fig. 10 have been normalized relative to the OH densities observed 10 mm above the burner surface. Within experimental error, the OH LIF signals for both flames were equal at this distance; the profiles appear to be identical for these flames.
Discussion
Several aspects, already mentioned, of the detection of REMPI electrons with ionization probes need further study at the present state of development of REMPI techniques for flame diagnostics.
1428
.~
LAMINAR FLAMES: FLAME STRUCTURE
o
1.0 •
•
o
o
o
o o
0.8 o 0.6
~o0 0.4
o
CH4/02
•
C2H4/0 2
8 8
0.2
0.0 1
2
3 Distonce
4
5
above
6 Burner
7
8
9
10
(ram)
FIG. 10. Profiles of normalized OH density (OH fluorescence intensity) as a function of the separation of the laser focus from the burner head with the laser tuned for absorption on the Q~(5) rotational line of the (v',v") = (1,0) vibronic band of the OH A2~+ <---X2II system. The frequency-doubled Nd-YAG pumped dye laser was operated with rhodamine 590 dye; the laser pulse energy near 281 nm was 2.0 mJ; the 5 ns laser pulses were focused to an area of 6 × 10-5 cmz. The methane and ethylene flame conditions were the same as for the data of Figs. 8 and 9 (see text).
with a laser pulse energy of 2.5 mJ at 397.4 nm; for CH3, with a pulse energy of 5 mJ at 333.5 nm, the SNR exceeded 103 at the density peaks. The reproducibility and good precision of these data may warrant comparisons of the qualitative features of the density profiles with existing flame models, even in the absence of absolute density calibrations. The significant lag in HCO formation with respect to CH3 formation for the two flames has been mentioned. The flames exhibit a second kinetic similarity; comparable amounts of HCO are present in the methane and ethylene flames with equal (per carbon) fuel flow rates. HCO is produced in the methane flame via CH3 and H2CO i n t e r m e d i a t e s through the welldocumented39 reactions CH3 + O ~ H2CO + H
(1)
and H2CO + R ~ HCO + RH, where R = O, H, and OH,
(2)
following CH3 production via H-atom abstraction from methane by the reactions CH4 + R ~ CH3 + RH, where R
If, as seems likely, these questions can be satisfactorily resolved, the two primary remaining limitations to the use of this method will be the lack of spectroscopic data for Rydberg states of combustion radicals and the need for accurate calibration procedures which yield quantitative measurements of absolute species densities. The need for absolute density data is illustrated by a dilemma faced by flame modelers in the assignment of rate constants and mechanisms for the description of the fate of HCO radicals in hydrocarbon combustion.31-33 Sensitivity analyses show that reactions consuming HCO have strikingly large effects on computed species profiles, rivaled in importance only by the well-studied reaction between CO and OH. 31 Absolute concentration measurements of HCO with conventional flame sampling and mass spectrometry have proved to be notoriously difficult;30 "34 - 36 available estimates of HCO densities in methane flames appear inconsistent with present estimates of HCO consumption rates. 31 33 This need for more reliable rate constants for reactions with HCO and better measurements of HCO flame profiles may be met with further development of the quantitative capabilities of REMPI spectroscopy applied to HCO. The excellent spatial resolution and high sensitivity of the REMPI method are exemplified by data for HCO and CH3 presented here. The SNR at the peaks of the density profiles for HCO exceeded 20
= O, H, and OH.
(3)
Thus for the methane flame Rxns. (1)-(3) provide an efficient route to HCO formation via intermediate formaldehyde; since HCO is formed with the consumption of CH3, the peaking of the HCO profile follows the decay of the CH3 profile in agreement with the data of Figs. 8 and 9. The kinetic similarities between the ethylene and methane flames suggest a comparably efficient mechanism for HCO formation in the ethylene flame. The product channels for the oxidation of C2H4 by O and OH have been much discussed. 37-44 The kinetics of the formation of CH3 and HCO in the ethylene flame is complicated by the low flame pressure, which lies within the falloff region for adduct stabilization, and by the wide variation of temperature in the preheat zone between the burner head and the luminous flame zone. Methyl radicals may be produced in the ethylene flame by the reactions 394°,45,46 C2H4 + OH ~ CH3 + HeCO
(4)
C.2H4 + O ~ CH3 + HCO
(5)
C2H4 + O ~ C2H30 + H
(6)
and
or
1429
PROFILES OF HCO AND CH3 followed by C2H30 ~ CH3 + CO
(7)
C2H30 + O ~ CH3 + CO2.
