The coadsorption of I and Cl on Pt(111)

Using LEED and AES, the coadsorption of iodine and chlorine on Pt(111) was studied. Five surface structures were observed: (1 × 1), (√3 × √3)R30°, (√7 × √7)R±19.1°, (3 × 3) and c(2 × 4). Fractional monolayer coverages for both Cl and I are assigned to these structures. The importance of the adatom-adatom steric interactions is discussed.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

Change in adsorption bond length with coverage for KAl(111)

The local bond geometry of K adsorbed on Al(111) at low temperature has been studied by photoelectron diffraction (PED) as a function of K coverage. It is found that K atoms occupy on-top sites in the coverage range 0.05-0.4 monolayer and that the KAl bond length increases by 0.17 Å over this coverage range. The reliability of this result is supported by PED studies of the (√3 × √3)R30° structures formed by adsorption of one-third monolayer Na and K at 300 K, and K at 150 K, which give results in quantitative agreement with previous structure determinations by SEXAFS and LEED.     

Azimuthal dependence of angle resolved photoelectron diffraction from I on Ag(111) -- multiple scattering analysis to determine the adsorption site

The azimuthal anisotropy from the 4d level of (√3 × √3)R30°I on Ag(111) is calculated and compared with the data of Farrell et al. The comparisons indicate the φ-patterns give a clear and unambiguous determination of the adsorption registry of I on Ag(111).     

RHEED study of In-induced superstructures on Ge(111) surfaces

When submonolayer and monolayer amounts of indium were deposited onto clean Ge(111) surfaces at room temperature and then heated, (13 × 2√3), (12 × 2√3), (11 × 2√3), (10 × 2√3), (4√3 × 4√3) R30°-related, (√31 × √31) R(±9°), (√61 × √61) R(30 ± 4°) and (4.3 × 4.3) structures appeared on the surfaces at fixed In coverages and at fixed surface temperatures. General intensity features of superlattice reflections are derived from intensity estimations by eye of superlattice spots in their RHEED patterns, and some structural characteristics of the superstructures are clarified from the analysis of the general intensity features. The former four superstructures are long-period (2 × 2)-related antiphase structures whose period changes, depending on the coverage. The wavevector characterizing the (13 × 2√3) structure, which appears at the smallest coverage, almost coincides with those of structural fluctuation emerging at the clean Ge(111) (1 × 1) surface around 350°C. The coincidence suggests that the longperiod (2 × 2)-related antiphase structures have a close relationship to the structural fluctuation and, besides, to the (2 × 8) structure in their origin.     

Study of Ag/Si(111) submonolayer interface: II. Atomic geometry of Si(111) (√3 × √3 )R30°-Ag surface

Final state diffraction of Ag 3d X-ray photoelectrons from the Si(111) (√3 × √3)R30°-Ag surface has been measured. From a kinematical analysis of the diffraction patterns, it is found that a buried honeycomb framework of Ag atoms is formed on the surface with lateral displacement of the first Si layer.     

Chemisorption of CO on differently prepared Cu(111) surfaces

The adsorption of CO on Cu(111) has been studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), work function measurements and thermal desorption spectroscopy. Two LEED overlayers of CO on Cu(111) have been found: $\text{√3 × √3}\text{R}\text{30°}$ and $\text{√}\text{7}\text{3}\text{× √}\text{7}\text{3}\text{R}\text{49.1°}$. Two different heats of adsorption were derived from thermal desorption spectra: 44.2 and 35.1 kj/mole. The isosteric heat of adsorption evaluated from work function measurements corresponds to the thermal desorption results. Energy losses due to CO adsorption have been found by means of EELS at 4.7, 7.7, and 13.8 eV.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Chlorine adsorption on copper: II. Photoemission from Cu(001)c(2 × 2)-C1 and Cu(111)(√3 × √3)R30°-Cl

