A Surface Core Level Shift Study of Hydrogen-Induced Ordered

Aug 20, 2008 - Valerio 2, 34127 - Trieste, Italy, Laboratorio Nazionale CNR ... Milano, Italy, and Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in A...
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J. Phys. Chem. C 2008, 112, 14475–14480

14475

A Surface Core Level Shift Study of Hydrogen-Induced Ordered Structures on Rh(110) Erik Vesselli,†,‡ Marco Campaniello,§ Alessandro Baraldi,*,†,‡ Laura Bianchettin,†,‡ Cristina Africh,†,‡ Friedrich Esch,‡ Silvano Lizzit,| and Giovanni Comelli†,‡ Physics Department and Center of Excellence for Nanostructured Materials, UniVersity of Trieste, Via A. Valerio 2, 34127 - Trieste, Italy, Laboratorio Nazionale CNR TASC-INFM, Area Science Park, SS 14 km 163.5, 34012 - BasoVizza (Trieste), Italy, Physics Department, Politecnico di Milano, p. L. da Vinci 32, 20133 - Milano, Italy, and Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34012 - Trieste, Italy ReceiVed: April 10, 2008; ReVised Manuscript ReceiVed: July 07, 2008

The interaction of hydrogen with Rh(110) was studied by means of high-resolution core level photoelectron spectroscopy, in combination with temperature-programmed desorption spectroscopy, low energy electron diffraction, and scanning tunneling microscopy measurements. By exploiting the H-induced Rh3d5/2 surface core level shift sensitivity to the local coordination environment, we propose here new structural models for the intermediate coverage 0.67 and 1.0 ML overlayers, while we confirm the previous results for the low coverage structures at 0.33 and 0.5 ML, and for the saturation phase at 2.0 ML. Surface core level shift values are interpreted on the basis of our previous data about the hydrogen adsorption on the (100) and (111) rhodium surfaces. The proposed structural models are supported by scanning tunneling images of the surface and provide a comprehensive description of all the available experimental data. Introduction Reconstruction, relaxation, and faceting phenomena induced upon hydrogen chemisorption, i.e. local and long-range hydrogeninduced restructuring of solid surfaces, are key steps for the understanding of the H surface chemistry but difficult to tackle experimentally. A wide range of surface structure-sensitive techniques has been developed to probe the geometrical, electronic, and chemical properties of the hydrogen-substrate complex.1,2 Besides the diffraction-based techniques such as low energy electron diffraction (LEED)3 and helium atom scattering (HAS),4 which are extensively employed to investigate the surface structural properties of H-covered surfaces, also spectroscopic probes such as high-resolution electron energy loss spectroscopy5 are commonly used to determine H adsorption sites.6,7 However, the small scattering cross-section of H atoms, almost 2 orders of magnitude below that of metal atoms, and the high surface mobility that often limits the formation of ordered phases are the most relevant experimental constraints in the application of these methods. A further problem is the difficulty to probe, when the strong overlap between the H 1s orbital and the metal wave functions takes place, electronic structure modifications in the first-layer atoms induced by H adsorption. In recent years, thanks to the improved energy resolution and surface sensitivity achieved with synchrotron radiation at soft X-ray wavelengths, core level photoelectron spectroscopy has been successfully used to probe hydrogen interaction with several metal surfaces, as in the case of beryllium8,9 and rhodium.10,11 The shift of the substrate core levels, induced by H adsorption, has been used to identify in a direct way the * Author to whom correspondence should be addressed. E-mail: [email protected]. † University of Trieste. ‡ Laboratorio Nazionale CNR TASC-INFM. § Politecnico di Milano. | Sincrotrone Trieste S.C.p.A.

