Surface Science 604 (2010) 2184–2189

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Surface Science

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Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

S. Kunz a,⁎, F.F. Schweinberger a, G. Kwon a, J. Kiermaier a, S. Le Moal b, C. Henry b, U. Heiz a

a Technische Universität München, Chair of Physical Chemistry, Catalysis Research Center, Lichtenbergstraße 4, 85748 Garching, Germanyb CRMC2-CNRS, Campus de Luminy, Case 913, F-13288 Marseille Cedex 9, France

⁎ Corresponding author. Tel.: +49 89 289 13394; faxE-mail addresses: sebastian-kunz@tum.de (S. Kunz),

(F.F. Schweinberger), gihan.kwon@mytum.de (G. Kwon(J. Kiermaier), lemoal@cinam.univ-mrs.fr (S. Le Moal), h(C. Henry), ulrich.heiz@ch.tum.de (U. Heiz).

0039-6028/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.susc.2010.09.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 June 2010Accepted 20 September 2010Available online 24 September 2010

Keywords:UPS (He I)MIESTPDMgOTrichloroethylene (TCE)Physisorption

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impactelectron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of theadsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCEinteracts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change fromone peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to thesingle peak is detected. Additionally, the observation of a local minimum for the work function (WF) for bothMIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption ofmetals with outer s valence electrons on transition metal substrates and adsorption of metals with outer svalence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system oforganic molecules adsorbed on an insulating surface. Two different models are discussed in order tounderstand the presented results.

: +49 89 289 13389.florian.schweinberger@tum.de), j.kiermaier@mytum.deenry@crmc2.univ-mrs.fr

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Metal oxides and their surface chemistry have found increasingattention in the surface science community over the past decades[1,2]. This is due to their important role in a number of applicationssuch as corrosion protection, thermal coatings, as a basis for porousmaterials and even more important as inert catalytic supports [3–6].Despite the efforts made on the characterization of metal oxides, onlylittle attention has been drawn towards the adsorption properties oforganic molecules on metal oxide films so far. Under well-definedconditions, standard surface science methods such as low-energyelectron diffraction (LEED), X-ray photoelectron spectroscopy (XPS)Auger-electron spectroscopy (AES) and ultraviolet photoelectronspectroscopy (UPS) are commonly used methods of choice for theinvestigation of such systems. For example, UPS, due to the escapedepth dependency of photoelectrons, allows for the investigation ofthe electronic structure of the top 2–3 atomic layers of a surface. UPSstudies enable the distinction between molecular and dissociativeadsorptions of molecules like CO [7]; also it has given importantinsight into the electronic promotion effect of potassium [8].

The previously described methods, however, lack the necessarysurface sensitivity for probing small amounts of adsorbates on a

surface. An improvement towards further sensitivity can be achievedby covering the surfacewith Xe, thereby revealing the adsorption sitesof the noble gas by means of UPS [9]. This technique is known asphotoemission of adsorbed xenon (PAX) and allows for site-selectivetitration, e.g. probing of bimetallic systems [10]. However, extremelylow temperatures are necessary to achieve xenon adsorption.

Another surface sensitive method that can be used at anytemperature is metastable impact electron spectroscopy (MIES),also known as Penning ionization electron spectroscopy (PIES) ormetastable atom electron spectroscopy (MAES), which uses metasta-ble rare gas atoms with low kinetic energy as projectiles. Thesemetastable atoms cannot penetrate into the surface and thus allow forexclusively probing the uppermost surface layer [11]. In particular,MIES is extremely sensitive to defective sites [12,13]. In combinationwith theoretical studies, MIES gives insight into the electronicstructure of the uppermost surface layer and its electronic properties[14,15].

Conveniently, bothMIES and UPS can bemeasured simultaneouslyby means of a UV discharge source and applying a time of flightseparation that has been previously described [16].

Combined MIES and UPS studies are a useful tool to exploresurfaces and adsorption phenomena. Interesting results such as phasetransition of adsorbed organic molecules or analyses of the orienta-tion of alkyl chains on flat surfaces have been reported [17,18].

In the presented work we focused on trichloroethylene (TCE), apollutant that presents severe problems as a contaminant of drinkingwater due to its poor degradability [19]. The photooxidation of TCEseems to be a promising option to convert TCE and thus attracted

TC

E io

n si

gnal

/a.u

.

