Selective detection of isomers with photoionization mass spectrometryfor studies of hydrocarbon flame chemistry

Terrill A. Cool,a) Koichi Nakajima, and Toufik A. MostefaouiSchool of Applied and Engineering Physics, Cornell University, Ithaca, New York 14886

Fei Qi and Andrew McIlroyCombustion Research Facility, Sandia National Laboratories, Livermore, California 94551

Phillip R. Westmoreland and Matthew E. LawDepartment of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003

Lionel Poisson, Darcy S. Peterka, and Musahid AhmedChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

!Received 6 May 2003; accepted 28 July 2003"

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, forflame-sampling photoionization mass spectrometry !PIMS". Synchrotron radiation offers importantadvantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superiorsignal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges ofcurrent VUV laser sources. Near-threshold photoionization efficiency measurements were used todetermine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressurelaminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomericcomposition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence ofsubstantial concentrations of ethenol !vinyl alcohol" and acetaldehyde. Ethenol has not beenpreviously detected in hydrocarbon flames. Absolute photoionization cross sections were measuredfor ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curvesare presented for several additional reaction intermediates prominent in hydrocarbon flames.© 2003 American Institute of Physics. #DOI: 10.1063/1.1611173$

I. INTRODUCTION

Progress in minimizing environmental pollution associ-ated with hydrocarbon combustion requires the continuingdevelopment of kinetic models for the combustion of ethyl-ene, ethane, propene, propane, propyne, and higher hydrocar-bons including 1,3-butadiene and benzene. Further kineticmodel development is also needed for oxygenated hydrocar-bon fuels !e.g., dimethyl ether, methanol, ethanol, propanol"with clean-burning characteristics !low NOx , low soot". Im-proved descriptions of solid propellant chemistry and thechemistry of the incineration of hazardous materials are alsoof current interest. Kinetic model development in all of thesereaction systems requires direct measurements of the abso-lute concentrations of combustion intermediates in labora-tory flames under carefully controlled and documented flameconditions.

Vacuum ultraviolet !VUV" photoionization mass spec-trometry !PIMS", applied to the selective detection of flamespecies, is a powerful new approach for studies of flamechemistry,1–4 which provides a valuable supplement to thetraditional use of electron-impact mass spectrometry !EIMS"for such studies. A flame-sampling molecular beam time-of-flight mass spectrometer !TOFMS", recently designed andconstructed for use with a synchrotron radiation lightsource,5 provides significant improvements over previous fa-

cilities that have employed tunable VUV laser sources.1–3These include superior signal-to-noise, low light intensity‘‘soft ionization’’ without the complications of multiphotonabsorption and parent ion fragmentation, and photon energiesreadily tunable over the 8–15 eV range required for compre-hensive flame species concentration measurements.

The new instrument is used for measurements of concen-tration profiles and photoionization efficiency !PIE" curvesfor species sampled from well-characterized low-pressurelaminar flames. Both stable and radical intermediates are de-tectable at mole fractions as low as 10!6. Individual iso-meric species may often be selectively detected and quanti-fied; this is one of the most important advantages of VUVPIMS.

We report here measurements of the absolute concentra-tions of the allene and propyne isomers of C3H4 (m/e 40) inpremixed laminar flames of ethylene–oxygen–argon andbenzene–oxygen–argon. The separate contributions ofethenol and acetaldehyde isomers of C2H4O (m/e 44) arealso distinguishable in fuel-rich ethylene–oxygen–argonflames. This is the first identification of ethenol !vinyl alco-hol" in a hydrocarbon flame and supports the hypothesis thatvinyl alcohol may be a highly stable product of the decom-position of the primary HOCH2CH2 adduct formed by theaddition of OH to ethylene at the CvC double bond.6

Direct measurements of the absolute concentrations offlame species are possible with VUV PIMS when speciesphotoionization cross sections are known.7 In our work wea"Electronic mail: tac13@cornell.edu

JOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 16 22 OCTOBER 2003

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measure photoionization cross sections for ‘‘target’’ speciesby comparing ion signals recorded from a binary mixture ofthe target with a ‘‘standard’’ species of known photoioniza-tion cross section. Photoionization cross sections for allene,propyne, ethylene and acetaldehyde are presented here, mea-sured with propene as a standard, for photon energies from9.7 to 11.75 eV. We have measured cross sections for severaladditional reaction intermediates observed in ethylene–oxygen–argon, benzene–oxygen–argon, and 1,3-butadiene–hydrogen–oxygen–argon flames.8

