Mechanism of ion and emitter formation due to cyanogen in hydrogen-oxygen-nitrogen flames

MECHANISM OF ION AND EMITTER FORMATION DUE TO CYANOGEN IN HYDROGEN-OXYGEN-NITROGEN FLAMES M. A. BREDO, P. J. G U I L L A U M E A N D P. J. V A N T I G G E L E N Laboratoire de Physieo-Chimic de la Combustion, Universit~ Catholique de Louvain, Louvain-la-Neuve, Belgium

A detailed investigation of cheml-ionization and chemi-]uminescence in Hz/O~/N~-i-CzN~ flames has led to the following eonchlsions: (a) A single molecule of CzN~ is required for the formation of an ion or excited species such as CN* or NH*. (b) The very lfigh ionic yield and the large over-all activation energy suggest a bimolecular process for the primary ionization. The variation of the ionic yield with pressure shows that the overall order of the cheml-ionizai,ion process is greater by one than that of the combustion process. (c) Since the thickness of the flame front depends on the pressure a.s p-o.~, the ovcr-all combustion reaction corresponds to a 1.4 order, and therefore a 2.4 apparent order for the over-all chend-lonization reaction can be deduced. (d) These results lead us to propose the following mechanism for the formation reactions of the primary ion (NO +) :

CN+O-~N(~P)+CO CN + ( ) ~ N (~D) + C O N (~P) -~N (ZD) +h~ N (~P) + O ~ N O + ~ - e N (~D) -I-O~NO+~-e Such a mechanism accounts for all our experimental results for NO + and the other ionic species detected by mass spectrometry. Some data for the excited CN-radical are also discuased.

Introduction The general consensus now is that ions form in hydrocarbons flames as suggested by Caicote ~ b y the elementary process: C H + O - + C H O + - [ - e - , in which the large exothermicity due to the C - O bond formation is used to ionize the formyl radical. This reaction has already received direct experimental support, e When only N a t o m s are present in the fresh gas mixture, the bond releasing the l~rgest a m o u n t of heat during its formation is the ~ 0 bond which plays a rolc in the ion fomlation? Therefore it is interesting to investigate what is specific to the simultaneous

presence of carbon and nitrogen atoms in a fuel. Cyanogen is particularly suited for this purpose. Although a recent detailed study of CzN~/Oz/H2 /]ames has bccn carried out b y Bulewicz and Padley, 4 the v e r y high ionic yield noticed in H~/O~/N2 flames see(ted with C2N.z cannot be explained. T h e ionic yield measured in seeded flames is about 10 -~ ion mole -~ at around 2200~ in flames of fuel-lean mixtures. This is quite close to the vahm ~eported b y Van Tiggelen et al.,s in CO/02/Nz-{-CzN2 flames burning at a higher terupcrature. T h e present study was performed with H:/O~/N2 flames seeded with cyanogen ( < 0.5%) so t h a t the properties of the flames were

1003

1004

ELECTRICAL PROPERTIES

.7,5

.,1o 1 N2

~ =

%

9 2.5

+

0.69

x

o.~5

o;~o

q.7~

~ c~ N2

I

FIG. 1. Saturation current i~ as a function of the initial cyanogen content at variable nitrogen dilutions in a H~/02 flame burning at 1 aim ~ith an equivalence ratio of ~=1.113. not perturbed by G2N2 addition. Different teehIriques were utilized for this investigation and are described below9

ions produced per unit time and per unit volume in the reaction zone. Therefore:

U+=iJES.e~ Experimental

Procedure

and Results

All the flames studied were stabilized on a Spalding-Botha type burner. The burner consists of a bundle of 200 tiny inconel tubes (od :0.04 cm) in a tube of 0.78 cm inner diameter and 16 em length. With this type of buruer, a flat flame was achieved. When a potential difference is applied between the burner (cathode) and a metal pLate (anode) parallel to and a few centimeters above the burner mouth, the positive ions and the electrons arc withdrawn from the flame reaction zone. If the electric field is sufficiently high, the recombination in the flame becomes negligible and saturation current (i,) is obtained (see Fig. 1). The saturation current is of great importance because it can be related directly to the ion formation rate (U+), as U+ represents the number of