(8)
and
The branching ratio between Kxns. (5) and (6) has been controversial. Buss, et al. 37 found no evidence for the CH3 + HCO product channel of Rxn. (5) under single collision conditions. Other studies indicate that Rxn. (5), which requires an intersystem crossing between triplet and singlet energy surfaces, 37 does indeed occur under typical flame conditions. 39,40 The equal per carbon atom production of HCO for the methane and ethylene flames would be consistent with strong contributions from reactions (4) or (5) followed by the formaldehyde route of Rxns. (1) and (2). Figures 8 and 9 exhibit little evidence tbr the direct formation of HCO coincident with the formation of CH3 as a result of Rxn. (5) in the ethylene flame. The delayed peaking of HCO would be more consistent with reaction (4) which is known to occur at low temperatures 4z and has been included in recent ethylene flame models. 45'46 Several studies, 43 most recently those of Tully and coworkers, 44 have shown, however, that the addition reaction C2H4 + OH --~ C2H4OH*
(9)
followed by adduct stabilization C2H4OH* + M ~ C2H4OH + M
(10)
dominates other OH reaction channels [including reaction (4)] for temperatures below 600-700 K and pressures above a few Torr. At higher temperatures the H-atom abstraction C2H 4 + OH --~ C2H3 + H20
(11)
reaction is expected to predominate. 43-46 The studies of Baldwin and Walker4v and of Gutman and coworkers 48 indicate that the subsequent reaction of vinyl radicals with 02. C2H3 + 02 ~ H2CO + HCO
(12)
may be quite important under the present flame conditions. Reactions (11), (12) and (2) provide an efficient mechanism for HCO formation in the ethylene flame consistent with the equal per carbon atom formation of HCO observed for the methane and ethylene flames. The kinetics of CH3 and HCO formation in the preheat zone of the low pressure stoichiometric
ethylene flame is thus apparently more cvmplex than for the comparable methane flame. A comparison of the CH3 profiles of Fig. 8 and the HCO profiles of Fig. 9 with flame model calculations, for the present flame conditions, would be informative concerning the above uncertainties in the mechanisms of HCO production and consumption for the two flames. An effort is in progress in our laboratory to place the REMPI density measurements of HCO on an absolute scale with a calibration based on a measurement of laser absorption on the HCO A2II(A") ~ XzII(A') band system. Acknowledgments
We wish to thank John E. M. Goldsmith for providing the design for the fiat flame burner facility. Support was provided by the U.S. Army Research Office under Contract DAAL-87-K-0066 and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Grant DE-FG02-86-ER13508.
REFERENCES 1. MALLARD, W. G., MILLER, J. H., AND SMYTH, K. C.: J. Chem. Phys. 76, 3483 (1982). 2. ROCKNEY, B. H., COOL, T. A., AND GRANT, E. R.: Chem. Phys. Lett. 87, 141 (1982). 3. SMrrH, K. C. AND MALLARD, W. G.: J. Chem. Phys. 77, 1779 (1982). 4. GOLDSMITH,J. E. M.: Opt. Lett. 7, 437 (1982). 5. GOLDSMITH,J. E. M.: J. Chem. Phys. 78, 1610 (1983). 6. TJOSSEM,P. J. H. AND COOL, T. A.: Chem. Phys. Lett. 100, 479 (1983). 7. MALLARD,W. G., MILLER, J. H. AND SMYTH, K. C.: J. Chem. Phys. 79, 5900 (1983). 8. TJOSSEM, P. J, H. AND COOL, T. A.: Twentieth Symposium (International) on Combustion, p. 1321, The Combustion Institute, 1985. 9. GOLDSMITH, J. E. M.: Twentieth Symposium (International) on Combustion, p. 1331. The Combustion Institute, 1985. 10. SMYTH, K. C. AND TAYLOR, P. H.: Chem. Phys. Lett. 122, 518 (1985). 11. TAYLOR, P. H., MALLARD, W. G. AND SMYTH, K, C.: J. Chem. Phys. 84, 1053 (1986). 12. MEIER, U. AND KOHSE-HOINGHAUS, K.: Chem. Phys. Lett. 142, 498 (1987). 13. BERNSTEIN, J. S., SONG, X.-M. AND COOL, T. A,: Chem. Phys. Lett. 145, 188 (1988). 14. TJOSSEM, P. J. H. AND SMYTn, K. C.: Chem. Phys. Lett. 144, 51 (1988). 15. DEGIUSEPPE, T. G., HUDGENS, J. W. AND LIN, M. C.: J. Phys. Chem. 86, 36 (1982); Chem.
