The reaction of O + ( 4 S ) and N 2 (X 1 Σ g + ) revisited: Recoil velocity analysis of theNO + productDale J. Levandier, Rainer A. Dressler, Yu-hui Chiu, and Edmond Murad Citation: The Journal of Chemical Physics 111, 3954 (1999); doi: 10.1063/1.479697 View online: http://dx.doi.org/10.1063/1.479697 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/111/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsedfield ionization-photoion method: Charge transfer reaction of N2 +(X 2Σg +; v+ = 0–2; N + = 0–9) + Ar J. Chem. Phys. 137, 104202 (2012); 10.1063/1.4750248 Reactions of O + with C n H 2n+2 , n=2–4: A guided-ion beam study J. Chem. Phys. 120, 6999 (2004); 10.1063/1.1667459 Ab initio analytical potential energy surface and quasiclassical trajectory study of the O + ( 4 S)+ H 2 (X 1 Σ g +)→ OH + (X 3 Σ − )+ H ( 2 S) reaction and isotopic variants J. Chem. Phys. 120, 4705 (2004); 10.1063/1.1638735 Guided-ion beam study of the O 2 + +C 2 H 2 charge-transfer and chemical reaction channels J. Chem. Phys. 110, 4291 (1999); 10.1063/1.478312 Collision energy dependence and product recoil velocity analysis of O + ( 4 S)+C 2 H 2 charge-transfer andchemical reaction channels J. Chem. Phys. 109, 5300 (1998); 10.1063/1.477148

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The reaction of O 1„

4S… and N2„X 1Sg1… revisited: Recoil velocity analysis

of the NO 1 productDale J. Levandier,a) Rainer A. Dressler, Yu-hui Chiu,b) and Edmond MuradAir Force Research Laboratory, Space Vehicles Directorate, 29 Randolph Road, Hanscom Air Force Base,Massachusetts 01731-3010

~Received 6 May 1999; accepted 8 June 1999!

The guided-ion beam method has been used to measure time-of-flight~TOF! spectra of the NO1

produced in the reaction of O1(4S) and N2(X 1Sg1). Particular care was taken to minimize the

participation of the O1(2D,2P) metastables in the reactions, the success of which is validated bymeasuring the absolute cross section for the reaction as a function of collision energy in thenear-thermal to 1.5 eV range. In this range, the cross section exhibits a region of low reactivity atnear-thermal energies, which can be obscured by reactions of metastables and other effects, and aregion of sharply increasing reactivity above a threshold of collision energy'0.3 eV. Thevelocity-transformed TOF spectra, at collision energies of 0.51 eV, 0.80 eV, 3.2 eV, and 6.3 eV,indicate two reaction mechanisms: in the first, product ions are scattered backward with inefficienttranslational to internal energy conversion; in the second, product ions arise from intermediatecollision complexes with efficient translational energy conversion. The first mechanism is associatedwith the passage through the spin-forbidden doublet ground state N2O1 intermediate, while thesecond involves passage through an endothermic quartet intermediate. ©1999 American Instituteof Physics.@S0021-9606~99!02233-3#

INTRODUCTION

The reaction

O1 1 N2 ˜ NO11N, DH521.09 eV, ~1!

which involves the most abundant ion and neutral in theionosphereF-region, has been studied extensively. All of thestandard experimental techniques have been applied to thestudy of this reaction through the energy range from thermalto a few eV1–14 and there have been some endeavors intheory.15–21The cross section for reaction~1! at low collisionenergies is bimodal, exhibiting a threshold at;0.3 eV,10

corresponding to a rapid increase in reaction efficiency to amaximum of ;4 Å2 at ;10 eV, where NO1 dissociationthen becomes accessible. In the pre-threshold region, thecross section is of the order of 0.1–0.3 Å2. The form of thecross section in this low energy region has been attributed tothe details of the N2O

1 potential,15–19 which indicates amechanism that involves spin-forbidden surface hops to andfrom the N2O

1 doublet ground state at pre-threshold colli-sion energies. The threshold is consistent with the availabil-ity of a quartet reaction pathway, via the N2O

1(1 4A9) statewhich forms an effective barrier of;0.2–0.3 eV.10,17–19

Despite the attention given to reaction~1!, there are openquestions regarding the dynamics involved, particularly withrespect to the energy disposal. Knowledge of the dispositionof energy in the products, whether through translational tointernal energy conversion or due to the exothermicity,which is sufficient to produce NO1 with three quanta of

vibrational excitation, will allow further insight into the re-action mechanism. Moreover, this information would be re-quired if efforts in modeling the ionosphere and other airplasmas are to include the effects of internal energy.

