Chemistry Reference
In-Depth Information
Fig. 5.9 Sequential images
of two (H
2
O)
2
(71 9 71 Å
2
).
a Two dimers show the
bi-stable fluctuation
corresponding to the donor-
acceptor interchange. b As
they migrate and come to
close each other, one of the
two configurations is strongly
preferred, indicating that the
interchange property is
affected by the intermolecular
interaction. c After they pass
each other the preference is
inverted. d The dimers
recover their inherent
dynamical properties as they
diffuse out of the range of the
interaction. All images were
acquired at I
t
= 0.5 nA and
V
s
= 24 mV
interchange results from the vibrational excitation of a dimer. The isotope ratio of
*1.1 on the threshold voltages suggests that the vibrational mode mainly involves
hindered translation of water molecules. The DFT calculations suggested the
motion of the donor-substrate stretch and the acceptor rotation is associated with
the excited mode (Fig.
5.8
e). By the normal-mode analysis, the corresponding
vibrational energies were determined to be 36 and 34 meV for an (H
2
O)
2
and
(D
2
O)
2
, respectively. The other intermolecular modes were much lower energy.
This mode directly correlates with the interchange reaction coordinate. The barrier
of the interchange (0.24 eV), however, is much larger than the energy of the
excited vibration, thus transferred from a tunneling electron (45 mV). This result
can be explained by vibrationally-assisted tunneling process. At the excited state,
the effective barrier of the interchange becomes relatively low and thin compared
to that in the ground state, and thus the interchange can proceed more efficiently
(indicated by red arrow in Fig.
5.8
f). A significant isotope effect is also found in
the interchange induced by tunneling electrons. Above the threshold, the slope of
the increasing rate for an H
2
O dimer is *20 times higher than that of a (D
2
O)
2
(note that the vertical scale in Fig.
5.8
b is logarithmic), indicating vibrationally-
assisted tunneling is more effective for an (H
2
O)
2
. For a gas-phase dimer, the
tunneling splitting was also drastically affected by the vibrational excitation [
10
].
Quantum tunneling is significantly affected by surrounding conditions. Figure
5.9
a-d
shows a sequence of the STM images of two (H
2
O)
2
. They diffuse on the surface with the
interchange. However, when they come close to each other, the fluctuation is temporarily
suppressed and one of the orientations is strongly preferred (Fig.
5.9
b). After they pass
by each other, the preference is reversed and another configuration becomes dominant
(Fig.
5.9
c). Subsequently as they diffuse away from each other, their normal interchange
