Coherent acoustic phonon generation in exciton self-trapping

Abstract

The coupled electronic and vibrational dynamics of exciton self-trapping are studied in the quasi-one-dimensional material $[\text{Pt}{(\text{en})}_{2}][\text{Pt}{(\text{en})}_{2}{\text{Br}}_{2}]\ensuremath{\cdot}{({\text{PF}}_{6})}_{4}$ ($\text{en}=\text{ethylenediamine}$) 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 $11\text{ }{\text{cm}}^{\ensuremath{-}1}$ in addition to the $110\text{ }{\text{cm}}^{\ensuremath{-}1}$ 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 $\ensuremath{\sim}5$ unit cells of the PtBr chain structure. This value is in good agreement with the localization length predicted by previous extended Peierls-Hubbard calculations.