Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

The chemisorption of carbon monoxide on a Co(0001) single crystal surface; studied by LEED,UPS, EELS,AES and work function measurements

The chemisorption of CO on Co(0001) has been investigated by LEED, UPS, EELS, Auger and sp measurements. CO is molecularly adsorbed on Co(0001) in the investigated temperature range from 100 to 450 K. This is deduced from the UPS and EELS results and the reversibility of the sp and LEED data. The isosteric heat of adsorption has a constant value of 128 kJ/mol up to a coverage of $\text{1}\text{3}$ and drops then to about 96 kJ/mol. This coincides with the completion of a (√3 × √3)R30° overlayer structure and the formation of a (2√3 × 2√3)R30° CO overlayer which is fully developed at 100 K.     

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.     

Chemisorption studies on cobalt single crystal surfaces: I. Carbon monoxide on Co(0001)

The chemisorption of CO on Co(0001) and on a polycrystalline specimen has been studied by LEED, Auger spectroscopy, and thermal desorption measurements. Annealing of the polycrystal was found to result in a surface dominated by crystallites of (0001) orientation in the surface plane, along with a few (101̄2) oriented crystallites. CO adsorbs on the clean surface at 300 K with an initial sticking probability of 0.9 and the system follows precursor state kinetics. The saturation coverage under UHV conditions corresponds to a well-ordered (√3 × √3)R30° structure; with P_CO>5 × 10^-9 a uniform compression of the adlayer takes place and a (√7 × √7)R19.2° structure begins to form. Models are proposed for these two ordered phases which are in agreement with the observed relative coverage data and the appearance of the corresponding desorption spectra. The desorption enthalpy of CO at low coverages is 103 ± 8 kJmol^-1, and a fairly sharp fall in this enthalpy occurs for coverages $\text{>}\text{1}\text{3}$. In many respects, the system's behaviour closely resembles that of Ni(111)-CO. Oxygen contamination leads to the appearance of a strongly adsorbed CO state with a desorption enthalpy of ~170 kJmol^-1. This is reminiscent of a strongly adsorbed non-dissociated state of CO on Ru(101̄1) which occurs under similar conditions.     

Iodine adsorption on a platinum stepped surface: Pt(s)[6(111) × (111)]

I₂ adsorption on Pt(s)[6(111) × (111)] surfaces under vacuum and atmospheric pressure conditions was studied by LEED, AES and thermal desorption. In contrast to smooth Pt(111), the surface structures were composed of multiple phase domains having (3 × 3) or ($\text{3}\text{×}\text{3}$)R30° local geometry and structural coincidence of the adjacent terraces. No special stability or instability of iodine adsorption at steps was observed.     

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ～0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The $\text{I}\text{4}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ core electron binding energy in both cases decreases by ～0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.     

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δ?, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δ? results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol^?1. These values, which depend on an assumed pre-exponential factor of 10¹³ s^?1, are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl₂ desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.     

The properties of K and coadsorbed CO + K on Ru(001): I. Adsorption,desorption, and structure

An extensive photoemission and LEED study of K and CO+K on Ru(001) has been carried out. In this paper the LEED and some XPS results together with TPD and HREELS data are presented in terms of adsorption, desorption. and structural properties, and their compatibility is discussed. Potassium forms (2 × 2) and $\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ overlayers below and near monolayer coverage, and multilayer bonding and desorption is similar to that of bulk K. The initial sticking coefficients for CO adsorption on K predosed surfaces are correlated with the initial K structure, and s₀ and CO saturation coverages decrease with increasing K coverage. Two well-characterized mixed CO+K layers have been found which are correlated with predosed (2 × 2) K and $\text{(}\text{3}\text{×}\text{3}\text{)R30°}\text{K}$. They have CO to K ratios of 3:2 and 1:1, and lead to LEED patterns with (2 × 2) and (3 × 3) symmetry, respectively. The molecule is believed to be sp² rehybridized under the influence of coadsorbed K, leading to stronger CO-Ru and weaker C-O bonds as indicated by the TPD and HREELS results, and to stand upright in essentially twofold bridges.     

The sorption of CO and O on Ni (111) studied by leed and iss

The adsorption of CO and O on Ni (111) was studied by low-energy ion scattering (ISS) and low-energy electron diffraction (LEED). For the ordered (√7/2) × (√7/2) R19.1° CO layer ion scattering gives a coverage greater than $\text{1}\text{2}$ monolayer, and for the (2 × 2) O layer a coverage of $\text{1}\text{4}$ monolayer. The CO is non-dissociatively adsorbed, with the C bound to the Ni. The molecules are oriented parallel to the surface normal. Island formation at lower CO coverages is possible.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.     

The adsorption of sulphur on Pd(111) I. A LEED analysis of the (2 × 3)R30° S adsorbate structure

The adsorption of sulphur on the Pd(111) surface is studied by low energy electron diffraction (LEED). Four different adsorbate structures are identified. LEED intensity analyses are performed for the clean surface and for the ordered initial adlayer, i.e. the $\text{(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}\text{S}$ adsorption phase. It is found that the sulphur atoms occupy threefold-symmetric hollow sites, with a SPd chemisorption bond length of 0.222±0.003 nm.     

