Suggested origin of the anomalous line-reversal temperatures in the reaction zone of hydrocarbon flames

Suggested Orig& of the Anomalous L&e-reversal Temperatures & the Reaction Zone of Hydrocarbon Flames* E. M. BULEWlCZand P. J. PADLEY Department of Physics, Duke University, Durham, North Carolin. ( Receired March 1961) Measurements of electron concentrations by the method of cyclotron resonance, and of no.-therm.I iron line intensities in emission, as a fanction of composition and dilation in the reaction zos~e qf low-pressure acetylene-oxygen--ar,qon mixtures, reveal a marked Mmilarity in behaviottr of these tu'. quan$itiea. On this and other circumstantial evidence, the possible causes of the attom,tlou.~' IDreversals van be narrowed down to two, neither of which can, however, be era,girl, red ohjectio.-[r~,¢ until further experintental evidence reveals in even more detail the nature of certain of the v¢,wtioJ, steps proposed. One of these schemes involves postulating the &imolecular production of an ¢.reitrd species Y* by the meclranism also responsible for electron production. Species Y * mu.~t he ,~uffieieml/! energetic, and be present in sufficient quantity, to produce excited metal atom M* b!! etterff.q tratt~.fi,i uqth M. The other---4, more specific sehenae--involves reaction betu'een M + and e to produce M*. the emitted radiation being discrete. Although there i.~ e~'perimental evidence for all the i m p o r t . . t steps involved in this second scheme, this schente, too. can fail if the cross sections ,[or e,*pture of . by M + prove to be too small. Schemes so far postulated involving termoleeular reactiotv~ ttrv elimimtted as not satisfying the ea'perimental ob,~ervations pre.~ented.

lntroductiott and Review of Previous Work IT HAS been known for some time that linereversal measurements, made on the spectral lines of various metal additives, in the reaction zone of premixed flames t-:' often give rise to temperatures considerably above those expected on the basis of full thermodynamic equilibrium. There is, however, relatively little in the way of an understanding of t h e cause of this phenomenon (except in the cast, of hydrogenoxygen-nitrogen mixtures 4) nor has it been studied in great quantitative detail. The effect is most marked for premixed acetylene-oxygennitrogen mixtures, with lines requiring up to 174 kcal/mole for their excitation appearing in the reaction zone, but not in the burnt gases. The ammonia-oxygen, ammonia-nitrous oxide and hydrogen-nitrous oxide flames also appear to behave in a similar manner. On the other hand, flames like those of hydrogen, carbon monoxide and carbon disulphide with oxygen have often been considered to give either essentially thermal, or only slightly anomah)us, line reversals.

Suggestions put forward to date t(, acc()u)]t tar this phenomenon include the fi)llo~ving. (1) A. G. GAYDON:', in an attempt to give general explanation for the phenomenon as observed in a wide variety of different flames, has con.,,idered whether or not la~s in equiF'artitian of energy between elecmmic and othe~ modes could be responsibh'. (2) H. P. BRain>, and K. E. S~:Ht'LE~C: have suggested a collision scheme nf tiw type CH {CH._,) -- (1+ Y - - ~ C ( ) - H l i t : l - Y* ....

11i

where Y is H,O, O 2, H, or C.H., [~dlowed bv the efficient bimolecular exchange of energy l)etween vii)rational and electronic lll()des Y* +

Ire (e.g.}

--~

F('*

-- Y

. . . . [2]

(3) A. (;. (;ArDOr" and It. G. WOLi;H,am)' have considered the possit)ility of three-l)ody collisions between a metal atmn and two free radicals. They point out that it is not difficult to postulate reactions of sufficitnt ener~: involving CH, C._,, CHO or C. (4) More recently, A. G. GA'CDoY; and A. G. GAYDON and H. G. WOLFIIARDt; have suggested that the effect could be connected

*This research was suoporied by the United States Air Furor under Contract No. AF 4~)(638g-765 monito.-ed by the Air Force Office of Scientific Research of the Air Research and Developmost Command. 331

332

E . M . Bulewicz and P. J. Padley

with the abnormally high ionization found in the reaction zone of hydrocarbon flames. Recent experimental evidence by I. R. KInG7, although not sufficiently detailed for any definite conclusions to be drawn, also suggests a qualitative parallel between these two phenomena when hydrocarbon flames are considered: for other systems (e.g. methane-nitric oxide or hydrogen-oygen), however, no such .correlation is evident. All these suggestions (1) to (4) either suffer from lack of conclusiveness in such experiments as have so far been carried out, or else need reviewing in the light of more recent data on fundame~tal flame processes '~-'4' ,.-..,,t First, at least two distinct types of anomalous line-reversal, or 'chemiluminescence', can now be distinguished, according to whether or not the effect persists into the burnt gases. The former type of anomaly is observed in hydrogenoxygen-nitrogen mixturesL and quantitative studies-of this". ~ revealed that operation of one or both of the processes: H~H+M lt-()tt:-M

> It..- M*

. . . . [3]

> tt/)+M*

....