Previous investigations into the dynamics of reaction~1!have met with limited success. The results of merged beamexperiments,5 which featured measurement of the componentof product kinetic energy along the beam axis, were affectedby the presence of O1 and N2 excited states. A recent state-selected, guided-ion beam~GIB! study of the reactions ofO1 and N2 illustrates the importance of avoiding theO1(2D,2P) metastables when ground state species are ofinterest.13 This is particularly true for O1(2P), which has alarger cross section below;0.5 eV that increases with de-creasing collision energy. The thermal rate of reaction~1!has also been shown to increase dramatically forN2(v>2!.1,2,12 In a crossed beam study,7 limited to collisionenergies of>1.50 eV, doubly differential cross sections wereobtained that were in poor agreement with the merged beamresults. A beam-cell study, limited to energies above;1.8eV, may also have been subject to contamination by O1

metastables.3

Although lower in resolution than crossed beam studies,the cylindrical symmetry and 100% product ion collectionefficiency of GIB time-of-flight ~TOF! measurements pro-vide many advantages when determining recoil velocity dis-tributions of ions.22 In the study presented here, a GIB in-strument is used to measure product TOF spectra from whichthe product velocity distributions are obtained. Care wastaken to minimize the participation of O1 excited states, thesuccess of which is confirmed by comparison of the mea-sured cross section to that from other studies.

a!Orion International Technologies, Albuquerque, NM 87110.b!Boston College, Institute for Scientific Research, Newton, MA 02159.

JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 9 1 SEPTEMBER 1999

39540021-9606/99/111(9)/3954/7/$15.00 © 1999 American Institute of Physics

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EXPERIMENT

The apparatus used in this study has been explained indetail previously,23 so only a brief description is given here.The instrument is a tandem mass spectrometer equipped witha rf octopole ion guide located between the mass filters. Theoctopole has two stages, the first of which is 7.4 cm in lengthand passes through a 3.5 cm long collision cell. The secondoctopole stage begins at the exit of the collision cell and is16.7 cm long. Lens systems exist for injecting ions into thefirst octopole stage and for extracting ions from the secondstage prior to analysis.

Primary O1 ions are produced in an electron impact ionsource using CO2 as a precursor gas for dissociative ioniza-tion. By working with an electron energy of;20 eV, 0.9 eVabove and 2.3 eV below the appearance potentials forO1(4S) and O1(2D), respectively, it is possible to form O1

almost exclusively~>99%! in its ground state. In the presentstudy, this is easily checked by measuring the O11N2

charge transfer cross section, which is negligible below the1.97 eV thermochemical threshold for charge transfer ontoO1(4S). For 1 eV collision energy, the charge transfer crosssections for O1(2D) and O1(2P) are;20 Å2 and 30 Å2,13

respectively, so even a 1% population of metastables in theprimary ion beam is discernable. The ion beam is turnedthrough 90° in a dc quadrupole bender, mass selected in aWien filter, and then injected into the first octopole at thedesired kinetic energy. Unreacted primary ions and ions re-sulting from reactions in the collision cell pass into the sec-ond octopole, which is typically biased 0.3–0.4 V below thefirst octopole to aid in the extraction of thermal product ionsfrom the cell. To minimize the occurrence of secondary re-actions in the collision cell, the N2 target gas is maintained ata pressure of;0.25 mTorr. The ion beam energy is knownto better than60.05 eV, with beam spreads typically of0.22–0.25 eV FWHM, as measured by the retarding poten-tial method and by time-of-flight.

