Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 68

0.60 eV probe 1.24 eV probe

a b

4

0

(i) (i)

3

-0.5

x ph = 0.025

2

-1

1

x ph = 0.025

0

-1.5

-0.2 0 0.2 0.4 -0.2 0 0.2 0.4

t d (ps) t d (ps)

0

4

(ii) (ii)

3

-0.5

2

-1

1

0 -1.5

(iii) (iii)

2

2

1

0 0

-1

-2

-2

-4 -3

0 0.8 1.6 2.4 0 0.8 1.6 2.4

t d (ps) t d (ps)

Fig. 5.12 (a, b) (i) Initial 0.025 ( open circles ) with probe photon energies

of (a) 0.60 eV and (b) 1.24 eV. Solid lines denote fitting curves. (ii) Time profiles of

D R response for x ph

¼

D R within
3 ps. (iii) Oscillatory components obtained by subtracting background rise and decay from time
profiles shown in (ii) ( dots ) (see text). Solid lines denote the time profiles of a damped oscillator

where erf is the error function. The calculated response with parameters for
t ¼

D R
(1.24 eV) are shown as solid lines in Fig. 5.11a . Equation ( 5.2 ) describes the
geminate recombination of a pair of excited species diffusing along a 1D chain
[ 65 ], and is consistent with the linear relationships between (|

0.85 ps and n ¼ 0.8 for

D R (0.60 eV) and

t ¼ 0.42 ps and n ¼ 0.61 for

D R |) and x ph

(Fig. 5.11b ). The parameter is related to the diffusion constant ( D ) and the

initial distance between two photogenerated species ( l o )by

t

t ¼ l o 2 /4 D .

0.5) indicate that the
decay rate of the excited species is larger than in an infinite case, possibly due to the
spatial confinement of the motion of the excited species, which increases the encoun-
ter rate [ 66 ]. A plausible candidate for the excited species is the pair of domain walls
(DWs) between the original CDW state and the photogenerated MH state (i.e., the
CDW-MH DWs). When the MH and CDW states are degenerate, a finite 1D MH
domain corresponds to a pair of CDW-MH DWs, similar to the case of DWs in the
neutral ( N )-ionic ( I ) transition system [ 67 ]. Strictly, the energy of the MH state is
slightly higher than that of the CDW state, so that an MH domain will be confined.
The coherent oscillations observed in the

For an infinite chain, n is 0.5. The evaluated values of n ( >

D R response [Fig. 5.12a (ii), b(ii)]
provide important information about the charge and lattice dynamics of the present
photoinduced CDW to MH transition. By subtracting the background rise and
decay from the

D R response, we extracted the oscillatory component and plotted