[4]

can result in overpopulation of the uplx'r state of the spectral transition by a factor as large as 1 O00-fold. The relative contributions of reactions 3 and 4 could be quantitatively assessed for each metal examined, and so far the ca.~es of sod':urn, thallium, lead, iron, silver and manganese have been discussed ~'". ~'~. With iron, lines with excitation energies up to only 122 kcal/mole have been observed ~')', while the 'reaction zone', chemiluminescence appears to originate from a more energetic source. This important difference is illustrated in Figure I, which giw.s, essentially, log (no. of ground state atoms)/(no, of excited atoms) as a function of the excitation energy E of the transition, for four hydrogen flames (iron, cobalt, nickel, copper, silver, thallium, lead, potassium, sodium and lithium transitions: for more details see ref. 4), and an acetylene flamO (iron, lead, thallium and sodium transitions), individual p~)ints are omitted for clarity. Regions of thermal and chemiluminescent excitation can be clearly distinguished, and an inspection of the

Vol. 5

15[1

i I tZ 10

oo ~

.

,"

t

I 5l

1 Energy

2

of

e x c ) t a t ion

3

~,

(c m -! x 10')

Fi~,ur~: 1. Degree o] excitation as a ]unction .i en,'rA,y o] excitation o] m v t a t lim'.~ in the rcactb.,~ zom's ,J] four hydrogcn-,)xygcn-nilrog,,n flames (I to 4) at atmospheric pressure It and an m e t v l e n e - a b ' flare, ~ (5) at 24 )tt;Jt o] m e r c u v y t , l h c r m a l t e m p , ~,~ turf.~ o[ lhc.~e ]la)tles (m('aszer('d hv l)-Iim' r,'v(.r.sai. anti c,)'resp,mding to the sl,l,cs oj lit,, I,i,'er ha H ,>f ,.mh pl,,tl: 1. 1 7 5 0 ° K , 2. 2 3 4 W ' K , 3. 2 6 0 O ' K . 4. 2 6 7 o : K , 5. 20OO'~K

plots reveals that for undiluted hy(lrog(,n ,,xygen mixtures n,) overexcitation would I)v expected (which explains the failure ~.~f earlier w(wkers to detect the cffec L while even for the h,)ttest acetyh,rw-oxygen flames the over~:,xcitation of high energy line, would still be ,mtstanding. Thus, a systematic study of each fuel is required; and an explanation which wouhl cover all flame sy,;tems seems highly unlikely. The above arguments make it easier to define the problem under discussion, for it is considered that both types of chemiluminescence are, in fact, operating in acetylene flames. Preliminary studies by P. J. PADLEY and T. M. StrGDES"t5 suggest that for cool, highly diluted (T .< 2 300 ° K, preburnt nitrogen / oxygen :> 4) acetylene-oxygen flames at atmospheric pressure the concentrations of species like H and OH are above the equilibrium level--as in hydrogen-oxygen-nitrogen flames--the effect being greatest near stoichiometry. The rate of

December 1961

Suggested origin of anomalous line-reversal temperatures

radical decay was found to be similar to that in hydrogen-oxygen-nitrogen flames s and consistent with the operation of reactions of the type 3 and 4. Evidence on the overexcitation of sodium and thallium lines in such flames is in line with the more detailed work on hydrogen". 8. Lines with excitation energies 40 to 120 kcal/ mole are observed, though for E > about 70 :
333

partition and persistence of metastable electronically excited species. Turning now to suggestions" (2) and (3). Broida and Schuler~ excluded the l~)ssibility that reactions 1 and 2 might occur effectively in one step, by the reaction CH+O+Fe

>CO+H+Fe*

...[5]

on the grounds that the concentration of Fe atoms is too low for iron to act as third body, and that the overexcitation is sensibly independent of pressure'. Closer inspection suggests, however, that neither exclusion of reactions of the type 5, nor the inchlsion of reaction~ 1, carl t)e satisfactorily justified on these two counts alone. First, the degree of overexei*ation is not a function of the concentration ~,f Fe atoms. Secondly. simple steady-state calculations of Fe*/lq', taking into account the-quvnching:!lnd radiation processe:~ : Y*-X---,-Y-X lq'* + ~

, l'-e ~ X

....

[';1

....

[71

....

is]

wlu.rc X is any third b¢,h', ~ll~.~'~t that. 'xnh.:.~ the ctpnCellll'ations ,~[ X ;tll(t ~1" l i f e vuI'V ditf,,rent (rather unlikely), and unless lhe rat,' o m s t a n t ~ (,f processes l, 2. 5 and 6 satisfy rath:.r sp..cial c,mdilif,ns, then it will n,~t be !~-s~ibh , It, distin~ guish kinvticallv ln,tw,.,.n pr~,'v~st-~ ! (o,uph'd with 2) aml 5. M,,rv,~v¢.r, such eXl-,1;mittion~ unfortunately d,, ,mr av,,id the diflh:ullv pr,,sented by the pressure-indvpet~dcnce of the &feet. As (;aydon and ~V¢,!fhard' p,,i,,t out, lhree-!,odv processes are unlikely CitllSeS of the phenomen, m for this very rcas,,n, thou.~h it would be tx~,;sible t,, inwdidate this argument {f the radicals pr, ntucing the ,verexcitation were lhemselv,'s deslmw'd pred.,mdnanlly by threel,ody pr,,cesses..This, howew'r, we c,msid,,r improlmble, since radicals like CIt d,, n,,t apt-war h~ t~.rsist into tile burnt gases (suggesting lfilngdecular destructionS, and since s!wcies like (t, ()It. etc., cannot be sufficiently out of ,.quilibrium under conditions when their calculated h,wds are of the order of one t-x'r cent of Ill{> iotal gases'". Thus suggestion.~ (2~ and (.t) need be considered no further here.

