VOLUME 79, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 NOVEMBER 1997

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Electron Capture and Loss Processes in the Interaction of Hydrogen, Oxygen, and FluorineAtoms and Negative Ions with a MgO(100) Surface

S. Ustaze, R. Verucchi,* S. Lacombe, L. Guillemot, and V. A. Esaulov†

Laboratoire des Collisions Atomiques et Moléculaires (Unité de Recherche associée au CNRS),Université de Paris-Sud, Orsay, France

(Received 13 March 1997)

A study of electron capture and loss processes during the scattering of H, O, and F atoms and anionson a MgO(100) surface is described. Large fractions of anions in the scattering of atoms are observed,indicating the existence of an efficient electron capture process. This is ascribed to a nonresonant,localized charge exchange mechanism between an atom and a MgO lattice oxygen anion. This chargetransfer becomes possible because of anion level shifts in the Madelung field. The existence of anelectron loss channel is demonstrated using incident anions and is ascribed to loss to the conductionband or Mg cations. [S0031-9007(97)04471-2]

PACS numbers: 79.20.Rf

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Oxide surfaces are of considerable technological aheuristic interest, and over the past years an increasamount of surface science work has been devoted to thstudy [1–4]. The interaction of gases with oxides haattracted much attention with the aim of understandinfundamental problems in catalysis, gas sensors, adhesetc. A fairly large number of experimental and theoreticstudies of chemisorption of various molecules have beperformed on some model oxide surfaces such as thoof magnesium, aluminum, zinc, etc. [3,4]. Chemisorptioand reactions on surfaces frequently involve electron tranfer processes [5], as has, for instance, been discussed wthe example of H1 and OH2 adsorption on a series of ox-ides [6]. Information about such fundamental interactioprocesses are difficult to obtain formin situ studies of ad-sorbates. However, as has been amply demonstrated incase of metal surfaces [7–9], information about these pcesses and important characteristics of atomic or moleclar levels in front of surfaces (positions and widths) can bobtained from ion beam scattering studies. Results of eletron transfer measurements in these studies have allowrigorous tests of theory. For the very important caseoxide surfaces, such studies are essentially nonexistenour knowledge. A notable exception is some ion backsctering studies [10,11], which gave a clear qualitative indcation that electron capture could occur on oxides, thouelectron transfer probabilities were not determined.

Besides the general fundamental motivation concernithe understanding of electron transfer on oxides, studof these processes on oxide surfaces are also importfrom a practical point of view, i.e., for a better analysiof quantitative structure data obtained by low energy ioscattering (LEIS), which is a powerful, highly sensitivetool for investigating surface structure [12]. Furthermorethe thorough understanding of the results of studieselectron capture on metal surfaces with chemisorbelectronegative species, which are well known poisonscatalytic reactions [13], also requires knowledge of thcorresponding dielectrics. This is clear from our rece

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study of the changes in electron capture rates durinoxygen adsorption, which involved negative ion formationin the scattering of H, O, and F [14–16]. Rather curiousvariations in the anion yield as a function of coveragewere observed. A detailed understanding of the observefeatures was only possible if a study of electron capturon oxide monocrystals was performed.

In this paper we present the first results of a quantitativstudy of electron capture and loss processes in the scatting of H, O, and F atoms and negative ions on MgO(100)which is one of the most extensively studied oxide surfaces, with a rather simple structure [1–4]. We report thmeasurements of the energy dependence of the yieldH2, O2, and F2 ions. The situation we consider is de-picted in Fig. 1, where the disposition of the anion levelsin front of MgO [17] is depicted. Note that the anion levelsare generally located in the gap region. For a high wor

FIG. 1. Schematic diagram of the MgO bulk band structure[17] and the H2, O2, and F2 affinity levels relevant for theelectron capture process is shown. The conduction bandindicated by the hatched area at the top of the figure.

