712 Matching Results

Search Results

Advanced search parameters have been applied.

Effect of oxygen recombination on one-dimensional flow at high Mach numbers

Description: Report presenting a theoretical analysis of air flow in a channel in which oxygen dissociation and recombination occur. An investigation is also made of the comparative relaxation times of the oxygen dissociation-recombination reaction in relation to molecular vibrations.
Date: January 1958
Creator: Heims, Steve P.
Partner: UNT Libraries Government Documents Department

Recombination Parameters in InGaAsSb Epitaxial Layers for Thermophotovoltaic Applications

Description: Radio-frequency (RF) photoreflectance measurements and one-dimensional device simulations have been used to evaluate bulk recombination parameter and surface recombination velocity (SRV) in doubly-capped 0.55 eV, 2 x 10{sup 17} cm{sup -3} doped p-InGaAsSb epitaxial layers for thermophotovoltaic (TPV) applications. Bulk lifetimes of 90-100 ns and SRVs of 680 cm/s to 3200 cm/s (depending on the capping layer) are obtained, with higher doping and higher bandgap capping layers most effective in reducing SRV. RF photoreflectance measurements and one-dimensional device simulations are compatible with a radiative recombination coefficient (B) of 3 x 10{sup -11} cm{sup 3}/s and Auger coefficient (C) of 1 x 10{sup -28} cm{sup 6}/s.
Date: March 17, 2003
Creator: Kumar, R.J.; Gutmann, J.J.; Borrego, J.M.; Dutta, P.S.; Wang, C.A.; Martinelli, R.U. et al.
Partner: UNT Libraries Government Documents Department

A Kinetic Study of the Recombination Reacton Na + SO₂ + Ar

Description: The recombination reaction Na + S02 + Ar was investigated at 787 16 K and at pressures from 1.7 to 80 kPa. NaI vapor was photolyzed by an excimer laser at 308 nm to create Na atoms, whose concentration was monitored by time-resolved resonance absorption at 589 nm. The rate constant at the low pressure limit is ko = (2.7 0.2) x 10-21 cm6 molecule-2 s~1. The Na-SO 2 dissociation energy E0 = 170 35 kJ mol1 was calculated with RRKM theory. The equilibrium constant gave a lower limit E0 > 172 kJ mol~ 1. By combination of these two results, E0 = 190 15 kJ mol~ 1 is obtained. The high pressure limit is k, = (1 - 3) x 10-10 cm3 molecule 1 s~1, depending on the extrapolation method used. Two versions of collision theory were employed to estimate k,.. The 'harpoon' model shows the best agreement with experiment.
Date: December 1990
Creator: Shi, Youchun
Partner: UNT Libraries

Dissociative-recombination product states and the dissociation energy D<sub>0</sub> of Ne<sub>2</sub><sup>+</sup>

Description: Final product states of Ne<sub>2</sub><sup>+</sup> dissociative recombination were studied using time-of-flight spectroscopy to determine the kinetic energies released. The dissociative recombination occurred in a sustained discharge in the presence of a variable magnetic field and discharge voltage, at pressures of 5-15 mTorr. Under different conditions various excited states were observed ranging from the lowest 3<i>s</i> metastable states to higher Rydbcrg states within 0.000 54 eV of the dissociation limit. From their narrow widths, it is deduced that these higher states arose from Ne<sub>2</sub><sup>+</sup> ions with subthermal energies. From two of these narrow distributions, we obtain an improved value for the dissociation limit D<sub>0</sub>(Ne<sub>2</sub><sup>+</sup>)= 1.26±0.02 eV.
Date: February 28, 1998
Creator: Hardy, K. A.; Peterson, J. R.; Ramos, G. & Sheldon, J. W.
Partner: UNT Libraries Government Documents Department

