Thesis (M.Sc.)--York University, 2005. Graduate Programme in Physics and Astronomy. / Typescript. Includes bibliographical references (leaves 48-50). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url%5Fver=Z39.88-2004&res%5Fdat=xri:pqdiss &rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR11883

Scattered light echoes provide a unique way to engage in late-time study of supernovae. Formed when light from a supernova scatters off of nearby dust, and arrives at Earth long after the supernova has initially faded from the sky, light echoes can be used to study the precursor supernova through both photometric and spectroscopic methods. The detection rate of light echoes, especially from Type II supernovae, is not well understood, and large scale searches are confounded by uncertainties in supernova ages and peak luminosities. We provide a novel spectroscopic search method for detecting light echoes, and test it with 4 hours of observations of NGC 6946 using the SITELLE Imaging Fourier Transform Spectrometer mounted on the Canada-France-Hawaii Telescope. Our procedure relies on fitting a sloped model to continuum emission, and identifying negatively-sloped continua with the downslope of the emission component of a highly-broadened P-Cygni profile in the H$\alpha$ line, characteristic of supernova ejecta. We find no clear evidence for light echoes from any of the ten known Type II supernovae in NGC 6946, and only one light echo candidate from potential historical supernovae predating 1917. We do however, present an upper limit in H$\alpha$ brightness of \SI{5e-17}{erg/s/cm^2/arcsec^2} for light echoes within this galaxy. We discuss extensions of our methodology, as well as viability of this type of study in other objects, and with other instruments. / Thesis / Master of Science (MSc)

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The Super Dual Auroral Radar Network (SuperDARN) is a global network of coherent high frequency (HF) radars located in the polar, high- and mid-latitudes of both the Northern and Southern hemispheres. This thesis deals with close-range SuperDARN echoes (oblique HF backscatter from the lower part of the ionosphere). The aim of this thesis is to shed light on the origin of these echoes. Previous studies have been content to propose explanations for the origin of these echoes without thorough checking of the proposed mechanisms against constraints available from various radars and other important information. For the purpose of clarifying the situation, a chain of SuperDARN radars in the Northern and Southern hemispheres and several years of daily statistics have been used. This has allowed for several findings. Notably, the close-range SuperDARN echoes show diurnal and seasonal variations and their properties with respect to signal-to-noise-ratio, Doppler velocity and Doppler width vary. Three distinct populations of close-range HF backscatter have been established: (1) a morning population (0400-0700 LT), (2) a midday summer population (0800-1300 LT) and (3) a pre-midnight (2100-2300 LT) population. The morning population is associated with meteor trails which are observed to be peaking near local dawn as expected, and already suggested by previous research. High latitude SuperDARN radars also had echoes (pre-midnight population) with higher Doppler velocities than the others yet the Doppler velocities are smaller than that expected from auroral E region echoes. Given the time and location of this population of echoes, it has been concluded that they are a special class of high latitude E region echoes at high aspect angle which have been termed ``high aspect irregularity region" echoes in the past. Lastly, the midday summer population was found to be too high for polar mesosphere summer echoes and too early for plasma instabilities. It is proposed that these SuperDARN echoes are produced either from contribution from meteors trails or by neutral turbulence which is suspected (from other work) to be present near 100 km. The properties of the midday summer population resembles those of meteor trails as they have the same power, and the same altitude and have high summer occurrence as expected for meteors. Their late morning occurrence could be due to particular look direction of individual radars which may change the occurrence statistics in the presence of meteor showers. With respect to neutral turbulence, the drift of the midday summer population is similar to that of neutral wind.

This thesis focuses on the ultrafast isomerization reaction of retinal in both rhodopsin and bacteriorhodopsin, examples of sensory and energy transduction proteins that exploit the same photoactive chromophore for two very different functions. In bacteriorhodopsin, retinal isomerizes from an all-trans to 13-cis conformation as the primary event in light- driven proton pumping. In the visual pigment rhodopsin, the retinal chromophore isomerizes from an 11-cis to all-trans geometry as the primary step leading to our sense of vision. This diversity of function for nominally identical systems raises the question as to just how optimized are these proteins to arrive at such drastically different functions? Previous work has employed transient absorption spectroscopy to probe retinal protein photochemistry, but many of the relevant electronic and nuclear dynamics of isomerization are masked by inhomogeneous broadening effects and strong spectral overlap between reactant and photoproduct states. This work exploits the unique properties of two-dimensional photon echo spectroscopy to deconvolve inhomogeneous broadening and spectral overlap effects and fully reveal the dynamics that direct retinal isomerization in proteins. In bacteriorhodopsin, vibrational coupling to the reaction coordinate results in a surface crossing event prior to the conventional conical intersection associated with isomerization to the J intermediate. In rhodopsin, however, a similarly early vibrationally-mediated barrier crossing event is observed, resulting in spectral signals consistent with the known photoproduct state appearing an order of magnitude faster than determined from conventional transient absorption measurements. The competing overlapping spectral signals that obscured the initial dynamics when probed with transient absorption spectroscopy are now clearly resolved with two-dimensional photon echo spectroscopy. These experiments illustrate the critical role of the protein in directing the outcome of retinal photochemistry. The protein controls the reaction pathway through steric interactions between the binding pocket and the retinal chromophore, the result of which directly sets the isomerization coordinate and indirectly controls the vibrational coupling to the reaction coordinate based on the local retinal structure. The new insight from this work is the extraordinary degree of selective vibrational coupling involved in directing the isomerization reaction in retinal proteins.

This thesis focuses on the ultrafast isomerization reaction of retinal in both rhodopsin and bacteriorhodopsin, examples of sensory and energy transduction proteins that exploit the same photoactive chromophore for two very different functions. In bacteriorhodopsin, retinal isomerizes from an all-trans to 13-cis conformation as the primary event in light- driven proton pumping. In the visual pigment rhodopsin, the retinal chromophore isomerizes from an 11-cis to all-trans geometry as the primary step leading to our sense of vision. This diversity of function for nominally identical systems raises the question as to just how optimized are these proteins to arrive at such drastically different functions? Previous work has employed transient absorption spectroscopy to probe retinal protein photochemistry, but many of the relevant electronic and nuclear dynamics of isomerization are masked by inhomogeneous broadening effects and strong spectral overlap between reactant and photoproduct states. This work exploits the unique properties of two-dimensional photon echo spectroscopy to deconvolve inhomogeneous broadening and spectral overlap effects and fully reveal the dynamics that direct retinal isomerization in proteins. In bacteriorhodopsin, vibrational coupling to the reaction coordinate results in a surface crossing event prior to the conventional conical intersection associated with isomerization to the J intermediate. In rhodopsin, however, a similarly early vibrationally-mediated barrier crossing event is observed, resulting in spectral signals consistent with the known photoproduct state appearing an order of magnitude faster than determined from conventional transient absorption measurements. The competing overlapping spectral signals that obscured the initial dynamics when probed with transient absorption spectroscopy are now clearly resolved with two-dimensional photon echo spectroscopy. These experiments illustrate the critical role of the protein in directing the outcome of retinal photochemistry. The protein controls the reaction pathway through steric interactions between the binding pocket and the retinal chromophore, the result of which directly sets the isomerization coordinate and indirectly controls the vibrational coupling to the reaction coordinate based on the local retinal structure. The new insight from this work is the extraordinary degree of selective vibrational coupling involved in directing the isomerization reaction in retinal proteins.

No description available.

Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. Description based on print version record. Includes bibliographical references.