Oxidation of Iron Deposited on PolycrystallineAluminum Surfaces

The interaction of oxygen with iron deposited on polycrystalline aluminum surfaces has been investigated using AES, XPS, and ARXPS. The growth of the ...
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J. Phys. Chem. B 2001, 105, 10805-10811

10805

Oxidation of Iron Deposited on PolycrystallineAluminum Surfaces C. Palacio* and A. Arranz Departamento de Fı´sica Aplicada, Facultad de Ciencias, C-XII, UniVersidad, Auto´ noma de Madrid, Cantoblanco, 28049-Madrid, Spain ReceiVed: February 9, 2001; In Final Form: June 29, 2001

The interaction of oxygen with iron deposited on polycrystalline aluminum surfaces has been investigated using AES, XPS, and ARXPS. The growth of the iron on the aluminum surfaces occurs in two stages: formation of FeAl islands 10-ML thick up to a coverage of θFeAl≈0.65, followed by the formation of metallic iron islands 8-ML thick that grow over the intermetallic islands previously formed. For surfaces containing FeAl islands alone, the chemical information obtained by the analytical techniques shows that oxygen exposure causes the formation of aluminum intermediate oxidation states Al2+ and Al1+, in addition to Al3+, which are attributed to the formation of Al-O-Fe cross-linking bonds at the interface. The analysis of the Fe 2p peak shape shows that no iron oxide is formed, the small changes observed in this band being attributed to Fe atoms in an aluminum depleted layer at the interface. The oxidation of surfaces containing iron islands leads to results consistent with the formation of an Fe2O3 oxide film that grow over a film containing a mixture of intermediate aluminum and iron oxidation states.

Introduction Intermetallic surfaces have attracted great attention during recent years as a consequence of the important changes observed in the physical and chemical properties of these systems with respect to the properties of the single elements.1-5 Changes in the reactivity, catalytic, and magnetic properties of ultrathin metallic multilayers are also well documented in the literature. In general, they are attributed to the formation of intermetallic compounds on the surface, as well as to the low dimensionality of these systems.1-9 Because many oxide catalysts are binary complex oxides, the growth and characterization of ultrathin mixed oxide layers are very important. Metal oxide thin films at nanometer scale have been prepared during the oxidation of metal/metal interfaces3-5,10-13. In particular, iron oxide thin films have been grown on several surfaces. These films possess new surface structures and a chemical reactivity different from that of the bulk oxide.3,12,14,15 In fact, the surface electronic properties of metal oxides can be altered by metal impurities in the materials or by adding metallic adsorbates to the surfaces13,16,17. An interesting group of binary oxides used in catalytic and magnetic applications is that of the oxides formed in the FeAl system14,18,19. Likewise, the surface oxide on bulk Fe-Al alloys plays an important role in high-temperature applications because it determines the corrosion resistance and bonding properties of the alloy. Results published recently by H. Graupner et al.20 on the oxidation of FeAl monocrystals at room temperature indicate the formation of an amorphous oxide film on top of an Al-depleted interlayer. However, no studies concerning the initial stages of the oxidation of the bimetallic system formed at the Fe/Al interface have been performed to our knowledge. The formation of the Fe/Al interface during the deposition of Fe on polycrystalline Al at room temperature has been already * To whom correspondence should be addressed. Fax: ++34 91 3974949. E-mail: [email protected]

