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CDW
M
2+
X
M
4+
X
M
2+
X
M
3+
X
M
3+
X
M
4+
X
M
2+
X
M
4+
X
Formation of 1D Pd
3+
do main (<< 180 fs)
(i)
M
2+
X M
3+
X
M
3+
X M
3+
X
M
3+
X M
3+
X
M
3+
X M
4+
X
Coherent oscillation of
lattice phonon (~ 360 fs)
(ii)
M
2+
X
M
3+
X
M
3+
X
M
3+
X
M
3+
X
M
3+
X
M
3+
X
M
4
+
X
Geminate recombination
process along 1D chain
CDW
X
M
4+
X
X
M
3+
X
X
M
4+
X
X
M
4+
X
M
2+
M
2+
M
3+
M
2+
Fig. 5.14 Schematic of proposed CDW-MH conversion process
response should be double the frequency of the Pd-Br stretching mode. This suggests
that some charge modulation
Pd
3
þ
r
Pd
3
r
Pd
3
þ
r
Pd
3
r
1)
remains even in the MH domains, which is likely related to the constrained finite size
of the MH domains (ca. 20 Pd sites).
On the basis of the results presented above, the dynamics for the photoinduced
CDW-to-MH transition can be interpreted in the scheme shown in Fig.
5.14
. Upon
excitation, a CT excited state is photogenerated, and a confined 1D MH domain
composed of ca. 20 Pd sites subsequently emerges (i). As this process is a sequence
of CT processes, the transition will complete on the time scale of the transfer energy
T
. For a typical value of
T
for the MX chains, 0.7 eV [
68
,
69
], the time scale is 6 fs,
which is much faster than the time resolution (180 fs). Therefore, the formation
process of the confined 1D MH domain could not be resolved by the present
pump-probe measurements. In the formation process of a 1D MH domain, the
bridging-Br displacements persist as shown in Fig.
5.14(i)
, since the period of the
Pd-Br stretching mode (360 fs) is much longer than the time scale of the transfer
energy
T
. The important finding is that the initial formation of a 1D MH domain is
purely electronic, driven through the effect of the intersite Coulomb repulsion
V
.
Namely, a photogenerated Pd
3
þ
ð
Þ
(0
< r
Pd
3
þ
pair decreases the energy gain due to
V
in
the background CDW state and then destabilizes the neighboring Pd
2+
and Pd
4+
sites. A large-size MH-like Pd
3+
domain can be realized without the release of the
Br displacements, indicating that
V
is essential for stabilization of the CDW state. In
Pd(chxn
Þ
2
Br
Br
2
, the effect of e-l interaction is suppressed compared with other
Pd compounds owing to the shorter Pd-Pd distance resulting from the chemical
pressure of the strong 2D hydrogen-bond network [
44
,
70
]. The suppression of e-l
interaction is considered to be the reason why the MH domain can be produced
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