ULTRAFAST SPECTROSCOPY OF SELF TRAPPED EXCITONS IN QUASI-ONE-DIMENSIONAL MATERIALS
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This dissertation studies the transition of electronic excitations from a delocalized state to a localized state. Such processes have a dramatic impact on optical and electronic properties and reflect fundamental interactions in the physics of condensed matter systems. An important example is the self-trapping of excitons: an initially extended free exciton interacts with a deformable lattice, resulting in the formation of a localized self-trapped exciton (STE).Experiments were performed in a series of quasi-one-dimensional materials called mixed-valence metal-halide linear chain (MX) complexes, in which variation of the chemical structure allows systematic control of the strength of the electron-phonon interaction that drives the dynamics. Results are compared in regimes ranging from the strong coupling limit (where electronic excitations are localized to nearly one unit cell) to the weak coupling limit (where electronic localizations are spread out over many unit cells). The MX materials studied here were [Pt(en)2][Pt(en)2(Cl)2]*Y or PtCl, and [Pt(en)2I2][Pt(CN)4] or PtICN. The following experiments were carried out:1 - Transient optical absorption experiments were carried out on PtCl, a material with a very strong electron phonon coupling. These studies revealed the presence of a low frequency 68 cm-1 modulation that carries the system to the self trapped state. This frequency is used to calculate the spatial extent of the self trapped exciton, which is estimated to be localized to nearly 1 unit cell. 2 - Transient optical absorption experiments were carried out on PtICN, a material with a strong electron phonon coupling. These studies revealed the presence of a low frequency 15 cm-1 modulation that carries the system to the self trapped state. Using this frequency, the STE is calculated to be confined to nearly 3 unit cells, reflecting the unusually strong coupling in this novel material.3 - The vibrational properties of the equilibrated STE were determined using 3-pulse pump-pump-probe techniques. Resonant impulsive Raman excitation of the equilibrated STE in both PtCl and PtICN revealed a downward shift in frequency as the lattice distorts to form the more localized STE.