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Further studies on radical and ion formation in the combustion of acetylene initiated by a strong flash
FURTHER STUDIES ON RADICAL AND ION FORMATION IN THE COMBUSTION OF ACETYLENE INITIATED BY A STRONG FLASH TOSIR0 KINBARA AND KAZUHIKO NODA
Department of Physics, Sophia University, 7 Kioicho, Chiyodaku, Tokyo, Japan
The time change of the intensities of emission spectra of C2, CH and OH of the CzH2/O~/NO2 mixture ignited by a flash laanp and burned in a closed tube hs.s been previously reported. A s~milar study of the absorption spectra was not undertaken at the time because of the complications caused by emission from the reacting inLxture. In the present work, two methods were combined to overcome this difficulty. The spectrometer was moved away from the combustion ~ube and nmltiple path absorption was used. Thus, the time changes of all the emission and absorption spectra of C~, CIt and OH, and that of the ion culu'ent t~hrough the burning mixture, could be compared quantitatively. In other experiments, the dependence on the mixing ratio of the fuel gas was established for (1) the intensity of the emission spectrum, (2) the light absorbence of the fuel for C2, CH, and OH and (3) the total charge produced in the combustion tube. Based upon thee experimental results, the following mechanism of the formation of C2, CH, OII and chemi-ious is discussed: C:H~+O-~C~H +OH~OH*
(i)
C.~H-~(C.~H),,~C.~, CH-~C2*, CH*
(2)
CHTO~CHO++e
(3)
I. Introduction
In previous work, the present authors applied a flash photolysis tecl~ique to study the time changes of emission and absorption spectra of the explosively burning mixture C2H2/O2/NO2. The ionic current through the ignited gas was also observed and the growth and decay of emission, absorption and ionic current were cornpured, t-3 In these experiments, the time changes of the emission spectra and the ionic cnrrent~ and accordingly, those of the concentrations of excited radicals and ions wcre compared precisely. However, the intensities of the absorption spcctra which correspoud to ~he concentration of the Unexcited radicals were not amenable to such a Comparison for the following reason. Light abseq)tion by the ignited gases, except CH (3143 ~), is very'weak compared with their 993
emission at the same wavelength. Therefore, the temperature of a continuously glowing xenon arc lamp is not high enough to be used as the light suurce of ubsorption spectra. The source required was a high temperature flash lamp. Iiowcver, this lamp emits light of varying intensities only for a short time. Thus, quantitative comparison of emission and ubsorption was not to be expected, although we attempted it in our previous work.~ We have now inlproved our apparatus by using a continuously glowing xenon arc lamp and have succeeded in measuring precisely the change in ab~rption spectra over the necessary time interval Norrish, et al./used a xenon arc lanlp as a light source for absorptio~l spectroscopy and obtained the time change of CN absorption, but thcir method was not applicable to C2, CH, and OH. In this paper, the new apparatus and the new observations on the absorption and
994
ELECTRICAL PROPERTIES
Fin. 1. Apparatus for obtaining absorption and emission spectra of burning mixtures. C, Combustion tube; F1, Xenon flash lamp for igniting fuel gases; L, Light source (Xenon arc lamp) for absorption spectra; Sp, Spectrometer; m~, m~, ms, Plane mirrors for enlarging D; M~, M~, Plane mirrors for double reflection; Phi, Photomultiplier for absorption and emission spectra; F-Ph2, Interference filter and a photomultiplier for emission spectra; Os~ Oscillograph; E, Electrode8 for the ion current; S, Electric source; R, Resistance to regulate the ion current. emission of C~, CH, OH are described, and the formation mechanism of these radicals and ions is discussed. II. ExperimentM (1) Mixing ratio of the fuel gases. The fuel gases used were mixtures of Call2 and 02 containLag a small quantity of NO~ as photosensitizer. The partial pressures of Call:, 02, and NO~ in the mixture are expressed respectively as p, p(O~) and p(N02). The earlier experiments had been carried out under the following conditions with several values of p:
p--Fp(02) = 21 Torr,
p(NO~)=4 Tort.
In the present experiments, the eontinmml accompanying the above band spectra was weakened by decreasing p(NO~), and the new experiments were performed under the conditions: pq-p(02) = 24 Torr,
p(N02) = 1 Torr.
In the following, the mixing ratio wilt be indicated by the partial pressure of C:H2, i.e., p. (2) Method of keeping the spectrometer at a distance from the combustion tube. With the conventional photoelectrical method, 2 except for CH (3143/~.), the absorption spectra of the burning mixtures could not be observed because of
their strong emission. In order to observe them, it was necessary to make the emitted light reachLag the spectrometer as weak as possible. This was achieved by making the distance between the combustion tube and the spectrometer entrance slit as large as possible. Since the ray from the source L (Fig. 1) was made nearly parallel, the intensity decrease was not as great as t h a t of the emission from gases. In fact, wlien the distance was extended to about 4 m, all the absorption spectra of Ctt (4315 s OH (3064 .~.), CN (3883 X) and the absorption continuum could be detected. (3) Method of double reflection. However, the absorption of Ca is so weak that this method was not enough to identify it. Therefore, we next increased the absorption of the gas mLxture by making the optical path inside the combustion tube longer by a double reflection method shown in Fig. 1. The tube length was 54 cm, and hence the optical path was 162 cm. By combining these "keep away" and "double reflection" methods, Ca absorption was finally observed. (4) Method of observing emission spectra. Emission spectra other than continua were observed through the side wall of the tube with an interference filter F, photomultiptier Ph2 and an oscillograph Os (Fig. 1). The filters were the stone as in our previous work.2 No differense was found, so far as the starting time was concerned, between the two emission spectra observed along the axis of the tube and through its side wall. (5) Method of comparison of two kinds of
RADICAL AND ION FORMATION
absorption spectra. Comparison of the time changes of the emission and absorption spectra of one kind of radical was thus made possible. On the other hand, accurate comparison of the emission spectra of any two rMicals was performed with thc method used in our previous study.~ From these two experiments we could compare the changes in the absorption of a n y two radicals. The starting times were also compared by taking photographs of the spectra, as Norrish et aI., did,5 with an interval of 10 ~sec to determine the order of appearance. (6) Method of measuring ionic current. The method used to measure the ionic current through the burning gases was the same as in our previous studies, ~ and is shown in Fig. 1. (7) Widlh of spectrometer inlet and exit slits. The width of the spectrometer inlet slit was adjusted to between 7 and 10 ~ depending on circumstances, and therefore the intensity of entering light was necessarily weak. We were obliged to set the width of exit slit at about 100 ~. This width corresponds to about 3.9 -~ at 5165 /~, and 0.8 .~ at 3143 ~ in terms of wavelength. Even with this broad slit, the osciilogram of the absorption was a broad line due to the weak intensity. No overlapping of any two band systems was seen in the regions we studied. (8) Absccption and emission continua. Norrish et al.,~ found that continuous spectra extending from below 3000 ~ to beyond 5000 ,~ exist in absorption. For our information, we attempted to study their time change, selecting the following four regions: (30404-16) .~, (43304-10) .~
995
in)
I
~'~
I
!
