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Picosecond time-resolved polarized infrared spectroscopic study of photoexcited states and their dynamics in oriented poly(p-phenylenevinylene).

The picosecond time-resolved polarized infrared absorption spectra of the photoexcited states of an oriented poly(p-phenylenevinylene) [-(C6H4CH=CH)n-, PPV] film have been recorded in the 3600-1050 cm(-1) region. The time-resolved photoinduced infrared absorptions due to an electronic transition in the wavenumber region of 3600-1700 cm(-1) and vibrational transitions in the region of 1700-1050 cm(-1) have fast and slow decay components under all polarization conditions of the pump and probe pulses with respect to the orientation of PPV. The picosecond time-resolved infrared absorption spectra are analyzed by reference to the photoinduced and doping-induced infrared difference spectra of PPV. The long-lived transient species, observed with the pump polarization perpendicular and the probe polarization parallel to the orientation of PPV, are assigned to positive and negative polarons. The short-lived transient species under the same polarization condition is assigned to bound polaron pairs. It is likely that singlet excitons are observed with both the pump and probe polarizations parallel to the orientation of PPV. The singlet excitons show fast decay due to the exciton-exciton annihilation under the high excitation density used in the present study.