Low-energy electron-induced processes in condensed CF2Cl2 films

JOURNAL OF CHEMICAL PHYSICS VOLUME 114, NUMBER 4 22 JANUARY 2001

Low-energy electron-induced processes in condensed CF 2Cl2 filmsM. N. Hedhilia)

Department of Physics and Astronomy, and Laboratory for Surface Modification,Rutgers,The State University of New Jersey, Piscataway, New Jersey 08854

M. Lachgar, Y. Le Coat, and R. AzriaLaboratoire des Collisions Atomiques et Mole´culaires, Universite´ Paris-Sud,Batiment 351, 91405 Orsay, France

M. TroncLaboratoire de Chimie Physique-Matie`re et Rayonnement, Universite´ Pierre et Marie Curie,75231 Paris Cedex 05, France

Q. B. Lub) and T. E. Madeyc)

Department of Physics and Astronomy, and Laboratory for Surface Modification,Rutgers,The State University of New Jersey, Piscataway, New Jersey 08854

~Received 18 July 2000; accepted 27 October 2000!

We report measurements of electron stimulated desorption yields and kinetic energy distributionsfor F2 and Cl2 anions from thin films of CF2Cl2 condensed on polycrystalline platinum at 20 K.Dissociative electron attachment processes cause a peak in the F2 yield at an electron energy of;4eV and a peak in the Cl2 yield at ;7 eV. After electron irradiation, a new dominant structureappears in the Cl2 yield at 5 eV incident electron energy, and Cl2 ions having high kinetic energy~.1 eV! are observed in the kinetic energy distributions. These changes are attributed to thesynthesis of Cl2 in the condensed phase induced by electron irradiation of the CF2Cl2 film. This Cl2formation is demonstrated by the observation in the Cl2 yield of the Cl2

2/Cl22Pu resonance state,

and by Cl2 kinetic energy distributions that are similar to those obtained from condensed pure Cl2

films. © 2001 American Institute of Physics.@DOI: 10.1063/1.1334347#

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I. INTRODUCTION

The interaction of low-energy electrons with molecuadsorbates on surfaces can produce highly reactive sp~radicals, cations, and anions! via reactions such as dissocitive electron attachment~DEA!, ion-pair formation, or mo-lecular~pre!dissociation. These species can react with neiboring molecules to form new products. Examples of iomolecule and atom–molecule reactions mediated by elecimpact have been observed in the condensed phase1,2 formolecules such as O2. In this paper we present a caseelectron-induced adsorbate modification via dissociationmolecules, or from the recombination of fragments resultfrom dissociation. In particular, we report evidence for chrine (Cl2) synthesis induced by low-energy electron irradtion of a condensed film of dichlorodifluoromethan(CF2Cl2).

The study of chlorofluorocarbons~CFCs! has receivedmuch attention due to their involvement in the destructionthe stratospheric ozone layer via very efficient catalyreactions.3 In addition to their importance in stratospherchemistry there is also evidence that CFCs are efficgreenhouse gases which may cause a change in the Ea

a!Present address: De´partement de Physique, Faculte´ des Sciences de Tunis1060 Tunis, Tunisie.

b!Present address: Group of the Medical Research Council of Canada iRadiation Sciences, Faculty of Medicine, University of Sherbrooke, Sbrooke, Canada J1H 5N4.

c!Author to whom all correspondence should be addressed; [email protected]

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climate.4 The present study is devoted to one of the classCFCs, i.e., the CF2Cl2 molecule~freon 12!.

We have studied Cl2 and F2 electron stimulated desorption ~ESD! from condensed CF2Cl2 molecules on polycrys-talline platinum; the anions are formed via dissociative eltron attachment~DEA! processes. Total ion yields, partiaion yields at fixed kinetic energies, and kinetic energy disbutions of Cl2 and F2 were obtained for incident electroenergies from 0 to 13 eV. DEA processes cause a peak inF2 yield at an electron energy of;4 eV and a peak in theCl2 yield at ;7 eV. The kinetic energy distributions shothat both Cl2 and F2 anions are ejected from condensCF2Cl2 into vacuum with low kinetic energy~,1 eV!. How-ever, the yields and kinetic energy distributions of C2

change with electron beam exposure, and a new dominstructure around;5 eV incident electron energy appearsthe Cl2 yield; these ions are released with high kinetic eergy ~.1 eV!. We explain these observations as arising frothe formation of Cl2 molecules induced by electron irradiation of CF2Cl2 and the subsequent ESD of Cl2 from con-densed Cl2 .

