Study of the absorption spectra of free radicals in flames

STUDY OF THE ABSORPTION SPECTRA F R E E R A D I C A L S IN F L A M E S

OF

P. F, lESSEN AND A. G. GAYDON I)epartntcnl of Chemical Engineering and ('hcmic:,l "lechnol(mgy, Imperial College, London, S.W.7

A nv.~.dif~¢dmttltiplc reflt.'dion sysl.cm in ',.vhich the image of the source is focused into lhe flame in,/.tcad or on I.o the mirrm surla¢cs has been devulolxd. It reduces troubles due to defocusing of the lighl beam by refraelive index gr:tdit, ms in Ih¢ ll,,me and gives belter sp~ltial resolution. The Sw~m band~ of C: and the 4050A C., band have been observed in the 'l~:;tlhcr' of;, rich ~xy-llcdyPenc I~ame, and all Ihree syslem~ of Cli. as well as C z in the reaction zone.

Introduction Tllli characteristic band spectra of the free radicals CH, OH and C, are prominent features in the emission spectrum of all hydrocarbon flames, Investigations of the occurrence of these spectra in flames under different conditions have led to a better understanding of the typos of combustion occurring and the existence or otherwise of temperature equilibrium, but by themselves provide only limiled information about the chemical kinetics of coi'~ibustion. In emission, the intensity of a band system is determined by the temix:rature and conditions of chemiluminescent excitation as much as by the concentration of the radical itself, and is furthermore a summation over different stages of the reaction. Absorption spectroscopy has the great advantage Ihal it gives information aboul the concentrations of species in the lower electronic state, However, its obvious theoretical advan. loges are subject to severe experinl(ntal limitations which have greatly h,amlxred the exploitation of this technique for the study of premixed flames, OH is the only radic,'d which lends itselfreadily to investigationby absorption techniques because of its relatively high equili. brium concentration in fla~esand concentration profiles and studies of decay meehani'~'~shave been made j-~'. Many transient intermediate chemicalspecies are found only in or near the primary reaction zone which is only a fraction of a millimetre

thick in hot flumes,and is often curved. Even if a fiat reaction zone is achieved, by suitable burner design, it is still necessary to reslricl the light to a narrow beam of very small angular aperture, and this pencil of light tends to be thrown out or the flame by refraction effects in the high temperature gradients existing in the reaction z,,ne, Long pathlengths are necessary to obtain appreciable absorption because the concentrations of the absorbing species are generally very low. For photographic detection it is necessaryto have a background source with a higher brightness temperature than the excitation temperature in the flame; this can be appreciably higher than the equilibrium flame temperature'*. The difficulties have Men discussed by Gaydon, Spokes and van Suchtelens. They studied low pressure oxy-acetylene Ilames and were. for the first time in flames, able to detect weak absorption in the 10,0) band of C~ at 5165 A, and the 3143 A band of CH. They employed a system of mirrors, first described by White°, in which the light was reflectedback and forth several times through the flame to increase the effectivepathlength. Although very compact and moderately easy r to adjust, this multiple refl¢ction system of White gives a wide light beam, and the cerise. quent poor spatial resolution greatly limits its usefulness. Only thick reaction zon~ such as tho:~eof low pressure flam¢sor near-limit names, can be studied, and even here the detailed flame 11

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slruclurc c;~nno! be resolved. The reason is IJ]~(

the light beant is lbcused oil tile ,,urfac¢ of one of the mirrors and ~ :airly wide bean1 acluully passes Ihrough the flame, Tlhs nc.'lrly par,'dlel beam is also very ~uhjcct to schlkren-typc deflections in file temperature gnldicnts of Ihc llamc and these cause deterioration of I[ICJnl:.lge Ibrmalion, limiting ll~c number of Im~,crs:ds which c;m be obtained. We ha~,,; employed a multiple ref~cclillll system whivh focuses the light iJ'to the fl~,mcarid consequently has much belier spatial resolu!io, and i~ less affected'by refractive index gradicnls lhau the White system. Exl~rim~tal Apart from the burner and multiple reflection system, our apparatus is similar to that employed by Gaydon et aL ~, The background sot:tee is a tlashtube and the absorptien spectra are recorded photographically with a 2 m grating speclrograph giving a reciprocal dispersion of 5 A:mm. A synchromzed rOl~lting sector device ensures thai the slit of the speclrogral~k exposed only at the moment the fla.~hl,bc ~-, triggered. Figure 1 shows a diagram tfl ',he optical arrangement.