Ibl Fm. 2. Oscfllograms for C~ (5165 .~_). (a) p = ] 5 Torr; A, Absorption, E, Emission; I, Ion current. (b) Oscillogram when the sensitivity of photomultiplier is raised, p = 13 Torr. The first and second stages are marked with arrows I and II respectively.
(39004-10) .~, and
(51904-10) ~.
The centers of these regions are separated by about 20 .~, respectively, from the band heads of OH, CN, CH (4315 .&) and C.~. These regions are free from overlap by adjacent bands. As the emission continuum was observed with the same spectrometer, the emission and absorption continua could not be obtained simultaneously. Fortunately, however, the starting times of emission and absorption, based on that of the ionic current, showed comparatively small fluctuations (less than • ~sec) for any given mixture. This made comparison of the behavior of emission and absorption possible, taking the ionic current as the basis. III. T i m e C h a n g e s
of S p e c t r a
The spectra of Ca, CH, OH, and CN, either in emission or absorption, do not appear during the
induction period. They then start almost simultaneously and disappear after a short time. The period during which the spectra are seen will be referred to as the "main reaction period." (A) Continua Before and After the Main Reaction Period (1) Before the Period. A faint absorption continuum 3000-5000 ~_ appeared during the induction period for all mixing ratios. It disappeared as the nmin reaction began. :Norrish et al.,5 attribute this to "hot NO2 molecules" which are not dissociated by the light flash into NO and 0. The continuum was much stronger for p(NO2)=4 Torr than for p(NO~)=l Torr. When combustion was initiated by microwave without the help of NO2, the continuum did not appear.3 The absorber is undoubtedly some kind of molecule closely associated with N02. (2) After the Period. After the main reaction
ELECTRICAL PROPERTIES
996
lal
Ib) F ~ . 3. Oscillograins f~r CH. (a) CH (4315 o\) E. and A. and CN E., p=14 Torr. (b) CH (3143.~) E. and A. and C~ E., p - 1 5 Torr. period, another absorption eontimmm appeared for several seconds and then disappeared. When p was increased to 16 Torr (upper lhnit of flammability), the absorption was greatly intensified and soot deposited on the wall of the combustion tube. The absorber is unknown, but it is considered to be a species associated with carbon formation, or to he a cart)on particle itself.
the absorption of CN was studied wi0~ a photo. electrical method. 4 It is from our experimental results that one can deters,line correctly the order of appearances and the emission and absorption behavior of C2, CH, OH, and the ion current. (1) C2 system (~II~--3IIu), observed at 5165 ~. One of the oscillogrtm~s obtah~ed by our method is shown in Fig. 2(a). From these oscillogrm-as we can draw the fallowhlg new conclusions: (i) Absorption begins earlier than emission. (it) Emission intensity seems to grow in t,wo stages. hi the first stage, the intensity grows slowly, but soon it proceeds to the second stage and begins to increase rapidly. Figure 2(b) is an oscillogram that shows this clearly. (ill) The ion current beconles detectable during the first stage emission. (2) ON 3883 system (2X--2Y~), observed at 3883 .~. Our results show that CN absorption beghts, attains maxilrlt~l intensity and disappears sinmltaneonsly with that of C2; the same holds for the CN and Cs emissions. (3) CH 4315 system (2A--2i-[), observed at 4315 _~. ClI 3143 system (2Z+--21I), observed at 3143 .~. Figures 3(a) and (b) are the oscillograms of CH 4315 and CII 3143, respectively. Emissions of CN and C: are showlt for reference. The following conclusions may bc derived from Fig. 3: (i) CH 3143 changes in step with CH 4315, both in emission and absorption; (ii) Both h~ emission and '~bsorption, C2 and Ctt start nearly simultaneously, become strongest at the same time, and disappear in step with each other; (iii) As in thc case of C~, the emission intensity seenls to grow in two stages, and Co. lags a little behind CII in the first stage. This is clearly shown in our previous paper." They enter the second stage simultancously.
(B) Spectra that Appear During the Main Reaction Period Emissions of C2, CH (4315 .~), CH (3143 .~) and OH from burner flames are well known, although CIt (3143 .~) is very faint. The absorption of these band systems by burner flames is very weak but has bccn rcported by Jessen and Gaydon, 7 a~d Porter et al.s Thne Oranges of the absorption of C2, CH, OiI, and CN were first studied by Norrish et al., ~ but they conht not obtain detailed changes because they used a photographic method. Only
FIG. 4. Oseillogram for OH (3064 A) and. (]2 emission, p - 8 Tort.
RADICAL AND ION FORMATION
1(~ -~ ~ \
~ EM[SSION---O
\
(4315)31431UFI ~'~ [] A r O
iON
\ '\//
997
I !
/,z,
"
_ X
Q~'
'\
G
\\ \\ \
l/I] 8
9
10
11 12 13 14 15 1~6 P R E S S U R E OF C2H2(Torr) Fin. 5. Dependence of co, A and Q in arbitrary milts upon the pressure of C,I-I2. co, Maximum photoelectron current through the photomultiplier; A, Absorbencc of the fuel gas, i.e., log (Io/I); Q, Total charge through the electrodes. (4) OH 3064 system (2~--2II), observed at 30fi4 .~. An oscillogram is shown in Fig. 4. The results are as follows: (i) OH absorption appears sonsiderably (more than 100 psec) earlier than that of C~ and CH; (it) At about the time when OH absorption becomes strongest, ions begin to appear, as previously reported.~ (5) Continuous spectra during the main reaction period. Two different continua were found to exist together with band spectra. (i) First continuum (cont. 1). The emission and absorption of this continuum ap!~ear throughoat the whole range of 3000-6000 ~, synehronimd with those of the band spectra only when p>15 Tort. This continuum, tentatively call~t cont. 1, is the one observed and attributed to a kind of polynuclear aromatic substance by Norris]] el al. 6 (it) ~Second continuum (cont. 2). This continuum also appears in emission as well as in absorption in the region of short wavelengths (less than 3020 ~) when p < 10 Torr. The shorter the wavelength, the stronger the intensity, and the intensity thne change is synchronous with that of OH. This is referred to as cont. 2.
