A cyclotron resonance study of ionization in low-pressure flames

A CYCLOTRON RESONANCE STUDY OF IONIZATION IN L O W - P R E S S U R E FLAMES E. M. BULEWICZ AND P. J. PADLEY Electron concentrations and electron-molecule collision cross sections have been measured by the cyclotron resonance method in flames of hydrocarbons, alcohols, esters, ketones, and ethers, all at reduced pressure. The following summarizes the main observations made in the reaction zone: 1. The average electron-flame gas molecule collision cross section varies little from fuel to fuel. 2. The electron concentration in hydrocarbon flames is proportional to the total pressure. 3. A plot of the ratio of the electron concentration per molecule of fuel to the total burned flame gas concentration against the number of carbon atoms in the molecule shows the following regularities: points for saturated hydrocarbons lie on a smooth curve; those for unsaturated hydrocarbons lie on various smooth curves displaced upwards to greater ionization levels; if the fuel contains one oxygen atom the ionization is lowered by an approximately constant amount with respect to the corresponding saturated hydrocarbon; when two oxygen atoms are present the effect is doubled. 4. The effect of inert additives such as argon~ and of nonhydrocarbon fuel additives such as hydrogen was studied. The results are shown to suggest that a very important step in the process of ion production is the reaction CH ~- O --~ CHO + ~- e-. Polymerization reactions as a means of ion production appear to be of secondary importance.

Introduction The understanding of the phenomenon of ionization in flames is a problem which has received increased attention in the last decade. 1-8 The experimental methods which have been used fall into three general groups: (a) measurements with probes, 1 giving quantitative information about the over-all ionization level; (b) studies of the effect of free electrons on the characteristics of microwave and radiofrequency circuits~,3; (c) recent, elegant applications of the mass spectrometer, by which the individual positive ions can be identified.4-7 Although it is well known that the ionization in the region of primary reaction in hydrocarbon flames is nonthermal, the process b y which the ions are produced is still a subject for considerable speculation. This situation exists mainly because the rapidity of the initial combustion processes has so far precluded kinetic studies of the individual reactions. In general, the nature of the elementary processes must still be inferred from their over-all effect, on the level of ionization [-methods (a) and (b)3 and from the type

of ions produced [method (c)l. So far, only the results of method (e) have led to any significant elucidation of the problem.

There has as y e t been no systematic, quantitative study of the behavior of the over-aU ionization level in a wide range of fuels under similar conditions with the view to understanding these processes in any detail. Such an a t t e m p t forms the basis of the present paper. The cyclotron resonance method used--essentially a type (b) method--is novel in its present application and will therefore be briefly outlined.

Theory of Method The principle utilized here is t h a t the cyclotron motion of free electrons, wMch takes place in an applied magnetic field, will cause power to be absorbed from a beam of electromagnetic radiation with its electric vector perpendicular to the field, provided t h a t the radiation frequency is equal to t h a t of the cyclotron motion. The product of power loss and line width at the resonance frequency can be related quantitatively to the concentration of electrons present in the flame. F r o m the width of the resonance curve, electron-molecule collision frequencies can be calculated.~13 The salient features of the theory of cyclotron resonance relevant to the present work are reproduced below.

638

IONIZATION IN LOW-PRESSURE FLAMES The attenuation, B, in db, of the intensity o f an electromagnetic wave passing through a partially ionized gas is related to the real part, a', of the electric conductivity by the expression = 40~rda'/c. log10 e

-

(9.)

where n is the number of electrons/cc, v the electron-molecule collision frequency, ~o and wc the microwave and cyclotron frequencies, respectively, and e and m have their usual significance. When w >> v, cyclotron resonance occurs near o~ =

~c

=

eH/mc,

where H is the magnetic field strength in gauss. From Eqs. (1) and (2) it can be shown that n = AU(r~'/ec = AHfl/4Ored

where No is Loschmidt's number and p is the pressure in mm Hg. Equation (6) reduces to A H / p ---- (6.8 X 1017) 9 Q/T 89