II. EXPERIMENT

Our flame-sampling photoionization time-of-flight massspectrometer is described in detail elsewhere.5 The apparatusconsists of a low-pressure flame chamber, a differentiallypumped flame-sampling system, and a time-of-flight massspectrometer !TOFMS". It is coupled to a 3 m Eagle mono-chromator used to disperse synchrotron radiation at terminal3 of the Chemical Dynamics Beamline of the AdvancedLight Source !ALS" at the Lawrence Berkeley NationalLaboratory. Premixed fuel–oxygen–argon flames are stabi-lized on a flat flame burner !McKenna" at pressures rangingfrom 20 to 40 Torr. Molecular-beam flame sampling is ac-complished with a quartz sampling cone of 0.2 mm diamorifice mounted on a water-cooled flange, which serves asthe end wall of the flame chamber. A nickel skimmer, with its2 mm diam aperture located 23 mm downstream from thesampling cone orifice, forms a molecular beam, which passeshorizontally through the 1.27 cm gap between the repellerand extractor plates of a conventional !Wiley–McLaren9"TOFMS. A grounded accelerator plate is located 1.27 cmabove the extractor plate; the extractor and accelerator plateshave 1.27 cm diam apertures, covered with wire mesh, cen-tered on the vertical flight tube axis of the TOFMS.

Pulse gating of the voltage of the repeller plate is used topropel ions up the 1.3 m flight tube to a multichannel plate!MCP" detector. Optimal plate voltages were found to be:Repeller, 2997 V !ungated"; extractor, 3005 V, accelerator, 0V. The 8 V difference in potential between extractor andrepeller plates, maintained between gating pulses, acts as adiscriminator to reduce a small random background signalcaused by ions escaping the source region. Pulsed gatingvoltages of 325 V amplitude and 15 kHz frequency, with 30ns rise and fall times and 1100 ns pulse duration, are appliedto the repeller plate. The gate pulse duration is sufficient todraw out most of the available ions located below the aper-ture of the extractor plate. The TOFMS mass resolution istypically m/%m"400.

A multiscaler !FAST Comtec P7886" records TOFMSmass spectra in 15 008 channels of 2 ns width. Useful spectrawith a dynamic range of 105 are obtained with 5#105 to 4#106 sweeps. The 3 m off-plane Eagle monochromator, witha 600 lines/mm tungsten grating, delivers photon fluxes ofabout 1014 photons/s with an energy resolution E/%E&400under usual operating conditions in the 9–12 eV photon en-ergy range. The monochromator has an ultimate resolution of2600 for these energies. A silicon photodiode !InternationalRadiation Detectors, Inc. SXUV-100" records the variation inphoton flux with photon energy. Flame temperatures were

measured with a platinum–platinum-rhodium thermocouplethat had been given a beryllium oxide–yttrium oxide anti-catalytic coating and had been carefully calibrated againstflame temperatures measured with the sodium D-line rever-sal method.10

III. PHOTOIONIZATION CROSS SECTIONS

The absolute concentrations of reaction intermediates arequite sensitive to the reaction mechanisms and rate constantsthat determine their rates of formation and consumption. Ac-curate kinetic modeling, therefore, requires measurements ofthe spatial profiles of the absolute concentrations of as manyflame species as possible. Current kinetic models for flamechemistry are largely based on concentration measurementsperformed by conventional electron-impact mass spectrom-etry !EIMS", which is universally applicable to flame speciesbecause it does not require the detailed spectroscopic infor-mation needed for application of less intrusive optical meth-ods such as LIF !laser induced fluorescence", REMPI!resonance-enhanced multiphoton ionization", CRDS !cavityring-down spectroscopy" or CARS !coherent anti-Stokes Ra-man spectroscopy". The universal use of EIMS or VUVPIMS for absolute concentration measurements requiresknowledge of either electron-impact ionization or photoion-ization cross sections, respectively, for each species of inter-est. In this section we describe photoionization cross-sectionmeasurements for allene, propyne, ethylene, and acetalde-hyde with an accuracy !$25% uncertainty" suitable for ki-netic modeling.

