Ultrafast spectroscopy in conjugated organic and biological materials.
Item
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Title
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Ultrafast spectroscopy in conjugated organic and biological materials.
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Identifier
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AAI9325167
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identifier
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9325167
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Creator
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Yan, Ming.
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Contributor
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Adviser: Robert R. Alfano
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Date
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1993
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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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Physics, Optics | Engineering, Electronics and Electrical | Biophysics, Medical
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Abstract
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The dynamics of two kinds of conjugated materials, the visual pigment rhodopsin and the organic polymer poly(p-phenylene vinylene), have been studied utilizing femtosecond spectroscopy.;The 11-cis to all-trans torsional isomerization of the retinal chromophore in rhodopsin for both protonated and deuterated aqueous environments have been studied by time-resolved absorption measurements at room temperature. The kinetic results are well modeled by rate equations based on the scheme which involves the isomerization along the torsional coordinate of the 11-cis bond of the retinal chromophore. A metastable intermediate 90 degree twisted state is formed within 200 fs on the excited state surface by rotation around the C{dollar}\sb{11}{dollar}-C{dollar}\sb{12}{dollar} double bond, and it takes 3 ps to form the fully isomerized all-trans photoproduct known as bathorhodopsin and to repopulate the ground state rhodopsin. These results agree well with the semiempirical energy level and molecular dynamics calculations. The observed dynamics are insensitive to deuteration of the exchangeable protons which suggest that proton translocation is unimportant at physiological temperatures.;The conjugated polymer, Poly(p-phenylene vinylene) (PPV) in a stretch oriented film, has been studied using polarized time-resolved absorption with subpicosecond resolution and transient luminescence measurements. Excitations are generated by photoexcitation near the band edge (500nm-540nm) with a 200 fs pulse and the resulting spectral changes are probed with a white light pulse. Lattice stabilized (singlet) self-trapped excitons are formed within 200 fs which are observed by measuring the stimulated gain in their emission band which decay at 10 ps. The agreement of the photoinduced exciton gain spectrum ({dollar}<{dollar}1ps), the transient luminescence spectrum (10 ps) and the steady state luminescence spectrum suggest that the singlet excitons are not further trapped after 200fs of their formation time. Excitation wavelength dependence measurements suggest that the polarization anisotropy effect is due to the different dipole-allowed absorptions for different polarized excitation. Raman gain is observed at 1170 cm{dollar}\sp{lcub}-1{rcub}{dollar} and 1560 cm{dollar}\sp{lcub}-1{rcub}{dollar} when the pump and probe pulses overlap in time, and are distinguished from stimulated exciton gain by tuning the excitation wavelength. The induced absorption is characterized by a fast decay ({dollar}\sim{dollar}1ps), a slow decay ({dollar}\sim{dollar}100ps) and a long-lived absorption process, which correspond to the decay time of free excitons and polaron pairs, respectively. The polaron pairs are formed within 800 fs and decay non-exponentially. Polaron pairs initially decay in about 100 ps to form (singlet) self-trapped excitons which contributes to the long tail of luminescence. A model for the relaxation kinetics in PPV is proposed. The intra- and inter-chain photoexcitations which involve different energy level transitions are discussed.
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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.