The ra~hiple reflection system The multiple reflection :ystem con:ists es.~cntially of two spherical cow,cavemirrors oJ' equal

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focal length..~¢p~tr~Lled by a di.~l;mCe equ~d IO !wice their nalins of eurv:tlur(l lsec I"igur¢ 21. 1'h¢ eentres of curvature of each mirror coincide at the poinl O. A n.'trrow beam1 ,t tight is brought Io a focus at ,1 pOJIH P Whidl is [alcrally displaced from 0 by a sm~lll di~lance dependenl ,~11Ihe nLllllber ~l i fll~¢]:~i.ll.,, required. "['h¢ l)C;liU is rclocu.~efl al'lcr rel]eclion from mirroJ- B. ~1 a Dilill[ (.~ on tile other sloe of the eelllrc oi' ¢urvalul'¢ an eqlia] dJsl;HlCe ~lway. Ftlrlher hn~L-,es are furnl¢'J al ]) ~Jl~(J () by SLiC~¢SSiV,; I'ell¢clions i"ronl mirrors A aJld J~ rcsp¢clivcl) ; this resulls in the light beam moving romtd II:c arc of the circle defined by the mirror,~. I:or a given angular aperture o1"11)cbe~nl, the ntlnlr:er of reflections possihle h determined by he distance OP ;nld the (lJl!lllClC[ OJ' |he nllfl:H~. 1"he lllJllJntuni length of OP. corresp~mdm., tu the maxinlunl nUltlbCf of f¢llL, gl;o11,:,. J.~s¢lh !h~ i~llgu]ar :lperltlrc of the bean') ()bviousiy it i~ possible t~) increase the number of reflecli¢ Is by increasing l.hc ~izc of the mirrors, but it .~:~ouJd be noted that the image lbrmation deteri,~rates with increasing number of reflections. St.,co we arc working slightly off Ihe axis of Ihe r:firrors and rays strike Ihcm obliquely, the sy.tem is subject ~o all the aberrations associat,d with spherical mirrors. Thus. after several ref' :etJons, the images will become blurred and e:flarged, and the poinls P ~ntd ¢.) increasingly le~,:,clearly defined. We theref(;rc h:~vr t() make a ~.'om-

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promise between the number of travers~ls and the spatial resolution required when studying a particular flame. Using four mirrors, each of 6 cm diameter and 10 cm radius of curvature, we found that up to twelve reflections were possible without adversely affecting ahe spatial resoluti,~n. With mirrors of longer focal length and by employing a light beam of still narrower aperture: it should be possible to increase this number ft~rther.

The burner To take fu[I advantage of the increa~d absorbing pathlength offered by the system, two flames must be used, or alternatively a single burner can be employed having a diameter greater than the distance PQ, e.g. a flat flame burner. We were particularly interested in investigating the luminous mantle or 'feather' surrounding lbe inner colic of a fuel-rich oxy. acetylene flame burning at atmospheric pressure, because appreciable concentrations of free radicals are known to exist there over a volume much larger than the reaction zone i t , l l ~. Several types of burner were tri•d. The most effective consisted of a pair of slobtype burners (3 mm× 3/4 mmj placed at the image points P and Q. The slot burners were bull! on to a modified commercial welding torch to prevent danger from flashbacL The mirror system inverts the images of one flame on to the other. This prevents simultaneous stud} of an extended zone of the flame, ha: does not interfere with the step.by-step investigation of the absorption

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spectrum wt;en a point source, such as a nashtube, is used as background, The intensity of the background source is reduced after each traversal due to reflection losses. The resulting fall in brightness temperature can be calculated using Planck s law5; it is rather less serious at short wavelengths if a uniform reflection coefficient is assumed, but in practice the reflection coeflieient of the mirrors decreases rapidly in the ultra-violet and the overall effect is a reduction of intensity at shorter wavelengths, Reasonable exposures could be obtained after twelve reflections with between twenty five and one hundred and fifty flashes depending on the type of plate and wavelength region examined. Results

Cz Weak C z absorption in the (0.0) band head at 5165 A was detected with nine traversals, through the tip of the reaction zone era luel-rich flame. With thnrteen traversals, the absorption was slrong enough to show up most of the band heads and the rotational structure of the stronger bands, Figure 3 shows a microphotometer trace of the absorption spectrum in the 5165Aregion; an emission spectrum is also presented for comparison. The C2 absorption was strongest in the reaction zone, with mixture ratios ofabJut three times stoichiometric, i.e. CzHz: O: = 6:5. It extended a few millimetres above the react/on zone into the luminous mantle although the intensity was reduced. CH CH absorption was observed in fhe reaction zone of slightly leaner flames ~ith thirteen traversals. The three main band systems at 4315 A, 39110A and 3143 A were all detected although the absorption was very weak in the 4315 A and 3900 A systems. A microphotomet ¢r trace of the 4315 A band is shown in Figure 4. No absorption was detected in the luminous mantle above the inner cone. C~

