A. P. Graham

Orientational ordering of two-dimensional ice on Pt(111)

Two highly ordered, epitaxially rotated phases of bilayer ice are observed on Pt(111) in high resolution helium atom diffraction. Analysis of helium diffraction patterns shows that the two phases differ slightly in their density and alignment with respect to the surface. The lack of any isotope effect for both phases indicates that the ice bilayers have structures and hydrogen bond lengths very similar to bulk ice.

A helium atom scattering study of the structure and phonon dynamics of the ice surface

The structure and phonons of an ordered ice surface, prepared in situ under ultra high vacuum conditions, have been studied by high resolution helium atom scattering. The angular distributions are dominated by sharp hexagonal (1×1) diffraction peaks characteristic of a full bilayer terminated ice Ih crystal. Additional, very broad and weak, p(2.1×2.1) peaks may indicate the presence of small domains of antiphase oriented molecules. An eikonal analysis of the 1×1 peaks is compatible with either a proton disordered or a proton ordered surface with corrugations of 0.76 A and 0.63 A, respectively. Inelastic time-of-flight spectra reveal not only a dispersionless phonon branch reported previously at 5.9 meV, but also the first evidence for the surface Rayleigh phonons, which are reproduced well by a Born–von Karman simulation of a full bilayer terminated ice surface using the unmodified force constants derived from neutron scattering bulk phonon measurements. Since the lattice dynamics simulations do not repro...

Quasielastic helium atom scattering measurements of microscopic diffusional dynamics of H and D on the Pt(111) surface

The microscopic diffusional dynamics of H and D on Pt(111) have been studied over length scales between 3 and 14 A by quasielastic helium atom scattering. Data taken over a wide range of parallel wave vectors (0.3u200aA−1⩽|ΔK|⩽3.1u200aA−1) in the surface temperature range 140u200aK⩽Ts⩽250u200aK and at coverages 0.05u200aML⩽ΘH⩽0.66u200aML, provide evidence for an isotropic single jump mechanism with an activation energy of 68±5u200ameV and a pre-exponential factor of D0=1.1±0.5×10−3u200acm2u200as−1 at low coverages. The diffusion coefficient is 2 to 3 orders of magnitude higher than found in an earlier laser induced desorption experiment, whereas the dependence on coverage is similar.

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=100u2002K 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±10u2002meV is compatible with the shape of the potential at the on-top site and makes ...

The low energy dynamics of adsorbates on metal surfaces investigated with helium atom scattering

Abstract High resolution helium atom scattering (HAS) has proved to be a useful probe of the low frequency (

Vibrational spectroscopy of water molecules on Pt(111) at submonolayer coverages

Abstract The adsorption behavior and vibrational dynamics of H 2 O (D 2 O) molecules on the Pt(111) surface at surface temperatures, T s , between 20 and 140xa0K have been investigated using high-resolution helium atom scattering. At T s T s >40xa0K, the water molecules cluster to form stable dimers or trimers. At higher temperatures of about 100–130xa0K, water adsorbs in large two-dimensional ice bilayer islands. From time-of-flight measurements the low-frequency external vibrations of the water monomers, dimers and 2D islands were determined, demonstrating substantial differences for these three water-binding configurations.

The dispersion curves of the three phonon modes of xenon, krypton, and argon monolayers on the Pt(111) surface

The dispersion curves of all three phonon modes, the shear horizontal (SH), longitudinal (LA), and perpendicular (S) vibrations of the two-dimensional monolayer inert gas solids of xenon, krypton, and argon on Pt(111) have been measured by inelastic helium atom scattering (HAS). Previous HAS measurements of xenon on other substrates revealed, in addition to the S mode, only a single dispersive mode which deviated strongly from the expected behavior of the LA branch, to which it had been assigned. The new measurements now identify this mode as the SH mode and all three modes are in excellent agreement with calculations based on the inert gas pair potentials determined from gas and bulk solid data. Possible mechanisms for the excitation of the SH branch, which should couple only very weakly to HAS on the basis of symmetry considerations, are discussed.

Determination of the lateral potential energy surface of single adsorbed atoms and molecules on single crystal surfaces using helium atom scattering

The lateral potential energy surface for adsorbed atoms and molecules is important for a detailed understanding of diffusion, sliding friction, catalysis, and molecular beam epitaxy. In this brief review, it is shown that the analysis of quasi-elastic helium scattering data, in addition to the complementary inelastic results for parallel translational vibrations, leads to a detailed description of the potential energy surface and the accompanying friction parameter describing energy transfer between the adsorbed particle and the surface. Several recent examples of the application of helium atom scattering to this determination are presented for Na and CO on Cu(001) and CO and Xe on Pt(111).

DIFFRACTION AND ROTATIONAL TRANSITIONS IN THE SCATTERING OF D2 FROM CU(001) AT ENERGIES UP TO 250 MEV

Absolute diffraction probabilities for the scattering of D2 from a clean Cu(001) surface along the [100] azimuth have been measured at incident kinetic energies between 20 and 250 meV. The measured attenuation of the diffraction intensities with surface temperature corresponds to a surface Debye temperature of ΘD=341u2009K. The high-resolution angular distributions show clear evidence of rotationally inelastic diffraction (RID) peaks. The RID probability increases with incident energy and represents as much as 30% of the elastic diffraction probability at energies above Ei=200u2009meV. An Eikonal approximation analysis gives a value h=0.075u2009A for the surface corrugation which is independent of incident energy. The rotational transition probabilities correspond to an effective value of δ=0.3 for the molecular eccentricity. The experimental results indicate that diffraction of D2 from Cu(001) can be accounted for by a hard-wall collision mechanism over the whole range of investigated energies.

A He‐atom scattering study of the frustrated translational mode of CO chemisorbed on defects on copper surfaces

The energies of the frustrated translational vibrations parallel to the surface (T‐mode) of isolated CO molecules adsorbed on various flat, regularly stepped and defective surfaces of copper have been measured using high resolution He‐atom scattering. Whereas on the Cu(111) and Cu(001) surfaces CO has a T‐mode energy of about 4.0 meV, at the steps of the Cu(211) and Cu(511) surfaces it is reduced to 3.0 meV. The same energy was observed for scattering parallel and perpendicular to the step edge directions. At defects, which were created by sputtering under different conditions, two additional T‐mode energies of 3.2 meV and 2.5 meV were observed and are attributed to CO molecules adsorbed at step edges and at ‘‘isolated’’ defects, respectively.