Chemistry Reference
In-Depth Information
s
peak is ascribed to an interface plasmon. Assuming that the aluminium
conduction electrons arewell described by a free-electron gas, the interface plasmon
frequency is related to the relative dielectric constant
The
h
ω
of themolecular film through
the relation:
ω
b
√
1
ω
=
+
.
(4.4)
s
Equation (4.4) stresses that the surface plasma wave propagates within the alu-
minium substrate whose frequency is modified by the dielectric response of the
molecular adlayer. From the measurements reported, with
h
ω
=
8
.
5 eV and
s
h
1 for CuPc, a value in agreement with those
measured for other planar organic molecules, with an extended delocalization of
π
ω
=
15 eV, one obtains
=
2
.
b
-electrons (Alonso
et al.
, 2003).
Electronic transitions due to the CuPc molecule become evident in the EELS
spectrum starting from a coverage of 0.3 nm. For this coverage two weak peaks cen-
tred at 1.9 and 3.7 eV (Q and B, respectively, in Fig. 4.25) appear together with the
broad structure between 5 and 8 eV. Increasing the coverage, the Q and B transitions
are always present: their intensities increase as a function of the coverage while
their shape, energy position and relative intensity remain essentially unperturbed.
On the contrary the broad structure at 5-8 eV shows a modest evolution reach-
ing its final shape at 0.6 nm. Above this coverage two features located at 5.8 and
7.1 eV appear that are weaker than the B and Q transitions. It is interesting to note
that starting from 1.0 nm the EELS spectrum does not show significant modifica-
tions, thus suggesting that the molecular film has reached a bulk-like configuration.
The transitions at lower energies (
<
5 eV) that appear in Fig. 4.25 are assigned
π
→
π
∗
electronic transitions of Pc molecules (Schaffer
et al.
, 1973). In
particular the peak at 1.9 eV, corresponding to the Q band, is associated to
a
1u
(
mostly to
π
)
→
π
∗
) and
b
2g
(
Np
σ
)
→
e
g
(
y
2
) transitions and related to the formation of
singlet excitons (S
0
and S
1
transitions) in the Pc ring. Although the two transitions
are almost degenerate in energy, the former has a dipole moment perpendicular to
the molecular plane while the latter, mostly from the copper
d
orbitals, has a dipole
moment in the plane of the molecule. The Q band energy coincides with
E
opt
,as
indicated in Table 1.6. The peak at 3.7 eV, associated to the B band, is related to
the
a
2u
(
b
1g
(
d
x
2
−
π
∗
) transition, thus having a well-defined symmetry with respect
to the plane of the molecule. Peaks at 5.8 eV and 7.1 eV correspond to the C and
X
1
bands, respectively, both assigned to
π
)
→
e
g
(
π
→
π
∗
transitions.
From Fig. 4.25, the Q and B transitions appear for coverages of 0.3 nm and
higher. This threshold value corresponds, in the hypothesis of a flat-lying adsorption
geometry, to the saturation of the surface with oneML. In this framework molecules
