Vibrations Localized at Metallic Step Edges:
A Route to Understanding Forces at Extended Surface Defects
The chemical and physical properties of atomic level surface defects play a crucial role in governing the outcome of many important interfacial processes such as chemical catalysis, corrosion, interface stability, and crystal growth. Professor Sibener's group has recently observed new collective surface vibrational modes which propagate along one-atom-high steps on a stepped nickel single crystal surface [59,60], Figure 1.
In these measurements low-energy neutral helium atoms reflect from a surface; excitation
or de-excitation of surface vibrations causes some of the reflected atoms to slow down
or speed up upon scattering, Figure 2.
Detection of these small energy changes allows us to characterize the vibrations, and
hence forces, which bind the atoms together at the surface. Analysis reveals that the
forces near the step-edge differ significantly from those elsewhere on the surface or
in the bulk of the material. Such measurements are particularly informative as they
give valuable new information on metallic bonding and interface stability in the vicinity of
extended surface defects. These vibrations also provide a stringent test for electronic
structure calculations which seek to explain bonding near extended
structural defects. When molecules such as oxygen are added, these step edges meander
over the surface and can produce new surface structures. Work is now
continuing on new structures that form during the initial stages of metallic
oxidation [68].
Theoretical efforts within the Chicago MRSEC have supported these pioneering measurements.
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