~i~

E.M.

Bulewicz and P. J. Padley

This leaves possibility (4) for comment. It is known from probe and microwave attenuation studies in flames ~--~° that the concentration of electrons in flames is often considerably above the value expected on equilibrium considerations, particularly when the reaction zone is considered. The level of ionization in hydrogen flames is still a matter of dispute, with H. F. CAI.COTE and I. R. KING Is and S. DE JAEGERE et al. ~'~ favouring a low level (in the region of l0 "~ to 10 6 ions/cm ~) and with K. E. SCHULER and J. J. W~BER~ and P. F. KNEWSTUBB and T. M. SUGDEN~ suggesting a higher value (in the region of l0 9 ions/cm'~). Both these values are, however, considerably lower than the generally agreed-upon level of l0 ~ to l0 ~-" ions/cm ~ in the reaction zone of acetylene flames at atmospheric pressure. The nature of the positive ions is now known'-'~-2% these being mainly H.~O+ and, in the reaction zone of hydrocarbon flames, a large number of hydrocarbon fragments of the type C.H~. In an attempt to establish a relationship, if any, between the anomalously high ionizatmn and line-reversal measurements we have noted the following point, illustrated in Figure 2. The

E"gi

st

(b)

."2

a:l t

05

10

15

l:tflur, 2. fSteclr~n concenlratz~N ~n the r c a t l : o n z,me o/ acetylcm'--air flame's at 22"~ ~n~n o.~ m e r c u r y

~',<- ~us

c~nnp~szto,n--laken

from

Calcotc'-':,.

,,tL~o

1%* / Fe (calculated /rmn temperature nzea.suremrnt.~, made on the 3020,4 line of Fe, q u , t e d by G a v d o n and IVolfhard~) versus eomposition in the reactton zone o.[ acelvlene--~tiv thone.s at 30 to 60 m m oi mercury. Th[,se two plots have a .qightly similar appearance

Vol.

upper curve is a plot of electron concentration against X, obtained from probe measuremem~ made by H. F. CnI.COrE2~ in the reaction z o ~ of a series of acetylene-air flames at 22-8 mr: of m e r c u r y pressure. The lower curve is a. ply: of the relative number of Fe atoms excited ~, the level giving rise to the S 020 A group again~ )t, recalculated from Gaydon and Wolfhard' temperature measurements on this group , lines in the reaction zones of a series of a c e t , lene-air flames between 30 and 60 mm , : mercury pressure. The qualitative parall~ between the two effects is quite striking, bearb : in mind that curves A and B were taken un& ,completely different experimental conditions, it is evident that the establishment of a quantit.::tive relationship here would be of the greate-t importance in elucidating the cause of the chemiluminescence; and the experimental section ~,f this work is devoted to this end. Theory and Experimental Electron concentrations were measured by tlw method of cyclotron resonance, described i, detail elsewhere '-'~-2". The advantages of this method are, first, that no solid objects or any other form of 'indii~ator' are introduced int, the flame. Secondly, the method to a large extent circumvents the old objections to the u.~' of conventional microwave absorption techniques, based on tile thinness of the reaction zone compared with the wavelength of ttw absorbed radiation, even at reduced pressure: in the latter case high sensitivity generally demands wavelengths of at least several centimetres (based boil-,,, en theoretical and experimental limitations), whereas the sharpness ,,f cyclotron resonance lines is improved if the. nficrowave frequency and the magnetic fiel,t strength are increa_,~ed. Thirdly (an important! advantage over conventional microwave absor I tion methods), no assumptions have to be mad about the electron-molecule collision frequency which appears in the expression relating th measured attenuation to the electron conce~ tration. When microwave radiation is passed throug: the partially ionized gases so that tile directio of propagation, the E vector and the magneti,

December 1961

Suggested t d g i n of anomalous line-reversal temperatures

field are mutually perpendicular, resonance ,bsorption takes place in the region of :, = to~= e H / m c , where o and (,)~ are the micro~,ave and cyclotron frequencies respectively, ~nd H is the field strength in gauss, provided ~5at '~, the electron-molecule collision frequency, It can be shown that the number of . 0). :!ectrons/cmL N, is given by N = AH-#,, d

