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.

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.