Events: Dissertation Defenses

• Census and Characterization of Nearby Infrared Excess Stars

  Guest: Tara Cotten
  Wednesday, November 9, 2016 3:00 pm - 4:00 pm
  Location: CSP Conference Room (322)

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  Debris disk stars are the workshops for planetary formation and evolution and excess emission in the infrared serves as the signpost. The disk of tenuous dust surrounds a main-sequence star analogous to our own Solar System. The goal of this study is to understand the evolution of the star in relation to the circumstellar dust through an unprecedented number of sources since there exist a plethora of theories with weak statistical influence.

  A comprehensive collection of stars which display infrared excess were first compiled from many published studies over the past three decades. Concurrent to the literature sources, a new search for excess using the recent release of the AllWISE catalog assembled an extensive catalog of stars. The development of algorithms to identify and exclude stars masquerading as infrared excess emission ensured the highest fidelity that these stars host a debris disk.

  Unanswered questions regarding relationships between a star and it's circumstellar material include evolution, angular momentum, the amount of metals contained in the star, as well as whether or not the star has a companion. In order to begin to address these claims, a large number of debris disk stars with reliable stellar parameters is needed. Archival optical spectroscopy and new observations for over 500 stars obtained many stellar parameters in- cluding rotational velocity, equivalent width of Hα and Li 6708Å, radial velocity, metallicity, activity indicators (Ca II H & K), and age. This investigation sought confirmation of the nature in which infrared excess decays over time and if there are observational signatures to confirm the idea that a stellar companion is expected to disrupt the star's ability to host a debris disk. In addition, the search for stars with debris disks offered an ideal sample to investigate the unanswered questions of whether a large stellar metal content inherently produces a large amount of circumstellar material that is ideal for planet formation or whether there is a direct connection between the debris disk and the stellar rotation where the disk serves as a means to remove angular momentum from the host star. The comprehensive catalog of debris disk stars confirms the nature of the evolution as dust decreases over time, but negated assertions regarding rotation, metallicity, and multiplicity of the star as primary influences on the dustiness of a disk alone. Thus, this work suggests that collisional grinding accounts for the main method of dust production and the initiator of these collisions must be tied to planetary evolution.

  Index words: Main Sequence Stars, Infrared Astronomy, Circumstellar Disks

• Tomas Koci Defends his Dissertation

  Guest: Tomas Koci, UGA Physics and Astronomy
  Friday, July 22, 2016 11:00 am - 12:00 pm
  Location: Room 322, Physics Building

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  Fundamental understanding of structure forming processes in the context of mesoscopic polymer systems is relevant for a number of technological and biomedical applications. In this thesis, extensive simulations of coarse-grained off-lattice polymer models are performed using advanced generalized-ensemble Monte Carlo methods. Microcanonical inflection-point analysis and structural order parameters are used to systematically examine the effects of bond confinement and short-range repulsion on the structural macrostates and transitions of elastic homopolymer chains. The effectiveness of the inflection-point analysis is also demonstrated in a detailed study of polymer aggregation. It is shown that in finite systems, the aggregation transition is a first-order process consisting of a sequence of substransitions between intermediate structural phases.

• Replica-exchange Wang–Landau simulations of lattice proteins for the understanding of the protein folding problem

  Guest: Guangjie Shi, UGA Physics and Astronomy
  Wednesday, July 20, 2016 1:15 pm - 2:15 pm
  Location: Room 322, Physics Building

