Werner Stocker

Low-energy electron and ion projection microscopy

Abstract We have built a projection microscope by positioning an ultrasharp tip, prepared by field-ion microscopy techniques, in close proximity to a partly transparent carbon foil. The short distance between the perforated carbon film and the tip, achieved with an STM-like approach mechanism, leads to electron emission from the emitter at only 30 V. This arrangement provides a bright source for low-energy electrons or noble gas ions. With a detector at a macroscopic distance opposite the emitter side, a projection image of the holes and structures in the foil of about 30 nm diameter is generated by the low-energy electron beam.

Investigations of the selective population of hydrogen subsurface sites on Pd(110) using He diffraction and thermal-desorption spectroscopy

Surface-structure models for the 2×1 and 1×2 hydrogen chemisorption phases formed on Pd(110) at 100 K have been derived from He-diffraction data. The respective coverages correspond to 1 and 1.5 monolayers (ML). Upon heating to 200 K, the 1×2 saturation phase transforms back into the 2×1, and 0.5 ML hydrogen moves subsurface. Based on structural arguments, we suggest that only the first available subsurface sites, i.e., the octahedral interstitials between topmost and second layers are populated by thermal activation. The subsurface movement is eased since H-chemisorption sites on top of the second Pd layer can be occupied in the 1×2 owing to substrate reconstruction. Structural considerations also explain that exactly 1 ML H can be accommodated subsurface by thermal cycling. New TDS measurements corroborate these notions: only the α2 desorption state, probably associated with Hin subsurface sites between first and second Pdlayers, is selectively filled by the thermal-cycling processes. The α1 state remains empty upon thermal cycling, and is very likely connected with hydrogen deeper in the bulk.

Coherent point source electron beams

The concept of a point source for electrons in view of quantum mechanical experiments is described. The engineering of such atomic electron emitters is discussed and recent experiments in holography with low energy electrons are presented.

Successive population of subsurface and surface sites upon hydrogen chemisorption on 1 × 2-reconstructed Pt(110)

Abstract He-diffraction measurements and intensity analyses based on close-coupled channels calculations show that the repulsive potential corrugation amplitude ζ m = 1.5 A of missing-row Pt(110)1 × 2 increases to 2.0 A upon saturating the stronger bonded β2-H chemisorption state without change of the attractive potential. We conclude that β2-H occupies sites below the topmost Pt rows, whereby the initial ∼ 15% inward relaxation changes into ∼ 20% outward relaxation. A pronounced decrease of the corrugation amplitude at higher coverages signals population of deep trough sites upon filling of the β1-H adstate.

Analyses of Ne-diffraction data from transition-metal surfaces based on charge-density calculations

Abstract We evaluate the proportionality constant β for neon in the Esbjerg-Norskov potential V R ( r ) = βϱ ( r ) from experimental data on Ni(110), Ni(113), Cu(110) and Pd(110). Our calculations using overlapping atomic charge densities require that β be material and surface-dependent. Although Ne diffraction is more sensitive to details of corrugation shapes than He, we found it to be insensitive to normal relaxations of the topmost layers.