Excitation of the shear horizontal mode in a monolayer by inelastic helium atom scattering

Inelastic scattering of a low-energy atomic helium beam (HAS) by a physisorbed monolayer is treated in the one-phonon approximation using a time-dependent wave packet formulation. The calculations show that modes with shear horizontal polarization can be excited near high symmetry azimuths of the monolayer, in agreement with recent experiments. The parameters of the calculations are chosen to match the conditions of HAS experiments for triangular incommensurate monolayer solids of xenon, krypton, and argon adsorbed on the (111) face of platinum, and the results show many of the systematic experimental trends for relative excitation probability of the shear horizontal and longitudinal acoustic phonon branches. The inelastic scattering at beam energies near 8 meV is exceedingly sensitive to small misalignment between the scattering plane and the high symmetry directions of the monolayer solid. The diffraction and inelastic processes arise from a strong coupling of the incident atom to the target and the calculated results show large departures from expectations based on analogies to inelastic thermal neutron scattering.     

Calculation of inelastic helium atom scattering from H2/NaCl(001)

The one-phonon inelastic low energy helium atom scattering theory is adapted to cases where the target monolayer is a p(1 × 1) commensurate square lattice. Experimental data for para-H(2)/NaCl(001) are re-analyzed and the relative intensities of energy loss peaks in the range 6 to 9 meV are determined. The case of the H(2)/NaCl(001) monolayer for 26 meV scattering energy is computationally challenging and difficult because it has a much more corrugated surface than those in the previous applications for triangular lattices. This requires a large number of coupled channels for convergence in the wave-packet-scattering calculation and a long series of Fourier amplitudes to represent the helium-target potential energy surface. A modified series is constructed in which a truncated Fourier expansion of the potential is constrained to give the exact value of the potential at some key points and which mimics the potential with fewer Fourier amplitudes. The shear horizontal phonon mode is again accessed by the helium scattering for small misalignment of the scattering plane relative to symmetry axes of the monolayer. For 1° misalignment, the calculated intensity of the longitudinal acoustic phonon mode frequently is higher than that of the shear horizontal phonon mode in contrast to what was found at scattering energies near 10 meV for triangular lattices of Ar, Kr, and Xe on Pt(111).     

Rotationally inelastic scattering of HD from Cu(100) and Pd(111)

Rotational excitation of HD scattered from Cu(100), Pd(111), and Pd(111):H(D) was measured using molecular beam and quantum-state-specific laser spectroscopy techniques. Greater than 91% of the incident HD population was in the v = 0, J = 0 state. The final rotational distributions from Cu(100), Pd(111), and Pd(111):H(D) were compared for a HD beam at an incident energy of 74 meV. For all the three surfaces studied, rotationally inelastic scattering probabilities were large. We find that the final HD rotational distributions are remarkably similar for the three surfaces even though Pd(111) is very reactive to dissociative adsorption of HD whereas Cu(100) and Pd(111):H(D) are chemically inert.     

Quantum and classical study of surface characterization by three-dimensional helium atom scattering

Exact time-dependent wavepacket calculations of helium atom scattering from model symmetric, chiral, and hexagonal surfaces are presented and compared with their classical counterparts. Analysis of the momentum distribution of the scattered wavepacket provides a convenient method to obtain the resulting energy and angle resolved scattering distributions. The classical distributions are characterized by standard rainbow scattering from corrugated surfaces. It is shown that the classical results are closely related to their quantum counterparts and capture the qualitative features appearing therein. Both the quantum and classical distributions are capable of distinguishing between the structures of the three surfaces.     

Oxygen reduction at platinum monolayer islands deposited on Au(111)

Atomic monolayer islands of Pt, namely, two-dimensional Pt nanoparticles, on a Au(111) electrode have been studied for the first time, focusing on their electrocatalytic activities for oxygen reduction in acid solutions. The Pt islands' electrodes were prepared using the self-assembled technique of thiols together with the replacement of Pt with a Cu monolayer. The states of adsorbed OH and the catalytic activities of oxygen reduction were sensitive to the Pt island size. As island size decreased, a delay in the reduction of surface oxide was observed. However, negligible influence of adsorbed OH on activity for oxygen reduction was observed. Pt islands of sizes ranging from 5 to 10 nm showed higher specific catalytic activities for oxygen reduction. Specific catalytic activities decreased by a factor of 10 with a decrease in island sizes from 5.5 to 3.1 nm. Size effects observed in Pt monolayer islands were discussed in comparison with three-dimensional nanoparticles, to obtain information concerning the size effects of metal nanoparticles.     