(7)

(1)

where d is the path length in cm, and c the velocity of light. In a uniform magnetic field, parallel to the Z axis, the attenuation of a microwave beam travelling with E vector perpendicular to the field will be determined by the real part of the (transverse) component, g~', of the conductivity, given by

+ ./I-. +

639

lOgloe

(3)

Experimental The premixed flames were burned at pressures between 8 and 200 mm Hg inside a cylindrical Pyrex vessel, 20 mm in diameter on burner tubes 10-15 mm in diameter--an arrangement essentially similar to that described by Gaydon and Wolfhard. 14 The vessel was air-cooled. Gaseous fuels were obtained from the Matheson Gas Company, and all were stated to be at least 99 per cent pure. The gases were metered at atmospheric pressure with rotameters, after which they were sucked into the reduced pressure part of the apparatus through controlled leaks. Liquid fuels were introduced by bubbling a measured part of the oxygen supply through the thermostated fuel, contained in a saturator at atmospheric pressure. The total gas flow (at room temperature and atmospheric pressure) did not exceed 800 cc/min. The flame compositions were varied between k = 0.6 and k = 2.0 (k, the mixture strength, = ]-oxygen present in unburned gasesl/['oxygen present at stoichiom-

where AH is the cyclotron line width between

etry]).

the half conductivity points and f~ the attenuation at the center of the line. Equation (3) holds provided that w2 >~ ~ = neS/m~, where r is the plasma frequency and 9 the dielectric constant. This condition is satisfied for values of n (1-5 X 101~electrons/cc) and used in this work. When the cyclotron line is sufficiently sharp, v is given by

The flame vessel was arranged vertically in the 2.3 cm gap of a 12-inch, 20,000 gauss maximum, Varian Associates' electromagnet (field homogeneity over 1 cu inch volume at the center of the gap better than 1 part in 104). Radiation at 48 kMc/sec (obtained from a standard 2K33 Raytheon klystron combined with frequency multiplier), and modulated at 2 kc/sec, was passed horizontally through the flame in the vicinity of the reaction zone, i.e., the region of maximum electron concentration. The transmission across the flame was effected b y two waveguide horns (I to K band transition sections) firmly clamped in a mount which surrounded tightly the flame vessel. The direction of flame propagation, the magnetic field and the E vector of the radiation were, therefore, all mutually perpendicular. After crystal detection and suitable amplification, the power transmitted through the flame was plotted automatically as a function of magnetic field strength, which was swept electrically over a range of about 3000 gauss in 2 minutes. This was sufficient to cover the whole of the resonance curve, the center of which lay near 17,000 gauss. Unless otherwise stated, all measurements were made in the reaction zone, which was several mm thick at the pressures used.

AH/Ho = 2v/r

(4)

where H0 is the value of the field at which maximum attenuation occurs. Hence the electronmolecule collision cross section, Q, can be obtMned from Q = v/N~

(5)

where ~ is the mean electron velocity [f~ = (8kT/~'m)i] and N the concentration of neutral molecules. Rearrangement of this equation leads to

= L',-m

-j

(6)

640

FUNDAMENTAL FLAME PROCESSES

TABLE 1 Electron Molecule Collision Cross Sections in Different Fuels

C:H ratio 1 :1 3:4 2 :3 1 :2 3: 7 5:12 2:5 3: 8 1 :3 1:4

Fuel Acetylene Methyl acetylene Ethyl acetylene Ethylene n-Hexane 2-Methyl butane Diethyl ether Propane Ethane Methane

