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Collisional quenching of A 2Σ+ NO and A 2Δ CH in low pressure flames
Volume 178, number 5,6
CHEMICALPHYSICSLETTERS
5 April 1991
Collisional quenching of A2Ef NO and A 2A CH in low pressure flames Dwayne E. Heard, Jay B. Jeff’ries and David R. Crosley Molecular
Physics Laboratory, SRI International, Menlo Park, CA 94025, USA
Received 19 November 1990
Quenching of electronically excited CH and NO radicals in methane/air flames between 30 and 70 Tot-r has been studied using the time decay of laser-induced fluorescence. Rates of 0.3 and 1 lo’ Torr-’ were determined for A2ACH and A*E+ NO respectively at a temperature near 1700 K. These are compared with rates calculated using previously determined bimolecular quenching cross sections. The agreement is good, within 20 to 30%. The temperature dependence of Hz0 quenching of CH and ofN, and CO, quenching of NO need further investigation.
1. Introduction
Laser-induced fluorescence has proven to be a popular and useful tool for measuring the concentrations fo trace reactive species in flames. In order to relate observed emitted fluorescence signal to the desired ground state concentration, even on a relative basis at different positions in the flame, it is necessary to know the fluorescence quantum yield. For most situations of interest, this is determined by quenching of the electronically excited state due to collisions with the gases present at the point of measurement. In the case of the well-studied OH molecule, quenching cross sections have been measured for many collision partners of importance in combustion. Importantly, it was found that the cross sections decreased with increasing temperature (collision velocity), a fact attributed to the influence of attractive forces in the quenching process for this radical [ 11. A simple dynamical model of the quenching process [ 2 ] then permitted the extrapolation of cross sections for OH, and provided the expectation that, given the flame gas composition and temperature, OH quenching could be estimated to within some 30% in most cases [ 31. For other species, considerably less information is available. We here consider the diatomic molecules CH and NO, important in the chemistry of prompt 0009-26 14/9 l/$ 03.50 0 199 I
nitric oxide formation [4] in hydrocarbon/air flames. Quenching of CH by selected collision partners has been measured at room [ 5,6] and elevated temperature [ 71. Aspects of the temperature dependence of NO quenching have recently been investigated using a heated cell [ 8 1, a low pressure flame [9], and a shock tube [lo]. We report here direct decay lifetime measurements made on CH and NO in methane/air flames operated at 30 and 70 Torr. The quenching rates deduced from these results are compared with those calculated using previously determined bimolecular quenching cross sections; the differences are 20% for NO and 30% for CH. Likely reasons for the differences, and the remaining key questions for future measurement, are identified.
2. Experimental details and results Low pressure flames of methane were burned in air at pressures of 30 and 70 Torr. All were supported on a 6 cm diameter porous plug McKenna burner, located inside an evacuable chamber suitable for optical probing and described in more detail in ref. [ 111. The flame was slightly rich, with an equivalence ratio of 1.13, and run at 30 and 70 Torr for the quenching studies. A determination of flame profiles of these radicals and of OH has been made
- Elsevier Science Publishers B.V. (North-Holland )
533
Volume 178,number 5,6
to study prompt-NO chemistry in these flames [ 121. There, the NO concentration in the 30 Torr flame was determined as 7~ 10” molecules cm-3, or 4 ppm, calibrated using a measured flow of NO and the present quenching results. CH was not measured absolutely, although the peak value was calculated to be 8 ppm using a detailed model of the pertinent combustion chemistry [ 121 based on the Chemkin computer code [ 131. For comparison, the calculated peak OH and NO concentrations were 1.6 and 3 times the measured values, respectively, whereas calculated concentrations of major species should be much more accurate. The temperature at the point of measurement is required, both for determination of total density and mean velocity, and also as input to the combustion chemistry code. Rotational temperature profiles in this flame were determined [ 121 using