Article,
Shape of molecular adsorbates in STM images: A theoretical study of benzene on Pt(111)
Phys. Rev. B, 53 (8): 4910--4925 (February 1996)
DOI: 10.1103/PhysRevB.53.4910
Abstract
The scanning tunneling microscope (STM) images of benzene on Pt(111)
have been calculated with different adsorption sites (hollow, top,
and bridge sites). Our aim was to get a qualitative understanding
with a molecular-orbital (MO) approach of the factors that govern
the STM image pattern and shape in the case of a molecular adsorbate.
The calculated images strongly depend on the chemisorption site and
they allow the assignment of each experimental image of benzene to
a given site and orientation of the molecule. The contributions to
the tunnel current of each molecular orbital were calculated and
analyzed with the help of a simple analytic model of tunneling through
a molecule. It is not only the orbitals close to the Fermi level
that have a significant contribution to the current. Indeed, the
shape of the orbital (especially the number of nodal planes perpendicular
to the surface) also has a great importance. The final image results
from the electronic interferences between these individual MO contributions
and with the direct tip surface electronic current, as explained
by the model. The main interference effect is between the \sigma
and the orbitals of benzene with a given symmetry, since these
orbitals have a different phase behavior across the tunnel junction
(respectively, symmetric and antisymmetric). The site differentiation
in the STM pattern results from the effective symmetry of the adsorption
site and, for the hollow case, only appears after interference of
the MO contributions.
