You limited your search to:
Partner:
UNT College of Arts and Sciences
Decade:
1990-1999
Year:
1994
Collection:
UNT Scholarly Works
- Are Net Discount Rates Stationary?: Some Further Evidence
- This article discusses net discount rates. Abstract: Gamber and Sorensen provide evidence suggesting that the net discount ratio experienced a level shift in the mean between 1977 and 1981. If such a shift occurred, the nonlinearity in the data shows up as a failure to reject the null hypothesis that a unit root is present; that is, the series is I(1). In this reply, evidence is presented - the Phillips-Perron test and a univariate version of the Stock-Watson q-test - suggesting that the net discount ratio is stationary. Hence, the mean is constant. In addition, if one extends the analysis to include the 1989 through 1993 period, the net discount ratio appears to be reverting. digital.library.unt.edu/ark:/67531/metadc71791/
- Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes
- This article discusses the calculation of a methane C-H oxidative addition trajectory. Abstract: An effective core potential (ECP), parallel supercomputing study of methane activation by 14-electron, Ir(PH₃)₂(X) complexes (X = H, Cl) is presented. Considerable weakening of the coordinated methane C-H bond occurs upon formation of an ɳ²-CH coordinated (X)(PH₃)₂Ir•••HCH₃ adduct. A more strongly bound adduct (with greater weakening of the coordinated C-H bond) occurs when X = Cl versus X = H. The calculated Ir(PH₃)₂(H) + CH₄ → Ir(PH₃)₂(H)₂(Me) reaction enthalpy is -12.8 kcal mol⁻¹, and -41.6 kcal mol⁻¹ for the chloro analogue. The intrinsic reaction coordinate is calculated and compared to an experimental trajectory. Analysis of the wave function along the intrinsic reaction coordinate (IRC) suggests that although donation of electron density from methane to metal is essential for adduct formation, it is not until backdonation to σ* сʜ increases that the C-H bond is activated and cleaved. The electronic and molecular structure of the reacting system along the IRC suggest a two-stage mechanism: substrate to complex donation is important in the early part of the reaction (electrophilic stage) while complex to substrate backdonation is necessary later on (nucleophilic stage) for C-H scission. Finally, comparison of IRCs for low- and high-valent methane-activating complexes shows similar topology in the early portion of the activation event; differentiation between oxidative addition and σ-bond metathesis occurs at the point at which there is a shift from the electrophilic to nucleophilic stage of the reaction. digital.library.unt.edu/ark:/67531/metadc107777/
- Control of chaos in a CO2 laser
- This article discusses the control of chaos in a CO2 laser. Abstract: We report the experimental control of chaos in an optically modulated CO2 laser. The CO2 laser was driven into chaos by injecting a feedback beam modulated by an electro-optical modulator. Control of chaos was achieved using a modified proportional feedback technique in which the control pulses were delayed by approximately one relaxation period. Using this technique, it was possible to control unstable periodic orbits up to period 6. digital.library.unt.edu/ark:/67531/metadc84149/
- An Effective Core Potential Study of Transition-Metal Chalcogenides. 1. Molecular Structure
- This article discusses an effective core potential study of transition-metal chalcogenides. Abstract: A structural analysis is reported of roughly 150 transition-metal (TM)-chalcogenido complexes in a variety of chemical environments. With few exceptions, agreement between calculated and experimental geometries is excellent. The research provides convincing evidence that computational methods employed are adequately describing the bonding in these diverse TM complexes. Interesting trends in relative TMCh (Rмсh-Rмсh) bond lengths are found. Experimental and computational data show that other than the zirconocene-and halfnocene-oxos there is similar behavior in relative bond lengths for widely varying TM-chalcogenido complexes. Relative bond lengths versus oxo (S-O, Se-O, and Te-O) in group IVB metallocenes tend to be larger than for other families of complexes and show less variation among the heavier chalcogens (Se-S, Te-S, and Te-Se). Analysis of localized wave functions for Cp₂ZrCh point to a greater contribution from a singly-bonded Zr-Ch structure (relative to Zr=Ch) when Ch is O compared to heavier chalcogens. Taken together, the data suggest that there is a fundamental difference in the Zr-oxo (and Hf-oxo) bond in relation to heavier chalcogens, consistent with recent experimental data. In previous studies of multiply bonded TM complexes the authors have focused on the ability of ECPs to make computations feasible for complexes incorporating even the heaviest transition metals. The present work also evaluates ECP methods for heavier main group (MG) elements. The chalcogens (CH) O, S, Se, and Te are included in this study. digital.library.unt.edu/ark:/67531/metadc107778/
- Fabrication of silicon-based optical components for an ultraclean accelerator mass spectronomy negative ion source
- This article discusses fabrication of silicon-based optical components for an ultraclean accelerator mass spectronomy negative ion source. Abstract: An ultraclean accelerator mass spectronomy negative ion source for semiconductor material mass analysis has been built and is in operation at the University of North Texas' Ion Beam Modification and Analysis Laboratory (IBMAL). The source is unique in that the active surfaces and apertures of the optical components in the ion source have been fabricated from high-purity single crystal silicon. This prevents both the 133Cs+ beam incident on the semiconductor samples and the negative ions from the sample surfaces from "seeing" and sputtering any metal surfaces (mostly stainless steel) in the beamline. The Cs+ beam can be rastered across the sample surface and the impact energy is adjustable to control depth-profiling rates. An ultraclean ion source of this type is necessary to prevent the injection of Fe and other beamline elements onto the sample or into the tandem accelerator, which is equivalent to putting an impurity signal into the mass analysis of the semiconductor sample. Suppression of these elements increases the sensitivity of the analysis to one part in 10¹² for many masses. The fabrication and alignment of the optical components-einzel lenses, steerers, raster/scanners, and Faraday cups-in the negative ion source will be presented. Also, simulation trajectories are presented to show (1) the interaction of the incident Cs+ ion beam, the semiconductor sample, and holder (biased up to -30 kV), the extracted beam in the sample chamber, and associated potential field lines; and (2) the potential field lines of the octupole steerers. digital.library.unt.edu/ark:/67531/metadc146573/
- The observation of silicon nanocrystals in siloxene
- In this article, the authors report the direct observation of silicon nanocrystals in unannealed siloxene using high resolution transmission electron microscopy. The microstructure consists of an amorphous matrix plus silicon crystallites with dimensions of a few nanometers. This is additional evidence that the photoluminescence of silicon-based materials is due to quantum confinement. digital.library.unt.edu/ark:/67531/metadc84364/