laser-induced fluorescence in the (0,O) band of the OH radical, employing a method of direct spectrum fitting of a rotational excitation scan [ 141. Comparison of replicate measurements indicated a temperature precision of about 30 K at the measured value of 1700 K, i.e. about 2%. The decay measurements were made using fluorescence excited via the (0,O) band of the A*C+-X*H system of NO at 226 nm, and the (0,O) band of the A2A-X*H system of CH at 431 nm. Conveniently, the same laser dye could be used for both, with and without a BBO doubling crystal. The emission was collected at a right angle to the laser beam and detected using a Heath monochromator with a 250 (500) nm blaze grating and a Hamamatsu RG166 ( I P28) photomultiplier, respectively. A 100 MHz transient digitizer captured the signal into a CAMAC crate controlled by a laboratory computer. A typical decay trace for CH is shown in fig. I. All such decays were fit as single exponentials over the range 90% to 10% of maximum amplitude. The decay rate results for both radicals are given in table 1, including 1 o error bars from the tits. The replicate measurements agree to within the error bars. After subtracting the well-established radiative decay rate for NO of 4.55 us-‘, we obtain a total quenching rate for NO in the burnt gases at 30 Torr flame of 29 ps-‘, or a rate constant of 0.95 us-’ Torr-‘. The measurement in the same region of the 534
5 April 1991
CHEMICALPHYSICSLETTERS
11
5
0
100
200
300
400
500
TIME(m)
Fig. I. Semi-logarithmicplot of the decay ofthe fluorescencesignal from the v’=O level of the A*Astate of CH, taken at 1.OOcm height above the burner surface in the 30 Torr flame. Excitation is via the QZZd (6) line. The data are taken at 10ns resolution and averaged over 5000laser shots. The tit, indicated by the dashed line, is taken to a single falling exponential from 90 to 10%of the maximum amohtude.
70 Torr flame data yields a value of 1.04 us-’ Torr- ‘, in excellent agreement. The decay rate of CH was measured at three positions in the 30 Torr flame, at the peak and at both half-maximum positions, and found to be nearly the same. The 11.4 us-’ decay rate, after subtracting the radiative rate of 1.85 ns- ‘, corresponds to a quenching rate constant of 0.32 us-’ Torr-‘. In the 70 Torr flame, a value of 0.28 us-’ Torr-’ was obtained, again in excellent agreement.
3. Discussion The measured quenching rates may be compared with those expected from current knowledge of bimolecular quenching cross sections. The calculated compositions [ 121 of the 30 Torr flame at the temperatures of each measurement point were used for this purpose. As examples, mole fractions of those species producing a significant amount of quenching of NO at the temperatures of 1190 and 1680 K and CH at 1370 K are listed in table 2. The quenching cross sections co used for each radical are given to-
Volume 178, number j,6
CHEMICAL PHYSICS LETTERS
5 April 199 1
Table I Decay rate measurements and quenching results in the 30 Torr flame
NO
Height
Temperature
Decay rate
Measured Q
Calculated Q
(cm)
(K)
W’)
W’)
W’)
0.85
1190 1680
30 33
k2 t2
26 28
23 30
4.70
1490
33 32 34 34 33
&I ?I k2 ?I +I
29
34
0.81 1.00
1090 1370
1.29
1610
1.73
CH
11.4+0.5 11.3kO.l 11.5kO.l 12.0+0.1
9.5 9.5
12.3 12.7
10.2
12.4
Table 2 Selected calculated quenching rates at 30 Torr ‘) Compound
NO
CH
u
N2 QZ H* Hz0 co CO*
1190K
b) 20 < 0.0 I 34 3.3 75
1680 K
(i
X
Q
X
Q
0.67 0.09 0.04 0.1 I 0.05 0.02
0.3 5.4
0.67 0.01 0.03 0.17 0.05 0.06
1.0 0.6 co.1 17.6 0.4 10.1
‘) Cross section uin AZ, mole fraction X, quench rate Q in ps-‘.
gether with the quenching rates due to each collider at the given flame conditions. We first consider the choice of cross sections for A2X+ NO. For H20, the primary quencher, a value 0~~34 A2 was chosen. This value is considerably lower than the room temperature value [8] of 100 A*. The smaller cross section is that measured in an Hz/O2 flame [ 91 at 1300 K, where H20 was assumed to produce all the quenching. The same value was obtained at 750 K in a heated cell [ 81. For CO*, the other main quencher for NO, a,=75 A’ was taken from two studies at room temperature [ 15,161. The value 3.3 A: for CO is also at room temperature [ 171. Measurements at higher temperature are not available for either of these two colliders. Because CO2 is nearly as important a quencher as HzO,
1.4 2.7 3.6 9.6 8.3 2.1
1370K X
Q
0.67 0.06 0.04 0.13 0.06 0.02
3.6 0.6 I.2 5.2 1.9 0.2
b, See text.
