J. Ellis

Helium atom scattering study of the frustrated translation mode of CO adsorbed on the Cu(001) surface

The structure and vibrational modes of CO adsorbed on Cu(001) have been investigated by helium atom scattering (HAS). Below coverages of θ=0.13, CO adsorbs as isolated molecules, and the time of flight (TOF) spectra of helium atoms scattered from the surface are characterized by energy losses and gains of 3.94 ± 0.07 meV. With the aid of isotope shift measurements, this mode is unambiguously assigned to the frustrated translation mode of the adsorbed CO. In the c(2×2) structure (θ=0.5) this mode is found to show dispersion in both the 〈100〉 and 〈110〉 azimuths with frequencies ranging from 3.9 meV at the zone origin to 5.6 meV at the zone boundary. A simple force constant model with a single nearest neighbor force constant within the layer is found to be sufficient to describe the dispersion. For coverages slightly less than θ=0.5 the angular distributions of scattered helium atoms show features characteristic of vacancies in the overlayer. In the TOF spectra an additional nondispersive mode at about 4 meV...

A high resolution helium atom scattering and far infrared study of the dynamics and the lateral potential energy surface of CO molecules chemisorbed on Cu(001)

Inelastic helium scattering (HAS) and infrared reflection adsorption spectroscopy (IRAS) have been used to measure the isotope shifts of the frequencies of both the parallel and perpendicular frustrated translation modes, as well as the frustrated rotation mode of CO molecules at on top sites on Cu(001). The measured isotope shifts for four different isotopomers indicates a significant rotational contribution to the parallel frustrated translation (T-mode), where the vibrational amplitude of the oxygen atom is significantly larger than for the carbon atom. Conversely, for the frustrated rotation the vibrational amplitude of the carbon atom was observed to be larger than for the oxygen atom. At surface temperatures above Ts=100 K a careful analysis of the peak shape of the HAS quasielastic peak shows a small broadening, which is attributed to a rapid diffusion of the CO molecules. The measured dynamic diffusion barrier of 31±10 meV is compatible with the shape of the potential at the on-top site and makes ...

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.

A molecular dynamics simulation of the diffusion of sodium on a Cu(001) surface

Abstract A molecular dynamics simulation of the diffusion of sodium adatoms on a Cu(001) surface has been performed. A pairwise Morse potential, fitted to the adatom adsorption geometry, binding energy, vibrational frequencies and activation energy for diffusion is used to model the sodium-copper interaction. Good agreement is obtained with quasielastic helium scattering data for the system, confirming the interpretation that sodium diffuses by jumps from one four-fold hollow site to another with an effective activation energy of 51 meV and a jump attempt frequency of 0.53 THz. The measured jump rate is lower than that predicted by transition state theory. The MD simulation suggests that this discrepancy arises because the rate of energy transfer between the adatom and the substrate is so slow that it controls the attempt rate rather than the vibrational frequencies of the adatom as transition state theory would predict.

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.

Two-photon laser observation of diffusion of Na atoms through self-assembled monolayers on a Au surface

Abstract The first direct laser observation of atomic sodium deposited on a self assembled monolayer (SAM) on a Au/Si(100) crystal is reported. The applied two-photon detection scheme allows a Doppler-free measurement of the change in linewidth, spectral peak position and peak intensity as function of time after depositing Na on top of the SAM. A small initial sticking probability of Na on CH 3 (CH 2 ) 21 SH is determined (⪡ 10 −6 ). From the data the mean distance of the Na atoms from the Au surface is calculated to decrease after deposition within 7 min from 32 to 20 A due to diffusion of the Na atoms through the SAM with a diffusion coefficient of 0.083 A 2 /s.

The use of quasielastic helium atom scattering to study correlated motion in adsorbate overlayers

Quasielastic helium atom scattering (QHAS) has been shown to be a powerful probe of surface diffusion on atomic length- and time-scales. So far, however, its application has been limited by data analysis techniques which rely on the assumption that there are no interactions between diffusing particles. It has therefore only been used for systems composed of low coverages of independent adsorbed particles. In this paper it is shown how molecular dynamics simulations can be used to overcome this limitation and realise the techniques potential for studying correlated motion in systems of strongly interacting particles. The simulations are applied to experimental data on the diffusion of Na on Cu(001), where it is shown that correlated adatom motions are the cause of the change observed in the momentum-transfer dependence of the quasielastic broadening as the coverage is increased, and that these correlated motions are sensitive to the form of the adatom-adatom interaction potential. The applicability of QHAS to studies of two-dimensional liquids and the two-dimensional melting phase transition is demonstrated with simulations of a monolayer of Xe adsorbed on a flat surface. The momentum dependence of the quasielastic broadening shows an abrupt change in behaviour at the melting transition, in contrast to the more continuous changes observed in the quantities usually measured, such as diffusion coefficient, diffraction peak shape and position.

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...