[Show abstract][Hide abstract] ABSTRACT: We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico-to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces. S ingle atomic steps are the dominant surface defect, even on a carefully prepared surface. It is widely accepted that atomic steps play an important role in surface diffusion of add-atoms and adsorbates, and hence their existence and density is expected to modify the growth mode of thin layers, heterogeneous catalysis, and many other surface systems, which are limited by the diffusion rate. Furthermore, it has been hypothesized that atomic steps are responsible for controversies and discrepancies between diffusion measurements performed with techniques that have different spatial resolutions. 1,2 Significant efforts have been made to measure the effect steps have on surface diffusion of adsorbates over the years. Macroscopic techniques that measure variations in coverage on a micron scale have shown the cumulative effect of a large number of atomic steps, clearly demonstrating the anisotropic nature of the motion and the changes in the activation energies for diffusion. 3,4 Unfortunately, experimental data for the atomic-scale motion of an adsorbate near a step edge are particularly scarce. The tendency of adsorbates to be trapped at step sites makes it difficult, if not impossible, to study the diffusion near a step using low-temperature scanning tunneling microscopy due to its limited dynamical range. Under certain conditions, adsorbates can be released from the steps using energetic photons in pump−probe experiments, providing valuable insight into the dynamics of the excited adsorbate. 5,6 In contrast, when it comes to studying how the steps modify the individual and collective atomic-scale motion of adsorbates in thermal equilibrium, the experimental database is essentially nonexistent, leaving us with an understanding based solely on theoretical approaches. 2,7 In this work, we present measurements of the atomic-scale motion of Na atoms on a Cu(115) surface. The well-defined geometry of a densely stepped vicinal surface such as Cu(115) 8 provides a controlled environment for studying the role atomic steps have on diffusion. Because the motion of Na on Cu(100) surface has been thoroughly studied in the last two decades, 9−12 working with the Cu(115) surface provides an excellent opportunity to focus on the changes induced by the presence of atomic steps. The transition from the relatively flat Cu(100) surface to a stepped surface raises several questions and unknowns, in particular: which are the available adsorption sites on this surface and to what extent do the atomic steps modify the diffusion, both parallel and perpendicular to the step direction? Another question, which is typically inaccessible to experiments, is whether the steps affect the lateral interactions between the adsorbed atoms, that is, the collective diffusion process. We combine experimental data with molecular dynamics (MD) simulations to address these questions and show that atomic steps radically change the nature of the atomic-scale diffusion. The Cu(115) sample studied in this work is a vicinal surface with (100) terraces and a very small step separation of 6.63 Å. The schematic drawing in Figure 1 illustrates the geometry of the surface viewed from above and from the side. The clean Cu(115) surface has been shown to be a stable vicinal surface below the roughening transition (380 K). 13 Adsorption of alkali metals at high coverages and temperatures can initiate a surface
[Show abstract][Hide abstract] ABSTRACT: We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces.
[Show abstract][Hide abstract] ABSTRACT: The vibrational excitation of molecules adsorbed on a surface is typically probed by spectroscopic techniques such as infrared or Raman spectroscopy. In the present article we demonstrate an alternative method to determine vibrational lifetimes of adsorbate molecules using quasi-elastic helium atom scattering (QHAS). As a probe of diffusive motion of molecules on surfaces QHAS is well established. Here, we demonstrate that QHAS can also be used to probe the vibrational lifetime of a molecule in its adsorption well. Measurements of cyclopentadienyl, C5H5, on Cu(111) allow us to distinguish two substrate phonon modes as well as two molecular vibrational modes, perpendicular and parallel to the surface. We further find that the dephasing of the vibrational motion corresponds to the friction determined in previous diffusion measurements.
[Show abstract][Hide abstract] ABSTRACT: An exact description of the interactions in aromatic carbon systems is a key condition for the design of carbon based nanomaterials. In this paper we investigate the binding and adsorbate structure of the simplest prototype system in this class - the single aromatic ring molecule benzene on graphite. We have collected neutron diffraction data of the ordered phase of deuterated benzene, C6D6, adsorbed on the graphite (0001) basal plane surface. We examined relative coverages from 0.15 up to 1.3 monolayers (ML) in a temperature range of 80 to 250 K. The results confirm the flat lying commensurate (√7 × √7)R19.1° monolayer with lattice constants a = b = 6.5Å at coverages of less than 1 ML. For this structure we observe a progressive melting well below the desorption temperature. At higher coverages we do neither observe an ordered second layer nor a densification of the structure by upright tilting of first layer molecules, as generally assumed up to now. Instead, we see the formation of clusters with a bulk crystalline structure for coverages only weakly exceeding 1 ML.