TC

E io

n si

gnal

/a.u

.

150145140135130125120115110

temperature /K

140135130125120115

temperature /K

0.1 TCE/SA 0.2 TCE/SA 0.4 TCE/SA 0.6 TCE/SA 0.7 TCE/SA

AC

B

Fig. 1. TPD spectra of different TCE coverages on MgO(100). (A) single peak at acoverage of 0.1 TCE/SA; (B) low temperature peak and (C) high temperature peak forcoverages above 0.1 TCE/SA. The inset shows the important details of the TPD spectralimited to the relevant temperature range.

2185S. Kunz et al. / Surface Science 604 (2010) 2184–2189

various groups to study the mechanistic details of this reaction [20].As a contribution to the ongoing studies we hereby present anadsorption study of TCE by means of combined TPD, MIES and UPSmeasurements under UHV conditions on MgO, a commonly appliedsupport for catalysts [21].

2. Experimental

Experiments were carried out in a UHV chamber with a basepressure of 2×10−10 mbar. The MgO substrates used for the presentstudy are thin films (∼10ML) grown by evaporatingMg in an isotropicoxygen background on a Mo(100) single crystal at 350 K, followed byflash annealing to a temperature of 800 K. The quality of the MgO(100) film is checked by AES as well as MIES and UPS and inagreement with spectra previously published in the literature[13,22,23].

Metastable helium atoms as well as UV photons (He I) weregenerated using a modified MIES/UPS source developed by Kempteret al. [16]. All modifications particularly regarding the separation ofthe two signals and gas feed of the source are the subject of a separatepublication, which is in preparation [24]. The incident angle of themetastable atom beam and the UV photons is 45°. Electrons aredetected using a hemispherical analyzer (VSW Scientific InstrumentsLtd., HA150), which is in line with the surface normal of the sample.An acceleration voltage of 30 V was applied to the sample. Spectra arerecorded using a LabView program of in-house design. All MIES andUPS spectra were recorded at low sample temperatures (∼100K).

To extract the work function (WF) of the investigated sample, onehas to examine the width of the spectrum, which is determined by thelow kinetic energy cut-off Ekinmin (high binding energy cut-off) and theFermi-edge, which is the highest occupied state and thus the highkinetic energy cut-off Ekin

max. Subtracting the width of the spectrumfrom the excitation energy yields the work function (Eq. (1)).

ϕ = hν−ðEmaxkin −Emin

kin Þ ð1Þ

In order to extract the low kinetic energy cut-off (high bindingenergy cut-off) of the spectrum, the point of inflection and thecorresponding slope were determined. The intersection point of thistangent with the x-axis was taken as the low kinetic energy cut-off ofthe spectrum.

The surface investigated in this work (MgO) is an insulatingmaterial and hence, there are no occupied states at the Fermi-edge. Inother words, the Fermi-edge lies in the band gap and thus, the highkinetic energy cut-off does not coincide with the Fermi-edge. In orderto define the Fermi-edge in the MgO spectrum, the work function ofthe analyzer has to be examined, which was done by measuring Mo(100) and Pt(111) single crystals. The twometals are typical d-metalsand hence exhibit a sharp Fermi-edge. From the spectra of the d-metals the work function of the analyzer can be elucidated, since thework function of the metal is obtained directly from the spectrumwidth. Knowing the work function of the analyzer, the MgO spectracan be plotted with respect to the Fermi-edge, as done in this workand the work function can be extracted by determining the cut-off asdescribed above. For further details we refer to the literature ofphotoelectron spectroscopy [25].

A calibratedmolecular beam doser has been applied to dose TCE onthe MgO(100)/Mo(100) surface, which allows a better control of theadsorbate coverage than the often used Langmuir dosage procedure[26]. TCE dosage was performed at low temperatures (∼100K) andTPD spectra were measured using a Balzers QMG 421 mass spec-trometer and applying a temperature ramp of 1 K/s. The integral ofTPD signals presented in this work with respect to the proposedcoverage can be found in the supporting information. The coveragevalue used in this work is TCE molecules per surface atom (TCE/SA),

assuming a density of 2.25×1015 atoms/cm2 for an MgO surface and asticking probability of one for TCE at low temperatures.

