Chemistry Reference
In-Depth Information
Fig. 5.8 a Typical current trace recorded with fixed the tip over an (H
2
O)
2
at V
s
= 24 mV and
I
t
= 0.5 nA. The tip was slightly displaced from the center of a dimer along [110] direction. The
current jumps between the two states correspond to the moments of individual interchange
events. b Distribution of the time intervals between the interchange events for an (H
2
O)
2
, which
were obtained at three different tunneling current. The vertical axis is logarithmic scale. The
distribution fitted to an exponential function in order to determine the interchange rate. c Voltage
dependence of the interchange rate for an (H
2
O)
2
(open circles) and (D
2
O)
2
(open triangles). The
tip height was adjusted to give 0.5 ± 0.1 nA for the high-current state during the measurement.
The arrows indicate the threshold voltages at which the rate starts to increase. The thresholds
were determined to be 45 ± 1 and 41 ± 1 mV for an (H
2
O)
2
and (D
2
O)
2
, respectively. The tip
height was varied so that the tunneling current was 0.06, 0.25, and 0.7 nA. The latter two data are
displaced vertically for clarity. d Current dependence of the interchange rate at V
s
= 24 mV
(open squares) and V
s
= 54 mV (open circles) for an (H
2
O)
2
in a logarithmic scale. The latter
rate indicates a linear dependence to the current. e A calculated normal mode for an (H
2
O)
2
that
couples with the interchange reaction. This mode involves the motions of donor-substrate stretch
and acceptor rotation. f A schematic diagram of the potential energy along the interchange
reaction pathway. The transition state is of C
2v
symmetry with the Cu-O distances of 2.26 Å. In
addition to the intrinsic tunneling between the ground states (blue or dashed line double-ended
arrow), the interchange is mediated by the vibrational excitation (red or solid line arrows). The
potential barrier is 23 kJ/mol (0.24 eV) while the vertical scale is not realistic
5.2.3 Quantitative Analysis of the Donor-Acceptor Interchange
of a Water Dimer on Cu(110)
The interchange can be directly monitored by recording tunneling current over a
water dimer. Figure
5.8
a shows a typical current-time spectrum measured over an
(H
2
O)
2
at V
s
= 24 mV. The feedback loop of the STM is turned off during the
measurement. The tunneling current shows a bi-stable fluctuation corresponding to
the interchange and current jumps correspond to the moments of individual