Results and Discussion

Average Sections

Electron-Molecule

Collision Cross

The third column in Table 1 lists observed AH/p values for a selection of stoichiometric fueloxygen flames; a more detailed list can be found elsewhere?3 Q values are not given since the temperatures of all these flames are not known. However, since T i is hardly likely to vary greatly from one stoichiometric flame to another, the AH/p values should be approximately proportional to Q. The fourth column in Table 1 lists the values of Q calculated for each fuel on the reasonable assumptions that only carbon dioxide and water contribute significantly to the over-all Q in stoichiometric mixtures, and that the values found for Qu2o and Qco2 in acetylene flames at 2000~ i.e., 80 =t= 4 ~2, and 37 4- 2 ~2, respectively, n are of general applicability. T h a t these approximations are reasonable is illustrated by the fact that the values of Q * p/AH (column 5) show no dependence on the carbon to hydrogen ratio. Further, the absence of significant deviations from the mean value of 1.06 (the closeness to unity is fortuitous) suggests t h a t the temperature factor in Eq. (7) is, indeed, unimportant. Thus the average electron-molecule collision cross section for a flame can apparently be predicted with some certainty from known cross section values for its constituents.

hH/p observed (gauss/ram Hg)

Average Q (calc., A)

Q.p/AH

49 50.5 52 55 56 58 54 57 58 60

51.5 54 55.5 58 60 60.5 61 61.5 63 65.5

1.05 1.07 1.07 1.05 1.07 1.03 1.13 1.08 1.09 1.09

constant. In this work this relationship was tested at lower pressures (between 12 and 45 mm Hg) with other hydrocarbons. A typical plot of the electron concentration, n, against the total pressure, p, for an acetylene-oxygen flame is shown in Fig. 1. I t is a good straight line (in agreement with King's observations at higher pressure) and appears to pass through the origin. Similar results are obtained at other mixture strengths and for other hydrocarbons--at least for those used in compiling Fig. 3. Thus R, the ratio of the electron pressure to the total burned gas pressure, is independent of the total pressure. This suggests that the most important processes of electron production and decay are kinetically of the same order, because at the point of measurement (maximum electron concentration in the reaction zone) steady state conditions for electrons are presumed to exist. Since termolecular reactions in flames (at atmospheric pressure) are known to require of the order of 10 milliseconds to reach completion, 16 and since a

;o

Electron Concentration as a Function of Pressure I t has been previously found b y King, 15 for propane-air flames bctwcen 100 mm Hg and 1 atmosphere, that the ratio of positive ion pressure to the total burned flame gas pressure is

i

~ p

~o

(mm Hg)

FIG. 1 Variation of electron concentration with total flame gas pressure in an acetylene-oxygen flame (k = 0.9).

IONIZATION

IN

LOW-PRESSURE

molecule passes through the reaction zone in about 50 microseconds, it is reasonable to conclude that bimolecular processes predominate.

641

FLAMES

C6H69

2.5

OXYGEN " D I L U E N T " O ARGON DILUENT

C3H 6

2.0

C2H 4

Ionization in the Presence of Nonreactive Additives The preburned mixtures were diluted with argon and nitrogen in such a manner that both the mixture strength, )~, and the total flow rate of unburned gases remained constant. Representative results for fuel-rich, stoichiometric, and fuel-lean acetylene-oxygen flames are shown in Fig. 2, which is a logarithmic plot of R (proportional to n (T/p), where T is the measured flame temperature) against the mole fraction of acetylene present in the unburned gases. The features of interest are: firstly, all plots for nitrogen and argon addition have the same slope of two; secondly (again for nitrogen and argon) all points for ~ ~ 1.0 lie on the same straight line; thirdly, if n is measured in pure acetyleneoxygen flames at various k, and the same type of graph plotted, then for oxygen-rich flames the experimental points fall exactly on this same straight line. Identical behavior is exhibited by all other hydrocarbons examined: Fig. 3 shows the results

0 ~ h > 1.0 2L.h < 1.0 N 2 ~ = 1.0 : 1.0 h ~ h : 0.7 k , k =1.4

fh

: . A 9 v

e1

.7/

0.5 A o <

I

I I t

0.0

2/ a

I | l o! i

I

V

! I t

!

i

-1.0

-0.5 C2H2

Fia. 2. Effect of inert diluents on ionization in acetylene flames. Plots of log (fuel fraction) against log (nT/p), where (nT/p) is proportional to R, the ratio of electron partial pressure to the total burned flame gas pressure.

