Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 75

Fig. 5.18 (a) Polarized Raman spectra obtained from a 1.96 eV excitation for [Pt(chxn) 2 I]I 2 ,
[Pt(en) 2 I](ClO 4 ) 2 , and [Pt(chxn) 2 I](ClO 4 ) 2 .(b) Frequencies of the M-X stretching Raman bands
as a function of the distortion parameter d ; a: [Pt(chxn) 2 I]I 2 , b: [Pt(en) 2 I](ClO 4 ) 2 , c: [Pt(chxn) 2 I]
(ClO 4 ) 2 , d: [Pd(en) 2 Br](C 5 -Y) 2 (H 2 O), e: [Pd(chxn) 2 Br]Br 2 , f: [Pd(en) 2 Br](C 5 -Y) 2 (H 2 O), g:

[Pd(en) 2 Br](C 4 -Y) 2 sulfosuccinate)

(a-c and e-h: 293 K and d: 206 K). The broken line and the dashed dotted line show the frequency of
the coherent oscillation for [Pd(chxn) 2 Br]Br 2 and [Pt(chxn) 2 I]I 2 , respectively

(H 2 O), and h: [Pd(en) 2 Br](ClO 4 ) 2 (en ¼ ethylenediamine and Y ¼

o i , t i , and y i are

oscillatory components are also shown in Fig. 5.17c . The evaluated

100 cm 1 , 1.2 ps, and

p

(cosine type) for the high-frequency component and

67 cm 1 , 5 ps and 3/2 (sine type) for the low-frequency one. The excitation-

density dependence of the amplitude for the 100-cm 1 oscillation (triangles in
Fig. 5.16 ) is the same as that of the amount of the MH phase characterized by D R
(0.54 eV) (open circles). In addition, the oscillation is of cosine type. These results
suggest that the 100-cm 1 oscillation is a displacive-type oscillation associated
with the CDW-MH transition. Since this transition should be accompanied by the
release of the displacements of the I ions, the oscillation can be assigned to the Pt-I
stretching mode in the photogenerated MH domains. As the 67-cm 1 oscillation is
of sine type, it will be due to an impulsive stimulated Raman process.
To clarify the origin for the coherent oscillation with 100 cm 1 in more detail,
we refer to the Raman spectra. In Fig. 5.18a , the Raman spectrum of [Pt(chxn) 2 I]I 2
is presented together with those of other PtI-chain compounds,

p

½ Pt(en Þ 2 I ð ClO 4 Þ 2

and ClO 4 Þ 2 . All of the Raman spectra were measured with the back-

scattering configurations and with the polarizations of incident and scattering lights
parallel to 1D chains. A small peak at 67 and 130 cm 1

½ Pt(chxn Þ 2 I ð

Þ 2 I]I 2 can be
assigned to the oscillation of the ligand molecules like the case of [Pd(chxn

in [Pt(chxn

Þ 2 Br]Br 2
[ 40 ]. The bands at 100-120 cm 1 are due to the symmetric Pt-I stretching mode. In
Fig. 5.18b , the frequency of this mode is plotted as a function of the distortion
parameter d by the solid circles. d is defined as d ¼

is the displacement of
the bridging X ions and L is the M-M distance). The frequency of the Pd-Br

2 d / L ( d