II. EXPERIMENT

The apparatus5 used in this experiment consists of aelectron monochromatized source, a polycrystalline platin~Pt! substrate fixed on the cold end of a helium cryostatquadrupole mass spectrometer~QMS I! in line with an en-ergy selective filter~referred to as the energy–mass analyz!to detect electrons or desorbing anions, and an indepenfixed quadrupole mass spectrometer~QMS II!. These differ-

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1845J. Chem. Phys., Vol. 114, No. 4, 22 January 2001 Electron-induced processes in CF2Cl2 films

ent elements are housed in an ultrahigh vacuum chamreaching a pressure below 5310211Torr, equipped with atwo layerm-metal foil to reduce the magnetic field belowmG. The electron source is a hemispherical monochromwith mean radius of 2.5 cm and pass energy of 4 eVcomputer controlled electrostatic zoom lens focuses the 1nA electron beam on the sample. The desorbed anions cadetected in two ways:~a! either energy selected with a hemspherical analyzer similar to the electron monochromatorseparated from scattered electrons before being identwith QMS I in line with the energy analyzer, or~b! directlymass analyzed by means of QMS II. The energies ofincident electron beam (Ei) and desorbed ions~KE! werecalibrated with respect to the vacuum level within60.2 eVby measurement of electron energy loss spectra fromplatinum substrate covered by a condensed molecCF2Cl2 film. These spectra present a sharp cutoff for zeenergy electrons; this provides a correction for the energthe scattered electrons in vacuum, which we assume to bsame for the desorbing anions. The energy differencetween the observed cutoff and the peak of elastically stered electrons gives the incident electron energyEi . Theoverall energy resolution of our system was about 200 m

The electron gun and energy–mass analyzermounted on rotatable stages having the cryostat on axis.quadrupole mass spectrometer~QMS II! is fixed normal tothe substrate. All the spectra presented were obtainedthe energy–mass analyzer normal to the substrate andelectron gun oriented at 60° from the normal.

The polycrystalline Pt can be heated by electron bobardment and cooled to 15 K by continuous flow of liquhelium. No ion signal was observed after evaporation ocondensed molecular film and cleaning the Pt substrat1300 K. The multilayers were prepared from an effusbeam doser directed to the sample and the film thicknessestimated by volumetric dosing~the thickness is estimateby allowing a given volume of the vapor to expand fromgiven pressure to the vacuum and estimating the numbemolecules which condensed on the metal, assuming a sing coefficient of unity! with an accuracy of 50%. All of themeasurements reported here are for CF2Cl2 films <10 mono-layers~ML ! in thickness.

The energy–mass analyzer was operated in two monamely, the ion energy mode in which a mass selected ntive ion signal is collected as a function of KE at fixed incdent electron energy, and the yield mode in which a partilar ion with a fixed KE is monitored as a function of electroenergy. The quadrupole mass spectrometer~QMS II! is usedto measure total ion yields as a function of incident electenergy.

III. RESULTS AND DISCUSSION

A. Previous gas and condensed phase results

As for all other chlorofluorocarbon compounds, electrattachment to gaseous CF2Cl2 leads, at low electron energieto the production of Cl2 as the most abundant fragmeanion.6,7 This can be easily understood knowing that theaction leading to the production of Cl2 is exothermic~by


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about 0.4 eV!, whereas the channel leading to the productof F2 is endothermic~by about 1.3 eV! due to the strongC–F bond~the electron affinities of Cl and F are 3.61 an3.45 eV, respectively!.8 The cross section for the productioof Cl2 by electron attachment to CF2Cl2 at zero electronenergy at 300 K is about 4.5310216cm2.9 The anion forma-tion via dissociative electron attachment to gaseous CF2Cl2molecules as a function of electron energy in the gas phhas been studied by Illenbergeret al.10–12The intensity ratioof Cl2 to F2 is approximately 8:1.10 The Cl2 yield spectrumshows resonances at 0.8 eV and near 3–4 eV. No signifiion signal was observed above 5 eV. However, Chen13 andrecently Kiendler9 using higher-energy resolution observedCl2 peak at zero energy interpreted as ans-wave capture bythe CF2Cl2 ; they also observe a small shoulder at 0.7 eIllenbergeret al.12 have measured the translational energythe Cl2 ion fragment in the incident energy range from 05 eV, with a time-of-fight technique together with a quadrpole mass filter. They attribute the low energy resonanc0.7 eV to the following process:

e21CF2Cl2→Cl21CF2Cl.