studied by A. Arranz et al..21 The purpose of the present work is to chemically characterize the oxide films formed at room temperature on different Fe/Al interfaces exposed to low oxygen pressure and to relate the morphological and compositional details of the different interfaces to the composition of the thin oxide film formed. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and angle resolved X-ray photoelectron spectroscopy (ARXPS) are being used to check in-situ the formation of the oxide film as a function of oxygen exposure. Experimental Section High-purity aluminum substrates (nominal composition 99.994% Al, 0.004% Si, 0.001% Fe and 0.001% Cu) manufactured by Toyo Aluminum Co., Japan, were used throughout this work. Before being introduced into the vacuum chamber, the high-purity aluminum substrates were degreased by successively boiling in carbon tetrachloride, acetone, and ethanol. Then, the sample was introduced into the UHV chamber. The aluminum substrates were sputter-cleaned in-situ with 3 keV Ar+ until no impurities were detected by AES. To minimize the surface roughness attained after sputter cleaning, the ion current density was kept below 2µA/cm2. Such a low current density develops small and uniform roughness that does not influence, from a qualitative point of view, the spectroscopic results. Experimental details for iron deposition and for AES and XPS characterization have been given in detail elsewhere,21 and will only be summarized here. Iron was deposited by sublimation of a directly heated filament of 99.5% purity onto high-purity Al substrates at room temperature. The iron deposition rate was ∼9.2 × 1014atoms•cm-2 min-1. Auger spectra were measured in the derivative mode using a cylindrical mirror analyzer (CMA) with a nominal resolution of 0.25%. A modulation voltage of 2Vp-p was supplied to the CMA. A constant primary electron beam current density of 1 × 10-3A/cm2 at 3 keV primary beam energy was used. XPS data were recorded using

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10806 J. Phys. Chem. B, Vol. 105, No. 44, 2001

Palacio and Arranz

Figure 1. Coverage of FeAl and Fe islands as a function of Fe deposition time on Al substrates. A schematic model of the Fe/Al interface formation is also given. The first stage of growth (top left panel) is characterized by the formation of FeAl islands of 10 ML thickness up to a coverage θFeAl ≈ 0.65. The second stage (bottom right panel) is characterized by the formation and growth of pure iron islands on the FeAl islands previously formed.

a hemispherical analyzer (SPECS EA-10 Plus). The pass energy was 15 eV giving a constant resolution of 0.9 eV. A twin anode (Mg and Al) X-ray source was operated at a constant power of 300W, using Mg KR (1253.4 eV) radiation. The oxidation was carried out at room temperature. For the oxidation experiments, oxygen 5 N quality, was introduced into the spectrometer chamber at a controlled partial pressure in the 10-8 to 10-5Torr range. The partial pressure of oxygen and the purity of the gas were controlled by means of a quadrupole mass analyzer. Results In an earlier work, the formation of the interface during the deposition of iron on polycrystalline Al substrates has been studied at room temperature using AES-FA, XPS and ARXPS.21 In that work, a two-stages mechanism for iron growth was found: a first stage characterized by the formation of FeAl islands 10 monolayers (ML) thick up to a coverage of θFeAl ) 0.65, followed by the formation of metallic iron islands 8 ML thick that grow over the FeAl islands formed previously. The simultaneous lateral growth of both kinds of islands is observed during the second stage. Figure 1 summarizes such a model, showing the coverages θFeAl and θFe as a function of the iron deposition time. As observed in Figure 1, no metallic iron is formed on the surface during the first stage (t e 11 min). However, both FeAl and Fe islands grow on the surface during the second stage. A schematic diagram of the model for the Fe/Al interface formation is also given in Figure 1. In our present work, different Fe/Al interfaces corresponding to both the first stage (t e 11 min) and the second stage (t > 11 min) of the Fe/Al interface formation, have been oxidized at room temperature. For the oxidation experiments oxygen exposure was varied up to 5000 L (1 L ≡ 10-6Torr•s). To characterize the thin oxide film formed, the Fe LMM and O KLL Auger transitions as well as the Al 2p, O 1s, and Fe 2p XPS bands, have been measured for different iron deposition times and subsequent oxygen exposure. Figure 2 shows the Fe LMM and O KLL Auger transitions of 5000 L oxide films measured for different interfaces. The AES spectra for Fe only exhibit important changes (indicated by arrows) when Fe/Al interfaces corresponding to the second stage (θFe >0) are exposed to oxygen. These changes can be attributed to the iron oxidation at this stage. The O KLL peaks

Figure 2. Derivative Fe LMM and O KLL Auger spectra of the 5000 L oxide films measured for different Fe/Al interfaces.