Absolute cross sections are determined by integratingthe mass spectrometer signal intensities for the product andtransmitted primary ions, monitoring the pressure of the tar-get gas, and using the low density limit of the Lambert–Beerexpression, as has been described before.23

To avoid the effect of very slow primary ions trappedwithin the octopole, the cross section measurements are car-ried out using a pulsed mode in which the ion beam andoctopole rf are turned off briefly. The trapping of low energyprimary ions, which may result from surface potential barri-ers, has the result of increasing the apparent cross sectionsince the ions can simply reside within the ion guide untilthey undergo a reactive collision.24 In the present study, a 5ms pulsing cycle is used, with the octopole rf turned off forthe last 25ms of the period to allow the slow ions to escapefrom the ion guide. The ion beam is turned on 5ms after therf potential is restored, and remains on for 4.5 ms, the re-mainder of the period being used to collect the product ionsderived from the latter part of the ion beam pulse. The puls-ing scheme is illustrated in Fig. 1~a!. Also shown in Fig. 1~b!are the results of an experiment in which the cross section forreaction ~1!, at 0.19 eV collision energy, is measured fordifferent pulsing cycle periods, with the rf and ion beam

off-time unvaried. The cross sections measured with thispulsing scheme are compared to continuous mode data.

TOF spectra are measured by pulsing the primary ionbeam, using a 3ms ion beam pulse width, and measuring theflight time of product ions arriving at the detector. Very slowions, which would otherwise result in a background signal insubsequent pulse cycles, are eliminated at the end of eachcycle by briefly turning off the octopole rf potential.

RESULTS AND ANALYSIS

Figure 2~a! shows the absolute cross section measured inthe present study for production of NO1 from collisions ofO1(4S) and N2(X 1Sg

1) at kinetic energies in the range ofnear-thermal to;1.6 eV. The data have been corrected forthe primary ion residence time effect using standard decon-volution procedures.22,25The data exhibit a threshold at;0.3eV, below which the cross section is a nearly constant 0.05–0.06 Å2. The cross section rises steeply from the thresholdthrough the remainder of the studied range; at 1 eV the crosssection is 1.0 Å2.

The same results are plotted in Fig. 2~b! in the form ofan energy dependent rate constant. The rate constant,k(^ET&), is obtained from the cross section,s(ET), by theexpressionk(^ET&)5v•s(ET), where ^ET& is the averagerelative translational energy andv is the relative velocity.Also illustrated on this plot is the agreement between thepresent data and the results of drift tube9 and high tempera-ture flowing afterglow experiments.12 The high temperatureflowing afterglow data were converted from temperature~T!

FIG. 1. ~a! Pulsing scheme used to eliminate effects of trapped primary ions.A 5 ms period is used, during which time the octopole rf potential is on forall but the last 25ms, the interval required for trapped ions to escape theoctopole volume. The ion beam is turned on 5ms after the rf potential isrestored, and it remains on for 4.5 ms.~b! Effect of the period length on theapparent cross section measured atET50.19 eV. The dashed line indicatesthe measured cross section when the normal experiment configuration isused, with a continuous octopole rf field and ion beam. The points are crosssections measured at different period lengths, while keeping the rf and ionbeam off intervals constant.

3955J. Chem. Phys., Vol. 111, No. 9, 1 September 1999 Reaction of O1(4S)and N2(X1Sg

1)

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to kinetic energy~KE! using the usual KE51.5kBT, wherekB is the Boltzman constant. The only noteworthy differ-ences are the slight rise of the flowing afterglow data at;0.2eV, due to vibrational enhancement,12 and the lack of a clearminimum in the present data, due largely to the width of thedistribution of collision energies in this work.

As indicated above, monitoring of the charge transferreaction O11N2˜N2

11O, which is negligible for O1(4S) atenergies below 2 eV, indicates that the ion beam used in thisstudy should include less than 1% of the O1(2D,2P) meta-stables. The cross section presented here compares well withthe first GIB studies of reaction~1! by Burley et al.,10 inwhich the primary ion beam was purged of metastable O1 bypassing the beam through an N2 charge transfer cell prior toreaction. The agreement between the present work and thedrift tube rate constant, illustrated in Fig. 2~b!, is similarlygood. The same reaction was also recently studied usingstate-selected GIB experiments,13 and agreement betweenthose results and the present data are reasonable. Moreover,the state-selected data, which include the cross sections forreaction with the O1 metastables, further confirm that thepresent study is not significantly affected by reactions ofthese species. Specifically, the O1(2P) cross section is ap-preciable at near thermal collision energy and decreases withincreasing energy, typical of exothermic ion-molecule reac-tions. At 1 eV the cross sections for O1(4S) and O1(2P) areof similar magnitude, and below this energy the latter crosssection is much larger, suggesting that the presence of

O1(2P) in our ion beam would severely affect our crosssection in the near-thermal region.

