Chemistry Reference
In-Depth Information
than the barrier of 0.14 eV, suggesting the vibrationally-assisted tunneling is
involved in the process.
A spatial mapping of IETS intensity enables us to visualize a spatial distribution
of a specific vibrational mode with a sub-molecular resolution [
37
]. Figure
7.9
show the two-dimensional mapping of the IETS peak intensity measured at
V
s
= 54 mV (Fig.
7.9
a) and the simultaneously acquired topographic image
(Fig.
7.9
b) for an OH. The intensity shows the highest over the depressions in the
topographic image. Furthermore the inversion of IETS intensity can be observed at
the center of molecule (between the two depressions). Either peak or dip in the
IETS was observed in various adsorbate systems [
38
], which was explained by the
coupling of the vibrational mode with the electronic structure in the tunnel junc-
tion [
39
]. In the present case, however, both a peak and dip coexist in the same
molecule and cannot be rationalized theoretically at the moment. I speculate the
enhancement of the switching induces the change of the mean distribution of H
atom at the transition state, which might temporally perturb the electronic structure
and eventually causes the inversion of the IETS intensity. The region of the
negative intensity is very narrow along the [001] direction (*1.5 Å) and it
depends on the tip conditions if the negative intensity can be observed or not.
7.3 Summary
Hydroxyl group was produced on a Cu(110) surface by the dissociation of a water
molecule. The structure and dynamics were investigated by a combination of STM
and DFT calculations. It was found that a hydroxyl has an inclined geometry
against the surface normal and switches back and forth between the two orienta-
tions via tunneling, resulting in the characteristic paired depression aligned along
the [001] axis in the STM image. This switching was directly observed only for an
OD as a bi-stable fluctuating current measured over a molecule. Theoretical
calculations predicted a significantly high barrier for the switching, which cannot
be overcome via a thermal activation at 6 K. However, the tunneling process was
acceptable because of a small mass of hydrogen. Due to its light mass the
switching rate of an OH is expected to be too fast to observe with the limited time-
resolution of STM. Furthermore, the switching was enhanced by the excitation of
the bending mode that directly correlates with the reaction coordinate. The
enhanced motion gives rise to a peak or a dip in IETS, depending on the position
of the tip over the molecule.
References
1. R.P. Bell, The Tunnel Effect in Chemistry (Chapman and Hall, London, 1980)
2. K.W. Kehr, Hydrogen in Metals I and II, ed. by G. Alefeld, J. Völkl (Springer, Berlin, 1978)
3. Y. Moritomo, Y. Tokura, N. Nagaosa, T. Suzuki, K. Kumagai, Phys. Rev. Lett. 71, 2833
(1993)
