Angle resolved photoemission study of surface states on the Pt(997) vicinal surface

One-dimensional atomic chains can be synthesized on stepped surfaces and the electronic structure of the high vicinal surface plays an essential role in determining the physical properties of atomic chains grown on top of it. We have applied surface analysis techniques to study the surface of a Pt(997) single crystal. The STM image of the surface showed that the surface was uniform with a well defined distance between the terraces. Angle resolved photoemission spectroscopy (ARPES) was used to characterize the electronic states of the Pt(997) surface, and confinement of electrons with wave vector perpendicular to the step direction was observed.     

Dynamics of confined Xe monolayers adsorbed on the Pt(997) vicinal surface

The dynamics of a confined discrete xenon monolayer adsorbed on the vicinal Pt(997) surface is analyzed within a semi-empirical potential approach. We have investigated two monolayer geometries, namely the commensurate phase and the quasi-hexagonal incommensurate phase, which seem to be stable on the (111) face of Pt, depending on the terrace size. We show that the number and the behavior of localized and side modes characteristic of the confinement are completely different in the two phases. Inelastic helium scattering experiments with a resolution better than 0.2 meV should be an efficient tool to detect the localized modes of the monolayer. This could be an additional way to elucidate the structure of Xe/Pt(997), which has not yet been well established.     

Inelastic scattering of He atoms from a Cu(110) surface

A He beam has been used to measure inelastic scattering from a Cu(110) surface. The scattering was a result of predominantly single phonon events and both energy gain and loss processes were observed. This was in contrast to Cu(001) where only energy loss was observed. For Cu(110), aligned in the [001] azimuth, partial dispersion relations have been measured which meant that phonons with ΔQ values up to the zone boundary value have not been detected under the present experimental conditions. The elastic incoherent component varied with azimuthal angle for Cu(110).     

He atom diffraction from a silicide precipitate on a Pt(100) surface

We have observed the formation of an ordered silicide at the surface of a nominally pure Pt(100) crystal. The silicide appears after thermally activated migration of a trace Si impurity (<10^?4 at%) and subsequent cooling. The silicide is about one monolayer thick and forms a large area unit mesh, $\text{(6}\text{2}\text{×6}\text{2}\text{)R45°}$, which is observed with both He diffraction and LEED. A thermal attenuation analysis of the specular and a diffracted He beam yields a consistent value for the attractive depth of the He-surface potential D=13 meV. From a simple classical analysis of the He diffraction pattern we have obtained an estimate for the corrugation of the He-surface potential, $\text{ξ=0.07}\text{A}\text{?}$. The well depth D is shown to be consistent with the wavelength dependence of the angular distribution of the diffraction pattern.     

Quantum interference effect on low-energy He atomic beam scattering at the surface of He II

The specular reflection coefficient for low-energy ⁴He atoms incident on the free surface of superfluid ⁴He is calculated as a single particle motion coupled to the ripplon field in an effective surface potential. We find a characteristic dip in the reflection coefficient as an interference effect for the truncated surface potential.     

Features of hydrogen adsorption on a Ni(997) surface

The change of the initial sticking coefficient for a Maxwellian beam of hydrogen on Ni(997) has been determined as a function of beam temperature and angle of incidence. At low temperatures adsorption is governed by the steps, at high temperatures adsorption on the terraces is more dominant. Adsorption on the terraces is probably an activated process whereas adsorption on the step sites is non-activated. There seems to be little diffusion of undissociated H₂ between steps and terraces.     

Theory of scattering of He by A W(112) surface

Experimental data (D.V. Tendulkar and R.E. Stickney, Surface Sci. 27 (1971) 516) on the diffraction of He by W(112) are interpreted using some of the most recent theoretical ideas. The model used is a combination of the stationary sinusoidal hard wall (for example, F.O. Goodman, J. Chem. Phys. 66 (1977) 976) and the Debye-Waller-type attenuation assumption (for example, F.O. Goodman, Surface Sci. 65 (1977) 37). Encouraging qualitative agreement of theory with experiment is obtained.     

Determination of a Cu(110) surface contraction by leed intensity analysis

Normal incidence LEED intensity spectra from six symmetrically inequivalent diffraction beams from a clean Cu(110) surface are presented and analyzed. The analysis has employed the results of two distinct sets of dynamical LEED calculations, which differ only in the choice of the electron scattering potential. It is indicated by the analysis that the Cu(110) surface region is in lateral registry with the truncated bulk, the first atomic layer is contracted toward the second by 10.0 ± 2.5% relative to the bulk spacing, and the second to third layer spacing is within 2.5% of the bulk value. A discrepancy of unknown origin has been found between the experimental and calculated I–V proflies for three beams in the 60 eV region; however, this discrepancy does not appear to affect the crystallographic conclusions of the analysis.     

Survival of He+ ions during grazing scattering from a Ag(111) surface

He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.     

Limitations in specular beam leed analysis due to measurement of the scattering angle: Examples from a study of Cu(110)

The I–V profiles for specular LEED beams from a clean Cu(110) surface have been measured for three selected angles of incidence of the primary electron beam with respect to the crystal orientation. A comparison of experimental and calculated profiles indicates that the first interlayer spacing is contracted by 8 ± 3%, a value in reasonable agreement with the result obtained by a previous study of non-specular beams. However, this study demonstrates some inherent limitations of LEED analyses due to difficulties in the precise measurement of the alignment of the primary electron beam.     

