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We have observed antiabsorption resonances when alkali metals are adsorbed on Cu(111). The resonances occur at [h bar][omega][sub 0][approx]0.1 eV and cannot be attributed to phonon modes. We show that the experimental data can be consistently explained by attributing the resonances to vibrations, parallel to the surface, of a pinned surface charge density wave (CDW). We estimate the CDW effective mass to be [ital m][sup *][approx]500[ital m][sub [ital e]] per electron in the condensate.

We study the fluid flow at the interface between elastic solids with randomly rough surfaces. We derive (approximate) analytical expressions for the fluid flow factors which enter in the equation describing the fluid flow, and for the frictional shear stress factors which enter in the equation for the frictional shear stress. Numerical results for a rubber cylinder with surface roughness sliding on a flat lubricated substrate, under "low" and "high" pressure conditions, are presented and discussed. Finally we discuss the role of the fluid-induced elastic deformations of the surface roughness profile.

Rubber wear typically involves the removal of small rubber particles from the rubber surface. On surfaces with not too sharp roughness, e.g. most road surfaces, this involves (slow) crack propagation. In this paper I shall present a theory of mild rubber wear. I shall derive the distribution of wear particle sizes Φ(D), which is in excellent agreement with experiment. I shall also show that the calculated wear rate is consistent with experimental data for tire tread block wear.

We study the average separation between an elastic solid and a hard solid with a nominal flat but randomly rough surface, as a function of the squeezing pressure. We present experimental results for a silicon rubber (PDMS) block with a flat surface squeezed against an asphalt road surface. The theory shows that an effective repulse pressure act between the surfaces of the form p proportional to exp(-u/u0), where u is the average separation between the surfaces and u0 a constant of order the root-mean-square roughness, in good agreement with the experimental results., Comment: 6 pages, 10 figures

We have measured the friction force acting on a rubber block slid on a concrete surface. We used both unfilled and filled (with carbon black) styrene butadiene (SB) rubber and have varied the temperature from -10 °C to 100 °C and the sliding velocity from 1 μm/s to 1000 μm/s. We find that the experimental data at different temperatures can be shifted into a smooth master-curve, using the temperature-frequency shifting factors obtained from measurements of the bulk viscoelastic modulus. The experimental data has been analyzed using a theory which takes into account the contributions to the friction from both the substrate asperity-induced viscoelastic deformations of the rubber, and from shearing the area of real contact. For filled SB rubber the frictional shear stress σ(f) in the area of real contact results mainly from the energy dissipation at the opening crack on the exit side of the rubber-asperity contact regions. For unfilled rubber we instead attribute σ(f) to shearing of a thin rubber smear film, which is deposited on the concrete surface during run in. We observe very different rubber wear processes for filled and unfilled SB rubber, which is consistent with the different frictional processes. Thus, the wear of filled SB rubber results in micrometer-sized rubber particles which accumulate as dry dust, which is easily removed by blowing air on the concrete surface. This wear process seams to occur at a steady rate. For unfilled rubber a smear film forms on the concrete surface, which cannot be removed even using a high-pressure air stream. In this case the wear rate appears to slow down after some run in time period.

We study the sliding of elastic solids in adhesive contact with flat and rough interfaces. We consider the dependence of the sliding friction on the elastic modulus of the solids. For elastically hard solids with planar surfaces with incommensurate surface structures we observe extremely low friction (superlubricity), which very abruptly increases as the elastic modulus decreases. We show that even a relatively small surface roughness may completely kill the superlubricity state., 11 pages, 17 figures, format revtex

We present a general theory of phononic heat transfer between two solids (or a solid and a fluid) in contact at a flat interface. We present simple analytical results which can be used to estimate the heat transfer coefficient (the inverse of which is usually called the 'thermal boundary resistance' or 'Kapitza resistance'). We present numerical results for the heat transfer across solid-solid and solid-liquid He contacts, and between a membrane (graphene) and a solid substrate (amorphous SiO(2)). The latter system involves the heat transfer between weakly coupled systems, and the calculated value of the heat transfer coefficient is in good agreement with the value deduced from experimental data.

