Raman spectra of molecules considered to be surface enhanced

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) have been used to study the structure of adsorbed benzene (C₆H₆ and C₆D₆) monolayers on the Rh(111) surface at 300K. A surface bonding geometry is proposed for benzene adsorbed to give a c(2?3×4) rectangular structure, which involves very little perturbation of the molecular structure with the ring plane parallel to the surface. Only one chemical environment for adsorbed benzene is indicated by a single frequency shift of the symmetric CH out-of-plane bending mode. The adsorption site is tentatively assigned to benzene centered over a single Rh atom.     

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.     

CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ_CO ～ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.     

Study of CO adsorption on Sn/Rh(1 1 1)

An ultra thin Sn layer (6 Å) was deposited onto Rh(1 1 1) single crystal surface. We followed changes in low energy electron diffraction (LEED) pattern during progressive annealing together with development of CO adsorption capacity and photoelectron spectra obtained using synchrotron radiation. Surface bimetallic alloy development with increasing temperature was followed by LEED and synchrotron radiation photoelectron spectroscopy (SRPES). LEED results show several surface structure of Sn/Rh(1 1 1) sample in dependence on sample temperature. If it increases, the surface structure develops to the stable ordered (√3 × √3)R30° structure. Surface CO adsorption depends strongly on the amount of Sn in the top sample layer then it corresponds to the development of the surface structure. The CO adsorption capacity raises with increasing temperature.Photoelectron spectra of Sn and Rh core levels and their shifts and shapes were studied during the annealing and CO adsorption. The resulting spectra are used to discuss the Sn-Rh surface alloy creation.The goal of this paper is to demonstrate the CO adsorption on the Sn/Rh(1 1 1) surface. Valence band spectra measured at different primary energies are presented to demonstrate this effect. These spectra show different adsorption properties of the studied system in dependence on the amount of Sn in the top layer and geometric structure of the surface.     

The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

NO在Cu(110)表面吸附的角分辨光电子能谱

本文用角分辨光电子能谱(ARUPS)(He Ⅱ),低能电子衍射(LEED)和俄歇电子能谱AES等方法研究了NO在Cu(110)表面吸附的光电子能谱。测量结果表明:在150K左右,NO在Cu(11O)表面是一个比较复杂的分解吸附过程。随着暴露量的不同,在Cu(110)表面形成的分解吸附分子是不同的。在NO5L暴露量时,主要形成O原子和N₂O分子吸附。吸附的LEED图形仍然是(1×1)。 关键词：     

LEED investigations of the Rh(111)-(2 × 2)-O surface structure: measurements with a video analyser in the presence of some electron beam disordering of the adsorbed layer

A multiple-scattering analysis of LEED intensities has been made for the Rh(111)-(2 × 2)-O surface structure. The measurements were made with a video LEED analyser under conditions where the O layer disorders in 2 min in the incident electron beam. The experimental and calculated intensities were compared with Pendry's reliability index, which was minimised with O atoms adsorbed 1.23 Å above the “expected” hollow sites of three-fold coordination.     

Interactions between coadsorbed species on the Rh(111) surface as a function of the separation distance between them

The interaction energies as a function of the separation distance between two molecules of CO and between CO and H or O coadsorbed on the Rh(111) surface have been calculated by the Extended Hückel Method. Weak interactions of the order of a few hundredths of eV occur between the adsorbates via the metal lattice. The interaction energy is found to change its signs as the distance between the adsorbates increases. For first neighbor occupation sites the interaction energies are positive. Second and third neighbor interaction energies between CO and CO are, however, negative, the latter being stronger than the former. The interaction energies between CO and H or O are positive for first and second order neighbors and, in general, negative for third order neighbors. The computed interaction energies allow one to explain the observed LEED patterns of adsorbed CO at low temperatures. The compression of the p(2 × 2) LEED structure of CO to the (√3 × √3)R30° structure, induced by the subsequent exposure to hydrogen or oxygen, is explained on the basis of the more favorable interaction energies when CO and H, or CO and 0 are segregated.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

A LEED and AES study of the structure,debye temperature,and oxidation of the (111) crystal face of thorium