Distinctive features of a pulsed beam scattering on a plane surface (two-dimensional case)

A two-dimensional problem of scattering of pulsed beams having arbitrary time and spatial form on a plane surface of a lossy dielectric halfspace is solved. A pulsed beam has been modeled by letting an E- or H-polarized plane pulse with homogeneous front pass through a space filter. The reflected pulse is found using an expansion of the incident pulsed beam over time-harmonic plane waves.     

A molecular beam study on the interaction of NO with a Pt(111) surface

NO adsorbs on Pt(111) with a (temperature independent) initial sticking coefficient S₀=0.88. The fraction of molecules not being chemisorbed is directly inelastically scattered back due to failure of translational energy accommodation. The nonlinear variation of s with coverage can well be described by a precursor-state model, the precursor state being formed by NO molecules translationally and rotationally accommodated in a physisorbed second layer. Dissociation is essentially restricted to defect sites and is negligible on perfect (111) planes. These defect sites (present in small concentration) are first populated and are also sampled by the modulated beam technique yielding an activation energy for desorption E_d = 33.1 kcal/mole and preexponential factor v_d = 10^15.5s^?1. Isothermal desorption measurements yielded E_d and v_d as a function of coverage: E_d rapidly drops from its initial value (at defect sites) to about 27 kcal/mole — which value is considered as representing the adsorption energy on a perfect (111) plane — and then decreases continuously due to effective repulsive interactions. Simultaneously v_d is decreasing to about 10¹² s^?1 at θ = 0.25 which marks the equilibrium coverage to be reached at 300 K. If the surface is precovered with oxygen atoms the NO sticking coefficient is reduced to 0.6, and the desorption parameters are lowered to E_d = 17.1 kcal/mole and v_d= 10^12.6s^?1 (at zero NO coverage).     

Lateral displacement by transient mobility in chemisorption of CO on Pt(997)

We investigated adsorbed states of CO on Pt(997) at 11 K using infrared reflection absorption spectroscopy. At 11 K, thermal migration is suppressed and thus the initial chemisorption at terrace sites and step sites is controlled by the transient mobility of the adsorbing molecule. The initial occupation ratio between atop CO on the terrace and atop CO at the step is directly determined to be 3.6:1. With a simple isotropic migration model, we estimated the mean lateral displacement from the first impact point to the initial chemisorption site to be 6.8 A. We also discuss the origin of transient mobility of CO on metal surfaces.     

Magnetism of Fe clusters formed by buffer-layer assisted growth on Pt(997)

The growth and magnetism of nanometer size Fe clusters on stepped Pt surfaces is investigated by scanning tunneling microscopy (STM) and magneto-optical Kerr effect measurements (MOKE). The clusters are formed on xenon buffer layers of varying thickness and then brought into contact with the substrate by thermal desorption of the Xe. The cluster size is controlled by the thickness of the Xe layer. It is found that clusters of diameter smaller than the Pt terrace width of 2 nm are aligned along the step edges of the Pt(997), thus forming linear cluster chains. In this arrangement, the clusters are ferromagnetic with an easy axis in the direction along the surface normal. If the cluster diameter is larger than the terrace width then the alignment along the step edges is not observed and rather large agglomerates are found which are randomly distributed over the surface. Despite their increased volume, such agglomerates are superparamagnetic with in-plane easy magnetization axis. The enhanced magnetic anisotropy energy in the smallest clusters is originating from hybridization effects at the Fe-Pt interface.     

Symmetry changes in He scattering from a Si(111) surface exposed to H atoms

Changes which occur on, and just below, the surface of a Si(111)-(7×7) crystal when it is dosed with H atoms are complex and still not well described. Measurements of He atom diffraction (HAS) from this surface in the [ 2 ] or [ 11] directions give scans which are asymmetric about the specular angle. The scans become symmetric when the surface is exposed to doses of H atoms sufficient to give coverages of 0.1 to 14 ML, if the atoms all stick. This means that the 3-fold rotational symmetry of the clean surface becomes 6-fold after the addition of the H atoms and shows that the faulted and unfaulted regions of the DAS model become equivalent as they are averaged by the He beam, although the large 49-atom unit cell remains. Thus this surface is not Si(111)-H(1×1), the equilibrated product of H-atom addition which is about 0.3 eV lower in energy, but a metastable intermediate state(s).

Comparison of results from HAS with those from other techniques, such as scanning tunneling microscopy, help to characterize this surface.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

Relaxation effects and thermal vibrations in a Pt(111) surface measured by medium energy ion scattering

Medium energy ion scattering, in conjunction with channeling and blocking has been applied to search for differences in the planar spacing of the first two layers of a Pt(111) crystal surface as compared to the spacing in the bulk. We find a small expansion of 1.5 ± 1%. The measurements have yielded a value for the transverse rms thermal vibration amplitude of a surface atom relative to its neighbour along the [001] direction and likewise for the [110] axis. The results are giving evidence for an enhanced amplitude for the surface atom perpendicular to the surface. A strong blocking effect has been observed along the [110] axis, which may be due to correlation effects, although the role of surface defects cannot be ruled out.