~ 1.5

~ 1.0 _o

"~

.9#~'.-e~.,yl,.. . ~ . . Jr o 9

"C~H 2 6 9

(3t

0.s

0.0

-1.0

-.O.S

I~ 0 [ Fuel ] / [ Total unburned gases ]

FIG. 3. Ionization as a function of fuel concentration. As for Fig. 2, but using other fuels. for methane, ethane, propane, butane, ethylene, propylene, and benzene. Since the temperatures of these flames were not studied in detail, the quantity nip was used to represent R; exclusion of the T factor, however, should not introduce more than about 20 per cent error over the dilution range examined. The dotted, curved portions on the right-hand side of each plot are obtained only when ~ ( 1.0; these will not be discussed here. These results confirm that the ionization is nonthermal, since flames containing the same proportions of nitrogen and argon differ in the temperature on account of the different specific heats of these two gases. Also, oxygen molecules in excess of those required for complete combustion of the fuel, in behaving in the same manner as an equivalent amount of either nitrogen or argon, apparently make no significant contribution to the primary ionization processes. The consistent dependence of R on the square of the fraction of fuel in the unburned gases was an unexpected result. If the differences in ionization level in different hydrocarbons are related to the ability of the hydrocarbon firstly to form intermediate species (such as acetylene or an acetylenic fragment) and secondly for this species to polymerize and then produce ions, as has been supposcd, s then some variation in the slopes of Fig. 3 might have been expected. As an example, since acetylene exhibits a much greater degree of ionization and more diversc spectrum of hydrocarbon ions than methane, 5 R for acetylene might have been expected to depend on a lower power

642

FUNDAMENTAL FLAME PROCESSES

of the fuel fraction than R for methane, which molecule--on the polymerization theory--would presumably have to degrade and form unsaturated linkages before producing ions. A reconciliation of the observed results with the polymerization theory would be possible if all hydrocarbons degraded to a single carbon atom species which could combine with another fragment also containing only one carbon atom, leading directly to ionization. Ferguson's observations on C~ emission from acetylene flames, using C 12 and C la isotopes, 1~in demonstrating the carbon atoms comprising the C2 to be randomized, have already suggested at least a limited breakdown of the fuel into single carbon atom species. This possibility will be examined shortly.

Ionization in the Presence of Nonhydrocarbon Fuel Additives The fuels chosen were hydrogen, carbon monoxide, hydrogen sulfide and carbon disulfide. None of these, when burned alone with oxygen, gave measurable ionization (which, with our apparatus implies n < 10s electrons/cc, i.e., at least 3 orders of magnitude lower than in hydrocarbon flames). During the substitution of diluent fuel for hydrocarbon, both h and the total rate of flow of unburned gases were kept constant, as in the previous section. Figure 4, a plot of relative R against hydrocarbon fuel fraction, illustrates typical results for a stoichiometric acetylene-oxygen-additive flame; points for argon, nitrogen, and oxygen additions are included for comparison. The lowest curve (nitrogen, argon or oxygen) is known from Figs. 2 and 3 to correspond to R depending on the square of the fraction of fuel. Inspection of the curves for carbon monoxide

and hydrogen shows that R now depends on a lower power of t h e fuel fraction. Furthermore, the value of the exponent varies with the flame composition--another difference from the behavior with inert diluents. For example, the exponents at ), 0.7, 1.0, 1.4, and 2.0 for acetyleneoxygen-hydrogen flames are approximately 1.4, 1.1, 0.85, and 0.85, respectively. Similar results are obtained for acetylene-oxygen-carbon monoxide flames, except that in this case the logarithmic plots are, in addition, slightly curved. LoW exponents are also obtained when either carbon disulfide or hydrogen sulfide are used (0.80 and 0.95, respectively, at ), = 2.0). These two cases are more complicated in that, for k <~ !.5, sulfur-containing fuels give rise to electron acceptors which modify considerably the free electron concentration, is For k > 1.5, however, such interferences have been shown to be negligible.ZS All these results are in broad agreement in the sense that they exhibit a pattern of behavior quite different from that found with "inert" additives. It is very difficult to see how a polymerization theory--already a little strained in order to explain the observations with inert additives--can meet the additional requirement of interpreting the above results. However, it is still necessary to postulate that from the ionization viewpoint all hydrocarbons degrade to a single carbon atom species in order to explain the consistency of the slopes of the plots of Fig. 2 and 3. The new evidence provided by the present section is that these single carbon atom species must be reacting, not with themselves or some other carbon-containing fragment, but with a species which does not contain carbon, the concentration of which in the presence of a non-hydrocarbon fuel addi-

