The coupled electronic and vibrational dynamics of exciton self-trapping are studied in the quasi-one-dimensional material $\left[\text{Pt}{\left(\text{en}\right)}_2\right]\left[\text{Pt}{\left(\text{en}\right)}_2{\text{Br}}_2\right]\cdot {\left({\text{PF}}_6\right)}_4$ ($\text{en}=\text{ethylenediamine}$) using femtosecond impulsive excitation techniques. We report transient absorption measurements at 77 K that are modulated by a large amplitude, strongly damped oscillatory component at a frequency of $11{\text{cm}}^{-1}$ in addition to the $110{\text{cm}}^{-1}$ excited-state optical-phonon wave-packet oscillation previously observed at room temperature. We find that the characteristics of the low-frequency oscillatory response are consistent with the theoretically predicted generation of a propagating coherent acoustic wave accompanying the formation of the localized lattice deformation that stabilizes the self-trapped state. The observed low-frequency oscillation, interpreted in the context of theoretical models for polaron formation via coupling to acoustic phonons, provides an estimate of the spatial extent of the resulting localized state of $\sim 5$ unit cells of the PtBr chain structure. This value is in good agreement with the localization length predicted by previous extended Peierls-Hubbard calculations.

• Received 1 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.094302