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Fast-moving nanoscale systems offer the tantalizing possibility for rapid processing of materials, energy or information 1. Frictional forces can easily dominate the motion of these systems, yet whereas nanomechanical techniques, such as atomic force microscopy, are widely used to measure static nanoscale friction 2 , they are too slow to measure the kinetic friction crucial for short-timescale motion. Here, we report measurements of frictional damping for a prototypical nanoscale system: benzene on a graphite surface, driven by thermal motion. Spin-echo spectroscopy is used to measure the picosecond time dependence of the motion of single benzene molecules, indicating a type of atomic-scale continuous Brownian motion not previously observed. Quantifying the frictional coupling between moving molecules and the surface, as achieved in these measurements, is important for the characterization of phononically driven nanomechanical tools. The data also provide a benchmark for simulations of nanoscale kinetic friction and demonstrate the applicability of the spin-echo technique. Recent nanoscale linear motors have shown that phonon flow, from hot to cold regions, can drive a section of nanotube along a coaxial 'track' 1,3 , in a reversal of the normal frictional process. Speeds are potentially ∼10 8 µm s −1 , enabling atomic-scale motion on picosecond timescales 1. Measurements of nanoscale friction have been carried out with friction force microscopy (FFM), using the remarkable ability of scanning probe techniques to manipulate objects and measure small forces 4. FFM is, however, limited by the instrument's inertia to ∼10 µm s −1 (ref. 5), and there is a need for much faster probes to study the kinetic friction associated with nanoelectromechanical systems. As the moving parts are not loaded laterally or normally in the conventional sense, one approach is to study the dynamics of molecules directly as they move over a surface. For example, Krim et al. 6 used a quartz-crystal microbalance to study the friction of layers of molecules and fluorescence correlation spectroscopy 7 provides diffusion information in the time domain. However, neither of these techniques, nor any other, offers spatial information on an atomic scale together with picosecond time resolution, as in the present work. Here, we demonstrate an alternative approach, carrying out in effect a nanotribological measurement of kinetic friction in the single-molecule limit. Friction and the coupling to surface phonons is deduced for a model nanoscale system from the way individual molecules move on the atomic scale as they are pushed around on picosecond timescales by thermal excitation. Our work is also driven by the knowledge that macroscale friction is fundamentally a microscopic phenomenon 8 , because real surfaces contact only at microscopic asperities, and that energy dissipation, just as in nanoscale systems, is often dominated by the creation of phonons 4. Simple theoretical models of lateral motion of surface species

Helium-3 spin-echo (3HeSE) is a powerful, new experimental technique for studying dynamical phenomena at surfaces with ultra-high energy resolution. Resolution is achieved by using the 3He nuclear spin as an internal timer, to enable measurement of the energy changes of individual atoms as they scatter. The technique yields a measurement of surface correlation in reciprocal space and real time, and probes the nanometre length scales and picosecond to nanosecond timescales that are characteristic of many important atomistic processes. In this article we provide an introductory description of the 3HeSE technique for quasi-elastic scattering measurements and explain how it can be used to obtain unique insights into the motion of adsorbates. We illustrate the technique by reviewing recent measurements, starting with simple hopping and then showing how correlations, arising from adsorbate interactions, can be observed. The final measurements demonstrate how the absence of such correlations, when expected, are used to question the conventional description that attributes the coverage dependence of surface processes entirely to pairwise forces between adsorbates. The emphasis throughout is on the characteristic signatures of adsorbate motion that can be seen in the data, without recourse to a detailed theoretical analysis. Numerical simulations using the Langevin equation are used to illustrate generic behaviour and to provide a quantitative analysis of the experiment.

3He spin-echo measurements are used to follow the picosecond motion of sodium atoms on a copper (001) substrate. 2D correlated motion arising from repulsive adsorbate interactions is observed for coverages as low as 0.04 ML. At coverages greater than 0.05 ML there is a pronounced onset of motion perpendicular to the surface. The perpendicular motion is thermally activated and seems related to the basic translational hopping diffusion process. The correlated motion is modeled successfully using a molecular dynamics simulation and a dipolelike lateral interaction. A simple model which relates the apparent height of the atom with its local coverage is shown to reproduce the experimental observations.

3He spin-echo (3HeSE) dynamics measurements of low coverages of Cs on Cu(001) both reveal quasi-elastic broadening of the helium beam due to aperiodic transport on the surface, and extend measurements of the previously observed low energy acoustic phonon mode, at coverages between 0.014 and 0.056 ML and temperatures of 130 and 80 K. The low energy phonons and quasi-elastic broadening occur on similar timescales and we separate the contributions by converting the spin-echo measurement to the energy domain. Langevin molecular dynamics simulations reproduce the variation of the quasi-elastic peak width, phonon position and amplitudes with momentum transfer, temperature and coverage. The main features in the experimental data require a potential corrugation of 20 ± 2 meV and a friction parameter of 1/40 ps−1. Our results indicate that the Cs dynamics are dominated by dipole–dipole repulsion and produce strongly correlated motion. However, contrary to previous expectations the transport proceeds through jump like behaviour within the Cs overlayer, and Cs moves much more freely than other alkali metals on copper. The unusual behaviour that we see requires three critical components; strong interadsorbate forces, a weak but finite substrate corrugation, and low adsorbate–substrate friction. Together, these key features manifest themselves as a distinct signature in the intensity distribution across the energy/momentum exchange spectrum.

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Modeling of intermolecular forces is a central theme in the physical sciences. The prototypical heterogeneous catalysis system, CO/Pt(111), is an extensively studied example where strong pairwise repulsive forces between the CO molecules have been used to explain the observed structure and dynamics. No direct measurements of these forces were available; yet, they offered a natural way of explaining various macroscopic observations assuming a separable adsorbate-substrate interaction and pairwise adsorbate-adsorbate interactions. In the present study, we measure intermolecular forces by following CO motion on a microscopic scale. The uncorrelated dynamics we observe throughout the coverage range of the measurements excludes the existence of the strong pairwise forces previously suggested. The increase in the rate of uncorrelated motion is explained by a nonlocal modification of the adsorbate-substrate interaction, reflecting a many-body system that cannot be described by the standard separable interaction approach.

Measurements of the atomic-scale motion of H and D atoms on the Pt(111) surface, above the crossover temperature to deep tunneling, are presented. The results indicate that quantum effects are significant up to the highest temperature studied (250 K). The motion is shown to correspond to nearest neighbor hopping diffusion on a well defined fcc (111) lattice. The measurements provide information on the adiabatic potential of both the adsorption site and the transition state and give strong empirical support for a dissipative transition-state theory description of the quantum contribution to the motion.