1 40 ~ e l o g e

here ~ is the attenuation, in dB, at the centre the resonance line, A H the line width, and d ~e length of microwave path, in era. through i~e flame. The Pyrex glass, low pressure, air-cooled vstem was similar to that described by Gaydon :~nd WolfharcP. Acetylene-oxygen-diluent mix~ures were burnt at pressures 8 to 40 mm of mercury, usually above a 15 mm diameter burner tube. Pressures below about 8 mm could not be used with the present arrangement, owing to the limitations imposed by the 2.3 cm gap of the magnet (12 in., 20 000 gauss maximum, Varian Research, Ser. No. 6). Gases were obtained from commercial cylinciters, and the Itow-rates measured by means of calibrated rotameters. The total volume of gas used wtried hetween 500 and 1 000 cm:'/min !usually 800 cm ~/min) at atnm:;pheric pressure and room temperatures. Temtx'rature measurements were made by the sodium D-line method';, anti havt: already been descrihed elsewhere'-'r. Microwave radiation at a frequency of about 18 kMc/s and modulated at 2 kc/s was obtained from a standard Raytheon K band klystron ~type 2K33) combined with frequency multi, ttr. The flame vessel was placed in the magnet ::,p between two waveguide horns. ,,he leading ~, the multiplier and the other to t l e detector. ~, that the ttame, the direction of radiation ,ropagation and the magnetic fiehl were ~utuaily perpendicular. The position of the urner tube was such that the nficrowaves aversed the flame in the immediate vicinity of :e reaction zone--fine adjustment could he ,ode by slight pressure alteration. In all the !~servations recorded below, the flame position

335

giving maximum attenuation was used, since this location was the most eaMly reproducible. The signal from the output crystal, after suitable amplification and demodulation, was plotted as a function of magnet field strength by a pen recorder. The magnetic field was swept electrically over aL range of about 3 000 gauss in two minutes, covering the whole of the cyclotron reson,::ce line, the centre of which was in the region of 17000 gauss. Iron was introduced in the form of iron pentaearbonyl. The lines chosen for this study were those at 4045,8 A, 4063.6 A and 4071-7 A (non-resonance lines, requiring about 105 kcal' mole for their excitation). Line intensities wero measured densitometrically (.larrell Asia .|A 2 3{}0 instrument), the photographs being taken ~m a conventional Hilger quarter-plate ~pectrograph. Results Figure 3(a) :h.ws the electron composition curve (corrected for pressure and Wmpcrature cha~:gcs) obtained, and Fig'.~re 3(t)) shows the corresponding, similarly corrected plot f.r the

co5

(a)

,

~'d 3 >

I/)

5

(b)

u_ ¢D t~

*_21/ o'-0--5 ....

l O-

i5

2o

l:t.t,,ur,. 3{al. Electron bartm/ pr,.,,u,, v,:~,a,, , , , m p,,mtmn "n th,' = o n e ,~! l,,,,.-pr,.,.,ur," a,., t v l , ' n , - , , L v . < ' , n mirtur,'. (bt. F r * F , ( l o t 4 0 6 4 . 4 . n o n . y , , o n , n, , I*n,'} vcr.~us c , m p , , s f f i , , n in th," sa~m" a,lm,.~ us,.d t., , # , t m n t.'~.~.,ur," 3!a). .\'~t, th,' str~k~n:~ .,imil,~r, t v ~,t ,':tgur,'., 3!a} and 311~i

3~6

E . M . Bulewicz and P. J. Padley

relative mmlber of non-thermally excited iron atoms. There is a remarkable similarity between these two diagrams; this is particularly outstanding when it is remembered that no quenching corrections have been applied to Figure 3(b). Unfortunately there seem to be few data availablC 9-'~ for the physical or chemical quenching of excited Fe by neutral molecules--these effects are very specific and it is difficult, therefore, to obtain information from comparisons with other metals, such as sodium, mercury, cadmium and thallium, which have been studied in detail. Using previous arguments s, the Stern~/olmer quenching factor for excited Fe in hydrogen flames, obtained from data similar to those presented elsewhere, is very probably about 10. For a radiative lifetime of about 10-s see, a quenching cross section of 10 to 15 ~,~- is calculated. The most likely quencher expected a priori in this case would be water. Also carbon monoxide, carbon dioxide and oxygen might be expected to have quenching cross sections for excited Fe in this general range-~% It seems quite probable that, owing to the way in which these molecules vary in concentration as a

/ -~

•

• , /

/

PeVge

ted for flame gas q Jer~ ~-:'qg

~,.o! I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ib !4 : 3 12 I-I I-,3 log(C2H 2 *O 2)/(tota! ga%e£~

/"igure "4. Eleclron partial prv~su~,~ and Fe* t:e (/or -Hie , t 0 6 4 j line) us a [unction o[ argon dilution. /liter currtction /or quenching, both plots have very .similar .slupes

Vol 5.

function of composition in low-pressure acet-lene flames 6, the average collisional cross sectk for all flame gas molecules with excited t ~. might be virtually independent of compositio . It might be observed, in this connection, th t the average cross sections for collisions , f electrons with flame gas molecules varies o n , slowly"' with ,X. We have also made observations on flare diluted with argon, at constant X and consta: t total gas flow of 800 cm3/min. The parti l pressure of electrons is found to be proportion ! to the square of the filel-fraction present--s, ,~ Figure 4 (this result, with respect to ionizatio;~, will be discussed elsewhere). The fraction ~,f iron atoms excited under the same conditiot~s is found to be pruportional to the fuel-fractiot,. in good agreement with the sqnare-dependen,-c. of electrons, since the quenching cross secti,,n for argon can be taken as effectively zero w h ~ compared with those of the flame gas molecuh,,. Discussion Mechanisms by which the connection betwe,~n the two quantities measured could arise can r,. narrowed down to two possible alternatives. (!) The excited metal atoms are producl.,] directly by the same reaction mechanism that produces tile electrmls; e.g. reaction 5 coupb,I with C H + O - ~CIt¢~ ~+e followed by CttO~+ H.,O--~ CO

+

It.,O' t'h:.