3. Results and discussion

3.1. Temperature programmed desorption

TPD spectra of different TCE coverages on MgO(100)/Mo(100) areshown in Fig. 1. For the lowest coverage of 0.1 TCE/SA a single peak at126 K is observed (A), which is assigned to physisorbed TCE. Anincrease in surface concentration leads to a two peak structure from acoverage of 0.2 TCE/SA on with one maximum at low (B=124 K) anda second at higher temperatures (C=132 K). The peak at highertemperatures is assigned to TCE physisorbed on MgO and the peak atlower temperatures to the condensation of TCE on top of thephysisorbed layer. Further increase in the coverage (up to 0.7 TCE/SA) does not change the shape of the spectra, however, a small shift ofthe maximum of the second peak towards lower temperatures (from132 K (0.2 TCE/SA) to 130 K (0.7 TCE/SA) is observed (see inset ofFig. 1). The intensities of both peaks increase with rising coverage;with the increase in the first peak being more pronounced.

In summary, only one adsorption state is observed at lowcoverages, which is accredited to physisorption, whereas at highercoverages two adsorption states are found: one at higher tempera-tures due to the physisorbed layer and a second, higher populated oneat lower temperatures originating from condensed TCE on top of thephysisorbed layer. Surprisingly, a shift of the physisorbed statetowards higher temperatures and hence a stronger bonded state isobserved for multilayer adsorption (≥0.2 TCE/SA). The physisorbedstate seems to not be saturated at any coverage, since the integral ofthe peak increases continuously with rising coverages. The temper-ature shifts indicate that lateral interactions inhibit the full saturationof the physisorbed surface state at lower coverages.

3.2. UPS/MIES

UPS aswell asMIES spectra were recorded for two different dosageseries in order to utilize the respective experimental techniques in

binding energy /eV

7

6

5

4

3

2

1

0

cove

rage

/ (T

CE

/SA

) (x

0.1)

151050

UPS

10

binding energy /eV

7

6

5

4

3

2

1

0

cove

rage

/ (T

CE

/SA

) (x

0.1)

151050

10

MIES

Fig. 3. UPS (left) and MIES (right) spectra of high coverage dosage (0.1 TCE/SA) of TCEon MgO(100)/Mo(100). The bottom spectra show the clean MgO(100) surfaces.

2186 S. Kunz et al. / Surface Science 604 (2010) 2184–2189

their specific sensitivity range. In the low coverage series, presented inFig. 2, dosage steps of 0.01 TCE/SA were used, whereas for the highcoverage series, shown in Fig. 3, TCE was dosed in steps of 0.1 TCE/SA.All spectra are plotted with respect to the Fermi-edge (EF=0 in unitsof binding energies).

At zero coverage typical UPS and MIES spectra for clean MgO(100)are observed (see Fig. 2). Both spectra are dominated by the O(2p)peak that appears at 6 eV binding energy. In the UPS spectra a secondO(2p) peak is located at 9 eV originating from p-orbitals, which areoriented parallel with respect to the surface. However, in MIES the p-orbitals oriented perpendicular to the surface are preferably ionizeddue to steric reasons [27]. The O(2p) peaks decrease with increasingTCE surface concentration in both spectra. The vanishing of the MgOfeatures is more pronounced in the MIES spectra due to its highersurface sensitivity. Two peaks can also be clearly observed at bindingenergies of 8 eV and 11 eV for increasing TCE coverages in the MIESspectra. Furthermore, a shoulder at lower binding energies, around5 eV is found. The peak at 8 eV splits into two peaks for increasingcoverages (from 0.1 TCE/SA on). In the UPS spectra only for thehighest coverage (0.15 TCE/SA) a clearly identifiable peak at 11 eV canbe seen.

As for the low coverage series, the bottom spectra in the largerdosing steps show a clean MgO film for MIES and UPS (see Fig. 3). Forincreasing coverages the MgO features disappear in the MIES spectra,as well as in the UPS spectra.

