Structure and Bonding of Ethylene Oxide on Si (100)

Aug 19, 2009 - The bonding configuration and electronic structure of ethylene oxide adsorbed on Si(100)-(2 × 1) is investigated with fully polarizati...
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J. Phys. Chem. C 2009, 113, 16077–16082

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Structure and Bonding of Ethylene Oxide on Si(100) F. Hennies,*,† Z. Bao,‡,§ O. Travnikova,‡,| and M. N. Piancastelli‡ MAX-lab, Lund UniVersity, Box 118, 221 00 Lund, Sweden, and Department of Physics and Materials Science, Uppsala UniVersity, Box 530, 751 21 Uppsala, Sweden ReceiVed: April 3, 2009; ReVised Manuscript ReceiVed: July 7, 2009

The bonding configuration and electronic structure of ethylene oxide adsorbed on Si(100)-(2 × 1) is investigated with fully polarization resolved X-ray absorption spectroscopy as well as with core level and valence band photoemission spectroscopy. Ethylene oxide is found to adsorb via a ring-opening reaction, where the molecule forms a five-membered ring together with the silicon surface dimer atoms inserting between a carbon and the oxygen atom. In the resulting geometry, the molecule is tilted out of the surface plane. The chemistry on the Si(100) surface has been studied in increasing extent over the last decades. This surface is technologically important in today’s silicon oxide-based semiconductor technology (with the foreseen limit given by the oxide thickness1), and connected to a deeper understanding of the silicon surface chemistry is the hope to be able to further push the applicability of this widely used substrate in all relevant areas including interfaces to biological and chemical systems. As is well-known, Si(100) exhibits a (2 × 1) surface reconstruction, where one dangling bond per surface atom is saturated by the formation of surface dimers, while one dangling bond per Si atom remains and accounts for the high reactivity of the surface. Related publications report the adsorption of organic molecules to Si(100) at room temperature or below through binding to the dangling bonds and while keeping the surface dimer intact.2-11 For small organic molecules such as ethylene and acetylene, a cycloaddition reaction9,10 is observed (in the case of acetylene, an additional bridging species coexists).3,4 Benzene adsorbs molecularly in a 1,4-cyclohexadiene-like structure.5 Recently, the chemistry of epoxy molecules on the Si(100) surface has drawn some interest when it could be demonstrated that the chiral nature of 2,3-butanediol is preserved upon adsorption on Si(100) and can be identified utilizing circular dichroism in photoelectron spectroscopy.12 For methyl oxirane as a first thoroughly investigated epoxy molecule adsorbed on Si(100), we have recently reported a ring-opening reaction.6 Ethylene oxide (EO) (Figure 1) is the simplest epoxy molecule and as such the central building block of more complex epoxy molecules. The adsorption of EO on metal surfaces has been the subject of a number of investigations;13-19 however, no report on semiconductor adsorption of EO is known to us. On nearly all investigated metals surfaces, molecular adsorption has been observed at low temperatures (Ag(110), Cu(110), Ni(110), Ni(111), and Fe(100)), sometimes followed by a ringopening reaction and further dissociation (Pd(110), Pt(111)).13,14,19 While a wealth of methods has been applied to EO on metals, we could not find any X-ray absorption measurements of EO on metals. In particular, polarization resolved X-ray absorption spectroscopy * To whom correspondence should be addressed. E-mail: [email protected]. † MAX-lab. ‡ Uppsala University. § Present address: Department of Physics, University of Oregon, Eugene, OR 97403-1274. | Present address: Synchrotron SOLEIL, 91192 Gif-sur-Yvette, France.

Figure 1. Ethylene oxide (EO) molecule in the gas phase.