As has been tx)inted out already, however, stlc~l a scheme does not explain the pressure ind,pendence of Fe*/Fe. Simple steady-state t r e a ment suggests that this difficulty could !,~ circumvented only if an appropriate bimolecul " reaction could be fimnd which would produ,. • a suitable Y* (see reactions 1 and 2) with hi,,. probability, ~> 0.1 (in order to explain raft of M*/M as high as 10-: found for some lin. reversal measurements' 3. (2) Gaydon and ~Aolflmrd" have obserw " that the intensity distribution of the iron ove excitation Sl~ctrum is similar to that in the i r e arc, which suggests that the overexcitati< might arise directly from the presence

December 1961

Suggested origin of anomalous line-reversal temperatures

charged particles. The possibility that the overexcitation is caused by electron impact is highly unlikely, particularly as some metals (e.g. the alkalis) do not, in general, show the effect at all. The closeness of the 174 kcal/mole, highest energy ICe line observed to the ionization potential of Fe t might imply that the operative reaction is the radiative recombination of M+ and e-. This could manifest itself in one of the ~vo following ways, based on other evidence which will be given shortly. (2, i) Metal ions are produced in non-thermal quantities by the same react,,.a mechanism producing the electrons. Again following reaction 5 as an example, we have: CH+O+M

>CO+H+M++e

k,

....[9]

M++ e----> M*

k,,, . . . .

M* + X -->- M + X

k~

M* -----> M + h'~

....

[71

. . [8]

k,

M+ + e- + X ----> M + X

[10]

kj, . . . .

[11]

which leads to

[M*]

k,,k,,, [CHI [0]

[MI = k, iX1) (k,,, + k;, iX1) (2, ii) Metal ions are produced in non-thermal quantities by charge exchange with positive ion fragments R + in the reaction zone 0°. Reactions 10, 7 and 8 occur as above. Once non-thermal metal ion production has ceased, i.e. immediately downstream of the reaction zone, the free metal ion must in some way t)e protected from direct attack by e-. Evidence is presented which suggests that the metal ion might becomc largely surrounded by a cluster of water molecules, and is also to some extent combined as hydrate Mlt:() ~- or hydroxide M()H +. Destruclion of M+ is now, effectively, 1)imolecular. The new reactions are" R ~+ M - - ~ 3:1÷ + R (disintegrates)k,:

....

[t"l

R ~ + e-

k,:,

....

I i:q

.M~+ lt.fl) (e.g.)- :> MII.J) +

k,,

....

[141

M/I .~0 ~ + X ~>- M+ + H..O + X

k ,:,

....

[Is]

k,,,

....

[l

-~ fragments

MH..O a + e- ---+ M + H.:O (or fragments, possibly excited)

q

337

This leads to an expression of the form

[M*] [i]

kl,,k,o ( k , ° [~(.] + k,~. [e-I) [R +]

(k, + k, [Xl) (k.,,k.,, [X] + k,

[H_.O])

for e~:cited M* in the reaction zone. The evidence on which the schemes (2, i) and (2, it) are based is the following" (a) The partial electron pressure, as measured by us, in an acetylene flame burning on tubes of various diameters over the pressure range 8 to 40 m m of mercury, is .sensibly independept of pressure. I. R. KING:'2 has made probe measurements of partial pressures of ions in propane-air flames burning from 100 mm of mercury to 1 atmosphere pressure, and came to a similar conclusion. It is not unreasonable to assume, then, tl,at the partial electron pressure in acetylene--oxygen mixtures (at constant A), too, is independent of pressure over the additional range 40 mm of mercury to 1 atmosphere, thus paralleli'ag h e behaviour' of the ratio Fe*/' Fe. (b) P. F. Kr,'~WSTUBB and T. M. SUGDEN"~:', working on the ionizatitm of various metal additives in the burnt gases of hydrogen and acetylene tlames at atmospheric pressure, found that the electron level given by certain m,,tals in acetylene flames was considerably greater, bv a factor of up to 50-fold, than that expected from the Saha equati(m; but, owing to expertmenial difficulties, a delaih'd study was n-t undertaken. The work was taken up rt.centh," by I'adley and Sugden"", wht~, using wellshielded flames and an improved cavity technique with spatial res~dution about '2 ram, were abh, to study the problem kinetically. They concluded that the eh'ctrons persisted into the tmrnt gases by virtue of the charge transfer reaction 12 [see scheme (2. it}]. t a, it wa.~ sub:gestt'd, n¢,rmally disappeared in the rvacfi~m zone by the bimcdecular process 1,q, but. if a proportion ,)f the positive charge t)ecomes locked :is M+. then charge destructi,m will pr,ceed by at m u c h si,~wer pr.cess. This was fmmd to bt, kinetieally of the second cwder, with h v experimental velocity constant l,w rec~,m!)ination k I~eing equal to 3"0× 10 -'~ cma' seC-, at ab~mt '2 380°K. These observations w e r e sh,)wn to be

338

E.M.

