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Ion formation in the photochemically initiated combustion of C2H2-O2-NO2 mixtures
ION FORMATION IN THE PHOTOCHEMICALLY INITIATED COMBUSTION OF C~H2-O2-NO2 MIXTURES Part II. Ion Current and Emission Spectra TOSIRO K I N B A R A AND KAZUHIKO NODA
Sophia University, Kioieho, Chiyodaku, Tokyo, Japan Mixtures of C2H2--O2-NO2in a silica tube were ignited by a xenon flash lamp adjacent to the tube and the time change of the C~, CH, and OH emission spectra from the mixtures was
studied, together with the time change of the ion current through them. The emission and absorption spectra and the ion current showed nothing significantly different from those of ordinary C2Hz-O2 flames, and it was concluded that the main combustion reactions and the mechanism of ion production, that is, CH + O --* CHO + -}- e, are essentially common to both cases. The ion current never failed to start more than 50 ~sec after the first radical appeared. The first radical to appear was CH* when the combustion tube was long, and C~* when it was short, the interval between them being roughly 30 ~sec. The relation of C~* and CH* is not such that one of them originates from the other. They were considered to come from a common species--probably polymerized C~H. The C~ thus produced does not seem to be directly linked with ion production, although it is responsible for ion formation through CH by the reaction: C~ + OH --* CH + CO. Radical emission and absorption, as well as ion production, were greatly dependent upon the mixture ratio of the gases. This dependence is discussed and it is concluded that CH*(~A) is more closely related to CHO + formation than is CH(2II). Based on our experimental results, a system of reactions for producing CHO + has also been proposed. Introduction
I t has been established that the most abundant ion in hydrocarbon flames is H30+. Various theories have been presented 1 for the mechanism of its formation. Of these, the most widely accepted is C H + 0 = C H O + -}- e
C H 0 + + H20 = HzO+ + CO.
(1)
(2)
The authors undertook a study to obtain direct evidence for React:on (1), and to examine various related reactions which, until now, have been assumed by many researchers. The investigation was carried out by a flash-photolysis technique developed by Norrish et al. 2 I n Part I of our study, 3 mixtures of C2tt2-O2 were sensitized with a small quantity of NO~, and after the combustion reaction was initiated by a strong argon flash. The growth of radicals was studied b y absorption spectroscopy. Ion produc-
tion was determined from the current through a pair of electrodes inserted into the combustion chamber. The absorption spectra observed by flash photolysis were, as Norrish et al. report s C2, CH, OH, CN, and NH bands, together with a cont i n u u m which appears only in the induction period and is due to hot NO, plus another which appears with the above radicals and is due to carbon particles.4 The authors studied mainly the (0, 0) bands of C2 (5165/~), CH (3143 .~), and OH (3064 /~). The most outstanding feature is that the C2 and CH bands appear and disappear in synchronism with each other, as has been already shown by Norrish et al. ~ The growth process of radical absorption varied greatly with the mixture ratio. When the pressure of C2H2 (Pc) exceeded the sum of the pressures of 02 (Po) and of NO2 (PN), the C2 and CH bands and the ion current grew, keeping step with each other. This is considered to be direct evidence of Eq. (1). Against this, the C2 and CH bands vanished when 1.3pc < (po ~- pN), but the ion current persisted. In this case, the OH band ap-
333
334
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
Sp
L
/ t
~
~
l i __
S r
Ph~
FIo. 1. Schematic diagram of apparatus. C--combustion vessel (50 em long, 3 cm diam); E-electrodes for the ion current (10 cm long); M--half-aluminized mirror; X--xenon flash tube; T--trigger; P--pulser (15 kV); S--electric flash source (6 kV, 60 gF); S'--electric ion current source (150 V); R--resistor (500 k~); Phi, Ph~--photomultiplier and amplifier; F--interference filter; Os--two-channel oscillograph; SF--spectrograph. peared, but the ion current began to flow while the OH band was decreasing, after having attained its maximum intensity, suggesting that OH has nothing to do with ion formation. These results show only the relations between radicals in their ground states and ions, and tell nothing about the relations between excited radicals and ions. In this paper, the relations of ions to excited radicals are studied by emission spectra, and a probable series of ion-producing reactions is proposed.
Apparatus The apparatus used in Part I was somewhat modified and is shown schematically in Fig. 1. The combustion silica tube C is similar to the one used in P a r t I and is 50 cm long with a diameter of 3 cm. In order to compare two emissions of, say, CH and C2, a half-aluminized mirror M was placed near the end of the combustion tube and the transmitted light was analyzed, as in Part I, by a spectrograph (Sp)-photomultiplier (Phi) system, while the reflected light was analyzed with an interference filter (F)-photomultiplier
(Ph2) system. As described below, the CH band is overlapped by a continuum, the intensity of which is high enough as compared with CH so that it can only be studied with the S p - P h l system. The outputs from these photomultipliers were fed into cathode followers of totempole type, having a rise time of less than 0.1 ttsec. As in Part I, the rise time of the ion circuit was also less than 0.1 ~sec. The current is not necessarily proportional to the ion concentration, but it does increase with the concentration, beginning within 1 #sec of the appearance of flame ions. A two-channel oscillograph (Os) was used to compare the growth process of radicals and ions, or of any two radicals.
Experimental Results The partial pressures Pc (Cett2), Po (O2), and p~ (NO2) always fulfilled the following conditions, as in P a r t I: Pc ~ Po ~ PN ----25 torr, PN ---- 4 torr.
ION FORMATION We studied mainly the following (0, 0) bands: C2 emission: 3IIg---~3II~
=
5165 ~;
CH emission: 2A--~ 2H = 4315 .~;
OH emission: 2Z+--~2II = 3064/~. C H has three excited levels 2A, 2Z-, 2Z+, in ascending order of energy levels above the ground state 2II. The CH spectrum which appeared strongly in absorption was ~H---~Z+ = 3143 .~, whereas, in emission, the corresponding transition was very weak, and instead, 2A ~ 2H was strong. This is a remarkable feature of the CH spectra. The CH emission, 22~----)2II, was very weak and was not studied. In addition to these bands, a continuum was observed which is attributed to carbon particles,4 and which overlaps the radical spectra. Therefore, the intensity due to the continuum should be deducted from the observed value in order to obtain the true emission caused by radicals. For instance, CH emission has its band head at 4315 .~ and is shaded to the violet. The net value of CH emission was obtained by subtracting the observed intensity of 4320 .~ from that of 4310 ~. Thus, when Pc > 13 torr, although a considerably strong emission is observed at 4315 .~, all of it
335
comes from the continuum and none of it is due to CH emission. The same holds for C2 and CN emissions, but because they are strong as compared with the continuum, the required correction is comparatively small. In the case of OH, the situation became complicated, and it was hard to separate the radical emissions from the continuum distinctly. The emission spectra depended greatly upon Pc. Figures 2 and 3 are oscillograms showing the time dependence of C2 emission and the ion current at Pc -- 13 torr, and t h a t of CH emission and the ion current at pc = 9 torr, respectively. Experiments with mixtures of Pc = 5-16 torr gave the following results: (A) With a 50-era-long combustion tube: (a) The start of the ion current almost coincides with that for C2 or CH emission, b u t precise analysis shows that it is always some 10 /~see behind them. (b) C2 emission appears over a whole range of Pc = 5-16 torr, and increases with Pc. (c) CH emission appears only in the range of Pc ----5-13 tort, and has its maximum intensity at Pc = 10 torr. (d) OH emission appears in about the same range as CH emission and seems to attain its maximum intensity at about pc --- 10 torr. (e) The total charge Q, which flows through the electrodes, is given by Q=
(ion current) dt,
(3)
and is ealculated from oscillograms. I t is not unreasonable to presume t h a t Q is a rough approximation of the total number of ions
I(~N
I()N
increase
( :
(~
increase
t},Sm~e(' FIG. 2. Oscillograms of intensities of C2 emission and ion current. In all oscillograms shown, the intensity of emission is in arbitrary units, one division for the current equals 0.1 mA, and one division for the time is shown in each oscillogram; pc = 13 torr; tube, 50 cm long.