Person and Nicole11 have carefully measured photoion-ization cross sections for propene, propyne, ethylene, andseveral other molecules. The cross section for propene overthe 9.7–11.75 eV range, shown in Fig. 1, is the standard weuse to calibrate the response of our system for the measure-ment of cross sections for various target molecules.7 Binarymixtures of nominally 10 Torr propene with 10 Torr of agiven target were prepared in a 3.8-liter stainless steelsample cylinder to which an additional 2300 Torr of argon

FIG. 1. Photoionization cross section for propene measured by Person andNicole !Ref. 11" !left-hand scale, 1 Mb"10!18 cm2).

8357J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Hydrocarbon flame chemistry

was added. The combined gases were allowed to mix for atleast eight hours and then were introduced as a room tem-perature flow into the flame chamber at a flow rate of 0.1 slm!standard liters per minute", along with a second flow ofargon !as a shroud gas" of 0.15 slm. The flame chamberpressure was kept constant at 9 Torr with a servo-controlledthrottle value on the chamber exhaust.

This arrangement enabled run times of about 1 hour,during which the ratio of target ion signal to propene ionsignal was recorded over the 9.7–11.75 eV photon energyrange. Multiplication of this ratio by the ratio of propene totarget species partial pressures yields the ratio of the photo-ionization cross section for the target to that of propene as afunction of photon energy. In this paper ‘‘ion signal’’ refersto the ion count at a given mass/charge ratio, obtained byintegration of the accumulated ion counts per channel over25 multiscaler channels !50 ns". This permitted integrationover the entire temporal profile of each mass peak, whileavoiding overlapping contributions from adjacent masspeaks. The ratio of cross sections, multiplied by the propenecross section of Fig. 1, gives the desired cross section for thetarget species. Although the photon flux was routinely moni-tored during these measurements, the ratio of recorded ionsignals for the target species and propene is independent ofvariations in photon flux with photon energy.

Photoionization cross sections measured for allene, pro-pyne, ethylene, and acetaldehyde are presented in Figs. 2–5.Tabulated values for these cross sections and the propenecross sections of Person and Nicole11 are available as supple-mentary material.12 The dashed lines shown with the propyneand ethylene data are the cross sections measured by Personand Nicole.11 The comparison between the solid and dashedlines for these two molecules provides a self-consistencycheck between our measurements and those of Person andNicole. The cross sections for allene were obtained by aver-aging the results for three separately prepared samples; twosamples each were used for the propyne, ethylene, and ac-

etaldehyde measurements. The error bars shown with thedata of Figs. 2–5 indicate sample standard deviations($'s) at several photon energies. The probable error in thepropene cross section data of Person and Nicole is unlikelyto exceed $20%.11 This value, when combined with oursample standard deviations, suggests an overall uncertaintyof $25% for our cross section measurements.

IV. CONCENTRATION MEASUREMENTS OF ISOMERSOF C3H4 AND C2H4O

Because the mechanisms for formation and destructionmay be greatly influenced by the molecular structure of agiven isomer, knowledge of the isomeric composition offlame species is needed for accurate kinetic modeling. In this

FIG. 2. Photoionization cross section for allene determined from measure-ments of ion signals at m/e"40 and 42 from binary mixtures of allene andpropene.

FIG. 3. Photoionization cross section for propyne determined from measure-ments of ion signals at m/e"40 and 42 from binary mixtures of propyne andpropene. The dashed curve refers to the cross-section measurements of Per-son and Nicole !Ref. 11".

FIG. 4. Photoionization cross section for ethylene determined from mea-surements of ion signals at m/e"28 and 42 from binary mixtures of ethyleneand propene. The dashed curve refers to the cross-section measurements ofPerson and Nicole !Ref. 11".

8358 J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Cool et al.

section we present flame concentration measurements of theallene and propyne isomers of C3H4 and the ethenol andacetaldehyde isomers of C2H4O.