We study the heat transfer between graphene and amorphous SiO(2). We include both the heat transfer from the area of real contact, and between the surfaces in the non-contact region. We consider the radiative heat transfer associated with the evanescent electromagnetic waves which exist outside of all bodies, and the heat transfer by the gas in the non-contact region. We find that the dominant contribution to the heat transfer results from the area of real contact, and the calculated value of the heat transfer coefficient is in good agreement with the value deduced from experimental data.

We study the distribution of interfacial separations P(u) at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for P(u) using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches., 25 pages, 12 figures

Quite recently Philbin et al present new theory of the van der Waals friction at zero temperature. Contrary to the previous theory they claimed that there is no "quantum friction" due to quantum fluctuation of the electromagnetic field between two parallel plates moving relative to each other. We show that this theory is incorrect., Comment: 4 pages

When an external force acts on an adsorbate structure, the structure may slide or flow relative to the substrate. The mechanism behind this sliding motion is of fundamental importance for the understanding of friction and lubrication between two flat macroscopic surfaces, and is also related to the question of what boundary condition should be used for the velocity field at a solid-liquid interface when solving the Navier-Stokes equations of fluid dynamics. Here I study the friction which occurs when adsorbate structures slide on surfaces. I present results of simulations based on Langevin or Brownian-motion dynamics, where the dependence of the linear sliding friction on the temperature and on the coverage is studied. I also present results for the nonlinear (in the external driving force) sliding friction, which is found to exhibit hysteresis giving rise to the well-known phenomenon of ``stick-and-slip'' motion. The theory predicts that for a class of sliding systems the ratio ${\mathit{f}}_{\mathit{k}}$/${\mathit{f}}_{\mathit{s}}$ between the kinetic and the static friction coefficient should equal 1/2, in good agreement with experimental data.

When an external force acts on an adsorbate structure, the structure may slide or flow relative to the substrate. The mechanism behind this sliding motion is of fundamental importance for the understanding of friction and lubrication between two flat macroscopic surfaces. Here I present a study of the nonlinear (in the external driving force) sliding friction which is found to exhibit hysteresis giving rise to the well-known phenomenon of ``stick-and-slip'' motion. The theory predicts that for a large class of sliding systems, the ratio ${\mathit{f}}_{\mathit{k}}$/${\mathit{f}}_{\mathit{s}}$ between the kinetic and the static friction coefficients equals 1/2, in good agreement with experimental data.

We report the observation of vibrational overtone for an adsorbate on a single crystal surface by high-resolution vibrational spectroscopy. A very weak absorption band at 2017 cm−1 is identified as the first overtone of the C—O stretching fundamental (at 1027 cm−1 of CH3O-Ni(111). The anharmonic shift in the absence of dipole coupling and mechanical renormalization is determined to be 8.5±2 cm−1, slightly smaller than that of the C—O stretching vibration in methanol, 12.5 cm−1. Surprisingly no C—O bond weakening is found in CH3O-Ni(111) compared to gas-phase methanol.

I study the influence of the matrix environments on the polarizability of small metallic particles. The experimental data by Charle et al. [Ber. Bunsenges. Phys. Chem. 88 (1984) 350; Z. Phys. D 12 (1989) 471] for small silver particles embedded in Ne, Ar, Kr, Xe, N2, O2, C2H4 and CO matrixes and the results of Kreibig et al. [Z. Phys. B 21 (1975) 339; Surf. Sci. 156 (1985) 678] for glass matrixes are discussed in the light of the theory and compared with the result of electron energy loss and resistivity measurements for the same atoms and molecules adsorbed on smooth silver surfaces.

We study the radiative heat transfer and the van der Waals friction between graphene and an amorphous SiO$_2$ substrate. We study the surface phonon-polaritons contribution to the low-field mobility as a function of temperature and of carrier density. We find that the electric current saturate at a high electric field, in agreement with experiment. The saturation current depends weakly on the temperature, which we attribute to the "quantum" friction between the graphene carriers and the substrate optical phonons. We calculate the frictional drag between two graphene sheets caused by van der Waals friction, and find that this drag can induce a high enough voltage which can be easily measured experimentally. We find that for nonsuspended graphene the near-field radiative heat transfer, and the heat transfer due to direct phononic coupling, are of the same order of magnitude at low electric field. The phononic contribution to the heat transfer dominates at high field. For large separation between graphene and the substrate the heat transfer is dominated by the near-field radiative heat transfer., 5 figures