Figure 3 presents the results of NO1 TOF experiments atcollision energies of 0.51 eV, 0.80 eV, 3.2 eV, and 6.3 eV.The raw TOF data have been transformed to the velocitydomain, as described previously,26 and are plotted as a func-tion of v1p8 , the component of product ion laboratory veloc-ity parallel to the ion beam axis. Also shown in each case isthe corresponding velocity distribution for the primary ionbeam, confirming the narrow ion beam energy spread. Oneach plot are indicated the velocity of the center of mass andthe energetic limit of forward and backward~in one case!laboratory velocities. In this work, the forward direction isdefined as the ion beam direction, and corresponds to veloci-ties greater than the velocity of the center of mass (vc.m.).The lowest energy TOF data, atET50.51 eV, represents apractical limit in sensitivity, because of the small cross sec-tion, and was attempted as a means of providing informationon the pre-threshold mechanism.

The TOF data suggest two mechanisms for reaction~1!in the included energy range. In the lower energy TOF spec-tra, there is a strongly back-scattered component that domi-nates atET50.51 eV and which decreases with increasingenergy, relative to the second component. The second com-ponent appears as a shoulder in the velocity distributions atET50.51 eV and 0.80 eV, and includes products which arescattered in both the forward and backward directions. In

FIG. 2. ~a! Absolute reaction cross section for O1(4S)1N2(X 1Sg1)

˜NO11N as a function of collision energy.~b! The same data have beenconverted to a rate constant~solid circles! as a function of average kineticenergy, as outlined in the text. The data are compared to data from drift tube~open triangles; Ref. 9! and high temperature flowing afterglow~opensquares; Ref. 12! experiments.

FIG. 3. Time of flight data for the NO1 product ion from reaction~1! for theindicated collision energies~solid circles!. The data have been transformedto the laboratory velocity reference frame. The velocity spectra represent thedistributions of the product velocity component parallel to the primary ionbeam direction. The solid curves are the ion beam velocity distributions. Thehorizontal lines indicate: short dash–velocity of the center of massvc.m.;long dash—minimum~in one case! and maximum possible product labora-tory velocities. In the plot forET56.3 eV, the arrow indicates the mostprobable product velocity for the Spectator Stripping mechanism.

3956 J. Chem. Phys., Vol. 111, No. 9, 1 September 1999 Levandier et al.

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both these spectra, the forward-scattered intensity decreasesmonotonically, with very little intensity at velocities ap-proaching the thermochemical maximum.

The shoulder, which appears to be located nearvc.m. inthe lower energy TOF data, grows in relation to the stronglyback-scattered component until it dominates the product ve-locity distribution atET53.2 eV. At this collision energy,this component exhibits similar intensities in the forward andbackward directions, with a discernible peak in the forwarddirection atv1p8 '4000 m/s. In the velocity spectrum atET

56.3 eV, the forward-backward component, now the onlymechanism, is asymmetric with respect tovc.m. and peaks inthe forward direction atv1p8 '5500 m/s with a broad tail tolower velocities.

The component with forward and backward scattering isreminiscent of a complex-mediated reaction. At the lowestcollision energies the details are difficult to discern due tothe back-scattered component and low velocity ‘‘pile-up’’from the time-to-velocity transform. AtET53.2 eV, how-ever, these product velocities are nearly symmetric aboutvc.m., indicating a complex with lifetime comparable to therotational period of the complex. The TOF spectrum alsoindicates a significant degree of internal energy in the prod-ucts, as evidenced by the peaking well below the thermo-chemical limit. The velocity distribution forET56.3 eV isconsistent with a complex whose lifetime is somewhatshorter than its rotational period, which is expected for com-plexes formed at higher collision energies. At still higherenergies such a reaction mechanism typically evolves to di-rect dynamics. Indeed, the peak in this velocity distributionis near the spectator stripping limit,27 as indicated by thearrow in Fig. 3, which is consistent with similar observationsin other studies for collision energies of this magnitude.3,5,7