adsorption sites and to evaluate the changes in the local environment of the substrate atoms. The adsorbate-substrate bond formation leads to changes in the density of states near the Fermi level (EF) and to the development of broad features, typically 5-10 eV below EF, which are reflected in the variation of the core electron binding energies of the first-layer atoms, giving rise to the so-called adsorbate-induced surface core level shifts (SCLS). A specific advantage of the SCLS approach is the possibility to detect surface morphological modifications without the requirement of long-range order and with adsorbate coverage that can be very low.12 In the case of H/Be(0001), the Be 1s surface core level shift was successfully applied to determine a phase diagram that could unequivocally determine vacancy formation and reconstructions, as well coordination assignments.9 Following a similar approach, by measuring the hydrogen-induced Rh 3d5/2 SCLS, it was possible to probe the hydrogen occupation of both 4-fold hollow and bridge sites on Rh(100).10 The potential of this method is even more relevant for the H-Rh(110) system, that is characterized by a large number of distinct sharp LEED patterns, corresponding to different commensurate phases. Detailed investigations by dynamical LEED, temperature programmed desorption (TPD), HREELS, and work function measurements have shown that hydrogen adsorption in 3-fold (TF) sites induces at low coverage a shift-buckling reconstruction of the Rh(110) surface.13-22 More specifically, hydrogen adsorption on Rh(110) yields the formation of five ordered structures with different periodicities for increasing coverage: (1 × 3) at 0.33 and 0.67 ML, (1 × 2) at 0.5 and 1.0 ML, and (1 × 1) at saturation (2.0 ML). In TPD spectra, hydrogen desorption is characterized by three distinct desorption features (R, β, γ) at 130-150, 220, and 270-300 K, showing zero, first, and second order desorption kinetics, respectively.21-23 Accurate structural determination of the lowest coverage layers, (1 × 3)0.33 ML and (1 × 2)0.50 ML, has been performed by using LEED I-V17,19 and HAS.6 In agreement with HREELS mea-

10.1021/jp803112q CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

14476 J. Phys. Chem. C, Vol. 112, No. 37, 2008 surements,7 it was found that H atoms are adsorbed in TF sites, being coordinated to two first layer and one second layer Rh atoms. The two structures are locally very similar: H atoms are aligned along the Rh rows in the [110] direction, yielding a buckling and a lateral displacement of the first-layer metal atom chains. As to the (1 × 3)0.67 ML and (1 × 2)1.0 ML structures, models have been proposed where hydrogen 2H (double) chains are formed. This was suggested by the first order desorption process of peak β, since a “molecule like” arrangement of hydrogen atoms, which are already paired, would explain that observation.22 The R desorption feature of the 2.0 ML structure, however, cannot be consequently explained by the same argument. This would rule out that a similar local hydrogen arrangement in double chains occurs in both cases. A further important feature for the discussion of the H adsorption structure regards the (1 × 3)0.67 ML phase that is formed in an activated process:22 the best long-range order is indeed obtained by annealing the layer at the desorption temperature boundary under hydrogen atmosphere. Finally, HREELS investigations lead to the conclusion that the formation of hydrogen layers at coverage above 0.5 ML yields to a drastic frequency increase of the mode polarized along the chain direction.7 In the present study we measured the Rh 3d5/2 core level shifts induced by hydrogen adsorption on Rh(110). Using the same model already adopted for other atomic adsorbates such as oxygen,24 nitrogen,25 and sulfur26 and by close comparison with data related to the (100) and (111) low index Rh surfaces,10-12 we confirm the previously proposed models for the (1 × 3)0.33 ML, (1 × 2)0.50 ML and (1 × 1)2.0 ML structures, while we find disagreement with the models reported for the (1 × 3)0.67 ML and the (1 × 2)1.0 ML layers. For these phases, we propose new structural models that are compatible with both previous literature data and our new photoemission results. Furthermore, we can explain the observed TPD features on the basis of the proposed structural models. Our interpretation is further supported by scanning tunneling microscopy (STM) measurements of the (1 × 3)0.67 ML and the (1 × 2)1.0 ML adlayers.

Vesselli et al.

Figure 1. (a) LEED patterns (E ) 104 eV) corresponding to the indicated structures, obtained before measuring the related TPD spectra reported below. For clarity, the (1 × 1) unit cell is depicted in the image of the clean surface pattern; (b) m/e ) 2 TPD spectra obtained after exposure of the clean Rh(110) surface to molecular hydrogen from the background at 90 K. The inset shows the desorption spectrum after preparation of the (1 × 2)1.0 ML structure at 90 and 160 K. In the latter case, better order is achieved and the R feature is no longer present in the spectrum. Heating rate is 2 K/s in all cases.