show a chemical shift as the iron content increases. This shift is clearly observed for interfaces corresponding to the second stage of iron growth. For an iron coverage θFe ) 0.9, the O KLL spectrum is similar to that measured on an iron substrate, labeled Fe in Figure 2, therefore indicating the iron oxidation. The Al 2p, O 1s, and Fe 2p XPS bands were measured for different Fe/Al interfaces at oxygen exposures up to 5000 L. Measured spectra of aluminum, oxygen, and iron, after background subtraction based on a modified Shirley method,22 are shown in Figure 3a-3c, respectively. Although oxygen exposures were carried out in the range 0-5000 L, for simplicity, only results corresponding to an oxygen exposure of 5000 L are given in Figure 3a-3c. As the oxygen exposure increases (not shown) a broad shoulder appears on the high binding energy (BE) side of the Al 2p band. Also, an attenuation of the metallic Al 2p band is observed. Results corresponding to an exposure of 5000 L show that the position of this shoulder shifts to the lower BE as the iron content increases. Figure 3b shows the measured spectra of the O 1s band for different iron deposition times. The band exhibits a shift from ∼532 eV for the Al substrate to ∼530 eV for pure Fe, indicating the presence of at least four components. On the other hand, the Fe 2p band show new peaks associated with FeO and Fe2O3 formation for iron deposition times above 11 min (θFe > 0). Below 11 min, the band shifts ∼0.2 eV to higher BE and losses its asymmetry, but these changes cannot be related to oxide formation. To determine the bands associated with the different Al, Fe, and O species by peak deconvolution, synthetic spectra and a least-squares optimization were used. Five synthetic bands were necessary to reproduce the Al 2p spectra in the whole range of iron deposition times and oxygen exposures. An asymmetrical Gaussian-Lorentzian function (GL) centered at 72.7 eV and 1 eV fwhm (full width at half-maximum); a symmetrical GL function centered at 72.3 eV and 0.9 eV fwhm and three bands centered at 73.9, 74.6, and 75.4 eV, respectively, and 1.8 eV fwhm. The Al 2p spectra of the clean interface are characterized by two bands centered at 72.7 and 72.3 eV, respectively. The band at 72.7 eV should be attributed to metallic aluminum and will be labeled Al0 and that at 72.3 eV should be associated with the intermetallic compound FeAl21 and will be labeled AlFe.

Iron on Polycrystalline Aluminum Surfaces

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10807

Figure 3. XPS spectra of the 5000 L oxide films measured for different Fe/Al interfaces. (a) Al 2p band; (b) O 1s band; (c) Fe 2p band.

TABLE 1: Relative Binding Energies (eV) with Respect to the Metallic Band (Al0) of the Aluminum Oxidation States Found in the Literature Marcus et al.23 McConville et al.24 Berg et al.25 refs 26-29 Faraci et al.30,31 this work

Al0 0 Al0 0 Al0 0 Al0 0 Al0 0 Al0 0

Alch1 0.5 Alch1 0.37 Al1+ 1.2 Al1+ 1.2

Alch2 1 Alch2 0.84 AlChem 1.4

Alch3 1.5 Alch3 1.37 Al2+ 2 Al2+ 1.9

Al3+ 2.7 Al3+ 2.5-2.7 Al3+ 2.66 Al3+ 2.7 Al3+ 2.8 Al3+ 2.7

Exposure to oxygen causes the appearance and development of new peaks at 73.9, 74.6 and 75.4 eV, respectively. There is a controversy on the assignment of these peaks. Table 1 shows a compilation of the different Al 2p bands found during Al oxidation, referred to the metallic Al0 band. Marcus et al.23 found

