The adsorption of sulfur on the platinum (100) surface

Clean platinum (100) surfaces of 1× 1 and 5 × 20 structure were exposed to H₂S. Surface coverage with Sulfur followed Langmuir kinetics, which, together with LEED data, points to a repulsive interaction between sulfur atoms. Sulfur adsorption causes a decrease in the work function of platinum by 0.7 eV at saturation coverage. This is attributed to polarization, rather than ionization, of the adsorbed sulfur. Photoemission measurements are difficult to interpret because of two-dimensional periodicity and the overlap of electronic structure of the adsorbate with the platinum d band. We observe peaks due to sulfur at 6.3, 4.5, and 2.5 eV below the Fermi level for the c(2 × 2) overlayer and at 6.8, 4.5, and 2.0 eV below E_F for the p(2 × 2) surface. A tentative interpretation in terms of sulfur orbitals is given. The decrease in work function and analogy with the properties of PtS₂ lead us to propose covalent bonding of sulfur to platinum, in which every sulfur atom is bonded to four Pt neighbors in both structures. The repulsive interaction between sulfur atoms is indirect through the platinum substrate.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.     

Initial stage of the adsorption of fluorofullerene molecules on Si surface

Spatially resolved images of an individual C₆₀F₁₈ fluorofullerene molecule on Si(100) − 2 × 1 surface have been obtained using scanning tunneling microscopy. Scanning tunneling microscopy results and ab initio calculations show that the fluorofullerene molecules interact with the Si(100) − 2 × 1 surface with F atoms pointing down towards the surface. The adsorption energy of a C₆₀F₁₈ molecule on Si(100) − 2 × 1 surface is ∼12.1 eV, which is much higher than the adsorption energy of the same molecule on Si(111) − 7 × 7 surface (6.65 eV). C₆₀F₁₈ molecules are located in the troughs in-between the dimer rows occupying the four-dimer site on Si(100) − 2 × 1 surface.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Theoretical studies of the bonding of sulfur to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for a S atom bonded to Ni clusters consisting of 1 to 4 atoms designed to model bonding to the Ni(100) surface. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. Modeling the (100) surface with four Ni atoms, we find the optimum S position to be 1.33 Å above the surface, in good agreement with the value (1.30 ± 0.10 Å) from dynamic LEED intensity calculations. The bonding is qualitatively like that in H₂S with two covalent bonds to one diagonal pair of Ni atoms. There is a S pπ pair overlapping the other diagonal pair of Ni atoms. [Deleting this pair the S moves in to a position 1.04 Å from the surface.] There are two equivalent such structures, the resonance leading to equivalent S atoms and a c(2 × 2) structure for the S overlayer. The Ni in the layer beneath the surface seems to have little effect (~0.03 Å) on the calculated geometry. Bonding the S directly above a single Ni atom leads to a much weaker bond (D_e = 3.32 eV) than does bonding in a bridge position (D_e = 5.37 eV).     

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N₇VV AES transition shows approximately 60% attenuation. Unlike for H₂ adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the $\text{1}\text{2}$-order LEED beams is strictly temperature dependent, and significant differences exist between the $\text{1}\text{2}$-order LEED spectra produced by cooling and those produced by H₂ adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I-V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (±h/2, ±h/2) are identical in relative intensity to the (±h/2, ±h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H₂ onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H₂ onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.     

Angular dependence of the electron energy loss spectra from a clean and adsorbate covered W(001) surface

A dispersion analysis of the EELS from a W(001) surface in the range 1 < ΔE < 35 eV has been performed and compared with recent and complete optical data for tungsten. The non-dispersive (k ～ 0) EELS correlated well with a combination of the surface and bulk loss functions calculated from the optical data. Losses at 1–5 eV and a pair at 32 and 34.5 eV were assigned to interband and N_6,7 core ionization excitations respectively. The principal bulk and surface plasmon losses were identified at 24.0 and 20.3 eV respectively. Two further losses at 14.0 and 9.6 eV were also observed and assigned to subsidiary plasmon losses. All four plasmon losses showed only minimal energy dispersion, never exceeding 1.5 eV. A momentum selectivity for separating bulk and surface interband losses was demonstrated with the non-dispersive losses arising from excitations within the bulk even with incident energies as low as 88 eV, whereas their dispersive counterparts were extremely sensitive to the chemical state of the surface. New adsorbate derived losses which develop during adsorption were associated with excitations from the new deep lying adsorbate levels to final state levels at or near the Fermi level. It was concluded that this final state was also responsible for the N_6,7 ionization losses.     

Molecular cluster calculations for the interpretation of CO chemisorption on a Ni(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Ni(100) surface are presented. In earlier calculations of this type the CO molecule has been assumed to be chemisorbed in a hollow position of C_4v symmetry. A recent EELS experiment shows however that in the most stable configuration CO is linearly bonded to the Ni atoms, i.e. a top position of the CO-molecule. This experiment indicates also that there exists an additional bridge bonding of the CO molecule to the two nearest neighbour Ni atoms. The variation of the energy levels, binding energies and charge distribution with the height of the CO molecule above the nickel surface is calculated for the top position using the NiCO and Ni₅CO clusters and for the bridge bonding configuration using the Ni₂CO cluster. The CO 1π level is found to be split by about 0.8 eV in bridge bonding geometry. For both hollow and top positions the 1π and 5σ levels are separated by about 0.5 eV. The energy separation to the 4σ level is about 3 eV, which is in good agreement with experimental data. Theoretical ionization energies relative to the Fermi energy for top position geometry at a bond distance of 3.5 au between the carbon atom and nickel surface were found to be 25.7, 11.7, 8.7 and 8.2 eV for the 3σ, 4σ, 5σ and 1π levels which should be compared with the experimental data of 29.0, 10.8, 8.4 and 7.8 eV, respectively. The corresponding ionization energies for a bond angle of 99° in bridge bonding were 23.7, 12.1, 7.3, 7.0 and 7.9 eV. The two last values represent the 1π level which is split into two levels in this geometry. The variation of the C-O stretch vibrational frequencies with the height of the CO molecule above the surface for the top-position geometry is estimated from the 5σ and 2π gross orbital populations and from the CO σ and π overlap populations.     

