17 Matching Results

Search Results

Advanced search parameters have been applied.

Studies of spin alignment in ferrocenylsilane compounds and in regiospecific oxidation reactions of 1,9-dimethylpentacyclo [5.4.0.02,6.03,10.05,9]undecane-8,11-dione.

Description: Part I. The syntheses of a series of stable ferrocenylsilane compounds and their corresponding polyradical cations are reported. Electron spin properties of these molecules were investigated by cyclic voltammetry, ESR, and magnetic susceptibility measurements. All the compounds presented, showed significant electronic communication (>100 mV) between the redox centers by CV. Part II. Baeyer-Villiger oxidation of (1,9-dimethyl-PCU-8,11-dione) was performed using m-chloroperoxybenzoic acid in 1:2 molar ratios. The product obtained was the corresponding dilactone 113. The structure of the reaction products was established unequivocally via single crystal X-ray diffraction methods. The reaction of the 1,9-dimethyl-PCU-8,11-dione with 1:1 molar ratio of m-chloroperoxybenzoic acid produced again the dilactone 113, and not the expected monolactone 114. Ceric ammonium nitrate (CAN) promoted oxidation reaction of 1,9-dimethyl-PCU-8,11-dione afforded a mixture of dimethylated lactones, which indicated unique reaction mechanism pathways. These individual isomers, 115 and 116, have been isolated from these mixtures via column chromatography by using silica gel as adsorbent followed by fractional recrystallization of individual chromatography fractions. Structures of these pure products have been established unequivocally by application of single crystal X-ray crystallographic methods.
Access: This item is restricted to UNT Community Members. Login required if off-campus.
Date: August 2006
Creator: Atim, Silvia
Partner: UNT Libraries

Magnetic Properties of Insulators : Quarterly Report No. 2 Covering the Period from May 15, 1961 to August 15, 1961

Description: Abstract: "The electron paramagnetic resonance (EPR) of color centers in additively colored KCl crystals is measured to observe the effects of optical bleaching at room temperature. Earlier measurements on the F-center are confirmed and the susceptibility is measured at 78 and 300 deg K over five decades of power, including the very low power region. The width and the saturation properties of the individual multiplets are studied in detail and the technique of making EPR measurements on inhomogeneously broadened lines is discussed. A calculation is presented which shows that a slight departure from a Lorentzian multiplet shape can account for the saturation data. The bleached crystals show a resonance which has a width of 35 gauss and a different rate of saturation than the F- center. This resonance is associated with the B-band which appears in the optical absorption."
Date: August 30, 1961
Creator: Markham, Jordan J. & Noble, Gordon A.
Partner: UNT Libraries Government Documents Department

Particle-in-cell Simulations of Raman Laser Amplification in Preformed Plasmas

Description: Two critical issues in the amplification of laser pulses by backward Raman scattering in plasma slabs are the saturation mechanism of the amplification effect (which determines the maximum attainable output intensity of a Raman amplifier) and the optimal plasma density for amplification. Previous investigations [V.M. Malkin, et al., Phys. Rev. Lett., 82 (22):4448-4451, 1999] identified forward Raman scattering and modulational instabilities of the amplifying seed as the likely saturation mechanisms and lead to an estimated unfocused output intensities of 10{sup 17}W/cm{sup 2}. The optimal density for amplification is determined by the competing constraints of minimizing the plasma density so as to minimize the growth rate of the instabilities leading to saturation but also maintaining the plasma sufficiently dense that the driven Langmuir wave responsible for backscattering does not break prematurely. Here, particle-in-cell code are simulations presented which verify that saturation of backward Raman amplification does occur at intensities of {approx}10{sup 17}W/cm{sup 2} by forward Raman scattering and modulational instabilities. The optimal density for amplification in a plasma with the representative temperature of T(sub)e = 200 eV is also shown in these simulations to be intermediate between the cold plasma wave-breaking density and the density limit found by assuming a water bag electron distribution function.
Date: June 27, 2003
Creator: Clark, Daniel S. & Fisch, Nathaniel J.
Partner: UNT Libraries Government Documents Department

Magnetic Properties of Insulators. Quarterly Report No. 2 Covering Period May 15, 1961 to August 15, 1961

Description: The electron paramagnetic resonance (EPR) of color centers in additively colored KCl crystals is measured to observe the effects of optical bleaching at room temperature. Earlier measurements on the F-center are confirmed and the susceptibility is measured at 78 and 300 deg K over five decades of power, including the very low power region. The width and the saturation properties of the individual multiplets are studied in detail and the technique of making E PR measurements on inhomogeneously broadened lines is discussed. A calculation is presented which shows that a slight departure from a Lorentzian multiplet shape can account for the saturation data. The bleached crystals show a resonance which has a width of 35 gauss and a different rate of saturation than the F- center. This resonance is associated with the B-band which appears in the optical absorption. (auth)
Date: August 30, 1961
Creator: Markham, J. J.
Partner: UNT Libraries Government Documents Department