Since for the second resonance at 3–4 eV the time-of-flspectra do not show considerable translational energy,conclude that at these electron energies multiple fragmetion processes take place. The processes

e21CF2Cl2→CF21Cl21Cl, DH052.1 eV,

e21CF2Cl2→CFCl1Cl21F, DH053.5 eV

fit very well to the onset of the second resonance and wthe weak structure observed at 3.5 eV.

Concerning the F2 yield spectrum, only a resonancaround 3 eV was observed in the gas phase,10,11attributed tothe following process:

e21CF2Cl2→F21CFCl2 .

In the condensed phase, only two studies of electstimulated desorption from multilayers of CF2Cl2 have beenreported.11,14 In the Illenberger study,11 the Cl2 yield showsthree peaks around 1, 2.5, and 7 eV. The broad peak at 7~not observed in the gas phase! has stronger intensity thathe low-energy resonances and was attributed to a negaion resonance associated with the first electronically excstate. However, recent results obtained by Meinke15 indicatethat the Cl2 signal below 4 eV is very weak. Concerning thF2 yield, a single negative ion resonance around 3 eVbeen reported in the Illenberger study,11 as in the gas phaseThe more recent Meinke measurements15 show that the F2

yield around 3 eV is one order of magnitude less intense tat 15 eV~where dipolar dissociation is the dominant mechnism for F2 formation!.

B. FÀ and Cl À ESD at low electron dose „maximum ofbeam dose: 1 Ã1014 eÀÕcm2

…

The F2 yields obtained for different CF2Cl2 film thick-nesses~Fig. 1! show the presence of only one resonance,in the gas phase. The F2 yield obtained for 6 ML of CF2Cl2is similar to that obtained by Meinke.15 The low intensity of


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1846 J. Chem. Phys., Vol. 114, No. 4, 22 January 2001 Hedhili et al.

the F2 signal from dissociative electron attachment at 0and 1 ML at 3 eV incident electron energy is due to tproximity of CF2Cl2 to the substrate. A ‘‘quenching’’ of theresonance may occur, which reduces the lifetime ofCF2Cl2

2* intermediate state; moreover, F2 ions which havelow kinetic energy cannot escape into the vacuum due toattractive effect of the net image potential induced insubstrate. The resonance observed in the gas phase atelectron energy in the F2 yield was connected with the resonant vibrational excitation of CF2Cl2 , and was assigned tthe formation of a transient negative ion havingA1 symme-try with s* ~C–F! character.16 The same transient negativion appears to be responsible for the F2 signal around 3 eVin the condensed phase. The F2 yields obtained for differentCF2Cl2 film thicknesses also show an increasing F2 signalwith an onset around 9 eV. We attribute this F2 signal to thedipolar dissociation process which takes place in this raof energy.

In order to probe the dynamics of desorption of aniofrom CF2Cl2 , we have measured the kinetic energy distribtions for F2 anions~Fig. 2!, from 6 ML of CF2Cl2 condensedon polycrystalline platinum, at different fixed incident eletron energies. The most probable kinetic energy for F2 an-ions is at low energy,;0.5 eV, and is nearly independentthe incident electron energy.

The Cl2 yield obtained in the present study from 6 Mof CF2Cl2 condensed on polycrystalline platinum at 20 K,shown in Fig. 3. The intensity ratio of F2 to Cl2 is approxi-mately 2:1. The Cl2 spectrum was obtained by summing tdata corresponding to several newly deposited films andeach film the data are accumulated only in one scan. Treduces the exposure time of the condensed CF2Cl2 to theelectron beam~maximum of 131014e2/cm2); as discussed

FIG. 1. F2 yields obtained for different CF2Cl2 film thicknesses at 0.1, 1and 6 monolayers~ML ! condensed on polycrystalline platinum at 20 K,function of incident electron energy.


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later in this paper the Cl2 yield is extremely sensitive toelectron beam damage of the CF2Cl2 film. The results aresimilar to those obtained by Meinke15 and show a broadstructure around 7 eV with very small signal at lower incdent electron energy. The resonance in the Cl2 yield ob-served at 7 eV in the condensed phase, but not in the

FIG. 2. Kinetic energy distributions for F2 anions obtained from 6 ML ofCF2Cl2 condensed on polycrystalline platinum at 20 K, at different fixincident electron energies.