only two peaks during Al oxidation. They interpreted the highest BE peak as due to Al2O3. McConville et al.24 and Berg et al.25 found five peaks. They interpreted the peak at highest BE as due to Al2O3 and the other peaks were attributed to aluminum bonded to one, two or three chemisorbed oxygen atoms, respectively. Flodstro¨m et al.26-28 and Norman et al.29 found a peak in addition to those associated with the metal and the oxide, and they interpreted this peak as due to aluminum bonded to chemisorbed oxygen. Faraci et al.30,31 found peaks at 1.2, 2 and 2.8 eV above the metallic Al 2p peak during aluminum deposition on SiO2 and graphite substrates in the presence of atomic oxygen. They interpreted these peaks as due to the 1+, 2+ and 3+ oxidation states. In present work three peaks at 1.3, 1.9 and 2.7 eV above Al0 were observed, in addition to Al0 and AlFe, for oxygen exposed interfaces. Therefore, they could be attributed to the 1+, 2+, and 3+ oxidation states. However, the presence of the peaks at 1.2 and 1.9 eV for exposures 11 min), the subtracted spectra show the formation of additional chemical states. In this figure, the features associated with the formation of Fe2+(FeO) and Fe3+(Fe2O3) have been indicated by arrows.32,33,36,37 Moreover, for lower oxygen exposures (∼4-20L), an additional broad feature, denoted as Fex+, can be observed at ∼707.4 eV in the subtracted spectra. Because both features, Fex+ and Fe0+, are close and Fex+ is much broader than Fe0+ no attempt to use the Fe0+ band in the peak-fitting procedure, for deposition times corresponding to the second stage of growth, has been done. Therefore, for t > 11 min, the band denoted as Fex+ should involve information associated with the formation of any FeOx (x < 1) suboxide, with oxygen chemisorption during the first stages of the oxidation, and with Fe atoms in the aluminum-depleted layer at the interface. The corresponding synthetic peaks are symmetrical GL functions for Fe0+ and Fex+ bands and asymmetrical GL for FeO and Fe2O3. The parameters defining the synthetic bands are given in Table 2. For the FeO and Fe2O3 species, the Fe 2p3/2 binding energy (Eo), the Fe 2p spin-orbit splitting (sos), the fwhm of the two bands of the doublet, w3/2 and w1/2, and the Fe 2p3/2/2p1/2 area ratio, R, are consistent with reference spectra and values reported in the literature32,33,36-38. Information on Fe0+ and Fex+ species is not available in the literature. Examples of the deconvolution carried out for Al 2p, O 1s and Fe 2p spectra, are given in Figure 3a-3c. Figure 5 shows (a) the evolution of the Al, O and Fe XPS peak areas as a function of the oxygen exposure, for an iron deposition time of 4 min, that corresponds to the first stage of the Fe/Al interface formation. In addition, in (b) the evolution

Figure 5. Normalized intensities (normalization is explained in the text) as a function of oxygen exposure of Al 2p, O 1s, and Fe 2p signals for (a) an Fe/Al interface corresponding to the first stage of iron deposition on aluminum; (b) an Fe/Al interface corresponding to the second stage of deposition.

of the same XPS signals as a function of the oxygen exposure for an iron deposition time of 16 min, corresponding to the second stage of iron deposition, is also given. The intensities I (peak areas) are normalized to the corresponding sensitivity factors, SAl ) 0.11, SO)0.63, and SFe ) 3.8.39 On the other hand, Figure 6 shows the evolution of Al, O, and Fe XPS signals as a function of the iron deposition time, for an oxygen exposure of 5000 L. Figures 5 and 6 show that the composition of the thin oxide film formed depends on both the iron deposition time, and the oxygen exposure. To obtain additional information on the in-depth distribution of the different Al and Fe species, ARXPS measurements were carried out for all Fe/Al interfaces exposed to 5000 L. Figure 7 shows different peak area ratios between Al and Fe species, as a function of the takeoff angle for (a) t ) 4 min (first stage of

Iron on Polycrystalline Aluminum Surfaces

Figure 6. Normalized intensities as a function of the iron deposition time of Al 2p, O 1s, and Fe 2p signals measured for the 5000 L oxide films.

Figure 7. Variations of different XPS peak area ratios as a function of takeoff angle for a 5000 L oxide film grown on (a) an interface corresponding to the first stage of deposition (t ) 4 min); (b) an interface corresponding to the second stage (t ) 28 min).