The characteristics in the MIES spectrum at a coverage of 0.1 TCE/SA are similar to those found in the low coverage series: peaks at 8 eV,11 eV and a shoulder at 5 eV. However, an additional peak appears at abinding energy of 12.5 eV for a coverage of 0.2 TCE/SA. This peakseems to shift to lower binding energies for increasing coverages andto overlap ormixwith the one at 11 eV at 0.4 TCE/SA. Again, a splittingof the peak at 8 eV is found; however, the splitting occurs at higherTCE surface concentrations in comparison to the low coverage series.We assume that differences in the coverage dependence of thesplitting cannot be attributed to a beam damage induced by themetastable helium atoms, since the kinetic energy is very low

binding energy /eV151050

UPS

binding energy /eV151050

14

12

10

8

6

4

2

0

cove

rage

/ (T

CE

/SA

) (x

0.01

)

14

12

10

8

6

4

2

0

cove

rage

/ (T

CE

/SA

) (x

0.01

)

MIES

Fig. 2. UPS (left) and MIES (right) spectra of low coverage dosage (0.01 TCE/SA) of TCEon MgO(100)/Mo(100). The bottom spectra show the clean MgO(100) surfaces.

(∼0.1eV). An error in the dosage in this range is also excluded dueto the accuracy of dosing. Instead, the feature seems to depend on themeasuring time that is very similar in both series and might thus berelated to small impurities due to co-adsorption from the backgroundor from the metastable helium atom beam. Nevertheless, all otherfeatures observed in the MIES spectra are in agreement as well as thework function determined for the different series (see Fig. 4).

The shoulder in the MIES spectrum, which appears at 5 eV for0.1 TCE/SA shifts for increasing coverages to a binding energy of 4 eV.Since this range of binding energies is similar to the O(2p) peak ofMgO and is referred to non-bonding orbitals, we assume that this peakcan be assigned to the non-bonding orbitals of the chloridesubstituents. In the UPS spectra the O(2p) peak structure completelyvanishes at 0.4 TCE/SA, giving rise to two new peaks, one exhibiting alow intensity at a binding energy of 4 eV and a second, morepronounced one at 7 eV. As discussed for theMIES spectra, we assumethat the one at the lower binding energies can be assigned to non-bonding orbitals of the chloride substituents of TCE. For coverageshigher than 0.5 TCE/SA no further changes in the UPS spectra arefound.

A further, more detailed interpretation of the obtained spectracannot be achieved without theoretical support. Hence, we will focusin the following discussion on the change of the work functionextracted from the spectra presented in this work.

0.4

0.3

0.2

0.1

0.0

-0.1

Δ w

ork

func

tion

/eV

0.70.60.50.40.30.20.10.0

coverage / (TCE/SA)

UPS low TCE coverage UPS high TCE coverage MIES low TCE coverage MIES high TCE coverage

Fig. 4. WF change (MIES and UPS) for increasing TCE surface concentrations.

2187S. Kunz et al. / Surface Science 604 (2010) 2184–2189

3.3. Work function changes

Since the 2s level of the metastable helium atom lies in the bandgap of MgO, the deexcitation processes are limited to Augerdeexcitation (AD). This allows for the determination of the workfunction from MIES spectra using the same approach as for UPS[27,28].

Work function values have been examined from both, UPS andMIES spectra presented in Figs. 2 and 3. The absolute value obtainedfor MgO is 3.4 eV, which is in good agreement with previous studieson MgO films on Mo(100) [29]. The relative change of the workfunction with respect to TCE coverage is shown in Fig. 4. Theexperimental error has been determined by measuring the similarMgO film on Mo(100) fifteen times and calculating the standarddeviation of the work function value (0.05 eV).

Interestingly, the value decreases by ∼0.1eV in the beginning andreaches a minimum at a coverage of around 0.05 TCE/SA. Furtherincrease of the surface concentration leads to a rise of the workfunction to the starting value of MgO at a coverage of 0.1 TCE/SA andreaches a saturation level for surface concentrations higher than0.4 TCE/SA. The appearance of a minimum is well-known foradsorption metals with outer s valence electrons on transition metalsor metal oxides and it has been observed for N2 adsorption onW(100)[30–32]. To our knowledge a similar effect for the adsorption oforganic molecules on insulators has not yet been reported.

4. Proposed adsorption models

In the following, two models are discussed in order to explain thepresented results. The first model is based on the possibility thatadsorbed molecules can undergo a phase transition as previouslyobserved for organic molecules [33], whereas the second model triesto explain the results as an effect of depolarization [34,35]. A modelthat has been used to explain the appearance of a work functionminimum on an insulator is based on the interaction of the adsorbatewith defects [36]. For example, color centers observed in MgO on Ag(100) have reportedly produced occupied and unoccupied stateswithin the band gap of MgO [37]. However, from the spectrapresented for MgO no defects within the band gap can be identified.Thus, the model based on defects will not be discussed at this point.