336
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
FIG. 3. OsciUograms of intensities of CH emission and ion current. Pc = 9 torr; tube, 50 cm long.
produced. I t was shown that ions existed over the whole range of Pc -- 5-16 torr, with a maximum at pc = 10 torr. (f) Figure 4 compares Ce and CH emission. Both appeared, attained maximum intensity, and disappeared almost simultaneously. (g) If investigated more precisely, however, it will be found t h a t CH never fails to appear before C2. Thus, CH*, C2", and the ion begin to appear in this order, each interval being 30-50 #sec. (B) With a 10-cm-long combustion tube: If we want to discuss the time difference of 30-50 ttsec, it is necessary to make sure, first of all, that the reaction proceeds uniformly throughout the entire volume. For this purpose, a set of electrodes was placed near one end of the combustion tube and another near the opposite end. The currents were compared and it was found t h a t they began to flow almost at the same time
(within 15 t~sec). This test was performed before beginning the studies in Part I. Norrish et al. ~ also found that, within 30 #sec, which was the duration of the photolysis flash, homogeneous initiation was produced throughout a 50-cm-long reaction vessel. To make the results more accurate, however, a short combustion tube was devised. This tube was 10 cm in length, with a diameter of 4 cm. A long xenon flash tube (diam = 8 ram) was helically wound to form a cylindrical space which lust contained the combustion tube. The electrodes inserted were 8 cm long. Figures 5 to 8 are the oscillograms obtained with this tube. Experiments were repeated with this short tube and the results were as follows: (h) Results ( a ) - ( f ) were the same, b u t there was a remarkable change in (g), with C2" invariably appearing before CH* (Fig. 8), contrary to the case with the 50-cm tube. The par-
Fro. 4. Comparison of C2 and CH emissions; Pc = 7 torr; tube, 50 cm long.
ION F O R M A T I O N
Fro. 5. OsciUograms of intensities of C~ emission and ion current; pc = 9 torr; tube, 10 cm long.
FIG. 6. OsciUograms of intensities of CH emission and ion current. Pc = 7 torr; tube, 10 cm long.
Fro. 7. Oscillograms of intensities of OH emission and ion current; pc -- 9 torr; tube, 10 cm long.
337
338
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
FIo. 8. Comparison of C2 and CH emissions; Pc = 7 torr; tube, 10cm long..
ticular factor that determines which appears first has not yet been identified. A similar phenomenon is seen in the reaction zone of flat-burner flames, where the appearance of either C2" or CH* at the flame base depends on the mixture ratio. 5 (i) According to Part I, OH absorption began to appear some 100 #sec prior to CH absorption, but OH emission made its appearance only slightly (about 10/~sec) before CH emission and persisted even after C~ and CH emission had disappeared completely: (j) A bend, although very small, was almost always found in the initial ascending slope of the CH emission oscillogram (Figs. 6 and 8), and this coincided with the start of the ionization current. It suggests that some of the excited CH* radicals turn into ions after this time. No such bends were found in the C2" and OH* oscillograms. Discussion 1. Combustion in Flash Pho~lysis and in Ordinary Flames
(i) If the studies of Gaydon and Wolfhard5 and of Jessen and Gaydon 6 are compared with our present work and the studies of Norrish et al., 2,4 it will be found that the emission and absorption spectra of C2H2-02 burner flames are not essentially different from those of C2Hs-O2-NO2 in flash photolysis. In the case of flash photolysis, NO2 decomposes into NO-]-O, and O attacks C2H2. Therefore, the initiation of the combustion reaction is considered to be the same as in ordinary combustion, although some species produced from NO occur as byproducts. (ii) It is common to both cases that intense
CH (3143 .~) appears in absorption spectra, whereas CH (4315 /~) appears strongly in emission spectra. (iii) According to Gaydon and Wolfhard, 5 the radiations of OH, C2, and CH from the reaction zone of hydrocarbon flames with strong C2 and CH emission are mainly of a nonthermal origin. In the case of flash photolysis, Norrish et al. ~ concluded that OH emission is nonthermal, and that, although CN emission is mainly thermal, it still has some chemi-luminescence. 7's We tried unsuccessfully to obtain the absorption spectra of C2, CH, and OH, using a xenon arc lamp with an 8-amp current and a color temperature of about 6000~ The photomultiplier system was devised only to respond to the variation of the input intensity but the oscillogram of these emission spectra showed no sign of being affected by the arc lamp. Absorption spectra appeared only when a xenon flash lamp (2 kV, maximum c u r r e n t - 1000 amp) was used. CN emission, however, turned into absorption when illuminated by this xenon arc lamp. Thus, not only OH but also C2 and CH emissions are considered to be mainly nonthermal, whereas CN emission is mainly thermal. (iv) Cummings 9 observed that when NO was increased in a C2H4-O2-NO burner flame, C2 emission disappeared. This was confirmed in our flash photolysis, and at 4 torr NO2, no C2 emission was detected. Knewstubb 1~has observed that the addition of NO to a C2He-O2-N2 flame causes a large reduction in the H30 + concentration and gives a high NO + concentration. This was interpreted by Cummings 9 as follows. NO reduces the concentration of H and, accordingly, that of O, and this slows down the ion producing reaction (1).
ION FORMATION
339
Thus, NO affects Reaction (1) to some extent, but does not essentially change it. (v) Kinbara et al. u studied ion distribution in the reaction zone of a hydrocarbon flat flame at low pressures. The result is just what is to be expected from the relations shown in Figs. 2, 3, and others. This is one of the reasons why we consider ionization in flash photolysis to be mainly nonthermal. F r o m all these facts, it does not seem unreasonable to assume that the combustion and ionproducing reactions in flash photolysis do not differ essentially from those in ordinary flames.
(e), C2 emission increases b u t ion current decreases as pc increases in the region pc > 10 torr. This seems to contradict Eq. (6), but in this case we must also take into account t h a t the partner of C2", i.e., 02, is also decreasing. Thus, at present, we have no reason to deny the existence of C3H3+ or CO +. However, Peeters and Van Tiggelen 16 have concluded from a comparison of ion yield and the CH and C2 photon yield that Reactions (4) and (6) do not coiltribute significantly to the formation of ions.
2. Appearance and Disappearance of C2, CH, and OH Emissions
A t the present stage in which the primary ions are not directly identified, the only basis for discussion is to assume t h a t the combustion reactions are the same in ordinary flames and in flash photolysis. Then the most widely accepted Reaction (1) comes into question. Gaydon et al.5,17,1s considered C2 as the parent radical of CH and have proposed the reaction
The emissions of C2 and CH appear, attain their maximum strength, and disappear almost at the same time. OH emission also keeps pace with C2 and CH emissions, although it persists after they have vanished. This suggests either that the rates of C2", CH*, and OH* formation are controlled by the burning gas temperature or, as Gaydon and Wolfhard 12have proposed, that these radicals are excited by a common unknown species.
4. Production of C2 and CH
c2 + OH = CH + CO.