Experimental Section High-resolution X-ray photoelectron spectroscopy (XPS) experiments of the Rh 3d5/2 core levels were carried out at the SuperESCA beamline of ELETTRA in Trieste.27,28 The experimental chamber is equipped with a double pass 96-channel detector electron energy analyzer.29 Rh 3d5/2 core level spectra were collected using photon energies ranging from 370 to 430 eV, at an overall experimental resolution of 70 meV (X-ray monochromator + energy analyzer). The base pressure in the UHV experimental chamber was 1 × 10-10 mbar. TPD experiments were performed in a multipurpose apparatus with a base pressure of 5 × 10-11 mbar, equipped with a LEED optics residual gas analyzer for desorption and reactivity measurements, a conventional Mg KR X-ray tube, a monochromatic Al KR source, and a VG MKII electron energy analyzer. In both systems, the sample was mounted on a four degrees of freedom manipulator, cooled with liquid nitrogen, and heated by irradiation and electron bombardment from a hot tungsten filament. The sample was cleaned following standard procedures by repeated Ar+ sputtering cycles at 2.5 keV, annealing up to 1300 K, oxygen treatments, and hydrogen reduction. Surface cleanliness was checked by measuring the C and O 1s, S, and Si 2p XPS signals, as well as by comparing (in the synchrotron radiation experiment) the Rh 3d5/2 SCLS value with data for a clean (110) surface from previous results.30 Surface order was monitored by LEED.

Rh 3d5/2 core level spectra were analyzed, after normalization and linear background subtraction, by means of a least-squares fitting procedure using a Doniach-Sˇunjic´ function,31 convoluted with a Gaussian envelope. The former function is described by the Anderson singularity index, correlated to the electron-hole pairs excitation, and by the Lorentzian width, depending on the core hole lifetime. The Gaussian broadening accounts for instrumental resolution, phonon broadening, and surface inhomogeneity. Microscopy experiments were carried out with an Omicron VT-STM in a dedicated UHV system, equipped with standard sample preparation and characterization facilities (sputter gun, LEED imaging, etc.). Base pressure was 1 × 10-10 mbar. Hydrogen surface coverage is reported in monolayer (ML) units throughout the work. One ML corresponds to one adsorbed atom per surface Rh atom. Experimentally, H coverage has been calibrated by means of TPD and LEED measurements assuming the (1 × 3)0.33 ML and the (1 × 2)0.50 ML structures as a reference on the basis of previous literature.6,17,19 Results Prior to the photoemission measurements we have performed a first set of LEED (Figure 1a) and TPD (Figure 1b) measurements in order to obtain a preliminary characterization of the

Hydrogen-Induced Ordered Structures on Rh(110)

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Figure 3. Intensities of the Rh 3d5/2 core level shifted components from a real-time uptake experiment at 90 K. Dotted lines indicate the coverage of the ordered structures.

Figure 2. High-resolution Rh 3d5/2 core level spectra for the clean surface and the five ordered adlayers formed upon hydrogen adsorption at 90 K. Photon energy is 370 eV.

system. In Figure 1b we plot the complete sequence of m/e ) 2 TPD spectra obtained upon exposure of the clean surface at 90 K to increasing doses of molecular hydrogen. We observe the progressive development of three desorption features, previously indicated as R, β, and γ,21-23 and related to peaks at 130-150, 220, and 270-250 K, respectively. We confirm a clear fingerprint of a zero-order desorption process for the low temperature R feature,22 which develops at a surface coverage higher than 0.85 ML when the uptake is performed at 90 K, and first and second order desorption for the β and γ peaks, respectively. The 1.0 ML hydrogen layer is characterized by the presence of the R peak in the TPD spectrum and a (1 × 2) periodicity in the LEED pattern. We note that the formation of this high coverage (1 × 2)1.0 ML phase is thermally activated: an improved long-range order, as judged from the full width at half-maximum of the diffracted beams, is indeed obtained upon exposure of the clean surface to hydrogen at 160 K, in agreement with previous findings.22 A comparison of the corresponding TPD spectra (see inset of Figure 1b) reveals that the R peak is not related to the (1 × 2)1.0 ML structure but exclusively to the (1 × 1)2.0 ML adsorption configuration, thus indicating that (1 × 1)2H islands are likely to be present in the poorly ordered (1 × 2) structure when hydrogen is adsorbed at 90 K. The β peak is due to desorption from the (1 × 3)0.67 ML and (1 × 2)1.0 ML structures, while the γ peak is related to the two low coverage (1 × 3)0.33 ML and (1 × 2)0.50 ML structures. The Rh 3d5/2 core level spectra collected at 90 K after preparation of the corresponding overlayers are reported in Figure 2. We show the experimental data (black dots) together with the best fit decomposition of the bulk (gray line) and the surface components (color filled). The clean surface spectrum can be accurately fitted using two peaks, due to the bulk and the terminal layer atoms, centered at 307.150 (B) and 306.475 eV (S0), respectively, yielding a SCLS of -675 ( 25 meV, in