(XAS) is an excellent technique to reveal the bonding configuration of adsorbed molecules, as, e.g., we have demonstrated for a number of semiconductor surface adsorbates.3-6 We here present a fundamental investigation of the adsorption of ethylene oxide on Si(100) with a combination of synchrotronbased spectroscopies, as core-level photoelectron spectroscopy (commonly denoted as XPS, X-ray photoelectron spectroscopy), valence-band photoelectron spectroscopy (commonly known as UPS, ultraviolet photoelectron spectroscopy), and XAS. Experimental Setup and Data Treatment The experiments have been performed at the Swedish national laboratory MAX-lab in Lund at beamline I511-1. The beamline is equipped with an end station operated in UHV dedicated for XAS, XPS, and UPS on surface systems. The station consists of a preparation chamber operated at a base pressure of high 10-10 Torr connected to an analysis chamber (at a base pressure of low 10-10 Torr). The analysis chamber is equipped with a hemispherical electron analyzer for XPS and constant final state XAS measurements. For the substrate, two different Si(100) surface preparations were used. All XPS and UPS measurements were measured on a crystal cut to the nominal (100) surface. This surface exhibits a mixture of two domains separated by monoatomic steps. Between the domains, the silicon dimers are rotated by an angle of 90° with respect to each other. The commercially available silicon crystal was n-type phosphor doped with 0.6-1.1 Ω · cm resistance; it was grown according to the floating zone (FZ) method. In order to fully utilize the symmetry selectivity of XAS, we measured additionally fully polarization resolved XAS in an experimental setup with a geometry where the alignment of the polarization vector is well-defined with respect to the silicon

10.1021/jp9030859 CCC: $40.75  2009 American Chemical Society Published on Web 08/19/2009

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Figure 2. Si(001)-(2 × 1) sample mounting for full angle and polarization resolution. Incoming photons are graphed as rippled arrows. The polarization vector is represented by a double arrow.

dimer orientation. This is possible in the setup schematically depicted in Figure 2. Here, a vicinal cut Si(100) crystal was used which exhibits a macroscopic single-domain reconstruction. This crystal is deviating 5° in the surface normal from the [001] direction toward the [110] direction. This surface has a single (2 × 1) domain with diatomic steps in 8.5 dimer wide terraces.20 The dimers are aligned parallel to the step edges. The crystal was mounted at 7° grazing incidence of the incoming photon beam (here, 7° is the angle between the incoming light and the actual crystal surface, which deviates by 5° from the nominal (100) surface). The sample can be rotated around the beam axis, allowing to orient the polarization vector freely in-plane or outof-plane. First, it was mounted as in Figure 2a. Here, the inplane alignment of the polarization vector corresponds to inplane, perpendicular to the dimer axis. Then, the sample was remounted after rotating it by 90° around the surface normal with respect to first mounting, Figure 2b. Now, the in-plane alignment of the polarization vector corresponds to in-plane, along the dimer axis. The commercially available crystal was p-type boron doped with 5 Ω · cm resistance, made with the Czochralski (CZ) method. To obtain a defect depleted, clean reconstructed surface, the crystals were sputtered with Ar gas at 1 × 10-6 Torr first at room temperature for 15 min, then at 800 K for 20 min, and finally flashed several times to 1400 K. Surface cleanliness was monitored with XPS, and no residual carbon was detected except for some XAS measurements on the single-domain surface (see below). EO gas from Fluka with a purity better than 99.0% was used; its quality was checked with a quadrupole mass spectrometer. Saturated monolayers of EO were prepared by exposing the silicon substrate at room temperature to the molecular vapor at a pressure of P ) 5 × 10-7 Torr for 200 s. The samples were mounted at grazing incidence of the synchrotron light, namely, at an angle of 7° with respect to the surface plane. The sample manipulator allows one to rotate the sample around the beam axis. Since the beamline delivers linearly polarized light in the horizontal plane, this rotatory degree of freedom allowed us to measure with the e-vector aligned nearly normal to the surface (minus the grazing angle) or lying in the surface plane. Photoelectrons were detected under an angle of 45° with respect to the polarization vector and in a plane perpendicular to the propagation direction of the incoming photon beam. The valence band XPS spectra were recorded at a photon energy of 62 eV with an overall instrumental resolution of 20 meV, the Si 2p and C 1s spectra at 150 and 330 eV with 30 and 70 meV resolution, respectively. For the C K-edge XAS measurements, the photon bandwidth was about 50 meV. The XAS spectra were normalized according to the double normalization scheme.21 The raw spectra were first divided by the photon flux curve measured on a gold net inserted in the photon beam. Then, a background spectrum measured on a clean silicon substrate was subtracted from the adsorbate spectrum. For the data presentation, all XAS spectra were normalized to

Figure 3. C 1s XPS of ethylene oxide adsorbed on Si(100)-(2 × 1).