(7)
There has been no direct evidence that the primary ion in flash photolysis is CHO +. Knewstubb and Sugden TM found a fairly high concentration of C3H3+ in C2H2 flames and assumed
For C:* and CH*, Gaydon and Wolfhard 12 assumed C2H to be the parent radical, which, after polymerizing, breaks up and then forms C2" and CH* with the assistance of some unknown species. C2H has been found in hydrocarbon flames, and for its production Hand and Kistiakowsky19 and Gardiner 2~proposed the reaction
C2He + CH* -- C~Ha+ -~ e.
C2H2 ~- 0 = C2H -[- OH.
3. Types oflons
(4)
This seems to conflict with our experimental results (e) and (e), because when Pc > 13 torr, there is no CH emission, while the ion current flows. But it will be shown below (Fig. 9) that ions m a y be produced from CH in the ground state, and therefore Eq. (4) does not necessarily contradict our results. However, the reaction proposed by Glass et al. 14 C3H3+ -~- e = C2" -~- CH~
(5)
is not consistent with our results, because if (5) is true, ions should appear before C2 emission. On the other hand, Calcote 15 proposed the reaction C2" -~- 02 = CO + -~- CO -~- e.
(6)
According to our experimental results (b) and
(8)
Our results (g) and (h) show that C2" appears either before or after CH*, and that this "breaking-up theory" seems more likely than Reaction (7). As already mentioned, Cummings, 9 using C2H,-O2-NO mixtures, has shown t h a t even when there is no C2 emission, an ion current appears. Similarly, in our flash photolysis experiments with C2H4, no C2 emission appeared when the C2H4 pressure was 6 torr (Po = 15, PN ~- 4 torr), but a current was found to flow. Therefore, taken in conjunction with what has been said concerning Eq. (6), it seems more reasonable to think that C2" is not directly responsible for ion formation, although it m a y contribute through CH by Reaction (7). 5. The Parent Radical: CH* or CH
The intensities of CH emission E, CH absorption A at their peaks and the total charge Q in
340
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
~_
(a)
Q
r.D
concentration of the ~H state, however, is very small when Pc < 10 torr, and frequently only the transition ~A--+ 2II (emission 4315/~) occurs. There is controversy over whether the CH in Eq. (1) is in the excited or the ground state. Fontijn et al. 21 assumed that CH* -~ O CHO + e, in C2H: flames, whereas Peeters and Tiggelen, 16 and Arrington e/ al. 22 considered that CH O=CHO+~e. Our result [-Fig." 9 ( b ) ] shows that CH* is indispensible for ion production, but this does not necessarily deny the existence of the reaction CH ~ O = C H O + T e, because when pc > 13 t0rr, CH (2H) can be considered to contribute to ion formation. As a matter of fact, Fontijn et al. 21 concluded that CH (2II) is responsible for ion formation in C2H4 flames. ---
P
,4
~Z ;> 5
I0
15
(b) V. Conclusion
o~ Z~
z
Based on the above considerations, we tried to present a probable series of reactions for ion production. The following system offers the simplest form consistent with all of our experimental results, including those obtained in Part I.
ION
Z.~
C2H2
C2H2
--* CH (2II) ~ H2 ~ CO 5
I i0
I
15
PRESSURE OF C=,H., (Torr)
Fro. 9. (a) Measured values of Q (total charge through electrodes), E (maximum intensity of the CH emission), and A (maximum intensity of CH absorption) in arbitrary units against pc (tort). (b) Supposed concentrations in arbitrary units of ions and CH radicals in 2II, 2A, ~Z: states against pc (torr).
<
OH
---) C H 0 + ~ e O
C2H--* C2", C H * - o CHO + ~- e --~ CH* (2A) ~ COs 03
O
--~ CHO § ~- e O
The reaction Eq. (3), all depend on the mixture ratio. The relative value of E is measured from the oscillogram, and the results of Norrish et al. 2 can be adopted for A. If they are plotted in arbitrary units against pc, Fig. 9(a) is obtained. Figure 9(b) shows the supposed concentrations of CH (2H), CH* (2A), CH* (22;+), and the ions against pc in order to explain Fig. 9(a). When Pc > 13 torr, the concentration of the 2II state is so large that the transition 2~+--~2H (emission 3143 /~) occurs only with difficulty, and the transition ~A--. 2II (emission 4315/~) takes place only very weakly. In this region, 2II--* 2A (absorption 4315 /~) is also impossible, and the only transition easily made is 2II--*2Z+ (absorption 3143 /I.). The
C2H2 -Jr- OH - CH ~ H2 -{- C02
(9)
is the one shown by Mukherjee et a[., 1 although objections to it have been raised by Fenimore and Jones. 2a The reaction C2H -~ 0 2 - - C H * (2A) _~- CO
(10)
is the one supported by Hand and Kistiakowsky. TM However, in order to confirm this system of reactions and the values of Fig. 9 (b), one must know the actual concentrations of ions and radicals. This problem has been left for further investigation.
ION FORMATION
Acknowledgments The authors express their cordial thanks to H. Sato for his assistance in conducting our experiments. This work was supported in part by the Matsunaga Science Foundation. REFERENCES 1. MUKHERJEE, N. R., FU~NO, T., EYRING, H., I ~ , T. : Eight Symposium (International) on Combustion, p. 1, Williams and Wilkins, 1962. 2. NoamsH, R. G. W., PONTES, G., AND THaUSH, B. A.: Proc. Roy. Soc. (London) AM6, 165 (1953). 3. KINBARA, T. AND NOD)-, K.: Twelfth Symposium (International) on Combustion, p. 395, The Combustion Institute, 1969. 4. NORRISH, R. G. W., PORTER, G., AND THRUSH, B. A.: Proc. Roy..Soc. (London) A227, 423 (1954). 5. GAVDON, A. G. ANn WOLFH~D, H. G. : Fourth Symposium (International) on Combustion, p. 211, Williams and Wilkins, 1953. 6. JESSEN, P. F. AND GAYDON, A. e . : Combust. Flame 11, 11 (1967). 7. NOaRISH, R. G. W., PORT, a, G., AND THRUSH, B. A.: Fifth Symposium (International) on Combustion, p. 651, Reinhold, 1955. 8. NORRISH, R. G. W., PORTER, G., AND THRUSH, B. A.: Nature 172, 71 (1953). 9. CUMMINGS, G. A. McD. AND Hu~roN, E.: Eleventh Symposium (International) on Combustion, p. 335, The Combustion Institute, 1967.
341
10. KNEWSTUBB, P. F.: Ninth Symposium (International) on Combustion, p. 656, Academic Press, 1963. 11. KINBARA,T., NAKAMURA,J., AND IK~GAm, H.: Seventh Symposium (International) on Combustion, p. 263, Butterworths, 1959. 12. GAYDON, A. G. ANn WOLFHARD, H. G.: Proc. Phys. Soe. (London) A64, 310 (1951). 13. KNEWSTUBB, P. F. AND SUGDEN, T. M.: Seventh Symposium (International) on Combust/on, p. 247, Butterworths, 1959. 14. GLASS,G. P., KISTIAKOWS~:V,G. B., MICH,1,, J. V., ANn NmI, H.: J. Chem. Phys. ~ , 608 (1965). 15. C~eoTE, H. F. : Combust. Flame 1, 385 (1957). 16. PEETERS, J. AND VAN TIGGELEN, A. : Twelfth Symposium (International) on Combustion, p. 437, The Combustion Institute, 1969. 17. PANNETmR, G. AND GAYDON, A. G.: Compt. rend. 225, 1300 (1947). 18. BROIDA, H. P. ANn GAYnON, A. G.: Proc. Roy. Soc. (London) A~18, 60 (1953). 19. HAND, C. W. ANn KISTIAKOWSKY,G. B.: J. Chem. Phys. 37, 1239 (1962). 20. GXRDINER, W. C., JR.: J. Chem. Phys. 40, 2410 (1964). 21. FONTIZN, A., MILLER, W. J., AND ttOOAN, J. M.: Tenth Symposium (International) on Combust/on, p. 545, The Combustion Institute, 1965. 22. ARaINGTON,C. A., BRENIVEN, W., GLASS,G. P., MICHAEL, J. V., AND NIKI, H. : J. Chem. Phys. 43, 1489 (1965). 23. FEmMORE, C. P. AND JONES, G. W.: J. Chem. Phys. 39, 1514 (1963).