The present TOF results agree well with the mergedbeam5 and crossed beam7 studies only at higher collisionenergies, where reaction~1! appears to proceed via a directmechanism to yield forward-scattered NO1. At lower colli-sion energies, the results of the three experiments differ sig-nificantly. The merged beam study spans the energy rangeexamined here, and includes product kinetic energy scans atET50.5 eV, 0.8 eV, and 3.0 eV, nearly coinciding with thelower energies for which the present TOF data were ob-tained. At the two lower energies, the merged beam experi-ments indicate nearly symmetric forward and backward scat-tering, with significant NO1 product intensity at thethermochemical limits. This clearly does not correspond withthe present TOF data atET50.51 eV and 0.80 eV, in whichthe forward scattered product intensity diminishes at veloci-ties well below the thermochemical limit, and in which thescattering is dominated by a strong backward component.The difference between the two experiments may be attrib-uted to contributions from excited states of the reactant ionand molecule in the merged beam study. Similarly, the 3.0eV merged beam results feature more forward-scattered in-tensity than in the present study.

In the crossed beam work, the product ions for the low-est collision energy studied, 1.5 eV, are primarily forwardscattered, with a peak well beyond the spectator strippingvalue and with considerable intensity near the thermochemi-

cal limit for reaction~1!. Due mostly to the absence of back-scattered product, this is in clear contrast to the present re-sults, and to the results of the merged beam study in whichdata are also available forET51.5 eV. The lack of back-scattered NO1 in the crossed beam experiment may relate tothe combination of the kinematics for this system and therestricted range of laboratory angles for detection.28 Theformer constraint dictates that back-scattered ions have verylow laboratory velocities in that study, making them verysusceptible to stray fields, and the latter limits the experimentto viewing only a fraction of back-scattered ions.

To quantify the TOF results with regard to reaction en-ergy disposal, which we consider in terms of the translationalto internal energy transfer,DET , we have modeled the TOFvelocity distributions for the experiments atET53.2 eV and6.3 eV using the osculating complex model.29 The simula-tions are compared with the experimental distributions inFigs. 4~a! and 5~a!. We have detailed our implementation ofthis model in previous work,26,30however an outline is givenhere. In this model, a reaction is regarded as proceeding viaa weakly bound, orbiting complex with an exponential decaytime proportional to exp(2t/tc), where tc is the complexlifetime. The product recoil energy distribution is assumed tobe independent of scattering angle, the distribution of whichis governed by the ratiot r /tc , where t r is the complexrotational period. Under these conditions, for an experimentat a given collision energy, a basis set of laboratory axial

FIG. 4. Results of analysis of TOF data forET53.2 eV using the osculatingcomplex model.~a! The solid and dashed curves are the osculating modelsimulations for the laboratory velocity distributions obtained with normaland low rf amplitudes, respectively~solid and open circles!. The dottedcurve represents the laboratory velocity distribution calculated using the lowrf basis functions and the energy transfer distribution derived for the normalrf experiment. The dashed vertical line isvc.m.. ~b! The solid and dashedcurves are the translational energy transfer distributions derived from the fitsshown in~a! for the normal and low rf experiments, respectively.

3957J. Chem. Phys., Vol. 111, No. 9, 1 September 1999 Reaction of O1(4S)and N2(X1Sg

1)

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velocity distributions,Si(v1p8 ;DETi ,t i), may be constructedin which the adjustable parameters aret5t r /tc and DET ,and which account for the experimental broadening mecha-nisms. The simulations are linear combinations of the basisfunctions derived by a least squares fit to the experimentallyobserved distributions:

f ~v1p8 !5(i

ciSi~v1p8 ;DETi,t i !, ~2!

where the fit coefficientsci are equivalent to the probabilityof a particular value of translational exoergicity,P(DET).The plots ofP(DET), in Figs. 4~b! and 5~b!, represent thedistributions of translational to internal energy transfer forthe reactions atET53.2 eV and 6.3 eV, respectively.