[1]

where ~ is the electron charge, S the flame area i.e., the burner area, and el the thickness of the flame ionization zone assumed to be equal to the thickness of the flame front. BData for a stoichicmetric hydrogen-oxygen flame burning at atmospheric pressure, diluted with 50% nitrogen and with 0.5% added cyanogen (T]~2400~ give a rate of ion production U+ of (1.5• 1@s ion em-3 see-~. Measurements have been carried out on hydrogen-oxygen nitrogen flames to which small amo~mts of cyanogen were added. The pseudo-order determined by this method of isolation is first order: U+~(C2N2)01. The addition of cyanogen tr~ces does not modify the combustion process significa~ltly and, therefore, we can consider that the concentrations of H, 0, and OH-radlcal remain practically unchanged. Thus, we can infer that the mechanism leading to ion formation involves a single cyanogen molecule.

CYANOGEN TRACES IN HrOrN= -h,l~

H2/O2/N 2 " C2N 2

FLAMES

1005

or_ 1.1

H2 -

-

H2§ 0 2

= 0.69

~"

0.9

- ~A §

.J

Fro. 2. Difference between the over-all apparent activation energies 5E+' for the chemi-ionization and mmbustion proce~es in H=/O~/N2+0.5%C2N~flames, ~ 1.113 mad P = 1 arm. The ionic yield is n+ and .4+ =v+ (T~/Xo2) ~'+q ~-5 where T~ is the mean temperature of the flame front. Curve 1, AE+'= 30 kcal/mole if p+q=y§ and curve 2, AE+'=26.5 keal/mole~ if pq-q=yTz+l.

We can also define another parameter: the ionic yield (~+) i.e., the number of ions produced per CaN~ molecule burned: ~+=i~/r -~ ion/ molecule of C~N2, where F(B) stands for the CaN2 flowin molecule sec-L The ionic yield can also be defined as the ratio of the over-all chemi-ioniza" tlon rate U+ to the over-all burning rate (U) and formally written as follows:

u+ ~+= U

k+EC2N~Jo",EH2]o,[O2]oq exp ( - E+'/R T~) k+EC~N~30"EH:]eEo~]r exp (--

E+'/RT.) [e3

Where E+ t and E' are the over-aft apparent tctivation energies for chemi-ionization and for cOmbustion, respectively. T~, the mean tempera. of the flame front, has been defined pre~ 1fiOusly.7 With this equation, it is possible to ~ p u t e hE+' which is the difference of the over-

all activation energies for chemi-ionizatlon and for combustion. In this case, r e = x = 1 , the ionic yield does not depend on the initial cyaaogen concentration C~N~ and y+ can be written as:

7+= K(1/Tm)cv+q't-'~ (Xo~)(P+q~-~)

X exp(--aE+'/R rm), where K = (k+/k) (X~JXc~) v--v.no~-c"v-L The latter is constant provided the hydrogen-oxygan ratio remains constant, since no is the total concentration of particles at standard conditions. In H2/O2/N~q-CaN2 flames, AE t is almost hidependent of the equivalence ratio r (r is defined as the ratio of fuel to oxidizer divided by the same ratio for a stoichiometric flame). Figure 2 shows that if p-{-q = yq- z, then AE+'= 30 4-2 kcal/mole, and if pq-q=yq-zq-1, then AE+'=26.5=t=2 kcal/mole. Varying the pressure between 0.2 to 1 arm and taking into account the temperature variations, the study of the ionic yieht shows that pq-q=yq-z-q-1 (Fig. 3, curve 2) and that there-