COMMENTS Authors'Comment. We would like to add these comments to our paper: 1. The order of appearance of C~* and CH* changes according to the shape of the combustion tube, as was stated. The cause of this change has not y e t been clarified. The rapid change of the magnetic field through the combustion tube, caused by the flash current through the helical xenon lamp, m a y affect the chemical reactions. 2. To discuss the system of elementary chemical reactions and ion-producing reaction, it is necessary to know the detailed time change not only of the emission spectra but also of the absorption spectra. However, absorption spectra are very hard to obtain; they can be obtained only when a xenon flash lamp (not a xenon arc ]amp) is used as a light source. I n Part I of our study, the time change of absorption spectra was studied, but they were obtained by a xenon flash lamp which emitted light for a very short time. Thus, no precise
information about the time change of absorption spectra could be obtained. We are now planning to obtain the detailed time change of absorption spectra by the method of multiple reflection, using a xenon arc lamp. I t would be too premature to present the sequence of elementary reactions as the mechanism of ion formation. The sequence given above is just an a t t e m p t to explain the results of our experiment.
P. J. Padley, University College of Swamea. If the only CH absorption system detectable by the authors is the X~l-I--+ C2Z + system at 314.2 nm, then this is a point of difference--not of similarity--between photolysis and flame results with C~H2 + 02. I t is now known, from three different groups of workers, 1-~ that all three systems--the X ~II---+ C~Z+ (314.3 nm), the X~H---+B2~ - (390 nm),
342
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
and the X2H--*A~A (431 n m ) - - a p p e a r in C2H2 -]- 02 flames with comparable intensities.
References l. BLEEKRODE, R. AND NIEUWPOORT, W. C.: J. Chem. Phys. 43, 3680 (1965). 2. JESSEN, P. F. AND GAYDON,A. G.: Twelfth Symposium (International) on Combustion, p. 481, The Combustion Institute, 1969. 3. BULEWICZ,E. M., PADLEY, P. J., AND SMITH, R. E.: Proc. Roy. Soc. (London) A315, 129 (1970).
Author's Reply. The intensities of absorption spectra of the 2H - o 2A and 2H - o 2Z- systems in ordinary hydrocarbon flames are so small that they have been detected only b y the method of multiple reflection. In our case, the combustion tube was 50 cm long and the gas pressure was 25 torr; the p a t h of light was not long enough to give distinct absorption spectra. The longer tube is not preferable, considering the homogeneity of reaction throughout the tube, and the higher pressure makes our experiment hazardous. Another factor t h a t makes these absorption spectra difficult to detect is that only flash lamps can be applied as the light source. I t s duration time is less than 1 msec, and is too short to analyze spectra electrophotometrically. However, Norrish et al. already reported (Ref. 2 in our paper) that these absorption systems were just detectable in their experiment on flash photolysis. These systems are believed to exist, but are too faint to be easily detected.
A. Fontijn, AeroChem Research Laboratories. The absence of strong emission from CH (C ~ -X 2II) can be readily explained on the basis of quantitative spectroscopic observations, which have shown t h a t CH (C 22;) has a shorter lifetime for predissociation than for emission.1
Reference 1. HESSER, J. E.: Phys. Rev. Letters, 1968.
Authors' Reply. The absence of strong emission from CH (22;+--~ 2II) may be explained by its short lifetime, but when we consider it in relation to ions, it seems more reasonable to suppose the population of ~2;+ is small even for lean mixture. The population of ~2;+ shown in Fig. 9 (b) corresponds to a part which can be ionized without being dissociated.
C. Birkby, Midlands Region, Sciemific Servieea Department. The apparatus we describe in our paper 1 was very sensitive to inhomogeneity in the time history of the reactants along the axis of the reaction cell. We observed the passage of detonation waves along the cell axis despite our attempts to ensure homogeneous initiation. Would the authors comment on the homogeneity of their initiation, in view of the serious effects the resultant lack of homogeneous explosion would be expected to have on the occurrence and time history of emission and ionization intensities which they measured?
Reference 1. BIRKBY, C. AND HUTTON, E.: This Symposium.
Authors' Reply. We observed the emission spectra of OH not only with light emitted from the end of the combustion tube but also with that emitted radially through the side wall, and confirmed the existence of the shock wave. However, the ion currents through two pairs of electrodes placed at different points along the tube axis appeared within 10 #sec. This shows that the initial state of ion current was not affected b y the shock wave, although the ion current of the later stage may be affected by the shock wave. It thus seems reasonable to consider the ions are not produced by the shock wave in the initial stage.
E. Hutton, Laporte Industries Ltd., Lancashire, England. From a review of the literature on flashinduced explosions, and from our work at Manchester University on this subject, we believe that the reactions occurring in fuel-rich mixtures of flash-induced explosions do not simulate those taking place in a conventional nonsoot-producing premixed flame. The relevant references are presented in our paper at this Symposium. 1 The reasons for our belief are: 1. The lifetimes of the carbon radicals C~, CH, and CN are very much longer than those of carbon radicals in the reaction zone of a premixed flame. To study the importance of such phenomena it is necessary to study weaker mixtures, where carbon radical concentrations are not readily observable in absorption. If we go to weak mixtures, then no indication is obtained on the concentration of ground-state radicals, and the possibility of their involvement in the reaction process. 2. The almost simultaneous occurrence of C2 and Ctt indicates that normal flame conditions are not duplicated. For acetylene/oxygen combustion, C2 normally exceeds CH. 3. Acetylene/oxygen flame systems give a high
ION FORMATION degree of chemi-excitation of OH in the reaction zone. There seems to be no evidence for chemiexcitation in the results reported. 4. The presence of the NO2 sensitizer produces high concentrations of CN, depletion of carbon radicals, and probably also depletion of 0, H, and OH radical concentrations. Thus, it is our view t h a t the N02 sensitized acetylene/oxygen systems do not simulate true combustion in a nonsoot-forming premixed flame, due to both the above factors and to the inhomogeneity of the reaction initiation. Reference 1. BIRKBY, C. AND HUTTON, E.: This Symposium.
Authors' Reply. 1. According to Norrish et al. (Ref. 8 in the paper), the temperature rises to more than 3000~ and keeps this value for about 1 msec during an explosion of C2H2, 0.2, and NO2 at pressures of 14, 10, and 1.5 torr, respectively.
343
Chemical excitation of carbon radicals is followed by thermal excitation. It is considered that this makes the lifetime of excited carbon radicals appear to be longer than that in the reaction zone of conventional flames. As to the latter half of this comment, the conclusion is the same as ours. 2. In ordinary acetylene/oxygen combustion, C2 does not necessarily precede CH (Ref. 5 in the paper). 3. In the case of flash photolysis, OH emission in the first stage is non thermal. This was concluded by Norrish et al. (Ref. 2 in the paper), and believed to be reasonable from our experimental result that no OH-absorption spectra could be observed with a xenon arc lamp (6000~ as the light source. 4. These reactions may take place in the combustion tube. But it is not unreasonable to consider that these are branched reactions, and the main reactions such as C H - ~ - 0 - - ~ CHO + -~ e still exist.