1430
16. 17.
18. 19.
20.
LAMINAR FLAMES: FLAME STRUCTURE
Phys. Lett. 82, 267 (1981; J. C h e m . Phys. 76, 3338 (1982). DANON, J., ZACHARIAS, Iq[., ROTFKE, H. AND WELGE, K. H.: J. Chem. Phys. 76, 2399 (1982). HUDGENS, J. W., DIGuISEPPE, T. G. AND LIN, M. C.: j, C h e m . Phys. 79, 571 (1983). TJOSSEM, P. J. H., GOODWIN, P. M. AND COOL, T. A.: J. C h e m . Phys. 84, 5334 (1986). TJOSSEM, P. J. H., COOL, T. A., WEBB, D. A. AND GRANT, E. R.: J. C h e m . Phys. 88, 617 (1988). ROHLFING, E, A. AND CHANDLER, D. W.: "'Laser
Spectroscopy of Jet-Cooled Chlorinated Aromatic Hydrocarbons, "" Proceedings of the 1986 International Laser Science Conference, (1986), in press. 21. H(JDCENS, J. W.: Advances in Multi-photon Processes and Spectroscopy (S. H. LIN, Ed.), p. xxx, World Scientific, 1988, in press. 22. DULCEY, C. S. AND HUDGENS, J. E.: J. C h e m . Phys. 84, 5262 (1986); J. Phys. Chem. 87, 2296 (1983). 23. LONG, G. R., JOHNSON, R. D, AND HUDGENS, J. W.: J. Phys. Chem. 90, 4901 (1986). 24. CooL, T, A.: Appl, Optics 23, 1559 (1984). 25. The K" = 1 vibronic level of the ground state of H C O is subject to a large asymmetry splitring. l~ The P-branch head o b s e r v e d at 50328 cm -1 arises from transitions to the lower d levels of the K-doubled pairs; a second head located near 50290 em ~, for transitions to the u p p e r c levels, does not appear prominently in the spectra of Fig. 2. 26. HAVRILLA, G. J., SCHENCK, P. K., TRAVIS, J. C. AND TURK, G. C.: Anal. C h e m . 56, 186 (1984). 27. BIORDI, J. C.: Prog. E n e r g y Combust, Sci. 3, 151 (1977); BIORDI, J. C., LAZZARA, C. P. AND PAPP, J. F.: Fifteenth Symposium (International) on Combustion, p. 917, The C o m b u s tion Institute, 1975; BIORDI, J. C., LAZZARA, C. P. AND PAPP, J. F.: Fourteenth Symposium (International) on Combustion, p. 367, The Combustion Institute, 1973. 28. W e are i n d e b t e d to one of the anonymous reviewers of this paper for pointing out this possibility to us. 29. PEETERS, J. AND VINCKIER, C.: Fifteenth Symposium (International) on Combustion, p. 969, The Combustion Institute, 1975.