© 1997 The American Physical Society

VOLUME 79, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 NOVEMBER 1997

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function (4–5 eV) metal target, the anion levels are usally located in front of the conduction band, but becausof image charge effects are downward shifted below thFermi level, making anion formation possible by resonaelectron capture. In the present case such a capture proappears highly unlikely on the basis of Fig. 1 because image potential shifts will be small for the insulator.

A striking feature of our results is the observation oflarge scattered negative ion charge fraction,using neutralprojectiles, indicating that electron capture occurs withhigh probability. We demonstrate the existence of anelec-tron loss processby using incident negative ions as projectiles and show that the scattered ion fraction candifferent from that of incident neutrals. Contrary to scatering on metals, the memory of the incident charge stais thus not always lost. Electron capture is discussedterms of nonresonant transfer at the oxygen anion site aelectron loss to backtransfer to Mg cations. The appliction of these results used to interpret the changes in eltron capture rates during Mg and Al oxidation is describeelsewhere [14–16].

The experiments were performed on a setup describeddetail elsewhere [18]. Briefly, the hydrogen, oxygen, anfluorine negative ions are produced in a discharge sourmass selected, and steered into the main UHV chambThe pressure in the chamber is of typically2 3 10210 torr.Neutral beams were prepared by passing negative iothrough a cell containing He, Ar, or O2 located before themain chamber. The MgO(100) crystals10 mm 3 10 mmdwas introduced from air into the chamber.In situprepara-tion consisted of repeated cycles of Ar1 grazing incidencesputtering, annealing, and cooling in oxygen. Note thit has been previously pointed out that Ar sputtering oMgO is stoichiometric [1]. Grazing incidence sputterinwas used to reduce surface roughness [8]. Surface cleaness was ascertained by performing a time-of-flight (TOanalysis of scattered and directly recoiled particles for Ar1

incident ion scattering. An important advantage of thmethod is that it is extremely sensitive to surface hydrogeInitially, the TOF spectra displayed a very strong H peawhich dominated the spectrum. The cleanest surface tcould be obtained had a H peak which represented ab1% in the integrated TOF spectrum.

In this paper we report H2, O2, and F2 ion fractionmeasurements made for a fixed scattering angle of 7± forspecular scattering conditions. A position sensitive 30 mdiameter channel plate detector, equipped with three dcrete anodes, set at the end of a time-of-flight analysis tuwas used. A deflector plate assembly located beforechannel plate allows one to separate the incident positinegative, and neutral particles, which are then detectedmultaneously by each of the anodes. Measurements wperformed for ion energies of 0.5–4 keV. The negativsF2d in fractions are defined as the ratio of the scattereflux fN2scdg in a given charge state to the total scattereflux fNtotalscdg into a given anglec with respect to thesurface plane, i.e.,F2 ­ N2scdyN totalscd.

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The atom/ion fluxes were maintained at a much lowevalue than the usual fluxes used in previous studies of metargets to avoid charging. Typical currents as measureon the sample side of a metal targets1 mm 3 5 mmdwere of the order of10211 A. A typical count rate forincoming F2 at an, e.g., 1 keV energy, was of the order oa few hundred counts per second. Under these conditioncharging for incident negative ions only occurs for quiteprolonged measurements. Its absence was controlledsuccessive measurements.

Results ofF2 measurements using incident neutral andnegative ions are shown in Fig. 2. For incidentneutralparticles the ion fractions increase rapidly from a thresholin the energy range of a few hundred eV to a rather largvalue for energies of a few keV. The firststriking resultwe wish to underline is thevery large ion fractionthatis obtained on the oxide surface using incident atomsindicating thatelectron capture can occur most efficiently.As a comparison in a similar scattering geometry for H2

and O2 scattering on a clean Mg surface, ion fractions oonly 1% [8] and 10% [19], respectively, were found for4 keV in our previous study, while here the correspondinfractions attain 7% and 35%. We do not believe that thilarge fraction could be due to eventual low contaminatio

FIG. 2. Negative ion fractions obtained in the scattering oH, O, and F atoms, and negative ions as a function of collisioenergy.

