Chemistry Reference
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Fig. 5.13 Polarized Raman
scattering spectra obtained
under excitation of 1.96 eV
for [Pd(chxn)
2
Br]Br
2
(
solid
line
) and [Pt(chxn)
2
Br]Br
2
(
dashed line
). The intensity of
excitation is the same for both
compounds
[Pt(chxn)
2
Br]Br
2
[Pd(chxn)
2
Br]Br
2
10
200
150
100
Raman shifts (cm
-1
)
by dots in Fig.
5.12a
(iii), b(iii). The data were analyzed assuming a damped
oscillator given by the following formula.
D
R
osc
¼
A
0
cos
ðo
0
t y
0
Þ
exp
ðt=t
0
Þ
(5.3)
y
0
is the
initial phase. The simulated results (solid lines) reproduce well the experimental
results. The evaluated frequency
o
0
is the oscillation frequency,
t
0
is the decay time, and
Here, where
o
0
(the period
T
0
) of the oscillation does not
depend on the probe energy (
o
0
¼
90 cm
1
,
T
0
¼
0.36 ps). The decay time
t
0
of
the oscillation is 1.4 ps (1.2 ps) for 0.6 eV (1.24 eV). The initial phase
y
0
at 1.24 eV
is small (ca.
p
/6), indicating that the oscillator is of a cosine type, and shifts by
p
at
0.60 eV, which corresponds to the sign reversal of
D
R
osc
. Such a change in the sign
of
D
R
response (Fig.
5.10e
), suggesting that the
coherent oscillation is associated with the CDW-MH conversion. A possible origin
of the oscillation is the symmetric Pd-Br stretching mode in the photogenerated
MH domains, since the CDW-MH conversion should be accompanied by release of
the bridging-Br displacements.
The polarized Raman spectrum with the backscattering configuration for [Pd
(chxn)
2
Br]Br
2
is shown in Fig.
5.13
, together with that for [Pt(chxn)
2
Br]Br
2
which
has larger bridging-Br displacements. The polarizations of the incident and scattering
lights are both parallel to the PdBr chains. The band is observed at ca. 85 cm
1
in
both compounds with the comparable intensities, indicating that this peak is not
related to an M-Br stretching mode but probably to a mode associated with the ligand
molecules. The band at 120 cm
1
(165 cm
1
) in Pd(chxn
D
R
osc
is in agreement with that in
Br
2
ð½
Þ
2
Br
Pt(chxn
Þ
2
Br
B
r
2
Þ
can be assigned to the symmetric Pd-Br (Pt-Br) stretching mode. The intensity
decrease and the lower frequency shift of the Raman band in [Pd(chxn)
2
Br]Br
2
are
due to the decrease of the bridging-Br displacements and the resultant softening of the
mode, respectively. In [Pd(chxn)
2
Br]Br
2
, the frequency of the coherent oscillation
(90 cm
1
) is lower than that of the Raman band (120 cm
1
), consistent with the
relative weakness of Pd
3+
-Br
bond in the photoinduced MH-like domains compared
to the original Pd
4+
-Br
bond in the CDW state. Here, it should be noted that in the
photogenerated MH domains, the Br ion is not located at the precise midpoint
between neighboring Pd ions. Otherwise, the frequency of the oscillation in the
D
R
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