Femtosecond Vibrational Dynamics of Self-Trapping in a Quasi-One-Dimensional System

We have directly time resolved the lattice motions associated with the formation of the self-trapped exciton in the quasi-one-dimensional system $\left[\mathrm{Pt}\right(\mathrm{en}{)}_2\left]\right[\mathrm{Pt}\left(\mathrm{en}{)}_2{\mathrm{Br}}_2\right]̇({\mathrm{PF}}_6{)}_4$ ( $\mathrm{en}=\mathrm{ethylene}-\mathrm{diamine}$, ${\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2$), using femtosecond impulsive excitation techniques. A strongly damped, low-frequency wave packet modulation at $\sim 110{\mathrm{cm}}^{-1\mathrm{}}$ accompanies the formation of the self-trapped exciton on a $\sim 200\mathrm{fs}$ time scale following excitation of the intervalence charge-transfer transition. Coherent oscillations at the ground state vibrational frequency and its harmonics are also detected.