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Far-infrared pump-probe measurement of an organic semiconductor β'-(BEDT-TTF)2ICl2using synchrotron radiation source

We have constructed a far-infrared pump-probe measurement system employing a time structure of synchrotron radiation (SR) source at UVSOR in the Institute for Molecular Science (IMS). The system consists of the SR source (operating at 90.115 MHz), a Michelson-type rapid scan interferometer, a silicon-composite bolometer detector, and a mode-locked Nd:YAG laser that is synchronized to the SR pulses. The system achieved full synchronization between the laser and the SR pulses without reducing the number of electron bunches in the storage ring, and can trace phenomena with pump/probe delays between 1ns (the SR pulse width) and 11ns (the interbunch separation). We chose in this study an organic semiconductor β'-(BEDT-TTF)2ICl2 as a candidate for pump-probe sample. In the β'-type crystal BEDT-TTF molecule are stacked along the b-axis, forming strongly dimerized chains. The E∣b optical conductivity derived from the polarized reflectance showed a strong absorption at 456 cm-1 (Ag mode) as a result of electron-molecular-vibration (EMV) coupling in the dimeric structure. A definite SR delay-time dependence of order nanoseconds was found in the frequency range 470-480 cm-1, possibly related to this Ag mode. This suggests that the EMV dynamics might extend to nanosecond time scale.