very good agreement with previous findings.30 Upon hydrogen adsorption for the formation of the low coverage (1 × 3)0.33 ML and (1 × 2)0.50 ML ordered structures, two additional features develop in the photoemission spectra at the expense of the clean surface component S0. A low binding energy peak (S′0) grows at -765 ( 25 meV from the bulk peak, while another feature (S1) is centered at -505 ( 25 meV from B. By increasing the hydrogen coverage, a fifth component (S2) grows at -285 ( 25 meV from B. The new components are shifted by +170 meV (S1), +390 meV (S2) and -90 meV (S′0), with respect to S0. In Figure 3 we plot, as a function of the hydrogen coverage, the intensities of the core level shifted components obtained by means of a real-time uptake experiment at 90 K. At 0.33 ML each of the S0, S′0, and S1 components accounts for about 1/3 of the total surface peak intensity. At 0.50 ML, the clean surface component has almost vanished, while the S′0 and S1 account each for 50% of the residual intensity. At 0.67 ML, S1 is about twice as high as S′0. At completion of 1.0 ML, the S1 component constitutes the majority of the surface core level intensity, while the S2 starts to appear. Only at higher hydrogen coverage, toward saturation, the intensity of S2 becomes significant at the expense of S1. The intensity trends of the Rh 3d5/2 surface core level shifted components are the key factor for the comprehension of the geometries of the ordered structures. In parallel, the values of the H-induced SCLS provide significant information about the hydrogen adsorption sites, as discussed in the following section. Discussion As we have already extensively proven, the measurement of the SCLS induced by hydrogen adsorption is indeed a tool for hydrogen adsorption site determination that can be conveniently applied when conventional surface science probes lack a direct sensitivity.10,11 Hydrogen coordination can be deduced on this basis,12,13 provided that final state effects in the photoemission process are small with respect to the initial state contribution, as is the case for rhodium and other metal surfaces. This means that the contribution to the core level shift by the geometric, chemical, and electronic configuration of the probed Rh atoms is dominant with respect to the electronic effects related to the screening of the core-hole, which is created by the photoionization process. The value of the additional shift of a surface component due to the bonding with H is then, to a first approximation, proportional to the coordination defined as the number of H atoms coordinated to the same substrate atom divided by the number of substrate atoms shared by each H atom.24 In Figure 4 we report the SCLSs for hydrogen adsorption on the (100) and (111) low index Rh surfaces,10,11 yielding a

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Figure 4. Relative shifts of the Rh 3d5/2 core level induced by hydrogen adsorption with respect to the clean surface peak. Data are reported for the (100),10 the (111),11 and the (110) Rh surfaces (this work). The dashed line indicates the best linear fit, yielding a relative shift of 300 meV/H atom.

Figure 5. Ball models for the five ordered single, coupled, and double chain structures formed by hydrogen on Rh(110).

shift of 300 ( 10 meV/H atom. The values found in the present study for the (110) plane are also shown in the same plot. On this basis, the S1 and S2 components can straightforwardly be assigned to shifts induced by hydrogen adsorption in TF sites, corresponding to 2/3 and 4/3 coordination, respectively. The surface Rh atoms associated with the S1 component are thus coordinated to two H atoms in a TF site, while the S2 component is due to coordination to four hydrogen atoms in TF site, as shown in Figure 4. On the basis of the considerations above, we can now focus on each of the hydrogen-induced structures on Rh(110). Concerning the low coverage (1 × 3)0.33 ML structure, several detailed studies agree in assigning to hydrogen the occupation of TF sites on the same side of one out of three [110] Rh rows,19,21-23 as depicted in Figure 5. In our analysis (see Figure 3) we find that 1/3 of the first-layer Rh population is bound to two H atoms adsorbed in TF sites (single chains), in good agreement with this model. Hydrogen desorption from this structure is associated with the second order γ peak in the TPD