1~6

ELECTRICAL PROPERTIES

"2/~

C2N2 T

,oS

2[/7

.,S

flame with an equivalence ratio r 1.113 (same flame as in Fig. 4). The last column in Table I is related to the current on the mass spectrometer collector detected for each individual mass. By means of aa appropriate calibration method described previously,s the absolute individual concentration of the different ehemi-ions can be inferred. The NO+ concentration peaks at (l=h0.1)Xl0 n ions em-a. When the equivalence ratio of the flame is modified from 0.7 to 1.2, a decrease of the relative concentration of NO+ is noticed sad the "order" dependence of NO+ varies from 0.7 to 0.9. With the Spaldh~g-Botha burner, the light emitted from the flame front can be focused on the entrance slit of a monochromator. The intensity of band emissions has been recorded in the visible and the near UV range. The spectrum exhibits mainly the following bands: the 7-N0 system (A~Z+-X21I) at 2478 ~, the OH system (A2Z+-X'ffI) at 3064~, the N H system (A3II-

1 P(atm)

FIO. 3. Comparison between experimental data and theoretical curves of the ionic yield n+ versus the pressure. Stoichiome%ric H~/O2 flame with 50% nitrogen dilutioa and 0.5% cyanogen traces added. Curve 1: p+q=y+z. Curve 2: p + q = y + z + l . Curve 3: computed with equation [3~ (see text).

fore AE+'= 26.5 kcal/mole must be considered the correct value. Identification, concentration measurements and profiles of the individual ions were determined with a mass spectrometer using a sampling technique such that the composition of the sample in the mass spectrometer is not altered, s The experiments were performed on H2/02/N2+ C2N2 and D2/O2/N2+C2N~ flames. The most important ions detected are: NO+, NHa+, H30+, I42CN+, H4CN+, and H6CN+. The concentration profiles are given in Fig. 4. From this graph, it appears that NO+, NHa+, and HsO+ disappear by recombination with electrons, although H2CN+, I~CN +, and H~CN+ are removed very quickly by reaction with a species in high concentration. If the initial cyanogen content is [C2N2]o, the exponent "a" in the expression [ X + ] , . ~ ['C~N:]0~ is obviously related ~o the number of C2N2 fragments directly or indirectly involved in the formation and disappearance of the considered species. Figure 5 shows the logaritimfic plot of the relative maximum current for some ions vs. [C2N~]0. The exponent "a" is equal to the slopes of such a plot. Table I (column 3) summarizes the results for an H~/O2/N2-t-C2N2

H2/O2/N 2 + C2N 2 H2 H + 0 = 0,69 2 2

H2CN§

~

"5

.~

9210 d {crnl

Fro. 4. Concentration profiles of the chemi-ions in a natural h)garitimfic scale for a flame tt~/O2/N2+ l%CsN~: ~=1.113, N2=60%, P = I nun.

CYANOGEN TRACES IN Hr-O~-N= FLAMES .X~) at 3360 ~, the violet CN system (B2Z+X ~ +) at 3590 A and the red CN system (A~rlX~2;+) at 6332/~. Neither C~* nor CH* could be detected in the emission. The measured intensities were proportional to the over-all concentration of excited species in the flame when the measurements were made throughout the burned gaS region. For any given equivalence ratio, the concentrations of excited species (except OH) are independent of the mixture strength (~) and are proportional to the initial concentrations of C2N~. Once again, this indicates that only one C~Ns molecule is involved in the mechanism leading to the formation of NO*, NH*, and CN*. The over-all activation energy of formation of these species can be deduced in a similar manner as for the chemi,ionization reaction. In the fuelrich mixtures, both AENK*' and AEc~; have a near zero value, but only AEN~,' has a significant magnitude in fuel-lean mixtures, AEc~. keeping a zero value even when ~ < 1. Moreover, with subatmospheric flames, the intensity of the CNband is directly proportional to the pressure; one

sz/s#o2

+ o~N2

H2 - - - H2 +

= 0.69

02

1007

TABLE I Values of the exponent "a" and the individual ionic current for different ions detected Mass