Fe deposition), and (b) t ) 28 min (second stage). In this figure, Alox corresponds to the Al1++Al2++Al3+ contributions, and Fesubox is the sum of Fex+ and FeO contributions. As discussed below, a comparative analysis of the evolution of these ratios will give qualitative information on the in-depth composition of the oxide film formed. Discussion Because the deposition of iron on aluminum is characterized by a two-stages mechanism for the interface formation, differences are expected for the oxidation of the interfaces in both stages.

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10809 For the Fe/Al interfaces corresponding to the first stage, neither of the species associated with iron oxidation, OFe, Fex+, FeO, or Fe2O3, are observed during oxygen exposure. The curves of Figure 5a related to Al1+, O2 and Fe0+ species exhibit a sigmoidal shape showing a change of slope at approximately 1 L and at 10-20 L followed by a region of saturation slowly decreasing. On the other hand, the O1 and Al2+ signals show a maximum at ∼6 L and at ∼40-100 L, respectively. Above the maximum both signals strongly decrease. In fact, O1 completely disappears for high oxygen exposures. The signals related to the stoichiometric Al2O3 (Al3+ and O3) appear at approximately 4 L and continuously increase above this oxygen exposure. Sigmoidal shapes of the kinetic curves have been also observed for pure Al40 and other metals41,42 during the first stages of oxidation. This behavior is commonly explained by using an oxidation model that involves chemisorption of oxygen, oxide islands nucleation and growth until coalescence, and thickening of the oxide film. It is interesting to notice that H. Graupner et al.20 observed only two different XPS bands for Al, in addition to the metallic one, during the oxidation of FeAl monocrystalline surfaces. The band at the highest BE was assigned to Al2O3, whereas a second band, shifted 0.7-1.6 eV to higher BE with respect to the metallic peak, was related to Al atoms at the alloy/oxide interface having lower coordination number with oxygen atoms. Intermediate oxidation states were also observed during the first stages of oxidation of NiAl and Ni3Al compounds43-45 and were attributed to precursor oxide species on the outer surface of the oxide and partially formed by Ni atoms that have not been able to diffuse inside the substrate.44 Also other interpretations, such as the presence of defects in the oxide film,43 or Al atoms at the oxide/alloy interface bonded to the Ni of the alloy substrate43,45 have been used to explain such states. In the present work, two intermediate oxidation states, Al1+ and Al2+, in addition to Al3+, are observed during the oxidation of FeAl interfaces, in good agreement with the results of Faraci et al.30,31 for pure Al. Furthermore, the O 1s band shows three features, O1, O2 and O3 following similar kinetic evolution that those of Al2+, Al1+ and Al3+bands, respectively. No comparable data have been reported in the literature. As observed in Figure 5a, not only Al3+ (Al2O3) but also Al intermediate oxidation states, Al1+ and Al2+, are formed with increasing oxygen exposures. Moreover, Figure 6 shows that the signals Al3+ and O3 decrease with increasing iron deposition time. This behavior can be related to the observed decrease of the available surface of the Al substrate, which is covered by FeAl islands, with increasing iron deposition time. This evolution is also accompanied by the formation of Al intermediate oxidation states, characterized by the Al1+, Al2+, O1, and O2 species (see Figure 6). During the first stage, all the deposited Fe would react to form the intermetallic compound, FeAl. Because no “free” metallic iron is available on the surface no chemical species associated with the iron oxidation, (OFe, Fex+, FeO, and Fe2O3) are expected to be produced during oxidation, in good agreement with observed experimental results. Only Fe0+ was identified, which should be attributed to Fe atoms in an Al depleted layer at the interface, in good agreement with the work of H. Graupner et al..20 These authors found the formation of a layer ∼1-2 Å of metallic iron at the alloy/oxide interface, during the room-temperature oxidation of FeAl monocrystals. The shift to lower BE in the nonmetallic part of the Al 2p band (see Figure 3a) may arise from a decrease in the positive charge on Al atoms in the Al2+ and Al1+ intermediate oxidation

