Electromagnetic effects in the Surface Enhanced Raman Scattering from a molecule at a liquid Hg surface

In a previous publication the image enhancement effect and the enhanced radiation due to near field excitation of surface roughness were calculated for a finite-size molecule above a metal described by a nonlocal dielectric relation [Phys. Rev. Letters 44 (1980) 1774]. When applied to a roughened Ag surface these calculations led to a predicted Surface Enhanced Raman Scattering (SERS) gain of ~ 10³ due to each effect, yielding an overall gain ~ 10⁶, in agreement with experiment. Here these calculations are extended to the case of a liquid Hg surface, the roughness corresponding to thermally excited ripplons. The SERS gain due to image enhancement is reduced to ~ 10, due primarily to the ~ 24 times greater electron scattering rate in Hg over that in Ag. The roughness gain is reduced to ~ 2 at room temperature, due to the difference between the ripplon spectrum and that of the boss-like surface structures assumed for the solid surface. The predicted overall SERS gain for Hg is ~ 20, far less than reported in recent observations [R. Naanan et al., J. Phys. Chem. 84 (1980) 2692]. The discrepancy raises serious questions about the electromagnetic explanations of SERS.     

Dynamics of geometric discord and measurement-induced nonlocality at finite temperature

By using geometric measure of discord (GMOD) [B. Dakić, V. Vedral, Č. Brukner, Phys. Rev. Lett. 105, 109502 (2010)] and measurement-induced nonlocality (MIN) [S. Luo, S. Fu, Phys. Rev. Lett. 106, 120401 (2011)], we investigate quantum correlation of a pair of two-level systems, each of which is interacting with a reservoir at finite temperature T. We show that, for a broad class of states of the system, GMOD and MIN can endure sudden death, and there is no asymptotic decay for MIN while asymptotic decay exists for GMOD. We also give the dynamics of GMOD and MIN with respect to the temperature and illustrate their different characteristics.     

Potential surface waves at the boundary of a metal with a magnetoactive plasma at finite pressure

The propagation of surface-type potential waves along the interfacial boundary of a plasma with an ideally conducting metal in an external magnetic field perpendicular to the boundary is examined. It is shown that a necessary condition for the existence of these waves in the system is a finite gas kinetic pressure. Dispersion relations for these waves and expressions for the penetration depth of the wave fields into the plasma are obtained, and they are studied numerically for various plasma parameters. The frequency region for propagation of these waves is found. It is also shown that in a nonzero external magnetic field a system of this kind has a range of frequencies in which the wave is a generalized surface wave. Zh. Tekh. Fiz. 69, 30–33 (November 1999)     

On the structure of a water molecule chemisorbed on a metal surface

A model using the electrostatic image force and the harmonic restoring force has been adapted to find the equilibrium structure of a chemisorbed water molecule on a metal surface. An estimate of the maximum possible increase in the HOH interbond angle upon chemisorption due to image forces is given.     

Electromagnetic coupling between a metal nanoparticle grating and a metallic surface

The electromagnetic coupling between a two-dimensional grating of resonant gold nanoparticles and a gold metallic film is investigated. We report on the observation of multipeaks in the extinction spectra attributed to resonant modes of the hybrid system, resulting from the coupling between the localized plasmon of the nanoparticles with the underlying surface plasmon mode. Simulations based on the Fourier modal method give good agreement with the experimental measurements and allow for the identification of the respective contributions.     

Molecular distortions and chemical bonding of a large pi-conjugated molecule on a metal surface

Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are 0.18 +/- 0.03 angstroms closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled--through charge transfer into the former lowest unoccupied molecular orbital--to the primary, extended chemisorption bond via the perylene skeleton.     

Comments on chemisorption at a metal surface

The effect of the gradient correction to the exchange and correlation energies on chemisorption properties is examined using the evaluation of B_χc arrived at by Rasolt and Geldart. Quantities common to the study of hydrogen and alkali chemisorption on a high density metal are calculated for various forms of the energy functional within the linear response approach. It is found that the gradient correction is a quantitatively significant term, lowering binding energies by ≈ 10% and increasing the dipole sizably.     

X射线在金属表面产生Compton电流引起的电磁辐射时空分布

研究了强X射线在金属表面产生的Compton电流时空分布,利用Compton散射公式计算了出射电子动量和微分散射截面随电子出射角的变化关系,发现在光子能量为1 MeV、入射光强为1021 W·m-2的条件下,沿入射光方向形成的Compton电流密度达到107 A·m-2量级。采用1维电动力学模型计算了斜入射的X射线在金属表面产生的Compton电流密度引起的电磁脉冲,结果表明：X射线在单位长度金属辐射产生的磁场强度达到106 A·m-1量级,脉冲宽度为ps量级;电磁脉冲沿X射线反射方向传播,具有良好的定向性。     

