Stabilization of surface reaction intermediates by added metal atoms on metal surfaces of low free energy

Dynamic restructuring of the Ag(111) surface occurs during the reaction of sulfur dioxide with Ag(111)-p(4 x 4)-O at 300 K, resulting in the incorporation of added silver atoms into the unit cells of both adsorbed sulfite and sulfate. This result clearly demonstrates that incorporation of metal atoms into the structures of adsorbates and reaction intermediates is not restricted to more open, higher free energy single crystal planes. These observations indicate that the participation of added metal atoms must be considered in the theoretical treatment of metal catalyzed reactions.     

Spin decoherence of magnetic atoms on surfaces

We review the problem of spin decoherence of magnetic atoms deposited on a surface. Recent breakthroughs in scanning tunnelling microscopy (STM) make it possible to probe the spin dynamics of individual atoms, either isolated or integrated in nanoengineered spin structures. Transport pump and probe techniques with spin polarized tips permit measuring the spin relaxation time $T_1$, while novel demonstration of electrically driven STM single spin resonance has provided a direct measurement of the spin coherence time $T_2$ of an individual magnetic adatom. Here we address the problem of spin decoherence from the theoretical point of view. First we provide a short general overview of decoherence in open quantum systems and we discuss with some detail ambiguities that arise in the case of degenerate spectra, relevant for magnetic atoms. Second, we address the physical mechanisms that allows probing the spin coherence of magnetic atoms on surfaces. Third, we discuss the main spin decoherence mechanisms at work on a surface, most notably, Kondo interaction, but also spin–phonon coupling and dephasing by Johnson noise. Finally, we briefly discuss the implications in the broader context of quantum technologies.     

Participation of preadsorbed atoms in heterogeneous recombination of hydrogen atoms on semiconductor surfaces

Atoms trapped from the vapor phase in the preadsorbed state were found to provide the catalytic activity of semiconductors in the heterogeneous recombination of hydrogen atoms.     

Kinetics of heterogeneous recombination of hydrogen atoms on solid surfaces

Heterogeneous recombination rates of hydrogen atoms on solid surfaces were measured for the first time. An anomalous form of these curves testifies that the reaction on surfaces of metals, semiconductors and dielectrics follows a universal mechanism with participation of preadsorbed atoms.     

Enthalpies of adsorption of metal atoms on single-crystalline surfaces by microcalorimetry

The heats of adsorption of metal gas atoms onto single crystalline surfaces can be measured directly as a detailed function of coverage using a unique microcalorimeter. The development of this experimental apparatus and the results obtained from these experiments will be reviewed. The heat detection is performed with a highly sensitive pyroelectric polymer ribbon pressed into contact with the thin (1 μ m thick) single crystal sample, onto which pulses of a metal atom beam impinge. Heats of adsorption have been measured for pulses of gas containing a few per cent of a monolayer with a pulse-to-pulse standard deviation as low as 1.5 kJ · mol ^− 1. The adhesion energy of multilayer metal films can be estimated from the integral enthalpy of adsorption.     

Electron emission from naphthacene crystal due to the impact of argon and krypton metastable atoms

Energy distributions of electrons emitted from polycrystalline naphthacene due to the impact of metastable argon or krypton atoms were measured. The energy distribution peaks, except for large peaks appearing near zero eV, correspond to the kinetic energies estimated from photoelectron spectra on the assumption that the excitation energies of the metastable atoms are transferred to the electrons in the valence bands. The results are interpreted as the occurrence of Penning ionization (Auger de-excitation) on the naphthacene surface.     

Hamiltonian theory for vibrational line shapes of atoms adsorbed on surfaces

The vibrational motions of atomic adsorbates on surfaces can be probed by helium atom scattering. The experimental observable is the dynamic structure factor, which shows an inelastic peak around the vibrational frequency of the isolated adsorbates known as the frustrated translational or T-mode peak. In this paper we develop a theory for the line shape of this peak, as well as for its temperature-dependent shift and broadening, based on a Hamiltonian equivalent of the generalized Langevin equation. The theory can be used to infer physical parameters of the adatom-surface interaction, such as the friction coefficient, the barrier height to diffusion, and the anharmonicity parameter. Numerical simulations are used to ascertain the range of validity of the theory, which is also generalized to describe multidimensional systems and to include quantum corrections. We compare the theoretical predictions for the shift and broadening with experimental results for the Na/Cu(001) system, showing quantitative agreement within experimental resolution.     