knowledge of the dependence of its quenching cross section on temperature is essential. One might expect [ 1,8 ] that a cross section this large at 300 K will decrease with increasing temperature. The cross section for O2 is taken from ref. [ 8 1, where it was found to be independent of temperature between 300 and 750 K. Quenching due to Hz is probably negligible; an upper limit of 0.0 I A2 has been measured at room temperature [ 181. Quenching of A*C+ NO by N2 collider is of considerable interest and importance, as nitrogen is by far the majority gas in any air flame. N2 is known [ 8,16,18] to be an extremely inefficient quencher of A2Ef NO at 300 K, and at a temperature as high [ 81 as 750 K. Recent preliminary experiments in a shock tube [ lo], however, have indicated that N2 does 535
quench NO at temperatures as high as 3500 & with a cross section between 2 and 3 .A’.These values can be explained by a mechanism involving an energy barrier, leading to a cross section small at room temperature which increases with increasing temperature, This barrier could be rationalized in terms of an NO+N2 curve crossing lying above the neutral curves, leading to quenching [ 191. The ratio of rate coefficients at 3500 and 750 K can be lit to an Arrhenius dependence with an activation energy of 12 kcal and a frequency factor of 3.2~ lo-” cm3 s-‘. This fit corresponds to a cross section of 0.15 A’ at 1190 K and 0.6 A” at 1670 K. The quenching contribution of Nz is thus small compared to Hz0 and C02, but constitutes the next most important quencher in the burnt gases and will become even more important for experiments at yet higher temperatures. The total quenching rates, calculated using this set of cross sections, are given in table 1 for each measurement point. These may be compared with the measured rates; excellent agreement is seen, even with the change in density by 40% and the variations in composition over the temperature range studied. However, given the large calculated contribution by COZ, and the total lack of information on the temperature dependence of its the agreement must be regarded as tentative. NO quenching has also been measured in methane/air flames at atmospheric pressure, using picosecond laser excitation [20]. .4lthough this was a richer (equivalence ratio 1.92) and slightly hotter ( 1900 K) flame, it forms an interesting comparison. A quenching rate of670& 90 ps-’ was determined. Scaling for both pressure and temperature differences from our 30 Torr, 1700 K flame, we would expect 720 ps-‘, in excellent agreement. The present measured value for CH in the 30 Torr flame may also be compared with calculated values. The lack of significant variation of CH quenching rate with flame position was anticipated in a 1986 survey of quenching in the diatomic hydrides in flames [ 3 1. Note that this again involves a signiticant range of composition and density. In the case of CH A2A, cross sections at the elevated temperature of 1300 K [ 7 ] are available for most of the colliders and are given in the table. The conspicuous exception is the important collider HzO. oQ,
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5 April 199I
CHEMICALPHYSICSLETTERS
Volume178,number5,6
Its cross section has been measured at room temperature [ 61, a,= 9.6 A’. A measurement in a flame [ 111 furnished an upper limit of 13 A* at about 1500 K. Using the room temperature value of 9.6 A*, we calculate quenching rates in the 30 Torr flame of 12.4 ps-I, about 30% larger than the measured value of 9.5 (see table 1). For another polar collider, NH,, the a, at 1300 K is smaller than at 300 K by a factor of ~0.6 [6,7] presumably due to the influence of attractive dipole-dipole forces [ 11. For quenching of A*C+ OH, the ratio for NH, collider at these temperatures is also 0.6 [21] while that for Hz0 is 0.3 [22]. Thus, one expects for H20 quenching of CH also to be smaller at flame temperatures. If the ratio were 0.3 as for OH, one would calculate quenching rates of 7 to 9 ps-‘, slightly below the measured values. Altogether, measured and calculated quenching rates may be considered to be in very realistic agreement. A potentially significant quencher, hydrogen atoms, has been ignored in this discussion. These are known to cause significant quenching of A*C+ OH in Barnes [ 23 ] at 1200 K, with an estimated cross section 16 k 5 A’. Because of the high collision velocity of light hydrogen atoms, this corresponds to a large rate coefficient. If H atoms quench A*C+ NO with the same cross section in the present flame, the mole fraction of only 1.7% in the burnt gases would quench at a rate of 3 us-‘, the third largest contribution after Hz0 and COZ. If the same cross section held also for A26 CH, H atoms would again be the third most important quencher at higher temperatures. Clearly, the possibility of quenching by this species needs investigation. oQ
4. Conclusion Quenching rates have been measured in low pressure methane/air flames for electronically excited A*C+ NO and A2A CH. In each case: comparison with calculations using computed flame composition and previously measured bimolecular quenching cross sections showed very reasonable agreement. Differences between measurement and calculation indicate useful future measurements. These include further data on the quenching of NO by Nz and CO2
Volume 178, number 5,6
CHEMlCAL PHYSICS LETTERS
at high temperature, measurement of the temperature dependence of quenching of CH by H20, and considering the quenching of both species by H atoms at any temperature.