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  Protein folding is studied within the context of two coarse-grained lattice models that separate all amino acids into only a few types. The hydrophobic-polar (HP) model is a simplified lattice protein model for simulating protein folding and for understanding many biological problems of interest. In this work, an "improved" model, the semi-flexible H0P model, was proposed by introducing a new type of "neutral" monomer, "0", i.e., neither hydrophobic nor polar and also taking into consideration the stiffness of bonds connecting monomers. Even though both models are highly simplified protein models, finding the lowest energy conformations and determining the density of states are extremely difficult. We applied replica-exchange Wang–Landau sampling with appropriate trial moves for determining the density of states of multiple HP and H0P proteins, from which the thermodynamic properties such as specific heat can be calculated. Moreover, we developed a heuristic method for determining the ground state degeneracy of lattice proteins, based on multicanonical sampling. It is applied during comprehensive studies of single-site mutations in specific lattice proteins with different sequences. The effects in which we are interested include structural changes in ground states, changes of ground state energy, degeneracy, and thermodynamic properties of the system. With respect to mutations, both extremely sensitive and insensitive positions in the protein sequence have been found. That is, ground state energies and degeneracies, as well as other thermodynamic and structural quantities may be either largely unaffected or may change significantly due to mutation. Moreover, comparison between the HP model and the semi-flexible H0P model have been performed based on two real proteins: Crambin and Ribonuclease A. We found that, compared with the HP model, the semi-flexible H0P model possesses significantly reduced ground state degeneracy, and rich folding signals as the proteins rearranging into native states from very compact structures at low temperatures. We calculated the free energy vs end-to-end distance as a function of temperature. The HP model shows a relatively shallow folding funnel and flat free energy minimum, reflecting the high degeneracy of the ground state. In contrast, the semi-flexible H0P model has a well developed, rough free energy funnel with a low degeneracy ground state. In both cases, folding funnels are asymmetric with temperature dependent shape.

  Index words: Monte Carlo simulations, Wang–Landau sampling, Replica-exchange Wang–Landau sampling, protein folding, protein folding funnel, hydrophobic-polar model, HP model, semi-flexible H0P model, protein mutation

• 2-Thiouracil Intersystem Crossing Photodynamics Study by Wavelenght Dependent Photoelectron Spectroscopy

  Guest: José Sánchez, UGA Physics and Astronomy
  Thursday, July 14, 2016 2:00 pm - 3:00 pm
  Location: Room 322, Physics Building

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  This work presents the results of wavelength dependent Time-Resolved Photoelectron Spectroscopy (TR-PES) and Ion Yield (TR-IY) studies of 2-thiouracil, which can potentially lead to a better understanding of the phototherapeutic effects of thiouracil-based drugs used in medical applications, like cancer treatment. The experiments provide more information about the relaxation dynamics of the molecule, especially its intersystem crossing pathways and high yield of long-lived triplet states that determines the effectiveness of drugs. The results provide evidence of UV excitation to the S₂(¹ππ*) state with ultrafast (<100fs) internal conversion to the S₁(¹nπ*) state and highly efficient intersystem crossing to the ³ππ* state, in hundreds of femtoseconds. Finally, relaxation to the ground state takes place in the hundreds of picosecond timescale. A trend is observed where excitation to higher vibrational levels of the S₂(ππ*) states results in overall faster dynamics through conical intersections that are accessed by out-of-plane distortion of the molecular ring.

  INDEX WORDS: 2-Thiouracil, Excited states, Time-resolved, Photoelectron spectroscopy, Relaxation dynamics

• Highly-Charged Ion Collisions with Atoms: Applications to X-ray Emission in the Universe

  Guest: Renata Cumbee, UGA Physics and Astronomy
  Wednesday, July 13, 2016 2:00 pm - 3:00 pm
  Location: Room 322, Physics Building

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  Atomic collisions play a fundamental role in astrophysics, plasma physics, and fusion physics. In this dissertation, we focus on charge exchange (CX) between hot ions and neutral atoms relevant to a selection of astronomical environments. Even though charge exchange calculations can provide vital information including neutral and ion density distributions, ion temperatures, elemental abundances, and ion charge state distributions in the environments considered, both theoretical calculations and laboratory studies of these processes lack the necessary reliability and/or coverage. In order to better understand the spectra we observe in astrophysical environments in which both hot plasma and neutral gas are present, includ- ing comets, the heliosphere, supernova remnants, galaxy clusters, star forming galaxies, the outflows of starburst galaxies, and cooling flows of hot gas in the intracluster medium, a thorough CX X-ray model is needed. Included in this model should be a complete set of X-ray line ratios for relevant ion and neutral interactions for a range of energies.

