Optical properties and electronic structure of copper atoms adsorbed on a platinum electrode

The optical properties of Cu atoms deposited in the underpotential region from electrolytic solution onto a polycrystalline Pt electrode have been studied by differential reflectance spectroscopy. Spectra of the normalized reflectance change ($\text{ΔR}\text{R}$) have been obtained in the photon energy range between 1.5 and 5.5 eV for p- and s-polarized light as a function of coverage θ and angle of incidence ?₁. Evaluation of the adsorbate optical constants by a simple 3-phasemodel reveals three absorption bands in the range studied, one of them being strongly angular dependent. From an assignment of these transitions the positions of the 3d-, 4s- and 4p-derived levels in the energy level diagram of the Cu adatom is determined. As expected these levels are considerably shifted and broadened in comparison to their unperturbed atomic counterparts. Optical evidence for the occurrence of a structural change in the adsorbate layer at about θ = 0.6 is also presented.     

Interpretation of a new feature in photoemission from Cu based alloys with polyvalent solutes

In our recent angle resolved photoemission studies of oriented single crystal surface of $\text{Cu}$Al and $\text{Cu}$Ge solid solutions we had found a new structure lying between 4 and 5 eV below the Fermi energy, which could not be related to either the bulk or the surface states in the random alloy. To understand its nature and origin we report and discuss photoemission measurements from Cu films, submonolayer to a monolayer thick, deposited on the Al(111) and $\text{Cu}\text{Al(111)(}\text{3}\text{×}\text{3}\text{R}\text{30°)}$ alloy surfaces and from Al and Zn alloys containing dilute Cu impurities. Some experiments on the (100) surfaces in the various cases were also carried out. On the basis of these results, we suggest that the aforementioned new photoemission feature is characteristic of Cu clusters and isolated Cu impurities in polyvalent impurity-rich environment. These clusters probably lie at the topmost layer of the $\text{Cu}$Al and $\text{Cu}$Ge alloy surfaces.     

Reactive scattering of small molecules from platinum crystal surfaces: D2CO,CH3OH,HCOOH, and the nonanomalous kinetics of hydrogen atom recombination

The decomposition of D₂CO, CH₃OD and HCOOH on Pt(110) and of D₂CO on Pt(S)-[9(111) × (100)] was studied by molecular beam relaxation spectroscopy. D₂CO and CH₃OD evolved CO and H₂ via a desorption limited sequence of elementary steps. The rate constant for CO desorption from Pt(110) was 6 × 10¹⁴exp(? 35.5 $\text{kcal}\text{gmol}$ · RT) s^?1, and from Pt(S)-[9(111) × (100)] it was 1 × 10¹⁵ exp(?36.2 $\text{kcal}\text{gmol}\text{·RT}$) s^?1. On Pt(110) the rate constant for hydrogen formation was 10^{0 ± 1}exp(?24 $\text{kcal}\text{gmol}\text{·RT}$) $\text{m}\text{?}{}^2\text{atom}$ · s. On Pt(S)-[9(111) × (100)] two pathways for H₂ formation existed with rate constants of 8.7 × 10^?2exp( ?24.9 $\text{kcal}\text{gmol}\text{· RT}$) $\text{cm}{}^2\text{atom}$· s and 3.2 × 10^?3 exp(?19.5 $\text{kcal}\text{gmol}\text{·RT}$) $\text{cm}{}^2\text{atom}\text{·}$ s. These pre-exponential factors are in order of magnitude agreement with values typical of hydrogen recombination on other metals. When a small amount of sulfur ( ~ 0.1 ML) was adsorbed on the stepped Pt surface, only one pathway for H₂ formation existed due to blockage of stepped sites. A similar result was obtained when a beam of CO was impinged on the surface. Formic acid decomposed via a branched process to form primarily CO₂ and H₂.     

