Acetylene Dynamics on Pd(111)
To date, the system has been devoted to the study of adsorbates on the
Pd(111) surface. Like other transition metals, Pd finds important
application as a catalyst. In particular, the Pd(111) surface is
highly selective in catalyzing the formation of benzene (C6H6)
from three acetylene molecules (C2H2). This
reaction has been investigated by numerous researchers using a wide variety
of techniques. As a prelude to applying variable temperature STM
to the study of the reaction, we recently examined the behaviour of the
acetylene at low temperature and low coverages on the Pd(111) surface.
It is known that acetylene adsorbs in 3-fold hollow sites on Pd(111) and
that the molecule is significantly rehybridized by the bonding interaction
with the metal surface. The sp hybridization of the gas phase triple
bond reduces to approximately sp2.5, and the linear symmetry
of the molecule is broken. The carbon-carbon bond lies parallel to
the surface, while the carbon-hydrogen bonds bend away from the surface.
As a result of this adsorption geometry, an unoccupied p*
orbital projects from the molecule. This orbital is tilted slightly
out of the surface plane and the reduced symmetry allows constructive overlap
with orbitals of the STM tip involved in tunneling. Our calculations
of STM tunnel currents using ESQC theory show enhanced tunnel probability
over the p* orbital, while destructive interference
between tunneling amplitudes through the molecule and through the surface
leads to diminished tunnel probability away from the orbital. Thus
the acetylene molecule appears to the STM as a bump offset by 1.5Å from
a depression. This prediction is confirmed by our experimental images
of the molecule at low temperature (< 45K).
For particular tip geometries,
the depression is split into two lobes, giving the molecule a "Mickey Mouse"
appearance. Six orientations of the bump-depression pair are observed,
indicating adsorption in both FCC and HCP hollow sites. Consistent with this
result, we have performed total energy calculations that yield a difference in
adsorption energy of only 10 meV for the two three-fold hollow sites.
At and above ~45 K, we find that the orientation of individual acetylene
molecules jumps discretely by 120° intervals indicating thermally activated
rotation of the molecule between the three possible adsorption geometries
on a single three-fold hollow site. To the STM, these jumps appear as
vertical discontinuities in the image. The sequence of images to the right
shows an acetylene molecule that rotated from one orientation to another
while the tip scanned over it.
Our total energy calculations
for the barrier to rotation are roughly consistent with the observed rate.
In addition to thermal activation, the STM tip can induce this rotation.
Estimations based on extended Heuckel calculations suggest that this effect
is not due to a reduction of the barrier height caused by the electric
field between tip and sample. Instead we believe the rotation is
enhanced by vibrational excitation of the molecule due to collisions with
tunneling electrons. We have performed preliminary measurements of
this effect as a function of tunneling conditions (tip-sample bias and
current) and the results are in accord with predictions based on this model.
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