Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 67

a b

4

4 0.75

t d = 0.15 ps

x ph = 0.025

0.60 eV

3

Φ

2

0
0.8

0.60 eV

2

0

1

1.24 eV

Φ 1

0.5

1.24 eV

0

0

0

0 10 20 0 1 2 3

10 -2 photons/Pd site) Delay time t d (ps) x ph ( ×

Fig. 5.11 (a) Time profiles of 0.025 ( dashed lines ).

The time profile of f is also shown ( open circles ). Solid lines are fitting curves (see text).
(b) Excitation-density dependence of D R (0.60 eV) and D R (1.24 eV) for t d ¼ 0:15 ps ( dots ).
Dashed lines are guides for the eye

D R (0.60 eV) and D R (1.24 eV) for x ph

¼

To obtain information about the nature of the photogenerated MH state, we also
measured

D R spectra under a resonant excitation of the CT transition (0.7 eV) (not
shown). The spectral shape and its time dependence for the 0.7-eV pump were
found to be almost the same as those for the 1.55-eV excitation. Previous electrore-
flectance [ 64 ] and photoconductivity [ 45 ] measurements for MX-chain compounds
in the CDW phase revealed that the excited state produced by resonant excitation is
an excitonic CT state. Therefore, it is natural to consider that the MH domain
consisting of ca. 20 Pd sites Pd 3 þ Pd 3 þ Pd 3 þ Pd 3 þ Þ

ð is also

Pd 2 þ

ð

generated by pumping at 1.55 eV from an excitonic (or local) CT state

Pd 3 þ Pd 3 þ Pd 4 þ Þ

via multiple CT processes.
Next, let us discuss the dynamical aspects of the photoinduced transition.
Figure 5.11a shows the time profile of f (open circles) together with those of the

transient reflectivity change (| D R |) at 0.60 and 1.24 eV (the dashed line). As two

profiles are in very close agreement with each other, the profiles of | D R | at these two

energies are considered to be suitable for use as a measure for the photogeneration
and decay of the MH domains. The initial responses of

D R (0.60 eV) and D R

(1.24 eV) are presented by open circles in Fig. 5.12a (i), b(i), respectively.

D R
(0.60 eV) increases immediately upon photoexcitation, while D R (1.24 eV)
decreases. This behavior is well reproduced by the temporal profile of the step
response calculated by the convolution assuming a Gaussian pulse response func-
tion with a width of 180 fs (solid line). This indicates that the formation of the 1D
MH domain occurs much faster than the time resolution of the present observations.
Immediately after the initial rise, a coherent oscillation is observed up to 3 ps [see
Fig. 5.12a (ii), b(ii)], which will be discussed later.
The time profiles of

D R (Fig. 5.11a ) excluding the oscillation can be reproduced

well by

n

t
t

D RðtÞ¼A

erf (5.2)