Large and ultrafast photoinduced reflectivity change in the charge separated phase of Et2Me2Sb[Pd(1,3-dithiol-2-thione-4,5-dithiolate)2]2
Résumé
A large photoinduced change in reflectivity has been observed in the low-temperature charge separated (CS) phase of a dimeric radical anion salt, Et2Me2Sb[Pd(dmit)2]2 ( dmit=1,3 -dithiol-2-thione-4,5-dithiolate). Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time (τ) and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, τ and the estimated domain size were enhanced up to the transition temperature (Tc) with increasing temperature. This phenomenon, a sort of critical slowing down around Tc , suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of "charge" and "molecular orbital" of the constituents, i.e., as a type of photoinduced phase transition.