Ultrafast spectroscopy in biological and organic materials.
Item
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Title
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Ultrafast spectroscopy in biological and organic materials.
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Identifier
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AAI9807902
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identifier
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9807902
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Creator
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Bai, Guang.
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Contributor
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Adviser: Robert R. Alfano
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Date
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1997
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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 | Chemistry, Biochemistry | Chemistry, Polymer
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Abstract
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This thesis consists of an experimental investigation of the dynamics of the biological material, visual pigment rhodopsin, and the persistent hole burning material, octaethylpophine-doped polystyrene (OEP/PS), utilizing femtosecond laser spectroscopy.;The cis-trans isomerization of the retinal chromophore in rhodopsin at ambient temperature has been studied by employing a novel three beam femtosecond transient absorption method, and a new model is proposed. Two-thirds of the excited rhodopsin molecules isomerize promptly via curve-crossing to form bathorhodopsin in {dollar}\sim{dollar}200 femtoseconds. The remaining third will miss curve-crossing and stay in the excited state, which never isomerizes and decays to the ground state rhodopsin in {dollar}\sim{dollar}3 picoseconds. These results are consistent with recent two-beam femtosecond transient experiments{dollar}\sp{lcub}\lbrack1-6\rbrack{rcub}{dollar} and agree well with molecular dynamics calculations{dollar}\sp{lcub}\lbrack7-8\rbrack{rcub}.{dollar} The three-beam pump-probe measurement is an important technical advance in the characterization of transient species in the initial step of vision, which directly measures the formation dynamics of the ground state species. Using this technique, we could drive the bathorhodopsin back into rhodopsin. This is the first experimental evidence of trans to cis formation of rhodopsin at ambient temperature.;The characteristic parameters and phototransformation pathway of OEP/PS have been studied for optical storage applications. Femtosecond accumulated photon echo and time-resolved absorption spectroscopy were used. The optical dephasing time T{dollar}\sb2{dollar} for a laser bandwidth covering the whole inhomogeneous zero-phonon absorption band is 200 {dollar}\pm{dollar} 50 ps at 1.4 K. T{dollar}\sb2{dollar} reduces significantly to 100 ps when the temperature increases to 4.2 K. This temperature dependence indicates that OEP/PS must operate at very low temperatures. The saturation dose is 6 J/cm{dollar}\sp2.{dollar} The maximum number of readings is equivalent to the same amount of energy of writing. 150 fs single-shot detection of a 4-bit packet stored in an OEP/PS sample has been demonstrated for the first time by a novel low-intensity interferometric cross-correlation technique{dollar}\sp{lcub}\lbrack9\rbrack{rcub},{dollar} which corresponds to a data readout speed of 27 Tbits/sec. With stimulated emission from the first excited state S{dollar}\sb1{dollar} at 680 nm in addition to a 620 nm writing beam, the phototransformation pathway is altered, leading to a hole burning rate twice as large as the one without stimulated emission.
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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.
