Surface crystallography plays the same fundamental role in surface
science which bulk crystallography has played so successfully in
solid-state physics and chemistry. The atomic-scale structure is one of
the most important aspects in the understanding of the behavior of
surfaces in such widely diverse fields as heterogeneous catalysis,
microelectronics, adhesion, lubrication, cor- rosion, coatings, and
solid-solid and solid-liquid interfaces. Low-Energy Electron Diffraction
or LEED has become the prime tech- nique used to determine atomic
locations at surfaces. On one hand, LEED has yielded the most numerous
and complete structural results to date (almost 200 structures), while
on the other, LEED has been regarded as the "technique to beat" by a
variety of other surface crystallographic methods, such as
photoemission, SEXAFS, ion scattering and atomic diffraction. Although
these other approaches have had impressive successes, LEED has remained
the most productive technique and has shown the most versatility of
application: from adsorbed rare gases, to reconstructed surfaces of sem-
iconductors and metals, to molecules adsorbed on metals. However, these
statements should not be viewed as excessively dogmatic since all
surface- sensitive techniques retain untapped potentials that will
undoubtedly be explored and exploited. Moreover, surface science remains
a multi-technique endeavor. In particular, LEED never has been and never
will be self- sufficient. LEED has evolved considerably and, in fact,
has reached a watershed.