3

o

H2 l

//'~""

9 :o j

//:~.

9 %,~>I.o

[

0.1

c~.__

0~2

~,,'~

0.3

tot'~a goses

Fro. 4. Ionization in C~Hr-O2 flames in presence of diluent,s. As for Fig. 2 (but not plotted logarithmically), and including curves obtained with carbon monoxide and hydrogen as diluents. Note the difference between these curves and that obtained with argon and nitrogen diluents.

IONIZATIOI~IN LOW-PRESSURE FLAMES tive does not decrease so rapidly as when inert additives are used. Now the non-hydrocarbon fuels chosen for this study were selected so as to eliminate the possibility that the additive fuel itself, or its combustion products, were reacting with the single carbon atom species. Thus hydrogen molecules and atoms, water, hydroxyl radicals, carbon monoxide or carbon dioxide cannot be participants in the primary ionization reaction. The only species left unconsidered are oxygen molecules and oxygen atoms. When an inert diluent is introduced into a hydrocarbon-oxygen mixture, not only the fuel concentration, but also the oxygen molecule and atom concentrations must necessarily decrease. On the other hand, it is in the nature of the experiments performed with nonhydrocarbon fuel additives that the oxygen molecule and atom concentrations need not be so affected. Indeed, depending on the diluting fuel and on the stoichiometry it is possible for the oxygen molecule and atom concentrations actually to increase: a quantitative analysis of this point would, of course, require experimentally measured values of [ 0 ~ and EO-]. Now the effect of oxygen molecules can be discounted immediately, for it was shown in the preceding section that, when in excess of the number required for the stoichiometric reaction, oxygen molecules behave effectively as an inert diluent. Therefore it is inferred that the single carbon atom species is reacting with an oxygen atom. The simplest explanation, then, of the observed square dependence of R on fuel fraction when inert diluents are used is that both the concentrations of the single carbon atom species and of oxygen atoms are proportional to the fraction of fuel present.

Ionization in Flames of Mixed Hydrogen-Nonhydrocarbon Fuels Fuels containing carbon, but not hydrogen, such as carbon disulfide, carbon monoxide, and cyanogen, were mixed with various proportions of hydrogen and burned with oxygen. This was to test the possibility of producing reactions similar to those responsible for electron production in hydrocarbon-oxygen flames. Mixtures with either carbon disulfide or carbon monoxide gave no measurable ionization at any composition (at a pressure of about 20 mm Hg, 10s electrons/cc would have been detectable). Mixtures with cyanogen gave a pronounced ionization peak at 10-20 per cent cyanogen in the fuel. The effect is, however, too complex for analysis here, and it is sufficient to note that the characteristics of this "induced" ionization are

643

quite different from that found in hydrocarbon flames. This negative evidence suggests the presence of the C - H bond in the original molecule to be an important prerequisite before any appreciable ionization can take place. In this case, the primary ionization process suggested by the results so far discussed is CH,~ -[- 0 ---*R + -t- e- + X

(I)

where R + represents the positive ion produced and X may be a fragment split off in the reaction. If the simple interpretation of the slopes of Figs. 2 and 3 is correct, then the species Cttm cannot itself contain oxygen. The value of m remains, as yet, unspecified.

Relative Ionization in Flames of Different Carbonand Hydrogen-Containing Fuels The electron concentration was measured as a function of preburned fuel-oxygen composition for a wide range of fuels; the maximum values of

o/~ o/

lO.