FIG. 3. Cl2 yield obtained from 6 ML of CF2Cl2 condensed on polycrys-talline platinum at 20 K with low electron dose, as function of incideelectron energy~maximum of beam dose: 131014 e2/cm2).


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phase~although the vibrational excitation levels of CF2Cl2have been observed to occur with high probability for indent electron energies ranging from 6 to 10 eV in thephase16!, can be attributed to the dissociation of a transinegative ion having a longer lifetime in the condensed thin the gas phase. Indeed, it is known that the energy ofresonance state in the condensed phase can be lowered dthe effect of the image potential in the substrate, whichsults in a reduction in the number of relaxation channelsthe resonance state and consequently a longer lifetime foresonance. We have also measured the kinetic energy dbutions for Cl2 anions~Fig. 4!, from 6 ML of CF2Cl2 con-densed on polycrystalline platinum, at different fixed indent electron energies. The most probable kinetic energyCl2 anions is at low energy,;0.5 eV, and is nearly independent of the incident electron energy~as in the case for F2

anions; Fig. 2!. This is probably because, during the dissciative electron attachment process, the excess energtransformed into rotational and vibrational excitation of tassociated neutral fragment. The same phenomenon waserved for O2 anion desorption from condensed N2O andCO2, where the O2 ions formed with low kinetic energy arassociated with N2 and CO fragments in rovibrationally excited states as a consequence of the potential energy suof the anion resonance state and details of its topology.17,18

C. ClÀ ESD at high electron dose „beam dose higherthan 2 Ã1014 eÀÕcm2

…

We find that the behavior of the Cl2 yield changes withtime during electron irradiation of the CF2Cl2 film. Figure 5shows Cl2 yields from 6 ML of CF2Cl2 obtained after dif-

FIG. 4. Kinetic energy distributions for Cl2 anions obtained from 6 ML ofCF2Cl2 condensed on polycrystalline platinum at 20 K with low electrdose, at different fixed incident electron energies~maximum of beam dose:131014 e2/cm2).


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ferent times of electron irradiation. The electron currentthe platinum sample is about 5 nA. We observe that a nstructure rises during electron irradiation and dominatesCl2 yield around 5 eV incident electron energy. We hameasured the kinetic energy distributions for Cl2 ions ~Fig.6! from 6 ML of CF2Cl2 condensed on polycrystalline plat

FIG. 5. Cl2 yields from 6 ML of CF2Cl2 condensed on polycrystallineplatinum at 20 K, as function of incident electron energy, obtained adifferent times of electron irradiation.

FIG. 6. Kinetic energy distributions for Cl2 ions obtained from 6 ML ofCF2Cl2 condensed on a polycrystalline platinum at 20 K, after electirradiation~beam dose: 231015 e2/cm2), at different fixed incident electronenergies.


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num after electron irradiation for 10 min with 5 nA (;231015e2/cm2), at different fixed incident electron energieWe observe that the kinetic energy distributions of Cl2 ionsare different from those obtained with low electron beadose~Fig. 4! and that the Cl2 ions desorb with higher kineticenergy~.1 eV!. For the F2 ions, no significant change iobserved in yields and kinetic energy distributions after eltron irradiation.

The appearance of a new structure in the Cl2 yield andthe modification observed in the Cl2 kinetic energy distribu-tions after electron irradiation can be related to a modifition of the chemical composition of the condensed film. TCl2 yield obtained after electron irradiation of solid CF2Cl2is reminiscent of the Cl2 yield obtained from a puremultilayer Cl2 film. Figure 7 shows the Cl2 yield obtainedfrom 5 ML of pure Cl2 molecules condensed on polycrystaline platinum at 40 K. The Cl2 yield shows a dominanstructure around 5 eV which is assigned~as in the gas phase!to the 2Pu core-excited resonance state.19 The Cl2 kineticenergy distributions from 5 ML of pure Cl2 condensed ontopolycrystalline platinum at 40 K are shown in Fig. 8 fodifferent fixed incident electron energies. The Cl2 kineticenergy distributions extend from 1 to 4 eV and are similarthe Cl2 kinetic energy distributions obtained after electrirradiation of condensed CF2Cl2 . Based on the similarity ofthe kinetic energy distributions and the yield of Cl2 from Cl2~Figs. 7 and 8! and the kinetic energy distributions and thyield of Cl2 from CF2Cl2 following electron irradiation~Figs. 5 and 6! we conclude that electron irradiation of codensed CF2Cl2 at ;20 K induces the formation of Cl2 mol-ecules.