30. HENNESSY, R. J., PEACOCK, S. J. AND SMITH, D, B.: Comb. F l a m e 58, 73 (1984). 31. OLSSON, J. O. AND ANDERSSON, L. L.: Comb. Flame 67, 99 (1987). 32. COFFEE, T. P.: Comb. Flame 55, 161 (1984). 33. BECHTEL, J. H., BLINT, R. J., DASCH, C. J. AND WEINBERGER, D. A.: C o m b . F l a m e 42, 197 (1981). 34. PEETERS, J. AND MAHNEN, G.: Fourteenth Symposium (International) on Combustion, p. 133, The Combustion Institute, 1973. 35. HARVEY, R. AND MACCOLL, A.: Seventeenth
Symposium (International) 36,
37.
38.
39.
40. 41. 42.
on Combustion, p.
857, The Combustion Institute, 1979. VANDOOREN, J., OLDENHOVE DE GUERTECH1N, L. AND VAN TIGGELEN,, P. J.: Comb. Flame 64, 127 (1986). Buss, R. J., BASEMAN, R. J., GUOZHONG, H. AND LEE, Y. T.: J. Photochem. 17, 389 (1981). CLEMO, A. R., DUNCAN, G. L. AND GraCE, R.: J. Chem. Soc., Faraday Trans. 2, 78, 1231 (1982). WARNATZ, J.: Combustion Chemistry (W. C. GARDINER, JR., Ed.), Chpt. 5, Springer-Verlag, 1984. HUCKNALL, D. J.: Chemistry of Hydrocarbon Combustion, C h a p m a n and Hall, 1985. MILLER, J. A. AND F1SK, G. A.: C h e m . & Engr, News 65, 22, Aug. 31, 1987. BARTELS, n . , HOYERMANN, K. AND SIEVERT, R.:
Nineteenth Symposium (International) on Combustion, p. 61, Tile Combustion Institute, 1983. 43. LIU, A.-D., MULAC, W. A. AND JONAH, C. D.: Int. J. C h e m . Kinet. 19, 25 (1987), and references therein, 44, TULLY, F. P. : Chem. Phys. Lett. 96, 148 (1983); Chem. Phys. Lett. 143, 510 (1988). 45. WESTBROOK, C. K., DRYER, F. L. AND SCHUG,
K. F.: Nineteenth Symposium (International) on Combustion, p. 153, The C o m b u s t i o n Institute, 1983. 46. WESTBROOK, C. K., DRYER, F. L. AND SCHUG, K. P.: Comb. Flame 52, 299 (1983). 47. BALDWIN, R. R., AND WALKER, R. W.: Eigh-
teenth Symposium (International) on Combustion, p. 819, The Combustion Institute, 1980. 48. SLAGLE, I. R,, PARK, J.-Y,, HEAVEN, M. C., AND GUTMAN, D.: J. Am. C h e m . Soc. 106, 4356 (1984).
PROFILES OF HCO AND CH3
1431
COMMENTS K. Kuhse-Hfinghous, DFVLR, Fed. Rep. of Germany. Do you have any direct evidence in the relative CH3 profiles in the CH4 and the CzFI4 flames, that your collection efficiency does not depend in any way on the chemical composition, even if you are free from perturbations of the flame by either the probe or the laser?
Author's Reply. The chemical composition of the flame may indeed have an influence on electron collection efficiency if negative ions are formed by electron attachment to eleetronegative species. The presence of negative ion formation by electron attachment may be revealed by observation of the temporal profile of the REMPI electron signal) We saw no indications of negative ion formation in the flame preheat zone where the CH, and HCO profiles were measured.
REFERENCE 1. COOL, T. A.: Appl. Optics 23, 1559 (1984).
K. C. Smyth, National Bureau of Standards, USA. Hydrocarbon/oxygen flames exhibit high ionization levels, which can significantly influence the electric field distribution when making multiphoton ionization measurements. Thus, the detection sensitivity may vary as a function of electrode separation and distort relative profile data. To check this, it is necessary to make flame measurements in which ionization profiles are compared against another detection method, such as laser-induced fluorescence of mass spectrometry. The NO molecule would seem to be a good candidate for such studies.
