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Fig. 5.19 Schematic of the CDW-MH conversion. After the formation of a 1D M
3+
domain shown in
(i), the equilibrium position of X comes on the midpoint between the neighboring M ions as shown in
(ii). At the same time, the coherent oscillation of X occurs as (i)
!
(ii)
!
(iii)
!
(ii)
!
(i).
r
corresponds to the charge modulation associated with the displacements of X
stretching mode in the PdBr chains was also presented by the solid triangles in the
same figure. The frequency of each mode sharply decreases with decrease in
d
.In
fact, in [Pd(chxn
Þ
2
Br]Br
2
, the frequency of the coherent oscillation (90 cm
1
)
within the photogenerated MH domains (broken line in Fig.
5.18b
) is much lower
than that of the Pd-Br stretching Raman band (120 cm
1
: e in Fig.
5.18b
)[
40
]. In
[Pt(chxn
Þ
2
I]I
2
, however, the frequency of the coherent oscillation (100 cm
1
:
dashed-dotted line in Fig.
5.18b
) is almost equal to that of the Raman band
(102 cm
1
) and is larger than that (90 cm
1
) in the PdBr chains, although the
mass of I is much larger than that of Br.
This contradiction can be explained as follows. Since the size of a photo-
generated MH domain is very large (ca. 70 Pt sites) in [Pt(chxn
Þ
2
I]I
2
, the equilib-
rium position of the bridging I ion should be the midpoint between the neighboring
Pt
3+
ions. In such a case, the frequency of the coherent oscillation on
D
R
is double
the frequency of the Pt-I stretching vibration. It is because the two electronic states
corresponding to phase
0 of the oscillation, which are illustrated
in Fig.
5.19(i)
, (iii), respectively, are the same with each other [
40
]. Judging from
the relation in Fig.
5.18b
, the vibrational frequency for
d ¼
¼ p
and phase
¼
0 should be much
smaller than 100 cm
1
and may be decreased to ca. 50 cm
1
in the Pt-I chain. In
this case, the frequency of the coherent oscillation on
D
R
is ca. 100 cm
1
(the
double of 50 cm
1
) as observed. In [Pd(chxn)
2
Br]Br
2
, in which the size of a MH
domain is not so large (ca. 20 Pd sites), the Br ions do not come on the midpoints
and therefore the frequency of the coherent oscillation on
D
R
is not doubled.
The probe-energy dependence of the 100-cm
1
-oscillation amplitude was plot-
ted by open circles in Fig.
5.17d
. The spectrum is similar not to
D
R
(
t
d
¼
0.16 ps)
(solid circles) but to the first derivative of
0.16 ps) (broken line). It
indicates that the gap energy in the MH state is modulated by the Pt-I stretching
vibration. This also supports the fact that the coherent Pt-I vibrations are generated
over the MH state. The observed doubling of the coherent oscillation frequency is
D
R
(
t
d
¼
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