10810 J. Phys. Chem. B, Vol. 105, No. 44, 2001 states as a consequence of the formation of Al-O-Fe crosslinking bonds at the interface. This interpretation is also supported by the presence of O2 and O1 bands at intermediate BE due to a decrease in the negative charge of the oxygen. The parallel evolution in the shift of the nonmetallic part of the Al 2p band and that of the O 1s band with increasing iron deposition time (figs. 3a-3b), suggests a decrease of the charge transfer from Al to O atoms at the interface as a consequence of the formation of cross-linking bond with Fe atoms. Such cross-linking bonds have been proposed by G. Lassaletta et al.35 to explain a similar behavior of the Ti 2p and O 1s bands during the deposition of thin TiO2 films evaporated on SiO2 substrates. Complementary information on the composition of the oxide film formed can be obtained from the ARXPS results in Figure 7a. The data can be interpreted by a simple and idealized model which considers the superposition of three layers: Aluminum oxide with Al3+ states on top of Al2+ and Al1+ states followed by a layer enriched in iron on top of the substrate. As indicated above, the second stage of the Fe/Al interface formation is characterized by the formation of metallic iron islands 8ML thick, that grow over the FeAl islands previously formed. The simultaneous lateral growth of both kinds of islands is also observed during the second stage. From Figures 3 and 5b, it is observed that the reaction of such a type of interfaces with low-pressure oxygen is characterized by the formation of OFe, Fex+, FeO, and Fe2O3 species. Also new features appear in the Fe LMM Auger transition (Figure 2) indicating the formation of different iron oxidation states. As observed in Figure 5b, which corresponds to the oxidation of an interface formed at the beginning of the second stage of growth, the first species detected are Al1+. For exposures above 1L Al2+, Fex+, and FeO are detected. The signal intensities of Al1+ and Fex+ reach smooth maxima at approximately 40 L. At this oxygen exposure, Al3+ and Fe2O3 appear. It is concluded, that at exposures above 40 L all the iron oxide species and those of aluminum oxide may probably coexist. It is interesting to point out that for iron deposition times above 22 min, when θFe is comparable to θFeAl, the evolution of the signal intensities (not shown) of Fex+, FeO and Fe2O3 as a function of the oxygen exposure is similar to that observed during the oxidation of a high-purity iron substrate, therefore indicating that the oxidation is dominated by the oxidation of the iron islands. Likewise, the ratio between FeO and Fe2O3 signal at 5000 L, calculated from Figure 6, decreases as the iron deposition time increases converging to the value observed for pure iron. XPS line shape analyses of Al 2p, O 1s and Fe 2p bands of Figure 6 indicate that the features O1, O2, O3, and OFe follow similar evolution that those of Al2+, Al1+, Al3+and FeO + Fe2O3 bands, respectively, with increasing iron deposition time. The Al1+, Al2+, O1, O2, and Fex+ species are observed, at high exposures, during the second stage of growth. This confirms that the Fex+ synthetic band, used for Fe 2p spectra deconvolution during the second stage, involves information associated not only with chemisorption and suboxides formation but also with the Fe atoms in the Al depleted layer at the interface. As observed in Figure 6 (bottom panel) only FeO and Fe2O3 are detected at high iron deposition times. The behavior is typical for an external layer of iron oxides covering the substrate. Additional insight into the film composition can be obtained from the ARXPS results in Figure 7b. These results show that the in depth distribution of species is consistent with the sequence Fe2O3, aluminum oxides (Al3+ + Al2+ + Al1+), iron suboxides (Fex+ + FeO) and substrate when going from the