In Fig. 4~a!, the osculating complex model~solid curve!recovers theET53.2 eV TOF data~solid circles! reasonablywell, especially at higher velocity where there is contributionfrom only one of the reaction mechanisms. This fit wasachieved with a single value oft 51, indicating a complexwith average lifetime similar to its rotational period. Al-though the present model may also be used to simulate directdynamics, by using very large values oft, the near thermalproducts were not modeled in this case. The energy transferdistribution for reaction atET53.2 eV, Fig. 4~b! ~solidcurve!, is very broad and peaks at approximatelyDET

522.2 eV. The distribution indicates a substantial degree ofenergy transfer, with typically only 1 eV of energy retainedin translation.

To confirm the collision complex mechanism, a TOFexperiment was carried out atET53.2 eV in which the oc-

topole rf trapping potential is reduced. When interpreting theTOF data, it must be remembered that the velocity distribu-tions consist of projections of the product ion recoil velocityvector onto the octopole axis. As a result, complicated, wide-angle scattering could hinder the extraction of informationon energy partitioning from the TOF data. In the presentexperiment, large transverse velocity components can be dis-criminated by reducing the radial trapping potential. The re-sult of this experiment is included in Fig. 4~a! ~open circles!.The distinguishing feature of the low rf velocity distributionis the diminished intensity atvc.m., which is the behaviorexpected for the decay of a weakly bound complex. Productions derived from an orbiting complex would be scatteredisotropically ~with respect tovc.m.!, with a velocity distribu-tion independent of scattering angle. In this case, productions with parallel laboratory velocity nearvc.m. may have alarge transverse velocity component and would escape theion guide when the lower trapping potential is used, as isobserved in this experiment. The greater loss of back-scattered ions, compared to forward-scattered ions, can beattributed to losses of low energy ions due to octopole rodsurface potential barriers, which are of greater consequenceat low rf amplitudes where the ion trajectories pass closer tothe rod surfaces.

To substantiate the validity of the osculating complexmodel, the low rf data atET53.2 eV was also modeled.Purely osculating dynamics would result in the same energytransfer distribution as seen in the high rf experiment. Abasis set of velocity distributions was used that has the samevalue of t and which also accounts for the loss of productions above a calibrated transverse velocity.31,32 The fit is,again, satisfactory at higher velocities, as illustrated in Fig.4~a! ~dashed curve!. The energy transfer distribution deriveddirectly from the fit to the low rf data, in Fig. 4~b! ~dashedcurve!, is in good agreement with the distribution determinedin the high rf experiment, further confirming the complex-mediated mechanism. The low rf data also compare wellwith a simulation~dotted curve! derived by using the basisfunctions for the low rf configuration and the energy transferdistribution for the high rf experiment.

Figure 5~a! and 5~b! depict the results of the osculatingcomplex modeling of theET56.3 eV TOF experiment. Atthis collision energy, the channel that dominates the low en-ergy reaction, giving rise to the strongly back-scattered prod-ucts, is no longer prominent, so the simulation reproducesthe experimental data over essentially the whole range ofproduct laboratory velocity. The resulting energy transferdistribution, for a set of basis functions constructed with asingle value oft 54, is similar in form to the translationallyexoergic distribution derived from the lower energy experi-ment. The distribution is broad and peaked at large values ofkinetic to internal energy transfer, again about 1 eV remain-ing as translational energy.

The energy disposal in the strongly back-scattered com-ponent, at the lower collision energies, is more difficult todiscern and only an upper limit of product internal excitationcan be provided. At these low collision energies, becausereaction~1! is exothermic, back-scattered product ions withvelocities approaching the thermochemical limit travel back-

FIG. 5. Results of analysis of TOF data forET56.3 eV using the osculatingcomplex model.~a! The osculating model simulation of the laboratory ve-locity distribution ~solid circles!. The dashed vertical line isvc.m.. ~b! Thetranslational energy transfer distribution derived from the fit shown in~a!.

