Laser Raman scattering by sensitizing dyes adsorbed on substrates.
Item
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Title
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Laser Raman scattering by sensitizing dyes adsorbed on substrates.
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Identifier
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AAI8821060
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identifier
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8821060
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Creator
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Akpabli, Cornelius Kwami.
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Contributor
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Adviser: Daniel L. Akins
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Date
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1988
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Physical | Chemistry, Analytical | Engineering, Chemical
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Abstract
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A Raman scattering enhancement theory based on the formation of molecular vibro-excitonic levels, developed to account for aggregating dyes, is described. Enhanced Raman scattering of 1,1{dollar}\sp\prime{dollar}-diethyl-2,2{dollar}\sp\prime{dollar}-cyanine and five other cyanine dyes adsorbed on some substrate electrodes have been studied. The excitation wavelength dependence of the Raman scattering; the surface potential, the supporting electrolyte, the electrode pretreatment and the pH effects on the Raman scattering intensity and the intrinsic enhancement dependence on the number of molecules in the aggregate are experimentally investigated. The theoretical model presents a unified picture of the enhancement effect and is found to be consistent with the experimental findings. Electrode pretreatment is found not to be a prerequisite for these dyes to give enhanced Raman scattering, but KI as the supporting electrolyte and silver as the substrate are found to give optimum spectra; surface potential and pH also affect the relative Raman intensities. All these findings are supported by uv/vis spectroscopy. The relative enhancement of Raman bands of these dyes adsorbed on silver electrodes for which the surface potential is varied suggests that a resonance can be turned by variation of surface potential. The relative intensities of Raman bands obtained indicate that the adsorption-desorption kinetics are affected by the surface potential and pH of the solution. A resonance effect on the Raman scattering intensity is also found when the exciting laser radiation's frequency approaches the aggregate absorption band of the adsorbed dye. When incident excitation is close to resonance with the J-aggregate absorption, Raman bands which are due to Franck-Condon overlap between intramolecular modes of the molecules in the aggregate and intermolecular lattice modes of the aggregate, and which are attributable to the A-term of Albrecht's polarizability expression, appear in addition to the bands attributed to the B-term present in the nonresonant case. Bands assigned to the A-term have been used as internal Raman standards to normalize the surface concentration, thereby isolating the electric potential dependence of Raman bands assigned to the B-term. Stark-effect shifting of the J-aggregate band has been used to explain the voltage dependence of concentration normalized Raman band intensities for 2,2{dollar}\sp\prime{dollar}-cyanine and 4,4{dollar}\sp\prime{dollar}-cyanine.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