Three-Body Breakup Dynamics in Dissociative Recombination

Description: Using the CRYRing Facility in Stockholm Coupled with an MCP-CCD detector, and a differential stopping foil, we have determined dynamic parameters in the three-body dissociative recombination of H<sub>2</sub>O<sup>+</sup>. These include the distribution between the O(<sup>3</sup>P) and O(<sup>1</sup>D) channels, the distribution of H atom recoil energies in the O(<sup>3</sup>P) channel and the distribution of angles between the two departing H atoms.
Date: June 16, 1999
Creator: Al-Khalili, A.; Datz, S.; Derkatch, A.; Larsson, M.; Rosén, S.; Shi, W. et al.
Partner: UNT Libraries Government Documents Department

FLP-mediated conditional loss of an essential gene to facilitate complementation assays

Description: Commonly, when it is desirable to replace an essential gene with an allelic series of mutated genes, or genes with altered expression patterns, the complementing constructs are introduced into heterozygous plants, followed by the selection of homozygous null segregants. To overcome this laborious and time-consuming step, the newly developed two-component system utilizes a site-specific recombinase to excise a wild-type copy of the gene of interest from transformed tissues. In the first component (the first vector), a wild-type version of the gene is placed between target sequences recognized by FLP recombinase from the yeast 2 μm plasmid. This construct is transformed into a plant heterozygous for a null mutation at the endogenous locus, and progeny plants carrying the excisable complementing gene and segregating homozygous knockout at the endogenous locus are selected. The second component (the second vector) carries the experimental gene along with the FLP gene. When this construct is introduced, FLP recombinase excises the complementing gene, leaving the experimental gene as the only functional copy. The FLP gene is driven by an egg apparatus specific enhancer (EASE) to ensure excision of the complementing cDNA in the egg cell and zygote following floral-dip transformation. The utility of this system is being tested using various experimental derivatives of the essential sucrose-proton symporter, AtSUC2, which is required for photoassimilate transport.
Date: December 2007
Creator: Ganesan, Savita
Partner: UNT Libraries

Laboratory Study of Hall Reconnection in Partially Ionized Plasmas

Description: The effects of partial ionization (ni/nn ≤ 1%) on magnetic reconnection in the Hall regime have been studied systematically in the Magnetic Reconnection Experiment (MRX). It is shown that, when neutrals are added the Hall quadrupole field pattern and thus electron flow is unchanged while the ion outflow speed is reduced due to ion-neutral drag. However, in constrast to theoretical predictions, the ion diffusion layer width does not change appreciably. Therefore, the total ion outflow flux and the normalized reconnection rate are reduced.
Date: May 15, 2012
Creator: Eric E. Lawrence, Hanto Ji, Masaaki Yamaada and Jongsoo Yoo
Partner: UNT Libraries Government Documents Department

Energy dependence of pi, p and pbar transverse momentum spectra for Au+Au collisions at sqrt sNN = 62.4 and 200 GeV

Description: We study the energy dependence of the transverse momentum (pT) spectra for charged pions, protons and anti-protons for Au+Au collisions at sqrt sNN = 62.4 and 200 GeV. Data are presented at mid-rapidity (lbar y rbar&lt; 0.5) for 0.2&lt; pT&lt; 12 GeV/c. In the intermediate pT region (2&lt; pT&lt; 6 GeV/c), the nuclear modification factor is higher at 62.4 GeV than at 200 GeV, while at higher pT (pT&gt; 7 GeV/c) the modification is similar for both energies. The p/pi+ and pbar/pi- ratios for central collisions at sqrt sNN = 62.4 GeV peak at pT _~;; 2 GeV/c. In the pT range where recombination is expected to dominate, the p/pi+ ratios at 62.4 GeV are larger than at 200 GeV, while the pbar/pi- ratios are smaller. For pT&gt; 2 GeV/c, the pbar/pi- ratios at the two beam energies are independent of pT and centrality indicating that the dependence of the pbar/pi- ratio on pT does not change between 62.4 and 200 GeV. These findings challenge various models incorporating jet quenching and/or constituent quark coalescence.
Date: March 26, 2007
Creator: Ritter, H
Partner: UNT Libraries Government Documents Department

Towards the Development of a Molecular Map in Switchgrass: I. Microsatellite Marker Development