6

,,,

i

"E

14

-~I s.

f.

II L

.-~.

/

9

9

$

"~.'~

/ ~ ,-" ^,

..>" ...-",,.-" --

J I

] 2

l 3

i 4

i 5

i 6

7

HO. OF r ATI)I4S IH MOLECULE

FIG. 5. Relative ionization in different fuels. Plot of relative nv/p (proportional to R) per molecule of fuel burned against the number of carbon atoms in the molecule. The points are identified as: (1) methane, (2) ethane, (3) propane,'*(4) butane, (5) n-pentane, (6) n-hexane, (7) isobutane, (8) 2-methyl butane, (9) cyclohexane, (10) methyl cyclohexane, (11) ethylene, (12) propylene, (13) 1-butene and also 2-butene, (14) butadiene, (15) benzene, (16) acetylene, (17) methyl acetylene, (18) ethyl acetylene, (19) methyl alcohol, (20) ethyl alcohol, (21) n-propyl alcohol, (22) dimethyl ether, (23) diethyl ether, (24) dioxane, (25) acetone, (26) 2-butanone, (27) methyl formate, (28) methyl acetate, (29) ethyl acetate. Of these fuels, 1-4, 7, 11-14, 16-18, and 22 are gases under normal laboratory conditions.

644

FUNDAMENTAL

n so obtained (found in slightly fuel-rich flames) were then compared. In Fig. 5 the value of R per molecule of fuel burned (given here the symbol EF) is plotted against the number of carbon atoms in the molecule: in this way the effect of molecular size and structure can be truly examincd. Every point in Fig. 5 usually rel>rcsents the mean of several determinations, each of which was accurate to about 10 per cent; and exccl)t for one or two of the least volatile compounds therc was never any ambiguity in the EF value obtained. Fuels investigated included: saturated hydrocarbons, normal 1-6 (see legcnd of Fig. 5 for the meaning of each number), isomeric 7, 8, and cyclic 9, 10; unsaturated hydrocarbons, if-14, 16-18, and benzene 15; alcohols, 19-21; ethers, 22-24; kctoncs, 25, 26; esters, 27-29. The range of compounds examined was limited only by considerations of volatility. Figure 5 shows a number of very striking regularities: (a) Points for all saturated hydrocarbons containing up to seven carbon atoms lie on a smooth curve which appears to pass through the origin. (b) If the fuel contains one oxygen atom, the ionization is lowered by an almost constant amount with respect to the corresponding hydrocarbon, apparently irrespective of the carbonoxygen link. (c) When two oxygen atoms are present the effect described in (b) is doubled. (d) Points for unsaturated hydrocarbons lie on various smooth curves (depending on the type of unsaturation) displaced upwards to greater EF values. The fact that the smooth plot obtained for hydrocarbons (points 1-10) is almost a straight line suggests that essentially the same process of electron production takes place in all these fuels; further, since the EF value for methane also lies on this curve, and not below it, this process probably involves mainly single carbon atom species. There is no determined gain or loss of the EF value if the hydrocarbon is cyclic or branchedchain rather than straight chain. A general rule, then, is that the level of ionization for saturated hydrocarbons depends only on the number of carbon atoms, and not in any detectable way on the molecular structure. The plot also suggests that the value of m in CHIn cannot be greater than two, because if it were three or foqr the EF values would most probably tail off with increasing numbers of carbon atoms in thc molecule. The cause of the slight upward curvature may be connected with the increasing carbon to hydrogen ratio as CH4-+