5 April 1991
[lo] U.E. Meier, G.A. Raiche, D.R. Crosley. G.P. Smithand D.J. Eckstrom, Western States Meeting of the Combustion Institute, La Jolla, California, October 1990. [ 1 I ] K.J. Rensberger, M.J. Dyer and R.A. Copeland, Appl. Opt. 27 (1988) 3679.
[ 121 D.E. Heard, J.B. Jeffries, G.P. Smith and D.R. Crosley, Acknowledgement We thank Gregory Smith for discussions of his flame model calculations. This work was supported by the Southern California Gas Company.
Wesrem States Meeting of the Combustion Institute, La Jolla, California.. October 1990; Combustion Flame, to be published. [ 131 R.A. Kee, J.A. Miller and T.A. Jefferson, Sandia National Laboratory Report SANDBO-8003 (1980) [ 141 K.J. Rensberger, J.B. Jeffries, R.A. Copeland, K. KohseHiiinghous, M.L. Wise and D.R. Crosley, Appl. Opt. 28 (1989) 3556.
[ 151 I.M. Campbell and R.S. Mason, J. Photochem. 8 (1978) 321.
References [I ] D.R. Crosley, J. Phys. Chem. 93 (1989) 6273. [21 P.W. Fairchild, G.P. Smith and D.R. Crosley, J. Chem. Phys. 79 (1983) 1795. [ 31 N.L. Garland and D.R. Crosley, Twenty-First Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1988) p. 1693. [41 C.P. Fenimore, Thirteenth Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 197 1) p. 373; J.A. Miller and CT. Bowman, Progr. Energy Combustion Sci. I5 (1989) 287. [ 51 C.J. Nokes and R.J. Donovan, Chem. Phys. 90 ( 1984) 167. [61 P. Heinrich, R.D. Kenner and F. Stuhl, Chem. Phys. Letters 147 (1988) 575. [71 N.L. Garland and D.R. Crosley, Chem. Phys. Letters 134 (1987) 189. [81 G.A. Raiche and D.R. Crosley, I. Chem. Phys. 92 ( 1990) 5211.
[ 161 I.S. McDermid and J.B. Laudenslager, J. Quant. Spectry. Radiative Transfer 27 (1982) 483. [ 171 A.B. Callear and M.J. Pilling, Trans. Faraday Sot. 66 ( 1970) 1618.
[ 181G.D. Greenblatt and A.R. Ravishankara,
Chem. Phys. Letters 136 (1987) 501. [ 191 M. Asscher and Y. Haas, J. Chem. Phys. 76 (1982) 2115. [20]R. Schwarzwald, P. Monkhouse and J. Wolfrum, Chem. Phys. Letters 158 (1989) 60. [21] J.B. Jeffries, R.A. Copeland and D.R. Crosley, J. Chem. Phys. 85 (1986) 1898. [22] R.A. Copeland, M.J. Dyer and D.R. Crosley, J. Chem. Phys. 82 (1985) 4022; G.P. Smith and D.R. Crosley, J. Chem. Phys. 85 (1986) 3896. [23]J.B. Jeffries, K. Kohse-Hoinghaus, G.P. Smith, R.A. Copeland and D.R. Crosley, Chem. Phys. Letters 152 (1988) 160.
[91 R.J. Cattolica, T.G. Matagaand J.A. Cavalowsky, J. Quant. Spectry. Radiative Transfer 42 (I 989) 499.
