Ultrafast Optical Switching from an Ionic to a Neutral State in Tetrathiafulvalene-p-Chloranil (TTF-CA) Observed in Femtosecond Reflection Spectroscopy

Ultrafast optical switching from an ionic ( $I$) to a neutral ( $N$) state in TTF-CA was observed in femtosecond reflection spectroscopy. Charge transfer excitation in the $I$ phase produces $N$ donor ( $D^0$) acceptor ( $A^0$) strings within 2 ps. These initial $N$ states decay with a lifetime of 300 ps at 4 K, but rather multiply leading to macroscopic $I-N$ conversion within 20 ps at 77 K just below the $N-I$ transition temperature $T_{\mathrm{NI}}$. Near $T_{\mathrm{NI}}$, we also found the evidence for the coherent motion of the macroscopic $N-I$ domain boundary with a period of 85 ps.