Norrish et al., s considering only from absorption spectroscopy, assumed the absorber to be carbon particles. However, this does not seem reasonable because the emission spectrum has its maximum intensity, if it exists, at a wavelength less than 3000 1~..The nature of this absorber will be discussed in Section V IV, Dependence ol Intensities of Spectra and Ionic Current Upon Mixing Ratio of Fuel Gases In order to conskler the mechanism of formation of ions and radicals, both excited and unexcited, one must know the concentrations of the radicals and ions. To determine these, the following three quantities were studied: (1) Intensity of emission spectra. The intensity of the emission spectra varies not only with the time but also with the mixing ratio of the fuel gases. After comparing various mixtures with each other, it was found that the stronger the maximum signal Co, the more prolonged the emission was. Therefore, co was taken as a quantity that corresponded to the concentration of the emitters, i.e., excited radicals.
998
ELECTRICAL PROPERTIES
(2) Absorbence of fuels. Expressing the intensities of the light incident upon and transmitted through a layer of gas as I0 and I respectively, the absorbence A = log(Io/I) is taken as a measure of the concentration of radicals in the ground state. (3) Net values of co and A. co and A for cont. 1 began to appear at p = 15 Torr and continued to increase with p up to about p = 16 Torr (upper limit of flammability). For cont. 2 they began at p = 9 Tort and continued, increasing as p decreased until p = 7 Torr (lower limit). Therefore, the net values of co and A of a band spectrum were obtained by subtracting those of the continuum adjacent to this band. This correction is especially large when p > 15 Torr. (4) Ion concentration. If i is the ionic current through the combustion tube at an instant t,
Bulewicz,n using burner flames of C~H~ of various mixing ratios, found that the maximura emission intensities of all the C~, CH, and OH were obtained when the mixture was stoiehio. metric. This corresponds to p=7.1 Torr in our experiment. The discrepancy might come from the difference in the condition of combustion, i.e., burner flame combustion and explosive combustion in a closed tube. V. Discus~on
In order to show clearly the differences of the behavior of the spectra of Ca, CH, OH, and two kinds of continua, the relation is schematically shown in Fig. 6 and approximate values of the starting time intervals are tabulated, in Table I. The time change behavior can be separated into three groups: (Ca,CN),
Q=
(CH, cont. 1)
and
(OH, cont. 2)
i dt
is the total charge transported between the electrodes. This Q was used as a rough measure of the total number of ions. Q was calculated from the oscillograms. (5) Dependence of co, A and Q upon the mixture ratio. The next problem is to find the dependence of these three quantities upon the partial pressures of C2H~. Our experimental results are smnmasized in Fig. 5. The sensitivity of a photomultiplier varies with wavelength, but the intensity and the absorbence of the spectrum of any one radical is proportional to Co and A, respectively. According to Fig. 5, the emission and absorption of all the carbon radicals (C~, CH, CN) are strongest for p = 14 to 15 Torr. Another maximum of emission seems to exist for p = 9 to 10 Tort. As Norrish, etal., ~'9 pointed out, the excitation of these radicals is considered to be partly thermal and partly chemical in nature. The mixture for p = 9 Torr is the one having the highest adiabatic temperature. TM Therefore, it would not be unreasonable to attribute the maximum brightness of carbon radicals when p = 9 to 10 Torr to the high temperature, whereas when p= 14 to 15 Tort it can be attributed to the large concentration of radicals. Both the emission and absorption of OH begin at p= 11 to 12 Torr and increase with the decreasing p. One of the important conclusions derived from Fig. 5 is that with a mixture having strong emission, strong absorption appears.
and the results may be summarized as follows. (i) Absorption begins earlier than emission. (ii) Absorption of OH begins earlier than that of C2 and CH. (iii) For C, and CtI, the emission intensity changes in two stages, but no such behavior is observed for OH. (iv) As regards first-stage emission, CH appears a little ahead of C2. (v) In second-stage emission, both C~ and CH start at the same time. (vi) Ions begin to appear with the first stage emission of C: and CH. The results from Fig. 5 are as follows: (vii) Emission and absorption of all kinds of carbon radicals attain maximums at p = 14 to 15 Torr. (viii) Emission and absorption of OH appear only when p < 1 0 Torr, and the smaller the partial pressure of C2H2, the stronger the emission and absorption are. (ix) When the emission and absorption of carbon radicals are strong (p= 14 to 15 Torr), the ionic current is weak, and when they are weak (p= l0 to 11 Torr), it is strong. On the basis of these experimental results, let us consider the formation mechanism of C2, CH, OH, and ions. (1) When a mixture is a good emitter of C2, CH, and OH spectra, it also acts as a good absorber for them. This suggests that when the concentration of any of these radicals in the ground state is large, its concentration in the excited state is large too. Considering also that
RADICAL AND ION FORMATION absorption always precedes emission, it is reasonablc to suppose that unexcited radicals are first produced and are then excited. (2) The excitation may be of chemical or thermal nature. The two-stage growth of C,2 and CH emission seems to show that there exist two kinds of excitation. (3) Both in emission and absorption, C2 and suggests that, as Gaydon and Wolfhardn proposed, they are produced by destruction of a parent radical (C2H). formed by polymerization of C2H. C~H has been found in hydrocarbon flames, and Hand and Kistiakowsky~a and Gardiner14 proposed the following reaction for its formation: C2H2+0= Call+OH.
999
Et
c v aN
|
1
A
~
all. cant. 1
tep
OH, e~nt.2
Thus, in flash photolysis, NOz+hv= N0+0,
(1)
C~H~+O = C2H-}-OH,
(2)
C2H--~(C2H)~---~C~,CH---*C~*,CH*.