4.1. Model A — phase transition

As seen in Fig. 4 the work function value first decreases to aminimum at 0.05 TCE/SA, reaches the level of MgO for 0.1 TCE/SA andthen shows saturation at a coverage of 0.4 TCE/SA exhibiting a higherwork function value (+0.3 eV) than clean MgO. Since the saturationlevel shows an increase of the work function value in comparison toMgO, it can be concluded that a condensed TCE film has a highersurface dipole moment than clean MgO. However, the question ariseswhy the work function decreases for low coverages (b0.1 TCE/SA).

It is assumed that at low surface concentrations the TCE moleculeslie flat on the surface with their dipole moment oriented almostparallel to the surface. Further filling of the physisorbed layer leads tolateral interactions between the molecules and hence the moleculesstart to lift with increasing coverage in such a way that the dipolemoment is shifted more and more parallel to the surface normalvector. This leads to an increase in the work function and to a differentadsorption strength, which generates a shift of the desorption peak inthe TPD spectrum. A similar filling and lifting process has also beenobserved for alkanethiols adsorbed on Au(111) with variation of thecoverage [38].

The different aspectswhich are gained from TPD aswell as UPS andMIES experiments are summarized in Fig. 5 together with theproposed model.

The adsorption properties can be divided into four differentregions depending on the coverage. For region A it is concluded thatthe TCE molecules are lying almost flat on the surface. In region B,with increasing TCE coverage, the molecules start to lift in such a waythat the dipole moment is aligned more and more with the surfacenormal vector, leading to a rise in the work function value.

Region C exhibits two peaks in the TPD and thus TCE condensationon top of the physisorbed layer occurs at these surface concentrations.Further dosage in region C leads to an increasing population of bothstates. Since the molecules in the physisorbed layer are now shiftedtowards the vacuum side additional free spaces become available.However, they depend on the strength of lateral interactions. Thisexplains the shift in the TPD of the physisorbed peak maximumtowards lower temperatures, because an increase in lateral interac-tions results in a decrease of adsorption strength.

In region D the increase in physisorbed molecules does notinfluence the work function anymore, since the adsorbed layers havereached a thickness at which only condensed TCE can be probed inUPS and MIES.

4.2. Model B — depolarization

Work function minima depending on the adsorbate coverage havebeen observed previously for alkali and earth alkali adsorption onmetal substrates. A charge transfer from the alkali atom to thetransition metal is assumed to occur at low coverages, which causes asurface dipole moment in the opposite direction compared to that ofthe transition metal [39]. For increasing coverages, the alkali atomsinteract with neighboring alkali atoms leading to the formation of analkali film and a depolarization of the surface dipole moment, whichwas generated by the charge transfer [40]. For high coverages thework function of the bulk alkali metal is observed.

In contrast to a transition metal, MgO films exhibit a band gap ofmore than 6 eV [41]. Hence, no unoccupied states are available abovethe valence band maximum and no charge transfer can occur.Nevertheless, a work function minimum has been observed foradsorption of outer s valence electron metals like Ag, Mg or Cu onMgO. The same metals adsorbed on transition metal substrates showdifferences of the work function that are much more pronounced(Dϕ∼3eV), than those observed onMgO (Dϕb0.5 eV) [32,35,36,42]. Acomparison of the interactions between the adsorbate and theadsorbent shows that instead of a charge transfer, only van-der-Waals interactions occur between outer s valence electron metals andMgO [43]. Thus, the smaller decrease might be related to the weakerinteraction of the metal atoms with the surface that generates only asmall change of the surface dipole moment, in contrast to the stronginteraction of the same metals with transition metals. For TCEadsorbed on MgO the same considerations hold as for the outer svalence electron metals: only van-der-Waals interactions occurbetween MgO and TCE due to the lack of unoccupied states of MgO,leading to a small change of the surface dipole moment and a smalldecrease of the work function at low coverages.

