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.

Fast atom diffraction from superstructures on a Fe110 surface.

Fast atoms with energies from 500 eV up to several keV are grazingly scattered from a Fe(110) surface covered with defined superstructures of sulfur or oxygen atoms. For scattering along low index azimuthal directions, we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the analysis of those patterns, we derive the widths of low indexed axial channels and the corrugation of the interaction potential across these channels. This allows us to estimate the positions of adsorbed atoms on the Fe(110) surface. By demonstration, for adsorbate induced superstructures on a metal surface, we show that fast atom diffraction, first observed for insulators, can be applied to studies on surface structures for a wide range of materials.

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.

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.

Auger neutralization of He ions at an Al(100) surface using isotope effect.

He atoms and ions of the isotopes 3He and 4He are scattered with keV energies under a grazing angle of incidence from a flat and clean Al(100) surface. For the two isotopes we investigate Auger neutralization of incident He+ and He2+ ions via fractions of surviving ions. Pronounced effects for the different isotopes are observed which can be attributed to different time scales concerning the neutralization process of He ions in front of a metal surface. From the analysis of the data obtained for singly and doubly charged ions we find evidence that charge fractions for scattering of He+ ions from an Al surface result predominantly from a direct (Auger) electron capture event.

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.