537
CHEMICALPHYSICSLETTERS
5 April 1991
Collisional quenching of A2Ef NO and A 2A CH in low pressure flames Dwayne E. Heard, Jay B. Jeff’ries and David R. Crosley Molecular
Physics Laboratory, SRI International, Menlo Park, CA 94025, USA
Received 19 November 1990
Quenching of electronically excited CH and NO radicals in methane/air flames between 30 and 70 Tot-r has been studied using the time decay of laser-induced fluorescence. Rates of 0.3 and 1 lo’ Torr-’ were determined for A2ACH and A*E+ NO respectively at a temperature near 1700 K. These are compared with rates calculated using previously determined bimolecular quenching cross sections. The agreement is good, within 20 to 30%. The temperature dependence of Hz0 quenching of CH and ofN, and CO, quenching of NO need further investigation.
1. Introduction
Laser-induced fluorescence has proven to be a popular and useful tool for measuring the concentrations fo trace reactive species in flames. In order to relate observed emitted fluorescence signal to the desired ground state concentration, even on a relative basis at different positions in the flame, it is necessary to know the fluorescence quantum yield. For most situations of interest, this is determined by quenching of the electronically excited state due to collisions with the gases present at the point of measurement. In the case of the well-studied OH molecule, quenching cross sections have been measured for many collision partners of importance in combustion. Importantly, it was found that the cross sections decreased with increasing temperature (collision velocity), a fact attributed to the influence of attractive forces in the quenching process for this radical [ 11. A simple dynamical model of the quenching process [ 2 ] then permitted the extrapolation of cross sections for OH, and provided the expectation that, given the flame gas composition and temperature, OH quenching could be estimated to within some 30% in most cases [ 31. For other species, considerably less information is available. We here consider the diatomic molecules CH and NO, important in the chemistry of prompt 0009-26 14/9 l/$ 03.50 0 199 I
nitric oxide formation [4] in hydrocarbon/air flames. Quenching of CH by selected collision partners has been measured at room [ 5,6] and elevated temperature [ 71. Aspects of the temperature dependence of NO quenching have recently been investigated using a heated cell [ 8 1, a low pressure flame [9], and a shock tube [lo]. We report here direct decay lifetime measurements made on CH and NO in methane/air flames operated at 30 and 70 Torr. The quenching rates deduced from these results are compared with those calculated using previously determined bimolecular quenching cross sections; the differences are 20% for NO and 30% for CH. Likely reasons for the differences, and the remaining key questions for future measurement, are identified.
2. Experimental details and results Low pressure flames of methane were burned in air at pressures of 30 and 70 Torr. All were supported on a 6 cm diameter porous plug McKenna burner, located inside an evacuable chamber suitable for optical probing and described in more detail in ref. [ 111. The flame was slightly rich, with an equivalence ratio of 1.13, and run at 30 and 70 Torr for the quenching studies. A determination of flame profiles of these radicals and of OH has been made
- Elsevier Science Publishers B.V. (North-Holland )
533
Volume 178,number 5,6
to study prompt-NO chemistry in these flames [ 121. There, the NO concentration in the 30 Torr flame was determined as 7~ 10” molecules cm-3, or 4 ppm, calibrated using a measured flow of NO and the present quenching results. CH was not measured absolutely, although the peak value was calculated to be 8 ppm using a detailed model of the pertinent combustion chemistry [ 121 based on the Chemkin computer code [ 131. For comparison, the calculated peak OH and NO concentrations were 1.6 and 3 times the measured values, respectively, whereas calculated concentrations of major species should be much more accurate. The temperature at the point of measurement is required, both for determination of total density and mean velocity, and also as input to the combustion chemistry code. Rotational temperature profiles in this flame were determined [ 121 using laser-induced fluorescence in the (0,O) band of the OH radical, employing