3958 J. Chem. Phys., Vol. 111, No. 9, 1 September 1999 Levandier et al.

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ward in the laboratory frame, and cannot be directly mea-sured in the TOF experiment. Although it is possible, in thepresent experiment, to recover the signal intensity due tosuch product ions by imposing a potential barrier that reflectsthese species into the forward direction, the velocity infor-mation is not preserved. The peaks atv1p8 50 in the TOFspectra at the lower collision energies,ET50.51 eV and 0.80eV, are narrower than the experimental resolution, indicatedby the ion beam widths. This suggests that the back-scatteredcomponent may feature larger center-of-mass velocities thanare indicated in the TOF spectra. The measured zero-velocitysignal can therefore be taken to represent an upper~lower!limit of internal ~translational! product energy for the rel-evant collision energy. For theET50.51 eV and 0.80 eVexperiments, the upper limit on product internal energyamount to;0.9 eV. TOF measurements of the NO1 frag-ment in threshold photoion-photoelectron spectroscopy stud-ies, with excitation into the N2O

1 ~X 2P! state at energiesjust above the O1 appearance energy, indicate somewhatlower fragment internal energy33 and, therefore, are consis-tent with the present evaluation.

DISCUSSION

Based on their relative behavior with respect to collisionenergy, we assign the strongly back-scattered component tothe pre-threshold reaction and the forward-backward compo-nent to the mechanism associated with the threshold. Theosculating complex model simulations and the comparisonsof high and low rf TOF experiments atET53.2 eV stronglysupport the suggestion that, as reaction~1! gains access tothe N2O

1(1 4A9) surface, the reaction proceeds primarily viaa collision complex. Both the broad energy transfer distribu-tions, with peaking nearer high values of translational to in-ternal energy transfer, and the values oft 51 and 4, derivedfrom the fits of the data atET53.2 eV and 6.3 eV, respec-tively, are consistent with this picture. Moreover, the in-crease oft with collision energy, implying decreased life-time, is typical for a reaction via an intermediate complex.

The ability to fit the data forET56.3 eV with a singlevalue oft 54, a value consistent with a short-lived, osculat-ing complex, indicates that the dynamics at this collisionenergy cannot be accounted for exclusively by the direct,forward scattering implied by the spectator stripping mecha-nism mentioned above. The spectator stripping model hasbeen discounted previously, based on the fact that the maxi-mum, at;10 eV, and decline at higher energies of the crosssection for reaction~1! is attributed to the loss of NO1 viadissociation; the spectator stripping model dictates a thresh-old for dissociation of 13.3 eV.10 The present results suggestthat the true threshold for dissociation may be obscured bythe usual decline of cross sections with collision energy inreactions proceeding via complexes.

Attributing the strongly back-scattered channel to thepre-threshold reaction calls into question the actual mecha-nism that gives rise to the near thermal species. Earlier flow-ing afterglow8 and flow-drift6 studies indicate the participa-tion of a weakly bound complex in the thermal to near-thermal energy regime, an interpretation based largely on the

decrease of the rate constant with temperature or collisionenergy. The TOF data presented here indicate that themechanism giving rise to these strongly back-scattered prod-ucts comprises a reversal of direction of the oxygen moiety,which indicates a mechanism that involves either low-impactparameter, rebounding collisions or a collision complexformed via large impact parameters. Either mechanismshould, however, yield a broaderv1p8 distribution than indi-cated here, encompassing the broad angular scattering andenergy release distribution expected in each case.

The unusual product velocity distribution associatedwith the pre-threshold channel for reaction~1! implies veryconstrained dynamics. The large kinetic energy release maybe explained, for either the rebounding or complex-mediatedmechanism, by an exit channel barrier, possibly due to aphase space bottleneck associated with the exit channel dou-blet to quartet surface hop. This picture is in keeping withthe fact that the exit channel to which the N2O

1 (1 4A9)surface correlates adiabatically is repulsive.17 Such a bottle-neck could also dictate that only very specific trajectoriessuccessfully negotiate both passages across the N2O1

X 2P(1 2A8,12A9)21 4A9(1 4P) crossing seam and samplethe chemical exchange region of the potential energy surface.In a complex-mediated mechanism, this spatial constraintcould manifest itself in the very particular range of impactparameters that give rise to the sharply backward angularbehavior observed. For a rebounding mechanism, the lack ofbroad angular scattering indicates a narrow range of smallimpact parameters, corresponding to those capable of over-coming this bottleneck. The behavior of the cross section atthermal energies, decreasing with increasing collision en-ergy, can also be explained on the basis of this limitation onreactive trajectories. For reactions occurring at fixed impactparameters, in either mechanism, the efficiency of the quartetto doublet transition while passing through the surface cross-ing region diminishes with increasing velocity, as has beenpointed out previously.10,15 Without the constraint on impactparameters, however, there would exist a preference for largeimpact parameter collisions, during which the radial velocityis very small and would enhance the reaction efficiency.

