[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. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: Classical diffusion-quantum barrier: On Cu(111), pyrrole diffuses in channels, hopping between adjacent bridge sites over a barrier above hollow sites. The motion of the center of mass can be described classically; however, the activation barrier arises from the quantum character of internal vibrational modes that are largely unexcited during the motion. The unique helium spin-echo experiment is indicated by the green sphere and arrows.
Angewandte Chemie International Edition 03/2013; · 13.73 Impact Factor
[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. · 1.52 Impact Factor
[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. · 2.77 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: Helium-3 spin-echo measurements of K/Cu(001) are presented, the
diffusional surface dynamics of the system at low coverages and on
picosecond time scales. Langevin molecular-dynamics simulations are
used, together with a potential-energy surface derived from density
functional theory calculations, to provide further understanding of the
experimental data. An anisotropic potential with a corrugation of 35-64
meV and a friction parameter of 1/4.5ps-1 are found to give
a good fit to the data for the lower coverages with the adsorbate
interactions modeled with a dipole-dipole repulsion. Additionally, at
the highest coverage, θ=0.084 , a component of motion is
observed perpendicular to the surface, analogous to that recently found
[show abstract][hide abstract] ABSTRACT: We describe an improved high intensity, recycling, supersonic atomic beam source. Changes address several issues previously limiting performance and reliability of the apparatus, including the use of newly available vacuum pumps and modifications to the recycling system. We achieve a source intensity of 2.5 x 10(19) atoms/s/sr, almost twice that previously achievable during recycling. Current limits on intensity are discussed.
The Review of scientific instruments 08/2009; 80(7):076110. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Helium-3 spin-echo measurements of resonant scattering from the Si(111)-(1 × 1)H surface, in the energy range 4-14 meV, are presented. The measurements have high energy resolution yet they reveal bound state resonance features with uniformly broad linewidths. We show that exact quantum mechanical calculations of the elastic scattering, using the existing potential for the helium/Si(111)-(1 × 1)H interaction, cannot reproduce the linewidths seen in the experiment. Further calculations rule out inelastic and other mechanisms that might give rise to losses from the elastic scattering channels. We show that corrugation in the attractive part of the atom-surface potential is the most likely origin of the experimental lineshapes.
[show abstract][hide abstract] ABSTRACT: The papers collected here arise from the Workshop on Dynamical Phenomena at Surfaces, held in Cambridge, UK, in July 2008. The meeting has grown from the, SURPHON workshops , which traditionally had an emphasis on surface lattice dynamics. Over time the range of scientific interest has broadened and the recent meeting covered a wide range of topics including diffusion of adsorbates and clusters, growth, wetting, nanotribology, surface reactions and molecule–surface interactions as well as adsorbate vibrations and phonons. The contributions that appear here offer some sense of the breadth of the meeting. The motion of adsorbates on a surface serves as a particularly sensitive way to study the various interactions that take place on a surface and is therefore of fundamental importance. Theoretical work, of the type presented in the meeting, supplies us with important tools to analyze diffusion studies. For example, theoretical investigations of particularly complex surface dynamics exhibited by clusters  or due to internal degrees of freedom  are presented in this issue. The use of ab-initio calculations , to understand the relationship of electron motion to the underlying nuclear motion, is another illustration of the role theoretical methods have in the interpretation of experimental surface dynamics data. A wide variety of experimental probes, can be used to study complimentary aspects of surface interactions, the contributions in this issue reflect this variety. The helium spin echo technique is used to study gas–surface interactions with a silicone surface , metastable de-excitation spectra provides insight into the complex growth dynamics of a self assembled monolayer  and the role co-adsorption has on surface bonds is studied using high resolution electron energy loss spectroscopy . The field of nanotechnology has driven a growing interest in atomic scale friction and dissipation mechanisms on a surface. This topic, which provides an interface between tribology and surface science was the subject of various contributions presented in the meeting. Theoretical assessment of the dissipation due to electron-hole excitation  and the role of phononic dissipation  provide two complimentary examples of studies in this field. Recent experimental data showing a discontinuity in sliding friction near a superconducting transition , supplies additional motivation for understanding the interaction of weakly bounded mono-layers, a topic studied in detail by Bruch et al . The goal of understanding, and eventually designing surface assisted reactions from first principles has been a strong motivator for studying interactions on surfaces for decades. The increased ability to measure surfaces made possible with modern technology, provides critical tests for validating and developing the predictive capabilities of first principle methods. Combining surface dynamics data with a better understanding of the different dissipation mechanisms which take place on a surface, will provide the much needed perspective for reaction studies of the type presented in this issue . Finally, we believe the level of synergy and close interaction between theory and experiment suggests a bright future for the field and for the next meeting in the series, provisionally scheduled for 2010 with a return to Schloss Ringberg. References  Toennies J P 2004 J. Phys.: Condens. Matter 16 264001  Ferrando R and Fortunelli A 2009 J. Phys.: Condens. Matter 21 264001  de Wijn A S and Fasolino A 2009 J. Phys.: Condens. Matter 21 264002  Trioni M I, Fratesi G, Achilli S and Brivio G P 2009 J. Phys.: Condens. Matter 21 264003  Tuddenham F E, Hedgeland H, Knowling J, Jardine A P, MacLaren D, Alexandrowicz G, Ellis J and Allison W 2009 J. Phys.: Condens. Matter 21 264004  Canepa M, Lavagnino L, Pasquali L, Moroni R, Bisio F, De Renzi V, Terreni S and Mattera L 2009 J. Phys.: Condens. Matter 21 264005  Politano A, Formoso V and Chiarello G 2009 J. Phys.: Condens. Matter 21 264006  Goikoetxea I, Juaristi J I, Alducin M and Diez Muino R 2009 J. Phys.: Condens. Matter 21 264007  Franchini A, Bortolani V, Santoro G and Brigazzi M 2009 J. Phys.: Condens. Matter 21 264008  Highland M and Krim J 2006 Phys. Rev. Lett. 96 226107  Bruch L W, Nabar R P and Mavrikakis M 2009 J. Phys.: Condens. Matter 21 264009  Fink C and Jenkins S J 2009 J. Phys.: Condens. Matter 21 264010
[show abstract][hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 06/2009; 11(18):3355-74. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a detailed analysis and simulation of solenoidal, magnetically confined electron bombardment ion sources, aimed at molecular beam detection. The aim is to achieve high efficiency for singly ionized species while minimizing multiple ionization. Electron space charge plays a major role and we apply combined ray tracing and finite element simulations to determine the properties of a realistic geometry. The factors controlling electron injection and ion extraction are discussed. The results from simulations are benchmarked against experimental measurements on a prototype source.
The Review of scientific instruments 01/2009; 79(12):123301. · 1.52 Impact Factor
[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.
Progress in Surface Science 01/2009; · 7.14 Impact Factor