Figure 4. O 1s XPS of ethylene oxide adsorbed on Si(100)-(2 × 1).

the same continuum step height at the highest measured photon energy where polarization anisotropy can be assumed to vanish. To avoid beam damage, the samples were scanned during the measurements and fresh systems were prepared in short intervals. XPS measurements assured that no photon-induced reactions occurred in the adsorbate complex during measurements. The scanning rate was defined ex post, i.e., first the sample was irradiated so much that sample damage occurred (manifestly by atomic carbon showing up in the XPS) and then the sample was scanned fast enough to avoid these changes. Results and Discussion I: Core Levels In Figure 3, we show the C 1s core level XPS from ethylene oxide adsorbed on single-domain Si(100). The spectrum shows two peaks with a relative shift of 2.15 eV. Gas phase EO shows only one single C 1s level22 due to its two equivalent carbon atoms. The strong splitting observed upon adsorption on Si(100) indicates a very different chemical environment of the two carbon atoms. Such a strong splitting would not be expected in an adsorption geometry with both carbon atoms coordinating each to one silicon surface dimer atom. We can thus conclude a configuration of the EO molecule where a C-O bond is broken in a ring-opening and one C and the O coordinate to the Si surface dimer atoms in a cycloaddition reaction. We assign the peak at 290.85 eV to the C-C*-O carbon and the peak at 288.7 eV to the Si-C*-C carbon. The O 1s spectrum (Figure 4) shows a single broad line, with no indication of additional species. For oxygen adsorbed in different configurations on Si, core level binding energy shifts

Structure and Bonding of Ethylene Oxide on Si(100)

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16079 i.e., we used a symmetric line shape not accounting for certain types of loss processes. The ratio of bulk atoms to surface atoms is significantly increased in the adsorbate case, which we attribute to an artifact of the complicated peak fitting, probably caused by the disregarded asymmetries. Two additional components show up upon adsorption, one at 1 eV binding energy shift with respect to the bulk component and one at 150 meV. The strongly shifted component can clearly be assigned to SiO, i.e., a Si atom coordinated to the O atom of the adsorbed EO. We tentatively assign the other new species to a SiC component. Due to the similar electronic nature of C and Si, such a component would not be expected to shift much with respect to the sublayer and bulk silicon atoms. The analysis of the core levels of all three elements participating in the surface adsorption unambiguously supports an adsorption mechanism with the EO molecule breaking a C-O bond and then binding with one C and the O each to one Si surface dangling bond, i.e., a ring-opening cycloaddition mechanism. Results and Discussion II: Valence Band

Figure 5. Si 2p XPS of the Si(100)-(2 × 1) surface (a) clean and (b) with the ethylene oxide adsorbed. Also shown is an analytic peak deconvolution (see text) to elucidate the different contributions to the peak shape.

of about 1 eV have been reported.23 We therefore judge the coexistence of different oxygen species in our case to not likely be present. To further elucidate the local chemical environment of the atoms participating in the surface bond, we measured the Si 2p core levels. Figure 5 shows the spin-orbit split Si 2p upon adsorption of EO (panel b) and of the clean surface for comparison (panel a). A peak fit has been performed in order to decomposite the spectra. The clean Si spectra (Figure 5a) show the well-known superposition24,25 of the bulk signal, the silicon surface dimer signal, and two subsurface layers. The peak fitting results are summarized in Table 1 and closely approximate the original fit of the clean Si(100)-(2 × 1) surface by Landemark et al.25 The intensity ratio between up and down atoms of the surface dimer resembles the reported behavior, taking the 45° emission into account. The branching ratio of Si 2p1/2 to Si 2p3/2 deviates from the theoretical value of 0.5 and differs between the clean surface and the absorbate system, which can be attributed to the known nonlinear behavior of the detector26 in connection to different signal levels for both measurements. Upon adsorption of EO, the Si 2p envelope line shape changes substantially. The fit reveals that the two Si surface dimer components vanish, clearly indicating a complete saturation of the dangling bonds and a leveling and relaxation of the dimers. The bulk, second, and third layer species remain mostly unchanged. In the adsorbate case, the Gaussian width of these three species increases slightly, also indicating an increase of intrinsic loss processes compared to the clean Si case. The subsurface components shift slightly in energy, which is most likely due to an ambiguousness in the fitting procedure;