FLAME PROCESSES

CvHt4, because this implies that the lower the hydrocarbon, ttm more advanced must the combustion process be before free CH~ can be released. On balance, then, the evidence of this section suggests that saturated carbon atoms behave 1)redominantly as independent units. The consistency of the effect of the presence of oxygen is remarkable. The presence of one oxygen atom (points 19-23, 25, 26), whether in alcohols, ethers or ketones, lowers the ionization compared to the eorrcspomling hydrocarbon with the same number of carbon atoms by an approximately constant amount, irrespective of the number of carbon atoms in the molecule. If there are two oxygen atoms, as in esters or dioxane (points 24, 27-29), the lowering is doubled. Any difference bctwecn the effect of a double and a single C-O bond Was too small to detect. This provides strong confirmation of the conclusions already reached, i.e. that from the ionization viewpoint the fuel molecules are indeed shattered into single carbon atom fragments in passing through the reaction zone, and that the species CH~ most probably does not involve oxygen. For unsaturated hydrocarbons (points 11-18), a less siml~le pattern of results is observed. The EF value, however, is certainly greater than that for the corresponding saturated hydrocarbon and, moreover, depends on the type of unsaturation involved. Simple manifestations of this effect, e.g., that an acetylene flame contains more ions than one of ethane, are already well known. ~ Sufficient data are presented in Fig. 5 to show three patterns of results: for olefins (points 1113), for molecules in which each carbon atom makes at least one double bond (points 11, 14, 15), and for acetylenes (points 16-18). These curves show that the property of the unsaturated bond responsible for enhanced ionization becomes impaired in the presence of saturated carbon atoms--the EF value for methylacetylene is found to be actually lower than that for acetylene itself. There appears to be no such impairing effect, however, when all carbon atoms in the molecule can exhibit the same unsaturated bond type: thus the E F values for ethylene, butadiene, and benzene all seem to lie on a good straight line passing through the origin. It is also interesting to note that the EF value for benzene, (CH)6, is not three times that for acetylene, (CH)~. Since both benzene and acetylene exhibit very high ionization levels, the value of m in CH~ is therefore most likely less than the maximum value of two, previously suggested. A value of zero is improbable on several grounds; for example in view of the absence of detectable ionization in mixed fuels (preceding section),

IONIZATION IN LOW-PRESSURE FLAMES

and also since the ionization potential of carl)on monoxide is 325 kcal/molc, whereas its disvociation energy is only 256 kcal/mole, m9 Thus the primary ionization step suggested by all these results is CH + O -* CtlO + -]- e-,

(II)

which is the reaction already proposed by Calcote. 2~ The process is energetically feasible, for, taking the heat of dissociation of CH as -}-80 kcal/mole, the heats of formation of O, II, and CHO + as -- 58.6, --51.9, and +203 kcal/mole, respectively, and the high (170 kcal/mole) value for the l a ~ n t heat of sublimation of carbon, 19'21 the heat of reaction (II) can be as large as + 2 kcal/mole. The C H 0 + ion has been recently identified mass spectromctrically as a minor constituent in flames, by Bascombe, Green, and Sugden,22 and kinetic calculations by the same authors have indicated the plausibility of a 1)roce~ such as reaction II. Once such an ion is formed, other ions can be produced by proton exchange reactions. Thus, following Calcote~~ CHO + + HzO -~ CO + It3() +

(III)

lI30 + + C2H~ --~ C3H3 ~ + H20

(IV)

Vaidya's hydrocarbon bands have recently been shown to arise from excited CHO. ~ This species might be an intermediate in reaction (II) CH + O --* CHO* --* CHO + + e--

(V)

Now recent work by the authors -04has indicated the possibility of a direct connection between the abnormally high electronic excitation temperatures measured in the reaction zones of hydrocarbon flames and the initial process of ionization. The simpler of two possible explanations was in terms of an unknown bimolccular l)rocess which had to be responsible for ion production, which also had to be capable of producing an excited species that could excite metal atoms by inelastic collision, and which furthermore had to be, apparently, at least 174 kcal/mole exothermic. It is perhaps, therefore, of interest to note that reaction (V) coupled with CLIO* + M - , M* -1- CO q- tt

(VI)

where M is a metal atom introduced into the flame, would satisfy all these requirements. Conclusions

1. From the ionization viewpoint the fuel molecules are predominantly shattered into species containing single carbon atoms.