[Show abstract][Hide abstract] ABSTRACT: Accurate experimental data of adsorbate potential energy landscapes are crucial as benchmarks for the evaluation of first-principles calculations. Here, we present a Bayesian method, analyzing the difference in forward and backward hopping rate in helium spin-echo measurements, that allows us to determine the binding-energy difference between two sites with unprecedented accuracy. Demonstrating the power of the method on the model system cyclopentadienyl/Cu(111), we find an energy difference between fcc and hcp hollow sites of (10.6±1.7) meV.
Physical Review B 02/2014; 89(12). DOI:10.1103/PhysRevB.89.121405 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.
The Journal of Chemical Physics 05/2013; 138(19):194710. DOI:10.1063/1.4804269 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molecules are commonly thought of as classical objects obeying Newtonian mechanics. However, in their Communication on page 5085 ff., B. A. J. Lechner et al. describe how the diffusion of pyrrole on a Cu(111) surface is determined by the quantum character of the internal vibrations of the molecule. Bending and torsion modes are found to be responsible for the tripling of the barrier to diffusion through lateral changes in their zero point energies.
[Show abstract][Hide abstract] ABSTRACT: Klassische Diffusion – Quantenbarriere: Pyrrol bewegt sich auf Cu(111) in Kanälen um die Kupferatome, indem es zwischen benachbarten Brücken-Positionen springt. Die Bewegung des Massenzentrums kann klassisch beschrieben werden, das Verhalten des aktivierten Zustands ist jedoch auf den Quantencharakter interner Schwingungsmoden zurückzuführen, die während der Bewegung größtenteils nicht angeregt sind. Das aufschlussreiche Helium-Spin-Echo-Experiment wird durch die grüne Kugel und die Pfeile angedeutet.
[Show abstract][Hide abstract] ABSTRACT: A high-intensity supersonic beam source is a key component of any atom scattering instrument, affecting the sensitivity and energy resolution of the experiment. We present a new design for a source which can operate at temperatures as low as 11.8 K, corresponding to a beam energy of 2.5 meV. The new source improves the resolution of the Cambridge helium spin-echo spectrometer by a factor of 5.5, thus extending the accessible timescales into the nanosecond range. We describe the design of the new source and discuss experiments characterizing its performance. Spin-echo measurements of benzene/Cu(100) illustrate its merit in the study of a typical slow-moving molecular adsorbate species.
The Review of scientific instruments 02/2013; 84(2):026105. DOI:10.1063/1.4791929 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-assembled monolayers of sulfur-containing heterocycles and linear oligomers containing thiophene groups have been widely employed in organic electronic applications. Here, we investigate the dynamics of isolated thiophene molecules on Cu(111) by combining helium spin-echo (HeSE) spectroscopy with density functional theory calculations. We show that the thiophene/Cu(111) system displays a rich array of aperiodic dynamical phenomena that include jump diffusion between adjacent atop sites over a 59–62 meV barrier and activated rotation around a sulfur–copper anchor, two processes that have been observed previously for related systems. In addition, we present experimental evidence for a new, weakly activated process, the flapping of the molecular ring. Repulsive inter-adsorbate interactions and an exceptionally high friction coefficient of 5 ± 2 ps–1 are also observed. These experiments demonstrate the versatility of the HeSE technique, and the quantitative information extracted in a detailed analysis provides an ideal benchmark for state-of-the-art theoretical techniques including nonlocal adsorbate–substrate interactions.
[Show abstract][Hide abstract] ABSTRACT: The coverage dependent dynamics of CO on a Cu(111) surface are studied on an atomic scale using helium spin-echo spectroscopy. CO molecules occupy top sites preferentially, but also visit intermediate bridge sites in their motion along the reaction coordinate. We observe an increase in hopping rate as the CO coverage grows; however, the motion remains uncorrelated up to at least 0.10 monolayers (ML). From the temperature dependence of the diffusion rate, we find an effective barrier of 98 ± 5 meV for diffusion. Thermal motion is modelled with Langevin molecular dynamics, using a potential energy surface having adsorption sites at top and bridge positions and the experimental data are well represented by an adiabatic barrier for hopping of 123 meV. The sites are not degenerate and the rate changes observed with coverage are modelled successfully by changing the shape of the adiabatic potential energy surface in the region of the transition state without modifying the energy barrier. The results demonstrate that sufficient detail exists in the experimental data to provide information on the principal adsorption sites as well as the energy landscape in the region of the transition state.