Ion

a

10**i~ (A)

18 19 28 30

NH4+ HzO+ H~CN+ NO + H4CN§ H~CN+

0.6 0.2 1 0.75 1 1

136 58.5 18.3 372 11 4

32

is forced to conclude that the excited cyanogen radical is produced in a nearly third-order reaction. When a low pressure flarae is stabilized so that the burner axis is perpendicular to the optical axis of the monochromator, the concentration of the excited species can be recorded at variable distance from the burner sutgace. Suhatnmspherie flames are sufficiently thick to detemline their profiles with accuracy. A pressuI~e variation allows modification of thickness of the CN* concentration profile and therefore of the flame front (e0). As has been demonstrated elsewhere, ~ the following expressions: U~Vop/eo and ec~(Vop) -1 relate the over-all burner rate (U) and the flame front thickne~ (e0) to the burning velocity (V0) and to the over-all pressure (p). U can also be related to the pressure as p= where n, the over-all order of the rate of combustion, equals x+y~-z. In this case, eo~p -~/~. Assuming that the flame front thickness (Co) changes as the half-width of the CN* emission profile, from the variation of the latter with pressure one obtains n = 1.4 and using data from Fig. 3 one deduces U+~p +2"4. Discussion

A.

The Primary Chemi-Ionization Process

In the H~/O~/N2 flame, only NO+ can be considered as the primary cherai-ion; thermodynamic considerations exclude a primary ionization reaction leading to direct formation of the other ions detected in this system. As we have noticed already, a single C2N2 molecule is involved in the mechanism leading to the formation of the primary ion, we can thus exclude reactions such as: FIG. 5. Determiuation of exponent "a" of the chemi-ions on the initial C2N2flow in emS/min. Both scales are in natural logarithms. Same flame as in Fig. 4.

NO+O+C---*NO++ CO+ e-

(1)

NO-{-N + N---*NO++ N2+ e-

(2)

1008

ELECTRICAL PROPERTIES TABLE II

Mechanism for primary eheml-ionization processes ~H Reaction (kcal/mole) No. Mechanism I CN+0+O~CN*+O~ C N + H +H-~CN*+tt~ CN+HWOH-~CN*+H~O CN*-kO2~NO++CO +e -

-46 -30 --45 +31

5a 5b 5e 6

--14

7

+6 --22

8 9 10 11 12 13a 13b

However, the primary ion mechanisre must verify all other experimental requirements such as pressure and composition dependence on the ionic yield ~+, as well as the over-all activation energy and the ove,~all rate of imdzatiom U + ~ I . 5 X 1 0 ~6ions cm-'a sec-~ at 2400~K.

Mechanism II CN +O +O---~NO~--}-CO+e Mechanism Ill C N + O ~ N (IP) +CO CNT0--~N (~D) +CO N (~P)~ N (~D) +hv N (~P) +0~N0-~+e N (~D)+O~N()++e N (~D)+~I-* N (2/') + M ~

--20 +8

where two or more C_~,,~fragments are necessary and which have already been suggested by several authors for much hotter flames a,w,n and in fast flow reactionsY The reactions betwecn an excited species and two radicals: NII*-~ O + O-~NO++ O H + e -

(3)

CN*-~ O-~ O'-~NO+d- C 0 + e-

(4)

must also be ruled out since concentrations of CN and NH excited species are definitely too low in H2/O2/N~ flames with cyanogen traces. The most plausible mechanisms for the primary chemi-ionizatiou processes are presented in Table II. All three mechanisms are in agreement with the experimental requirement of a single cyanogen molecule buplied in the ion formation. The cyanogen radical is produced in an elementary step~a CaNzJFR--~RCNq-CIY

(R=0,0H,H).