(3)
TIME FIG. 6. Typical time dependence of the intensities of absorption and emission spectra, and of ion current. E, Intensity of emission spectra; A, Absorbenee of fuel gases (intensity of absorption spectra).
is a mechanism postulated from our experimental TABLE I results. As mentioned above, cont. 2 appears always in Values of intervals (vsec) step with OH. The responsible species are asp=$ T o r t ~l-& T o r t mm~ed, from Eq. (2), to be C~H or (C~H)~, but this is still uncertain. OH a b s o r p t i o n ~ - - - 200 A small difference between the starting times C2, CH a b a o r p t l o n of C2 and CH emissions suggests that part of -3 1O0 CH f i r s t e m i t s i o n J CH is formed by other reactions, such as +hose 5 ZO shown in the following Sects. (4) and (5). e Z flrst emission t 10 100 (4) OH in the ground state is fomled more than 100 ~see ahead of all other radicals. This is conIon i IO tOO sistent with the assumption that Eq. (2) is the C Z. CH s e c o n d e m l s ~ o ZOO Z000 first step in the series of reactions producing E m l s gion M a x i m t~m radicals and ions. However, if this is true, OII should appear fox" all nlixing ratios, whereas no OH is observed is considered to occur, and the CH thus produced when p > 12 Ton'. This may be interpreted as is txansformed into CHO + by a well known follows. When p is large, OH radlcais have Inauy reaction: opportunities of nleeting C2H~ and disappearing before being detected. The reaction in this ease C H + O = C H O + + e. (6) may be the one shown by Mukherjee ct o23~
]
0 H + C 2 H 2 = CH-}- H~-}-CO.
(4)
(5) When p(fJh) is eonlparatively large ( p < 11 Tort), the following reaction, supported by Hand and Kistiakowsky,~ and Gardiaer,'4 C21I+ 02= C H + CO
(5)
This causes the concentration of Cull to decrease, and as a result the intensity of C2, CH, C2", CH* decreases. (6) Equation (6) is widely accepted. However, there has been controversy over whether the CH in Eq. (6) is in the excited or ground state. Fontijn el al., 1~ considered CH to be excited
ELECTRICAL PROPERTIES
1000
while Peeters and Van TiggelenTMand Arrington el at., ~ did not. From our result (vi), it seems more resonable to be on the side of Fontijn. (7) The reaction of Eq. (2) is endothermic (AH< 20 Keal/mole)ta and does not occur at low temperatures. This is why the emission, absorption and ion production aft are weak witch p > 15 Torr (Fig. 5). Except for this ease of rich mixture, ion production is generally weak (strong) when emission and absorption are strong (weak). This seems at first glance to conflict with Eq. (6), but we eaz~ assume that CH emission and absorption are not observable when Eq. (6) occurs rapidly. If it does, CH will disappear before being observed, leaving ions behind. Thus, Fig. (5) and Eq. (6) are not inconsistent. Thus from several points of view, it seems reasonable to consider that Eq. (2) is the initial reaction in the formation of C~, CH, OH, and ions, and that C~ and CH form from C~H. However, with these experimental results alone, it is impossible to discuss further the formation mechanism of the radicals and ions. The authors are planning to carry out new experiments for the purpose of establishing the elementary reactions by which radicals and ions are formed. I t is needless to say that the above conclusions will play an important role in these developments. REFERENCES 1. KINRARA,T. ~ND NOD2,,~t~.: Twelfth Symposium (International) on Combustion, p. 395, The Combustion Institute, 1969. 2. /KINBAnA,T. AND NODA, K.: Thirteenth ~ymposture (International) on Combustion, p. 333, The Combustion Institute, 1971.
3. KINRAaA,T. aND NODA, ]~. : Feur~e~th Symposium (international) on Com~.stion, p. 321, The Combustion Institute, 1973. 4. No~us~, R. G. W., PORTER, G., XNDTHRUSH, B. A. : Fifth Symposium (International) on Combustion, p. 651, Reinhold, 1955. 5. No~ISH, R. G. W., PORTER, G., ANDTHRUSH, B. A.: Proc. R. Soc. A216, 165 (1953). 6. NORRISH,R. G. W., PORTER, G., ~kNDTHRUSH, B. A.: Proc. R. Soc. Lond. A227, 423 (1955). 7. JESSEN, P. F. AND GAYDON, A. G.: Combtlst. Flame 1I, 11 (1967). 8. PORTER,R. P., CL~RK, A. H., KaSv~'~, W. E., AHD BROW~F~,W. E.: Eleventh ~ymposium (International) an Combustion, p. 907, The Combustion Institute, 1967. 9. NORmSH,R. G. W., PORTEH,G., aHn THRUSH, B. A.: Nature 17~, 71 (1953). 10. MU~HSaZEE, N. R., FUE~o, T., EYnmO, H., ANn R~E, T.: Eighth Symposium (International) on Combustion~ p. 1, Williams and Wilkins, 1962. 11. BULEwlcz, E. M.: Combust. Flame 11, 297 (1967). 12. GAYDON,A. G. AND WOI~'HA~D, H. G.: Prec. Phys. Soc. Lo~d. A f t , 310 (1951). 13. HANDj C. W. AND KISTIAKOWSKYjG. B.: J. Chem. Phys. 37, 1239 (1962). 14. GARDINER,JR., W. C.: J. Chem. Phys. 60, 2410 (1964). 15. FO~T1JN, A., M~LLEE, W. J., AsH HOGAn, J. M.: Temh S~posium (International) o~ Combustion, p. 545, The Combustion Institute, 1965. 16. PEETEH, J. A~a VXN TIGGELEr~,A.: Twelfth Symposium (I~dernatianal) on Combustion, p. 437, The Combustion Institute, 1969. 17. ARRINGTO.'~,C. A., B R ~ N ~ , W., GLASS,G. P., MICHAEL~J. V., A.~DNt~t, H.: J. Chem. Phys. ~8, 1489 (1965).
COMMENTS P. J. Van Tiggelen, University of Louvain, Belgium. In your opinion, which of the two, excited or ground state CH radicals, contributes most to ion formation?
ground state CH. I t is difficult to definitely determine which one plays the more important role.
Authors' Reply. In flash photolysis, CH in the ground state appears first and then excited CH* appears, followed by the ion current. The ion current does not appear in the earliest stage when only CH absorption is observed. This is the reason we considered that probably a large part of ions are produced from CH*. However, we do not deny the possibility of ion production through
A. Fontijn, AeroChem Research Laboratories, U S A . Your suggested mechanism for the 0~C2H2 reaction appears at variance with that demonstrated by Bayes' group at the last symposium. I t is necessary to find a mechanism that is both consistent with your work as well as that of others. You erroneously quote me as favoring C H * + 0
RADICAL AND ION FORMATION over C H + O; see, e.g., m y review in "Progress in Reaction Kinetics," 6, 75 (1972). Authors' Reply. Our experimental results concerning the order of appearances of CH, CH* and ions do not coincide with those obtained by Zallen et al., 1 using a shock tube. The meclm~fism of combustion might be different depending on the manner of ignition. At this stage of our investigation, we cannot establish a complete scheme of elementary reactions applicable to the combustion of all kinds of hydrocarbons. The elementary reactions found by many authors are consistent with our experimental results.
1001
In your paper, 2 you show that when C2H2 is the fuel, the reaction CH*+ O - ~ H O + + e dominates C H + O--*CH0++ e. REFERENCES 1. ZALLEN,D. M., HIRL~MA~-,E. D., AND WlTrlO, S. L. K. : This Symposium. ~. FONTIJN~A., MILLER,~V. J., ~-~o tIoGAN~J. M.: Tenth Symposium (I.rdernational) on Combuslion, p. 545, The Combustion Institute, 1965.