Valence band spectra are a good indicator for to what extent the molecular nature of the adsorbing molecule is preserved on the substrate. In Figure 6, we show the valence band photoemission spectra of EO adsorbed on Si(100) as well as the clean Si spectra in the same energy range. We start looking at the contribution of the substrate states to the overall valence structure of the adsorbate system. The substrate spectra exhibit a small kink at 5.5 eV attributed to the dangling bond states.24 This feature vanishes upon adsorption when the dangling bonds get saturated. The remaining states contribute to the total valence structure at 7-8 eV and around 11 eV. We can now compare the valence electronic structure of the adsorbate system with gas phase measurements from the literature.27,28 The strong feature at 10 eV binding energy can be attributed to the orbital which stems from the 2b2 of the free molecule. There it belongs to the σCH2 bonds. Its still strong apparency indicates the persistence of the H bonds and that the carbon atoms remain sp3 hybridized, which will further be supported by the XAS measurements and supports the cycloaddition reaction. Upon adsorption, the features arising from the 6a1 and 3b1 states broaden and merge, indicating the changes in the σC-C and σC-O bonds. Results and Discussion III: Polarization Resolved X-ray Absorption Spectroscopy To finally determine the adsorbate configuration, we performed polarization-resolved C K-edge XAS on EO/Si(100). Here, the probability of the transitions from the C 1s core orbitals to the C 2p derived unoccupied molecular states is probed as a function of incident photon energy. Due to the dipole selection rules applying, only transitions into those unoccupied molecular orbitals are possible where the exciting e-vector is aligned along the respective orbital. We have measured C K-edge XAS on a single-domain Si(100) crystal (Figure 7). On the top (panel a), we show the spectrum recorded with the polarization vector pointing out of the surface. The inset shows the two possible orientations of the surface dimer, which in that case cannot be distinguished. Below that, we show the spectra recorded with the polarization vector lying in the surface and aligned parallel (panel b) and perpendicular (panel c) to the Si dimer axis. The spectra show clear polarization anisotropy. For the discussion of the features,

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TABLE 1: Peak Fitting Parameters for Si 2p XPS of the Clean Si(100)-(2 × 1) Surface and for EO Adsorbed on Si(100)a sample

bulk

Si(100)-(2 × 1)

0 (243)

EO/Si(100)

0 (258)

second layer

third layer

dimer (up atoms) 228 (251) -226 (251) -518 (251) SiC 237 (299) -231 (299) 154 (299) values: binding energy shift (meV) (Gaussian width (meV))

dimer (down atoms) 75 (251) SiO 1005 (299)

a Common for all components are the spin orbit splitting (600 meV), the Lorentzian width (85 meV), and the intensity ratio of Si 2p1/2 to Si 2p3/2 (0.43 for the clean surface and 0.48 for the adsorbate system).

Figure 6. UPS of ethylene oxide adsorbed on Si(100)-(2 × 1) and of a clean Si substrate recorded under the same conditions.

Figure 7. Polarization resolved XAS of EO/Si(100)-(2 × 1) measured on a single-domain substrate. The insets illustrate the relative orientation of the e-vector, Si dimer axis, and substrate plane for the different spectra. For peak assignment, see text.

it is important to recall that the experimental geometries are not perfectly pure (due to the 7° grazing incidence) and that the used single-domain surface has a fraction of about 15% step edge atoms. Our discussion in the following bases therefore on the clear trends visible rather than on minute intensity variations. The spectrum recorded with the e-vector normal to the surface exhibits three clearly pronounced features at 285.7 eV (peak A), 286.8 eV (B), and 288.8 eV (D). These are followed by two broader resonances at 290.8 eV (E) and 292.8 eV (F). Upon