(14)

An activation energy of about 11 keal/mole is obtained, if R is an oxygen atom.** CN radical is consumed mainly by: ON+ O~-~CN+O

which is a quite rapid process with a very sInall activation energy (1 kcM/mole)2~ Applying the steady-state assumption to the CN- or, even. tually, to the CN* concentrations, we obtain:

~ H = --28 kcM/mole (15)

For instance, mechanism I could almost account for AE+' and for the pressure dependence, [CN*'] being approximately second order with respect to pressure, but reaction (6) disagrees with the composition dependence. As shown in Table III, the CN* yield ((PEN,) is hardly dependent on the mixture strength and the ionic yield (77+)increases by a facto," of 2 from fuel-rich to fueMean mixtures, whereas the equilibrium O2 concentration increases by more than two orders of magnitude in the saree composition range at constant temperature (Table IV). Although process (7) wmfld take into account the experimental pressure and composition dependence (the ratio [-R]~O32/'[-02~increasing by a factor of 2 iH the investigated composition range, (see Table IV), one can neglect reaction (7) because of the relatively large value of the over-an activation energy for these flames, since termolecular reactions usually have a low activation energy, if any. Moreover, ~ssm~ing CN- and O concentrations of 6XI0 ~ and 1.5X10~ mole cm~ , respectively, and with the measured overall rate of innization, U+~_l.5X10 ~ ion em-~ see-% the rate constant k7 should have been

k~-----U+/CCNXO~= ~0-., cm~ mole-~ sec-', i.e., a value more than 50 times larger than the one usually recommendedTM even for the oxygen recombination: O+O+M-+O2~-M. Mechanism I I I involves two reactions (lI) and (12) to produce the primary ions. Reaction (12) has already been considered as tile elementary step responsible for ion production in ammonia flames~v where the over-Ml order of chemiionization rate was 2. When there are C2Nx traces in the mixture, the presence of the energy-rich CN radical (5Hf ~ 104 kcal/mole) in the flame can yield nitrogen atoms in eP state with a relatively low activation energy. Since a radiative lifetime of 10-s sec for N(~P) is a reasonable assumption for a spin-Mlowed transition (2p~D) of an atomic species,~

CYANOGEN TgACES IN H:-0z-N~ FLAMES

1009

TABLE 1II Variation of ionic yield and CN* yield with the equivalence ratio. XC:N~= 0.38% and at P = 1 arm.

T~ (~

%N~ 0.8 0.9 1 1.113 1.2

52 54 58 56 54

1722 1715 1736 1728 1718

TABLE IV ~en!peratnre and eqn}librinm concentration for aN mospheric H#0~ flames diluted wi~h 60% nitrogen and with 0.5~: cyanogen traces

Xo Xo~ X~ T (~

,~=0.76

~-1.18

6.2X19 -4 4.1X10 -~ 6,6 X 10-a 2221

2.8X10 -~ 6.9 X10 ~ 2.8 X 10-a 2238

the bulk of N(2P) will disappear by radiation rather than by eollisiomd deactivation, even with ~her~te constants quoted hy Husaln st a i Y The primary ions are then fro'reed hy reaction (11) but ir must be pointed out that the N(~D) atoms prodused in reaetio, (9) and (10) react also with oxygen atoms to form ions. Although the concentration of the metastable state N(~D) is, of course, larger than the concentration of N(~P), the reaction (12) is endothermle by 8 kcal/molc and N(2D) will thus react with 0 atoms much less rapidly than N(~P) to create ions. A combination of processes (11) and (12) can lead to a noninteger order of the over-all ehemi-ionization rate. When the steady state assumption applies to Iq(~D)and N(P), meehafism I I I leads to an overall rate of ion formation:

u+=A[I+~O+c)]

[3]

~ith

A=kuksECN]EO]'-'/'(l~,o+k,~bEi~I])

[4]