The adsorption of benzene and naphthalene on the Rh(111) surface: A LEED,AES and TDS study

The adsorption of benzene and naphthalene on the Rh(111) single-crystal surface has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). Both benzene and naphthalene form two different ordered surface structures separated by temperature-induced phase transitions: benzene transforms from a ($\text{3}\text{1}\text{1}\text{3}$) structure, which can also be labelled c($\text{2}\text{3}\text{× 4}$)rect, to a (3 × 3) structure in the range of 363–395 K, while naphthalene transforms from a ($\text{3}\text{3}\text{× 3}\text{3}$)R30° structure to a (3 × 3) structure in the range 398–423 K. Increasing the temperature further, these structures are found to disorder at about 393 K for benzene and about 448 K for naphthalene. Then, a first H₂ desorption peak appears at about 413 K for benzene and 578 K for naphthalene and is interpreted as due to the occurrence of molecular dissociation. All these phase transitions are irreversible. The ordered structures are interpreted as due to flat-lying or nearly flat-lying intact molecules on the rhodium surface, and they are compared with similar structures found on other metal surfaces. Structural models and phase transition mechanisms are proposed.     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

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.     

Interactions of CO molecules adsorbed on oxidised Cu(111) and Cu(110)

Reflection absorption infrared spectroscopy has been used in conjunction with LEED and surface potential measurements to study low temperature CO adsorption on the oxidised Cu surfaces Cu(111)O|$\text{3}\text{2}\text{?}\text{2}$|, Cu(110)O(2 × 1) and Cu(110)Oc(6 × 2). On all three surfaces adsorption at 80 K yields surface potential changes in excess of 0.6 V and does not lead to the formation of an ordered overlayer. At high coverages the adsorption enthalpy is lower than on the clean surfaces. Infrared spectra show the growth of a doublet band with components initially at 2100 and 2117 cm^?1 on the oxidised Cu(111) surface. Similar features seen on the oxidised Cu(110) surfaces are accompanied by a band at 2140 cm^?1: a very weak band at the same frequency on oxidised Cu(111) is attributed to defect sites. Studies of the temperature dependence of the spectrum from oxidised Cu(111) lead to the conclusion that two different binding sites are occupied. Spectra of ${}^{12}\text{CO}{}^{13}\text{CO}$ mixtures show that the molecules occupying these sites are in close proximity to each other, and that the spectrum is subject to large but opposing coverage-dependent frequency shifts.     

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.     

Angle resolved photoemission measurements on AgSi(111) 7 × 7 interfaces

The AgSi(111) interface is investigated by LEED, AES and angle resolved photoemission spectroscopy using 50 eV synchrotron radiation in p-polarization. Results on room temperature (RT) silver growth on Si(111) 7 × 7 are characterized by an evolution of the LEED pattern and of the d band shape which is consistent with 2D island formation in the submonolayer range. When the Ag coverage (Θ) is increased, a progressive build-up of Ag layers occurs with a possible interdiffusion of the atomic constituents. The ordered $\text{Si(111)}\text{3}\text{×}\text{3}\text{R(30°)Ag}$ structure (R₃) obtained by annealing a 1 ML RT deposit gives rise to new interface states near E_F. In contrast to the RT deposit at the same Θ, two well defined d band peaks are present while the bulk Si emission near 3.4 eV is clearly seen. The R₃ data would favour recent crystallographic models which conclude to an embedment of the Ag atoms in a threefold hollow adsorption site.     

Superlattices formed by interaction of hydrogen bromide and hydrogen chloride with Pt(111) and Pt(100) studied by LEED,Auger and thermal desorption mass spectroscopy

HBr and HCl react with Pt(111) and Pt(100) surfaces to form adsorbed layers consisting of specific mixtures of halogen atoms and hydrogen halide molecules. Exposure of Pt(111) to HBr yielded a (3×3) LEED pattern beginning at $\text{Θ}{}_{\mathrm{Br}}\text{=}\text{2}\text{9}$ and persisting at the maximum coverage which consisted of $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{=}\text{1}\text{3}$ plus $\text{Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{9}$. The most probable structure at maximum coverage, Pt(111)[c(3 × 3)]-(3 Br + HBr), nas a rhombic unit cell encompassing nine surface Pt atoms, and containing three Br atoms and one HBr molecule. On Pt(100) the structure at maximum coverage appears to be Pt(100)[c(2√2 × √2)]R45°-(Br + HBr), $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{= Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{4}$; the rectangular unit cell involves four Pt atoms, one Br atom and one HBr molecule. Each of these structures consists of an hexagonal array of adsorbed atoms or molecules, excepting slight distortion for best fit with the substrate in the case of Pt(100). Treatment of Pt(100) with HCl produced a diffuse Pt(100)(2 × 2)-(Cl + HCl) structure at the maximum coverage of Θ_Cl = 0.13, Θ_HCl = 0.11. Exposure of Pt(111) to HCl produced a disordered overlayer. Thermal desorption, Auger spectroscopy and mass spectroscopy provided coverage data. Thermal desorption data reveal prominent rate maxima associated with the structural transitions observed by LEED. Br and HBr, Cl and HCl were the predominant thermal desorption products.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

Distinct binding states in adsorption on a homogeneous surface: Te on W(100)

The adsorption of Te on a W(100) surface is studied by thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change (Δ?) measurements. Three distinct binding states are observed in the first monolayer corresponding the coverages from 0 to $\text{1}\text{2}$ monolayers (ML), $\text{1}\text{2}$ to $\text{2}\text{3}$ ML and$\text{2}\text{3}$ to 1 ML. Within each state a coverage dependence of the desorption parameters is found. The three binding states are discussed in terms of heterogeneity induced by lateral interactions and in terms of inherently different adsorption sites.