Chemistry Reference
In-Depth Information
Fig. 22 Re(
Δr
/
r
) spectrum
measured at a 20-ML-thick
Langmuir-Blodgett layer of
H
2
THOPP porphyrin
deposited onto silver. The
absolute sign of the
spectrum is arbitrary (From
Goletti et al. [
52
])
the axis perpendicular to the substrate has been measured. The maximum amplitude
has been always recorded when directions
were aligned with the edges of
the substrate. Spectra reported in Figs.
21
and
22
have been measured with this
orientation of the sample. The relation connecting the signal amplitude to the angle
˕
ʱ
and
ʲ
˕
) (Fig.
23
).
It is clear from Eq. (
4
) that in general the RAS signal contains both the real and
the imaginary anisotropies of the organic layer. When B is zero or negligible, the
above expression can be simplified. This is the case for both materials utilized as
substrates in Goletti et al. [
52
,
53
]. The quartz is transparent above 300 nm. For
silver, B is zero above 360 nm [
58
]. Consequently, in both cases, Re(
displays the expected dependence upon sin (2
Δ
r
/
r
) (and then
Δε
00
porph
are proportional and exhibit the same line shape. Since the
value of coefficient A is higher for quartz than for silver in the Soret band region,
the anisotropy
Δ
R/R
RAS
) and
Δε
00
porph
of the porphyrin layer results higher in the latter case.
Also in this case, the substantial lack of reliable data (when Goletti et al. [
52
,
53
]
have been published) about the optical anisotropy of the porphyrin layer
represented a significant limit to interpret the data. The authors assumed that the
RAS spectrum could be simulated by using Lorentz oscillators to convey the optical
properties of the molecule. The optical transition photon energies for the porphyrin
layer were obtained from the absorption spectrum of the same samples. In partic-
ular, the experimental RAS spectrum has been reproduced with two Lorentz
oscillators at 400 and 440 nm, fitting successfully the data by the energy derivative
of the layer dielectric function, similarly to the case of vicinal Si(001) surfaces
reported in literature [
22
,
23
]. The interpretation of the RAS spectra of Figs.
21
and
22
as the derivative of the dielectric function of the organic layer would mean that
increasing the thickness of the film, the optical properties of the outer layer