VOLUME 79, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 NOVEMBER 1997

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by H. Indeed, a strongly polluted surface yielded a lowefraction.

Equally important is the observation, using an incidenegative ionbeam, of the existence of anefficient electronloss process.Rather different behaviors are actually observed for the different systems. For H2 the ion fractionsfor incident H and H2 are similar. For F2, reflection ofthe negative ions is complete for low energies. As the eergy increases, the ion fraction decreases, and, for energof the order of 1.5 keV, the ion fractions for incident ionand neutrals become identical within the limits of statistcal error. At higher energies,F2 starts to decrease. ForO2 an intermediate situation is encountered. At low energies the ion fraction for incident O2 is somewhat largerthan for incident O atoms. Above 1.5 eV the ion fractionare similar.

Recent experiments on large angle H1 and He1 ionscattering on alkali halides and oxides have presentevidence for a local nature of charge transfer on thesurfaces [10,11], which is reasonable given the strolocalization of the charges in an ionic crystal. Receexperiments on grazing atom scattering on a LiF surfaare of interest here [20]. They also show efficient negtive ion production. It has been suggested [20] that thelectron capture process could be described by analowith the description of nonresonant charge transfer in ioatom collisions in the gas phase and consider capture on2

sites. Such atomiclike descriptions have been considepreviously in some cases of localized states (narrow banin solids [21,22]. In the gas phase a frequently usemodel is the one proposed by Demkov [23]. In thimodel, which corresponds to a molecular descriptionthe ion-atom system, the exchange termsH12d between theinitial and final states (corresponding to A1B and AB1)is given by an exponential form H12 , e2lr . Herel canbe approximately taken asl ­

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andEB are the ionization potentials of the two atoms. Thcharge transfer probabilitysPd is given by [23,24]

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whereRc is a critical distance where charge transfer occurs andysbd is the radial velocity for a given impact pa-rameter. The charge transfer probability depends on tenergy defect (DE) and the radial velocity. Using thismodel, a rough estimate of the dependence of negativeformation on ion velocity was made [20], noting an important point, viz. that very close to the F2 center in thecrystal the energy level of the anion will be downwarshifted due to the Madelung potential. This can strongreduce the otherwise large energy defect between thesition of the anion level and the energy level of the Fs2pdelectrons in LiF at infinite separation. A second assumtion is that following electron transfer the correspondinhole remains localized at the neutralized F± site. A moresophisticated approach was subsequently developed, baon a Hartree-Fock description [25]. Note that in these a

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proaches no electron loss processes were consideredonly the threshold dependence of the electron capture chnel was thus investigated.

We suggest that a similar approach can be used toscribe electron capture in the case of anion formationMgO(100) and consider capture at the oxygen anion sNote that one usually considers that the cation and aion in MgO are doubly charged, i.e., we should be deaing with Mg11 and O22 in the crystal. However, insome model chemisorption calculations, somewhat diffeent charges have been cited (charge close to unity; se.g., a discussion in Refs. [6,26,27]). We shall not enupon a discussion of this question here, but it is clear ththis model case of capture on an oxide surface, which ismuch general interest, will be an interesting challenge ftheory.

The incident energy dependence of the threshold regof the electron capture curve will be determined by bothe ion velocity and the distance of the approach of tatom (H, O, or F) to the anion in the crystal, which wildetermine the energy defectDE. Note that the former ina binary collision model of the gas phase type is the radvelocity in the atom-atom collision, while the distance oapproach to the atom of a surface will also be determinby the perpendicular component of the velocity. Forfinite energy defect, in the threshold region the aboformula predicts a rapid increase (close to exponential)transfer probability with increasing velocity.

At high energies the F2 fraction starts to decrease. Inthe gas phase nonresonant charge transfer, cross secusually also display a maximum as a function of collisioenergy, and a decrease of anion formation on the oxmay be expected at high energies.