Basis-Modified hydrogen atoms as embedding atoms in ab initio chemisorption cluster model calculations on Si surfaces

A comparative and systematic ab initio study of different models simulating the Si (111) surface has been carried out for a variety of embedding hydrogen atoms including unmodified hydrogen atoms and modified hydrogen atoms described with a STO-4G basis set and a Slater exponent optimized to have the cluster atoms as neutral as possible. The study has been extended to some chemisorption processes as Ag and Al on Si (111). The main conclusion of the present work is that neither the electronic structure of the isolated cluster models nor the nature of the chemisorption bond depend on the kind of embedding hydrogen atoms used to saturate the free valences of the cluster edge atoms. © John Wiley & Sons, Inc.     

Absolute total cross sections for scattering of hydrogen atoms by argon,krypton and xenon

Absolute total cross sections for scattering of hydrogen atoms by argon, krypton and xenon were measured as a function of velocity in the range from 1.8 to 6.2 km/sec. An analysis in terms of Lennard-Jones (12.6) and (8.6) model potentials leads to estimates of the interatomic forces.     

Preferential hydration and the exclusion of cosolvents from protein surfaces

Protein stability is enhanced by the addition of osmolytes, such as sugars and polyols and inert crowders, such as polyethylene glycols. This stability enhancement has been quantified by the preferential hydration parameter which can be determined by experiments. To understand the mechanism of protein stability enhancement, we present a statistical mechanical analysis of the preferential hydration parameter based upon Kirkwood-Buff theory. Previously, the preferential hydration parameter was interpreted in terms of the number of hydration waters, as well as the cosolvent exclusion volume. It was not clear how accurate these interpretations were, nor what the relationship is between the two. By using the Kirkwood-Buff theory and experimental data, we conclude that the contribution from the cosolvent exclusion dominantly determines the preferential hydration parameters for crowders. For osmolytes, although the cosolvent exclusion largely determines the preferential hydration parameters, the contribution from hydration may not be negligible.     

Capillary theory of free fluid surfaces

The traditional formulation of capillary theory does not explicitly contain the general dimension equation that is valid also for its own scalar variables. Its introduction enables the experimentally determinable physical properties to be interpreted. These properties individually characterize the bulk phases generating the layers that enter into the capillary interaction. Not only empirically known approximate relationships, such as the van der Waals and Walden equations, the Watson’s formula, and the Cailletet–Mathias rule, can be derived through them, but also new findings can be made. By extending the formulation with a new type of parameters, the relationships of temperature, density-dependence, etc. may be directly generated. The individual concept, which differs from the traditional theory in only one extra power law, is compatible with other capillarity methods based on material structure and outperforms the heuristic power of the traditional theory in terms of operability.     

Diffusive dynamics on multidimensional rough free energy surfaces

The dynamics of processes relevant to chemistry and biophysics on rough free energy landscapes is investigated using a recently developed algorithm to solve the Smoluchowski equation. Two different processes are considered: ligand rebinding in MbCO and protein folding. For the rebinding dynamics of carbon monoxide (CO) to native myoglobin (Mb) from locations around the active site, the two-dimensional free energy surface (FES) is constructed using extensive molecular dynamics simulations. The surface describes the minima in the A state (bound MbCO), CO in the distal pocket and in the Xe4 pocket, and the transitions between these states and allows to study the diffusion of CO in detail. For the folding dynamics of protein G, a previously determined two-dimensional FES was available. To follow the diffusive dynamics on these rough free energy surfaces, the Smoluchowski equation is solved using the recently developed hierarchical discrete approximation method. From the relaxation of the initial nonequilibrium distribution, experimentally accessible quantities such as the rebinding time for CO or the folding time for protein G can be calculated. It is found that the free energy barrier for CO in the Xe4 pocket and in the distal pocket (B state) closer to the heme iron is approximately 6 kcal/mol which is considerably larger than the inner barrier which separates the bound state and the B state. For the folding of protein G, a barrier of approximately 10 kcal/mol between the unfolded and the folded state is consistent with folding times of the order of milliseconds.     