We have studied high-resolution angle-resolved and photon-polarization dependent photoemission from chlorine adsorbed on Cu(OOl) and Cu(111). Chlorine forms a c(2 × 2) saturation overlayer on Cu(OO1) and adsorbs dissociatively as revealed by LEED and XPS. Several two-dimensional energy bands on Cu(001)c(2 × 2)-Cl could be iden along the $\text{}\text{?}$gG M? and $\text{}\text{?}$gG M? lines of the surface Brillouin zone, their respective mirror symmetry and their orbital character could be determined. An interpretation of these bands is given in terms of the interaction of the ordered overlayers with particular substrate bulk bands. Besides the appearance of adsorbate-induced two-dimensional bands drastic changes are resolved in the substrate d-band emission region. These can be explained almost exclusively by surface umklapp processes involving reciprocal lattice vectors of the ordered adsorbate mesh. Supplementary studies of the Cu(111) (√3 × √3)R30°-Cl system support our ideas. We discuss some important implications of our results for the interpretation of angle-resolved photoenussion spectra from ordered adsorbate layers.     

Adsorption and desorption of hydrogen on bare and Xe-covered Cu(111)

Using polarization-modulated ellipsometry to monitor adsorbate coverage in-situ, we studied the activated adsorption of filament-heated molecular hydrogen on Cu(111) and subsequent isothermal desorption of hydrogen adatoms. The adsorption is characterized by a zeroth-order kinetic with a constant sticking probability of S₀=0.0062 up to θ=0.25, followed by a Langmuir kinetic until the saturation coverage θ_s=0.5 is reached. The desorption follows a second-order kinetic with an activation energy of 0.63 eV and a pre-exponential factor of 1×10⁹ /s. A pre-adsorbed monolayer of Xe atoms on Cu(111), with a desorption activation energy of 0.25 eV and a pre-exponential factor of 8×10¹⁴ /s, efficiently blocks the subsequent adsorption of hot molecular hydrogen, making physisorbedXe useful as templates for spatial patterning of hydrogen adatom density on Cu(111). PACS 68.43.Jk; 78.68.+m; 81.15.-z; 82.40.Np     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

An STM study of desorption-induced thallium structures on the Si(111) surface

The scanning tunneling microscopy is used to study morphology of a Tl adlayer in various stages of Tl desorption from the Si(111) surface. Transition from the Si(111)/(1 × 1)-Tl structure through the (√3 × √3)R30° mosaic phase to domains of metastable Si reconstructions is observed. Silicon substitutional atoms are found to be intrinsic to the (√3 × √3)R30° structure. The temperature dependence of the amount of residual Tl atoms on the surface is successfully fitted by a model using the first order desorption. The same desorption energy of (2.1 ± 0.3) eV and frequency prefactor 5 × 10^14 ± 2 s^? 1 during all stages of the desorption are sufficient for the fitting. It is concluded that bonding of Tl in both (1 × 1) and (√3 × √3) configurations is of the same nature.     

Electronic structure of Ag adsorbed on Si(111); experiment and theory

Experimental results (low energy electron loss spectroscopy) and band structure calculations relating to the early stages of Ag growth on a Si(111) surface are presented. Crystallography and thermal desorption kinetics studies of this interface, previously published, gave rise to the following conclusions. At room temperature and below 200°C, two-dimensional (2D) (111) epitaxial layers develop on top of a first ordered layer (√3 × √3), while at higher temperatures three-dimensional (3D) clusters develop over this first layer. Low energy electron loss experiments were performed at various surface coverages θ. They display different evolutions according to the growth modes. For the 2D epitaxial growth, one observes the disappearance of the peaks characteristic of a Si surface and the onset of Ag induced peaks located at 7.1 and 4.6 eV at completion of the √3 layer. These peaks narrow and shift to the bulk Ag excitation energies at 7.5 and 4 eV when a second Ag layer is deposited. In order to explain these results, we present a theoretical calculation of the electronic density of states of the interface using a tight binding approximation. This calculation accounts for the development of the Ag d band from the √3 coverage range to the (111) epitaxial Ag planes. The evolution of the spectra when θ is increased is discussed in view of these results.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