A series of measurements of the spatial profiles of flamespecies were performed for a fuel-rich !("1.9" ethylene–oxygen–argon flame at a pressure of 30 Torr at photon ener-gies of 10.3, 11.0, 12.0, 13.0, and 14.0 eV. The flame tem-perature profile and spatial profiles !concentrations versusdistance from the burner face"13 for the m/e"28, 40, and 44species, recorded at 11.0 eV, are presented in Fig. 6. Them/e"28 data are for C2H4 ; m/e"40 is the sum of the alleneand propyne isomers of C3H4 ; m/e"44 is the sum of theethenol and acetaldehyde isomers of C2H4O. The speciesconcentrations !mol/cm3" are directly proportional to the re-corded species ion signals divided by the species photoion-

ization cross sections at 11.0 eV. Because the initial concen-tration of C2H4 is known, the absolute concentration profilefor C2H4 is directly calculated from the profile of C2H4

% ionsignal. Determination of the concentrations for m/e"40 and44 of Fig. 6 required measurements of ion signals for allthree masses at 11.0 eV, knowledge of the photoionizationcross sections at 11.0 eV for C2H4 and each of the isomers atm/e"40 and 44, and the isomeric compositions for m/e"40and 44. The 11.0 eV cross section for ethenol was estimatedat 10.9Mb (1 Mb"10!18 cm2) !cf. Fig. 11"; 11.0 eV crosssections for the other species, taken from the data of Figs.2–5, are 7.6, 8.0, 27.2, and 43.5 Mb for ethylene, acetalde-hyde, allene, and propyne, respectively. The remainder ofthis section is devoted to an explanation of the proceduresused for the determination of the isomeric compositions form/e"40 and 44, at a 5 mm distance from the burner face, ofthe !("1.9" ethylene–oxygen–argon flame of Fig. 6.

A. Allene and propyne

Figure 7 displays the results of an experiment conductedto test our procedure for determinations of isomeric compo-sition from photoionization efficiency measurements. A mix-ture of 10.8 Torr allene and 11.4 Torr propyne with 2300 Torrargon was prepared in a 3.8 liter stainless steel sample cyl-inder and allowed to mix for 13 hours. This mixture was thenused to maintain a room temperature flow through the flamechamber as described in Sec. III. The symbols of Fig. 7 arethe ion signals at m/e"40 recorded for the mixture as thephoton energy was varied over the range from 9.6 to 11.05eV. The ion signals have been normalized point-by-point bythe photon flux to remove the influence of flux variationswith photon energy.

The ion signals of Fig. 7 are compared with the photo-ionization cross section for a mixture of allene and propyne!solid curve" constructed with the cross section data of Figs.2 and 3 according to the relationships:

FIG. 5. Photoionization cross section for acetaldehyde determined frommeasurements of ion signals at m/e"44 and 42 from binary mixtures ofacetaldehyde and propene.

FIG. 6. Temperature profile and concentration profiles for C2H4 , theethenol/acetaldehyde isomers of C2H4O, and the allene–propyne isomers ofC3H4 . For a fuel-rich (("1.9) C2H4–O2–Ar flame at 30 Torr with flowrates: C2H4"0.493 slm, O2"0.778 slm, and Ar"1.27 slm.

FIG. 7. A comparison of the m/e"40 ion signal !symbols", recorded from abinary mixture of allene !48.6%" and propyne !51.4%", with the photoion-ization cross section !solid curve" calculated with Eq. !1". The arbitraryscale of the ion signal has been adjusted to match the cross section com-puted for the mixture.

8359J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Hydrocarbon flame chemistry

'"xa'a%xp'p , !1"

xa%xp"1. !2"

Here the photoionization cross section ' is expressed as acombination of cross sections 'a and 'p for allene and pro-pyne weighted by their respective mole fractions xa and xp .The comparison was facilitated by adjusting the arbitraryscale for the ion signals as required to match the calculatedphotoionization cross section.

The agreement between the flux-normalized ion signalsand photoionization cross sections for the allene–propynemixture of known composition, demonstrated in Fig. 7, vali-dates our method for determinations of isomeric composi-tion. Application of this approach to flame species requiresphotoionization cross sections for all isomers of a given m/e.Occasionally it may also be necessary to assign cross sec-tions for the potential formation of photofragment ions withthis m/e from precursor species of greater mass.

Figure 8 displays the ion signal !symbols" for the C3H4species at m/e"40 for a fuel-rich !("1.9" 30 Torr ethylene–oxygen flame recorded for photon energies from 9.7 to 10.6eV at a position 5 mm !after correction for probe-samplingeffects13" from the burner face. At this position the concen-tration of C3H4 species approaches its maximum value !cf.Fig. 6" reached at 6.5 mm. The values for allene and propynemole fractions were varied by trial and error to best-fit theobserved flux-normalized ion signals with the calculated #cf.Eqs. !1" and !2"$ photoionization cross sections for the as-sumed mixture composition. Here again the arbitrary scale ofthe ion signals was adjusted to assist in the curve fitting.

