Direct observations of the photoinduced change in dimerization in K-TCNQ

The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 ${\mathrm{cm}}^{\ensuremath{-}1}$. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system.