Sophia University, Kioieho, Chiyodaku, Tokyo, Japan Mixtures of C2H2--O2-NO2in a silica tube were ignited by a xenon flash lamp adjacent to the tube and the time change of the C~, CH, and OH emission spectra from the mixtures was
studied, together with the time change of the ion current through them. The emission and absorption spectra and the ion current showed nothing significantly different from those of ordinary C2Hz-O2 flames, and it was concluded that the main combustion reactions and the mechanism of ion production, that is, CH + O --* CHO + -}- e, are essentially common to both cases. The ion current never failed to start more than 50 ~sec after the first radical appeared. The first radical to appear was CH* when the combustion tube was long, and C~* when it was short, the interval between them being roughly 30 ~sec. The relation of C~* and CH* is not such that one of them originates from the other. They were considered to come from a common species--probably polymerized C~H. The C~ thus produced does not seem to be directly linked with ion production, although it is responsible for ion formation through CH by the reaction: C~ + OH --* CH + CO. Radical emission and absorption, as well as ion production, were greatly dependent upon the mixture ratio of the gases. This dependence is discussed and it is concluded that CH*(~A) is more closely related to CHO + formation than is CH(2II). Based on our experimental results, a system of reactions for producing CHO + has also been proposed. Introduction
I t has been established that the most abundant ion in hydrocarbon flames is H30+. Various theories have been presented 1 for the mechanism of its formation. Of these, the most widely accepted is C H + 0 = C H O + -}- e
C H 0 + + H20 = HzO+ + CO.
(1)
(2)
The authors undertook a study to obtain direct evidence for React:on (1), and to examine various related reactions which, until now, have been assumed by many researchers. The investigation was carried out by a flash-photolysis technique developed by Norrish et al. 2 I n Part I of our study, 3 mixtures of C2tt2-O2 were sensitized with a small quantity of NO~, and after the combustion reaction was initiated by a strong argon flash. The growth of radicals was studied b y absorption spectroscopy. Ion produc-
tion was determined from the current through a pair of electrodes inserted into the combustion chamber. The absorption spectra observed by flash photolysis were, as Norrish et al. report s C2, CH, OH, CN, and NH bands, together with a cont i n u u m which appears only in the induction period and is due to hot NO, plus another which appears with the above radicals and is due to carbon particles.4 The authors studied mainly the (0, 0) bands of C2 (5165/~), CH (3143 .~), and OH (3064 /~). The most outstanding feature is that the C2 and CH bands appear and disappear in synchronism with each other, as has been already shown by Norrish et al. ~ The growth process of radical absorption varied greatly with the mixture ratio. When the pressure of C2H2 (Pc) exceeded the sum of the pressures of 02 (Po) and of NO2 (PN), the C2 and CH bands and the ion current grew, keeping step with each other. This is considered to be direct evidence of Eq. (1). Against this, the C2 and CH bands vanished when 1.3pc < (po ~- pN), but the ion current persisted. In this case, the OH band ap-
333
334
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
Sp
L
/ t
~
~
l i __
S r
Ph~
FIo. 1. Schematic diagram of apparatus. C--combustion vessel (50 em long, 3 cm diam); E-electrodes for the ion current (10 cm long); M--half-aluminized mirror; X--xenon flash tube; T--trigger; P--pulser (15 kV); S--electric flash source (6 kV, 60 gF); S'--electric ion current source (150 V); R--resistor (500 k~); Phi, Ph~--photomultiplier and amplifier; F--interference filter; Os--two-channel oscillograph; SF--spectrograph. peared, but the ion current began to flow while the OH band was decreasing, after having attained its maximum intensity, suggesting that OH has nothing to do with ion formation. These results show only the relations between radicals in their ground states and ions, and tell nothing about the relations between excited radicals and ions. In this paper, the relations of ions to excited radicals are studied by emission spectra, and a probable series of ion-producing reactions is proposed.
Apparatus The apparatus used in Part I was somewhat modified and is shown schematically in Fig. 1. The combustion silica tube C is similar to the one used in P a r t I and is 50 cm long with a diameter of 3 cm. In order to compare two emissions of, say, CH and C2, a half-aluminized mirror M was placed near the end of the combustion tube and the transmitted light was analyzed, as in Part I, by a spectrograph (Sp)-photomultiplier (Phi) system, while the reflected light was analyzed with an interference filter (F)-photomultiplier
(Ph2) system. As described below, the CH band is overlapped by a continuum, the intensity of which is high enough as compared with CH so that it can only be studied with the S p - P h l system. The outputs from these photomultipliers were fed into cathode followers of totempole type, having a rise time of less than 0.1 ttsec. As in Part I, the rise time of the ion circuit was also less than 0.1 ~sec. The current is not necessarily proportional to the ion concentration, but it does increase with the concentration, beginning within 1 #sec of the appearance of flame ions. A two-channel oscillograph (Os) was used to compare the growth process of radicals and ions, or of any two radicals.
Experimental Results The partial pressures Pc (Cett2), Po (O2), and p~ (NO2) always fulfilled the following conditions, as in P a r t I: Pc ~ Po ~ PN ----25 torr, PN ---- 4 torr.
ION FORMATION We studied mainly the following (0, 0) bands: C2 emission: 3IIg---~3II~
=
5165 ~;
CH emission: 2A--~ 2H = 4315 .~;
OH emission: 2Z+--~2II = 3064/~. C H has three excited levels 2A, 2Z-, 2Z+, in ascending order of energy levels above the ground state 2II. The CH spectrum which appeared strongly in absorption was ~H---~Z+ = 3143 .~, whereas, in emission, the corresponding transition was very weak, and instead, 2A ~ 2H was strong. This is a remarkable feature of the CH spectra. The CH emission, 22~----)2II, was very weak and was not studied. In addition to these bands, a continuum was observed which is attributed to carbon particles,4 and which overlaps the radical spectra. Therefore, the intensity due to the continuum should be deducted from the observed value in order to obtain the true emission caused by radicals. For instance, CH emission has its band head at 4315 .~ and is shaded to the violet. The net value of CH emission was obtained by subtracting the observed intensity of 4320 .~ from that of 4310 ~. Thus, when Pc > 13 torr, although a considerably strong emission is observed at 4315 .~, all of it
335
comes from the continuum and none of it is due to CH emission. The same holds for C2 and CN emissions, but because they are strong as compared with the continuum, the required correction is comparatively small. In the case of OH, the situation became complicated, and it was hard to separate the radical emissions from the continuum distinctly. The emission spectra depended greatly upon Pc. Figures 2 and 3 are oscillograms showing the time dependence of C2 emission and the ion current at Pc -- 13 torr, and t h a t of CH emission and the ion current at pc = 9 torr, respectively. Experiments with mixtures of Pc = 5-16 torr gave the following results: (A) With a 50-era-long combustion tube: (a) The start of the ion current almost coincides with that for C2 or CH emission, b u t precise analysis shows that it is always some 10 /~see behind them. (b) C2 emission appears over a whole range of Pc = 5-16 torr, and increases with Pc. (c) CH emission appears only in the range of Pc ----5-13 tort, and has its maximum intensity at Pc = 10 torr. (d) OH emission appears in about the same range as CH emission and seems to attain its maximum intensity at about pc --- 10 torr. (e) The total charge Q, which flows through the electrodes, is given by Q=
(ion current) dt,
(3)
and is ealculated from oscillograms. I t is not unreasonable to presume t h a t Q is a rough approximation of the total number of ions
I(~N
I()N
increase
( :
(~
increase
t},Sm~e(' FIG. 2. Oscillograms of intensities of C2 emission and ion current. In all oscillograms shown, the intensity of emission is in arbitrary units, one division for the current equals 0.1 mA, and one division for the time is shown in each oscillogram; pc = 13 torr; tube, 50 cm long.
