Chemistry Reference
In-Depth Information
Figure 4.6. (a) d
2
I
t
/
d
V
t
spectra acquired above a single C
6
H
6
molecule (continu-
ous line) and on the bare Ag(110) surface (dashed line). The peaks at 4 and 19 meV
represent a change of the junction conductance of about 1 and 8%, respectively.
(b) d
2
I
t
/
d
V
t
spectra acquired on a C
6
H
6
ML grown at 4 K (STM image shown
in the inset: 2
5nm
2
,
I
t
=
.
5
×
2
.
0
.
1 nA,
V
t
=
100 meV). The peaks at
±
7 and
±
44 meV represent a change of the junction conductance of about 6 and 1%, re-
spectively. The authors define 1 ML as a fully covered Ag(110) surface. Reprinted
with permission from J. I. Pascual, J. J. Jackiw, Z. Song, P. S. Weiss, H. Conrad
and H.-P. Rust,
Physical Review Letters
,
86
, 1050 (2001). Copyright (2001) by the
American Physical Society.
molecule-substrate interactions, verify the extraordinary sensitivity of ISTS to the
chemical environment of an adsorbate (Pascual
et al.
, 2001). The energies of these
excited modes are well below the expected values for internal vibrations of C
6
H
6
adsorbed on
sp
-metal surfaces and are attributed to the excitation of vibrations
of the C
6
H
6
molecule with respect to the silver surface. In the case of single C
60
molecules adsorbed on Ag(110) the internal
H
g
(
ω
2
) mode is identified at 54 mV
with ISTS (Pascual
et al.
, 2002).
Inelastic tunnelling electrons can also be used to selectively induce either the
translation over a metallic surface or desorption from the metallic surface of indi-
vidual molecules, as has been shown for NH
3
on Cu(100) surfaces (Pascual
et al.
,
2003). Activation of either the stretching vibration of ammonia (
408 meV) lead-
ing to lateral translation on the surface, or the inversion of its pyramidal structure
(umbrella mode
δ
s
(NH
3
)
139 meV) leading to desorption, can be achieved by
adjusting
I
t
and
V
t
.
A STM-induced Ullmann's reaction between two molecules has also become
possible as demonstrated for C
6
H
5
I on Cu(111), producing a single biphenyl
molecule (Hla
et al.
, 2000). Here
I
t
is used to dissociate the C
6
H
5
I molecule into
