Phonons

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 [1,2], Figure 1. These step-localized transverse surface phonon modes have been detected using inelastic neutral helium atom scattering, using the apparatus of Figure 2.

Figure 2a. Diagram of the neutral atom scattering apparatus that was used in this study. The nearly monoenergetic stream of incident atoms is formed in the large chamber on the left. The beam then enters the target chamber in the center where scattering occurs from the sample. Finally, these atoms enter the detector section of the instrument which rotates about the collision center. Figure 2b. A photo of the apparatus used in the study.

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 3. 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[3]. Theoretical efforts within the Chicago MRSEC have supported these pioneering measurements.

Figure 3. Illustrative time-of-flight spectrum which shows the number of scattered atoms versus their time-of-flight for a given set of experimental conditions: incident energy 17.9 meV, incident polar angle 37.4 degrees, and reflected polar angle 33.4 degrees. The diffuse elastic peak, labelled DE, is due to atoms scattered without exchange of energy with the surface. The peak labelled E1 is due to one of the new step-edge modes. Finally, the peak labelled R is due to scattering from the surface Ray leigh wave, a collective interface oscillation which gives information on bonding throughout the surface.

S. Sibener, 8/96

Reference:

  1. "Phonons Localized at Step Edges: A Route to Understanding Forces at Extended Surface Defects" L. Niu, D. J. Gaspar, and S. J. Sibener, Science 268, 847 (1995).
  2. "Vibrational Dynamics of a Stepped Metallic Surface: Step-Edge Phonons and Terrace Softening on Ni(977)" L. Niu, D.D. Koleske, D.J. Gaspar, and S.J. Sibener, J. Chem. Phys. 102, 9077 (1995).
  3. "Reconstruction kinetics of a stepped metallic surface: step doubling and singling of Ni(977) induced by low oxygen coverages" L. Niu, D. D. Koleske, D. J. Gaspar, S. F. King, and S. J. Sibener, Surf. Sci. 356 144 (1996).

For more information, see Sibener group web site