Previous theoretical efforts have focused on the mecha-nism for reaction~1! at higher collision energies, above theobserved threshold, and have mostly treated the pre-threshold reaction qualitatively,15–19 suggesting either along-lived complex mechanism15 or a three-bodymechanism,17 the latter of which has been disproved as theprincipal pre-threshold mechanism.10 Theoretical studiesaimed at explaining photoionization and fragmentation ofN2O

1 were not concerned with also explaining the behaviorobserved in the reaction studies.20,34 A better understandingof the thermal mechanism will require a renewed effort intheory, with the accurate ground and excited N2O

1 surfacesneeded to account for the effects on the dynamics of both thechemistry, and the surface hopping transitions, including thetype of motion involved in the coupling, radial, vibrational orrotational.

The present results, combined with those from otherstudies, permit some broad suggestions regarding productstate distributions for reaction~1!. The energy transfer distri-

3959J. Chem. Phys., Vol. 111, No. 9, 1 September 1999 Reaction of O1(4S)and N2(X1Sg

1)

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butions derived with the osculating complex model for theTOF experiments atET53.2 eV and 6.3 eV indicate a highdegree of product internal excitation. The energy transferdistributions are very broad and peak approximately 1 eVbelow the maximum available energy. This is reminiscent ofa process that is nearly statistical, in keeping with a complex-mediated reaction in which the complex survives longenough for extensive energy randomization to occur. It maybe expected that at lower collision energies, still abovethreshold, the complex lifetimes are sufficiently large to al-low a statistical energy distribution in the products.

Statistical considerations would suggest that in the inter-mediate energy range, represented here by the experiments atET53.2 eV and 6.3 eV, the dominant complex-mediatedmechanism should result in the product internal energy re-siding primarily on the diatomic product. Moreover, for re-actions proceeding on the N2O

1(1 4A9) surface, productionof atomic nitrogen in its lower excited states (2D,2P) in-volves a spin-forbidden transition. The importance of thislast point is supported by the observation of stripping-likebehavior at the higher collision energies,3,5,7 the modeling ofwhich assumes a ground state atomic product. At still largercollision energies, where dissociation of the diatomic prod-uct is energetically possible and results in the eventual de-cline in cross section,10,13 the NO1 that does survive shouldhave high internal excitation.

CONCLUSION

The absolute cross section of the reaction O1(4S)1N2(X 1Sg

1)˜NO11N has been remeasured in a guided-ion beam experiment, in the threshold region between near-thermal energy and 1.5 eV. This measurement agrees wellwith previous guided-ion beam measurements, in which par-ticular care was taken to avoid reactions with O1

metastables,10,13 and with the results of drift tube9 and hightemperature flowing afterglow12 studies. Time-of-flight spec-tra of the NO1 product were obtained at four collision ener-gies, 0.51 eV, 0.80 eV, 3.2 eV, and 6.3 eV. The TOF dataindicate two distinct mechanisms in the energy range stud-ied. At thermal to near-thermal energies, reaction proceedsby a mechanism that may feature highly constrained dynam-ics yielding products with large backward velocities~in thecenter of mass frame! and, consequently, with little internalenergy. At energies above the cross section threshold, analy-sis using the osculating complex model shows that reactionoccurs via a weakly bound collision complex, which gener-ates products with substantial internal excitation. These find-ings are of considerable importance in the context of model-ing of ionospheric processes involving NO1, in both thethermal and hyperthermal energy ranges.

ACKNOWLEDGMENTS

This work was solely funded by the Air Force Office ofScientific Research~AFOSR! under task 2303EP2 and incooperation with DDR&E Air Plasma Ramparts MURI pro-gram. During this study, Y. Chiu held an NRC postdoctoralfellowship.

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