Chemistry Reference
In-Depth Information
The last example is the PIPT of the iodine-bridged platinum compound
Pt(chxn
I
2
[
25
]. This compound is also located close to the CDW-MH phase
boundary [
44
,
45
]. In this compound, the electron transfer energy between the
neighboring M ions is larger than that in the PdBr-chain compound mentioned
above. Therefore, we can expect that the CDW-MH transition is more continuous
[
49
] and the transition efficiency is enhanced as compared to the PdBr-chain
compound. It was indeed demonstrated that the photoinduced CDW-MH transition
occurs with high efficiency leading to a complete phase conversion in the PtI-chain
compound. For a strong photoexcitation, the low-energy spectral weight is
increased, which is discussed in terms of the MH insulator to metal transition.
The dynamics and mechanism of PIPTs observed in PdBr-chain compound and PtI-
chain compound is elucidated in Sect.
5.4
by means of femtosecond pump-probe
reflection spectroscopy. In Sect.
5.5
, we give the summary of this chapter.
Þ
2
I
5.2 Experimental Methods
5.2.1 Femtosecond Pump-Probe Reflection Spectroscopy
In this chapter, we discuss the ultrafast dynamics of PIPTs in MX-chain compounds
investigated on the basis of the results of femtosecond (fs) pump-probe (PP)
reflection spectroscopy. Here, we briefly explain the experimental method. The
experimental setup of fs PP reflection spectroscopy is schematically illustrated in
Fig.
5.2
. As a light source of the fs PP reflection spectroscopy, a Ti: sapphire
(Al
2
O
3
) regenerative amplifier system operating at 1 kHz was employed. The
fundamental output from the amplifier (800 nm: 1.55 eV) with a pulse width of
130 fs was divided into two beams by a beam splitter. In most of the experiments
described in this chapter, one beam was used for a pump light, and the other for the
excitation of an optical parametric amplifier (OPA) system from which probe lights
ranging from 0.1 to 2.5 eV were obtained. When it is necessary to change the
photon energy of the pump light, the fundamental beam for the pump light from the
amplifier is introduced to another OPA system from which we can obtain tunable
pump lights ranging from 0.5 to 2.5 eV.
In the fs PP measurements, the probe beam was focused at around the center of
the excitation area on the sample surface. The reflected beam was detected by a Si,
an InGaAs, or a HgCdTe photodetector. A suitable detector was selected depending
on the measured energy range. A part of the probe beam is split before it is delivered
to the sample and detected by another photodetector as shown in Fig.
5.2
. The
output signals (sample signal and the reference signal) of two photodetectors were
analyzed with three boxcar integrators and an analogue processor to get a photoin-
duced reflectivity change. To adjust the delay time (
t
d
) of the probe light with
respect to the pump light, the optical-pass length of pump light was changed by a
computer-controlled stepping motor. The time resolution of the apparatus is about
180 fs.
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