Ultrafast Dynamics of Photo-Induced Cooperative Phenomena: Neutral-Ionic Phase Transition in Charge Transfer Complex.

Abstract

Primary dynamics of photo-induced ionic (I)-neutral (N) phase transition in a charge transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) was investigated by femtosecond reflection spectroscopy with varying excitation densities. In I-to-N transition, charge transfer excitation in the I phase produces N donor-acceptor (D0A0) strings within 200 fs immediately after the weak photo-excitation. These initial N states decay with a life time of 300 ps at 4 K, whereas those cause multiplication leading to macroscopic I-to-N conversion within 20 ps at 77 K just below the N-I transition temperature TNI. Such a multiplication process is not observed for the N-to-I transition at 90 K which is a reverse process of I-to-N transition. Ionic (D+A−) string with a lifetime of several picosecond was observed even for strong excitation in N-to-I transition. Vibrational coherence due to the collective motion of the D+A− dimerization as well as of N-I domain boundary in several tenth ps and sub-ps time regions is observed.