This procedure has also been applied to benzene–oxygen–argon flames; the results are shown in Figs. 9!a" and9!b". The ratios of propyne to allene concentrations, #CH3C

wCH$/#CH2vCvCH2$ , obtained with the fits of Figs. 8and 9 are quoted, with estimated uncertainties, in Table I.

The very rapid forward and reverse rates for the allene-propyne isomerization reactions14–20

CH2vCvCH2!CH3CwCH, !3"

drive the isomeric composition toward thermal equilibriumin competition with isomer-specific chemical reactions thatconsume allene and propyne. The cyclopropene isomer is anintermediate in the allene-propyne isomerization reactions!3".17–20 The ionization energies21 of cyclopropene !9.67 eV"and allene !9.69 eV" are too similar to distinguish betweenallene and cyclopropene in our experiments. A contributionto the ion signal from cyclopropene seems unlikely, however,

FIG. 8. A comparison of the m/e"40 ion signal !symbols", recorded for thefuel-rich (("1.9) C2H4–O2–Ar flame of Fig. 6 with the photoionizationcross section !solid curve" calculated with Eq. !1". The computed crosssection is for a best-fit isomeric composition of 42% allene and 58% pro-pyne. Consideration of the uncertainties in the curve fitting yields molefractions xa"0.42$0.04 and xp"0.58$0.04. These data correspond to con-ditions 5 mm from the burner face !Ref. 13".

FIG. 9. A comparison of the m/e"40 ion signal !symbols" with the photo-ionization cross section !solid curve" calculated with Eq. !1". !a" Recordedfor a fuel-rich (("1.4) C6H6–O2–Ar flame !30 Torr, C6H6"0.149 slm,O2"0.80 slm, Ar"0.57 slm". The computed cross section is for a best-fitisomeric composition of 45% allene and 55% propyne. Consideration of theuncertainties in curve fitting yields mole fractions Xa"0.45$0.04 and Xp"0.55$0.04. The burner position was fixed at 1.75 mm from the samplingposition !Ref. 13". !b" For a fuel-lean (("0.7) C6H6–O2–Ar flame !20Torr, C6H6"0.093 slm, O2"1.00 slm, Ar"0.60 slm". The best-fit composi-tion is 50% allene and 50% propyne, Xa"0.50$0.04 and Xp"0.50$0.04. The burner was 1.5 mm from the sampling position !Ref. 13".

8360 J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Cool et al.

because the concentration of this transient intermediate at1520 K, the flame temperature 5 mm from the burner surface!cf. Fig. 6", is probably negligible compared with those ofthe allene and propyne isomers of much greater thermo-chemical stability.17–20 The equilibrium constant for theisomerization reaction !3", calculated with current thermo-chemical properties21,22 is

Keq"#CH3CwCH$/#CH2vCvCH2$"1.9, !4"

at 1520 K. The smaller ratios given in Table I for the ethyl-ene and benzene flames signify departures from thermalequilibrium. These results are qualitatively similar to thefindings of Davis et al.,23 who used a water-cooled gas sam-pling probe and gas chromatograph to study the oxidation ofpropyne in a turbulent flow reactor and a premixed laminarflame. They cited the reactions

CH3CwCH%H"C2H2%CH3 , !5"

CH3CwCH%OH"C3H3%H2O, !6"

as possible sources for selective depletion of propyne incompetition with the isomerization reactions !3".

B. Ethenol and acetaldehyde

An interesting result of this study is the observation ofethenol, a species never before identified in a hydrocarbonflame, although Ruscic and Berkowitz have observed ethenoland acetaldehyde as products of reactions between atomicfluorine and ethanol.24

The ion signals for m/e"44 displayed in Fig. 10 exhibita dependence on photon energy qualitatively similar to thatshown for the allene–propyne data of Figs. 8 and 9. Theonset of ionization of acetaldehyde near 10.2 eV is clearlyvisible, superimposed on the ion signal for a second specieswith apparent ionization energy near 9.3 eV. The only plau-sible assignment for this species is ethenol with an IE of 9.33eV.24 The data show no clear evidence of a potential contri-bution to the ion signal from ethylene oxide above its thresh-old !IE"10.56 eV21". The solid curve shown with the data ofFig. 10 is based on the cross sections for acetaldehyde of Fig.5 and estimated cross sections for ethenol presented in Fig.11. Dashed curves shown in Fig. 11 bracket the range!$25%" of probable uncertainty associated with the esti-mated cross section !solid curve". This estimate is based onthe observation that cross sections for ionization from the

