A. P. Jardine

Quasi-elastic helium-atom scattering from surfaces: experiment and interpretation

Diffusion of an adsorbate is affected both by the adiabatic potential energy surface in which the adsorbate moves and by the rate of thermal coupling between the adsorbate and substrate. In principle both factors are amenable to investigation through quasi-elastic broadening in the energy spread of a probing beam of helium atoms. This review provides a topical summary of both the quasi-elastic helium-atom scattering technique and the available data in relation to the determination of diffusion parameters. In particular, we discuss the activation barriers deduced from experiment and their relation to the adiabatic potential and the central role played by the friction parameter, using the CO/Cu(001) system as a case study. The main issues to emerge are the need for detailed molecular dynamics simulations in the interpretation of data and the desirability of significantly greater energy resolution in the experiments themselves.

Thermal energy He3 spin-echo spectrometer for ultrahigh resolution surface dynamics measurements

We present details of a He3 spin-echo spectrometer, designed to make possible a wide range of new surface dynamics measurements. The apparatus operates at beam energy of 8meV, sufficiently high to enable processes such as surface Bragg diffraction and permit inelastic and quasielastic scattering measurements at up to momentum transfers of about 4A−1. We describe the requirements for the machine, details of the major components used to fulfil these requirements, and the performance of the overall spectrometer. The machine can access a Fourier time range of 0.01ps–1ns, and yields a resolution of 3μeV for inelastic spectrum reconstruction, although under favorable circumstances quasielastic broadenings as narrow as 20neV can be resolved, allowing correspondingly slower processes to be studied.

Hexapole magnet system for thermal energy 3He atom manipulation

We present design and construction details for a novel high field, small bore permanent hexapole magnet. The design is intended for focusing atomic beams of 3He at thermal energies. The magnet uses an optimized polepiece design which includes vacuum gaps to enable its use with high intensity atomic and molecular beams. The 0.3 m long, 1 mm internal radius magnet achieves a polepiece tip field of 1.1 T using NdFeB permanent magnets and Permendur 49 polepieces. The polepiece shanks are designed to saturate so that the hexapole properties are determined predominantly by the shape of the polepiece tip. The performance of the hexapole assembly is demonstrated with an 8 meV 3He beam in the beam source of the Cambridge spin echo spectrometer and the measured focused beam results show excellent agreement with theoretical predictions and negligible beam attenuation.

Vibration and diffusion of Cs atoms on Cu(001)

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.

Observation of uncorrelated microscopic motion in a strongly interacting adsorbate system

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.

Simulation and analysis of solenoidal ion sources

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.

Low aberration permanent hexapole magnet for atom and molecular beam research

We present details of an 800-mm-long, 4.80 mm bore, 1.32 T pole-tip field permanent hexapole (sextupole) magnet system with exceptionally low field aberrations. The assembly was developed as an atom optical element for use in the Cambridge 3He Spin-Echo Spectrometer. Our 12 segment magnet is an improved version of the well-known Halbach design, in which we refine the pole piece shape to improve the field characteristics. Semi-analytic simulations and finite element modeling were performed to optimize the pole piece shape, in order to maximize field strength while minimizing higher order multipole aberrations. High precision machining and assembly techniques were used to construct the device and the resultant field was measured. The measured pole-tip field of 1.25 T is in good agreement with the simulations (5% error) and the measured aberrations that are at least 5 times smaller than those theoretically possible with an ideal 12 segment system using the conventional design. Finally, the application of the...

Unlocking new contrast in a scanning helium microscope

Delicate structures (such as biological samples, organic films for polymer electronics and adsorbate layers) suffer degradation under the energetic probes of traditional microscopies. Furthermore, the charged nature of these probes presents difficulties when imaging with electric or magnetic fields, or for insulating materials where the addition of a conductive coating is not desirable. Scanning helium microscopy is able to image such structures completely non-destructively by taking advantage of a neutral helium beam as a chemically, electrically and magnetically inert probe of the sample surface. Here we present scanning helium micrographs demonstrating image contrast arising from a range of mechanisms including, for the first time, chemical contrast observed from a series of metal–semiconductor interfaces. The ability of scanning helium microscopy to distinguish between materials without the risk of damage makes it ideal for investigating a wide range of systems.

Behaviour of sand during release from a shocked state

The dynamic response of granular materials to an applied shockwave is of wide ranging importance. While the shock Hugoniot has been studied, the shock-release of granular systems has never been experimentally characterised. Here, we present a simple approach to such measurements and present a series of plate impact experiments providing release data for a well characterised dry sand. We discuss the origin of the release behaviour, which we support with further measurements on a weakly bound sandstone.

Probing the non-pairwise interactions between CO molecules moving on a Cu(111) surface

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.