336
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
FIG. 3. OsciUograms of intensities of CH emission and ion current. Pc = 9 torr; tube, 50 cm long.
produced. I t was shown that ions existed over the whole range of Pc -- 5-16 torr, with a maximum at pc = 10 torr. (f) Figure 4 compares Ce and CH emission. Both appeared, attained maximum intensity, and disappeared almost simultaneously. (g) If investigated more precisely, however, it will be found t h a t CH never fails to appear before C2. Thus, CH*, C2", and the ion begin to appear in this order, each interval being 30-50 #sec. (B) With a 10-cm-long combustion tube: If we want to discuss the time difference of 30-50 ttsec, it is necessary to make sure, first of all, that the reaction proceeds uniformly throughout the entire volume. For this purpose, a set of electrodes was placed near one end of the combustion tube and another near the opposite end. The currents were compared and it was found t h a t they began to flow almost at the same time
(within 15 t~sec). This test was performed before beginning the studies in Part I. Norrish et al. ~ also found that, within 30 #sec, which was the duration of the photolysis flash, homogeneous initiation was produced throughout a 50-cm-long reaction vessel. To make the results more accurate, however, a short combustion tube was devised. This tube was 10 cm in length, with a diameter of 4 cm. A long xenon flash tube (diam = 8 ram) was helically wound to form a cylindrical space which lust contained the combustion tube. The electrodes inserted were 8 cm long. Figures 5 to 8 are the oscillograms obtained with this tube. Experiments were repeated with this short tube and the results were as follows: (h) Results ( a ) - ( f ) were the same, b u t there was a remarkable change in (g), with C2" invariably appearing before CH* (Fig. 8), contrary to the case with the 50-cm tube. The par-
Fro. 4. Comparison of C2 and CH emissions; Pc = 7 torr; tube, 50 cm long.
ION F O R M A T I O N
Fro. 5. OsciUograms of intensities of C~ emission and ion current; pc = 9 torr; tube, 10 cm long.
FIG. 6. OsciUograms of intensities of CH emission and ion current. Pc = 7 torr; tube, 10 cm long.
Fro. 7. Oscillograms of intensities of OH emission and ion current; pc -- 9 torr; tube, 10 cm long.
337
338
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
FIo. 8. Comparison of C2 and CH emissions; Pc = 7 torr; tube, 10cm long..
ticular factor that determines which appears first has not yet been identified. A similar phenomenon is seen in the reaction zone of flat-burner flames, where the appearance of either C2" or CH* at the flame base depends on the mixture ratio. 5 (i) According to Part I, OH absorption began to appear some 100 #sec prior to CH absorption, but OH emission made its appearance only slightly (about 10/~sec) before CH emission and persisted even after C~ and CH emission had disappeared completely: (j) A bend, although very small, was almost always found in the initial ascending slope of the CH emission oscillogram (Figs. 6 and 8), and this coincided with the start of the ionization current. It suggests that some of the excited CH* radicals turn into ions after this time. No such bends were found in the C2" and OH* oscillograms. Discussion 1. Combustion in Flash Pho~lysis and in Ordinary Flames
(i) If the studies of Gaydon and Wolfhard5 and of Jessen and Gaydon 6 are compared with our present work and the studies of Norrish et al., 2,4 it will be found that the emission and absorption spectra of C2H2-02 burner flames are not essentially different from those of C2Hs-O2-NO2 in flash photolysis. In the case of flash photolysis, NO2 decomposes into NO-]-O, and O attacks C2H2. Therefore, the initiation of the combustion reaction is considered to be the same as in ordinary combustion, although some species produced from NO occur as byproducts. (ii) It is common to both cases that intense
CH (3143 .~) appears in absorption spectra, whereas CH (4315 /~) appears strongly in emission spectra. (iii) According to Gaydon and Wolfhard, 5 the radiations of OH, C2, and CH from the reaction zone of hydrocarbon flames with strong C2 and CH emission are mainly of a nonthermal origin. In the case of flash photolysis, Norrish et al. ~ concluded that OH emission is nonthermal, and that, although CN emission is mainly thermal, it still has some chemi-luminescence. 7's We tried unsuccessfully to obtain the absorption spectra of C2, CH, and OH, using a xenon arc lamp with an 8-amp current and a color temperature of about 6000~ The photomultiplier system was devised only to respond to the variation of the input intensity but the oscillogram of these emission spectra showed no sign of being affected by the arc lamp. Absorption spectra appeared only when a xenon flash lamp (2 kV, maximum c u r r e n t - 1000 amp) was used. CN emission, however, turned into absorption when illuminated by this xenon arc lamp. Thus, not only OH but also C2 and CH emissions are considered to be mainly nonthermal, whereas CN emission is mainly thermal. (iv) Cummings 9 observed that when NO was increased in a C2H4-O2-NO burner flame, C2 emission disappeared. This was confirmed in our flash photolysis, and at 4 torr NO2, no C2 emission was detected. Knewstubb 1~has observed that the addition of NO to a C2He-O2-N2 flame causes a large reduction in the H30 + concentration and gives a high NO + concentration. This was interpreted by Cummings 9 as follows. NO reduces the concentration of H and, accordingly, that of O, and this slows down the ion producing reaction (1).
ION FORMATION
339
Thus, NO affects Reaction (1) to some extent, but does not essentially change it. (v) Kinbara et al. u studied ion distribution in the reaction zone of a hydrocarbon flat flame at low pressures. The result is just what is to be expected from the relations shown in Figs. 2, 3, and others. This is one of the reasons why we consider ionization in flash photolysis to be mainly nonthermal. F r o m all these facts, it does not seem unreasonable to assume that the combustion and ionproducing reactions in flash photolysis do not differ essentially from those in ordinary flames.
(e), C2 emission increases b u t ion current decreases as pc increases in the region pc > 10 torr. This seems to contradict Eq. (6), but in this case we must also take into account t h a t the partner of C2", i.e., 02, is also decreasing. Thus, at present, we have no reason to deny the existence of C3H3+ or CO +. However, Peeters and Van Tiggelen 16 have concluded from a comparison of ion yield and the CH and C2 photon yield that Reactions (4) and (6) do not coiltribute significantly to the formation of ions.
2. Appearance and Disappearance of C2, CH, and OH Emissions
A t the present stage in which the primary ions are not directly identified, the only basis for discussion is to assume t h a t the combustion reactions are the same in ordinary flames and in flash photolysis. Then the most widely accepted Reaction (1) comes into question. Gaydon et al.5,17,1s considered C2 as the parent radical of CH and have proposed the reaction
The emissions of C2 and CH appear, attain their maximum strength, and disappear almost at the same time. OH emission also keeps pace with C2 and CH emissions, although it persists after they have vanished. This suggests either that the rates of C2", CH*, and OH* formation are controlled by the burning gas temperature or, as Gaydon and Wolfhard 12have proposed, that these radicals are excited by a common unknown species.
4. Production of C2 and CH
c2 + OH = CH + CO.
(7)
There has been no direct evidence that the primary ion in flash photolysis is CHO +. Knewstubb and Sugden TM found a fairly high concentration of C3H3+ in C2H2 flames and assumed
For C:* and CH*, Gaydon and Wolfhard 12 assumed C2H to be the parent radical, which, after polymerizing, breaks up and then forms C2" and CH* with the assistance of some unknown species. C2H has been found in hydrocarbon flames, and for its production Hand and Kistiakowsky19 and Gardiner 2~proposed the reaction
C2He + CH* -- C~Ha+ -~ e.