Figure 9 shows Cl2 kinetic energy distributions obtaineat 5.5 eV incident electron energy after electron irradiat~beam dose: 231015e2/cm2) of a CF2Cl2 film, at two tem-

FIG. 7. Cl2 yield obtained from 5 ML of pure Cl2 condensed on polycrystalline platinum at 40 K, as function of incident electron energy.


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peratures~20 and 100 K!. The Cl2 kinetic energy distribu-tion obtained at 100 K after electron irradiation is peakedlow energy and is similar to the kinetic energy distributioobtained before electron irradiation~Fig. 4!. At 20 K, theCl2 kinetic energy distribution obtained after electron irrdiation shows that the Cl2 ions are formed with high kinetic

FIG. 8. Cl2 kinetic energy distributions from 5 ML of pure Cl2 condensedonto polycrystalline platinum at 40 K, at different fixed incident electrenergies.

FIG. 9. Cl2 kinetic energy distributions obtained at 6 eV incident electrenergy after electron irradiation~beam dose: 231015 e2/cm2) of a CF2Cl2film, at two temperatures: 20 and 100 K.


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energy, which we relate to the contribution of Cl2 molecules.At 100 K, Cl2 ions originate exclusively from condenseCF2Cl2 , rather than from the formed Cl2 , which desorbs~thetemperature of condensation of Cl2 is around 60 K!. Figure 9also shows a significant difference in the relative intensityCl2 ions obtained at 100 and 20 K. This difference in trate of desorption of Cl2 anions can be related to differeneffects, including the fact that the cross section for formatof Cl2 anions from Cl2 molecules (1310212cm2 at Ei

50 eV in the gas phase20! is much higher than the crossection for formation of Cl2 ions from CF2Cl2 molecules(4.5310216cm2 at Ei50 eV in the gas phase9!. Moreover,the probability that Cl2 ions issuing from Cl2 molecules canescape the film is higher compared to Cl2 ions formed fromCF2Cl2 molecules because of their higher kinetic energy.

In order to extract the contribution of Cl2 ions issuingfrom CF2Cl2 molecules from the total Cl2 yield at 20 K afterelectron irradiation, we have measured the Cl2 ion yieldwith fixed 0.5 eV kinetic energy~Fig. 10!. The 0.5 eV Cl2

ion yield shows a behavior similar to the total Cl2 yieldobtained before electron irradiation~Fig. 3!. A small Cl2

yield at low ~1–6 eV! incident energy is observed, but thdominant feature is a large structure extending from 6 toeV incident energy with a maximum around 7.5 eV. Thresult indicates that the appearance of a new low-enestructure in the Cl2 yield with high kinetic energy after electron irradiation~Figs. 5 and 6! is not due to a charging effecMoreover, notice that a charging effect would modify the F2

yield and F2 kinetic energy distribution, which is not observed.

Different processes can be involved in the formationCl2 species. Cl2 molecules can be formed via the recombnation of Cl atoms: Cl1Cl→Cl2 or the recombination of Cl1

FIG. 10. Yield of Cl2 ions having 0.5 kinetic energy obtained from 6 ML oCF2Cl2 condensed on polycrystalline platinum at 20 K, after electron irdiation ~bean dose231015 e2/cm2).


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and Cl2 ions: Cl11Cl2→Cl2 . The neutral and ionic chlo-rine atoms can be formed via dissociative electron attament to CF2Cl2 , and via dipolar dissociation. Cl2 moleculescan also be formed directly from dissociation of CF2Cl2 mol-ecules:

e21CF2Cl2→Cl21CF2.

The threshold for Cl2 formation by the recombination oCl atoms is related to the threshold for formation of Cl netral fragments resulting from the interaction of electrons wCF2Cl2 molecules. This threshold is equal to 2.1 eV acorresponds to the following reaction:

e21CF2Cl2→CF21Cl21Cl, DH°52.1 eV,

DH° 5 DH f(CF2) 1 2DH f(Cl)2A f(Cl) 2 DH f(CF2Cl2)whereDH f is the enthalpy of formation,Af is electron affin-ity of Cl, and DH° enthalpy of the reactionDH f (CF2!521.89 eV, DH f (CF2Cl2!525.11 eV,21