Description: The long-term goal of the switchgrass breeding program is to improve regionally adapted varieties and increase biomass yield and feedstock quality. Although, to some extent, biomass yields are dependent on environmental constraints, increased yield can be achieved through the development of genotypes with improved seasonal adaptation, tolerance to unfavorable environmental conditions, and improved resistance to pest and disease. To date, improvement in switchgrass has relied on recurrent breeding strategies based on phenotypic or genotypic selection. Yield improvements have been modest by this method. If we expect to make significant increase in yields, we need tools that will allow us to map complex traits and uncover the genes that influence them. A genetic linkage map could be a powerful tool for accelerating switchgrass development through marker-assisted selection, breeding and recombination. This type of mapping requires the development of markers that can be associated with phenotypic traits in a population of known pedigree. The most commonly used markers for mapping include restriction fragment length polymorphisms (RFLP) and simple sequence repeats (SSR). At ORNL, we have been concentrating on the development of SSR markers, while our colleagues at the University of Georgia are developing RFLP markers in order to select parents to produce a mapping population and from there to create a framework map from {approx}100 F1 progeny.
Date: August 23, 2001
Creator: Gunter, L.E.
Partner: UNT Libraries Government Documents Department

Optical properties of colloidal germanium nanocrystals

Description: Highly crystalline germanium (Ge) nanocrystals in the size range 2--10 nm were grown in inverse micelles and purified and size-separated by high pressure liquid chromatography with on-line optical and electrical diagnostics. The nanocrystals retain the diamond structure of bulk Ge down to at least 2.0 nm (containing about 150 Ge atoms). The background- and impurity-free extinction and photoluminescence (PL) spectra of these nanocrystals revealed rich structure which was interpreted in terms of the bandstructure of Ge shifted to higher energies by quantum confinement. The shifts ranged from {minus}0.1 eV to over 1 eV for the various transitions. PL in the range 350--700 nm was observed from nanocrystals 2--5 nm in size. The 2.0 nm nanocrystals yielded the most intense PL (at 420 nm) which is believed to be intrinsic and attributed to direct recombination at {Gamma}. Excitation at high energy (250 nm) populates most of the conduction bands resulting in competing recombination channels and the observed broad PL spectra.
Date: May 1, 2000
Partner: UNT Libraries Government Documents Department

Nonhomologous Mechanisms of Repair of Chromosomal Breaks

Description: Discovered three new proteins involved in DNA damage assessment. Interestingly they are all proteins involved in recombination, but they have very different roles in that process and other proteins that might be expected to be equivalently involved are not. This is developing into a very significant area of research.
Date: December 19, 2001
Creator: Haber, J. E.
Partner: UNT Libraries Government Documents Department

Molecular mechanisms of extensive mitochondrial gene rearrangementin plethodontid salamanders

Description: Extensive gene rearrangement is reported in the mitochondrial genomes of lungless salamanders (Plethodontidae). In each genome with a novel gene order, there is evidence that the rearrangement was mediated by duplication of part of the mitochondrial genome, including the presence of both pseudogenes and additional, presumably functional, copies of duplicated genes. All rearrangement-mediating duplications include either the origin of light strand replication and the nearby tRNA genes or the regions flanking the origin of heavy strand replication. The latter regions comprise nad6, trnE, cob, trnT, an intergenic spacer between trnT and trnP and, in some genomes, trnP, the control region, trnF, rrnS, trnV, rrnL, trnL1, and nad1. In some cases, two copies of duplicated genes, presumptive regulatory regions, and/or sequences with no assignable function have been retained in the genome following the initial duplication; in other genomes, only one of the duplicated copies has been retained. Both tandem and non-tandem duplications are present in these genomes, suggesting different duplication mechanisms. In some of these mtDNAs, up to 25 percent of the total length is composed of tandem duplications of non-coding sequence that includes putative regulatory regions and/or pseudogenes of tRNAs and protein-coding genes along with otherwise unassignable sequences. These data indicate that imprecise initiation and termination of replication, slipped-strand mispairing, and intra-molecular recombination may all have played a role in generating repeats during the evolutionary history of plethodontid mitochondrial genomes.
Date: June 1, 2005
Creator: Mueller, Rachel Lockridge & Boore, Jeffrey L.
Partner: UNT Libraries Government Documents Department

