Far-infrared pump-probe measurement of an organic semiconductor beta '-(BEDT-TTF)(2)ICI2 using 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 Insti tute for Molecular Science (IMS). The system consists of the SR source (ope rating at 90.115 MHz), a Michelson-type rapid scan interferometer, a silico n-composite bolometer detector, and a mode-locked Nd:YAG laser that is sync hronized to the SR pulses. The system achieved full synchronization between the laser and the SR pulses without reducing the number of electron bunche s in the storage ring, and can trace phenomena with pump/probe delays betwe en Ins (the SR pulse width) and 11ns (the interbunch separation). We chose in this study an organic semiconductor beta '-(BEDT-TTF)(2)ICI2 as a candid ate for pump-probe sample. In the beta ' -type crystal BEDT-TTF molecule ar e stacked along the b-axis, forming strongly dimerized chains. The E/b opti cal conductivity derived from the polarized reflectance showed a strong abs orption at 456 cm(-1) (A(g) mode) as a result of electron-molecular-vibrati on (EMV) coupling in the dimeric, structure. A definite SR delay-time depen dence of order nanoseconds was found in the frequency range 470-480 cm(-1), possibly related to this A(g) mode. This suggests that the EMV dynamics mi ght extend to nanosecond time scale.