Department of Physics, Sophia University, 7 Kioicho, Chiyodaku, Tokyo, Japan
The time change of the intensities of emission spectra of C2, CH and OH of the CzH2/O~/NO2 mixture ignited by a flash laanp and burned in a closed tube hs.s been previously reported. A s~milar study of the absorption spectra was not undertaken at the time because of the complications caused by emission from the reacting inLxture. In the present work, two methods were combined to overcome this difficulty. The spectrometer was moved away from the combustion ~ube and nmltiple path absorption was used. Thus, the time changes of all the emission and absorption spectra of C~, CIt and OH, and that of the ion culu'ent t~hrough the burning mixture, could be compared quantitatively. In other experiments, the dependence on the mixing ratio of the fuel gas was established for (1) the intensity of the emission spectrum, (2) the light absorbence of the fuel for C2, CH, and OH and (3) the total charge produced in the combustion tube. Based upon thee experimental results, the following mechanism of the formation of C2, CH, OII and chemi-ious is discussed: C:H~+O-~C~H +OH~OH*
(i)
C.~H-~(C.~H),,~C.~, CH-~C2*, CH*
(2)
CHTO~CHO++e
(3)
I. Introduction
In previous work, the present authors applied a flash photolysis tecl~ique to study the time changes of emission and absorption spectra of the explosively burning mixture C2H2/O2/NO2. The ionic current through the ignited gas was also observed and the growth and decay of emission, absorption and ionic current were cornpured, t-3 In these experiments, the time changes of the emission spectra and the ionic cnrrent~ and accordingly, those of the concentrations of excited radicals and ions wcre compared precisely. However, the intensities of the absorption spcctra which correspoud to ~he concentration of the Unexcited radicals were not amenable to such a Comparison for the following reason. Light abseq)tion by the ignited gases, except CH (3143 ~), is very'weak compared with their 993
emission at the same wavelength. Therefore, the temperature of a continuously glowing xenon arc lamp is not high enough to be used as the light suurce of ubsorption spectra. The source required was a high temperature flash lamp. Iiowcver, this lamp emits light of varying intensities only for a short time. Thus, quantitative comparison of emission and ubsorption was not to be expected, although we attempted it in our previous work.~ We have now inlproved our apparatus by using a continuously glowing xenon arc lamp and have succeeded in measuring precisely the change in ab~rption spectra over the necessary time interval Norrish, et al./used a xenon arc lanlp as a light source for absorptio~l spectroscopy and obtained the time change of CN absorption, but thcir method was not applicable to C2, CH, and OH. In this paper, the new apparatus and the new observations on the absorption and
994
ELECTRICAL PROPERTIES
Fin. 1. Apparatus for obtaining absorption and emission spectra of burning mixtures. C, Combustion tube; F1, Xenon flash lamp for igniting fuel gases; L, Light source (Xenon arc lamp) for absorption spectra; Sp, Spectrometer; m~, m~, ms, Plane mirrors for enlarging D; M~, M~, Plane mirrors for double reflection; Phi, Photomultiplier for absorption and emission spectra; F-Ph2, Interference filter and a photomultiplier for emission spectra; Os~ Oscillograph; E, Electrode8 for the ion current; S, Electric source; R, Resistance to regulate the ion current. emission of C~, CH, OH are described, and the formation mechanism of these radicals and ions is discussed. II. ExperimentM (1) Mixing ratio of the fuel gases. The fuel gases used were mixtures of Call2 and 02 containLag a small quantity of NO~ as photosensitizer. The partial pressures of Call:, 02, and NO~ in the mixture are expressed respectively as p, p(O~) and p(N02). The earlier experiments had been carried out under the following conditions with several values of p:
p--Fp(02) = 21 Torr,
p(NO~)=4 Tort.
In the present experiments, the eontinmml accompanying the above band spectra was weakened by decreasing p(NO~), and the new experiments were performed under the conditions: pq-p(02) = 24 Torr,
p(N02) = 1 Torr.
In the following, the mixing ratio wilt be indicated by the partial pressure of C:H2, i.e., p. (2) Method of keeping the spectrometer at a distance from the combustion tube. With the conventional photoelectrical method, 2 except for CH (3143/~.), the absorption spectra of the burning mixtures could not be observed because of
their strong emission. In order to observe them, it was necessary to make the emitted light reachLag the spectrometer as weak as possible. This was achieved by making the distance between the combustion tube and the spectrometer entrance slit as large as possible. Since the ray from the source L (Fig. 1) was made nearly parallel, the intensity decrease was not as great as t h a t of the emission from gases. In fact, wlien the distance was extended to about 4 m, all the absorption spectra of Ctt (4315 s OH (3064 .~.), CN (3883 X) and the absorption continuum could be detected. (3) Method of double reflection. However, the absorption of Ca is so weak that this method was not enough to identify it. Therefore, we next increased the absorption of the gas mLxture by making the optical path inside the combustion tube longer by a double reflection method shown in Fig. 1. The tube length was 54 cm, and hence the optical path was 162 cm. By combining these "keep away" and "double reflection" methods, Ca absorption was finally observed. (4) Method of observing emission spectra. Emission spectra other than continua were observed through the side wall of the tube with an interference filter F, photomultiptier Ph2 and an oscillograph Os (Fig. 1). The filters were the stone as in our previous work.2 No differense was found, so far as the starting time was concerned, between the two emission spectra observed along the axis of the tube and through its side wall. (5) Method of comparison of two kinds of
RADICAL AND ION FORMATION
absorption spectra. Comparison of the time changes of the emission and absorption spectra of one kind of radical was thus made possible. On the other hand, accurate comparison of the emission spectra of any two rMicals was performed with thc method used in our previous study.~ From these two experiments we could compare the changes in the absorption of a n y two radicals. The starting times were also compared by taking photographs of the spectra, as Norrish et aI., did,5 with an interval of 10 ~sec to determine the order of appearance. (6) Method of measuring ionic current. The method used to measure the ionic current through the burning gases was the same as in our previous studies, ~ and is shown in Fig. 1. (7) Widlh of spectrometer inlet and exit slits. The width of the spectrometer inlet slit was adjusted to between 7 and 10 ~ depending on circumstances, and therefore the intensity of entering light was necessarily weak. We were obliged to set the width of exit slit at about 100 ~. This width corresponds to about 3.9 -~ at 5165 /~, and 0.8 .~ at 3143 ~ in terms of wavelength. Even with this broad slit, the osciilogram of the absorption was a broad line due to the weak intensity. No overlapping of any two band systems was seen in the regions we studied. (8) Absccption and emission continua. Norrish et al.,~ found that continuous spectra extending from below 3000 ~ to beyond 5000 ,~ exist in absorption. For our information, we attempted to study their time change, selecting the following four regions: (30404-16) .~, (43304-10) .~
995
in)
I
~'~
I
!