Raman spectroscopy and time-resolved photoluminescence of BN and BxCyNz nanotubes

Description: We report Raman and time-resolved photoluminescence spectroscopic studies of multiwalled BN and B{sub x}C{sub y}N{sub z} nanotubes. The Raman spectroscopy shows that the as-grown B{sub x}C{sub y}N{sub z} charge recombination, respectively. Comparison of the photoluminescence of BN nanotubes to that decay process is characterized by two time constants that are attributed to intra- and inter-BN sheet nanotubes as predicted by theory. nanotubes are radially phase separated into BN shells and carbon shells. The photoluminescence of hexagonal BN is consistent with the existence of a spatially indirect band gap in multi-walled BN.
Date: January 21, 2004
Creator: Wu, J.; Han, Wei-Qiang; Walukiewicz, W.; Ager III, J.W.; Shan, W.; Haller,E.E. et al.
Partner: UNT Libraries Government Documents Department

A Study of Minority Atomic Ion Recombination in the Helium Afterglow

Description: Electron-ion recombination has been under study for many years, but comparisons between theory and experiment have been very difficult, especially for conditions where the ion under evaluation was a minority in concentration. This study describes a direct measurement of the recombination-rate coefficient for the recombination of minority as well as majority ions in the afterglow.
Date: August 1972
Creator: Wells, William E.
Partner: UNT Libraries

Widespread Discordance of Gene Trees with Species Tree inDrosophila: Evidence for Incomplete Lineage Sorting

Description: The phylogenetic relationship of the now fully sequencedspecies Drosophila erecta and D. yakuba with respect to the D.melanogaster species complex has been a subject of controversy. All threepossible groupings of the species have been reported in the past, thoughrecent multi-gene studies suggest that D. erecta and D. yakuba are sisterspecies. Using the whole genomes of each of these species as well as thefour other fully sequenced species in the subgenus Sophophora, we set outto investigate the placement of D. erecta and D. yakuba in the D.melanogaster species group and to understand the cause of the pastincongruence. Though we find that the phylogeny grouping D. erecta and D.yakuba together is the best supported, we also find widespreadincongruence in nucleotide and amino acid substitutions, insertions anddeletions, and gene trees. The time inferred to span the two keyspeciation events is short enough that under the coalescent model, theincongruence could be the result of incomplete lineage sorting.Consistent with the lineage-sorting hypothesis, substitutions supportingthe same tree were spatially clustered. Support for the different treeswas found to be linked to recombination such that adjacent genes supportthe same tree most often in regions of low recombination andsubstitutions supporting the same tree are most enriched roughly on thesame scale as linkage disequilibrium, also consistent with lineagesorting. The incongruence was found to be statistically significant androbust to model and species choice. No systematic biases were found. Weconclude that phylogenetic incongruence in the D. melanogaster speciescomplex is the result, at least in part, of incomplete lineage sorting.Incomplete lineage sorting will likely cause phylogenetic incongruence inmany comparative genomics datasets. Methods to infer the correct speciestree, the history of every base in the genome, and comparative methodsthat control for and/or utilize this information will be valuableadvancements for the field of comparative genomics.
Date: August 28, 2006
Creator: Pollard, Daniel A.; Iyer, Venky N.; Moses, Alan M. & Eisen,Michael B.
Partner: UNT Libraries Government Documents Department

Nonlinear broadband photoluminescence of graphene induced by femtosecond laser irradiation

Description: Upon femtosecond laser irradiation, a bright, broadband photoluminescence is observed from graphene at frequencies well above the excitation frequency. Analyses show that it arises from radiative recombination of a broad distribution of nonequilibrium electrons and holes, generated by rapid scattering between photoexcited carriers within tens of femtoseconds after the optical excitation. Its highly unusual characteristics come from the unique electronic and structural properties of graphene.
Date: July 1, 2010
Creator: Liu, Wei-Tao; Wu, S.W.; Schuck, P.J.; Salmeron, Miquel; Shen, Y.R. & Wang, F.
Partner: UNT Libraries Government Documents Department