We study the heat transfer between weakly coupled systems with flat interface. We present a simple analytical result which can be used to estimate the heat transfer coefficient. As an application we consider the heat transfer between graphene and amorphous SiO2. The calculated value of the heat transfer coefficient is in good agreement with the value deduced from experimental data., 4 pages, 2 figures

Within the framework of unified approach we study the Casimir-Lifshitz interaction, the van der Waals friction force, and the radiative heat transfer at nonequilibrium conditions when the interacting bodies are at different temperatures and when they move relative to each other with the arbitrary velocity V. The analysis is focused on the surface-surface and surface-particle configurations. We show that relativistic effects give rise to a mixing of the contributions from the electromagnetic waves with different polarization to the heat transfer and the interaction forces. We find that these effects are of the order (V/c)(2). The limiting case when one of the bodies is sufficiently rarefied gives the heat transfer and the interaction forces between a moving small particle and a surface. We also calculate the friction force acting on a particle moving with an arbitrary velocity relative to the black body radiation.

We study the time dependency of the (average) interfacial separation between an elastic solid with a flat surface and a rigid solid with a randomly rough surface, squeezed together in a fluid. As an application we discuss fluid squeeze-out between a tire tread block and a road surface. Some implications for the leakage of seals are discussed, and experimental results are presented to test the theory. The theoretical prediction for the leak rate as a function of the fluid pressure difference is in good agreement with the experimental data for all fluid pressures up to “lift-off”, where the fluid pressure equals the nominal rubber-substrate squeezing pressure.

The separation of solids in adhesive contact, or the fracture of solid bodies, often results in the emission of high-energy photons, e. g., visible light and X-rays. This is believed to be related to charge separation. We propose that the emission of high-energy photons involves surface roughness and surface diffusion of ions or electrons, resulting in the concentration of charge at the tips of high asperities, and to electric-field enhancement, which facilitate the discharging process which result in the high-energy photons. If the surface diffusion is too fast, or the separation of the solid surfaces too slow, discharging starts at small interfacial separation resulting in low-energy photons. Copyright (C) EPLA, 2010

Seals are extremely useful devices to prevent fluid leakage. We present experimental result which show that the leak-rate of seals depend sensitively on the skewness in the height probability distribution. The experimental data are analyzed using the critical-junction theory. We show that using the top-power spectrum result in good agreement between theory and experiment., 5 pages, 9 figures

A Comment on the Letter by Michael Mehlhorn, Heiko Gawronski, and Karina Morgenstern, Phys. Rev. Lett. 104, 076101 (2010).

I present a theory for how dipole-forbidden adsorbate vibrations can be excited in infrared reflection absorption spectroscopy from molecules adsorbed on metallic surfaces, and in the light of the theory I discuss the experimental data of Hirschmugl et al. for the CO/Cu(100) chemisorption system. The frequency-dependent background absorption observed for this system is explained and a qualitative insight into the origin of the various observed effects is given.

I discuss the concept of surface resistivity and present a number of applications. The origin of the recent observations of anti-absorption peaks in infrared reflection absorption spectroscopy (IRAS) of atoms and molecules adsorbed on metal surfaces is discussed and it is shown that parallel frustrated translations of adsorbates always give rise to relative strong anti-absorption peaks in IRAS, even though modes are dipole forbidden with respect to a normal electric field. The excitation mechanism is indirect, involving the metal electrons, and is related to the surface resistivity. I discuss a simple relation between the change in DC resistivity Δϱ of a thin metallic film due to adsorption of molecules on the film surface and the electron-hole pair damping (life time τc−h) of the parallel frustrated translations of the adsorbates. From the measured Δϱ for several different adsorbate systems we deduce the corresponding τc−h which ranges from ∼ 10−12 s for chemisorption systems to ∼ 10−8 s for physisorption systems. The experimental data are discussed in the light of a simple model calculation for the damping of parallel frustrated translations. Finally I present a number of applications related to surface resistivity, including surface diffusion and atomic scale friction.

This work focuses on ordered structures and phase transitions in adsorbed layers. I treat both order-disorder transitions and the properties of incommensurate structures. The aim is to review the fundamental aspects of these topics, but experimental results are presented as illustrations of theoretical concepts and results. An extensive discussion of numerical simulations based on molecular dynamics and on the Monte Carlo method is also presented.