The chemisorption of N2 on the (110) surface of iridium

The molecular chemisorption of N₂ on the reconstructed Ir(110)-(1 × 2) surface has been studied with thermal desorption mass spectrometry, XPS, UPS, AES, LEED and the co-adsorption of N₂ with hydrogen. Photoelectron spectroscopy shows molecular levels of N₂ at 8.0 (5σ + 1π) and 11.8 (4σ) eV in the valence band and at 399.2 eV with a satellite at 404.2 eV in the N(1s) region, where the binding energies are referenced to the Ir Fermi level. The kinetics of adsorption and desorption show that both precursor kinetics and interadsorbate interactions are important for this chemisorption system. Adsorption occurs with a constant probability of adsorption of unity up to saturation coverage (4.8 × 10¹⁴ cm^?2), and the thermal desorption spectra give rise to two peaks. The activation energy for desorption varies between 8.5 and 6.0 kcal mole^?1 at low and high coverages, respectively. Results of the co-adsorption of N₂ and hydrogen indicate that adsorbed N₂ resides in the missing-row troughs on the reconstructed surface. Nitrogen is displaced by hydrogen, and the most tightly bound state of hydrogen blocks virtually all N₂ adsorption. A p1g1(2 × 2) LEED pattern is associated with a saturated overlayer of adsorbed N₂ on Ir(110)-(1 × 2).     

The GaP(001) surface and the adsorption of Cs

GaP(001) cleaned by argon-ion bombardment and annealed at 500°C showed the Ga-stabilized GaP(001)(4 × 2) structure. Only treatment in 10^?5 Torr PH₃ at 500°C gave the P-stabilized GaP(001)(1 × 2) structure. The AES peak ratio $\text{P}\text{Ga}$ is 2 for the (4 × 2) and 3.5 for the (1 × 2) structure. Cs adsorbs with a sticking probability of unity up to 5 × 10¹⁴ Cs atoms cm^?2 and a lower one at higher coverages. The photoemission measured with uv light of 3660 Å showed a maximum at the coverage of 5 × 10¹⁴ atoms cm^?2. Cs adsorbs amorphously at room temperature, but heat treatment gives ordered structures, which are thought to be reconstructed GaP(001) structures induced by Cs. The LEED patterns showed the GaP(001)(1 × 2) Cs structure formed at 180°C for 10 h with a Cs coverage of 5 × 10¹⁴ atoms cm^?2, the GaP(001)(1 × 4) Cs formed at 210°C for 10 hours with a Cs coverage of 2.7 × 10¹⁴ atoms cm^?2, the GaP(001)(7 × 1) and the high temperature GaP(001)(1 × 4), the latter two with very low Cs content. Desorption measurements show three stability regions: (a) between 25–150°C for coverages greater than 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.2 eV; (b) between 180–200°C with a coverage of 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.8 eV; (c) between 210–400°C with a coverage of 2.7 × 10¹⁴ atoms cm^?2, and an activation energy of 2.5 eV.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

Interaction of iron atoms with the oxidized silicon surface

The room-temperature interaction of iron atoms with the oxidized Si(100)2×1 surface at a coverage from a submonolayer to four monolayers is studied by core-level photoelectron spectroscopy using synchrotron radiation. Computer simulation of the Si 2p core electron spectra demonstrates that iron atoms penetrate beneath the silicon oxide even at room temperature. This process causes the initial silicon phases at the SiO_x/Si interface to disappear; gives rise to a complex ternary phase involving Fe, O, and Si atoms; and favors the formation of a Fe-Si solid solution at the interface.     

Dangling bond surface state of Si(111): New results

By angle resolved photoemission on Si(111) 2 × 1 surfaces, the main surface structure at ?0.85 eV below E_F is shown to exhibit a strong s?p_z character in contradiction with previous measurements. The similarity of the results on the 2 × 1 and 7 × 7 surfaces, together with theoretical calculations lead us to favor a buckled model for the 2 × 1 surface and a model based on ring like arrangements of lowered and raised atoms for the 7 × 7 surface.     

Interaction of aluminum with the rhenium surface: Adsorption, desorption, and growth of surface compounds

The interaction of aluminum with the ( $10\bar 10$ ) rhenium surface was studied experimentally within a broad temperature range, 300–2000 K. Surface aluminide (SA) ReAl with a concentration of adsorbed Al atoms N _Al=1.6×10¹⁵ cm^?2 was found to form. It was shown that aluminum escapes from the surface by thermal desorption at temperatures from 1300 to 1600 K, with the desorption activation energy changing abruptly from ～3.6 to ～4.2 eV when passing through the concentration corresponding to the SA.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.