D abstraction by H at a D-saturated Ru(0 0 1) surface

D abstraction (ABS) by H at Ru(0 0 1) surfaces initially saturated with D adatoms has been investigated using in situ mass spectrometry. HD and D₂ desorption rates are measured at various surface temperatures T as a function of H exposure time. Yield of D₂ desorption increases with T, while that of HD is little affected. Analyzing the measured rate curves, HD and D₂ desorption orders are evaluated to be 1.7 ± 0.1 and 2.5 ± 0.1, respectively, with respect to D coverage θ_D. To pursue the origin of the derived non-integral reaction orders the rate curves are further analyzed with the rate equations constructed to involve several ABS channels. Consequently, we find that the HD desorption is mainly governed by a second-order rate law in θ_D rather than the conventional hot atom-mediated ABS reaction even when it is corrected to include an isotope effect on ABS. We argue that such second-order ABS kinetics becomes important when the H atoms in excited state of chemisorption have energetically relaxed to some extent, and thereby tend to reside at, e.g. hexagonal closed packed hollow sites, interacting with nearby adatoms. On the other hand, the D₂ rate curves can be fit with third-order kinetics, consistent with the Langmuir-Hinshelwood mechanism in a super-saturation state. The isotope effect plays an essential role in the ABS reaction of D abstraction by H which competes with H abstraction by H as D adatoms are replaced by H atoms.     

Photoelectron spectroscopy for probing surface reactions at the CO/K/Cu(1 1 5) interface

The adsorption of carbon monoxide on the potassium modified Cu(1 1 5) surface was investigated using photoelectron spectroscopy based on synchrotron radiation. From detailed analysis of the 1s core levels in combination with existing knowledge, the assignment of surface species is performed. It is demonstrated that in dependence of the alkali coverage, several adsorption states of CO are present on the interface at 135 K. From the temperature dependence of the C 1s and O 1s profiles it is established that surface reactions based on CO dissociation start from 223 K over an interface with a potassium coverage close to half a complete K overlayer. The role of potassium as a reordering environment of adsorbed CO, leading to molecule dissociation and disproportionation is proposed. It is observed that a higher density of potassium on the substrate surface blocks adsorption sites for incoming CO molecules and no dissociation takes place.     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

Optical solitons in (1 + 1) and (2 + 1) dimensions

This paper addresses the nonlinear Schrödinger's equation that serves as the model to study the propagation of optical solitons through nonlinear optical fibers. The main focus of this paper is the aspect of integrability. There are a couple of integration tools that are employed to obtain the exact solutions to the model. Fan's F-expansion approach is applied to extract several forms of solutions to the model. This integration mechanism displays cnoidal waves, snoidal waves and several other solutions; needless to mention that these solutions, in the limiting case, leads to bright, dark and singular soliton solutions. The study then rolls over to the (2 + 1)-dimensions where, in addition, the semi-inverse variational principle is applied to extract a bright soliton solution, along with the necessary constraint conditions. There is also a display of several numerical simulations.     

Rate coefficients measurement for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F)

We report experimental rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). In the experiment the Cs(5D) state was populated via photodissociation of Cs₂ molecules using an argon-ion laser at wavelength 488.0 nm. We also consider the competing process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F) that might also populate 9D, 11S and 7F. An intermodulation technique was used to select the fluorescence contributions due only to the process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F). The excited atom (nl_J) density and spatial distribution were mapped by monitoring the absorption of a counterpropagating probe laser beam tuned to various transitions. The measured excited atom densities are combined with measured fluorescence ratios to yield rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). The rate coefficients for nl = 9D, 11S, 7F are (4.1 ± 2.0) × 10⁻¹⁰ cm³ s⁻¹, (1.6 ± 0.8) × 10⁻¹⁰ cm³ s⁻¹ and (3.6 ± 1.8) × 10⁻¹⁰ cm³ s⁻¹, respectively. The contributions to the rate coefficients from other energy transfer processes are also discussed.     