a method of direct spectrum fitting of a rotational excitation scan [ 141. Comparison of replicate measurements indicated a temperature precision of about 30 K at the measured value of 1700 K, i.e. about 2%. The decay measurements were made using fluorescence excited via the (0,O) band of the A*C+-X*H system of NO at 226 nm, and the (0,O) band of the A2A-X*H system of CH at 431 nm. Conveniently, the same laser dye could be used for both, with and without a BBO doubling crystal. The emission was collected at a right angle to the laser beam and detected using a Heath monochromator with a 250 (500) nm blaze grating and a Hamamatsu RG166 ( I P28) photomultiplier, respectively. A 100 MHz transient digitizer captured the signal into a CAMAC crate controlled by a laboratory computer. A typical decay trace for CH is shown in fig. I. All such decays were fit as single exponentials over the range 90% to 10% of maximum amplitude. The decay rate results for both radicals are given in table 1, including 1 o error bars from the tits. The replicate measurements agree to within the error bars. After subtracting the well-established radiative decay rate for NO of 4.55 us-‘, we obtain a total quenching rate for NO in the burnt gases at 30 Torr flame of 29 ps-‘, or a rate constant of 0.95 us-’ Torr-‘. The measurement in the same region of the 534
5 April 1991
CHEMICALPHYSICSLETTERS
11
5
0
100
200
300
400
500
TIME(m)
Fig. I. Semi-logarithmicplot of the decay ofthe fluorescencesignal from the v’=O level of the A*Astate of CH, taken at 1.OOcm height above the burner surface in the 30 Torr flame. Excitation is via the QZZd (6) line. The data are taken at 10ns resolution and averaged over 5000laser shots. The tit, indicated by the dashed line, is taken to a single falling exponential from 90 to 10%of the maximum amohtude.
70 Torr flame data yields a value of 1.04 us-’ Torr- ‘, in excellent agreement. The decay rate of CH was measured at three positions in the 30 Torr flame, at the peak and at both half-maximum positions, and found to be nearly the same. The 11.4 us-’ decay rate, after subtracting the radiative rate of 1.85 ns- ‘, corresponds to a quenching rate constant of 0.32 us-’ Torr-‘. In the 70 Torr flame, a value of 0.28 us-’ Torr-’ was obtained, again in excellent agreement.
3. Discussion The measured quenching rates may be compared with those expected from current knowledge of bimolecular quenching cross sections. The calculated compositions [ 121 of the 30 Torr flame at the temperatures of each measurement point were used for this purpose. As examples, mole fractions of those species producing a significant amount of quenching of NO at the temperatures of 1190 and 1680 K and CH at 1370 K are listed in table 2. The quenching cross sections co used for each radical are given to-
Volume 178, number j,6
CHEMICAL PHYSICS LETTERS
5 April 199 1
Table I Decay rate measurements and quenching results in the 30 Torr flame
NO
Height
Temperature
Decay rate
Measured Q
Calculated Q
(cm)
(K)
W’)
W’)
W’)
0.85
1190 1680
30 33
k2 t2
26 28
23 30
4.70
1490
33 32 34 34 33
&I ?I k2 ?I +I
29
34
0.81 1.00
1090 1370
1.29
1610
1.73
CH
11.4+0.5 11.3kO.l 11.5kO.l 12.0+0.1
9.5 9.5
12.3 12.7
10.2
12.4
Table 2 Selected calculated quenching rates at 30 Torr ‘) Compound
NO
CH
u
N2 QZ H* Hz0 co CO*
1190K
b) 20 < 0.0 I 34 3.3 75
1680 K
(i
X
Q
X
Q
0.67 0.09 0.04 0.1 I 0.05 0.02
0.3 5.4
0.67 0.01 0.03 0.17 0.05 0.06
1.0 0.6 co.1 17.6 0.4 10.1
‘) Cross section uin AZ, mole fraction X, quench rate Q in ps-‘.
gether with the quenching rates due to each collider at the given flame conditions. We first consider the choice of cross sections for A2X+ NO. For H20, the primary quencher, a value 0~~34 A2 was chosen. This value is considerably lower than the room temperature value [8] of 100 A*. The smaller cross section is that measured in an Hz/O2 flame [ 91 at 1300 K, where H20 was assumed to produce all the quenching. The same value was obtained at 750 K in a heated cell [ 81. For CO*, the other main quencher for NO, a,=75 A’ was taken from two studies at room temperature [ 15,161. The value 3.3 A: for CO is also at room temperature [ 171. Measurements at higher temperature are not available for either of these two colliders. Because CO2 is nearly as important a quencher as HzO,
1.4 2.7 3.6 9.6 8.3 2.1
1370K X
Q
0.67 0.06 0.04 0.13 0.06 0.02
3.6 0.6 I.2 5.2 1.9 0.2
b, See text.
