Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition; the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition; optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results.