Chemistry Reference
In-Depth Information
a
CDW to C P [A]
2.41 eV exc.
1
5
4
0.5
3
2
1
0
1
2
3
4
1.96 eV exc .
Photon Energy (eV)
0
1
b
CP to CDW [B]
E exc // chain axis
T = 8 ms
0.8
4
0.6
3
2
0.4
1
0
0.2
2.41 eV exc.
1
2
3
4
Photon Energy (eV)
0
0
1
2
3
4
5
6
Excitation photon density (10 25 /cm 3 ┬Ěpulse)
Fig. 11.9 Excitation-photon-density dependence of the conversion efficiency from CDW to CP at
the point A (a) and from CP to CDW at the point B (b). The insets show the absorption spectra
obtained from the polarized reflectivity spectra. The arrows indicate the energy positions of the
excitation lights
To clarify whether this PIPT is induced by an optical process or a laser heating,
the dependence of the photoconversion efficiency on the excitation photon density
N was investigated for two excitation energies E ex of 1.96 and 2.41 eV. A He-Ne
laser is used for the 1.96-eV light. The result is presented in Fig. 11.9a . In the
measurements, the excitation photon density N (photon density per pulse) was
calculated considering the absorption coefficient and the reflection loss of the
excitation light, both of which are obtained from the polarized reflection spectra.
The converted fractions were estimated from the photoinduced changes in the
integrated intensity of the Pt-Pt stretching Raman bands. The photoconversion
efficiency shows a clear threshold N th , the values of which are strongly dependent
on E ex ; N th ~ 1.4
10 24 cm 3 /pulse at
2.41 eV. The absolute values of N th seem to be large. However, the irradiated power
of the lights is very small. Note that N th is calculated for one pulse with a long
duration of 8 ms. This characteristic excitation energy dependence of N th
demonstrates that the observed PIPT is driven not by a laser heating but by an
optical process. At the point B , PIPT could not be driven by the irradiation of 1.96-
eV or 2.41-eV light (Fig. 11.9b ), even if the intensity and duration of the light was
changed. However, irradiation with 2.71-eV light for 30 s did result in a PIPT, as
10 25 cm 3 /pulse at 1.96 eV and ~ 3
 
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