alignment of the e-vector in the surface plane (panel b), the three sharp features vanish. We here instead observe a weak shoulder at 286.8 eV (B) and one at 288 eV (C) in the onset of the photoabsorption. Feature C can be estimated to contribute to the spectrum recorded with normal polarization (panel a) as well. The broad features at 290.8 eV (E) and 292.8 eV (F) become much more strongly pronounced with respect to the continuum step. The shoulder observed at 286.8 eV (B) is notably more pronounced when the polarization vector is aligned along the Si dimer axis (panel b). The shoulder at 288 eV (C) does not vary in intensity much. Finally, the features E and F are also more intense upon alignment of the polarization vector in the direction of the Si dimer bond. We attribute a weak feature at 284.4 eV to a small residual of atomic carbon on the clean surface which showed up in our XPS surface preparation control measurements at the detection limit. This feature has not been visible in some XAS reference spectra we measured on a multidomain crystal. In agreement with the literature,29 we assign the broad features at 290.8 eV (E) and 292.8 eV (F) to transitions from the two shifted carbon 1s core levels into a σ*-type resonance which is commonly observed in the continuum of heterocyclic hydrocarbons. In free gaseous EO, this feature appears at 295.4 eV. It is well-known from simple linear hydrocarbons that the position of this feature shifts toward lower energies with increasing hybridization state of the carbon atom and accordingly increasing bond C-C bond length.30,31 In EO, we observe a shift upon adsorption compared to the free molecule, which is small compared to the shifts observed upon rehybridization.30,31 We can therefore conclude that the carbon atoms do not change their hybridization and that the C-C bond length increases slightly. This confirms the conclusion drawn from the valence band spectra that the molecule adsorbs without further dissociation. Gaseous EO exhibits a strong photoadsorption resonance29 comprised of transitions into the 7a1, 4b1 unoccupied molecular orbitals and a 3s Rydberg state. Free EO has C2V symmetry. Accordingly, orbitals representing the C-C and C-O bond have a1 symmetry, whereas the CH2 bonds have b1 symmetry. Upon adsorption, this resonance becomes much weaker and splits up into the three identified features. This splitting is of twofold origin: It (a) is a direct consequence of the changes in the valence electronic structure of the molecule itself upon adsorption. It (b) also stems from the core level shift between the two inequivalent carbon atoms observed with XPS; now transitions from the two separated C 1s levels into the same unoccupied molecular state, as long as both are dipole-allowed, occur at different photon energies. However, since XAS probes transitions from the localized core levels into the molecular valence structure, only those transitions are pronounced where the participating wave functions have significant spatial overlap; in general, XAS can therefore be considered a local probe. This means that only

Structure and Bonding of Ethylene Oxide on Si(100) transitions from a localized core level into valence orbitals representing a bond involving that particular atom are visible in the spectra. In the absorption spectra, we find the antibonding counterparts of the tetrahedrally oriented carbon sp3 bonds showing up. The C-C bond related σ*-type resonance we already identified at higher energy; left to be identified are the sign of a C-Si bond, a C-O bond, and the C-H bonds. The silicon bound carbon atom shifts by 2.15 eV toward lower binding energy (see XPS above) with respect to the other carbon. Consequently, the lowest energy transitions at 285.7 eV (A) and 286.8 eV (B) correspond to bonds involving this carbon, i.e., the C-Si bond and the CH2 bonds of this carbon. The higher energy features at 288 eV (C) and 288.8 eV (D) then belong to bonds involving the carbon atom coordinating to the oxygen, i.e., the C-O bond and the CH2 bonds of this carbon. However, there is no further reason for which bond should be assigned to which of the observed features in the first place. Additionally, XAS involves different final states compared to XPS, since the near edge features in XAS belong to core-excited neutral states, whereas XPS depicts the core ionized final state. Since XAS probes transitions from localized core orbitals into the local unoccupied density of states, the final state screening of the different carbon species, and therefore the binding energy shifts, differ from the XPS case. From the fully polarization resolved XAS measurements presented here, however, we can resolve the last obscurities in the peak assignment and further elucidate the adsorption geometry. The first feature at 285.7 eV (A) has intensity only in the out-of-plane spectra. It must therefore belong to the C-Si bond, which is accordingly oriented nearly perfectly perpendicular to the surface plane. The energy position of this peak agrees nicely with the location of the same bond in C2H4;3 there the C atom is sp3 hybridized as well. This peak A cannot be assigned to the CH2 bonds of this carbon, since these orbitals have to have components in the other symmetries as well; it is impossible that the CH2 bond has only a component perpendicular to the surface plane. As a consequence, we can assign peak B at 286.8 eV to this CH2 bond, again in agreement with C2H4. The H2 group is accordingly pointing slightly upward but with clear components in the other two spacial directions as well. The feature C appearing at 288 eV we assign to the C-O bond, which is accordingly mostly lying in the surface plane under some azimuthal and polar angle to the Si surface dimers. It appears from the spectra to have a small component in the perpendicular direction as well. We can then finally assign peak D at 288.8 eV to the CH2 bonds of the oxygen coordinated carbon. We therefore see this H2 group more pronounced sticking out of the surface plane. An alternative assignment of this peak D to the C-O bond would result in a bond being almost vertically aligned; this would result in a very distorted adsorption geometry. It would, additionally, contradict the findings from the polarization dependence of the C-C bond related σ*-type resonance (E and F). This bond clearly is tilted little out of the surface plane, with an azimuthal angle tending toward the Si surface dimer orientation. With the strong support of the ring-opening and cycloaddition mechanism from the XPS measurements and the more detailed bond orientations obtained from the polarization resolved XAS, we propose a conceptual adsorption structure, as shown in Figure 8. Conclusion We have performed a core-level and valence-band photoemission study and polarization dependent X-ray absorption

