Chemistry Reference
In-Depth Information
Fig. 10.6 a Side and top view of the H
2
O-(OH)
2
chain optimized by DFT calculations. b STM
simulation for (a)
To explore the mechanism of the H-atom relay, the bias voltage, tunneling
current, and spatial dependence of the relay rate is investigated for an H
2
O-(OH)
2
,
D
2
O-(OD)
2
, and H
2
O-(OH)
3
. Figure
10.8
a shows the quantum yield [reaction
probability per tunneling electron] as a function of bias voltage. The STM tip is
positioned over the water molecule during the measurements. The yield are
observable at the range from 10
-12
to 10
-6
per electron within the experimental
time scale. The lowest limit is determined by a realistic time scale of experimental
tolerance and the highest limit by the bandwidth of a preamplifier for STM. For
H
2
O-(OH)
2
the yield shows an initial increase around 180 mV, a moderate
increase beyond *220 mV, and a sharp enhancement around 430 mV. For D
2
O-
(OD)
2
obvious isotope effects can be observed, where the yield shows an initial
increase around 200 mV and a sharp enhancement around 320 mV. Figure
10.8
b
shows the current dependence of the relay rate. The rate shows a linear dependence
onto the current at several voltages, indicating that the transfer is induced via one-
electron process over the whole bias range. I confirmed that the same results are
obtained with inversed the bias polarity, ruling out the possibility that the electric
field induces the reaction. Above results unambiguously suggest that the relay is
triggered by the vibrational excitation of the adsorbate molecules. On the other
hand, the reaction yield of H
2
O-(OH)
3
(Fig.
10.8
a, green triangles) is found to be
much smaller than that of H
2
O-(OH)
2
by several orders of magnitude.
Figure
10.8
c shows the spatial dependence of the relay yield at V
s
= 243 and
443 mV, where the yield is plotted as a function of the distance along the
H-transfer axis (the [001] direction) as indicated in the inset. At both voltages the
