Ultrafast lattice dynamics in excitonic self-trapping of quasi-one dimensional materials

Francis Xavier Morrissey

doi:10.7273/000005726

This dissertation presents an investigation of the localization of electronic excitations via electron-lattice interactions. Electronic localization is an important fundamental process in molecular-based electronic materials. Femtosecond vibrationally impulsive excitation techniques were used to directly time-resolve the lattice motion associated with the self-trapping dynamics in halogen-bridged mixed-valence linear chain complexes, [Pt(en)2X2][Pt(en)2], en = ethylenediamine (C2H8N2) and (X = Br - and I - ), or PtX. Three sets of experimental studies were conducted: (1) The investigation of PtI, representing the weak electron-phonon coupling limit, revealed a low frequency modulation of the induced absorbance that was assigned to the motion that drives the system to the self-trapped state. A formation component, which decays within approximately a single period of the low frequency motion, is assigned to the transfer of population from the free exciton to the self-trapped state. Previous impulsive excitation measurements on PtBr and PtCl, in conjunction with the PtI measurements in this dissertation, uncover the dependence of the self-trapping dynamics on electron-phonon coupling strength.; (2) The vibrational frequency of a characteristic mode of the self-trapped exciton in its equilibrated structure was determined using a multiple pulse pump-pump-probe sequence. Timedomain excited state resonant stimulated impulsive Raman excitation of PtBr revealed a vibrational mode associated with the equilibrated self-trapped exciton at 125 cm-1. The upward shift in frequency from the final state of the intervalence charge transfer transition at 110 cm-1 to the equilibrated configuration is consistent with an increased degree of localization in the stabilized structure. (3) Low temperature pump-probe measurements on PtBr uncovered an acoustic phonon response seen as a large amplitude 11 cm-1 modulation of the induced absorbance. Using this frequency, the localized lattice deformation was estimated to have a spatial extent of ~ 5 unit cells, which reflects the size of the self-trapped exciton, consistent with theoretical models for polaron formation. An enhancement of the excited state wavepacket dephasing time indicates that the vibrational coherence properties of the exciton at room temperature are not simply limited by the excited state structural distortion.

Ultrafast lattice dynamics in excitonic self-trapping of quasi-one dimensional materials

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