knowledge of the dependence of its quenching cross section on temperature is essential. One might expect [ 1,8 ] that a cross section this large at 300 K will decrease with increasing temperature. The cross section for O2 is taken from ref. [ 8 1, where it was found to be independent of temperature between 300 and 750 K. Quenching due to Hz is probably negligible; an upper limit of 0.0 I A2 has been measured at room temperature [ 181. Quenching of A*C+ NO by N2 collider is of considerable interest and importance, as nitrogen is by far the majority gas in any air flame. N2 is known [ 8,16,18] to be an extremely inefficient quencher of A2Ef NO at 300 K, and at a temperature as high [ 81 as 750 K. Recent preliminary experiments in a shock tube [ lo], however, have indicated that N2 does 535
quench NO at temperatures as high as 3500 & with a cross section between 2 and 3 .A’.These values can be explained by a mechanism involving an energy barrier, leading to a cross section small at room temperature which increases with increasing temperature, This barrier could be rationalized in terms of an NO+N2 curve crossing lying above the neutral curves, leading to quenching [ 191. The ratio of rate coefficients at 3500 and 750 K can be lit to an Arrhenius dependence with an activation energy of 12 kcal and a frequency factor of 3.2~ lo-” cm3 s-‘. This fit corresponds to a cross section of 0.15 A’ at 1190 K and 0.6 A” at 1670 K. The quenching contribution of Nz is thus small compared to Hz0 and C02, but constitutes the next most important quencher in the burnt gases and will become even more important for experiments at yet higher temperatures. The total quenching rates, calculated using this set of cross sections, are given in table 1 for each measurement point. These may be compared with the measured rates; excellent agreement is seen, even with the change in density by 40% and the variations in composition over the temperature range studied. However, given the large calculated contribution by COZ, and the total lack of information on the temperature dependence of its the agreement must be regarded as tentative. NO quenching has also been measured in methane/air flames at atmospheric pressure, using picosecond laser excitation [20]. .4lthough this was a richer (equivalence ratio 1.92) and slightly hotter ( 1900 K) flame, it forms an interesting comparison. A quenching rate of670& 90 ps-’ was determined. Scaling for both pressure and temperature differences from our 30 Torr, 1700 K flame, we would expect 720 ps-‘, in excellent agreement. The present measured value for CH in the 30 Torr flame may also be compared with calculated values. The lack of significant variation of CH quenching rate with flame position was anticipated in a 1986 survey of quenching in the diatomic hydrides in flames [ 3 1. Note that this again involves a signiticant range of composition and density. In the case of CH A2A, cross sections at the elevated temperature of 1300 K [ 7 ] are available for most of the colliders and are given in the table. The conspicuous exception is the important collider HzO. oQ,
536
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CHEMICALPHYSICSLETTERS
Volume178,number5,6
Its cross section has been measured at room temperature [ 61, a,= 9.6 A’. A measurement in a flame [ 111 furnished an upper limit of 13 A* at about 1500 K. Using the room temperature value of 9.6 A*, we calculate quenching rates in the 30 Torr flame of 12.4 ps-I, about 30% larger than the measured value of 9.5 (see table 1). For another polar collider, NH,, the a, at 1300 K is smaller than at 300 K by a factor of ~0.6 [6,7] presumably due to the influence of attractive dipole-dipole forces [ 11. For quenching of A*C+ OH, the ratio for NH, collider at these temperatures is also 0.6 [21] while that for Hz0 is 0.3 [22]. Thus, one expects for H20 quenching of CH also to be smaller at flame temperatures. If the ratio were 0.3 as for OH, one would calculate quenching rates of 7 to 9 ps-‘, slightly below the measured values. Altogether, measured and calculated quenching rates may be considered to be in very realistic agreement. A potentially significant quencher, hydrogen atoms, has been ignored in this discussion. These are known to cause significant quenching of A*C+ OH in Barnes [ 23 ] at 1200 K, with an estimated cross section 16 k 5 A’. Because of the high collision velocity of light hydrogen atoms, this corresponds to a large rate coefficient. If H atoms quench A*C+ NO with the same cross section in the present flame, the mole fraction of only 1.7% in the burnt gases would quench at a rate of 3 us-‘, the third largest contribution after Hz0 and COZ. If the same cross section held also for A26 CH, H atoms would again be the third most important quencher at higher temperatures. Clearly, the possibility of quenching by this species needs investigation. oQ
4. Conclusion Quenching rates have been measured in low pressure methane/air flames for electronically excited A*C+ NO and A2A CH. In each case: comparison with calculations using computed flame composition and previously measured bimolecular quenching cross sections showed very reasonable agreement. Differences between measurement and calculation indicate useful future measurements. These include further data on the quenching of NO by Nz and CO2
Volume 178, number 5,6
CHEMlCAL PHYSICS LETTERS
at high temperature, measurement of the temperature dependence of quenching of CH by H20, and considering the quenching of both species by H atoms at any temperature.
