Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

The MgO(001) surface studied by helium diffraction

The diffraction of He atoms at thermal energy (~64 meV) from the (001) cleavage plane of MgO was studied. A hard-wall intensity analysis of in-plane spectra obtained at near normal incident angles and in the main azimuthal planes yields the corrugation function, which is simply sinusoidal in both unit-cell directions with a total corrugation amplitude of 0.47 ± 0.02 Å. This value is larger than the one previously reported by K.H. Rieder. A more detailed study of the surface is in progress.     

High-resolution H atom scattering from NaCl(001)

The NaCl(001) surface is studied using an H atom beam at energies of 76.3 and 86.3 meV. Elastic diffractive scattering is investigated under different incident angles and crystal orientations. Selective adsorption resonances are measured, determining experimentally the energy levels of the H atom bound to the surface. The energy spectrum is reproduced reasonably by a Morse potential with a well depth D = 29.2 meV and a range parameter k = 0.04 nm^?1.     

Interaction potentials for diffraction of a He atom by a NaCl(001) surface

There exist two quite different sets of experimental results for the He-NaCl(001) interaction potential, inferred from bound-state resonance measurements of associated ⁴He-NaCl(001) bound-state energies E_n: Frankl's group has found a lowest bound-state energy E₀ ≈ −4.1 meV, whereas Benedek's group has found evidence of a deeper interaction potential well, with E₀≈ −7.2 meV. Furthermore, there are strong physical arguments for the existence of the deeper well. In the present paper, interaction potentials, based on empirical pairwise He-Na⁺ and He-Cl⁻ interactions, for diffraction of a He atom by a NaCl(001) surface are considered. Comparisons of close-coupling diffraction calculations with experimental data of Frankl's group are made, and values of the empirical parameters are inferred. On the basis of these comparisons, it is concluded that the lowest bound state proposed by Benedek's group is absent, and that the correct bound-state energies are those proposed by Frankl's group.     

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast ³He and ⁴He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.     

Coherence during scattering of fast H atoms from a LiF(001) surface

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.     

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.     

Inelastic scattering of Ne atoms from a LiF(001) surface

Scattering experiments with a ²⁰Ne nozzle beam from a LiF(001) surface in the 〈100〉 azimuth are reported. The (11) and (1̄1̄) Bragg reflections show broad tails due to inelastic scattering. These tails can be attributed by time-of-flight measurements to single phonon scattering on acoustic modes. The inelastic contribution decreases rapidly with increasing energy of the phonons involved.     

Strongly Z-dependent Auger and photoelectron diffraction in MgO(001)

We have measured angular distributions of Mg KL_2,3L_2,3 Auger and O 1s core-level photoemission intensities at high angular resolution for MgO(001). We find substantial differences in the two types of scans over many regions of k space, despite the very similar local structure of Mg and O in the rocksalt lattice. These diffraction scans are quantitatively predicted by spherical-wave, single-scattering theory, assuming an s wave for the Mg KL_2,3L_2,3 Auger transition. The differences that are observed between Mg KL_2,3L_2,3 Auger and O 1s diffraction scans are shown to be due purely to differences in the elastic scattering phase shifts for Mg and O atoms in the solid.     

An atom scattering study of the kinetics of oxygen adsorption on a nickel (001) surface

A kinematic analysis of the scattering of helium atoms from oxygen adsorbed on Nickel (001) has been performed. For a substrate temperature of 375 K and an oxygen coverage of θ ～ 0.21 a metastable mixture of p(2×2) without long range order and some c(2×2) is obtained. Annealing at 475 K results in a well ordered p(2×2) structure with long range order. An order-disorder transition is identified at ～ 535 K.     

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.     

Neutralization of charged fullerenes during grazing scattering from a metal surface

The neutralization of C(60)(+) and C(60)(2+) fullerenes with keV energies is studied for grazing scattering from a clean and flat Al(001) surface. From the measured shifts between the angular distributions for scattered projectiles of different incident charge, we derive image-charge interaction energies, which relate to the distances of electron transfer for C(60)(+) and C(60)(2+). These neutralization distances are in accord with a classical over-the-barrier model taking into account the image-charge effects of the Al target and the polarization of the fullerene.     

AB initio hartree-fock study of the MgO(001) surface

The MgO(001) surface has been studied with an ab initio Hartree-Fock crystalline orbital LCAO program. An optimized basis set containing nine atomic orbitals (three s and six p) per atom has been used. The semi-infinite crystal has been simulated by a slab containing three planes (six atoms per cell). In agreement with the most recent LEED experiments, no relaxation is found and the “rumpling” is very small (1% of the nearest neighbour separation in the bulk). The analysis of the electron charge distribution and of the ion multipoles shows that the fully ionic character found for the bulk is maintained at the surface, and that the anion deformation is very small. A surface energy of 1.43 J/m² was obtained.     

Characterization of epitaxial MgO growth on Si(001) surface

MgO epitaxial growth on a Si(001) surface by ultrahigh-vacuum molecular beam epitaxy was investigated. Epitaxial orientation and crystalline quality were characterized based on the three-dimensional reciprocal map obtained by Weissenberg RHEED. The epitaxial orientation and crystallinity were strongly dependent on the initial condition of the substrate. When MgO was deposited on a clean Si(001) surface at room temperature a MgO(001) film grew on the Si(001) substrate with two in-plane orientations:MgO[110]//Si[100] and MgO[100]//Si[100]. This is the first observation of MgO epitaxy with the former orientation, which has a smaller mismatch than the latter orientation. When the substrate was exposed to O₂ or thermally oxidized, the latterorientation predominantly grew on the substrate. Deposition of Mg on the substrate also produced the latter orientation. These results imply that nucleation sites on the initial substrate play an important role in determining the epitaxial orientation.     

Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.