[Show abstract][Hide abstract] ABSTRACT: We present helium scattering measurements of a water ad-layer grown on a O(2 × 1)/Ru(0001) surface. The adsorbed water layer results in a well ordered helium diffraction pattern with systematic extinctions of diffraction spots due to glide line symmetries. The data reflects a well-defined surface structure that maintains proton order even at surprisingly high temperatures of 140 K. The diffraction data we measure is consistent with a structure recently derived from STM measurements performed at 6 K. Comparison with recent DFT calculation is in partial agreement, suggesting that these calculations might be underestimating the contribution of relative water molecule orientations to the binding energy.
The Journal of Physical Chemistry A 06/2011; 115(25):7205-9. DOI:10.1021/jp200221b · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the dynamics of low-coverage ethanethiolate on Cu(111) using helium spin-echo spectroscopy. Above 210 K, the measurements are dominated by translational hopping with an activation energy of only 86 ± 5 meV. At lower temperatures (150-210 K) a further process becomes apparent which has the signature of confined motion. We demonstrate the experimental results are consistent with scattering from an anchored rotor, enabling identification of sixfold jump rotation of the ethyl tail group around a static sulfur adsorption site, with a rotational activation energy of 18 ± 8 meV. Our approach represents a new form of rotational spectroscopy which can be used to study rotational surface diffusion.
[Show abstract][Hide abstract] ABSTRACT: The dissociative adsorption of cyclopentadiene (C(5)H(6)) on Cu(111) yields a cyclopentadienyl (Cp) species with strongly anionic characteristics. The Cp potential energy surface and frictional coupling to the substrate are determined from measurements of dynamics of the molecule together with density functional calculations. The molecule is shown to occupy degenerate threefold adsorption sites and molecular motion is characterized by a low diffusional energy barrier of 40±3 meV with strong frictional dissipation. Repulsive dipole-dipole interactions are not detected despite charge transfer from substrate to adsorbate.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The helium spin echo spectrometer is a powerful apparatus for measuring surface dynamics and can be used in several different modes of operation. In this paper we present the first two-dimensional measurements of the wavelength intensity matrix, offering a new approach for studying surface phonons. The approach that we present is completely independent of the incident beam energy distribution and hence can be used to study inelastic scattering with ultra-high resolution. The additional insights obtained by using this new approach and its technical difficulties are discussed, and a comparison with other existing methods is given.
[Show abstract][Hide abstract] ABSTRACT: We present an analytic model applied to quasi-elastic scattering from an adsorbed surface species undergoing jump diffusion between adsorption sites described by a Bravais lattice combined with a basis of multiple points. The model allows for hops between adsorption sites which are both symmetrically and energetically inequivalent. We give results for 1-D hopping, which are applicable to a species jumping between the top and bridge sites along the [11̅0] direction on an fcc-(110) surface or for jumps along a step edge. In 2-D, results for hopping between fcc and hcp hollow sites and between the bridge sites of an fcc-(111) surface are presented. These examples give characteristic signatures which will allow these forms of motion to be recognized in experimental data and will enable the underlying physical parameters to be extracted by comparison with the analytical forms derived here.
[Show abstract][Hide abstract] ABSTRACT: We review recent advances in neutron and helium spin-echo spectroscopy, applied to the diffusion of atoms and simple molecules adsorbed on surfaces. A comparative introduction to the techniques is given, including advances in theory and simulation techniques, followed by a Summary of several recent sets of experimental results.
[Show abstract][Hide abstract] ABSTRACT: In this review we give a detailed description of the recently developed helium-3 spin-echo technique and its application to several classes of surface dynamic measurements. We review existing surface dynamical probes briefly and illustrate the need for new experimental tools that measure on nanoscale distances and over picosecond timescales. We then describe the helium-3 spin-echo method, which is one such tool, together with the approaches used to describe such measurements and the instrumentation developed to realise its application. The main application of helium-3 spin-echo is the study of surface dynamics, hence we review the approaches which have been established to interpret dynamical data and the signatures for various forms of motion, before going on to summarise the experimental studies to date. We also describe Fourier transform atom spectroscopy, a new method for measuring elastic and resonant scattering that is facilitated by the availability of spin-echo instruments. Finally, we look towards future scientific challenges for the technique.