We also detected weak absorption at 4050 A just above the tip of the reaction zone. in Ilames

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STUD"/Ill: Till! AfISORP] I(}N SPECIRA OF FRH! RADICALS IN FLAMES

flame using the Whitemultiple reflectionsystem. They employed photoelectric recording of the spectrum using phase-sensitivedetection to discriminate between the background continuum and flame emission,and thus were able to use a background source of lower brightness temperature, namelya high pressure mercury lamp.

burning with mixture strengths between three and fo~,rtimes stoichiometric+The absorption is due to the C+ radical which has a diffuseband systc, i in this wavelength region and can also be observed in emission.Figure 5 showsa weak bu! :eproduciblc peak at 4051.5 Aon top of the background noise+ P ----I-

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Discussion Although several attempts have been made by a number of workers in the past, we are aware of only two previous reports of Cz and CH absorption in flames, Gaydon et el. 5 studied an oxy-acetylene flame at low pressure using an effectivepathlength of over 140 cm+They were able to detect Cz and CH absorption in the reaction zone only at the (0.0) band head at 5165 A and the piled up Q branch at 3143 A respectively. Bleekrode and Nieuwpoorts also studied the reaction zone of a low pressure oxy-acetylene

The improved sensitivity gained ;,, this way enabled them to detect the rotatio,,al structure in the Swan bands or"C2 and weak absorptiou in the 4315 Aand 3900A bands of CH. They also estimated the concentrationsof the C2 and CH radicalsin the reactionzone to be 1013 era" 3 and 1012 cm "a respectively. There do not appear to be any previous reports of C.~ absorption in flames but it has been observed in absorption in the flash photolysis of oxy-acetylenen.q~tures~. as also have Cz and CH. The presence of appreciable concentrations

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of C z and C~ radicals m the lumi~lous mantle above the lip of the reaction zone of fuel-rich flames is of interest in connection with the mechanisms of carbon formation in flames. The mantle first becomes noticeable at mixture strengths greater than about twice stoichiometric, becoming whiter and extending farther up the flame a.~the fuel supply is iocrea~d, until it finally merges with the characteristic yellow carbon luminosity at mixture strengths greater than about four times stoichiometrie, Emission studies ~ have revealed that the region contains appreciable concentrations or free radicals but depa~'tures from equilibrium are not as great as in ~.hereaction zone, Free carbon atoms and the radicals C~ and C~ have been postulated to act as nuclei for lhe formation of soot particles in hot flames~0. We are hoping to make some measurements of the concentrations of these radicals as a function or mixture strength and distance above the react ion zone. As yet we have only been able to delect the presence of carbon atoms by the emission line at 2478 A. Improvements to the apparatus arc

being made which ~hould incrca.~ the sensitivity of detection ar,d facilitate quantitative measure. ments. 0.,, qf ..~ I1'.1:,,I.~ i.~ i.dehted In llle Ga,~; CoUm'il /or thlcmrial .'~.l~l~t,rt iRe,'eired April 19h6;ame.ded May 19661 Re(erences I ()I.I~'J:~';ILI]R,(;. O, ;..nd I(ll Ki, R. |:. ,./, ,~,,/l,c,,#l. P~/I':L ~. 43 ~-)

l)wYiJ~. R. J. ~i,d ()J.JJl "~,~,R(;. O. J. d,'m. Plli'~. 12, 351 ~J944) K^SKA~;, W E. (',m,h.~ti,,~r & I.'hJmc, 2, ~'29 (195:~) ' GAvlx)~,. A. (.;. The Sp,,clr.~<,,pr i~/l:hmt,,~, Chw[ m:,n and llall: L~Jndon (1')571 ~]AYIX),%. A, (,,,;,, ~I'OKI:~,. G. ~. and v~,N ,~I;I'II]I!I,I!N..J. Pr.c, R,Jy. .~'.~'~/I, "t55, 323 (19(~)] ~' Wltlll,. J. U, L opt S.c" Anp,'r .12. 2~5 (1')42) ' M~I~P.,G. U ('.mu/. J, I'lO's, 3.~. 1265 '~19571 I'~l.lilKg(~l)l:, R ~l|d NIFI WlcNFI~L W ('~ .L