Photo-induced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the transformation between ioni c and neutral phases in tetrathiafuvalen-p-chloranil crystals, induced by e xciting the electronic states of the crystal. Spectroscopic studies have re vealed that N phase domains are formed in the ionic-host crystal only above threshold-excitation intensity in the case of charge-transfer excitation; the threshold intensity depends strongly on the wavelength. On the other ha nd, N-phase domains can be formed without threshold in the excitation inten sity in the case of intra-molecular excitation. Also, femtosecond time-reso lved study has shown the intensity-dependent dynamics of the N-phase domain formation, and revealed the presence of short-lived intermediate states pr ior forming the N-phase domain. The mechanism of the photo-induced phase tr ansition in this crystal is discussed based on these results.