Ibl Fm. 2. Oscfllograms for C~ (5165 .~_). (a) p = ] 5 Torr; A, Absorption, E, Emission; I, Ion current. (b) Oscillogram when the sensitivity of photomultiplier is raised, p = 13 Torr. The first and second stages are marked with arrows I and II respectively.
(39004-10) .~, and
(51904-10) ~.
The centers of these regions are separated by about 20 .~, respectively, from the band heads of OH, CN, CH (4315 .&) and C.~. These regions are free from overlap by adjacent bands. As the emission continuum was observed with the same spectrometer, the emission and absorption continua could not be obtained simultaneously. Fortunately, however, the starting times of emission and absorption, based on that of the ionic current, showed comparatively small fluctuations (less than • ~sec) for any given mixture. This made comparison of the behavior of emission and absorption possible, taking the ionic current as the basis. III. T i m e C h a n g e s
of S p e c t r a
The spectra of Ca, CH, OH, and CN, either in emission or absorption, do not appear during the
induction period. They then start almost simultaneously and disappear after a short time. The period during which the spectra are seen will be referred to as the "main reaction period." (A) Continua Before and After the Main Reaction Period (1) Before the Period. A faint absorption continuum 3000-5000 ~_ appeared during the induction period for all mixing ratios. It disappeared as the nmin reaction began. :Norrish et al.,5 attribute this to "hot NO2 molecules" which are not dissociated by the light flash into NO and 0. The continuum was much stronger for p(NO2)=4 Torr than for p(NO~)=l Torr. When combustion was initiated by microwave without the help of NO2, the continuum did not appear.3 The absorber is undoubtedly some kind of molecule closely associated with N02. (2) After the Period. After the main reaction
ELECTRICAL PROPERTIES
996
lal
Ib) F ~ . 3. Oscillograins f~r CH. (a) CH (4315 o\) E. and A. and CN E., p=14 Torr. (b) CH (3143.~) E. and A. and C~ E., p - 1 5 Torr. period, another absorption eontimmm appeared for several seconds and then disappeared. When p was increased to 16 Torr (upper lhnit of flammability), the absorption was greatly intensified and soot deposited on the wall of the combustion tube. The absorber is unknown, but it is considered to be a species associated with carbon formation, or to he a cart)on particle itself.
the absorption of CN was studied wi0~ a photo. electrical method. 4 It is from our experimental results that one can deters,line correctly the order of appearances and the emission and absorption behavior of C2, CH, OH, and the ion current. (1) C2 system (~II~--3IIu), observed at 5165 ~. One of the oscillogrtm~s obtah~ed by our method is shown in Fig. 2(a). From these oscillogrm-as we can draw the fallowhlg new conclusions: (i) Absorption begins earlier than emission. (it) Emission intensity seems to grow in t,wo stages. hi the first stage, the intensity grows slowly, but soon it proceeds to the second stage and begins to increase rapidly. Figure 2(b) is an oscillogram that shows this clearly. (ill) The ion current beconles detectable during the first stage emission. (2) ON 3883 system (2X--2Y~), observed at 3883 .~. Our results show that CN absorption beghts, attains maxilrlt~l intensity and disappears sinmltaneonsly with that of C2; the same holds for the CN and Cs emissions. (3) CH 4315 system (2A--2i-[), observed at 4315 _~. ClI 3143 system (2Z+--21I), observed at 3143 .~. Figures 3(a) and (b) are the oscillograms of CH 4315 and CII 3143, respectively. Emissions of CN and C: are showlt for reference. The following conclusions may bc derived from Fig. 3: (i) CH 3143 changes in step with CH 4315, both in emission and absorption; (ii) Both h~ emission and '~bsorption, C2 and Ctt start nearly simultaneously, become strongest at the same time, and disappear in step with each other; (iii) As in thc case of C~, the emission intensity seenls to grow in two stages, and Co. lags a little behind CII in the first stage. This is clearly shown in our previous paper." They enter the second stage simultancously.
(B) Spectra that Appear During the Main Reaction Period Emissions of C2, CH (4315 .~), CH (3143 .~) and OH from burner flames are well known, although CIt (3143 .~) is very faint. The absorption of these band systems by burner flames is very weak but has bccn rcported by Jessen and Gaydon, 7 a~d Porter et al.s Thne Oranges of the absorption of C2, CH, OiI, and CN were first studied by Norrish et al., ~ but they conht not obtain detailed changes because they used a photographic method. Only
FIG. 4. Oseillogram for OH (3064 A) and. (]2 emission, p - 8 Tort.
RADICAL AND ION FORMATION
1(~ -~ ~ \
~ EM[SSION---O
\
(4315)31431UFI ~'~ [] A r O
iON
\ '\//
997
I !
/,z,
"
_ X
Q~'
'\
G
\\ \\ \
l/I] 8
9
10
11 12 13 14 15 1~6 P R E S S U R E OF C2H2(Torr) Fin. 5. Dependence of co, A and Q in arbitrary milts upon the pressure of C,I-I2. co, Maximum photoelectron current through the photomultiplier; A, Absorbencc of the fuel gas, i.e., log (Io/I); Q, Total charge through the electrodes. (4) OH 3064 system (2~--2II), observed at 30fi4 .~. An oscillogram is shown in Fig. 4. The results are as follows: (i) OH absorption appears sonsiderably (more than 100 psec) earlier than that of C~ and CH; (it) At about the time when OH absorption becomes strongest, ions begin to appear, as previously reported.~ (5) Continuous spectra during the main reaction period. Two different continua were found to exist together with band spectra. (i) First continuum (cont. 1). The emission and absorption of this continuum ap!~ear throughoat the whole range of 3000-6000 ~, synehronimd with those of the band spectra only when p>15 Tort. This continuum, tentatively call~t cont. 1, is the one observed and attributed to a kind of polynuclear aromatic substance by Norris]] el al. 6 (it) ~Second continuum (cont. 2). This continuum also appears in emission as well as in absorption in the region of short wavelengths (less than 3020 ~) when p < 10 Torr. The shorter the wavelength, the stronger the intensity, and the intensity thne change is synchronous with that of OH. This is referred to as cont. 2.