Helium atom scattering study of the interaction of water with the BaF2(111) surface

The interaction of water with the BaF2(111) single crystal surface is investigated using the helium atom scattering technique. It is found that H2O forms a long-range ordered two-dimensional (2D) phase with (1 x 1) translational symmetry already after an exposure of 3 L (1 L=10(-6) Torr s) at temperatures below 150 K. The activation energy for desorption of the saturated 2D phase, which is assigned to a bilayer, is estimated to be 46+/-2 kJ mol(-1), corresponding to a desorption temperature of 165 K. The desorption of multilayers was observed at 150 K, consistent with a binding energy of 42+/-2 kJ mol(-1). Before completion and after desorption of the saturated 2D phase, a superstructure consistent with a disordered (square root of 3 x square root of 3)R30 degrees lattice has been observed, which is attributed to the first layer of water with a coverage of one molecule per surface unit cell, in accordance with recent theoretical studies. Desorption of this phase is observed at temperatures above 200 K, consistent with an unexpectedly strong bonding of the molecules to the substrate.     

Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

Delocalized electron resonance at the alkanethiolate self-assembled monolayer/Au(111) interface

We probe the electronic structure of alkanethiolate self-assembled monolayers (SAMs) on Au(111) using two-photon photoemission spectroscopy. We observe a dispersive unoccupied resonance close to the vacuum level with a lifetime shorter than 30 fs. The short lifetime and the insensitivity of the energy level and dispersion to molecular length (and thus layer thickness) suggest that the probability density of the electron wave function is concentrated inside the molecular layer close to the SAM/Au interface. Such an interfacial resonance results from the image like potential at the SAM/Au interface.     

Adsorption energies, inter-adsorbate interactions, and the two binding sites within monolayer benzene on Ag(111)

The adsorption of monolayer and multilayer benzene on the Ag(111) surface was characterized using temperature programmed desorption (TPD). TPD spectra revealed two broad peaks at approximately 205 and approximately 150 K at submonolayer coverage and a sharper, multilayer peak at 140 K. Analysis of the coverage-dependent shape and shift of the two submonolayer peaks has resulted in their assignment to desorption from two different binding geometries on threefold-hollow sites with symmetries C(3v)(sigma d) and C(3v)(sigma v). The TPD peak analysis incorporated inter-adsorbate repulsive interaction that resulted from the local dipole moment at the adsorption site induced by the adsorbate-surface charge transfer bonding. The analysis has yielded desorption energies of 54.9 +/- 0.8 and 50.4 +/- 0.4 kJ/mol for the C(3v)(sigma d) and C(3v)(sigma v) configurations, respectively. The interface dipole and polarizability of the benzene-silver complex have been determined to be 5.4 +/- 1.8 D and 14 +/- 10 A3, respectively. Repulsive interactions in the monolayer were found to lower the desorption energy from the zero-coverage value by 14.8 kJ/mol. Leading edge analysis of the multilayer peak yielded a desorption energy of 40.9 +/- 0.7 kJ/mol.     

Ultrathin monolayer of rubrene on Au(111) induced by charge transfer

An ultrathin self‐assembly monolayer of rubrene on Au(111) has been fabricated and studied by scanning tunneling microscopy. The apparent thickness of such monolayer is 0.08 nm and close to the radius of a carbon atom. Moreover, the rubrene molecules within the second layer prefer adsorbing on to the positions corresponding to the herringbone structure underneath the Au(111)–() while the Au surface is fully covered by a monolayer of rubrene. With the assistant with theoretical simulations, we reveal that small apparent height of such monolayer is due to the coupling between the molecular orbitals and the gold surface. About 0.237 electron per rubrene molecule is transferred to the surface, and as a consequence, an interfacial dipole is formed on the rubrene/Au interface. The formation of such interfacial dipole induced by charge transfer from molecules to surfaces is believed to be applied in organic molecules adsorbed on metal surfaces. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.     

Self-organized monolayer of meso-tetradodecylporphyrin coordinated to Au(111)

The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au(111), H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-C=N-) nitrogen atoms of the porphyrin macrocycle to Au(111).     

Efficient CO oxidation at low temperature on Au(111)

The rate of CO oxidation to CO2 depends strongly on the reaction temperature and characteristics of the oxygen overlayer on Au(111). The factors that contribute to the temperature dependence in the oxidation rate are (1) the residence time of CO on the surface, (2) the island size containing Au-O complexes, and (3) the local properties, including the degree of order of the oxygen layer. Three different types of oxygen--defined as chemisorbed oxygen, a surface oxide, and a bulk oxide--are identified and shown to have different reactivity. The relative populations of the various oxygen species depend on the preparation temperature and the oxygen coverage. The highest rate of CO oxidation was observed for an initial oxygen coverage of 0.5 monolayers that was deposited at 200 K where the density of chemisorbed oxygen is maximized. The rate decreases when two-dimensional islands of the surface oxide are populated and further decreases when three-dimensional bulk gold oxide forms. Our results are significant for designing catalytic processes that use Au for CO oxidation, because they suggest that the most efficient oxidation of CO occurs at low temperature--even below room temperature--as long as oxygen could be adsorbed on the surface.     

Quantitative analysis by (p,X) and (p, γ) reactions at low energies

Production of atomic X-rays and nuclear γ-rays by bombardment with 0–3 MeV protons of thick targets is described. In the case of low-Z atomic X-rays, the absorption in the target is very large, while in the case ofK X-rays from high-Z atoms or for γ-rays, this phenomenon is negligible. Both of these reactions can be used for analysis of elements from F to U, and the sensitivities and the accuracies of the determinations are discussed. A table is given showing the γ-ray energies observed in 11 substances and the limits of sensitivity.     