The Luminescence of Chlorophyll-Containing Plant Material

Description: The luminescence of various chlorophyll-containing plant materials has been investigated under a variety of conditions. The results have been shown to be consistent with a mechanism involving the recombination of electrons and holes trapped in a quasi-crystalline lattice. Some details of such a mechanism have been proposed which suggest the mode of entry of the light energy into the photosynthetic pathway.
Date: July 1, 1957
Creator: Tollin, Gordon & Calvin, Melvin
Partner: UNT Libraries Government Documents Department

Up-Dating Of Atomic Data Needed For Ionisation Balance Evaluations of Krypton and Molybdenum

Description: Atomic data for both ionization and recombination of Kr and Mo ions are reviewed, since the rate coefficients for these processes need to be regularly up-dated following the publication of new theoretical calculations and new experimental data. Kr is used in magnetic-confinement-fusion devices to produce a peripheral radiating mantle meant to spread the heat confinement-load on the plasma-facing components. In a few tokamaks Mo tiles cover the plasma-facing surfaces, acting in most cases as a plasma-column limiter. The collected atomic data represent the state of the art on the ionization and recombination data for the two considered elements. Samples of rates are proposed for both ionization and recombination along with tables of the fractional abundances at ionization equilibrium. The proposed rates should be included in codes that simulate the impurity behavior in magnetic-confinement-fusion devices, i.e., when radial transport is added to ionization and recombination to predict spatially resolved charge-state distributions that are to be compared with experimental results.
Date: June 1, 2006
Creator: Mattioli, M; Mazzitelli, G; Fournier, K B; Finkenthal, M & Carraro, L
Partner: UNT Libraries Government Documents Department

Simple model relating recombination rates and non-proportional light yield in scintillators

Description: We present a phenomenological approach to derive an approximate expression for the local light yield along a track as a function of the rate constants of different kinetic orders of radiative and quenching processes for excitons and electron-hole pairs excited by an incident {gamma}-ray in a scintillating crystal. For excitons, the radiative and quenching processes considered are linear and binary, and for electron-hole pairs a ternary (Auger type) quenching process is also taken into account. The local light yield (Y{sub L}) in photons per MeV is plotted as a function of the deposited energy, -dE/dx (keV/cm) at any point x along the track length. This model formulation achieves a certain simplicity by using two coupled rate equations. We discuss the approximations that are involved. There are a sufficient number of parameters in this model to fit local light yield profiles needed for qualitative comparison with experiment.
Date: September 24, 2008
Creator: Moses, William W.; Bizarri, Gregory; Singh, Jai; Vasil'ev, Andrey N. & Williams, Richard T.
Partner: UNT Libraries Government Documents Department

Overexpressed of RAD51 suppresses recombination defects: a possible mechanism to reverse genomic instability

Description: RAD51, a key protein in the homologous recombinational DNA repair (HRR) pathway, is the major strand-transferase required for mitotic recombination. An important early step in HRR is the formation of single-stranded DNA (ss-DNA) coated by RPA (a ss-DNA binding protein). Displacement of RPA by RAD51 is highly regulated and facilitated by a number of different proteins known as the 'recombination mediators'. To assist these recombination mediators, a second group of proteins also is required and we are defining these proteins here as 'recombination co-mediators'. Defects in either recombination mediators or comediators, including BRCA1 and BRCA2, lead to impaired HRR that can genetically be complemented for (i.e. suppressed) by overexpression of RAD51. Defects in HRR have long been known to contribute to genomic instability leading to tumor development. Since genomic instability also slows cell growth, precancerous cells presumably require genomic restabilization to gain a growth advantage. RAD51 is overexpressed in many tumors, and therefore, we hypothesize that the complementing ability of elevated levels of RAD51 in tumors with initial HRR defects limits genomic instability during carcinogenic progression. Of particular interest, this model may also help explain the high frequency of TP53 mutations in human cancers, since wild-type p53 represses RAD51.
Date: October 15, 2009
Creator: Schild, David & Wiese, Claudia
Partner: UNT Libraries Government Documents Department