C2H2 ~- 0 = C2H -[- OH.
3. Types oflons
(4)
This seems to conflict with our experimental results (e) and (e), because when Pc > 13 torr, there is no CH emission, while the ion current flows. But it will be shown below (Fig. 9) that ions m a y be produced from CH in the ground state, and therefore Eq. (4) does not necessarily contradict our results. However, the reaction proposed by Glass et al. 14 C3H3+ -~- e = C2" -~- CH~
(5)
is not consistent with our results, because if (5) is true, ions should appear before C2 emission. On the other hand, Calcote 15 proposed the reaction C2" -~- 02 = CO + -~- CO -~- e.
(6)
According to our experimental results (b) and
(8)
Our results (g) and (h) show that C2" appears either before or after CH*, and that this "breaking-up theory" seems more likely than Reaction (7). As already mentioned, Cummings, 9 using C2H,-O2-NO mixtures, has shown t h a t even when there is no C2 emission, an ion current appears. Similarly, in our flash photolysis experiments with C2H4, no C2 emission appeared when the C2H4 pressure was 6 torr (Po = 15, PN ~- 4 torr), but a current was found to flow. Therefore, taken in conjunction with what has been said concerning Eq. (6), it seems more reasonable to think that C2" is not directly responsible for ion formation, although it m a y contribute through CH by Reaction (7). 5. The Parent Radical: CH* or CH
The intensities of CH emission E, CH absorption A at their peaks and the total charge Q in
340
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
~_
(a)
Q
r.D
concentration of the ~H state, however, is very small when Pc < 10 torr, and frequently only the transition ~A--+ 2II (emission 4315/~) occurs. There is controversy over whether the CH in Eq. (1) is in the excited or the ground state. Fontijn et al. 21 assumed that CH* -~ O CHO + e, in C2H: flames, whereas Peeters and Tiggelen, 16 and Arrington e/ al. 22 considered that CH O=CHO+~e. Our result [-Fig." 9 ( b ) ] shows that CH* is indispensible for ion production, but this does not necessarily deny the existence of the reaction CH ~ O = C H O + T e, because when pc > 13 t0rr, CH (2H) can be considered to contribute to ion formation. As a matter of fact, Fontijn et al. 21 concluded that CH (2II) is responsible for ion formation in C2H4 flames. ---
P
,4
~Z ;> 5
I0
15
(b) V. Conclusion
o~ Z~
z
Based on the above considerations, we tried to present a probable series of reactions for ion production. The following system offers the simplest form consistent with all of our experimental results, including those obtained in Part I.
ION
Z.~
C2H2
C2H2
--* CH (2II) ~ H2 ~ CO 5
I i0
I
15
PRESSURE OF C=,H., (Torr)
Fro. 9. (a) Measured values of Q (total charge through electrodes), E (maximum intensity of the CH emission), and A (maximum intensity of CH absorption) in arbitrary units against pc (tort). (b) Supposed concentrations in arbitrary units of ions and CH radicals in 2II, 2A, ~Z: states against pc (torr).
<
OH
---) C H 0 + ~ e O
C2H--* C2", C H * - o CHO + ~- e --~ CH* (2A) ~ COs 03
O
--~ CHO § ~- e O
The reaction Eq. (3), all depend on the mixture ratio. The relative value of E is measured from the oscillogram, and the results of Norrish et al. 2 can be adopted for A. If they are plotted in arbitrary units against pc, Fig. 9(a) is obtained. Figure 9(b) shows the supposed concentrations of CH (2H), CH* (2A), CH* (22;+), and the ions against pc in order to explain Fig. 9(a). When Pc > 13 torr, the concentration of the 2II state is so large that the transition 2~+--~2H (emission 3143 /~) occurs only with difficulty, and the transition ~A--. 2II (emission 4315/~) takes place only very weakly. In this region, 2II--* 2A (absorption 4315 /~) is also impossible, and the only transition easily made is 2II--*2Z+ (absorption 3143 /I.). The
C2H2 -Jr- OH - CH ~ H2 -{- C02
(9)
is the one shown by Mukherjee et a[., 1 although objections to it have been raised by Fenimore and Jones. 2a The reaction C2H -~ 0 2 - - C H * (2A) _~- CO
(10)
is the one supported by Hand and Kistiakowsky. TM However, in order to confirm this system of reactions and the values of Fig. 9 (b), one must know the actual concentrations of ions and radicals. This problem has been left for further investigation.
ION FORMATION
Acknowledgments The authors express their cordial thanks to H. Sato for his assistance in conducting our experiments. This work was supported in part by the Matsunaga Science Foundation. REFERENCES 1. MUKHERJEE, N. R., FU~NO, T., EYRING, H., I ~ , T. : Eight Symposium (International) on Combustion, p. 1, Williams and Wilkins, 1962. 2. NoamsH, R. G. W., PONTES, G., AND THaUSH, B. A.: Proc. Roy. Soc. (London) AM6, 165 (1953). 3. KINBARA, T. AND NOD)-, K.: Twelfth Symposium (International) on Combustion, p. 395, The Combustion Institute, 1969. 4. NORRISH, R. G. W., PORTER, G., AND THRUSH, B. A.: Proc. Roy..Soc. (London) A227, 423 (1954). 5. GAVDON, A. G. ANn WOLFH~D, H. G. : Fourth Symposium (International) on Combustion, p. 211, Williams and Wilkins, 1953. 6. JESSEN, P. F. AND GAYDON, A. e . : Combust. Flame 11, 11 (1967). 7. NOaRISH, R. G. W., PORT, a, G., AND THRUSH, B. A.: Fifth Symposium (International) on Combustion, p. 651, Reinhold, 1955. 8. NORRISH, R. G. W., PORTER, G., AND THRUSH, B. A.: Nature 172, 71 (1953). 9. CUMMINGS, G. A. McD. AND Hu~roN, E.: Eleventh Symposium (International) on Combustion, p. 335, The Combustion Institute, 1967.
341
10. KNEWSTUBB, P. F.: Ninth Symposium (International) on Combustion, p. 656, Academic Press, 1963. 11. KINBARA,T., NAKAMURA,J., AND IK~GAm, H.: Seventh Symposium (International) on Combustion, p. 263, Butterworths, 1959. 12. GAYDON, A. G. ANn WOLFHARD, H. G.: Proc. Phys. Soe. (London) A64, 310 (1951). 13. KNEWSTUBB, P. F. AND SUGDEN, T. M.: Seventh Symposium (International) on Combust/on, p. 247, Butterworths, 1959. 14. GLASS,G. P., KISTIAKOWS~:V,G. B., MICH,1,, J. V., ANn NmI, H.: J. Chem. Phys. ~ , 608 (1965). 15. C~eoTE, H. F. : Combust. Flame 1, 385 (1957). 16. PEETERS, J. AND VAN TIGGELEN, A. : Twelfth Symposium (International) on Combustion, p. 437, The Combustion Institute, 1969. 17. PANNETmR, G. AND GAYDON, A. G.: Compt. rend. 225, 1300 (1947). 18. BROIDA, H. P. ANn GAYnON, A. G.: Proc. Roy. Soc. (London) A~18, 60 (1953). 19. HAND, C. W. ANn KISTIAKOWSKY,G. B.: J. Chem. Phys. 37, 1239 (1962). 20. GXRDINER, W. C., JR.: J. Chem. Phys. 40, 2410 (1964). 21. FONTIZN, A., MILLER, W. J., AND ttOOAN, J. M.: Tenth Symposium (International) on Combust/on, p. 545, The Combustion Institute, 1965. 22. ARaINGTON,C. A., BRENIVEN, W., GLASS,G. P., MICHAEL, J. V., AND NIKI, H. : J. Chem. Phys. 43, 1489 (1965). 23. FEmMORE, C. P. AND JONES, G. W.: J. Chem. Phys. 39, 1514 (1963).