Bulewicz and P.

consistent with operation of the 'sticky' termoleculax process 11 [see scheme (2, i)]. A possible explanation not then considered is one in terms of an 'inefficient' bimolecular process of the type 15 (the inefficiency being caused by the low proportion of MH~O + and the large shielding effect of the surrounding H.O molecules), postulated in scheme (2, it). It was inferred, however, that water is involved in the recombination, although it was, unfortunately, not possible to determil,e what the precise state of the water was immediately after charge destruction. {c) The above-mentioned work of Knewstubb and Sugden "~ lists the excess electron levels for a number of elements at a point above 3 msec after primary, combustion in a typical, fuel-rich acetylene flame at 2500°K. The number of metals examined was extended and the level of disequilibrium confirmed in the course of the work of Padley and Sugden -~° (e.g. with tests on lead, manganese, lithium, sodium, potassium and caesium). If all these results are plotted as a function of ionization l~tential, the curve shown in Figure 5 is obtained. For metals with ionization 50 m

~ 40

/"'~,Pb

~3o

/

' 20

/

/.,

10

.-Cr ,, N[~Mg

c;hco

us 0 "

/00

l"zgur,, 5. .ll,.qsur~d t, r i u m ~.nt~mlralzon.~ ¥1( h

Te Zn

50 60 70 80 lonmation potential

acvtylcne-ox),.,cn

90

Se

100

eV , I c c l r o n c o n c c n t r a D , m ~ tequzltJ,;r vari,,u m,'tals ~n a ]ucllt~l~',JL't'n

IJlt.t'ittl'e'

;It

23(10°[(

( t a k e n p.,1~t l-~nri.,.stittpt, a n d Nu~,,L.n aa ~otd l ' a d l c r a n d Sugdcn~:') . . a f u m t m n .] mnz:atu,n potcntml rj¢ lllC IHrtetl';

p-temial below about 5 cV, and also. apparently, for those abort-abottt 9 eV, tl~,. ex~,ct,,d equilibrium c~mcentrations of electrons are found, t:.r metals of intermediate ionizati~,n potentials, however, the above-nwntioned deviations occur. A %irly srncioth curve can be drawn through the exl~Mmental p.int% reachin~ a

J. Padley

Vol. ,-"

m a x i m u m for metals with ionization potential near 7 eV. Although a rather fuel-rich flam~ ( X = 0 . 5 ) was used in this study, it is possib/ that chromium (ionization potential=6-74 eV should be at the maximum of this curve, sinc no correction f r (oxide) compound formatio, was made in the cases of chromium, tin. silicon aluminium and zinc The value of this evidenc will be discussed shortly. (d) The mobilities of univalent metal ions i' flames under a constant impressed electric riel, i are all the same, at least for the alkali metals ,~' For the divalent alkaline earths the mobilitic are equal among themselves though smaller than those of the univalent alkali metals. Thi~ suggests that the ions become the nucleus ~f some kind of cluster. By analogy with the rather similar behaviour of metal ions in the aqueo,~ phase:':', water might be suspected to fulfil such a role. Support for this idea is provided by the work of R. J. MUNSOS and A. M. TYNDAH.::'~ . n the mobilities of the alkali ions in the raw gases containing various quantities of wahr vapour, who found that the mobility, and hence size, ,,f the clustered ion for slow moving ictus, was indewndent of the amount of water present. Simple ion-dipole calculations of the stabili W of such an arrangement (using a valw,~ of 1"8 I~ h,r water and reasonable ion-dipole distances), however, suggest a value of about 40 keal/m,h. for a divalent ion, and only half ¢his for ;t univalent ion. Although such a cluster, then, cannot be visualized as a stable unit at ttam: gas temperatures (for whieh its stability wonl,! have to be at least 70 kcal/mole'~7), it can I, considered in terms of an increased probabilit5 of collision between 5I ÷. and il.,O. This conclu sion is in line with the early observatkm ,,' .[. A. MCC.LELLAN|)~ that the mobility of th, ions decreases by a!mut IO-fold in the o , , r,,gions, a c~msiderablc distance above the thm~t Direct evidence for clustering of this .¢W i provided by the mass-spectroscopic studies i~ flames, math, 1,y P. F. KyFws'itr[m and T. ?~1 SU(;I)EN::: :'",

in

which

m e l al

ion

(and

t,v~'~

metal ion hydroxide) hydrates. ~.~,nlt~ti/n~'~; it, voh,'ing several molecules of water, have ]wc~ defected. The proporti,m of metal i, ms t:~m~ bined in this way is, however, small.

December 1961

Suggested origin of anomalous line-reversal t e m p e r a t u r e s

(e) F. W. Hot.'mam~ and H. KonN 'Lo have obtained direct evidence, from measurements of atom and ion emission spectra of barium, coupled with electron measurements, in flames, that the concentration of free Ba + is, in general, less than one per cent of the free electron concentration. They suggest that this result might be caused by formation of BaOH +. How general this effect is will, of course, have to await future evidence, hut it is interesting to note that this type of effect would be in line with operation of scheme (2, it). (]) It is very important to know what the efficiency of process I0 is, but ;mfortunately there appear to be very few data available on the cross sections for (discrete emission) radiative capture of an dectron by a positive ion. Experimental and theoretical data assembled by H. S. W. MASSEY and E. H. S. ]3URItOP"-'9 for similar processes resulting in continuous emission suggest a cross section in the region l0 -I'J to 10 -21 cm 2 for electrons with flame gas energ; ~. Combination of this with the known elec rt,n-motecule collision frequency for acety-. lene-oxygen-nitrogen mixtures at 10 mm of Inercury 27, i.e. 5'0 x 10~ see-' and the electronttame gas molecule collision cross section -~7 of average valae 30 x 10 -~n cm', leads to a value of kin,, for the process 10 of about 10-~' to 10 -v-' Cllq3 / s~_'C.