Vscm/sec

10sX& (A)

10s•

256 237 205 241 245

24 20 10.5 12.I 14

11.35 9.63 4.54 5.6 6.4

10sX@ex, 4.28 3.76 3.12 3.82 2.71

At atmospheric pressure, with a collisiona.l deactivation of excite,4 nitrogen atoms mainly due to water molecules and making use of k~= 10a sec-t and of the values of the order rate constants and concentrations stated in Table V, the over-all rate (U+= 1.5X 10~6 ion/eraa sec) of ion formation in a stoichlometrlc hydrogen-oxygen flame diluted with 50% nitrogen and with 0.5% cyanogen (TI~2400~ is quite consistent with the experimental value. Furthermore, taking into account the pressure dependence of equation [3~, curve 3 was computed for the dependence of the ionic yield on the pressure and agrees reaSonably well with the measured data (Fig. 3). The compuhttion carried out with the CN concentration mentioned in Table V corresponds to g

ste~dv-~tate apprnxim~tio:~ [CN~=k.[C~Ns [R]/t(~.,]hmwith/~t4and kl~ given by Refs,14 and 15, respectively. Based on this dependence of [-CN] and using the equations (3) and (4), we find that U+ is proportional to ['C2N~I[-0"]2~R~/ ~0~. At a constant conccntrat, ion of C~h'2 (0.5%), we have computed the ratio [-0-]S[Rff ]-Os'l for mixture strengths ~ = 0.76 and 1.18 (see Table IV). This ratio increases by a factor of 2 between these values. On the other hand, meaSurements on flames with ~,= 0.8 and ~ = 1.2 (sec Table II1) also exhibit an increase of about a factor of 2 for the saturation current i,, and for the chcmi-ionization rate U+ as well Indeed, eoremains nearly constant, as seen from the wdues of V0. Mechanism I I I m~com~tsfor the composition dependence of U+. I t also gives a better agreement t,lum mechanism I I for the over-all activation energy, since in mechanism 11I either reaction 8 or reaction 12, at least, requires an activation energy.

B. TI~e Secondary Chemi-lonization Processes The ion-molecule reactions of Table VI are responsible for the other ions detected in the flame front. This emphasizes once again the primary character of NO+ .

ELECTRICAL PROPERTIES

1010

TABLE V

Reaction No. 8

k~ cmS/mole-sec 10-n exp ( - 10 O00/RT)

9 1O 12

Species

Concentration mole/cm ~

15"

(M) .~

10TM

10-1~

15

(O) ~

1.5 X 10x6

10-n

Our estimate

(CN)

6 X l0 W

10-x~ exp ( -- 10 O00/RT)

13a or 13b

Ref.

17

3 10-n

19

For a flaane burning at atanospherlc pressure in a stoichiometric mixture diluted with 50% nitrogen and with 0.5% cyanogen.

* Activation energy', Es, is an estimate. TABLE VI Ion-molecule reactions AH Reaction (kcal/mole) No. NO ++H---*HNO+ NO ~ +e---~N +O HNO++H~O~H~O* +NO NO+ + H +H~O-~H~O++NO H,O++HCN--~H2CN++H~O H~CN++YI2--~H4CN+ H4CN++H,--*H,CN + H~CN++H2--*NH4+ + CH~ NH~++e---~NHs+H

--27 -62 --40 -67 -3 -50 ? ? --107

16 17 18 19 20 21 22 23 24

The heats of reaction were estimated from a survey of the heats of formation for ions reported by Fralaklin et o2., ~ for H4CN+ and NO+; by Kohout el al., 21 for HNO+; and by Halley~2 for I-L30+, H2CN+, and NHa+. Applying tim steady state assmnption to the maximmn concentration of each individual ion, it is possible to compare the experimental "order" dependence with the one deduced from the mechanism involving reactions (I6)-(24). For instance: (a) NO + ions may disappear by recombination or by reaction with hydrogen atoms, which leads to

ENO+]~U+/EH]-I-[e-] ~FO2NS/Kq- EC2N~]~

with b between 0.4 and 1 depending on the relative hnportance of reactions 16 and 17; the e:cperimental value of b= .75 fits this scheme very well.

