Ultrafast Lattice Relaxation Dynamics of Exciton in a QUAISI-1-D Metal-Halogen Complex

A quasi-one-dimensional halogen-bridged mixed-valence metal complex is studied by time-resolved pump and probe spectroscopy with sub-5 fs time resolution. Two kinds of oscillatory signals are observed, which are attributed to the wave packet motions both in an electronic ground state and in a self-trapped exciton (STE) state. The onset of the wave packet motion is found to be delayed by about 50 fs, comparing with the ideal wave packet in the electronic excited state. The delay reflects the thermalization process in a free exciton (FE) state and a lattice relaxation process from FE to STE states.