Chemistry Reference
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Fig. 5.5 Production of a water dimer on a Cu(110) surface a Constant current image of H
2
O
monomers. The image was acquired at V
s
= 24 mV, I
t
= 0.5 nA. b Scanning with V
s
= 54 mV
and I
t
= 0.5 nA induces the hopping of monomer along [110] direction. Two monomers
encounter and yield a dimer. c The image after (b). The dimer is characterized by its fluctuating
image. The apparent heights of the monomer and dimer are 0.6 and 0.8 Å, respectively. The size
of these images is 40 9 160 Å
2
5.2 Results and Discussions
5.2.1 Production of a Water Dimer on Cu(110)
Figure
5.5
show the production of a water dimer. Round protrusions in Fig.
5.5
a
are water monomers. In order to associate water monomers, hopping along the
atomic row direction is induced by scanning the surface at V
s
= 54 mV
(Fig.
5.5
b). Then two monomers encounter with each other, and yield a dimer
(Fig.
5.5
c). A dimer is characterized by a continuously fluctuating image. A dimer
was never dissociated into monomers spontaneously under the same conditions as
its production, suggesting it is energetically preferred to isolated monomers due to
H bond formation. The dissociation of dimer into monomers can be induced by
applying a voltage pulse above 0.2 V over a dimer. This energy implies the H
2
O
scissors mode is associated with the dissociation mechanism.
Upon the deuterium (D) substitution, the STM image is drastically affected.
Figure
5.6
a shows the high resolution image simultaneously acquired for an
(H
2
O)
2
and (D
2
O)
2
. Since the STM tip is scanned horizontally from the top to
bottom, the fluctuating image of the (H
2
O)
2
corresponds to the motion along the
atomic row direction. As discussed later in details, this fluctuation corresponds to
the donor-acceptor interchange of a dimer. In contrast to the (H
2
O)
2
, the (D
2
O)
2
is
imaged stationary in Fig.
5.6
a. Although the fluctuation takes place in a (D
2
O)
2
,