(b) [I-I,O+]m~.~EHNO+] EnvOI/[11ON] ~[C~N2] ~ has to be compared with measured [H30+l~.. ~ EC2N2"l~ the slight discrepancy can be accounted for, since part of H80+ also disappears by recombiamtion.

(e) EH~CN*]o,~EH,o*] EHCN]/E~] ~[C~N~] '.~ must be compared with ['II~CN+],~[Ce_N2]l'~ experimental; agreement is also very good for [HaCN+']m~xand ['H6CN+J ......

(d)

[NI-I,+].,~ EH,CN+]EH2]/[c-]

It shouht be pointed out that diffusion phenomena have not been taken into account in the above considerations and could explain the small discrepancies observed. The HNO + ion plays an importmlt role in this mechanism, althongh it has not been detected by mass spectrometry. This is probably because the charge transfer reaction (18) is much faster than reaction (16) which produces the HNO+ ion, so that the concentration remains below the detection limit of our equipment. The mechanism [reactions (8)-(24)] which

CYANOGEN TRACES IN Hr-Ov-N~ FLAMES accounts for the behavior of all ions detected in hydrogen-oxygen flames seeded with cyanogen is somewhat different from the mechanism sugbested by Van Tiggelen, 5 which is valid in a carbon monoxide-oxygen flame with cyanogen traces, where NO + was at the same time a primary and a secondary ion and where CO+ was the precursor of all other ionic species. Lastly, reactions (5a), (5b), and (5c) account for all the experimental observations on the intensity of emission of the violet C N - system, i.e., the pressure dependence, the partial order with respect to the cyanogen concentration, the zero value of the over-all activation energy for CN* radical, and the almost constant CN*emission yield when the equivalence ratio is varied at constant temperature. Acknowledgments The authom thank C. Bertrand for valuable discussions. Suppor~ was received from the Fends de la Recherche Fondamentale et Collective. One of us (M.B.) is much indebted to the Institut, pore" l'Encouragement de la Recherche dana l'Industrie et l'Agricukure for the allocation of a postgraduate fellowship. REFERENCES 1. CALCOTE, H. F.: 20th AGARD Propulsion and Energetics Panel, (1965). 2. PEETESS, J. L. : Combustion Institute, European Symposium Sheffield, p. 245 (1973). 3. BREDO, .'V[.A., FRANCOIS, C. A., AND VAN TIGGELEN, P. J. : Combustion Instlute, European Symposium Sheffield, p. 285 (1973). 4. BtlL~WICZ, E. M., PADLEY, P* J., ax,m SMITH,

R. E.: Fourteenth Symposium (Into.national) on Combustion, p. 329, The Combustion Institute (1973).

1011

TIGGELEN, A., PEETERS, J., AND VINCKIER~ C.: Thirteenth Symposium (International) on Combustion, p. 311, The Combustion Institute (1971). 6. WORTB~RO, G.: Tenth Symposium (International) on Combustion, p. 651, The Combustion Institute (1965). 7. VAN TIGGELEN, A., et al.: Oxydations et Combustions, p. 480, Technip, (1968). 5. ~r

