Chemistry Reference
In-Depth Information
Fig. 9.6 a STM images of a
water molecule and an OH
group located on adjacent
rows (dashed lines). b The
complex produced under this
geometry shows an oval
shape. c Schematic
illustration of the reaction.
d An STM image of the
complex on which the lattice
of Cu(110) is superimposed.
e The height profiles along
the solid ([001]) and dashed
([110]) lines in d. The sample
bias voltage and tunneling
current during the image
acquisition were V
s
= 24 mV
and I
t
= 0.5 nA, respectively,
and the size is 47 9 22 Å
2
for a and b, and
17.5 9 17.5 Å
2
for d
hydroxyl. Thus two isomers of water-hydroxyl complex can be produced selec-
tively depending on the initial geometries of the reactants. No interconversion
between the two complexes was observed. The enlarged image of a [001]-complex
with the lattice of Cu(110) is shown in Fig.
9.6
d. As shown in Fig.
9.6
e, the height
profiles along the [001] (solid line) and [110] (dashed) directions clearly indicate
C
2v
symmetry of the appearance.
The dissociation of a [001]-complex into a water and hydroxyl requires a voltage
pulse higher than 0.5 V, suggesting the larger binding energy than that of a [110]-
complex. On the other hand, below 0.5 V, a [001]-complex shows a hopping along
the atomic row direction. Using the atom tracking routine, the average hopping rate
was investigated. A typical trace measured at V
s
= 453 mV is shown in Fig.
9.7
a,
where the displacement of the [001] (red line) and [110] (blue line) directions is
plotted as a function of the measurement time. This result clearly indicates the
hopping occurs exclusively along the [110] direction. The time intervals between
the hopping events were collected by repeating the tracking routine, and the average
hopping rates were obtained. Figure
9.7
b shows the voltage dependence of the rate,
where the thresholds are observed around V
s
= 440 and 330 mV for an H
2
O-OH
and D
2
O-OD, respectively. The energy and isotope ratio of *1.3 suggests the
excitation of the O-H(D) stretch mode [m(OH(D))] triggers the hopping. The
threshold
energy
is
comparable
to
the m(OH)
of
a
free
water
molecule