Study of the low-frequency motions in poly(ethylene terephthalate) (PET) by neutron inelastic scattering

By neutron inelastic scattering, a complex low-frequency spectrum is obtained for PET. This is attributed to the complicated structure of the monomer unit, a large number of intramolecular modes being possible. Most of them were identified using infrared and Raman data. Some indication of the presence of intermolecular modes below 200 cm^?1 was found in the dependence of resolution and intensity on temperature in this frequency domain. The existence of these intermolecular modes was further supported by the spectra obtained from oriented or annealed crystalline samples.     

Potassium-benzene interactions on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra obtained by thermal collisions of He(?)(2(3)S) metastable atoms with C(6)H(6)/Pt(111), C(6)H(6)/K/Pt(111), and K/C(6)H(6)/Pt(111) were measured in the temperature range of 50-200 K to elucidate the adsorption/aggregation states, thermal stabilities of pure and binary films, and local electronic properties at the organic-metal interface. For C(6)H(6)/Pt(111), the He(?)(2(3)S) atoms de-excite on the chemisorbed overlayer predominantly via resonance ionization followed by Auger neutralization and partly via Penning ionization (PI) yielding weak emission just below the Fermi level (E(F)). We assigned this emission to the C(6)H(6) π-derived states delocalized over the Pt?5d bands on the basis of recent density functional calculations. During the layer-by-layer growth, the C(6)H(6)-derived bands via PI reveal a characteristic shift caused by the final-state effect (hole response at the topmost layer). C(6)H(6) molecules chemisorb weakly on the bimetallic Pt(111) (θ(K)=0.1) and physisorb on the K multilayer. In both cases, the sum rule was found to be valid between the K?4s and C(6)H(6)-derived bands. The band intensity versus exposure plot indicates that the C(6)H(6) film grows on the K multilayer by the Volmer-Weber mechanism (island growth), reflecting the weak K-C(6)H(6) interactions. In case of K/C(6)H(6)/Pt(111), the K atoms are trapped on the topmost C(6)H(6) layer at 65 K, forming particlelike clusters. The surface plasmon satellite was identified for the first time and the loss energy increases with increasing cluster size. The K clusters are unstable above ～100?K due to thermal migration into the C(6)H(6) film. When the cluster coverage is low, the K?4s band extends below and above E(F) of the Pt substrate and the anomaly is discussed in terms of vacuum level bending around the cluster.     

Scattering of neon atom by nitrogen molecule at thermal and subthermal energies

Collision of Ne atom with N₂ molecule in the thermal and subthermal energy regions (0.01–260 meV) has been thoroughly investigated. Centrifugally decoupled exponential distorted wave (CDEDW) method and the BSTT interaction potential have been used to calculate elastic as well as inelastic (rotational) scattering cross sections. The resonances in the very low energy region,R —Tenergy transfer mechanisms and nature of the rotational excitation cross sections near the threshold have been examined very critically. In general CDEDW method has been found to work quite well in this energy region.     

Structure of incommensurate gold sulfide monolayer on Au(111)

We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450 K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S-Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates.     

Ni/Pt(111) bimetallic surfaces: unique chemistry at monolayer ni coverage.

We have utilized the dehydrogenation and hydrogenation of cyclohexene as probe reactions to compare the chemical reactivity of Ni overlayers that are grown epitaxially on a Pt(111) surface. The reaction pathways of cyclohexene were investigated using temperature-programmed desorption, high-resolution electron energy loss (HREELS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our results provide conclusive spectroscopic evidence that the adsorption and subsequent reactions of cyclohexene are unique on the monolayer Ni surface as compared to those on the clean Pt(111) surface or the thick Ni(111) film. HREELS and NEXAFS studies show that cyclohexene is weakly pi-bonded on monolayer Ni/Pt(111) but di-sigma-bonded to Pt(111) and Ni(111). In addition, a new hydrogenation pathway is detected on the monolayer Ni surface at temperatures as low as 245 K. By exposing the monolayer Ni/Pt(111) surface to D2 prior to the adsorption of cyclohexene, the total yield of the normal and deuterated cyclohexanes increases by approximately 5-fold. Furthermore, the reaction pathway for the complete decomposition of cyclohexene to atomic carbon and hydrogen, which has a selectivity of 69% on the thick Ni(111) film, is nearly negligible (<2%) on the monolayer Ni surface. Overall, the unique chemistry of the monolayer Ni/Pt(111) surface can be explained by the weaker interaction between adsorbates and the monolayer Ni film. These results also point out the possibility of manipulating the chemical properties of metals by controlling the overlayer thickness.     

Water activated by atomic oxygen on Au(111) to oxidize CO at low temperatures

The Au(111) surface was populated with atomic oxygen [16O] followed by oxygen-labeled water [H218O] at surface temperatures as low as 77 K. When a CO beam was impinged on this surface, both [C16O16O] and [C16O18O] were produced. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111) at low temperatures.