Inverse Bremsstrahlung Stabilization of Noise in the Generation of Ultra-short Intense Pulses by Backward Raman Amplification

Description: Inverse bremsstrahlung absorption of the pump laser beam in a backward Raman amplifier over the round-trip light transit time through the sub-critical density plasma can more than double the electron temperature of the plasma and produce time-varying axial temperature gradients. The resulting increased Landau damping of the plasma wave and detuning of the resonance can act to stabilize the pump against unwanted amplification of Langmuir noise without disrupting nonlinear amplification of the femtosecond seed pulse. Because the heating rate increases with the charge state Z, only low-Z plasmas (hydrogen, helium, or helium-hydrogen mixtures) will maintain a low enough temperature for efficient operation.
Date: November 6, 2003
Creator: Berger, Richard L.; Solodov, Daniel S. Clark Andrei; Valeo, Ernest J. & Fisch, Nathaniel J.
Partner: UNT Libraries Government Documents Department

Magnetic Separations with Magnetite: Theory, Operation, and Limitations

Description: This dissertation documents the theory development and experimental plan followed to describe how a magnetite-based column under the influence of an external magnetic field functions as a magnetic separator. Theoretical simulations predict that weekly paramagnetic particles in the sub-micron range can be magnetically separated while diamagnetic particles as large as 2 microns in diameter may pass. Magnetite-based columns were evaluated as magnetically-controllable enhanced filtration devices. There was no evidence of enhanced filtration for diamagnetic particles by the magnetite-based bed. Magnetite-based magnetic separators have proven to be effective in specific laboratory experiments, indicating a potential feasibility for scale-up operations. Column media-filter type filtration effects indicate a magnetite-based column would not be suitable for treatment of a waste stream with a high diamagnetic solids content or high volume throughput requirements. Specific applications requiring removal of sub-micron para- or ferromagnetic particles under batch or Stokes flow conditions would be most applicable.
Date: August 1, 2000
Creator: Cotten, G. B.
Partner: UNT Libraries Government Documents Department

Comment on ''Generation of Electromagnetic Pulses from Plasma Channels Induced by Femtosecond Light Strings''

Description: In a recent Letter, Cheng et al. calculated/predicted several new effects: that (a) fraction of the short laser pulse momentum can be imparted to plasma electrons via collisional damping of the laser, thereby exciting a long-lived (longer than an oscillation period) plasma wave, which (b) gives rise to a spatially uniform dipole moment of a plasma, which (c) emits far-field narrow-band radiation at the plasma frequency omega subscript ''p'' over the recombination time of the plasma. We claim that the calculation of the effect (a) is in error and the predicted effects (b,c) do not occur as described. In fact, predicted narrow-band emission at omega subscript ''p'' would not occur even if the momentum transfer and the dipole excitation were calculated correctly.
Date: September 3, 2002
Creator: Shvets, Gennady; Kaganovich, Igor & and Startsev, Edward
Partner: UNT Libraries Government Documents Department

Calcium EXAFS establishes the Mn-Ca cluster in the oxygen-evolving complex of Photosystem II

Description: The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca K-edge using active PS II membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared subsequent to Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 angstroms, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is {approx}; 3.5 angstroms distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster.
Date: August 2, 2002
Creator: Cinco, Roehl M.; McFarlane Holman, Karen L.; Robblee, John H.; Yano, Junko; Pizarro, Shelly A.; Bellacchio, Emanuele et al.
Partner: UNT Libraries Government Documents Department

Nuclear magnetic resonance imaging of water content in the subsurface

Description: Previous theoretical and experimental studies indicated that surface nuclear magnetic resonance (NMR) has the potential to provide cost-effective water content measurements in the subsurface and is a technology ripe for exploitation in practice. The objectives of this investigation are (a) to test the technique under a wide range of hydrogeological conditions and (b) to generalize existing NMR theories in order to correctly model NMR response from conductive ground and to assess properties of the inverse problem. Twenty-four sites with different hydrogeologic settings were selected in New Mexico and Colorado for testing. The greatest limitation of surface NMR technology appears to be the lack of understanding in which manner the NMR signal is influenced by soil-water factors such as pore size distribution, surface-to-volume ratio, paramagnetic ions dissolved in the ground water, and the presence of ferromagnetic minerals. Although the theoretical basis is found to be sound, several advances need to be made to make surface NMR a viable technology for hydrological investigations. There is a research need to investigate, under controlled laboratory conditions, how the complex factors of soil-water systems affect NMR relaxation times.
Date: January 21, 1999
Creator: Hendricks, J.; Yao, T. & Kearns, A.
Partner: UNT Libraries Government Documents Department