X射线在金属表面产生Compton电流引起的电磁辐射时空分布

 研究了强X射线在金属表面产生的Compton电流时空分布,利用Compton散射公式计算了出射电子动量和微分散射截面随电子出射角的变化关系,发现在光子能量为1 MeV、入射光强为10²¹ W·m^-2的条件下,沿入射光方向形成的Compton电流密度达到10⁷ A·m^-2量级。采用1维电动力学模型计算了斜入射的X射线在金属表面产生的Compton电流密度引起的电磁脉冲,结果表明：X射线在单位长度金属辐射产生的磁场强度达到106 A·m^-1量级,脉冲宽度为ps量级;电磁脉冲沿X射线反射方向传播,具有良好的定向性。     

Dynamics of a crystal containing a molecular impurity—I. Rigid molecule approximation

The dynamics of a crystal containing a rigid, isolated substitutional molecular impurity is discussed using the Green’s function method. The dynamical matrix for the problem is set up, and the various constraints on the force constants are pointed out. The application of the matrix partitioning technique is then indicated, and the possibility of a resonance mode arising from molecular librations is pointed out. Comparison is made with the earlier work of Wagner, and finally the relevance of the present formalism to the interpretation of a recent neutron scattering experiment is discussed.     

Real-time STM observation of molecular dynamics on a metal surface

Dynamic processes of molecular assembly on a metal surface were examined using scanning tunneling microscopy (STM). Molecules of a porphyrin derivative were deposited on a Cu(1 1 1) surface and were found to be highly mobile at room temperature. The real-time STM observation enabled visualization of molecular activity such as surface diffusion, domain formation and phase transition. The high mobility of the molecules caused build-up and break-down of molecular domains. Metastability of the molecular assembly caused various domain formations with different molecular alignments, including square and hexagonal motifs. A phase transition from a hexagonal to a square domain structure was successfully observed by sequential STM imaging.     

Conductivity of a quasi-one-dimensional metal at finite temperatures

The longitudinal conductivity of a quasi-one-dimensional metal is calculated for the case T?ω_D (ω_D being the limiting phonon frequency) and ω_Dl¹/v?1 where l¹ is the effective mean free path determined by impurity and phonon scattering: $\text{l}{}^1\text{= (l}{}^{\mathrm{?1}}{}_{\mathrm{ph}}\text{+ l}{}^{\mathrm{?1}}{}_{\mathrm{i}}\text{)}{}^{\mathrm{?1}}\text{, l}{}_{\mathrm{ph}}\text{= v/λT, l}{}_{\mathrm{i}}$ is the impurity mean free path. The conductivity is σ = (c₁e²/πS)l³_iv^?2ω_DλT for l_i?l_ph, σ = (c₂e²/πS)vω_D(λT)^?2 for l_i?l_ph, λ being the dimensionless electron-phonon interaction constant, c₁, c₂ ～ 1, S = a_xa_y is the (xy) area per one chain.     

Van der Waals interaction between a molecule and a metal surface: Effect of spatial dispersion

In this paper the effect of spatial dispersion on the dielectric response of a semi-infinite metal is investigated in the hydrodynamic approximation, and the result is used to evaluate the van der Waals force between a molecule and a metal surface. It is shown that corrections to the image potential result arising out of spatial dispersion can be evaluated systematically.     

Shift and broadening of the vibrational mode for a diatomic molecule adsorbed on a metal surface

The shift and broadening of the vibrational frequency of a diatomic molecule adsorbed on a metal surface are calculated on the basis of electromagnetic interactions. Finite molecular size and nonlocal metal response are included in a calculation of the response function, whose pole is the complex normal mode frequency. Finite molecular size is described by a model of a point-like vibrational mode within a polarizable sphere. Nonlocal metal response is described by a semiclassical infinite barrier model with a Lindhard dielectric constant generalized to include core polarization and finite electron lifetime. When applied to the case of CO adsorbed on Cu and if the molecule is positioned so as to fit the observed width, it is found the predicted shift is a factor of three or more greater than that observed.     

Elastic interactions of adatoms or clusters on a surface: Effects due to anisotropy of the substrate and finite size of the clusters

The interactions of atoms or clusters of atoms adsorbed on a surface due to elastic strains created in the substrate is investigated. At large distances it falls off ass ^–3. The strength of the interaction which is repulsive for elastically isotropic substrates becomes angular dependent in general and becomes even attractive in certain directions for substrates with sufficient anisotropy. For a certain model of a cluster the interaction among clusters is calculated at all distances for the isotropic as well as for the anisotropic case. A repulsive potential barrier is found at distances of the order of the cluster diameter even in situations where the long-range behaviour is attractive.     

Some effects of inhomogeneous strain on surface properties of metals: I. Flat surface

Jelium model calculations using the many electron formalism and a parameterized charge density distribution have been used to evaluate the change in surface energy, barrier height and work function due to the presence of a tensile and compressive strain gradient at the surface of Cs, Rb, K and Na. The method is useful for short range strain fields (? 4–5 atom spacings), but not for larger fields. The surface energy change due to inhomogeneous strain is almost twice as large as that due to homogeneous strain. The work function changes due to inhomogeneous strain are ~10–10² times larger than those due to homogeneous strains. The results are of considerable importance to general fracture and stress corrosion cracking studies.