As shown above, our experiments demonstrate the extence of an electron loss channel, which has not been preously considered. This channel is in fact quite importanIt has been suggested [20] that coherent resonance iontion could be responsible for electron loss. While this maplay some role, we propose that electron loss is due totransfer of electrons to the cation Mg site. One could viethis process as a neutralization process between the incing ion and Mg11. In calculations of OH2 chemisorptionon MgO (see, e.g., Ref. [6]), one considers adsorptionthe Mg cation site involving electron transfer. Our proposal seems quite reasonable in this sense. This is inequivalent to loss to theconduction bandof MgO, whichcorresponds to Mgs3sd electrons. For H2 this may appeartrivial (see Fig. 1) but is not so for other cases. In ordfor loss to the conduction band to occur, the F2 level hasto be “upward” shifted. This should indeed occur for thcase of an occupied anion level in the field of lattice anionfor a sufficiently close approach. As for capture, this cloapproach implies a sufficiently high perpendicular velociand can explain the clearly visible energetic threshold floss for F2. If one discusses losses as due to transitioto the conduction band, the energy dependence shoulddetermined by the time during which this loss is possib

VOLUME 79, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 NOVEMBER 1997

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and should be favored for smaller velocities. If one considers a binary gas phaselike model for mutual neutralizatiowith Mg11, then the energy dependence could be morcomplex as, for instance, in H1 H2 collisions [28].

Without exact calculations on these systems, it is difficult to predict the atom-surface distance dependence of tmovement of the corresponding energy levels and makdefinite statements about the threshold energy dependenof capture and loss channels. However, some qualitativstatements can be made. Since the O and F velocities anot significantly different, on the basis of the dispositionof the energy levels in Fig. 1, electron capture should onset for lower energies for fluorine than for oxygen, becausthe initial energy defect is smaller. In the case of a smalleDE, P will be higher. Therefore a higher fraction may beexpected for F2. Furthermore, for oxygen, because of itslower electron affinity, an earlier onset and more efficienloss process may be expected, giving another reason folower fraction. The relatively low threshold for H2 is notin contradiction with these statements. Indeed, althougthe energy shift required for capture for H2 is greater thanfor the other ions, the velocity is also higher in this energyrange. The similarity between the results for incident Hand H2 can be ascribed to its low binding energy, whichmakes efficient loss to the conduction band possible.

In conclusion, we have presented a study of electrocapture and loss processes in the interaction of H, O, anfluorine atoms and negative ions with a MgO(100) surfaceOur experiments show that electron capture occurs veefficiently on this insulating oxide surface. It is suggestedthat, as for the case of negative ion formation in thescattering on LiF, electron capture could be treated asnonresonant charge transfer process at the oxygen ansite. The downward shift of the affinity level of the atomdue to the Madelung potential reduces the energy defecmaking nonresonant charge transfer possible. Use of boatoms and negative ions as projectiles has allowed uto demonstrate the existence of an electron loss channWe suggest that this is due to electron transfer to thconduction band or the Mg cation. Both capture and loschannels must be properly taken into account to arrivean accurate description of negative ion formation.

We hope that these results on a prototype oxide surfacof much practical interest and which has served as a bsis for many model studies, will stimulate a detailed theoretical study of these phenomena on oxides. While thsimple description we proposed for the capture process acounts qualitatively for the general features of the experments, a full treatment would involve a cluster calculationtaking exactly into account the electron exchange in thsolid. These effects, more consistent with a band structu(Fig. 1), would be particularly important for oxides with agreater degree of covalency, which will be the objects ofuture study.

The authors are indebted to M. Bernheim for providingthe MgO crystal. Interesting discussions with A. Borisov

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C. Henry, J. Jupille, C. Noguera, and V. Sidis are gratefully acknowledged.

*Present address: Dipartimento di Fisica, Universita dModena, Modena, Italy.

†Corresponding author.Electronic address: esaulov@veofl.lcam.u-psud.fr

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