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solitons. Considering that bands
a
and
b
are associated with spins, solitons
responsible for
a
and
b
should be spin-solitons. A simple model shown in
Fig.
2.12
expects that a single midgap absorption arises for a spin-soliton. However,
the observed PA consists of a doublet structure
a
and
b
and is located at the higher
energy than
E
CT
/2. These experimental features could be explained by the theoreti-
cal studies; Iwano et al
.
showed that the absorption band of a spin-soliton is
split under the presence of quantum lattice fluctuations [
33
] and its energy is higher
than
E
CT
/2 due to the effect of on-site Coulomb repulsion
U
[
51
]. Thus, PA bands
a
and
b
were reasonably assigned to spin-solitons.
The energy of band
g
is small, being about 0.3
E
CT
, so that band
g
was
assigned to the lower energy transition of polarons, which correspond to a
2
band in
Pt-Br-Pd. In fact, the lower energy transitions of polarons are observed at around
0.37
E
CT
in Pr-Br-Pd. In another Pt-Br chain compound, [Pt(chxn)
2
][Pt
(chxn)
2
Br
2
]Br
4
(chxn
ΒΌ
cyclohexanediamine), the similar low energy PA band
was also observed at 0.35
E
CT
(not shown) [
37
]. Moreover, the excitation profile
of polarons (band a
1
) in Pt-Br-Pd is similar to that of band
g
, as shown in Fig.
2.10
.
This also supports the assignment of band
g
to the lower energy transition of
polarons. The higher energy transition of polarons could not be identified in these
studies. It was considered to be obscured by the strong PA band
b
.
2.3.3 Conversion of an Exciton to a Spin-Soliton Pair
In this section, we discuss the relaxation dynamics of photoexcited states in MX-
chain compounds associated with excitons, solitons, and polarons. As mentioned
above, two relaxation processes are considered to exist; radiative decay (PL) from
STE and nonradiative decay via conversions to soliton or polaron pairs and their
recombination. In order to clarify the interrelation of these two processes, excitation
profiles of PL were measured and compared to those of PA signals in Pt-Br-Pt-II
and Pt-Br-Pd, which were shown in Fig.
2.10
[
5
,
52
].
The excitation profiles of PL from STE were shown by thick solid lines in
Fig.
2.10
. In Pt-Br-Pt-II, the intensity of PL begins to increase from the absorption
edge at about 1.9 eV and then decreases sharply at around
E
CT
. Such a sharp
decrease of PL corresponds to the increase of the generation efficiency of spin-
soliton pairs. This suggests that a conversion from an exciton to a spin-soliton pair
occurs. This interpretation was supported by the fact that in Pt-Br-Pd the PL
intensity did not decrease for the excitations with the energy of
E
CT
and that the
relative PL intensity in Pt-Br-Pd is more than one order of magnitude larger than
that in Pt-Br-Pt-II (see Figs.
2.8
and
2.10
).
Dynamical aspects of the exciton to spin-soliton conversion were also
investigated by the time-resolved PL measurements. In Fig.
2.13
, the time-
integrated PL spectra due to STEs at 10 K were shown by solid lines, and time
characteristics of PL were shown in the insets. PL dynamics were reproduced by a
single exponential decay, as shown by the broken lines. The decay time
t
is
220 ps
in Pt-Br-Pt-II and
390 ps in Pt-Br-Pd. Those values are at least one order of
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