Vesselli et al. spectra, typical for a surface diffusion, recombination, and desorption process. Of the remaining clean metal surface atoms, one-half yields a negative shift in the photoemission spectra (S′0). This can be explained as due to the relaxation of the clean Rh row adjacent to the H-atoms. Indeed, while on the clean surface the first-tosecond layer distance is contracted by 6.9% and the hydrogenfree Rh rows of the (1 × 2)0.50 ML structure relax only by 5.5%.17 This 1.4% difference can be viewed as a decreased coordination of the clean Rh atoms. It is well-known that changes in the interatomic distances strongly influence core level binding energies, as in the case of lattice-strain in clusters33 or surface thermal expansion.34,35 Also in the case of the (1 × n) missing row reconstructed phases on Rh(110), the first-layer atoms produce surface core level shifted components which vary from -715 to -685 meV,30 this difference being most probably due to changes in the local relaxation. We have recently shown that for a large set of nonequivalent Rh configurations the position of the different Rh 3d5/2 core level components is correlated with the calculated changes of the individual interatomic bond lengths.36 In the present case, the 1.4% different relaxation is expected to contribute with an additional negative shift of the core level binding energy, as found in the experiment. The case of the ordered structure at 0.50 ML is similar to that at 0.33 ML. In the (1 × 2)0.50 ML the local adsorption geometry is exactly the same of the (1 × 3)0.33 ML. The same arguments concerning the core level shifted components and the γ feature in the desorption spectra apply also to this case, again in agreement with the literature.19,21,22 It has to be noted that the S0 component has almost vanished in the spectra and S′0 and S1 dominate. This is in agreement with the model of Figure 5, where only the relaxed clean Rh rows and the H-bonded Rh rows (single chains: each Rh coordinated to two H atoms) are present. At 0.67 ML, the main H-induced core level shifted component is still S1, thus clearly indicating that the local coordination of Rh to hydrogen atoms is unchanged. Therefore, the previously reported structural model for this layer21,22 is not compatible with our experimental data. It was indeed proposed that in the (1 × 3)0.67 ML and (1 × 2)1.0 ML structures, the hydrogen atoms are paired in TF sites along the Rh rows, yielding a local surface coverage of 2.0 ML (double chains). The distribution of these Rh-2H rows along the [001] direction would then originate the ×2 periodicity of the structure at saturation by alternating Rh2H rows (with the same local structure as in the (1 × 1)2.0 ML) and clean Rh rows. Our results indicate that this is not the case, since the corresponding CLS data show no evidence of firstlayer Rh atoms with coordination 4/3. It is worth noting that the intensity of the S′0 of the (1 × 3) structure, measured after dosing hydrogen at 90 K (Figure 2), presents a quite low intensity compared to the spectrum of the equivalent dose in the real-time uptake experiment (see Figure 3). This difference can be due to postdose adsorption from the residual hydrogen background pressure for the spectrum in Figure 2 or to kinetic limitation in the formation of the structure which is known to be a thermally activated process. Indeed a well-defined (1 × 3)0.67 ML phase develops only upon annealing at the desorption boundary.22 We propose two alternative structural models for the (1 × 3)0.67 ML (Figure 5, parts a and b), where the local H adsorption geometry and Rh coordination are exactly the same as in the (1 × 3)0.33 ML and (1 × 2)0.50 ML structures. In the corresponding TPD spectrum, the β feature is associated with this structure, showing first order desorption kinetics. This typically occurs

Hydrogen-Induced Ordered Structures on Rh(110)

Figure 6. STM images of the (1 × 3)0.67 ML and (1 × 2)1.0 ML structures. (a) 150 × 150 Å2 image of the (1 × 2)1.0 ML structure after hydrogen saturation at 160 K; (b) (1 × 3)0.67 ML structure (54 × 70 Å2) with (1 × 2)1.0 ML defects; (c) line profile along the [001] direction across neighboring (1 × 3)0.67 ML and (1 × 2)1.0 ML stripes (80 × 33 Å2 image): the vertical dashed lines indicate the expected positions of the Rh atoms of the clean surface. Images were collected at Vbias ) 0.4 V(a), 0.6 V(b), 0.6 V(c), I ) 1 nA and T ) 140 K.

