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Two-photon photoelectron emission studies of metal/conjugated polymer interfaces.

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Two photon photoelectron spectroscopy (2PPE) has been used in the investigation of the electronic structure of interfaces between metals and conjugated polymers. This technique probes excited intermediate states in the polymers. The systems studied in this thesis are regioregular poly(3-hexlythiophene-2,5-diyl) (P3HT) and MEH-PPV polymers on a Au(111) substrate. Electron kinetic energy distributions and quantum yields were recorded with varying polymer thicknesses to investigate the influence of the metal substrate. Spectra and yields were also measured as a function of various photon energies and incident laser power. The kinetic energy distribution increases in width by an amount equal to the increment in photon energy, characteristic of the 2PPE process with photoexcitation from a fixed intermediate state. Kinetic modeling of the emission yield as a function of incident laser power and photon energy suggests that the intermediate state accessed during the 20 ns laser pulse is the negatively charged polaron. The binding energy of the intermediate state decreases for films thinner than 4 nm. This behavior can be understood by polarization at the interface, or alternatively by charge transfer. Also, thickness has a very large effect on quantum yield. Intermediate state pumping efficiency is lowered more than ∼1000-fold for thin films compared to films more than 4 nm thick. Change in lifetime quenching with polymer film thickness maybe a consequence of diffusion and dipole coupling to the metal.
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