  In this work, theoretical charge exchange emission spectra are produced using cross sec- tions calculated with widely applied approaches including the quantum mechanical molecular orbital close coupling (QMOCC), atomic orbital close coupling (AOCC), classical trajectory Monte Carlo (CTMC), and the multichannel Landau-Zener (MCLZ) methods. When possible, theoretical data are benchmarked to experiments. Using a comprehensible, but still far from complete, CX database, new models are performed for a variety of X-ray emitting environments. In an attempt to describe the excess emission in X-rays of the starburst galaxy M82, Ne X CX line ratios are compared to line ratios observed in the region. A more com- plete XSPEC X-ray emission model is produced for H-like and He-like C-Al ions colliding with H and He for a range of energies; 200, 300, 500, 700, 1000, 2000, 3000, and 5000 eV/u. This model is applied to the northeast rim of the Cygnus Loop supernova remnant in an attempt to determine the contribution of CX to that region. The database is also applied to a preliminary model of the fast and slow solar wind interacting with the heliosphere.

  Index words: atomic processes, galaxies: individual: M82, galaxies: starbursts, ISM: supernova remnants, line: formation, X-rays: general, X-rays: ISM, X-rays: galaxies

• Site Specific Relaxation Dynamics of Fluorescent Centers

  Guest: Mayuri Perera, UGA Physics and Astronomy
  Friday, June 24, 2016 1:00 pm - 2:00 pm
  Location: Room 204B, Physics Building

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  The relaxation dynamics of Ce³⁺ ions in phosphor materials relevant to solid state lighting has been investigated. Of particular interest was the temperature dependence of the emission efficiency in heavily doped yttrium aluminum garnet (YAG) materials, which show a decreased quenching temperature compared to low concentration samples. Using site se- lective spectroscopy, we demonstrate that the lowering of the quenching temperature in high concentration samples is due to cerium ions in distorted sites. Our results can be generalized that high concentrations of cerium and/or an addition of optically mute rare earth ions to broaden the emission spectrum will lead to a lowered quenching temperature. We studied also Na₂Gd₂O(BO₃)₂ doped with Ce³⁺, a system where Ce³⁺ ions occupy two distinct lattice sites. In this case it is shown that the luminescence quenching is due to photoionization.

  Index words: Lifetime Measurements, Luminescencence, Level Crossing, Rare Earth Ions, Photoionization

• The Near-IR Spectrum of NO(X ̃²&Pi;)-He Detected Through Excitation Into the A-State Continuum

  Guest: Volker Beutner, UGA Physics and Astronomy
  Thursday, April 28, 2016 10:00 am - 11:00 am
  Location: Room 204B, Physics Building

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  Nitric Oxide (NO) plays an important role in areas such as Chemistry, Physics and Biology. In order to better understand the interactions of NO, its van der Waals complexes are the target of experimental and theoretical studies. Different NO-Rare gas complexes have been studied, however there has yet to be a report of any structured spectrum of the NO-He complex. In this thesis, a measurement of the near IR spectrum of NO-He in the region of the first NO overtone transition is presented. The IR absorption is detected by exciting the vibrationally excited complex into the A ̃-state continuum. The comparison to the theory of Klos, Chalasinski et al. (J. Chem. Phys. 112, 2195 (2000)) is in good agreement with the observed spectrum. Given the results, this thesis opens up the possibility of using Helium as a seed gas in order to study other NO van der Waals complexes. Dr. Mather now serves as Senior Project Scientist (95-present) for the James Webb Space Telescope, the successor to the great Hubble Space Telescope.

• Statistical and Conformational Analysis of Structural Transitions in Finite Systems

  Guest: Kai Qi
  Wednesday, April 20, 2016 4:00 pm
  Location: Room 322, Physics Building

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