Chemistry Reference
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Fig. 5.16 Excitation-density
dependence of D R (0.54 eV)
and e 2 (0.6 eV) at t d ¼ 1.7 ps
and the amplitude of the
coherent oscillation
(100 cm 1 )on D R (0.69 eV)
in arbitrary unit. The dashed
line shows a linear relation
0.15
Δ R = 0.139
30
0.1
20
0.05
10
Δ
R(0.54 eV)
Δε 2 (0.6 eV)
100-cm -1 osc. amp.
0
0
0
0.02
0.04
0.06
x ph (ph/Pt)
the space filling of the photoinduced MH states and the saturation density of x ph
¼
0.015 ph/Pt, the size of the photoinduced MH domain is evaluated to be 70 Pt sites/
ph. In [Pd(chxn) 2 Br]Br 2 ,
D R is proportional to x ph at least up to 0.025 ph/Pd which is
about four times as large as 0.006 ph/Pt and
(ca. 20 Pd sites/ph) is about 1/3.5 of
70 Pt sites/ph in the PtI compound [ 40 ]. These results are fairly consistent with each
other. Using the value of
f
f
and the linear relation between
D R and x ph ,wecan
evaluate the
D R value for the complete CDW to MH conversion to be 0.139, which
was shown by the solid line in Fig. 5.16 .Fromthisline,theratio C of the photoin-
duced MH state relative to the original CDW state for x ph ¼
0.05 is estimated to
be 0.9.
Now, we can deduce the
e 2 spectrum for the MH state using the evaluated C value.
Thin broken lines in Fig. 5.15b show the spectra of
e 2 (CDW)
þ D e 2 ð
0
:
05
Þ=C with
C ¼
0.6-1.0, which give the hypothetical spectra of the MH phase. Here,
e 2 (CDW) is
the original spectrum and
D e 2 (0.05) is
D e 2 at t d ¼
1.7 ps for x ph ¼
0.05 ph/Pt. In the
Br 2 in the MH phase is
inset of Fig. 5.15b ,the
e 2 spectrum of Ni 0 : 16 Pd 0 : 84 ð
chxn
Þ 2 Br
presented together with that of [Pd(chxn
Þ 2 Br]Br 2 in the CDW phase [ 47 ]. The former
provides a single peak with a Lorentzian shape. Therefore, it is reasonable to consider
that the C values of 0.6-0.8 giving the split or distorted spectra are not valid and the
appropriate C value is 0.9-1.0. This is also consistent with the C value (0.9) estimated
from the excitation density dependence.
To clarify the nature of the metallic state produced just after the photoirradiation
for x ph ¼
0.05 ph/Pt, we normalized the
D e 2 spectra for x ph ¼
0.05 and 0.02 ph/Pt
( t d ¼
0.16 ps) at the absorption peak (0.65 eV) for the MH state and calculated the
differential spectrum between them, which is shown by the thick broken line in
Fig. 5.15b . The spectrum shows a monotonous increase with decrease in energy,
demonstrating the formation of a metallic state. As discussed above, the conversion
to the MH state is almost complete for x ph ¼
0.05 ph/Pt. It is therefore natural to
consider that the metallic state is produced by the carrier doping to the MH state.
Such a carrier doping after the photogenerations of MH domains may be understood
possibly by the following processes (1) the residual CDW domains or the boundary
between the MH domains has finite charges, acting as additional carriers in the MH
state or (2) the MH state is formed by the first half of the pump pulse and the carriers
are generated in the MH state by the second half of the pump pulse, while it is
difficult to discriminate these two processes.
 
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