Structure of a Langmuir film on a liquid metal surface

The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers.     

Characterization of excess electrons in water-cluster anions by quantum simulations

Water-cluster anions can serve as a bridge to understand the transition from gaseous species to the bulk hydrated electron. However, debate continues regarding how the excess electron is bound in (H2O)-n, as an interior, bulklike, or surface electronic state. To address the uncertainty, the properties of (H2O)-n clusters with 20 to 200 water molecules have been evaluated by mixed quantum-classical simulations. The theory reproduces every observed energetic, spectral, and structural trend with cluster size that is seen in experimental photoelectron and optical absorption spectra. More important, surface states and interior states each manifest a characteristic signature in the simulation data. The results strongly support assignment of surface-bound electronic states to the water-cluster anions in published experimental studies thus far.     

Comment on "Characterization of excess electrons in water-cluster anions by quantum simulations"

The conclusion by Turi et al. (Reports, 5 August 2005, p. 914) that all experimental spectral and energetic data on water-cluster anions point toward surface-bound electrons is overstated. Comparison of experimental vertical detachment energies with their calculated values for (H2O)n- clusters with surface-bound and internalized electrons supports previous arguments that both types of clusters exist.     

Imaging surface atomic structure by means of auger electrons

Measurements of the complete angular distribution of Auger electrons emitted from well-defined platinum[111] single-crystal surfaces have led to the discovery that the distributions are composed of "silhouettes" of surface atoms "back lit" by emission from atoms deeper in the solid. Theoretical simulations of Auger electron angular distributions based upon atomic point emitters and spherical atomic scatterers of uniform cross section are in close agreement with these experimental results, but opposite to previous theoretical predictions. In view of the definitive results obtained and the straightforward agreement between theory and experiment, angular distribution Auger microscopy (ADAM) is useful for direct imaging of interfacial structure and investigation of electron-solid interactions in the physical and biological sciences and engineering. Applicability of ADAM is illustrated by images obtained for monolayers of silver and iodine on platinum[111].     

Real-time observation of adsorbate atom motion above a metal surface

The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10(-15) seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photoemission spectroscopy, are attributed to atomic motion in a copper-cesium bond-breaking process. Describing the change in energy of the cesium antibonding state with a simple classical model provides information on the mechanical forces acting on cesium atoms that are "turned on" by photoexcitation. Within 160 femtoseconds, the copper-cesium bond extends by 0.35 angstrom from its equilibrium value.     

Time-resolved investigation of coherently controlled electric currents at a metal surface

Studies of current dynamics in solids have been hindered by insufficiently brief trigger signals and electronic detection speeds. By combining a coherent control scheme with photoelectron spectroscopy, we generated and detected lateral electron currents at a metal surface on a femtosecond time scale with a contact-free experimental setup. We used coherent optical excitation at the light frequencies omega(a) and omega(a)/2 to induce the current, whose direction was controlled by the relative phase between the phase-locked laser excitation pulses. Time- and angle-resolved photoelectron spectroscopy afforded a direct image of the momentum distribution of the excited electrons as a function of time. For the first (n = 1) image-potential state of Cu(100), we found a decay time of 10 femtoseconds, attributable to electron scattering with steps and surface defects.     

Energy transfer across a metal film mediated by surface plasmon polaritons

Coupled surface plasmon polaritons (SPPs) are shown to provide effective transfer of excitation energy from donor molecules to acceptor molecules on opposite sides of metal films up to 120 nanometers thick. This variant of radiative transfer should allow directional control over the flow of excitation energy with the use of suitably designed metallic nanostructures, with SPPs mediating transfer over length scales of 10(-7) to 10(-4) meters. In the emerging field of nanophotonics, such a prospect could allow subwavelength-scale manipulation of light and provide an interface to the outside world.     

Inverse velocity dependence of vibrationally promoted electron emission from a metal surface

All previous experimental and theoretical studies of molecular interactions at metal surfaces show that electronically nonadiabatic influences increase with molecular velocity. We report the observation of a nonadiabatic electronic effect that follows the opposite trend: The probability of electron emission from a low-work function surface--Au(111) capped by half a monolayer of Cs--increases as the velocity of the incident NO molecule decreases during collisions with highly vibrationally excited NO(X(2)pi((1/2)), V = 18; V is the vibrational quantum number of NO), reaching 0.1 at the lowest velocity studied. We show that these results are consistent with a vibrational autodetachment mechanism, whereby electron emission is possible only beyond a certain critical distance from the surface. This outcome implies that important energy-dissipation pathways involving nonadiabatic electronic excitations and, furthermore, not captured by present theoretical methods may influence reaction rates at surfaces.     

Tuning the quantum stability and superconductivity of ultrathin metal alloys

Quantum confinement of itinerant electrons in atomically smooth ultrathin lead films produces strong oscillations in the thickness-dependent film energy. By adding extra electrons via bismuth alloying, we showed that both the structural stability and the superconducting properties of such films can be tuned. The phase boundary (upper critical field) between the superconducting vortex state and the normal state indicates an anomalous suppression of superconducting order just below the critical temperature, Tc. This suppression varies systematically with the film thickness and the bismuth content and can be parametrized in terms of a characteristic temperature, Tc* (less than Tc), that is inversely proportional to the scattering mean free path. The results indicate that the isotropic nature of the superconductive pairing in bulk lead-bismuth alloys is altered in the quantum regime.     

The estuarine surface microlayer and trace metal cycling in a salt marsh

The aqueous surface microlayer in a Delaware salt marsh carries an average of 10 percent of the copper, 19 percent of the zinc, and 23 percent of the iron relative to the total metal flux including the dissolved and seston components. Such trace metals cycle in the salt marsh by net import on the surface microlayer and net export in the dissolved and seston components during maximum monthly tides.     

The response of electrons to structural changes

The properties of a molecule are determined by the distribution of its electrons. This distribution can be described by the charge density, which is readily obtained from the wave functions derived by ab initio molecular orbital calculations. The charge density may be analyzed in a number of different fashions to give information about the effects of substituents, structural changes, and electronic excitation on the properties of molecules; one common procedure makes use of projection density or charge difference plots. Charge density also may be partitioned among atoms, and by numerical integration over appropriate volume elements one may obtain atomic charges, dipoles, kinetic energies, and other properties of the atoms in a molecule. Many chemical phenomena have been analyzed in terms of charge densities.     

Selective growth of metal tips onto semiconductor quantum rods and tetrapods

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.     

Reactive and nonreactive scattering of H2 from a metal surface is electronically adiabatic

The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation.     

金属材料抗植物磨料磨损表面轮廓分形维数计算

基于分形理论和磨损表面轮廓分析,采用盒计数法、变差法、功率谱法和结构函数法4种算法,计算了植物磨料对金属材料磨损表面轮廓曲线的分形维数.结果表明:磨损后的表面分形维数与磨损质量损失存在密切的关系,质量磨损越大,分形维数也越大;4种算法中结构函数法与变差法计算的分形维数误差较小,可用于计算植物磨料对金属材料磨损表面的分形...     

蔬菜重金属污染的叶面控制技术研究进展

对蔬菜重金属污染的现状、重金属污染对蔬菜生长和品质的影响、重金属污染的叶面控制技术研究进行了概述,并对蔬菜重金属污染控制提出了未来研究方向.     

The force needed to move an atom on a surface

Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip. We found that the force that it takes to move an atom depends strongly on the adsorbate and the surface. Our results indicate that for moving metal atoms on metal surfaces, the lateral force component plays the dominant role. Furthermore, measuring spatial maps of the forces during manipulation yielded the full potential energy landscape of the tip-sample interaction.