DH f(Cl!51.26 eV, andAf~Cl!53.61 eV.22

Since the Cl2 ions can be formed via a dissociative eletron attachment process at 0 eV, the threshold for formaof Cl2 molecules by recombination of Cl1 and Cl2 ionscorresponds to the threshold for Cl1 formation. This thresh-old is 14.59 eV and corresponds to the reaction:

e21CF2Cl2→CF2Cl21Cl11e2, DH°514.59 eV,

DH° 5 DH f(CF2Cl) 2 A f(CF2Cl) 1 I (Cl)1DH f(Cl)2DH f(CF2Cl2), whereDH f is the enthalpy of formation,Afis electron affinity of CF2Cl, I is the ionizationpotential of Cl, and DH° enthalpy of the reactionwith DH f(CF2Cl2!525.11 eV,21 DH f(CF2Cl!522.79 eV,23

A f(CF2Cl!52.00 eV,12 I (Cl!513.01 eV, and DH f(Cl!51.26 eV.22

The direct formation of Cl2 molecules via the dissociation of CF2Cl2 molecules is also possible. In fact, irradiatioof CF2Cl2 with an ArF ~6.4 eV! laser beam shows that Cl2

molecules can be formed directly by dissociationCF2Cl2 ;24

hn1CF2Cl2→Cl21CF2, DH°53.2 eV.

The same reaction may possibly be stimulated by elecimpact and be responsible for the formation of Cl2 mol-ecules.

We have measured the efficiency of the formation of C2

molecules as a function of the energy of electron irradiatof a CF2Cl2 film ~Fig. 11!. This efficiency of formation ofCl2 is defined as the integral of the Cl2 kinetic energy dis-tribution obtained at an incident electron energy of 6 eV afelectron irradiation. We observe that the efficiency showmaximum between 10 and 20 eV, which can be related tocontribution of specific states leading to the dissociationCF2Cl2 molecules. King and McConkey25 have measured theenergy-loss spectrum of CF2Cl2 between 6 and 16 eV foincident electron of 500 eV. Multiple structures were oserved in this range of energy and it was difficult to identall of them. Multiple fragmentation and ionization of thCF2Cl2 molecule are possible. In particular, a large structwas observed around 7 eV with an onset at 5.7 eV, cosponding probably to the following reaction:

e21CF2Cl2→CF21Cl1Cl1e2, DH°55.7 eV.

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Other reactions are also possible:

e21CF2Cl2→CF1F1Cl21e2, DH°58.5 eV,

e21CF2Cl2→CF1FCl1Cl1e2, DH°58.4 eV.

At higher energies, ionization of the CF2Cl2 molecule and itsfragments take place. Ajelloet al.26 have studied the photoionization of CF2Cl2 and have measured the thresholdformation of the CF2Cl2

1 , CFCl21 , CF2Cl1, CFCl1 and CF2

1

cations. The following reactions lead to the formation ofneutral fragments,

hn1CF2Cl2→CF2Cl11Cl1e2, DH°511.99 eV,

hn1CF2Cl2→CF211Cl1Cl1e2, DH°516.98 eV,

hn1CF2Cl2→CFCl11F1Cl1e2, DH°517.76 eV.

These reactions can be initiated by electron impact and cthe formation of Cl2 molecules by recombination of Cl aoms. These processes, in addition to the recombinatioCl1 and Cl2 ions which has an onset at 14.59 eV, can eplain the maximum of the efficiency of Cl2 formation ob-served between 10 and 20 eV electron irradiation ofCF2Cl2 film.

IV. SUMMARY

We report measurements of electron stimulated destion ~ESD! yields and kinetic energy distributions of F2 and

FIG. 11. Integrated Cl2 intensity as a function of electron irradiation ofCF2Cl2 film at different energies; this plot is a measure of the efficiencyformation of Cl2 molecules~see the text!.


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Cl2 from thin films of CF2Cl2 condensed on polycrystallinplatinum at 20 K. Dissociative electron attachment procescause a peak in the F2 yield at an electron energy of;4 eVand a peak in the Cl2 yield at ;7 eV. Electron irradiationinduces changes in the yield and kinetic energy distributiof Cl2. These changes are attributed to the synthesis of2

in the condensed phase during electron irradiation ofCF2Cl2 film. Cl2 formation is demonstrated by the observtion after electron irradiation of the Cl2

2 2Pu resonance statein the Cl2 yield characteristic of condensed Cl2 , and by Cl2

kinetic energy distributions that are similar to those obtainfrom pure condensed Cl2 molecules.

ACKNOWLEDGMENTS

This work has been supported, in part, by the NatioScience Foundation, Grant No. CHE-9705476, and byCNRS.

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Low-energy electron-induced processes in condensed CF2Cl2 films

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