It is now l~ssit,le t- discuss schemes (2, i) and (2, ii) with reference to the evidence (a) to (f). Scheme (2, i) was proposed on the assumption that charge destruction proceeds according to process 11 [see evidence (b)]. Since k, ~< k, [X], anti k,0 <~k,, [X], then it can readily be seen that such a scheme, suggesting termolccular production of M +, would be in accord with the known pressure independence of the partial pressures of M* and e-. The scheme does not, however, explain why the overexcitation is contined to the reaction zone. Moreover, [M+] in the reaction zone is estimated, from data quoted b y Padley anti Sngden'-'% to he up to about 10-'-' [M]. With the reaction zone :it most abont I00 l~sec thick, such a proportion must be reached in probably less than 50 /tsec, whereas calculations of [M +] from 9. making very generous assumptions about flit concen-

339

trations of the species reacting with M, suggest that in 50 l~sec only one M in 10 -~ will be ionized. Thus scheme (2, i) can be considered highly unlikely. Fewer such objections can be raised to scheme (2, it). The expression for M*/M, bearing in mind that, under flame gas conditions, k, ~ kr [X], and 1,:~.~[X] > k,, [e-I, shows that M*/M should be proportional to re-] and should be independent of pressure, as observed. The extremely low mobility of the positive ion cluster arrangement (particularly noticeable in comparison with that for electrons ::~ in flames) suggests that, once M* is formed (i.e. once the reaction zone is leftL the metal ion might become to a large degree surrounded with H,O molecules. Thus the probability of captm'e of an electron t,y M ~ would be considerably reduced" the anomaly could thus be effectively contined to the reaction zone. This evidence is, further, qualitatively in line with observation (e) and also the observation (b) that the overall processes 14 to 16 postulated for charge destruction are considered inefficient. This sclaeme would fail if such processes were predominantly termolecular. - T h e impcwtance ~d l:ifcure ti c a n n o w l,e assvs~;t'd. 'With metal i.ns pr(,duced by the charge exchange reaction 12 Is,.e Masst'y and Burh-p"" f,,r 1,any examples ,,f pr, mvss), thi~ curve sug~e...ts lhal R" with which reaclitm 12 takes pl.tcc has an average i,,nizati,m pc~tential in the region of 7 eV. This value is c~msiderably lower than tilt, i~,nizalum p,~tvi~tials measured fi~r hwlr~marbon Inolecx~h's "t 1 i 0 to, 12 eW} bill is of the salne .rder -f magnitude as the ianization Fotentials of hvdr~carbon radicais. Thus, truly th~se metal~ which produce tho ap,m~a!ou,dy hitdl electron concentration effect [see ii~) ;in5 (c)] would shrew anomalous line-rever:~,ds, h is interesting to n.te, then, that of the metals sn far examined ~''', te!lurium, h,ad, iron and tin detinitelv do, whereas sodium definitely does nat. An apparent exception is '~tlg (ionization p~tential,: lO',qq eV), but this ,nay weli be caused by charge-exchange with hydrocarbon m,~lecule ions. Having due regard for lhe arguments presented above, it seems that the weakest l!t'k in

340

E . M . Bulewicz and P. J. Padley

scheme (2,ii) is reaction 10. If experiments should reveal k~0 to be too small, then the scheme will fail. A value of l0 -~t to l0 - ~ cma/sec [see (0] was suggested for k~o on the basis of evidence from similar systems. ,Substitution of reasonable numbers into the expression for M*/M suggests a value of about 4 x l0 -s for this ratio at ?0 nut-, of mercury, which would mean that lines with excitation energies greater than 80 kcal/mole in flames with thermal temperature ~Z000°K will be chemiluminescent in emission at this pressure. Values of reversal temperatures of iron lines quoted by Gaydon and Wolfhard ~ under comparable conditions suggest that, while this ratio i s sufficient to account for the excitation of lines of high

energies, above

ca.

105 kcal/mole, at lower

excitation energies it can be as much as 100-fold too small, Thus, for this scheme to operate, the efficiency of reaction l0 must be higher than the rather low value suggested in (f).

