Photoinduced transitions from ionic ͑I͒ to neutral ͑N͒ and neutral ͑N͒ to ionic ͑I͒ phases in an organic charge transfer (CT) complex, tetrathiafulvalene-p-chloranil (TTF-CA), were investigated by femtosecond pumpprobe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced N ͑I͒ states in the I ͑N͒ phase. The results reveal that the I to N ͑IN͒ transition induced by the resonant excitation of the CT band at 4 K is composed of three processes; (1) formation of a confined one-dimensional (1D) N domain, that is, a sequence of D 0 A 0 pairs, just after the photoexcitation, (2) multiplication of the 1D N domains to the semimacroscopic N states up to 20 ps within the absorption depth of the excitation light, and (3) proceeding of the IN transition along the direction normal to the sample surface. At 77 K near the NI transition temperature ͑T c =81 K͒, the size of the 1D N domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D N domain to the positively and negatively charged N states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring I state strongly unstable. The dynamics of the photoinduced N to I ͑NI͒ transition has also been investigated. The 1D I domains are initially produced by lights, however, they decay within 20 ps even if the density of the I domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced IN and NI transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of ϳ0.6, ϳ50, and ϳ85 ps have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the NI domain boundary. On the basis of the results, dynamical aspects of the photoinduced IN and NI transitions have been discussed in detail.

Ultrafast photoinduced phase transition in a spin-Peierls (SP) system of K-tetracyanoquinodimethane (K-TCNQ) was studied by femtosecond (fs) reflection spectroscopy. Photocarriers destabilize the SP phase, resulting in a decrease in molecular dimerization within 400 fs. Such a melting of the SP phase drives three kinds of coherent oscillations. By comparing the oscillations with the Raman bands activated by the dimerization, we show that the oscillation of 20 cm-1 is due to an LO phonon, and it plays an important role for the stabilization of the SP phase.

Coherent control of a charge-lattice coupled oscillation associated with the neutral (N)-ionic (I) transition was successfully demonstrated for a charge-transfer (CT) compound, tetrathiafulvalene-p-chloranil, with use of double femtosecond laser pulses. By changing the interval of the two laser pulses, amplitude of the oscillation in the reflectivity for the intramolecular transition band sensitive to the degree of CT was periodically changed. With increase of the pump intensity, the oscillation amplitude was dramatically enhanced. Such an enhancement is discussed in terms of the cooperative nature of the N-I transition.

Photoinduced phase transitions in spin-Peierls ͑SP͒ systems of alkali ͑M =K,Na͒-tetracyanoquinodimethane ͑TCNQ͒ have been studied by a reflection-type femtosecond ͑fs͒ pump-probe spectroscopy. The SP phase is destabilized by the generation of photocarriers through the breaking of the spin-singlet states in dimers. It results in the decrease of the dimeric molecular displacements within a few hundred of femtoseconds over several tens of TCNQ molecules. It is accompanied by the displacive-type coherent oscillations, which consist mainly of three modes with the frequencies of 20, 49, and 90 cm −1 in K-TCNQ and of two modes with the frequencies of 49 and 99 cm −1 in Na-TCNQ. By taking into account the temperature dependence of the Raman scattering spectra, the mode with 20 cm −1 in K-TCNQ and the modes with 49 and 99 cm −1 in Na-TCNQ are assigned to the phonon modes in the SP ground state, while the modes with 49 and 90 cm −1 in K-TCNQ are assigned to the local modes originating from the photoexcited states. Polarization dependence of the Raman scattering signals shows that the 20-cm −1 mode of K-TCNQ and the 49-cm −1 mode of Na-TCNQ are longitudinal optical ͑LO͒ modes, whereas the 99-cm −1 mode of Na-TCNQ is a transverse optical ͑TO͒ mode. Namely, the LO mode plays an important role on the stabilization of the SP phase in K-TCNQ, while both the LO and TO modes in Na-TCNQ. Such a difference is discussed by scrutinizing the difference of the crystal structures and the nature of the SP transitions in the two compounds.