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The study covers the development and testing of the alternative mechanics survey (AMS), a modified force concept inventory (FCI), which used automatically marked free-response questions. Data were collected over a period of three academic years from 611 participants who were taking physics classes at high school and university level. A total of 8,091 question responses were gathered to develop and test the AMS. The AMS questions were tested for reliability using classical test theory (CTT). The AMS computer marking rules were tested for reliability using inter-rater reliability (IRR). Findings from the CTT and IRR studies demonstrated that the AMS questions and marking rules were overall reliable. Therefore, the AMS was established as a physics concept inventory which uses automatically-marked, free-response questions. The approach used to develop and test the AMS could be used in further attempts to develop concept inventories which make use of automatically-marked, free-response questions.

The study investigated student reaction to the alternative mechanics survey (AMS), a modified force concept inventory, which used automatically marked free-response questions and offered limited feedback to students after their answers had been submitted. Eight participants were observed in completing the AMS, and they were interviewed to gain insight into what had been observed; the resultant data set was analyzed by thematic analysis. This revealed six key themes: “use of free-response questions supported deep learning”, “interpretation of the AMS instructions affected answer length”, “the idea of being marked by a computer did not affect answer structure”, “participant reaction to the usability of the AMS was mostly positive”, “reactions to the AMS depended upon what participants thought it was for”, and “limited feedback was a useful addition to the AMS”. Participants gave answers of differing length, being guided by the question wording as well as by their own preferences. It was found that participants valued being given feedback on their performance. Participants reacted positively to the free-response questions and could see potential for the use of this question type, opening up possibilities for the use of automatically marked free-response questions in concept inventories in the future.

The adsorption configurations of a technologically relevant model organic adsorbate on the silicon (001) surface were studied using energy scanned x-ray photoelectron diffraction (PhD). Previous work has established the existence of an interesting vertically-aligned (‘flagpole’) configuration, where the acetophenone attaches to Si(001) via the acetyl group carbon and oxygen atoms. Density functional theory calculations have predicted two energetically similar variants of this structure, where the phenyl ring is orientated parallel or perpendicular to the rows of silicon dimers on this reconstructed surface. However, previously published experimental measurements, including scanning tunnelling microscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure investigations were unable to distinguish between these two configurations. Here, we apply the unique experimental capabilities of the PhD technique to this system and demonstrate that the dominant adsorption configuration has the phenyl ring parallel to the dimer rows (the end-bridge structure). This information in turn facilitates the determination of the dominant reaction pathway for acetophenone on Si(001), which has remained elusive until now. Information about subtle preferences in reaction pathways that affect the alignment and orientation of organic adsorbates such as acetophenone on technologically-relevant semiconductor surfaces such as Si(001) is critical for the fabrication of future atomically-precise atomic and molecular-scale electronic devices utilising the organic-silicon interface, and this work demonstrates the unique and complementary capabilities of PhD for providing this information.

Measuring understanding is notoriously difficult. Indeed, in formulating learning outcomes the word ‘understanding’ is usually avoided, but in the sciences, developing understanding is one of the main aims of instruction. Scientific knowledge is factual, having been tested against empirical observation and experimentation, but knowledge of facts alone is not enough. There are also models and theories containing complex ideas and interrelationships that must be understood, and considerable attention has been devoted across a range of scientific disciplines to measuring understanding. This case study will focus on one of the main tools employed: the concept inventory and in particular the Force Concept Inventory (FCI). The success of concept inventories in physics has spawned concept inventories in chemistry, biology, astronomy, materials science and maths, to name a few. We focus here on the FCI and ask how useful concept inventories are for evaluating learning gains. Finally, we report on recent work by the authors to extend conceptual testing beyond the multiple-choice format.

We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico to nano-seconds 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 one dimensional hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms which lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces., 3 figures

We use apparatus based on two Geiger–Muller tubes, a simple electronic circuit and a Raspberry Pi computer to illustrate relativistic time dilation affecting cosmic-ray muons travelling through the atmosphere to the Earth's surface. The experiment we describe lends itself to both classroom demonstration to accompany the topic of special relativity and to extended investigations for more inquisitive students.

We address previous hypotheses about possible factors influencing the gender gap in attainment in physics. Specifically, previous studies claim that scaffolding may preferentially benefit female students, and we present some alternative conclusions surrounding this hypothesis. By taking both student attainment level and the degree of question scaffolding into account, we identify questions that exhibit real bias in favor of male students. We find that both multidimensional context and use of diagrams are common elements of such questions.