5 April 1991
[lo] U.E. Meier, G.A. Raiche, D.R. Crosley. G.P. Smithand D.J. Eckstrom, Western States Meeting of the Combustion Institute, La Jolla, California, October 1990. [ 1 I ] K.J. Rensberger, M.J. Dyer and R.A. Copeland, Appl. Opt. 27 (1988) 3679.
[ 121 D.E. Heard, J.B. Jeffries, G.P. Smith and D.R. Crosley, Acknowledgement We thank Gregory Smith for discussions of his flame model calculations. This work was supported by the Southern California Gas Company.
Wesrem States Meeting of the Combustion Institute, La Jolla, California.. October 1990; Combustion Flame, to be published. [ 131 R.A. Kee, J.A. Miller and T.A. Jefferson, Sandia National Laboratory Report SANDBO-8003 (1980) [ 141 K.J. Rensberger, J.B. Jeffries, R.A. Copeland, K. KohseHiiinghous, M.L. Wise and D.R. Crosley, Appl. Opt. 28 (1989) 3556.
[ 151 I.M. Campbell and R.S. Mason, J. Photochem. 8 (1978) 321.
References [I ] D.R. Crosley, J. Phys. Chem. 93 (1989) 6273. [21 P.W. Fairchild, G.P. Smith and D.R. Crosley, J. Chem. Phys. 79 (1983) 1795. [ 31 N.L. Garland and D.R. Crosley, Twenty-First Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1988) p. 1693. [41 C.P. Fenimore, Thirteenth Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 197 1) p. 373; J.A. Miller and CT. Bowman, Progr. Energy Combustion Sci. I5 (1989) 287. [ 51 C.J. Nokes and R.J. Donovan, Chem. Phys. 90 ( 1984) 167. [61 P. Heinrich, R.D. Kenner and F. Stuhl, Chem. Phys. Letters 147 (1988) 575. [71 N.L. Garland and D.R. Crosley, Chem. Phys. Letters 134 (1987) 189. [81 G.A. Raiche and D.R. Crosley, I. Chem. Phys. 92 ( 1990) 5211.
[ 161 I.S. McDermid and J.B. Laudenslager, J. Quant. Spectry. Radiative Transfer 27 (1982) 483. [ 171 A.B. Callear and M.J. Pilling, Trans. Faraday Sot. 66 ( 1970) 1618.
[ 181G.D. Greenblatt and A.R. Ravishankara,
Chem. Phys. Letters 136 (1987) 501. [ 191 M. Asscher and Y. Haas, J. Chem. Phys. 76 (1982) 2115. [20]R. Schwarzwald, P. Monkhouse and J. Wolfrum, Chem. Phys. Letters 158 (1989) 60. [21] J.B. Jeffries, R.A. Copeland and D.R. Crosley, J. Chem. Phys. 85 (1986) 1898. [22] R.A. Copeland, M.J. Dyer and D.R. Crosley, J. Chem. Phys. 82 (1985) 4022; G.P. Smith and D.R. Crosley, J. Chem. Phys. 85 (1986) 3896. [23]J.B. Jeffries, K. Kohse-Hoinghaus, G.P. Smith, R.A. Copeland and D.R. Crosley, Chem. Phys. Letters 152 (1988) 160.
[91 R.J. Cattolica, T.G. Matagaand J.A. Cavalowsky, J. Quant. Spectry. Radiative Transfer 42 (I 989) 499.
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