  We observe a terahertz radiation from a hydrogen-bonded organic molecular
ferroelectric 5,6-dichloro-2-methylbenzimidazole excited by a femtosecond laser
pulse at room temperature. The emitted terahertz wave consists of three
oscillatory components, the frequencies of which agree with those of Raman- and
infrared-active phonon modes. This suggests that the terahertz radiation is
attributed to polarization modulations by infrared-active phonons excited via
the impulsive stimulated Raman scattering processes. By taking into account the
Raman polarizability tensor and dipole-moment for each phonon, we succeeded in
reproducing not only the spectrum of the terahertz radiation, but also its time
characteristic. The analysis method is discussed in detail. Our result provides
a new way for the light-induced terahertz radiation in organic ferroelectrics.
