Chemistry Reference
In-Depth Information
Fig. 2.10 STM-AS for cis-2-
Butene on Pd(110). Abrupt
increases of the motion rate
are associated with molecular
vibrations. [The number of
arrow, mode, energy
measured by EELS] = [(1),
Pd-C stretch, 21, 36 meV],
[(2), C-C stretch, 125 meV],
[(3), C-H stretch, 357 meV].
Reprinted with permission
from Ref. [
53
]. Copyright
2005, American Physical
Society
(rate) is increased by a factor of 10-100 after a specific mode excitation. Inter-
estingly, the STM-AS of cis-2-butene showed the excitation of m(M-C), d(CH
3
),
m(C-C), and m(C-H) modes but only m(M-C) and m(C-H) are detected in the STM-
IETS. Therefore the STM-AS can be considered as a complementary method of
the STM-IETS to conduct the chemical identification with the STM. The STM-AS
is applied to identify several chemical adsorbates [
54
-
63
]. More recently, the
practical method to extract vibrational information from the STM-AS was
developed by Motobayashi et al. [
64
] at RIKEN, which can be generally applied to
vibrationally mediated motions and/or reactions of adsorbates. The experimental
techniques to obtain the rates of reaction and motion of a single molecule are
mentioned in next chapter.
I now turn to the mechanism of STM-induced motions and reactions. The
process resulting in the molecular motion or reaction can be classified into the
direct and indirect process. The direct process is quite simple where tunneling
electron from STM excites a vibration mode that is directly associated with the
coordinate of a motion or reaction. In this case we need to consider a simple
vibration ladder climbing model as shown in Fig.
2.11
a. The questions are the
barrier height E
B
and the number of vibrational levels in the potential well, which
can be determined by investigating the number of electrons, n, required inducing
the motion or reaction. The rate is given by the so-called power law, R I
n
, here
I is the tunneling current. If a tunneling electron has enough energy to overcome
the barrier, the process proceeds with one-electron (n = 1; solid arrow). On the
other hand, if the energy is not sufficient, multiple electrons are required to
overcome the barrier (dashed arrows). The direct process was observed in Xe atom
switching between a tip and a Ni(110) surface [
65
] and the dissociation of single
oxygen molecules on a Pt(111) surface [
41
]. The power law dependence of R