Adsorption of C 2 H 4 and C 2 H 6 on smooth silver films increases the DC resistivity R in a way which correlates with the adsorbate induced density of states at the Fermi energy ϵ F . The strongest increase in the film resistivity is observed after formation of atomic scale roughness by submonolayer quantities of cold-deposited silver. In good agreement with theory we find that the DC cross section of a silver adatom on smooth silver is about 14 A 2 . When atomically roughened films are gradually exposed to C 2 H 4 , R decreases first and eventually increases, as observed for smooth films. The decrease of R is explained by a decrease of the Ag adatom induced density of states at ϵ F by adatom-C 2 H 4 -bonding via the π ∗ orbital of C 2 H 4 and the adatom 5s-resonance. The initial relatively strong increase of R of an atomically roughened silver film by adsorption of C 2 H 6 is explained by counteracting the smoothening of the surface profile by the electrons (Smoluchowski effect), because electrons at ϵ F can hardly penetrate C 2 H 6 .

In this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory.

We present molecular dynamics friction calculations for confined hydrocarbon solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are considered: a) polymer sliding against a hard substrate, and b) polymer sliding on polymer. In the first setup the shear stresses are relatively independent of molecular length. For polymer sliding on polymer the friction is significantly larger, and dependent on the molecular chain length. In both cases, the shear stresses are proportional to the squeezing pressure and finite at zero load, indicating an adhesional contribution to the friction force. The friction decreases when the sliding distance is of the order of the molecular length indicating a strong influence of molecular alignment during run-in. The results of our calculations show good correlation with experimental work.

Theory of heat transfer between adsorbate vibrational degrees of freedom and ultrafast laser heated hot electrons including vibrational intermode coupling is applied to calculate two-pulse correlation, laser fluence dependence and time dependence of lateral hopping of CO molecules from a step to terrace site on a stepped Pt (111) surface. The intermode coupling is a key ingredient to describe vibrational heating of the frustrated translation mode responsible for the CO hopping. The calculated results are in good agreement with the experimental results, especially if we scale down the experimentally determined absorbed fluence. It is found that CO hopping is induced by indirect heating of the FT mode by the FR mode with a strong frictional coupling to hot electrons.

Seals are extremely useful devices to prevent fluid leakage. We present experimental results for the leak-rate of rubber seals, and compare the results to a novel theory, which is based on percolation theory and a recently developed contact mechanics theory. We find good agreement between theory and experiment., Comment: 6 pages, 10 figures

We explain the origin of recent observations of anti-absorption peaks in infrared-reflection-absorption spectroscopy (IRAS) of atoms and molecules adsorbed on metal surfaces. It is shown that parallel frustrated translations of adsorbates always give rise to relatively strong anti-absorption peaks in IRAS, even though these modes are dipole-forbidden with respect to a normal electric field. The excitation mechanism is indirect, involving the metal electrons. The practical importance of this result is emphasized.

For a localized vibration of a linearly bonded adsorbate, with a relative high-frequency displacement coordinate qa, coupled to a substrate mode with displacement coordinate qs, we propose a new contribution to the linewidth arising from the cubic terms q2aqs and qaq2s by going to third and higher order in perturbation theory. For third-or higher-order multiphono contribution is shown through a specific example to be much more important than the leading-order damping contribution from the term qaqns (n>2), which has been considered by others.

I derive a simple relation between the change in dc resistivity \ensuremath{\Delta}\ensuremath{\rho} of a thin metallic film due to adsorption of molecules on the film surface and the electron-hole pair damping (lifetime \ensuremath{\tau}) of the frustrated translations of the adsorbates. From the measured \ensuremath{\Delta}\ensuremath{\rho} for several different adsorbate systems, I deduce the corresponding \ensuremath{\tau}, which ranges from \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}12}$ s for chemisorption systems to \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}9}$ s for physisorption systems. Theories for the damping of parallel frustrated translations are developed for three limiting cases of the adsorbate-substrate bond; namely, for covalent, ionic, and van der Waals bonds. I study the change in the broadband infrared (ir) light reflectivity caused by the adsorption of molecules on a semi-infinite metal and I discuss the recent ir study by Hirschmugl et al. for the CO-Cu(100) system. Finally, the observation by ir-spectroscopy by Chabal et al. of dipole-forbidden frustrated translations and rotations for H and CO chemisorbed on metals is discussed and the contribution from the excitation of the parallel frustrated translations to the surface resistivity is calculated.

