A study on the photoinduced dynamics of conjugated polymers by means of phase-modulation Raman spectroscopy

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We have constructed a phase-modulation Raman measurement system by the use of two continuous-wave laser lines. One laser line is used for generating elementary excitations (charged solitons, polarons, etc.) in conjugated polymers ; the other is used for probing the Raman scattering from these photo-generated species. A thin film of trans-polyacetylene (PA) or poly (2,5-thienylenevinylene) (PTV) was set at the head of a cryostat. The temperature of the sample was kept at 77K during the measurements of Raman spectra. The 514.5-nm laser light for photo-irradiation and the laser light for measuring Raman spectra were incident on the same area of the sample. The wavelengths of probing laser lines are 830 and 1064 nm for PA and PTV,respectively. The difference Raman spectrum between the laser-on and laser-off spectra was measured for each sample. Photoinduced Raman spectra have been obtained. It has been shown that these spectra are not due to the thermal effect induced by laser-irradiation. Thus, photoinduced Raman spectra have been tentatively attributed to charged species generated by photo-irradiation.