Chemistry Reference
In-Depth Information
The first unoccupied peak in the total DOS of neutral TTF (see Fig. 6.5(b)) lies
around 2 eV from the TTF HOMO peak. This peak originates from the
a
g
orbital
(see Fig. 6.15), a
-type orbital, which is essentially antibonding for all carbon-
sulfur bonds and to a lesser extent between all carbon-hydrogen bonds. The reason
why it is the lowest lying empty orbital is that it mixes in carbon-carbon bonding
contributions thus making the level less antibonding. This feature will be labelled
as
σ
σ
∗
(
a
g
) owing to its antibonding character. This feature is not observed in the
C1
s
NEXAFS spectrum of TTF (dashed line in Fig. 6.14 (b)) as if transitions from
an initial
s
-state to a final
σ
∗
(
a
g
)-state were forbidden. This is the first example of
the breakdown of the atomic dipolar selection rules.
The next peak in the DOS of TTF (see Fig. 6.5(b)), which is the most prominent
one, in fact consists of two contributions, a larger one at around 2.5 eV above the
HOMO maximum and a smaller one, appearing as a shoulder slightly below. This
is also the case for the C and S PDOS. The two contributions originate from
-type
orbitals of TTF. The lower contribution originates from the
b
3g
orbital lying at 2.58
eV from the TTF HOMO (see Fig. 6.15). The large peak slightly above originates
from the pair of
a
u
and
b
2g
orbitals in Fig. 6.15, which in the molecule are very close
in energy, i.e., at 2.94 and 3.07 eV from the HOMO, and thus appear as a single
peak in the DOS. The associated feature will be labelled as
π
π
∗
(
b
3g
,
b
2g
), and
it can be observed in the NEXAFS curves for both S2
p
(since it has a significant
d
-character in the sulfur atoms) and C1
s
(
s
a
u
,
→
π
∗
transitions in carbon are allowed)
spectra.
The next three peaks in the DOS (see Fig. 6.5(b)), seen as two peaks and a
shoulder, are due to
-type orbitals. The first one is due to the lower
b
2u
orbital of
Fig. 6.15, which lies at 3.26 eV from the HOMO in the molecule and is another
antibonding carbon-sulfur level which will be labelled
σ
σ
∗
(
b
2u
). The next two peaks
originate from two orbitals that are quite similar in energy in the molecule: the
b
3u
and upper
b
2u
orbitals of Fig. 6.15, which lie at 3.61 and 3.86 eV from the HOMO in
the molecule. These two orbitals are also carbon-sulfur antibonding but also include
some carbon-carbon antibonding character and will be termed
σ
∗
(
b
3u
,
b
2u
).
It is important to note that although not shown in Fig. 6.15, the lower
b
2u
orbital
contains a sizeable contribution of the appropriate symmetry adapted combination
of
d
z
2
orbitals of the sulfur atoms. This is not the case for any of the orbitals
considered up to now. This observation will be important in order to understand the
angular variation of the S2
p
spectra discussed later. At higher energies, the spectra
are rather structureless because states lie in a quasi-continuum and electrons can
be considered as nearly free. In fact the crystal field may have some influence even
at such high lying states. In graphite, for instance, this influence remains even for
surprisingly high energies,
c
. 90 eV above the VBM (Bianconi
et al.
, 1977), while
for III-V semiconductors it goes up to
c
.30eV(Faul
et al.
, 1993).
