Chemistry Reference
In-Depth Information
(a) (b) (c)
Figure 6.16. Angular dependent NEXAFS spectra of TTF-TCNQ for (a) S2
p
,
(b) C1
s
and (c) N1
s
. The
θ
E
angle is defined as the angle between the light polar-
ization vector and the surface plane. Reprinted with permission from J. Fraxedas,
Y. J. Lee, I. Jimenez, R. Gago, R. M. Nieminen, P. Ordejon and E. Canadell,
Phys-
ical Review B
,
68
, 195115 (2003). Copyright (2003) by the American Physical
Society.
The origin of the different peaks of the TTF-TCNQ S PDOS (Fig. 6.4(b)) can
be understood exactly as for neutral TTF (Fig. 6.5(b)). This is not that surprising
since the TTF 1D stacks in the two solids are very similar and TCNQ does not
possess sulfur atoms. Summarizing, the first peaks above
E
F
in the S PDOS of
Fig. 6.4(b) originate respectively from the
σ
∗
(
a
g
), the
π
∗
(
b
3g
b
2g
) (the two
peaks which were seen as a peak and a shoulder for TTF appear as a single peak
now), the
,
a
u
,
σ
∗
(
b
2u
) and the
σ
∗
(
b
3u
,
b
2u
) orbitals of TTF (see Fig. 6.14(a)).
Figure 6.16 shows the NEXAFS spectra as functions of the incidence angle
θ
E
for the oriented TTF-TCNQ films. For
θ
E
close to 90
◦
E
lies
parallel and perpendicular to the molecular
ab
-plane, respectively. In this case the
planar geometry of both TTF and TCNQ molecules is a clear advantage strongly
simplifying the analysis.
The most relevant trends in Fig. 6.16(a), corresponding to the S2
p
spectra, are
the intensity reduction of
0
◦
and
θ
E
=
σ
∗
(
b
2u
) and the increase of
σ
∗
(
a
g
) with increasing
θ
E
.
According to the geometry of the TTF molecules and their molecular orbitals (see
Figs. 1.15 and 6.15), the absorption intensity should exhibit a maximum when the
light electric field coincides with the molecular plane, i.e. for large
θ
E
values. This
σ
∗
(
a
g
), but not for
σ
∗
(
b
2u
). The reason for this behaviour is
is indeed observed for