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Figure 8. Molecular adsorption configuration of ethylene oxide (EO) on Si(100): (left) drawing plane coplanar to the (110) crystallographic plane; (right) drawing plane coplanar to the (100) crystallographic plane.

measurement on ethylene oxide adsorbed on Si(100)-(2 × 1). Summarizing, we find that • Ethylene oxide adsorbs molecularly, i.e., without further dissociation on Si(100)-(2 × 1). This is manifest from the valence-band spectra, and has been confirmed by the X-ray absorption measurements. • The molecular ring opens and the adsorbate molecule binds in a cycloaddition reaction to the silicon surface dangling bonds. The silicon dimer inserts between a carbon and the oxygen atom of the ethylene oxide molecule. This finding is evident from both the Si and C core level spectra. • We determined the resulting adsorption geometry by fully polarization resolved XAS measurements. The resulting five-membered ring is almost perpendicular to the surface, with the outermost carbon atom lying slightly out of plane. Acknowledgment. Valuable support from the MAX-lab staff is gratefully acknowledged. References and Notes (1) Schulz, M. Nature 1999, 399, 729. (2) Nishijima, M.; Yoshinobu, J.; Tsuda, H.; Onchi, M. Surf. Sci. 1987, 192, 383. (3) Hennies, F.; Fo¨hlisch, A.; Wurth, W.; Witkowski, N.; Nagasono, M.; Piancastelli, M. N. Surf. Sci. 2003, 529, 144–150. (4) Pietzsch, A.; Hennies, F.; Fo¨hlisch, A.; Wurth, W.; Nagasono, M.; Witkowski, N.; Piancastelli, M. N. Surf. Sci. 2004, 562, 65–72. (5) Witkowski, N.; Hennies, F.; Pietzsch, A.; Mattsson, S.; Fo¨hlisch, A.; Wurth, W.; Nagasono, M.; Piancastelli, M. N. Phys. ReV. B 2003, 68, 115408. (6) Piancastelli, M. N.; Bao, Z.; Hennies, F.; Travnikova, O.; Ce´olin, D.; Kampen, T.; Horn, K. Electronic and geometric structure of methyl oxirane adsorbed on Si(100)2 × 1; 2007; pp 108-112. (7) Cranney, M.; Comtet, G.; Dujardin, G.; Kim, J. W.; Kampen, T. U.; Horn, K.; Mamatkulov, M.; Stauffer, L.; Sonnet, P. Phys. ReV. B 2007, 76, 075324. (8) Kim, J. W.; Carbone, M.; Tallarida, M.; Dil, J. H.; Horn, K.; Casaletto, M. P.; Flammini, R.; Piancastelli, M. N. Surf. Sci. 2004, 559, 179–185. (9) Hovis, J. S.; Liu, H.; Hamers, R. J. Surf. Sci. 1998, 402-404, 1–7. (10) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1–56. (11) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830–2842. (12) Kim, J. W.; Carbone, M.; Dil, J. H.; Tallarida, M.; Flammini, R.; Casaletto, M. P.; Horn, K.; Piancastelli, M. N. Phys. ReV. Lett. 2005, 95, 107601. (13) Shekhar, R.; Barteau, M. A. Surf. Sci. 1996, 348, 55. (14) Shekhar, R.; Barteau, M. A.; Plank, R. V.; Vohs, J. M. Surf. Sci. 1997, 384, L815–L822. (15) Weinelt, M.; Zebisch, P.; Steinru¨ck, H. P. Surf. Sci. 1993, 287/ 288, 471. (16) Grosche, U.; Hamadeh, H.; Knauff, O.; David, R.; Bonzel, H. P. Surf. Sci. Lett. 1993, 281, L341. (17) Nieber, B.; Benndorf, C. Surf. Sci. 1991, 251/252, 1123. (18) Benndorf, C.; Nieber, B.; Kru¨ger, B. Surf. Sci. 1987, 189/190, 511. (19) Kim, J.; Zhao, H.; Panja, C.; Olivas, A.; Koel, B. E. Surf. Sci. 2004, 564, 53–61.

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