Adsorption of atoms on cu surfaces: a density functional theory study

The chemisorption of atoms (H, N, S, O, and C) on Cu surfaces has been systematically studied by the density functional theory generalized gradient approximation method with the slab model. Our calculated results indicate that the orders of the adsorption energy are H < N < S < O < C on Cu(111) and H < N < O < S < C on Cu(110) and Cu(100). Furthermore, the adsorption energies of the given atoms on Cu(100) are larger than those on Cu(111) and Cu(110). The preferred adsorption sites are a 3-fold hollow site on Cu(111) and a 4-fold hollow site on Cu(100), but the preferred adsorption sites on Cu(110) are different for different adatoms. The energy, as well as the geometry, is in good agreement with the experimental and other theoretical data. In addition, this study focuses on the electronic and geometric properties of the metal-atom (M-A) bond to explain the difference in adsorption energies among adatoms. A detailed investigation of the density of states curves explains the nature of the most stable site. Finally, we test the effect of the coverage and find that the surface coverage has no influence on the preferred adsorption sites of the given adatoms on Cu(110) with the exception of hydrogen and oxygen, but has much influence on the value of the adsorption energy.     

Classical stochastic theory for the sticking probability of atoms scattered on surfaces

A stochastic theory is formulated for the sticking probability of a projectile scattered from a surface. The theory is then explored by applying it to a generalized Langevin equation model of the scattering dynamics. The theory succeeds in describing the known features of trapping on surfaces. At low energies sticking will occur only if there is an attractive interaction between the projectile and the surface. The probability of sticking at low energies is greater the lower the temperature and the deeper the attractive well of the particle as it approaches the surface. The sticking probability in the absence of horizontal friction tends to be lower as the stiffness of the surface increases. However, in the presence of horizontal friction, increased stiffness may lead to an increase in the sticking coefficient. A cos(2)(θ(i)) scaling is found only in the absence of corrugation and horizontal friction. The theory is then applied successfully to describe experimentally measured sticking probabilities for the scattering of Xe on a Pt(111) surface.     

Preferential site of attack on fullerene cations: frontier orbitals and rate coefficients

An analysis of reaction efficiency is presented for reactions of carbonaceous ions and molecules. Our results show that the combination of experimental rate-coefficient measurements and computations of the condensed Fukui functions of frontier molecular orbitals and pyramidal angles of pi orbitals is very useful for elucidating the reactive sites on fullerene carbon clusters in the gas phase.     

Spectrum of electronic excitations due to the adsorption of atoms on metal surfaces

The time-dependent, mean-field Newns-Anderson model for a spin-polarized adsorbate approaching a metallic surface is solved in the wide-band limit. Equations for the time evolution of the electronic structure of the adsorbate-metal system are derived and the spectrum of electronic excitations is found. The behavior of the model is demonstrated for a set of physically reasonable parameters.     

X-Ray emission modulation by electron channelling and site occupancy in garnets

Axial channelling effects in a pyrope garnet for three zone axis orientations: [111], [211], and [311] have been investigated. The variations of the characteristic X-ray emission under the [111] zone axis diffracting conditions was correlated with dynamical n-beam calculations. Site occupancies determinations obtained by comparing X + Z and Y sites have been made using ALCHEMI techniques.     

Preferential site of solvation of the furan ring in acidic solvents

A study of the correlation of solvent effects on the chemical shifts of the methyl protons of sylvan, toluene and α-picoline led to the conclusion that the preferential site of solvation of the furan ring in acidic solvents is the π-electron cloud and not the n-electron orbital of the oxygen atom.