When
a beam of fast ions or neutral atoms impinges on a surface, the particles
may lose energy and suffer deflections as a result of momentum transfer to
the translational degree of freedom of the atoms of the material and by a
variety of inelastic processes. On a crystalline surface, provided the
normal energy of the beam to the surface is chosen to be sufficiently
small, the beam will not penetrate the surface layer. Provided the beam is
incident along a high index azimuth of an atomically flat surface, any
significant deviation from planar elastic scattering will reveal the
influence of charge transfer and inelastic processes, i.e. plasmon and
electron-hole pair excitation of the substrate and electronic excitation
of the beam.
Our aim in this project is to theoretically describe and experimentally
identify the inelastic channel or channels responsible for the energy loss
of fast beams scattered at grazing angles from crystalline surfaces. Dr.
Stefan Tzanev has measured the energy lost by neutral Ar and He incident
at several degrees to an Al(111)-surface at velocities between 5 and 25%
of the Fermi velocity of the conduction electrons in aluminium. The
magnitude of the loss is difficult to assign to phonon excitation nor can
it be accounted for by a molecular dynamics simulation which includes only
energy transfer to translational modes of the substrate atoms. No discrete
loss features corresponding to electronic excitation of the projectile
atom or the plasmon modes of the substrate are observed. We conclude
therefore that the translational energy of the projectile is dissipated
primarily via excitation of electron-hole pairs in the substrate. The
intrinsic corrugation of the ‘electronic surface’ of the metal gives
rise to a high frequency modulation (~1015 s-1) in
the coupling strength, which enhances the efficiency of this excitation
process. Our Zagreb collaborators Dr. Branko Gumhalter and Ante Bilic have
successfully described our experimental results using these concepts.
Information provided by: http://surface.ncl.ac.uk