Palacio and Arranz outer surface to the substrate. It must be pointed out that this model is idealized. In reality, concentration gradients are to be expected in the layers. In fact, at high iron deposition times, the aluminum oxide layer is mainly formed of intermediate aluminum oxidation states with Fe atoms incorporated in the network. Conclusions The oxidation of iron deposited on polycrystalline aluminum surfaces has been studied at room temperature and low oxygen pressures, using AES, XPS, ARXPS. The growth of the iron on the aluminum surfaces occurs in two stages. The reaction of the oxygen with the interfaces formed during the first stage leads to the formation of aluminum intermediate oxidation states Al2+ and Al1+, in addition to Al3+. These intermediate oxidation states are attributed to the formation of Al-O-Fe cross-linking bonds at the interface. The analysis of the Fe 2p band shows small changes that can be attributed to Fe atoms in an aluminum depleted layer at the interface. However, no iron oxide is detected. The oxidation of the interfaces formed during the second stage of iron growth can be described by a model postulating an outer film of Fe2O3 on top of a film containing a mixture of intermediate aluminum and iron oxidation states. Acknowledgment. The authors thank D. Dı´az for technical assistance. This work is a part of a research project supported by the Spanish Comisio´n Interministerial de Ciencia y Tecnologı´a (Project MAT99-0830-CO3-02). References and Notes (1) Rodriguez, J. A. Surf. Sci. Reports. 1996, 24, 223. (2) Ranga Rao, G.; Rao, C. N. R. J. Phys. Chem. 1990, 94, 7986. (3) Den Daas, H.; Passacantando, M.; Lozzi, L.; Santucci, S.; Picozzi, P. Suf. Sci. 1994, 317, 295. (4) Takehiro, N.; Yamada, M.; Tanaka, K.; Stensgaard, I. Surf. Sci. 1999, 441, 199. (5) Maeda, T.; Kobayashi, Y.; Kishi, K. Surf. Sci. 1999, 436, 249. (6) D′Addato, S.; Binns, C.; Finetti, P. Surf. Sci. 1999, 442, 74. (7) Kralj, M.; Pervan, P.; Milun, M. Surf. Sci. 1999, 423, 24. (8) Ealet, B.; Gillet, E.; Nehasil, V.; Moller, P. J. Surf. Sci. 1994, 318, 151. (9) Sotiropoulou, D.; Ladas, S. Surf. Sci. 1998, 408, 182. (10) Della Negra, M.; Sambi, M.; Granozzi, G. Surf. Sci. 1999, 436, 227. (11) Huang, H. H.; Jiang, X.; Siew, H. L.; Chin, W. S.; Sim, W. S.; Xu, G. Q. Surf. Sci. 1999, 436, 167. (12) Stierle, A.; Zabel, H. Surf. Sci. 1997, 385, 310. (13) Maetaki, A.; Yamamoto, M.; Matsumoto, H.; Kishi, K. Surf. Sci. 2000, 445, 80. (14) Gota, S.; Guiot, E.; Henriot, M.; Gautier-Soyer, M. Phys. ReV. B 1999, 60, 14 387. (15) Di Castro, V.; Ciampi, S. Surf. Sci. 1995, 331-333, 294. (16) Diebold, U.; Tao, H.; Shinn, N. D.; Madey, T. E. Phys. ReV. B 1994, 50, 14 474. (17) Andersson, S.; Bru¨hwiler, P. A.; Sandell, A.; Frank, M.; Libuda, J.; Giertz, A.; Brena, B.; Maxwell, A. J.; Ba¨umer, M.; Freund, H. J.; Mårtensson, N. Surf. Sci. 1999, 442, L964. (18) Hoffmann, D. P.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1991, 95, 5552. (19) Paparazzo, E.; Dormann, J. L.; Fiorani, D. Phys. ReV. B 1983, 28, 1154. (20) Graupner, H.; Hammer, L.; Heinz, K.; Zehner, D. M. Surf. Sci. 1997, 380, 335. (21) Arranz, A.; Palacio, C. Surf. Interface Anal. 2000, 29, 392. (22) Proctor, A.; Sherwood, M. P. A. Anal. Chem. 1982, 54, 13. (23) Marcus, P.; Hinnen, C.; Olefjord, I. Surf. Interface Anal. 1993, 20, 923.

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