FORMATION AND EVOLUTION OF NONLINEAR EXCITATIONS  IN LOW DIMENSIONAL MATERIALS

Jason A Leicht

This dissertation presents studies of the formation and evolution of localized electronic excitations using femtosecond transient absorption measurements. Experiments were conducted on mixed-valence halide-bridged transition metal linear chain complexes. In these materials, the strength of the electron-phonon coupling that drives the localization dynamics can be systematically controlled via the chemical composition. The creation of photogenerated excitations can be controlled by the optical excitation energy, with excitation well above the optical gap energy resulting in the formation of polarons in addition to the self-trapped excitons (STEs) that form following excitation near the band edge. Studies were carried out on platinum bromide linear chain materials, which have intermediate coupling strengths, and on a platinum-chloride linear chain material, which is in the strong coupling regime. Three studies were conducted: 1) Exciton dynamics were investigated in [Pt(en)2][Pt(en)2Br2]·(ClO4)4 and [Pt(en)2][Pt(en)2Br2]·(PF6)4 (en = ethylenediamine, C2H8N2) using low fluence excitation near the band edge. The measurements resolve the spectral evolution associated with the transition from an initially excited delocalized exciton to form a localized STE on a femtosecond time scale, followed by its conversion to a soliton/antisoliton pair via charge transfer processes on a picosecond time scale. Interpretation of the dynamics is aided by global analysis, a signal processing technique that extracts time-dependent spectral components, allowing the overlapping spectra associated with each type of excitation to be identified. 2) Polaron dynamics were investigated in the platinum-bromide complexes. In these experiments, polarons were generated by excitation well above the band edge and by two-photon absorption from excitation at high fluence near the band edge. Self-trapping of polarons was observed on a femtosecond time scale, and was accompanied by wavepacket oscillations that reflect the vibrational motions that create the lattice deformation that stabilizes the self-trapped state. The decay of the polaron population was successfully modeled as geminate recombination constrained to a one-dimensional geometry. 3) Exciton and polaron dynamics were examined in [Pt(en)2][Pt(en)2Cl2]·(ClO4)4. Both excitations were found to form by barrierless self-trapping. The decay of the polaron population followed geminate recombination in one-dimension. The conversion of STEs into solitons was observed, and rates associated with these processes were determined.

FORMATION AND EVOLUTION OF NONLINEAR EXCITATIONS IN LOW DIMENSIONAL MATERIALS

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