Norrish et al., s considering only from absorption spectroscopy, assumed the absorber to be carbon particles. However, this does not seem reasonable because the emission spectrum has its maximum intensity, if it exists, at a wavelength less than 3000 1~..The nature of this absorber will be discussed in Section V IV, Dependence ol Intensities of Spectra and Ionic Current Upon Mixing Ratio of Fuel Gases In order to conskler the mechanism of formation of ions and radicals, both excited and unexcited, one must know the concentrations of the radicals and ions. To determine these, the following three quantities were studied: (1) Intensity of emission spectra. The intensity of the emission spectra varies not only with the time but also with the mixing ratio of the fuel gases. After comparing various mixtures with each other, it was found that the stronger the maximum signal Co, the more prolonged the emission was. Therefore, co was taken as a quantity that corresponded to the concentration of the emitters, i.e., excited radicals.
998
ELECTRICAL PROPERTIES
(2) Absorbence of fuels. Expressing the intensities of the light incident upon and transmitted through a layer of gas as I0 and I respectively, the absorbence A = log(Io/I) is taken as a measure of the concentration of radicals in the ground state. (3) Net values of co and A. co and A for cont. 1 began to appear at p = 15 Torr and continued to increase with p up to about p = 16 Torr (upper limit of flammability). For cont. 2 they began at p = 9 Tort and continued, increasing as p decreased until p = 7 Torr (lower limit). Therefore, the net values of co and A of a band spectrum were obtained by subtracting those of the continuum adjacent to this band. This correction is especially large when p > 15 Torr. (4) Ion concentration. If i is the ionic current through the combustion tube at an instant t,
Bulewicz,n using burner flames of C~H~ of various mixing ratios, found that the maximura emission intensities of all the C~, CH, and OH were obtained when the mixture was stoiehio. metric. This corresponds to p=7.1 Torr in our experiment. The discrepancy might come from the difference in the condition of combustion, i.e., burner flame combustion and explosive combustion in a closed tube. V. Discus~on
In order to show clearly the differences of the behavior of the spectra of Ca, CH, OH, and two kinds of continua, the relation is schematically shown in Fig. 6 and approximate values of the starting time intervals are tabulated, in Table I. The time change behavior can be separated into three groups: (Ca,CN),
Q=
(CH, cont. 1)
and
(OH, cont. 2)
i dt
is the total charge transported between the electrodes. This Q was used as a rough measure of the total number of ions. Q was calculated from the oscillograms. (5) Dependence of co, A and Q upon the mixture ratio. The next problem is to find the dependence of these three quantities upon the partial pressures of C2H~. Our experimental results are smnmasized in Fig. 5. The sensitivity of a photomultiplier varies with wavelength, but the intensity and the absorbence of the spectrum of any one radical is proportional to Co and A, respectively. According to Fig. 5, the emission and absorption of all the carbon radicals (C~, CH, CN) are strongest for p = 14 to 15 Torr. Another maximum of emission seems to exist for p = 9 to 10 Tort. As Norrish, etal., ~'9 pointed out, the excitation of these radicals is considered to be partly thermal and partly chemical in nature. The mixture for p = 9 Torr is the one having the highest adiabatic temperature. TM Therefore, it would not be unreasonable to attribute the maximum brightness of carbon radicals when p = 9 to 10 Torr to the high temperature, whereas when p= 14 to 15 Tort it can be attributed to the large concentration of radicals. Both the emission and absorption of OH begin at p= 11 to 12 Torr and increase with the decreasing p. One of the important conclusions derived from Fig. 5 is that with a mixture having strong emission, strong absorption appears.
and the results may be summarized as follows. (i) Absorption begins earlier than emission. (ii) Absorption of OH begins earlier than that of C2 and CH. (iii) For C, and CtI, the emission intensity changes in two stages, but no such behavior is observed for OH. (iv) As regards first-stage emission, CH appears a little ahead of C2. (v) In second-stage emission, both C~ and CH start at the same time. (vi) Ions begin to appear with the first stage emission of C: and CH. The results from Fig. 5 are as follows: (vii) Emission and absorption of all kinds of carbon radicals attain maximums at p = 14 to 15 Torr. (viii) Emission and absorption of OH appear only when p < 1 0 Torr, and the smaller the partial pressure of C2H2, the stronger the emission and absorption are. (ix) When the emission and absorption of carbon radicals are strong (p= 14 to 15 Torr), the ionic current is weak, and when they are weak (p= l0 to 11 Torr), it is strong. On the basis of these experimental results, let us consider the formation mechanism of C2, CH, OH, and ions. (1) When a mixture is a good emitter of C2, CH, and OH spectra, it also acts as a good absorber for them. This suggests that when the concentration of any of these radicals in the ground state is large, its concentration in the excited state is large too. Considering also that
RADICAL AND ION FORMATION absorption always precedes emission, it is reasonablc to suppose that unexcited radicals are first produced and are then excited. (2) The excitation may be of chemical or thermal nature. The two-stage growth of C,2 and CH emission seems to show that there exist two kinds of excitation. (3) Both in emission and absorption, C2 and suggests that, as Gaydon and Wolfhardn proposed, they are produced by destruction of a parent radical (C2H). formed by polymerization of C2H. C~H has been found in hydrocarbon flames, and Hand and Kistiakowsky~a and Gardiner14 proposed the following reaction for its formation: C2H2+0= Call+OH.
999
Et
c v aN
|
1
A
~
all. cant. 1
tep
OH, e~nt.2
Thus, in flash photolysis, NOz+hv= N0+0,
(1)
C~H~+O = C2H-}-OH,
(2)
C2H--~(C2H)~---~C~,CH---*C~*,CH*.
(3)
TIME FIG. 6. Typical time dependence of the intensities of absorption and emission spectra, and of ion current. E, Intensity of emission spectra; A, Absorbenee of fuel gases (intensity of absorption spectra).