The growth and alloy formation of copper on the platinum (111) and stepped (553) crystal surfaces; characterization by Leed,AES, and CO thermal desorption

Epitaxial layers of copper were formed on Pt(111) and Pt(553) single crystal surfaces by condensation of copper atoms from the vapor. Surface alloys were formed by diffusing the copper atoms into the platinum substrate at temperatures above 550 K. The activation energy for this process was found to be ~ 120 $\text{kJ}\text{mol}$. These Pt/Cu surfaces were characterized by LEED, AES, and TDS of CO. The copper grows in islands on the Pt(111) surface and one monolayer is completed before another begins. There is an apparent repulsive interaction between the copper atoms and the step sites of the Pt(553) surface which causes a second layer of copper to begin forming before the first layer is complete. Epitaxial copper atoms block CO adsorption sites on the platinum surface without affecting the CO desorption energy. When the copper is alloyed with the platinum however, the energy of desorption of CO from the platinum was reduced by as much as 20 $\text{kJ}\text{mol}$. This reduction in the desorption energy suggests an electronic modification that weakens the Pt-CO bond.     

Optical spectroscopy of surfaces: Reflectance studies of chemisorption

A high precision technique has been used to study the small changes in near-normal incidence reflectance R caused by chemisorption of gases on an atomically clean metal surface maintained in ultrahigh vacuum. Examples are given for O₂, CO, and H₂ on Mo(100) in the photon energy range 1.9 <?ω < 4.8eV. The relative reflectance change $\text{ΔR}\text{R}$ is negative and reaches as much as 1%. Structure that appears in ΔR(e), where e=exposure (pressure × time), is attributed to different binding configurations of the adatoms as observed, e.g., in LEED experiments. The dependence of $\text{ΔR}\text{R}$ on ?ω suggests the presence of adsorbate-induced surface states within a few eV of the Fermi energy. Possible approaches to determining the dielectric function of the surface region from the data are discussed.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

The epitaxy of gold on (110) tungsten studied by leed

Growth of gold condensed on the (110) plane of tungsten has been studied using LEED and AES. Three ordered surface structures were observed when condensation takes place at or above 700 K, and no detectable order is seen below this temperature. Structure 1 is developed as the coverage approaches one monolayer and has gold atoms held in the W(110) array with a resultant 2% reduction in gold atom diameter. The second gold layer adopts the Au(111) structure with $\text{Au}\text{[1}\text{2}\text{1]}$ rotated by 2.5° from $\text{W}\text{[1}\text{1}\text{0]}$ and the first gold layer may also be constrained to adopt this structure. Deposition of more gold produces three dimensional crystallites with Au(111)∥W(110) which are double-positioned with their 121 directions parallel to the 121 directions of tungsten. Addition of half a monolayer of oxygen before condensation, completely prevents formation of structures 1 and 2. Instead, at coverages of 3 or more monolayers, three dimensional crystallites develop with Au(111) ∥ W(110) and $\text{Au}\text{[1}\text{2}\text{1] ∥}\text{W}\text{[1}\text{1}\text{0]}$. This behaviour is compared with the reported behaviour of copper and silver on W(110).     

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.     

Anisotropic surface effects in Au and Cu monocrystals

Electroreflectance spectra at normal incidence of (100) and (110) faces of gold and copper monocrystals are given, in the spectral range from 0.22–0.7 μm. The fractional change in reflectance is different with (110) faces when light is polarized parallel to the [001] direction and parallel to the [$\text{1}\text{10}$] direction while no anisotropy is seen on (100) faces. This shows that electroreflectance is a powerful tool to investigate metal surfaces where the optical electrons are sensitive to the distribution of the surface atoms.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).     

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.     

Band structure of Ag,Au and Pt along the Δ line near X from angle resolved photoemission spectroscopy (ARUPS) : Location of transitions via evanescent gap surface states

The zero-slope (ZS)_method and the triangulation method of angle resolved photoemission for the location of transitions in

-space have been used on (111) and (110) faces of Ag, Au and Pt in the ΓLK(1$\text{1}$0) mirror plane with hν = 21.2 eV. It is found that the experimental ZS-points correspond to emission from around the X₁₁₀-point is due to excitation into evanescent states localized on the Γ₁₁₁-Δ-X₁₁₀ line, allowing an energy-mapping of the occupied bands. A comparison with calculated bands is given.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

The absorption of Ag on Ge(100)-(2 × 1)

The initial stages of interface formation for Ag deposited onto Ge(100)-(2 × 1) were studied with high-energy electron diffraction and high- resolution photoemission. The surface core-level energies for clean Ge(100)-(2 × 1) were not changed with the deposition of about one monolayer of Ag, indicating that there was no chemical reaction or atomic intermixing. The Ag nucleated at a coverage of about $\text{1}\text{3}$ monolayer and showed three-dimensional growth for higher coverages.     