Thermal reactions of 2-iodoethanol on Cu(1 0 0)

Temperature-programmed reaction/desorption, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy have been employed to investigate the reactions of ICH₂CH₂OH on Cu(1 0 0) under ultrahigh-vacuum conditions. ICH₂CH₂OH can dissociate on Cu(1 0 0) at 100 K, forming a -CH₂CH₂OH surface intermediate. Density functional theory calculations predict that the -CH₂CH₂OH is most probably adsorbed on atop site. -CH₂CH₂OH on Cu(1 0 0) further decomposes to yield C₂H₄ below 270 K. No evidence shows the formation of -CH₂CH₂O- intermediate in the reactions of ICH₂CH₂OH on Cu(1 0 0) in contrast to the decomposition of BrCH₂CH₂OH on Cu(1 0 0) and ICH₂CH₂OH on Ag(1 1 1) and Ag(1 1 0), exhibiting the effects of carbon-halogen bonds and metal surfaces.     

Cu(1 1 1) and Cu(0 0 1) surface electronic states. Comparison between theory and experiment

Recent advances in both the experimental resolution and in the computational capabilities motivate new studies of surface properties. In particular, a detailed comparison between theoretical and experimental data is expected to provide a better insight into surface and image states. In this work we present a joint effort analyzing such features of the Cu(1 1 1) and Cu(0 0 1) surfaces. The experiments are performed by using both linear and non-linear angle-resolved photoemission. From the theoretical point of view, we make use of the Green function embedding technique within density functional theory. We are able to account for the image states by suitably modifying the effective potential in the Kohn-Sham equation and the generalized boundary conditions on the Green function. Comprehensive theoretical and experimental results on the effective mass and the binding energy of the Shockley state and the first image states are reported.     

Anisotropic diffusion of Cu adatoms on strained Cu (1 1 1) surface

Diffusion of Cu atoms on a strained Cu (1 1 1) surface was studied by molecular dynamic simulation using an EAM potential. The anisotropic diffusion behaviour is found when the uniaxial strain is imposed on the surface, which does not exist under the biaxial strain. The migration of the adatom is suppressed along the tensile strain direction. The results suggest that different island morphology can be obtained by controlling anisotropic diffusion of adatoms on the strained surfaces during film growth.     

Assignment of surface IR absorption spectra observed in the oxidation reactions: 2H + H2O/Si(1 0 0) and H2O + H/Si(1 0 0)

Infrared reflection absorption spectroscopy that used buried metal layer substrates (BML-IRRAS) and density functional cluster calculations were employed to investigate the water related oxidation reactions of 2H + H₂O/Si(1 0 0)-(2 × 1), 2D + H₂O/Si(1 0 0)-(2 × 1), and H₂O + H/Si(1 0 0)-(2 × 1). In addition to the oxygen inserted coupled monohydrides, which were previously reported in the former reaction system, we report several other oxidized Si hydride species in our BML-IRRAS experiments. Three new pairs of vibrational bands are identified between 900 and 1000 cm⁻¹. These vibrational frequencies were calculated using Si9 and Si10 cluster models that included all possible structures from zero to five oxygen insertions into the top layer silicon atoms using a B3LYP gradient corrected density functional method with a polarized 6-31G** basis set for all atoms. The three pairs of vibrational modes are assigned to the scissoring modes of adjacent and isolated SiH₂ with zero, one, and two oxygen atoms inserted into the Si back bonds. All the other newly observed vibrational peaks related to Si oxidation are also assigned in this study. The Si-O stretching bands observed in the reaction 2D + H₂O/Si(1 0 0)-(2 × 1) show an isotope effect, which suggests that in the system 2H + H₂O/Si(1 0 0)-(2 × 1) also, hydrogen atom tunneling plays an important role for the insertion of oxygen atoms into Si back bonds that form oxidized adjacent dihydrides.     

