Chemistry Reference
In-Depth Information
Fig. 26 2 Re(
r
/
r
) spectra measured at Langmuir-Schaefer layers of NiC
10
OAP porphyrin
deposited onto gold (
Δ
30 mN/m). Total coverage ranges from 0 to 16 monolayers. The absolute
sign of the quantity 2 Re(
ʠ ¼
R
b
)/(
R
a
+
R
b
) is arbitrary (From Goletti et al. [
59
].
Reprinted with permission. Copyright 2002. American Chemical Society.)
Δ
r
/
r
)
¼
2(
R
a
organic overlayer. The main structure always falls in the energy region of the Soret
band (at about 390 nm), while the peaks at lower photon energies (522 and 561 nm)
agree with the Q bands of the molecule.
Just after 2 monolayers, the anisotropy spectrum exhibits a well-defined peak at
395 nm, which is in fair agreement with the position of the absorption maximum
(as reported in Fig.
25
), but not totally. At 4 and 6 monolayers, apart from a negative
broad minimum appearing above 420 nm, the spectra have a similar aspect. But at
8 monolayers, the change is evident: the positive maximum shifts to a lower
wavelength (about 374 nm), and an almost symmetric minimum appears at
394 nm. On the whole, the spectrum assumes a “derivative” line shape. Up to
12 monolayers, no appreciable spectral changes occur. Above 14 monolayers, the
amplitude diminishes at all photon energies, rapidly converging to a limit value
which remains steady up to the maximum thickness of 22 monolayers.
When PdC
10
OAP layers are deposited on gold at the lower value of surface
pressure
ʠ¼
10 mN/m, RAS spectra exhibit a completely different behavior: in the
whole range of thickness (2-22 monolayers), the appearance is always peaklike,
with a maximum at about 390 nm and amplitude increasing with coverage. No
transition to the derivative-like line shape is observed. In Fig.
26
, 2Re(
r
/
r
) spectra
of NiC10OAP layers deposited onto gold are reported. The dependence of the RAS
signal upon thickness (here ranging from 0 to 16 monolayers (ML)) and the spectral
characteristics are completely analogous to the case of PdC
10
OAP. The main
difference is the thickness value (10 ML) at which the spectral line shape varies,
Δ