8. P E E T E ~ , J. L., VINCKIER, C., AND VAN TIG-

OF~LF~N,A.: Oxid. Combus. Rev. 4~, 93 (1969). 9. VAN TmGELEN, A., et al.: Oxydatious et Combustions, p. 107, p. 479, Tecimip (1968). 10. BULEWICZ, E. M. AND PADLEu P. J.: Ninth Symposium (International) on Combustion, p. 647, Academic Press (1963). I1. BVLEWICZ, E. M.: Twelfth SymposiuTn (International) on Combustion, p. 957, The Combustion Institute (1969). 12. FONTIJN, A. ANn VR~E, P. H.: Eleverdh Symposium (Into'national) on Combuslion, p. 343, The Combustion Institute (1967). 13. AiLucv, M. aND ~%N T m G E ~ , A.: Bull. Soc. Chim. Belg., 77, 433 (1968). 14. SETSEn, D. W. A~u THRUSH, B. A.: Prec. R. Soc. Lend., A288, p. 275 (1965). 15. SCHACKEI 12I., SCHMATJKO, K., AND V~rOLFRUM, J. : Third International Symposium on Combtmtion Processes, Abstract No. 9, 25, Kazimierz (1973). 16. J(mNs~o~, H. S.: NBS-NSRD~20 (1968). 17. BERT1L~ND, C. AND VAN TIGGELEN, P. J.: J.

Phys. Chem., 78, 2320 (1974). 18. HERZBERG, a . : Atomic Spectra and Atomic Structure, p. 51, Dover, 1944. 19. H~SAIN, D., ]~IRSCH~ L. J., A~l) WIESENFELD 1 J. R.: Disc. Faraday Soc. 53, 201 (1972). 20. FRANKU~, J. L., et al.: NSRDS-NBS-26 (1969). 21. Kellog% F. C. AND L ~ e E , F. W.: J. Chem. Phys. ~5, 1074 (1966). 22. HANEY, M. A. AND FRANKLIN, J. L.: J. Phys. Chem. 73, 4328 (1969).

COMMENTS A. N. Hayhurst, She~eld Uni~,ersity, England. I am interested in your finding the ions H2CN+, HaCN+ and H~CN+, as well as in their role as intermediates in the formation of the principal positive ion NH4+. I wo/fld have guessed that these ions were fah'ly unstable with respect to dlssociatiun by loss of H~. Do you have any evidence that these al~e genuine flame species as opposed to ions formed by clustering reactions occurring during

the cooling of the flame gases as they are sampled into your mass spectrometer? Authors' Reply. NH4+ is not the pr[nclpal ion, and seems to play a role only in rich mixtures. When a potential difference of 5 V is applied between the sampling orifice and the skimmer, cluster-ions such as NO+(H20), NO+(HCN), " ' " were present in addition to the ions mention~t in the paper (NH4+, HzO+, tI~CN+, NO +, H4CN +,

1012

ELECTRICAL PROPERTIES

and H6CN+). If the potential is increased to 35 V, the cluster ions disappear, but the collector currents corresponding to H~CN+, H~CN+, and H~CN+ ions, with respect to the voltage variation indicates that they are genuine flame ions. Moreover, this is consistent with. the proposed mechanism for secondary ion fonnatlon,

than by collision, whereas ttm metastable species N(:D) disappear only by collisional removal as pointcd out by Kaufman.2,a The mechanism we suggest takes into account the literature data for quenching of excited nitrogen atoms and agrees with all our experimental results (pressure, equivalence ratio and dilution influences on cyanogen induced chemi-ionization).

A. Fo~ijn, AeroChem Research Laboratories, USA. Is your nlechanism compatible with known N(~P,2D) electronic and reactive quenching data?

REFERENCES

Authors' Reply. Definitely yes. The rate constants for eollisioital deactivation of both excited states of the nitrogell atoms have been measured by Husain el al., 1 who fotmd a value of kls around 3X 10- n cm~/mole sec. The N(~P) atoms disappear by radiation (kl0~10 s see-a) rather

1. See Ref. 19 of paper. 2. LIN, C. L. A ~ K~UmIAN, F. : J. Chem. Phys. 55, 3760 (197I). 3. t~VFMA~', F.: "Atmospheric Research Involving Neutral Species--An Evaluation," (Amer. Geophys. Union ~Ieeting, San Francisco 1968).