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 x √7) R19.1° monolayer with lattice constants a = b = 6.5 Å 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.

In this work we combine scanning tunneling microscopy, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy and density functional theory to resolve a long-standing confusion regarding the adsorption behaviour of benzonitrile on Si(001) at room temperature. We find that a trough-bridging structure is sufficient to explain adsorption at low coverages. At higher coverages when steric hindrance prevents the phenyl ring lying flat on the surface, the 2+2 cycloaddition structure dominates.

Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin−echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

Self-assembled monolayers containing conjugated π systems find application in organic electronics to functionalize and modify the electronic properties of metals and metal oxides. Isolated cyclopentadienyl is an aromatic molecular anion similar in size to benzene that, unlike benzene, adsorbs quite strongly even on coinage metal surfaces. In this study, the electronic structure, bonding, and minimum energy configuration of cyclopentadienyl (c-C5H5 or Cp) adsorbed on Cu{111} are calculated via first-principles density functional theory (DFT). The Cu{111} surface has been modeled within a (2√3 × 2√3)R30° cell, and the adsorbed Cp has been found to reside preferentially on the hollow sites, with a binding energy of 1.73 eV. Electronic population analysis reveals a net charge transfer of ∼1.1 electrons from the metal to the Cp, indicating that the adsorption is dominated by ionic bonding. The surface diffusion barrier between two adjacent hollow sites was calculated to be 55 meV, in good agreement with previously reported measurements by helium spin echo (HeSE) spectroscopy. It was found that lateral interactions do not significantly influence the binding energy and mobility of the adsorbate. The physical–chemical properties of this strongly bound but weakly mutually interacting molecular adsorbate suggest that Cp could become a model system for ionically adsorbed molecular adsorbates.

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 [110] 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.

Previous studies have claimed that male advantage may arise from multiple choice question types; we have made a detailed evaluation of this hypothesis, finding limited evidence that female students are disadvantaged by multiple choice questions in summative assessment. Additionally, we find no significant evidence of a gender gap around the use of multiple choice type questions, including variants such as multiple response questions, in formative assessment. Our findings suggest that the use of a multiple choice question format is not a significant factor in the gender gap in assessment.

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.

A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite basal planes has been performed in the sub-monolayer coverage regime. Atomistic calculations were performed using the second generation forcefield COMPASS as well as the general purpose forcefields Dreiding and Universal. The COMPASS based calculations show evidence for a Brownian nature of the diffusion with a very small diffusion activation barrier of 11 ± 2 meV in agreement with recent helium and neutron spin-echo spectroscopy data [Fouquet P, Hedgeland H, Jardine AP, Alexandrowicz G, Allison W, Ellis J. Measurements of molecule diffusion on surfaces using neutron and helium spin echo. Physica B 2006;385–386:269-71]. Reasonable agreement is also found for the general purpose forcefields if screened charges are used in the description of the Coulombic non-bond interaction. The less computationally intensive Dreiding forcefield was shown to give a good qualitative description of the diffusion validating its applicability for future large scale calculation, i.e., for long times or enlarged systems. A potential energy surface (PES) has been established for the translational and rotational diffusive motion. The PES shows a peculiar dependence of the lateral diffusion barriers on the rotation angle of the benzene molecule which leads to a preferential selection of certain rotational states in the MD trajectories.

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.

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.

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.

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 ($10.6\ifmmode\pm\else\textpm\fi{}1.7$) meV.

We present a new approach to gain detailed surface-molecule potential information by combining neutron and helium atom scattering. First results are presented for a prototype study of benzene molecule diffusion on graphite surfaces. The measurements show good agreement and confirm the transferability of the results between the two methods. Future instrumental improvements are described that will improve the sensitivity of NSE by a factor of 50 for studies of molecules adsorbed on surfaces and mesoporous substrates.

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 (Cp), 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.

Pyrrole diffuses in channels on Cu(111), hopping between adjacent bridge sites over a barrier above hollow sites. Strong lateral interactions alter the lineshapes in helium-3 spin-echo measurements from a predicted double exponential toward an apparent single exponential decay. Molecular dynamics simulations reproduce the centre-of-mass motion of pyrrole and reveal a friction coefficient of 2.0 \(\pm \) 0.4 \(\mathrm{ps}^{-1}\). Density functional theory calculations reveal that a large contribution to the experimentally determined activation barrier of 53 \(\pm \) 4 meV arises from the quantum character of internal vibrational modes.