CvC )-orbitals of alkenes !e.g., ethylene, propene, buteneisomers" typically reach values 1 eV above threshold ofabout 8–10 Mb.25

A reasonable fit of the solid curve to the m/e"44 ionsignals, displayed in Fig. 10, is obtained for an ethenol–acetaldehyde concentration ratio #CH2CHOH$/#CH3CHO$"0.6. Similar fits using the upper and lower dashed curves

TABLE I. Propyne–allene concentration ratios.

Flame #CH3CwCH$/#CH2vCvCH2$

ethylene–oxygen !("1.9"a 1.38$0.25benzene–oxygen !("1.4"b 1.22$0.22benzene–oxygen !("0.7"c 1.0$0.2

aEthylene–oxygen–argon !0.493/0.778/1.27" #Respective flow rates in stan-dard !1 atmosphere, 273 K" liters per minute !slm"$, 30 Torr, 5.0 #Positionafter correction for probe sampling effects !Ref. 13"$ mm from burner face.bBenzene–oxygen–argon !0.015/0.80/0.57" #Respective flow rates in stan-dard !1 atmosphere, 273 K" liters per minute !slm"$, 30 Torr, 1.75 #Positionafter correction for probe sampling effects !Ref. 13"$ mm from burner face.cBenzene–oxygen–argon !0.093/1.00/0.60" #Respective flow rates in stan-dard !1 atmosphere, 273 K" liters per minute !slm"$, 20 Torr, 1.5 #Positionafter correction for probe sampling effects !Ref. 13"$ mm from burner face.

FIG. 10. A comparison of the m/e"44 ion signal !symbols", recorded for thefuel-rich (("1.9) C2H4–O2–Ar flame of Fig. 6, with the photoionizationcross section !solid curve" calculated for an best-fit isomeric composition of37$5% ethenol and 63$5% acetaldehyde. This calculation is based on thecross section for acetaldehyde of Fig. 5 and the cross section !solid curve"for ethenol of Fig. 11; the error limits reflect the uncertainty !$25%" of theestimated cross section for ethenol.

FIG. 11. Estimated photoionization cross sections for ethenol. The photo-ionization efficiency of ethenol for photon energies ranging from thresholdnear 9.3 eV to the ionization energy of acetaldehyde at 10.23 eV !Ref. 21" isassumed to be proportional to the ion signals of Fig. 10 over this range!hence the wiggles". We estimate a value of 8 Mb for the cross section !solidcurve" at 10.2 eV, based on the cross sections observed for photoionizationfrom the CvC )-bonds of ethylene, propene, and isomers of butene !Ref.25". The dashed curves indicate the probable range of uncertainty !$25%"in the estimated cross section !solid curve".

8361J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Hydrocarbon flame chemistry

of Fig. 11 give values of 0.47 and 0.73, respectively, for theethenol–acetaldehyde ratio.

If we estimate the difference in Gibbs free energy be-tween ethenol and acetaldehyde at 1520 K to be 8.5kcal/mol,21,26 then the ethenol–acetaldehyde ratio#CH2CHOH$/#CH3CHO$ for thermal equilibrium at 1520 Kis only 0.06. Despite the thermochemical stability of acetal-dehyde relative to ethenol, a very high barrier !*56 kcal/mol" to the unimolecular tautomerization of ethenol toacetaldehyde exists.27 Indeed, ethenol is known to be highlystable in the gas phase.24,28–30

How is ethenol formed in the ethylene–oxygen flame?Further experimental and modeling efforts are needed to an-swer this question. Re-examination of formation mechanismsfor acetaldehyde may reveal previously unidentified productbranching channels leading to ethenol. Another hypothesis isthat vinyl alcohol may be a highly stable product of thedecomposition of the primary HOCH2CH2 adduct formed bythe addition of OH to ethylene at the CvC double bond.6Indeed, Kirchner et al.31 and Howard32 have studied the re-action of chloroethylenes with the OH radical and concludethat OH addition at the carbon–carbon )-bond results in Clatom elimination, e.g.,

