Initial excitation process of ferroelectric B₂ soft mode in KD₂PO₄

Femtosecond Impulsive stimulated Raman scattering of KD2PO4with an optical heterodyne detection has been applied to elucidate the ultra short-time dependence of the response function G(t) associated with a ferroelectric B2soft mode above Tc. In a time region 0200 fs, where the B2mode was excited at t=0 fs, G(t) shows clearly the existence of an initial excitation process followed by a relaxation process well explained in terms of the Debye model. The relaxation process indicates a critical-slowing down with lowering temperature toward Tcin contrast to the initial process independent of temperature due to its mechanical property.