We derive a simple relation between the change in dc resistivity Δϱ of a thin metallic film due to adsorption of molecules on the film surface and the electron—hole damping (lifetime τ) of the parallel frustrated translations of the adsorbates. From the measured Δϱ for several different adsorbate systems, we deduce the corresponding τ which ranges from ∼ 10 −12 for chemisorption systems to ∼ 10 −9 s for physisorption systems. The result of model calculations for the damping of parallel frustrated translations are presented for three limiting cases of the adsorbate-substrate bond, namely for covalent, ionic, and van der Waals bond.

Within the framework of unified approach we study the Casimir-Lifshitz interaction, the van der Waals friction force, and the radiative heat transfer at nonequilibrium conditions when the interacting bodies are at different temperatures and when they move relative to each other with the arbitrary velocity V. The analysis is focused on the surface-surface and surface-particle configurations. We show that relativistic effects give rise to a mixing of the contributions from the electromagnetic waves with different polarization to the heat transfer and the interaction forces. We find that these effects are of the order (V/c)(2). The limiting case when one of the bodies is sufficiently rarefied gives the heat transfer and the interaction forces between a moving small particle and a surface. We also calculate the friction force acting on a particle moving with an arbitrary velocity relative to the black body radiation.

We study the heat transfer from femtosecond laser-heated hot electrons in a metal to adsorbates in the presence of vibrational-mode coupling. The theory is successfully applied to the experimental result of atomic oxygen hopping on a vicinal Pt(111) surface. The effective friction coupling between hot electrons and the vibrational mode relevant to the hopping motion depends on the transient temperature of the partner mode excited by hot electrons. The calculated two-pulse correlation and fluence dependence of the hopping probability reproduce the experimental results, which were previously analyzed using the hot-electron temperature (Te)-dependent friction eta(a)(Te) in a conventional heat transfer equation. A possible elementary process behind such a hypothetic modeling using eta(a)(Te) is discussed in terms of an indirect heating of the vibrational mode for hopping at the surface.

Seals are extremely useful devices to prevent fluid leakage. However, the exact mechanism of roughness induced leakage is not well understood. We present a theory of the leak-rate of seals, which is based on percolation theory and a recently developed contact mechanics theory. We study both static and dynamics seals. We present molecular dynamics results which show that when two elastic solids with randomly rough surfaces are squeezed together, as a function of increasing magnification or decreasing squeezing pressure, a non-contact channel will percolate when the (relative) projected contact area, A/A_0, is of order 0.4, in accor dance with percolation theory. We suggest a simple experiment which can be used to test the theory., 12 pages, 19 figures

We present a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. The numerical calculations mainly focus on the contact area and the interfacial separation from small contact (low load) to full contact (high load). For small load the contact area varies linearly with the load and the interfacial separation depends logarithmically on the load. For high load the contact area approaches the nominal contact area (i.e., complete contact), and the interfacial separation approaches zero. The numerical results have been compared with analytical theory and experimental results. They are in good agreement with each other. The present findings may be very important for soft solids, e.g., rubber, or for very smooth surfaces, where complete contact can be reached at moderate high loads without plastic deformation of the solids., 15 pages, 23 figures

We study the van der Waals frictional drag force induced by liquid flow in low-dimensional systems (2D and 1D electron systems, and 2D and 1D channels with liquid). We find that for both 1D and 2D systems, the frictional drag force induced by liquid flow may be several orders of magnitude larger than the frictional drag induced by electronic current., Comment: 10 pages, 4 figures

We present a theory for the energy transfer between adsorbates and ultrafast laser heated hot electrons in the presence of the vibrational-mode coupling. This mode coupling provides another source of vibrational heating, in addition to the direct heating by hot electrons via friction coupling. The theory is general and accurate and easy to implement to analyze adsorbate motions induced by femtosecond laser heating of the adsorbate and/or metal systems. The heat transfer equation for the coupled harmonic oscillators is applied to the recent time-resolved studies of hopping of carbon monoxide and atomic oxygen on Pt surfaces. It is found that the temperature of the frustrated translation mode responsible for hopping can become high enough, even when its direct friction coupling to the hot electrons is too weak to induce motion.