OH%CHCIvCCI2"HO–CHCl–CCI2

"CCI2vCHOH%Cl. !7"

Evidence for reaction !7" in a methane–oxygen flame dopedwith trichloroethylene has been presented.33 An analogousreaction of the chemically activated adduct of the OH attackon ethylene may also occur34

OH%C2H4"HO–CH2–CH2"CH2CHOH%H, !8"

despite the stronger C–H bond and a higher barrier toH-atom elimination. Howard32 has suggested that the adductmay also react with O2 to form ethenol and HO2 :

OH%C2H4"HO–CH2–CH2 , !9"

HO–CH2–CH2%O2"CH2CHOH%HO2 . !10"

At elevated flame temperatures (T&800K), however, theHO–CH2–CH2 adduct may not survive rapid unimoleculardecomposition6 to react with O2 by reaction !10". Moreover,at flame temperatures above 650 K, the H-atom abstraction

OH%C2H4"H2O%C2H3 , !11"

FIG. 12. PIE curves measured in benzene–oxygen flames. Solid circles: fuel-rich !("1.4"; open circles: fuel-lean !("0.7". !a" m/e"42, !b" m/e"50, !c"m/e"52, !d" m/e"66. The labels give the accepted ionization energies !Ref. 21" for species identified by the observed ionization thresholds.

8362 J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Cool et al.

dominates OH addition,6 and the production of ethenol byeither !8" or !10" will be inhibited. Nevertheless, reactions!8" and possibly !10" provide plausible sources for the pro-duction of ethenol in low-temperature regions of ethylene–oxygen flames.

The high barrier for tautomerization suggests that onceethenol is formed, it is quite stable; the decompositions ofthe two C2H4O isomers are uncoupled, and may thereforeoccur at different rates and by different flame reactionmechanisms.

V. FLAME SPECIES IDENTIFICATIONSWITH NEAR-THRESHOLD PIMS

The ease with which the photon energy may be preciselytuned near the ionization thresholds for flame species withthe synchrotron light source is a feature unmatched by lasersources. The available energy resolution (E/%E*400), farsuperior to that of conventional EIMS, facilitates speciesidentification with observations of apparent ionizationthresholds.

Nevertheless there are questions that arise regarding theinfluence that rotational and vibrational excitation of flame-

sampled molecules may have on observed ionization thresh-olds. Moreover, field ionization from high-lying Rydbergstates, induced by the 250 V/cm fields used to extract ionsfrom the source region, must be considered. Rotational exci-tation ensures that Rydberg levels lying within !3/2" kT!+0.04 eV for our apparatus35" of the ionization limit con-tribute to the ion signal.36,37 Similarly, field ionization ofRydberg states lying within ,6.1!250"96 cm!1 !0.012 eV"of the ionization limit38,39 may also contribute to an apparentthreshold below the true IE. These effects are comparable tothe energy spread !,0.025 eV at 10 eV" of the dispersedsynchrotron radiation used in these studies. A more seriouscomplication may be the influence of hot bands associatedwith incomplete vibrational relaxation during the gas dy-namic expansion of the molecules sampled by the quartzcone. We originally feared that such effects might place se-vere limitations on the ability to use apparent ionizationthresholds for species identification. As it happens, in prac-tice these problems do not seriously compromise species dis-crimination based on apparent ionization thresholds.4 In theremainder of this section we present a few examples in sup-port of this conclusion.

FIG. 13. !a"–!c" PIE curves measured in benzene/oxygen flames. Solid circles: Fuel-rich !("1.4"; open circles: Fuel-lean !("0.7". !a" m/e"78, !b" m/e"92,!c" m/e"94. The labels give the accepted ionization energies !Ref. 21" for species identified by the observed ionization thresholds. !d" PIE curve for1,3-butadiene measured in 30 Torr. 1,3-butadiene–hydrogen–oxygen–argon flames. Solid circles: Respective flow rates"!0.17/1.00/1.00/3.00" !slm"; opencircles: Respective flow rates"!0.05/1.00/0.65/2.00" !slm".

