Coherent acoustic phonon generation in exciton self-trapping

The coupled electronic and vibrational dynamics of exciton self-trapping are studied in the quasi-one-dimensional material [Pt(en)₂][Pt(en)₂Br₂]·(PF₆)₄ (en=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 cm^-1 in addition to the 110 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 ~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.