We would like to thank Dr Waiter Gordy for his continued interest in this work, for generously making equipment available to us, and Dr T. M. Sugden, in whose laboratory and under whose guidance some o] the work referred to in this paper was carried ovd. References (;AVDO,~, A. (;. a n d ~*VOLFHARI;, II. G. Proc. Rov. Soc. A, 195~, 205, 118 o WOLrHARD, I t . G. a n d Paax~:a. W. G. Fifth Symposium (International) on Combustion, p 718. R e i n h o l d : New Y o r k . 1955 :: Br~omA, tl. P. a n d SCHt:L~.a, K. F. ]. chem. Phys. 1957, 27, 933 ~" PADLEY, P. J. a n d S~tiDEY, T. M. S e v e n t h 5;,mposium (International) . n Conlbtlslion. p 235. B u t t e r w o r ~ h s : L o n d o n , 1959 s t;AYDON, A. G . The Spectroscopy of Flanles. W i l e y : New York, 1957 QAVDON', ;k. G . a n d WZ)LFHARD; tt. G. l:lame~, 2nd ed. C h a p m a n anti t t a l l : l.ondon, 19~;0 " KL~r;, I. R. ]. chem. t'h$'s. 1959, 3 1 , 8 5 5 8 PADLEY, P. J. a n d Siyt;t)t-x. T. M. Pr,,c. Roy. Se~c. A, 1958, 248, 248 !' |{lrI.EWICT, E. ~,|., JAsll:s. C. (;. an,1 .~,UGDEN, T. M. Pr,,, . Roy. S,,c. .4. 1956, 235, 89 ~".|x:,It=s, ( . (;. an,1 .
Vol. s

-la PADLEY, p . J . and SUGDEN, . T . M . Trans. F a r a d ,

Soc. 1959, $$, 2054

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~_" published work PADLEY, P. J. and BULEWXCZ, E. M. Bull. Am, phys. Soc. 1961, Ser. II, 6, 258 17 SCHULER, K. E. Filth Symposium (Internation: on Combustion, p 86, Reinhold: New York, 19 18 CALCOTE, H. F. and K i l o , I. R. Filth SymposU: (International) on Combustion; p 429. Reinho], New York, 1955 19 SCHUtma, K. E. and WEBER, J. J. J. chem. Ph) 1954, 22, 491 20 PADLEX', ~:~. J. and SUGDEN, T. M. Eighth S y : . posium (International) on Combustion, i[~60. ,l press ~t DECKEnS, J. and VAN TW,GELEN, A. Seventh Sy~ ~. posium (International) on Combustion, p 2~J Butterworths: L o n d o n , 1959 -°2 IC,~Ewsxu~u, P . F. a n d SU6DE~, T. M. Sevrn:h S y m p o s i u m (International) on Combustion, p " " Butterworths: L o n d o n , 1959 0:~ KNF.WSTUnn, P. F. a n d SUGD~:N, T. M. Proc. R,,v Soc..4, 1960, 255, 520 -°t DE JAEGERE, S., I)F.CKERS, J. anti VAN "I'It,G.L:I.I~', A. Eighth S y m p o s i u m (International) on Comlm. tion, 1960. I n press "-':,CAtco'rE, H. F. C~mlbustion 6"y Flame, 1957, l, 383 26 .t:~nNEIDER, j . a n d HOFMANN, F . "~V. P h y s . R r , . 1959, 116, 244 ": BULEWlCZ, E . M. ] . chem. Phys. 1961. In p.',,-"~ BULEWlCZ, E. M. a n d P A D I . F . Y . P . J . f . ch,'m. P h y s . 1961. I n press 2~ MASSEV, II. S. W. a n d B u m t o p , E. H. S. l i b , trontc and lor.,c I m p a c t Phenomena. Claren,h,n Press: Oxford, 1952 :I,, I.AIDI.ER, K. J. Chemical Kinetics .1 I- rctt~,t Ntatrs. t;|arendon l~ress : { )xford, 1955 :~l ]~|ITCHEI.I.,

~2 33

at s~ sg al

~s a,J

A.

('.

(;.

and

ZEMAN:-,KV,

3 I.

~,~,

lie.sonance ltadiatron and Excited Atom~. (;,,: bridge [Vnivt..rsitv Press: l.ond~m, 1934 K i s , , I. R . / . chem. Phys, 1958, 29, 681 I~.NEtASTt;liB. p . F . a n d SUGDEN, T . . ' ~ [ . ](t'~'tl*, h Correspondence, 1958, 9, A I TtloMsoS, J. J. and THOMSON° G. P. (tmduct:,,,~ o[ Elcctricity Through Gases. Cambridge l ' t f i v - r s : : Press : i.ondon, [928 I'h':R~AL, .[ and I"OWLr,:U, R. tl. J. chem. l'h',, 19";3, I , 51.5 Mu.~so.~, R. J. a n d "I'v.xDm.i.... k. M. I're~,. it',,. Soc..4, 1939. 172, 28 SUt;,DI.;N, T . M. Trans. Faraday Soc. 1956, 5 , 146.'3 ,MC{'Li':/.LAXlJ, J. A. Phil. ,liar. 1898, 46, 29 K.~v.w.~.rt,m~. I'. V. a n d St:tmt-.'~, T. M. i n p u b l i .!,

l~ta ~o I..tOI,MANN' 1;. XV. a n d KOIIN, It. ]. opt. %,..-1,,',

19.81. Ill press .it Pnlt E. ~V. C., I~RAI.SFOICD. |(.. |{,xP,R:q, I' a n d Rlt,L~':v, 1¢. (;. Spcctr,, hint. .q, t , , I~.~ 14, 45 t2 AhEK.%EI,:VA, V. (;. ;tlld ),'IANIIEI~IITAM, S. 1.. J. ,

theor, Phys,, Moscow, 1947, 17, 759 t:; IiROII3A, II. P~ J. chem. Phys. 1951, 19, 1383