when the recombinative desorption process originates from atom pairing, as suggested in refs 21 and 22. From this point of view, the (1 × 3)0.67 ML-b model, where H atom pairing occurs across the troughs (coupled chains), appears to be the one consistent with both our XPS and TPD data and the observed shifts of the HREELS losses.7 The structure at 1.0 ML, indicated as (1 × 2)1.0 ML, yields exclusively the S1 component. By applying the same arguments presented for the (1 × 3)0.67 ML structure, we propose for this layer the model in Figure 5, in agreement with all the available data. Again, also this hydrogen layer contributes to the β desorption peak in the TPD spectra. Our conclusions for the (1 × 3)0.67 ML and (1 × 2)1.0 ML structures are further supported by STM images. In Figure 6(a) we show a (1 × 2)1.0 ML structure, characterized by protruding stripes oriented in [110] direction on a uniform background. In Figure 6(b), (1 × 2)1.0 ML stripes are shown to coexist with (1 × 3)0.67 ML stripes at the transition between both structures. As can be seen, the appearance of the (1 × 3)0.67 ML structure is characterized by a similar two-level pattern as the (1 × 2)1.0 ML, with a periodicity of three lattice constants in the [001] direction and with the addition of a dark row in the middle. The arrow in the figure points to a bright row extending continuously from the (1 × 2)1.0 ML into the (1 × 3)0.67 ML, suggesting the presence of the same structural feature in both

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14479 cases. The only structural elements coming into consideration, both in the (1 × 2)1.0 ML and in the (1 × 3)0.67 ML models, are the hydrogen atoms in coupled chains, that are hence associated with the bright stripes. Figure 6c shows a line profile along the [001] direction across both structures. The vertical dashed lines indicate the expected positions of the Rh atoms of the clean surface, according to the interpretation above. Consequently, the dark line in the image of the (1 × 3)0.67 ML structure corresponds to the clean [110] rhodium row, the half-tone to rhodium rows coordinated to H atoms and the bright rows to hydrogen coupled chains. These considerations confirm that the only suitable structural model for the (1 × 3)0.67 ML structure is the (1 × 3)b depicted in Figure 5. The fact that the H atoms are imaged as protruding is not necessarily due to a true topographic contrast but might be due to relaxation of the H atoms under the tip. A typical example is the imaging of STM in the case of H adsorbed on Rh(100),32 where the interaction with the microscope tip gives misleading results. In contrast to the clean rhodium rows and to the hydrogen coupled chains, the rhodium rows coordinated to H atoms could not be resolved as distinct lines. This might be explained by interactions with the tip, by strong changes in the density of states induced by the hydrogen adsorption, or by the limited resolution of these STM images. Finally, concerning the high coverage (1 × 1)2.0 ML phase, our data are consistent with the previous structural model proposed in refs 21 and 22, illustrated in Figure 5. At saturation, which is difficult to obtain and requires very large H2 exposure at low temperature, this structure is characterized by a single H-induced core level shifted Rh component (S2), originating from Rh atoms with local coordination 4/3. Moreover, TPD spectra indicate a zero-order desorption mechanism, typical for 2D gases with very fast exchange and equilibration with islands, making the desorption rate independent of the coverage. Thus, islands of (1 × 1)-2H (double chains) coexist on the surface with (1 × 2)1.0 ML regions on which the 2D gas forms by fast diffusion of additional hydrogen atoms (evident from the S1 and S2 core level intensities in Figure 3). This is exactly what happens when preparing a not well ordered (1 × 2)1.0 ML structure, for example when dosing hydrogen at 90 K (see inset of Figure 1). In this case we indeed observe the growth of the R desorption feature in the TPD spectra already before the (1 × 2) structure is completed (see Figure 1 inset). Conclusions We demonstrated how the Rh 3d5/2 surface core level shifts, measured as a function of the hydrogen exposure, unveil the structure of ordered hydrogen layers on Rh(110) and the local coordination environment. The previous models for the (1 × 3)0.33 ML, (1 × 2)0.50 ML, and (1 × 1)2.0 ML structures are confirmed, while we propose on the basis of our SCLS results new arrangements for the (1 × 3)0.67 ML and (1 × 2)1.0 ML layers. The proposed structural models conceal the new photoemission data and previous experimental findings and exclude the formation of Rh-2H rows (double chains), while the H atom pairing of adjacent Rh rows across the troughs occurs in coupled chains. Acknowledgment. We acknowledge financial support from Fondazione CRT Trieste, in the framework of a research grant assigned to G. Comelli. Technical support from Osram SpA is gratefully acknowledged. References and Notes (1) Christmann, K. Prog. Surf. Sci. 1988, 9, 1.

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