Chemistry Reference
In-Depth Information
Fig. 4.4 a The diffusion rates as a function of bias voltage for H
2
O(red circles) and D
2
O(blue
crosses). The tip height was adjusted to give I
t
= 12 ± 3 pA during the voltage pulses. The inset
shows typical tunneling current taken with the tip fixed over the molecule at V
s
= 54 mV. b The
rate of induced motion as a function of I
t
at V
s
= 54 mV gives a slope of 1.0 ± 0.1 in a
logarithmic scale
While the rate is constant below *40 mV, the rate increases above *40 mV
without any isotope effects. The same behavior is observed at a negative bias
voltage. The current dependence of the rate at V
s
= 54 mV is also investigated
(Fig.
4.4
b). In Fig.
4.4
b the contribution of the thermally-activated hopping
(R
0
= 0.13 s
-1
) is subtracted to evaluate the rate purely induced by STM. If a
reaction or motion of adsorbates is induced by electrons, its rate is proportional to
the power low: R I
N
. Here power low index N corresponds to the reaction order,
thus, the number of electron required to induce a reaction. The rate shows a linear
dependence (N * 1) onto tunneling current at V
s
= 54 mV, indicating that the
hopping is induced via a single-electron process. The linear dependence also
excludes the effect of electric field. These observations indicate increase of the
hopping rate stems from the vibrational excitation of a water molecule via inelastic
electron tunneling (IET) process.
In the IET framework, the rate of a molecular motion or reaction, R, is expected
to be proportional to the inelastic current, I
inelastic
, above a specific threshold [
20
]
R
¼
KI
inelastic
and I
inelastic
is proportional to bias voltages
I
inelastic
¼
K
0
V
s
Accordingly, the voltage dependence in Fig.
4.4
(a) can be fitted to a linear
function and constant above and below the threshold, respectively (red curve)
R
0
V
s
V
threshol
ð Þ
K
0
V
s
V
s
V
threshold
R
¼
ð
Þ
