Dynamics of acoustic phonons in exciton self-trapping in a quasi-one-dimensional system

The localization of electronic excitations via electron-lattice interactions is an important fundamental process in molecular-based electronic materials. In our previous work, we directly time-resolved the electronic and vibrational dynamics of the exciton self-trapping process in the quasi-one-dimensional mixed-valence metal-halide linear chain (MX) complexes [Pt(en)$_{2}$][Pt(en)$_{2}$X$_{2}$], (X = Cl, Br, I) using femtosecond coherent phonon techniques. In this work, we present transient absorption measurements on PtBr(en) at low temperature that reveal a large amplitude, strongly damped oscillatory component at a frequency of 11 cm$^{-1}$ that is consistent with the generation of a coherent acoustic wave associated with the formation of the localized lattice deformation that stabilizes the self-trapped state. Comparison with models for polaron formation provides an estimate of the spatial extent of the local deformation of $\sim $ 5 unit cells. This work is supported by the NSF under grant DMR-0305403.