A single crystal study of the initial stages of silver sulphidation: The chemisorption and reactivity of molecular sulphur (S2) on Ag(111)

Molecular sulphur undergoes rapid dissociative chemisorption on Ag(111) with an essentially constant sticking probability of unity up to the completion of the first layer of S atoms. At this stage a $\text{(√39 R 16.1° × √39}\text{R}\text{?}\text{16.1°)}$ structure is formed in which the S atom arrangement and spacing is similar to that in the (100) plane of γ-Ag₂S (the high temperature form of silver sulphide). Further dosing with S₂ leads to continued rapid uptake of sulphur and the appearance of a (√7 × √7) R 10.9° structure, the Auger, Δφ and thermal desorption data all indicate that fast formation of Ag₂S now occurs. Very well-ordered growth of γ-Ag₂S(111) is now observed, and low-temperature S₂ desorption spectra appear which show that the activation energy for S₂ desorption is ~175 kJ mol^?1 ; this value is in excellent agreement with that observed for the enthalpy of decomposition of bulk Ag₂S (2 Ag₂S(s) → 4 Ag(s) + S₂(g), ΔH = +179 kJmol^?1). All the properties of the Ag(111)-S system imply that the material characterised by the √39 structure (i.e. the first adsorbed layer of S) is very different from bulk Ag₂S. This is discussed and compared with the results of other studies on metal-sulphur systems.     

The adsorption of chlorine and chloridation of Ag(111)

The adsorption of chlorine on the Ag(111) surface has been studied using LEED, Auger and temperature programmed desorption. Chlorine adsorbs dissociately with an initial sticking probability of ~ 0.4, and a precursor state is implicated in the chemisorption process. The chlorine appears to form a close-packed monolayer with the same packing density as in AgCl(111), and is epitaxially related to the substrate mesh. Chlorine continues to adsorb above a monolayer in coverage, though the sticking probability drops precipitately, being ~ 0.01 after the adsorption of 5 monolayers at 300 K. There is little increase in the chlorine Auger signal above one monolayer coverage at 300 K, but when adsorption is carried out at 240 K the chlorine signal is more than doubled. This is interpreted as being due to the formation of a layer structure of alternate Cl and Ag layers at the lower temperature, while adsorption at 300 K results in dissolution of subsurface Cl into the bulk of the crystal. Upon heating, the low temperature layer structure is destroyed, the chlorine signal diminishes to a limiting value at 450 K equivalent to the value for one adsorbed monolayer — apparently due to the dissolution of the near surface Cl layers into the bulk. However, the chlorine re-emerges at the surface at ~ 600 K, probably due to an exothermic heat of solution of Cl in the silver lattice. Desorption from the multilayers peaks at 670 K and both AgCl and Ag are desorbed coincidently with kinetics identical to those for the sublimation of bulk AgCl (ΔH = 235 kJ mol^?1, ΔS = 90 JK^?1 mol^?1). After the multilayers have desorbed, the final Cl layer desorbs in a higher temperature peak ( ~ 760 K) as AgCl (no silver desorption) which shows complex desorption kinetics indicative of the strong influence of a precursor state in the desorption process.     

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω_CO = 1817 cm^?1. At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω_CO = 1910 cm^?1. At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm^?1. The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.     

Ga- and As-Adatom phases on the Ge(111) and Si(111) surfaces: analogies and differences

The (1 × 1) and (√3 × √3)R30° (T4) structures of Ga and As adatoms on the Ge(111) and Si(111) surfaces are studied using first-principles calculations. The surface energetics predicts, in some cases, a transformation of the T4 structure (surface covered with 1/3 monolayer (ML) of adatoms) into domains of the 1-ML covered (1 × 1) structure and areas of clean reconstructed suface. For As adatoms, such phase separation is favored on both substrates, while for Ga adatoms, it is only preferred on the Ge(111) surfaces. These results compare well with experimental observations.     

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule-substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.