The structure, and reactivity towards O₂ and CO, of the (111) crystal face of a single crystal of high purity thorium metal was studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After the sample was cleaned in vacuum by a combination of ion bombardment and annealing, a (1 × 1) LEED pattern characteristic of a (111) surface was obtained. Extended annealing of the cleaned sample at 1000 K produced a new LEED pattern characteristic of a (9 × 9) surface structure. A model of a reconstructed thorium surface is presented that generates the observed LEED pattern. When monolayer amounts of either O₂ or CO were adsorbed onto the crystal surface at 300 K, no ordered surface structures formed. Upon heating the sample following these exposures the (111) surface structure was restored accompanied by a reduction in the amount of surface carbon and oxygen. With continued exposure to either O₂ or CO and annealing, a new LEED pattern developed which was interpreted as resulting from the formation of thorium dioxide. Debye-Walter factor measurements were made by monitoring the intensity of a specularly reflected electron beam and indicated that the Debye temperature of the surface region is less than it is in bulk thorium. Consequently, the mean displacement of thorium atoms from their equilibrium positions was found to increase at the surface of the crystal. The presence of chemisorbed oxygen on the crystal surface affected the Debye temperature, raising it significantly.     

Low energy electron diffraction studies of the surfaces of molecular crystals (ice,ammonia, naphthalene,benzene)

Low Energy Electron Diffraction (LEED) has been used to study the surface structures of thin films of molecular crystals. The samples were grown epitaxially on metal single crystal substrates at low temperatures. Both Pt(111) and Ag(111) surfaces were used as substrates in order to identify the influence of the substrate on molecular film structure. Previous observations of ice (0001) and naphthalene (001) surfaces on films grown on Pt(111) substrates [Surface Sci. 55 (1976) 413], were confirmed using the Ag(111) substrate. The NH₃(111) and benzene (111) surfaces were also studied on films grown on either substrate. All observed molecular crystal surfaces showed no evidence of surface reconstruction. To minimize sample charging and electron beam induced damage, LEED experiments were performed on samples of thickness less than 10?10² nm, with low energy electron exposures less than 1 C cm^?2. The maximum thickness and exposure values were characteristic of the particular molecular crystal. The relationship between the structure of the initial adsorbed monolayer and the molecular crystal orientation is discussed.     

Carbon monoxide induced ordering of benzene on Pt(111) and Rh(111) crystal surfaces

Carbon monoxide induced ordering of an organic molecule, benzene, has been studied on the Pt(111) and Rh(111) crystal surfaces using low-energy electron diffraction, high-resolution electron energy loss spectroscopy, and thermal desorption spectroscopy. We propose detailed geometries for all the ordered structures of coadsorbed CO and benzene. Ordering in the adsorbed overlayer is proposed to result from the interactions between adsorbed CO molecules in the presence of benzene.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

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.     

Photoemission and LEED study of the Sn/Rh(111) surface--early oxidation steps and thermal stability

We have deposited two monolayers of Sn onto Rh(111) single crystal. After the deposition, no ordered structure was revealed by low energy electron diffraction (LEED). We oxidized the obtained system in a low-pressure oxygen atmosphere at 420 K. The oxidized sample was then gradually heated to study the thermal stability of the oxide layer. We characterized the system by synchrotron radiation stimulated photoelectron spectroscopy and LEED. Valence band and core level photoelectron spectra of rhodium, tin and oxygen were used to study the oxidation of the Sn-Rh(111) surface and its behaviour upon annealing. A low stoichiometric oxide of Sn was created on the surface. The oxidation process did not continue towards creation of SnO(2) with higher oxygen dose. The annealing at 970 K caused decomposition of the surface oxide of Sn and creation of an ordered (√3 × √3)R30° Sn-Rh(111) surface alloy.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

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.     

Molecular beam epitaxy of monotype CrSi2 on Si(111)

Low energy electron diffraction (LEED), angle-resolved ultraviolet (ARUPS), and X-ray (XPS) photoemission spectroscopy and work function measurements were used to investigate the growth of epitaxial CrSi₂ on a Si(111) surface. The CrSi₂layers ) (~ 100 Å) are formed by the MBE technique, in which Cr and Si are coevaporated in their stoichiometric ratio on the Si(111) substrate maintained at ~450°C. In comparison with the CrSi₂ epitaxy previously obtained by the SPE technique, where two kinds of CrSi₂ domains with equal formation probability are always observed, the epitaxial CrSi₂ layers obtained by the MBE technique essentially present one definite orientation characterized by CrSi₂(0001)∥Si(111) and CrSi₂[112&#x0304;0] ∥[112&#x0304;].