is a mechanism postulated from our experimental TABLE I results. As mentioned above, cont. 2 appears always in Values of intervals (vsec) step with OH. The responsible species are asp=$ T o r t ~l-& T o r t mm~ed, from Eq. (2), to be C~H or (C~H)~, but this is still uncertain. OH a b s o r p t i o n ~ - - - 200 A small difference between the starting times C2, CH a b a o r p t l o n of C2 and CH emissions suggests that part of -3 1O0 CH f i r s t e m i t s i o n J CH is formed by other reactions, such as +hose 5 ZO shown in the following Sects. (4) and (5). e Z flrst emission t 10 100 (4) OH in the ground state is fomled more than 100 ~see ahead of all other radicals. This is conIon i IO tOO sistent with the assumption that Eq. (2) is the C Z. CH s e c o n d e m l s ~ o ZOO Z000 first step in the series of reactions producing E m l s gion M a x i m t~m radicals and ions. However, if this is true, OII should appear fox" all nlixing ratios, whereas no OH is observed is considered to occur, and the CH thus produced when p > 12 Ton'. This may be interpreted as is txansformed into CHO + by a well known follows. When p is large, OH radlcais have Inauy reaction: opportunities of nleeting C2H~ and disappearing before being detected. The reaction in this ease C H + O = C H O + + e. (6) may be the one shown by Mukherjee ct o23~
]
0 H + C 2 H 2 = CH-}- H~-}-CO.
(4)
(5) When p(fJh) is eonlparatively large ( p < 11 Tort), the following reaction, supported by Hand and Kistiakowsky,~ and Gardiaer,'4 C21I+ 02= C H + CO
(5)
This causes the concentration of Cull to decrease, and as a result the intensity of C2, CH, C2", CH* decreases. (6) Equation (6) is widely accepted. However, there has been controversy over whether the CH in Eq. (6) is in the excited or ground state. Fontijn el al., 1~ considered CH to be excited
ELECTRICAL PROPERTIES
1000
while Peeters and Van TiggelenTMand Arrington el at., ~ did not. From our result (vi), it seems more resonable to be on the side of Fontijn. (7) The reaction of Eq. (2) is endothermic (AH< 20 Keal/mole)ta and does not occur at low temperatures. This is why the emission, absorption and ion production aft are weak witch p > 15 Torr (Fig. 5). Except for this ease of rich mixture, ion production is generally weak (strong) when emission and absorption are strong (weak). This seems at first glance to conflict with Eq. (6), but we eaz~ assume that CH emission and absorption are not observable when Eq. (6) occurs rapidly. If it does, CH will disappear before being observed, leaving ions behind. Thus, Fig. (5) and Eq. (6) are not inconsistent. Thus from several points of view, it seems reasonable to consider that Eq. (2) is the initial reaction in the formation of C~, CH, OH, and ions, and that C~ and CH form from C~H. However, with these experimental results alone, it is impossible to discuss further the formation mechanism of the radicals and ions. The authors are planning to carry out new experiments for the purpose of establishing the elementary reactions by which radicals and ions are formed. I t is needless to say that the above conclusions will play an important role in these developments. REFERENCES 1. KINRARA,T. ~ND NOD2,,~t~.: Twelfth Symposium (International) on Combustion, p. 395, The Combustion Institute, 1969. 2. /KINBAnA,T. AND NODA, K.: Thirteenth ~ymposture (International) on Combustion, p. 333, The Combustion Institute, 1971.
3. KINRAaA,T. aND NODA, ]~. : Feur~e~th Symposium (international) on Com~.stion, p. 321, The Combustion Institute, 1973. 4. No~us~, R. G. W., PORTER, G., XNDTHRUSH, B. A. : Fifth Symposium (International) on Combustion, p. 651, Reinhold, 1955. 5. No~ISH, R. G. W., PORTER, G., ANDTHRUSH, B. A.: Proc. R. Soc. A216, 165 (1953). 6. NORRISH,R. G. W., PORTER, G., ~kNDTHRUSH, B. A.: Proc. R. Soc. Lond. A227, 423 (1955). 7. JESSEN, P. F. AND GAYDON, A. G.: Combtlst. Flame 1I, 11 (1967). 8. PORTER,R. P., CL~RK, A. H., KaSv~'~, W. E., AHD BROW~F~,W. E.: Eleventh ~ymposium (International) an Combustion, p. 907, The Combustion Institute, 1967. 9. NORmSH,R. G. W., PORTEH,G., aHn THRUSH, B. A.: Nature 17~, 71 (1953). 10. MU~HSaZEE, N. R., FUE~o, T., EYnmO, H., ANn R~E, T.: Eighth Symposium (International) on Combustion~ p. 1, Williams and Wilkins, 1962. 11. BULEwlcz, E. M.: Combust. Flame 11, 297 (1967). 12. GAYDON,A. G. AND WOI~'HA~D, H. G.: Prec. Phys. Soc. Lo~d. A f t , 310 (1951). 13. HANDj C. W. AND KISTIAKOWSKYjG. B.: J. Chem. Phys. 37, 1239 (1962). 14. GARDINER,JR., W. C.: J. Chem. Phys. 60, 2410 (1964). 15. FO~T1JN, A., M~LLEE, W. J., AsH HOGAn, J. M.: Temh S~posium (International) o~ Combustion, p. 545, The Combustion Institute, 1965. 16. PEETEH, J. A~a VXN TIGGELEr~,A.: Twelfth Symposium (I~dernatianal) on Combustion, p. 437, The Combustion Institute, 1969. 17. ARRINGTO.'~,C. A., B R ~ N ~ , W., GLASS,G. P., MICHAEL~J. V., A.~DNt~t, H.: J. Chem. Phys. ~8, 1489 (1965).
COMMENTS P. J. Van Tiggelen, University of Louvain, Belgium. In your opinion, which of the two, excited or ground state CH radicals, contributes most to ion formation?
ground state CH. I t is difficult to definitely determine which one plays the more important role.
Authors' Reply. In flash photolysis, CH in the ground state appears first and then excited CH* appears, followed by the ion current. The ion current does not appear in the earliest stage when only CH absorption is observed. This is the reason we considered that probably a large part of ions are produced from CH*. However, we do not deny the possibility of ion production through
A. Fontijn, AeroChem Research Laboratories, U S A . Your suggested mechanism for the 0~C2H2 reaction appears at variance with that demonstrated by Bayes' group at the last symposium. I t is necessary to find a mechanism that is both consistent with your work as well as that of others. You erroneously quote me as favoring C H * + 0
RADICAL AND ION FORMATION over C H + O; see, e.g., m y review in "Progress in Reaction Kinetics," 6, 75 (1972). Authors' Reply. Our experimental results concerning the order of appearances of CH, CH* and ions do not coincide with those obtained by Zallen et al., 1 using a shock tube. The meclm~fism of combustion might be different depending on the manner of ignition. At this stage of our investigation, we cannot establish a complete scheme of elementary reactions applicable to the combustion of all kinds of hydrocarbons. The elementary reactions found by many authors are consistent with our experimental results.
1001
In your paper, 2 you show that when C2H2 is the fuel, the reaction CH*+ O - ~ H O + + e dominates C H + O--*CH0++ e. REFERENCES 1. ZALLEN,D. M., HIRL~MA~-,E. D., AND WlTrlO, S. L. K. : This Symposium. ~. FONTIJN~A., MILLER,~V. J., ~-~o tIoGAN~J. M.: Tenth Symposium (I.rdernational) on Combuslion, p. 545, The Combustion Institute, 1965.