Formate-induced destabilization of the Cu(1 1 0) surface

Scanning tunneling microscopy (STM) images show that adsorbed formate has a profound affect on the step edges of Cu(1 1 0) surfaces at room temperature. For low exposures, the presence of formate enhances step fluctuations as confirmed by a correlation function analysis. For formate coverages approaching 0.5 monolayers, drastic restructuring of step edges is observed. Quantum chemical calculations help to explain this behavior.     

Diffusion of single adatom Cu on Cu (0 0 1) and (1 1 0) surfaces

With static relaxation, the surface diffusion activation energies of a single Cu adatom migrated by both atomic exchange and hopping mechanisms and the forces acted on the diffusing adatom from other atoms of Cu (0 0 1) or (1 1 0) surface are calculated by using the MAEAM. When adatom migrated on Cu (0 0 1) or (1 1 0) surface, the increment curves of the system energy by hopping mechanism are symmetrical and the saddle points are in the midpoints of the migration path, but the ones by the exchange mechanism are dissymmetrical and the saddle points are always close to the initial hole positions of the adatom and away from the initial equilibrium positions of the exchanged atom. From minimization of both the diffusion activation energy and the force acted on the diffusing adatom from other atoms, we found that, on Cu (0 0 1) surface the favorable diffusion mechanism is hopping mechanism, however, on Cu (1 1 0) surface, hopping via long bridge is easier than the exchange mechanism but the hopping via short bridge is more difficult than the exchange mechanism.     

(6 × 2) Reconstruction of the Ag/Si(1 1 1) surface at 77 K

The ground state of the Ag/Si(1 1 1)-(3 × 1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6 × 2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3 × 1). The most stable (6 × 2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains.     

A first-principles study of C + O reaction on NiCo(1 1 1) surface

A density-functional theory method has been conducted to investigate the association of C + O on (1 1 1) facets of ordered NiCo alloy and the results have been compared with those obtained on pure Ni(1 1 1) surface. In reaction of C + O, the favorable reaction path is that C adsorbed on HCP-1 site moves to the nearest Ni-Co bridge site, and associates with O migrating from FCC-1 site to result in CO adsorbed on the bridge site of Ni-Co. However, the reaction barrier is higher by 0.35 eV than that on pure Ni(1 1 1), which indicates that the incorporation of Co into the Ni crystal is not in favor of the reaction of carbon delimination.     

Synchrotron photoemission studies of pentacene films on Cu(1 1 0)

We have investigated the nature of the interaction between pentacene and Cu(1 1 0) for films grown by two different methods, and the energy level alignment at the resulting pentacene/Cu(1 1 0) interface. The first film was grown in a stepwise fashion at room temperature while the second film was grown using an annealed monolayer to template the growth of the subsequent layers. Synchrotron based photoemission techniques have been used to compare the initial stages of the growth of the two films. A thorough examination of the growth of both films indicates a strong bond between the initial monolayer of pentacene and the Cu(1 1 0) substrate, in addition to a change in molecular orientation for subsequent layers after the completion of the initial monolayer. A comparison of the two films indicates identical film growth for both films with the film templated by the annealed monolayer exhibiting a more uniform growth mode for the film thicknesses investigated.     

Non-collinear magnetisation of V clusters supported on a Cu (1 1 1) surface: Theory

Magnetic properties and electronic structure of V clusters supported on a Cu (1 1 1) substrate, have been calculated from a first principles method. We observe in general non-collinear magnetic structures that are the result of antiferromagnetic interactions on a frustrated lattice. The values of the magnetic moments range from ∼0 to 2.7 μ_B/atom, depending on cluster geometry.     

A semi-empirical study of polyacene molecules adsorbed on a Cu(1 1 0) surface

To describe the adsorption of large organic molecules on metal surfaces, to calculate the corresponding diffusion and rotation barriers, the semi-empirical mono-electronic Hamiltonian of the ASED molecular orbital method have been completed to take into account three body interaction terms. The full re-parametrization of this ASED+ version of ASED was determined on the specific case of benzene adsorbed on Cu(1 1 0) and a full transferability assumed for the member of the polyacene series also adsorbed on Cu(1 1 0). The adsorption energies, geometries, diffusion and rotation barriers are very well described by this new semi-empirical technique of calculation opening the way of optimizing larger conjugated molecule on surface for uni-molecular mechanics or electronics.     