Increasing coverages lead to rising lateral interactions between theadsorbates. Thus, for the adsorbed metals, metallic bonds are formedat higher surface concentrations and the interaction between themetal atoms and theMgO surface is reduced, yielding a depolarizationof the surface dipole moment that was generated by the adsorbedmetal atoms. For TCE it is assumed that dipole–dipole interactionsbetween the adsorbed molecules, which occur for molecules withpermanent dipole moments, lead to a weakening of the interactionbetween TCE with MgO and thus to a depolarization of the surfacedipole moment [44]. These conclusions are in agreement withadsorption studies we performed for cyclohexene on MgO (seesupporting information). For this type of organic molecule acontinuous decrease of the work function was observed that reachessaturation at a work function change of 0.3 eV. The lack of a strong

A B C D

A B C D

Δ w

ork

func

tion

/eV

coverage / (TCE/SA)

UPSMIES

Fig. 5. Assumed four step adsorption model for TCE on MgO(100) due to a phase transition (Model A).

2188 S. Kunz et al. / Surface Science 604 (2010) 2184–2189

dipole moment excludes the possibility of a depolarization of thesurface dipole moment that is generated by the physisorbedcyclohexene. In Fig. 6 the second proposed model is summarized.

At low coverages TCE physisorbs on MgO due to van-der-Waalsinteractions (see region A in Fig. 6), leading to a decrease of the workfunction. The increase of the TCE coverage leads to dipole–dipoleinteractions between the adsorbed TCE molecules (region B in Fig. 6).In terms of the depolarization model it is assumed that these lateralinteractions reduce the interaction of TCE with the substrate leadingto the depolarization of surface dipole momentum that was generatedby the individual adsorbed TCEmolecules. In region C the physisorbedstate is nearly saturated and TCE condensation on the physisorbedlayer occurs, leading to the two peak structure observed in the TPDspectra. The peak of the physisorbed state is shifted to higher tempe-

Δ w

ork

func

tion

/eV

coverage

A B C

A

Fig. 6. Assumed four step adsorption model for TCE o

ratures, since the dipole–dipole interaction between the moleculesexceeds the van-der-Waals interaction of the molecules with MgO. Ifthe surface concentration is further increased, the amount of con-densed TCE on the physisorbed layer rises and the physisorbed layer iscompressed leading to the observed shift of the second desorptionpeak to lower temperatures and the increase of the peak area (regionC in Fig. 6). At high surface concentrations the spectra are determinedby a thick TCE film (D) and no further changes of the work functionare observed.

5. Conclusion

The adsorption properties of TCE on MgO(100)/Mo(100) havebeen investigated by means of TPD, MIES and UPS. Three adsorbed

/ (TCE/SA)

D

B C D

UPSMIES

n MgO(100) due to a depolarization (Model B).

2189S. Kunz et al. / Surface Science 604 (2010) 2184–2189

states depending on the TCE coverage were identified in the TPDspectra. For the change of the work function with respect to the TCEsurface concentration a minimum has been found at low coverages,which has been previously observed for different adsorption systems,but not for the adsorption of organic molecules on insulatingmaterials. At higher coverages a saturation of the work functionvalue occurs.

Based on these results two possible models for the adsorptionmechanism have been proposed. Model A is based on a phasetransition of adsorbed molecules and the second on the effect ofdepolarization. Both models are in accordance with the experimentalresults and further theoretical studies are in progress in order to verifythe two models. The obtained results yield an understanding of thecomplex adsorption properties of TCE as an organic moleculeadsorbed on a typical metal oxide (MgO). From this work furtherstudies of TCE adsorption are currently being planned on morecomplicated systems, e.g. size selected clusters supported on MgO.

6. Author contributions

S.K., F.F.S. and U.H. planned the experiments; S.K., F.F.S., G.K. and J.K. performed the MIES/UPS and TPD experiments. S.K., F.F.S., S.LM., C.H. and U.H. discussed the results, S.K., F.F.S. and U.H. wrote the paper.

Acknowledgements

The authors would like to thank Richard Schröter and FlorianSpirkl for solving many technical problems, Yuri Suchorski for fruitfuldiscussions, Hans-Peter Steinrück for useful remarks, the group ofVolker Kempter for the support in the beginning of the MIES/UPSproject. S. K. would like to thank the “Fonds der ChemischenIndustrie” and F. F. S. the “European Graduate School on SustainableEnergy Technology” for financial support. Furthermore we acknowl-edge financial support from the “Deutsch-Französischen Hochschule”(DFH Project G2R-FA-31-07 and the DFG (Project He 3454/902).

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.susc.2010.09.012.

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