CO chemisorption on PtCu surfaces I. CO on (1 × 3) Pt0.98Cu0.02(110)

Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt_0.98Cu_0.02(110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 Å, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).     

AES,photoemission and work function study of the deposition of Cs on (100) and (111)B GaAs epitaxial layers

Work function, photoemission and AES measurements have been made on (100) and (111)B epitaxial GaAs layers as a function of caesium coverage. It has been shown that the photoemission maximum and work function minimum occur at a coverage of $\text{2}\text{3}$ of a monolayer. The reduction of the work function takes place in distinct stages with changes in the rate of fall occurring at $\text{1}\text{8}$ and $\text{1}\text{2}$ of a monolayer for the (111)B face and $\text{1}\text{8}$, $\text{1}\text{4}$, $\text{1}\text{3}$ and $\text{1}\text{2}$ of a monolayer for the (100) face. This indicates the presence of specific adsorption sites which have not been observed in previous work on these faces.     

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.     

Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η2(C,N) species

Acetonitrile (CH₃CN) coordination to a Pt(111) surface has been studied with electron energy loss vibrational spectroscopy (EELS), XPS, thermal desorption and work function measurements. We compare data for the surface states with known acetonitrile coordination complexes. For CH₃CN adsorbed on Pt(111) at 100 K, the molecule is rehybridized and adsorbs with the CN bond parallel or slightly inclined to the surface plane in an η²(C, N) configuration. The ν(CN) frequency is 1615 cm^?1 and the C ls and N ls binding energies are 284.6 eV and 397.2 eV respectively. By contrast, weakly adsorbed multilayer acetonitrile exhibits a ν(CN) vibrational frequency of 2270 cm^?1, and C ls and N ls binding energies of 286.9 eV and 400.1 eV respectively. Both the EELS and XPS results are consistent with rehybridization of the CN triple bond to a double bond with both C and N atoms of the CN group attached to the surface. In addition to this majority η²(C, N) monolayer state, evidence is found for a second, more strongly bound minority molecular state in thermal desorption spectra. As a result of the low coverage of this state, EELS was unable to spectroscopically identify it and we tentatively assign it as an η⁴(C, N) species associated with accidental step sites. By contrast to the surface complexes, almost all of the known platinum-nitrile coordination complexes are end-bonded via the N lone-pair orbital. Several cases of side-on bonding are known, however, and we compare the results with the known complex Fe₃(η²-NCCH₃)(CO)₉. The difference in the coordinative properties of a Pt(111) surface versus a single Pt atom must be due to the increased ability of multi-atom arrays to back-donate electrons into the π¹ system of acetonitrile. Previously published EELS and XPS results for monolayer acetonitrile on Ni(111) and polycrystalline films are almost identical to the present results on Pt(111). We believe that the monolayer of $\text{CH}{}_3\text{CN}\text{Ni}\text{(111)}$ is also an η²(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.     

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.     

Influence of the surface morphology of single-crystal Si(111) substrates on the magnetic properties of epitaxial cobalt films

The epitaxial films Co(111)/Cu(111)-R30°/Si(111) have been grown on the atomically smooth and vicinal Si(111) surfaces. The roughnesses of the substrate and the cobalt film have been determined using scanning tunneling microscopy. The dependence of the coercive force has been investigated as a function of the azimuthal angle. The dependence of the magnetic anisotropy and the coercive force on the surface roughness has been determined. It has been shown that, in the epitaxial cobalt films deposited on the atomically smooth silicon surfaces, crystalline anisotropy of the 〈110〉 type leads to the isotropy of the magnetization reversal processes. The step-induced uniaxial anisotropy has been observed upon deposition on the vicinal surfaces. The films deposited on the atomically smooth surfaces have a complex domain structure.