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.

The dissociative adsorption of cyclopentadiene (C5H6) 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.

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±5meV. 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±8meV. Our approach represents a new form of rotational spectroscopy which can be used to study rotational surface diffusion. © 2011 American Physical Society.

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.

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.

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.

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.

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.

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\char21{}64 meV and a friction parameter of $1/4.5\text{ }{\text{ps}}^{\ensuremath{-}1}$ 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, $\ensuremath{\theta}=0.084$, a component of motion is observed perpendicular to the surface, analogous to that recently found for Na/Cu(001).

A picosecond technique for measuring the kinetic friction of a single benzene molecule on graphite reveals continuous Brownian motion, rather than the jerky hopping observed on most other surfaces. Fast-moving nanoscale systems offer the tantalizing possibility for rapid processing of materials, energy or information1. 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 friction2, 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.

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.

Measurements of (3)He scattering from the Cu(001)c(2 x 2)CO surface using (3)He spin-echo spectroscopy show a number of selective adsorption resonance features. The features cannot be reproduced by close coupled scattering calculations based on the existing Cu(001)c(2 x 2)CO-He interaction potential. An empirical potential is created by adjusting the shape, depth, and width of the existing potential to improve agreement with the experimental data.

3He spin echo measurements of the atomic scale motion of propane on a Pt(111) surface are presented. The measurements provide both the height of the energy barriers to diffusion and the strength of the frictional coupling of propane to the substrate. We show that both the rate and the nature of the dynamics we measure cannot be reproduced by an existing empirical potential. Using numerical simulation we derive a potential energy surface which is capable of reproducing the main features of our dataset.

3 He spin-echo ( 3 HeSE) 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.

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.

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 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 [1], 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 [2] or due to internal degrees of freedom [3] are presented in this issue. The use of ab-initio calculations [4], 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 [5], metastable de-excitation spectra provides insight into the complex growth dynamics of a self assembled monolayer [6] and the role co-adsorption has on surface bonds is studied using high resolution electron energy loss spectroscopy [7]. 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 [8] and the role of phononic dissipation [9] provide two complimentary examples of studies in this field. Recent experimental data showing a discontinuity in sliding friction near a superconducting transition [10], supplies additional motivation for understanding the interaction of weakly bounded mono-layers, a topic studied in detail by Bruch et al [11]. 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 [12]. 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 [1] Toennies J P 2004 J. Phys.: Condens. Matter 16 264001 [2] Ferrando R and Fortunelli A 2009 J. Phys.: Condens. Matter 21 264001 [3] de Wijn A S and Fasolino A 2009 J. Phys.: Condens. Matter 21 264002 [4] Trioni M I, Fratesi G, Achilli S and Brivio G P 2009 J. Phys.: Condens. Matter 21 264003 [5] 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 [6] 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 [7] Politano A, Formoso V and Chiarello G 2009 J. Phys.: Condens. Matter 21 264006 [8] Goikoetxea I, Juaristi J I, Alducin M and Diez Muino R 2009 J. Phys.: Condens. Matter 21 264007 [9] Franchini A, Bortolani V, Santoro G and Brigazzi M 2009 J. Phys.: Condens. Matter 21 264008 [10] Highland M and Krim J 2006 Phys. Rev. Lett. 96 226107 [11] Bruch L W, Nabar R P and Mavrikakis M 2009 J. Phys.: Condens. Matter 21 264009 [12] Fink C and Jenkins S J 2009 J. Phys.: Condens. Matter 21 264010

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.

High-intensity supersonic helium beam sources are a critical component in modern thermal energy helium atom scattering apparatuses. In common with sources developed elsewhere, we have observed beam attenuation at low temperature as the helium flow through the nozzle is increased. A detailed analysis of the measurements demonstrates that direct backscattering in the nozzle-skimmer region is likely to be the major attenuation source. A numerical model, which describes the principle sources of scattering in the region between source and skimmer, is shown to provide a good description of the experimental data for a range of flow conditions and for stagnation temperatures between 37 and 300K. The model includes partial accommodation of atoms scattered from surfaces of the skimmer and chamber, together with a realistic scattering potential. We use the model to predict the effect of various geometric alterations, aimed at improving the ultimate beam intensity.