Chemistry Reference
In-Depth Information
experiments. NEXAFS and constant-initial-state spectroscopy need synchrotron
radiation, because the photon energy has to be scanned and they differ in the en-
ergy range of the initial occupied states: core levels and valence band for NEXAFS
and constant-initial-state, respectively. We will now discuss in detail the electronic
structure of the unoccupied states of TTF-TCNQ as determined by NEXAFS exper-
iments and compare it to the theoretical calculations discussed before in this section
(Fraxedas
et al.
, 2003). Absorption techniques involving core levels have the great
advantage of being element-selective. TTF-TCNQ is a nice example (again!) since
sulfur atoms are exclusively found in TTF and nitrogen atoms in TCNQ.
The further advantage of working with planar molecules was discussed in Section
4.4. Information on the spatial distribution of empty states is obtained by performing
measurements at different angles of incidence of the X-ray radiation,
θ
E
, taking
advantage of the intrinsic linear polarization of synchrotron radiation. The example
of graphite is illuminating, since from initial
s
states,
σ
, symmetric with respect to
the molecular plane, or
, antisymmetric with respect to the molecular plane, final
states are selected when the polarization vector
E
lies parallel or perpendicular to
the basal plane, respectively. Apart from this purely geometrical constraint, dipolar
selection rules also apply. According to these selection rules atomic levels with an
orbital quantum number
l
can be excited only to those levels with
l
π
±
1. Hence
s
initial states can only be excited to
p
final states, while from
p
the achievable
final states are
s
and
d
. This selectivity is mainly accomplished when working with
MOs, and usually simply replacing
s
by
σ
and
p
by
π
does the job. However, this
approximation is insufficient as will be evident below.
The calculated total DOS and PDOS for the relevant atoms of TTF-TCNQ, TTF
and TCNQ were introduced in Figs. 6.4(b), 6.5(b) and 6.5(d), respectively, and
compared to experimental NEXAFS data (grey lines in the figures). The S2
p
,C1
s
and N1
s
NEXAFS spectra obtained from thin films of TTF-TCNQ, TTF and TCNQ
are also shown in Fig. 6.14 where the relevant MOs of TTF and TCNQ associated
with the different features of the spectra are indicated. The differences found in
the energies of the peaks in the NEXAFS spectra compared with the peaks in the
calculated PDOS are in part due to the fact that the calculated PDOS corresponds to
the electronic structure of the ground state while the NEXAFS spectra correspond
to the excited state with a core hole.
TTF-derived spectral features
Let us start with neutral and charged TTF. The experimental spectrum for S2
p
(Fig.
6.14(a)) corresponding to neutral TTF is disappointingly unstructured, whereas that
for charged TTF is clearly rich in structure. As shown in Fig. 1.31, the S2
p
core
levels are composed of two spin-orbit split components, 2
p
3
/
2
and 2
p
1
/
2
, separated
by 1.3 eV, so that the S2
p
NEXAFS spectra consists of the superposition of both
