Chemistry Reference
In-Depth Information
π
−
π
∗
C
2
H
2
MOs: (top) free molecule and (bottom) molecule
interacting with a surface. (b) Schematic interaction diagram between orbitals
Figure 4.3. (a)
π
σ
from C
2
H
2
and
d
z
2
from platinum. Adapted from Hoffmann, 1988.
becomes modified, weakening the metal-metal bonding. Some of the metal-metal
bonding levels that were at the bottom of the band are pushed up (lower binding
energies) while some metal-metal antibonding levels are pushed down. Therefore,
the molecule-surface bonding is accomplished at the expense of bonding within
the surface and the chemisorbed molecule.
Hoffmann's simplified model explains the most important features but has to be
considered, because of its simplicity, as an approximation. In fact, high-resolution
X-ray emission spectroscopy results performed on the N
2
/Ni(100) system, where
the N
2
molecules exhibit an upright adsorption geometry, have shown that the fron-
tier orbitals approximation is insufficient because the chemisorptive bonds affect
all valence states, down to the inner 2
σ
states (see Fig. 1.1), which are located about
25 eV below
E
F
(Nilsson
et al.
, 1997).
Let us now review some relevant measurements performed on individual
molecules adsorbed on metallic surfaces with a STM. The first chosen molecule
is C
2
H
2
, thus connecting with the discussion given above. Figure 4.4(a) shows a
STM image taken in UHV of a single C
2
H
2
molecule on a Pd(111) surface held at
28 K. The molecule appears as a combination of a protrusion (brighter part) with
two shallower depressions (darker parts), suggesting a famous cartoon character.
A cross section through the protrusion and depressions is shown in the lower part
of Fig. 4.5(a). The maximum is 0.013 nm high, the minimum 0.007 nm deep, and
