W. Allison

An atom-focusing mirror

The recent interest in atom-optics has mainly been directed at the manipulation of atomic beams by static fields or lasers. Using an alternative approach we have succeeded in focusing in two dimensions a neutral atomic helium beam at room temperature with a reflective optical element (an atom mirror). Such focusing relies on specular elastic scattering, which leaves the coherence of incoming wavepackets unchanged.

Quantum Effects in the Diffusion of Hydrogen on Ru(0001)

An understanding of hydrogen diffusion on metal surfaces is important not only for its role in heterogeneous catalysis and hydrogen fuel cell technology but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin–echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature-independent jump rate of 1.9 × 109 s–1, many orders of magnitude faster than previously seen. Quantum transition-state theory calculations based on ab initio path-integral simulations reproduce the temperature dependence of the rate at higher temperatures and predict a crossover to tunneling-dominated diffusion at low temperatures. However, the tunneling rate is underestimated, highlighting the need for future experimental and theoretical studies of hydrogen diffusion on this and other well-defined surfaces.

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.

He diffraction studies of Pt(110) (1 × 2)

Abstract Results are presented on the diffraction of 4 He atoms from the 1×2 reconstructed surface of Pt(110). The diffraction pattern observed indicates that the surface can be considered to have a one-dimensional corrugation. From the position of the rainbow maximum the corrugation amplitude is estimated to be ∼1.5 A. Structural models for the surface are used to derive corrugation funtions from the superposition of atomic charge densities. The results of hard wall scattering calculations from these large corrugations are compared with the experimental data. The results confirm that the large corrugation amplitude could arise from a displaced or missing row model. However, limitations in the present analytical theory prevent any definite distinction to be made concerning the detailed nature of the atomic displacements. These limitations are discussed in the light of the present results and previously published data.

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.

Graphene on Ni(111): Electronic Corrugation and Dynamics from Helium Atom Scattering

Using helium atom scattering, we have studied the structure and dynamics of a graphene layer prepared in situ on a Ni(111) surface. Graphene/Ni(111) exhibits a helium reflectivity of ∼20% for a thermal helium atom beam and a particularly small surface electron density corrugation ((0.06 ± 0.02) Å peak to peak height). The Debye–Waller attenuation of the elastic diffraction peaks of graphene/Ni(111) and Ni(111) was measured at surface temperatures between 150 and 740 K. A surface Debye temperature of θD = (784 ± 14) K is determined for the graphene/Ni(111) system and θD = (388 ± 7) K for Ni(111), suggesting that the interlayer interaction between graphene and the Ni substrate is intermediary between those for strongly interacting systems like graphene/Ru(0001) and weakly interacting systems like graphene/Pt(111). In addition we present measurements of low frequency surface phonon modes on graphene/Ni(111) where the phonon modes of the Ni(111) substrate can be clearly observed. The similarity of these findings with the graphene/Ru(0001) system indicates that the bonding of graphene to a metal substrate alters the dynamic properties of the graphene surface strongly and is responsible for the high helium reflectivity of these systems.

Initial growth morphology in molecular beam epitaxy of fcc iron on Cu(100)

Abstract Specular helium atom scattering has been used to study the early stages of growth in epitaxial iron films on a copper (100) substrate. Results are reported for films, up to five monolayers thick, grown at 298 K. The data show that, in the case of helium scattering, the specular intensity can give a picture of the vertical growth morphology even in situations when it is not possible to separate the sharp, coherently scattered contribution to the specular peak from the broad, incoherently scattered contribution to the peak. We conclude that film growth begins with the formation of three-dimensional islands having a small mean size. As the film thickens, there is a trend towards layer-by-layer growth and an increased island size. The results enable us to role out earlier suggestions that the growth begins in a layer-by-layer fashion with a single or bi-layer structure.

Nonlinearities in sensitivity of quadrupole partial pressure analyzers operating at higher gas pressures

We show, by direct numerical simulation, how space charge effects in an ion source give rise to nonlinearities in quadrupole gas analyzers operating at high pressures. Numerical simulations were performed on a model for a generic electron impact ionizer and the calculations included the effects of both the electron and the ion space charge. By comparing the results of the simulations with previous experimental work we conclude that the most important effects arise in the ion source itself rather than at later stages in the mass analysis process. It was not necessary to invoke ion–molecule scattering mechanisms to reproduce the trends in the experimental data. The results show how competition between the negative and positive space charges can lead to either an increase or a decrease in relative sensitivity as the pressure of gas in the source is raised. Our conclusions are extended to a discussion of nonlinearities in other types of partial pressure analyzer and to extractor gauges measuring total pressure.