Author's Reply. Variations in detection sensitivity caused by changes in the electric field distribution as a function of electrode separation must indeed be considered when interpreting REMPI profile measurements of species concentrations. Electric field variations of geometrical origin arising as the electrode separation is changed must be distinguished from variations caused by flame ionization. For the ionization probes we have used, the influence on detection sensitivity caused by changes in the electric field geometry is slight. This was determined through measurements of the probe response as a function of probe-burner separation when the cold burner housing was uniformly filled with air containing a trace (<0.1%) of NO. We have not yet investigated changes in probe response under actual flame conditions. NO is indeed a good candidate for studies in which REMPI flame profiles
are compared with LIF or mass spectrometric profiles. Another possibility would be the use of a chemically inert calibration species whose flame density profile could be calculated from measurem e n t s of the t e m p e r a t u r e profile, e.g. (3 + 1) REMPI measurements on N2 near 287 nm t. This latter approach would not be subject to uncertainties inherent in the LIF or mass spectrometric procedures.
REFERENCE 1. PRATT, S. T., DEHMER, P. M., and DEHMER, J. L.: J. Chem. Phys., 81, 3444 (1984).
D. R. Crosley, SRI International, USA. I assume you measure electrons, not the heavy ions. Why, then, do the signals vary differently with bias voltage, etc., between CH:3 and HCO? How does the electron remember where it came from? How does this affect your ability to calibrate the signals, using NO, at different points in the flame? Author's Reply. The small differences in the probe bias curves (the variation in ionization signal with probe voltage) between CH3 and HCO may be caused by the tendency for the effective volume from which electrons are drawn to increase with increases in probe voltage. Since the focal volume associated with laser-induced ionization decreases as the number of photons per molecule required for multiphoton ionization increases, the probe bias curves are sensitive to the dependence of the ionization on laser intensity. High bias voltages favor the collection of electrons produced by ionization processes which have a low order dependence on laser intensity; at low probe bias voltages, higher order processes (including nonresonant background ionization) are favored. Changes in lsaer intensity distributions near the probe will also cause changes in the probe bias curves. Markedly different focal geometries and spatial mode patterns existed between the CH3 and HCO cases; moreover, background ionization processes of high order in laser intensity will make different contributions at the differing laser intensities and wavelengths for the two cases. In practice the plateau regions of both probe bias curves overlap the range from about 80 to 150 volts so that a single probe voltage chosen within this range will serve for both CH3 and HCO. We anticipate no problems with REMPI measurements made for a calibration species, e.g. NO, provided that care is taken to maintain an appropriate probe voltage, determined for each species of in-
1432
LAMINAR FLAMES: FLAME STRUCTURE
terest with laser parameters appropriate for optimal detection of that species.
D. J. Seery, United Technologies Research Center, USA. Are your experimental conditions (stoiU. Meier, DFVLR, Fed. Rep. of Germany. Did you observe differences in the qualitative shapes of the CH3 profiles in the flame for the two different types of ion probes? This could be possible since for the type that uses the burner housing as ground electrode, the field and hence the detection efficiency may change as the burner is moved with respect to the anode.
Author's Reply. We have observed no qualitative differences in the CH3 and HCO profiles for the two different types of ion probes. Measurements of the detection eflqciency as a function of probe-burner separation were performed for both probes, Both probes had an essentially constant response, indep e n d e n t of p r o b e - b u r n e r separation. For t h e s e measurements the cold burner housing, filled with an air/NO mixture as described in the paper, were performed with a fixed separation between the laser focus and the probe surface of less than 0.5 ram.
chiometry, pressure, and temperature) close enough to those of previous workers (e.g., Biordi et al. or Hennessey et al.) to permit detailed comparisons?
Author's Reply. Comparisons of the CH3 and HCO profiles with the results of some previous workers is discussed in the paper. We were unaware of the results of Hennessy, et al. l which were still in press when this paper was prepared. The shape and position of the CH3 profile for the stoichiometric CH4/ O J A r flame at 18 Torr presented here is in excellent agreement with that given by Hennessy et al. for a similar 20 Torr flame.
REFERENCE
1. HENNESS't', R. J., ROBINSON, C., AND SMITH, D. B.: Twenty-first Symposium (International) on Combustion, p. 761, The Combustion Institute, 1988.