Chemistry Reference
In-Depth Information
Fig. 8.4 Side (upper) and
top (lower) views of the
calculated structure for an
OH dimer on Cu(110). The
H-bond OH-O distance is
1.83 Å. The OH bond length
and the tilt angles to the
surface normal for H-bond
donor (acceptor) are 1.00
(0.98) Å and 78.6 (43.4),
respectively
obviously indicates that H-bonding interaction exists in a dimer. For H-bonded OH
groups on Pd(100), the OH stretch mode was observed at 403 meV [
37
].
On the other hand, Fig.
8.5
b shows the current dependence of the switching
rate. The rate shows a linear dependence at high bias voltages, but it changes into
second-order dependence at low bias. Thus the switching is induced by one- and
two-electron processes at the high and low bias voltages, respectively. As shown in
the inset of Fig.
8.5
b the crossover from one- to two-electron processes takes place
around V
s
* 200 mV. The mechanism of the first increase around 200 mV is still
unclear. There is no specific vibration modetrun 1698around this energy region for
either of (OH)
2
and (OD)
2
. Furthermore, a significant isotope effect is also
observed in the switching rate; the rate of (OD)
2
is smaller than that of (OH)
2
by
two orders of magnitude. Although this origin is also still open question, I spec-
ulate the tunneling at the excited state might be included in the process.
8.2.3 Non-linear Characteristics of the Averaged I-V Curve
of a Hydroxyl Dimer
Figure
8.6
shows the averaged I-V curve and the conductance (dI/dV) spectra
measured over an (OD)
2
. The STM tip was fixed over the depression at a gap
corresponding to V
s
= 24 mV and I
t
= 5 nA before opening the feedback loop.
The I-V curve (dashed line) exhibits a rapid increase at the energy of m(OD). This
non-linear character is observed as a sharp peak in the dI/dV spectra (solid lines).
The same feature can be observed at a negative bias (inset of Fig.
8.6
) as well as
for an (OH)
2
. It is clear that the molecular vibration associated with non-linear