Characterisation of the interaction of glycine with Cu(1 0 0) and Cu(1 1 1)

Reflection-absorption infrared spectroscopy (RAIRS) has been used to characterise the interaction of standard and fully deuterated glycine with Cu(1 0 0) and Cu(1 1 1). RAIRS shows clearly that the surface interaction leads to formation of the adsorbed deprotonated glycinate (NH₂CH₂COO-) species, with some evidence for changes in orientation with coverage previously seen on Cu(1 1 0). Qualitative low energy electron diffraction observations were also conducted to characterise the long-range ordering, although effects of electron-beam-induced radiation damage limited the information obtained. Nevertheless, the results do suggest some subtle isotopic-mass-related structural variations. The results are discussed in the context of previously published scanning tunnelling microscopy and photoelectron diffraction measurements.     

LCAO study of the Cu(1 1 0)-p(2 × 1)O surface

We have performed semi-empirical LCAO calculations of the electronic structure of the Cu(1 1 0)-p(2 × 1)O surface. This has been done accounting for the Cu-Cu interactions by means of a recently proposed set of parameters, which give very good results for the bulk as well as for the surfaces of lowest Miller indices. Furthermore, the O-O interactions, which have been neglected in the preceding similar studies, have been taken into account. The resulting surface bands are in very good agreement with the overall set of the available experimental data. Several issues concerning the physical properties of this surface are addressed in the present paper: the changes induced on the clean surface bands by the adsorption and the reconstruction; the arrangement of the Cu and O atoms in the added rows; the position of the p_y antibonding band of the oxygen. In particular, we have found that the latter has an energy of −0.2 eV at the point. This result confirms an experimental indication in the same direction previously reported by Courths et al. [R. Courths, S. Hüfner, P. Kemkes, G. Wiesen, Surf. Sci. 376 (1997) 43].     

Molecular view of copper deposition chemistry: (Hexafluoroacetylacetonate)Cu(vinyltrimethylsilane) on a Si(1 0 0)-2 × 1 surface

The chemistry of a common copper deposition precursor, (hexafluoroacetylacetonate)Cu(vinyltrimethylsilane), (hfac)Cu(VTMS), on a single crystal Si(1 0 0)-2 × 1 surface is described at the molecular level using a combination of experimental surface analytical techniques under ultra-high vacuum conditions with computational analysis. At a cryogenic temperature of 100 K, (hfac)Cu(VTMS) adsorbs on this surface molecularly, without noticeable decomposition. Upon surface annealing, VTMS is easily released into the gas phase below the room temperature, while the hfac ligand is bound to the surface through the copper atom. When (hfac)Cu(VTMS) is adsorbed at room temperature, VTMS is released into the gas phase immediately, leaving surface adsorbate analogous in structure to the one formed by adsorption at cryogenic temperature and a brief annealing to room temperature. Upon surface annealing, the hfac ligand decomposes and constitutes the main source of impurities in copper deposition process.     

Molecular assembly of tetracene on the (2 × 1)O reconstructed Cu(1 1 0) surface

Using a combination of scanning tunneling microscopy (STM) and density functional theory calculations, we have studied the adsorption of tetracene on the Cu(1 1 0) (2 × 1)O substrate. At monolayer coverage the adsorbed molecules are in the flat-laying geometry with their long axis along the close-packed [0 0 1] direction of the substrate and a long-range ordered structure on the length scale up to 100 nm has been observed. DFT calculation results indicate a stronger interaction between tetracene molecules and Cu(1 1 0) substrate than Cu(1 1 0) (2 × 1)O substrate. The preferential adsorption sites have also been pointed out on both substrates. The observed wavelike structure is explained by the interdigitation of C-H bonds of adjacent molecules.