The linewidth of adsorbate vibrational modes are often strongly influenced by disorder. However, it is shown in this work that if strong dipole coupling occurs between the adsorbates, this tends to narrow the absorption band. This effect is clearly seen be comparing infrared spectra of the CO, PtCO and the combination band of the two modes for the c(4×2) CO/Pt(111) system. The dynamical dipole moment of the two latter modes is small with a negligible narrowing effect. The width of the combination band is rather large indicating a strong inhomogeneous broadening, which can be attributed to a wide distribution of CO stretch frequencies. The infrared spectrum of the CO stretch mode, on the other hand, shows a relatively sharp, almost Lorentzian peak, because of the line narrowing due to the strong dipole interaction.

Vibrational spectroscopy has indicated that the ordered overlayers formed by CO on metal surfaces at high coverage are not “compression” structures, or “floating” phases, but rather coincident site lattices in which the molecules remain adsorbed on high symmetry sites. In an equivalent description such layers may be regarded as a mixture of phase and anti-phase domains of a lower coverage structure separated by regularly spaced domain walls (solitons), at which the local CO density is higher. Due to the flat potential energy surface the molecules at the domain walls can adjust to the repulsive CO-CO interaction by moving off the high symmetry sites. The soliton model has the advantage of providing a convenient description of the formation of such phases and of the transitions from one phase to another. In the present paper low energy electron diffraction (LEED) and infrared spectroscopy are used to illustrate these phenomena in the adsorption systems Pt{111}-CO and Pd{111}-CO.

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root mean square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces., Comment: 15 pages, 25 figures. Minimal changes with respect to the previous one. A few small improvements, references updated, added the reference to the published paper. Previous work on the same subject: arXiv:cond-mat/0604052

Strong short laser pulses can give rise to strong increase in the electronic temperature at metal surfaces. Energy transfer from the hot electron system to adsorbed molecules may result in adsorbate reactions, e.g., desorption or diffusion. We point out the limitations of an often used equation to describe the heat transfer process in terms of a friction coupling, and present a simple theory for the energy transfer between the adsorbate and hot electrons using a proposed heat transfer coefficient.

I study the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. I derive a relation between the (average) interfacial separation $u$ and the applied normal squeezing pressure $p$. I show that, for nonadhesive interaction and small applied pressure, $p\ensuremath{\sim}\mathrm{exp} (\ensuremath{-}u/{u}_{0})$, in good agreement with recent experimental observations.

We calculate the (average) temperature increase in an adsorbed molecule resulting from heat transfer (photon tunneling) from a tip located at a short distance (nanometers) from the adsorbate. For adsorbates on insulating substrates, the temperature increase may be so large as to induce local reactions-e.g., diffusion or desorption of the adsorbate. We conclude that photon tunneling may be used for the manipulation of adsorbed molecules or for the modification of thin adsorbed films.

We propose an action spectroscopy for single-molecule motion induced by vibrational excitation with a scanning tunneling microscope (STM). Calculations of the inelastic tunneling current for excitation of the C-O stretch mode of the CO molecule on metal surfaces are combined with a theory which describes how the energy in the vibrational mode is transferred to a reaction coordinate mode to overcome the activation barrier. The calculated rate for CO hopping on Pd (110) as a function of the bias voltage agrees with the experimental result. It is proposed that the second derivative of the reaction rate with respect to the bias voltage is related to the vibrational density of states, which usually cannot be directly observed in STM inelastic electron tunneling spectroscopy when a molecule motion is induced by vibrational excitation.

Publisher Summary This chapter investigates the sliding of elastic solids in adhesive contact with flat and rough interfaces. The atoms in the bottom layer of the block form a simple square lattice with lattice constant a . Periodic boundary conditions are applied in the xy plane. The mass of a block atom is 197 amu and the lattice spacing of the block is a = 2.6 A, so as to get the same atomic mass and density of gold. The lattice spacing of the substrate is b = 1.6 A. In the absence of adhesion, contact mechanics theories predict that the area of real contact A between two elastic solids with randomly rough (but nominally flat) surfaces is proportional to the squeezing force. Extremely low sliding friction is possible only in the absence of elastic instabilities, and this is possible for stiff enough solids with incommensurate (or nearly incommensurate) surface structures. However, any types of imperfections may “lock” the surfaces together and introduce elastic instabilities during sliding. One type of defect is surface roughness and another possibility is adsorbed molecules.