8363J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Hydrocarbon flame chemistry

Benzene–oxygen–argon flames provide a rich source ofreaction intermediates that may be unambiguously identifiedwith near-threshold VUV PIMS. Photoionization efficiency!PIE" curves !near threshold variation in ion yield with pho-ton energy" are presented in Figs. 12!a"–12!d" and 13!a"–13!c" for several prominent intermediates observed in bothfuel-rich !("1.4" and fuel-lean !("0.7" flames. Figure13!d" presents a PIE curve for 1,3-butadiene, a species ob-served for both ethylene and benzene flames, measured witha 1,3-butadiene–hydrogen–oxygen–argon flame. The appar-ent ionization thresholds of the PIE curves of Figs. 12 and 13are seen to closely match the accepted ionization energyvalues21 for the species labeled on the figures. With conven-tional EIMS, some of these species would be indistinguish-able from several possible isomers.

Similar PIE curves have been recorded for a number ofstable and radical intermediates in ethylene–oxygen,benzene–oxygen, and 1,3-butadiene–hydrogen–oxygenflames; a complete tabulation of these species is given inTable II. In every case the apparent ionization thresholds areobserved to closely match the known ionization energies21tabulated in the last column of Table II.

VI. CONCLUSION

In this paper we have presented two interesting examplesof the use of flame-sampling VUV PIMS for quantitativemeasurements of the concentrations of isomer pairs of im-portance in hydrocarbon combustion. The concentrations ofthe ethenol–acetaldehyde isomers at m/e"44 shown in Fig.6 are comparable to those of many key intermediates in thefuel-rich ("1.9 ethylene–oxygen flame and exceed those of

the allene–propyne m/e"40 isomers. These data suggest thatthe mechanisms of formation and consumption of ethenolneed to be incorporated in continuing development40 of mod-els for the combustion of C2 hydrocarbons. The ability tosupplement measurements of spatial profiles of photoions,created at a fixed photon energy, with measurements of spe-cies PIE curves, over a range of photon energies at fixed-flame positions, adds an important second dimension to theuse of flame-sampling mass spectrometry for the develop-ment of kinetic models of flame chemistry.

While the focus of this paper is on the isomeric selectiv-ity of VUV PIMS, another significant advantage of this ap-proach is the ability to precisely tailor the photon energy tominimize parent ion fragmentation. An extensive database21giving the ionization energies of parent species along withthe appearance energies for photofragment ions is a valuableresource available to the experimenter.

Spatial profiles of the absolute concentrations of flamespecies are directly measurable when photoionization crosssections are known over an appropriate range of photon en-ergies. The data of Fig. 6 exemplify this approach. Photoion-ization cross sections for many of the stable intermediateslikely to be observed in hydrocarbon flames can be measuredwith an accuracy !$25% uncertainty" suitable for combus-tion modeling, as discussed in Sec. III, with the use of binarymixtures of a target species with propene. In related work,Neumark and co-workers41 are exploring the use of synchro-tron radiation for the measurement of photoionization crosssections of common combustion radicals, e.g., C2H3 andC3H3 . Theoretical and semiempirical methods may be veryuseful for cross section estimates when reliable experimentalmeasurements are unavailable.

ACKNOWLEDGMENTSThis research was supported by the U.S. Department of

Energy, Office of Basic Energy Sciences, Division of Chemi-cal Sciences, Geosciences, and Biosciences, and by the U.S.Army Research Office, Chemical Sciences Division.

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TABLE II. Hydrocarbon flame species for which PIE curves have beenrecorded.

Species Formula m/e IE !eV"a

methyl radical CH3 15 9.84water H2O 18 12.62acetylene C2H2 26 11.40ethylene C2H4 28 10.51formaldehyde H2CO 30 10.88oxygen O2 32 12.07hydrogen peroxide H2O2 34 10.58propargyl radical C3H3 39 8.67allene C3H4 40 9.69propyne C3H4 40 10.36ketene CH2CO 42 9.62ethenol CH2CHOH 44 9.33acetaldehyde CH3HCO 44 10.23dimethyl ether (CH3)2O 46 10.03formic acid HCOOH 46 11.33diacetylene C4H2 50 10.17vinylacetylene C4H4 52 9.581,3-butadiene C4H6 54 9.07glyoxal (CHO)2 58 10.21,3-cyclopentadiene C5H6 66 8.57diethyl ether (C2H5)2O 74 9.51benzene C6H6 78 9.24toluene C7H8 92 8.83phenol C6H6O 94 8.49phenylacetylene C8H6 102 8.82

aReference 21.

8364 J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 Cool et al.

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