645

2. If the fuel molecule is a saturatx~(l hydrocarl)on, each carbon atom behaves ahnost as an independent unit, thc ionization depending essentially on the number of carbon atoms and not in any mc~surablc way on the molccular structure. 4. The primary ionization step suggested is CIt + O -~ CHO § + e-

(II)

5. The effect of unsaturation in increasing the ionization level does not seem to arise from introduction of new charge producing reactions related to the appearancc of ion polymers. If the effect is not simply one of modifying the concentrations of reactants in process II, then it could be rela~d possibly to the ability of the uncharged polymer fragments to provide a wide spectrum of positive ions in which charge could l)e stored. ACKNOWLEDGMENTS

We would like to thank Dr. Walter Gordy for his continued interest in this work and for making av'filable to us the facilities of his laboratory. We also wish to express our gratitude to I)r. T. M. Sugden, who read the draft of this paper and made many valuable comments. This work was supported by the U.S. Air Force ()ffice of Scientific Research of Air Research and Development Command. REFERENCES

1. CALCOT~:,H. F.: Combustion and Flame 1, 385 (1957). 2. KNEWSTUBB,P. F. and SUGI)Z~, T. M.: Trans. Farad'~y Soc. 54, 372 (1958). 3. PADIIEY,P. J. and SUGDE~, T. M.: Eighth Symposium (International) on Combustion. Williams and Wilkins, 1962. 4. DECKERS, J. and VAN TIGGEI.EN, A.: Seventh Symposium (International) on Combustion, p. 254. But~rworths, 1959. 5. KNEWSTUBB, P. F. and SUODZN,T. M.: Seventh Symposium ([n~rnational) on Combustion, p. 247. Butterworths, 1959. 6. KNEWSTUBB,P. F. and SUGOEN, T. M.: Prec. Roy. Soc. (London) A255, 520 (1(,)(')0). 7. I)E JAb]GERE, ,~., ])ECKERS, J., and VAN TIGOELEN, A.: Eighth Symposium (lniernational) on Combustion. Williams and Wilkins, 1962. 8. Eighth Symposium [International) on Combustion: Discussions (ionization section). Williams and Wilkins, 1962. 9. KELLY, D. C., MARGENAU, H., and BROWN, S. C.: Phys. Rev. 108, 1367 (1957). 10. SerXNnXDER, J. and HOFM^SN, F. W.: Phys. Rev. 116, 244 (1959).

646

FUNDAMENTAL FLAME PROCESSES

11. BULEWlCZ, E. M.: J. Chem. Phys. 36, 385 1962). 12. BvL~.wtcz, E. M. and PADLEV, P. J.: J. Chem. Phys. 85, 1590 (1961). 13. BULEWlCZ,E. M. and P~LsY, P. J.: J. Chem. Phys. (in press). 14. GAYDON,A. G. and WOLFHARD,H. G.: Flames (2nd ed.). Chapman and Hall, 1960. 15. KlsG, I. R.: J. Chem. Phys. 81, 855 (1959). 16. BULEWmZ, E. M. and SUGDES, T. M.: Trans. Faraday Soc. 54, 1855 (1958). 17. F~-RGUSON, R. E.: J. Chem. Phys. ~5, 2085 (1955). 18. BULEWmZ, E. M. and PADL~.Y, P. J.: Unpublished data, 1961.

19. G~-YDON,A. G.: Dissociation Energies. Chapman and Hall, 1953. 20. CALCOTE, H. F.: Eighth Symposium (International) on Combustion. Williams and Wilkins, 1962. 21. FmL9, F. M. and FRANKLIN, J. L.: Electron Impact Phenomena. Academic Press Inc., 1957. 22. BASCOMS~, K. N., GREE~, J. L., and SUGDEN', T. M.: Symposium on Mass Spectrometry. Pergamon Press, 1961. 23. V~q)YA, W. M.: Eighth Symposium (In~ernational) on Combustion. Williams and Wilkins, 1962. 24. BULEWmZ,E. M. and P~.DL~y, P. J.: Combustion and Flame 5, 331 (1961).

Discussion Comments relevant to this paper will be found on pp. 654-658.