COMMENTS Authors'Comment. We would like to add these comments to our paper: 1. The order of appearance of C~* and CH* changes according to the shape of the combustion tube, as was stated. The cause of this change has not y e t been clarified. The rapid change of the magnetic field through the combustion tube, caused by the flash current through the helical xenon lamp, m a y affect the chemical reactions. 2. To discuss the system of elementary chemical reactions and ion-producing reaction, it is necessary to know the detailed time change not only of the emission spectra but also of the absorption spectra. However, absorption spectra are very hard to obtain; they can be obtained only when a xenon flash lamp (not a xenon arc ]amp) is used as a light source. I n Part I of our study, the time change of absorption spectra was studied, but they were obtained by a xenon flash lamp which emitted light for a very short time. Thus, no precise
information about the time change of absorption spectra could be obtained. We are now planning to obtain the detailed time change of absorption spectra by the method of multiple reflection, using a xenon arc lamp. I t would be too premature to present the sequence of elementary reactions as the mechanism of ion formation. The sequence given above is just an a t t e m p t to explain the results of our experiment.
P. J. Padley, University College of Swamea. If the only CH absorption system detectable by the authors is the X~l-I--+ C2Z + system at 314.2 nm, then this is a point of difference--not of similarity--between photolysis and flame results with C~H2 + 02. I t is now known, from three different groups of workers, 1-~ that all three systems--the X ~II---+ C~Z+ (314.3 nm), the X~H---+B2~ - (390 nm),
342
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
and the X2H--*A~A (431 n m ) - - a p p e a r in C2H2 -]- 02 flames with comparable intensities.
References l. BLEEKRODE, R. AND NIEUWPOORT, W. C.: J. Chem. Phys. 43, 3680 (1965). 2. JESSEN, P. F. AND GAYDON,A. G.: Twelfth Symposium (International) on Combustion, p. 481, The Combustion Institute, 1969. 3. BULEWICZ,E. M., PADLEY, P. J., AND SMITH, R. E.: Proc. Roy. Soc. (London) A315, 129 (1970).
Author's Reply. The intensities of absorption spectra of the 2H - o 2A and 2H - o 2Z- systems in ordinary hydrocarbon flames are so small that they have been detected only b y the method of multiple reflection. In our case, the combustion tube was 50 cm long and the gas pressure was 25 torr; the p a t h of light was not long enough to give distinct absorption spectra. The longer tube is not preferable, considering the homogeneity of reaction throughout the tube, and the higher pressure makes our experiment hazardous. Another factor t h a t makes these absorption spectra difficult to detect is that only flash lamps can be applied as the light source. I t s duration time is less than 1 msec, and is too short to analyze spectra electrophotometrically. However, Norrish et al. already reported (Ref. 2 in our paper) that these absorption systems were just detectable in their experiment on flash photolysis. These systems are believed to exist, but are too faint to be easily detected.
A. Fontijn, AeroChem Research Laboratories. The absence of strong emission from CH (C ~ -X 2II) can be readily explained on the basis of quantitative spectroscopic observations, which have shown t h a t CH (C 22;) has a shorter lifetime for predissociation than for emission.1
Reference 1. HESSER, J. E.: Phys. Rev. Letters, 1968.
Authors' Reply. The absence of strong emission from CH (22;+--~ 2II) may be explained by its short lifetime, but when we consider it in relation to ions, it seems more reasonable to suppose the population of ~2;+ is small even for lean mixture. The population of ~2;+ shown in Fig. 9 (b) corresponds to a part which can be ionized without being dissociated.
C. Birkby, Midlands Region, Sciemific Servieea Department. The apparatus we describe in our paper 1 was very sensitive to inhomogeneity in the time history of the reactants along the axis of the reaction cell. We observed the passage of detonation waves along the cell axis despite our attempts to ensure homogeneous initiation. Would the authors comment on the homogeneity of their initiation, in view of the serious effects the resultant lack of homogeneous explosion would be expected to have on the occurrence and time history of emission and ionization intensities which they measured?
Reference 1. BIRKBY, C. AND HUTTON, E.: This Symposium.
Authors' Reply. We observed the emission spectra of OH not only with light emitted from the end of the combustion tube but also with that emitted radially through the side wall, and confirmed the existence of the shock wave. However, the ion currents through two pairs of electrodes placed at different points along the tube axis appeared within 10 #sec. This shows that the initial state of ion current was not affected b y the shock wave, although the ion current of the later stage may be affected by the shock wave. It thus seems reasonable to consider the ions are not produced by the shock wave in the initial stage.
E. Hutton, Laporte Industries Ltd., Lancashire, England. From a review of the literature on flashinduced explosions, and from our work at Manchester University on this subject, we believe that the reactions occurring in fuel-rich mixtures of flash-induced explosions do not simulate those taking place in a conventional nonsoot-producing premixed flame. The relevant references are presented in our paper at this Symposium. 1 The reasons for our belief are: 1. The lifetimes of the carbon radicals C~, CH, and CN are very much longer than those of carbon radicals in the reaction zone of a premixed flame. To study the importance of such phenomena it is necessary to study weaker mixtures, where carbon radical concentrations are not readily observable in absorption. If we go to weak mixtures, then no indication is obtained on the concentration of ground-state radicals, and the possibility of their involvement in the reaction process. 2. The almost simultaneous occurrence of C2 and Ctt indicates that normal flame conditions are not duplicated. For acetylene/oxygen combustion, C2 normally exceeds CH. 3. Acetylene/oxygen flame systems give a high
ION FORMATION degree of chemi-excitation of OH in the reaction zone. There seems to be no evidence for chemiexcitation in the results reported. 4. The presence of the NO2 sensitizer produces high concentrations of CN, depletion of carbon radicals, and probably also depletion of 0, H, and OH radical concentrations. Thus, it is our view t h a t the N02 sensitized acetylene/oxygen systems do not simulate true combustion in a nonsoot-forming premixed flame, due to both the above factors and to the inhomogeneity of the reaction initiation. Reference 1. BIRKBY, C. AND HUTTON, E.: This Symposium.
Authors' Reply. 1. According to Norrish et al. (Ref. 8 in the paper), the temperature rises to more than 3000~ and keeps this value for about 1 msec during an explosion of C2H2, 0.2, and NO2 at pressures of 14, 10, and 1.5 torr, respectively.
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Chemical excitation of carbon radicals is followed by thermal excitation. It is considered that this makes the lifetime of excited carbon radicals appear to be longer than that in the reaction zone of conventional flames. As to the latter half of this comment, the conclusion is the same as ours. 2. In ordinary acetylene/oxygen combustion, C2 does not necessarily precede CH (Ref. 5 in the paper). 3. In the case of flash photolysis, OH emission in the first stage is non thermal. This was concluded by Norrish et al. (Ref. 2 in the paper), and believed to be reasonable from our experimental result that no OH-absorption spectra could be observed with a xenon arc lamp (6000~ as the light source. 4. These reactions may take place in the combustion tube. But it is not unreasonable to consider that these are branched reactions, and the main reactions such as C H - ~ - 0 - - ~ CHO + -~ e still exist.