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Including Quantum Effects in the Dynamics of Complex (i.e., Large)Molecular Systems

Description: The development in the 1950's and 60's of crossed molecular beam methods for studying chemical reactions at the single-collision molecular level stimulated the need and desire for theoretical methods to describe these and other dynamical processes in molecular systems. Chemical dynamics theory has made great strides in the ensuing decades, so that methods are now available for treating the quantum dynamics of small molecular systems essentially completely. For the large molecular systems that are of so much interest nowadays (e.g. chemical reactions in solution, in clusters, in nano-structures, in biological systems, etc.), however, the only generally available theoretical approach is classical molecular dynamics (MD) simulations. Much effort is currently being devoted to the development of approaches for describing the quantum dynamics of these complex systems. This paper reviews some of these approaches, especially the use of semiclassical approximations for adding quantum effects to classical MD simulations, also showing some new versions that should make these semiclassical approaches even more practical and accurate.
Date: April 27, 2006
Creator: Miller, William H.
Partner: UNT Libraries Government Documents Department

Determination of ionization energies of CnN (n=4-12): Vacuum-ultraviolet (VUV) photoionization experiments and theoretical calculations

Description: Results from single photon vacuum ultraviolet photoionization of astrophysically relevant CnN clusters, n = 4 - 12, in the photon energy range of 8.0 eV to 12.8 eV are presented. The experimental photoionization efficiency curves, combined with electronic structure calculations, provide improved ionization energies of the CnN species. A search through numerous nitrogen-terminated CnN isomers for n=4-9 indicates that the linear isomer has the lowest energy, and therefore should be the most abundant isomer in the molecular beam. Comparison with calculated results also shed light on the energetics of the linear CnN clusters, particularly in the trends of the even-carbon and the odd-carbon series. These results can help guide the search of potential astronomical observations of these neutral molecules together with their cations in highly ionized regions or regions with a high UV/VUV photon flux (ranging from the visible to VUV with flux maxima in the Lyman- region) in the interstellar medium.
Date: June 10, 2010
Creator: Kostko, Oleg; Zhou, Jia; Sun, Bian Jian; Lie, Jie Shiuan; Chang, Agnes H.H.; Kaiser, Ralf I. et al.
Partner: UNT Libraries Government Documents Department

Determination of ionization energies of CnN (n=4-12): Vacuum-ultraviolet (VUV) photoionization experiments and theoretical calculations

Description: Results from single photon vacuum ultraviolet photoionization of astrophysically relevant CnN clusters, n = 4 - 12, in the photon energy range of 8.0 eV to 12.8 eV are presented. The experimental photoionization efficiency curves, combined with electronic structure calculations, provide improved ionization energies of the CnN species. A search through numerous nitrogen-terminated CnN isomers for n=4-9 indicates that the linear isomer has the lowest energy, and therefore should be the most abundant isomer in the molecular beam. Comparison with calculated results also shed light on the energetics of the linear CnN clusters, particularly in the trends of the even-carbon and the odd-carbon series. These results can help guide the search of potential astronomical observations of these neutral molecules together with their cations in highly ionized regions or regions with a high UV/VUV photon flux (ranging from the visible to VUV with flux maxima in the Lyman-a region) in the interstellar medium.
Date: March 2, 2010
Creator: Kostko, Oleg; Zhou, Jia; Sun, Bian Jian; Lie, Jie Shiuan; Chang, Agnes H.H.; Kaiser, Ralf I. et al.
Partner: UNT Libraries Government Documents Department

Photoelectron spectroscopy of molecular beams

Description: From chemical spectroscopy and photochemistry in the vacuum ultraviolet conference; Valmorin, Quebec, Canada (5 Aug 1973). The development of the field of photoelectron spectroscopy and the intensities and angular distributions from which one can infer transition probabilities and phase shifts are discussed. (JFP)
Date: January 1, 1973
Creator: Berkowitz, J.
Partner: UNT Libraries Government Documents Department

THREE AND FOUR CENTER ELIMINATION OF HC1 IN THE MULTIPHOTON DISSOCIATION OF HALOGENATED HYDROCARBONS

Description: Three and four center unimolecular elimination reactions of HCl have been investigated for CHF{sub 2}Cl, CHFCL{sub 2}, CH{sub 3}CCl{sub 3}, CH{sub 3}CF{sub 2}Cl and in a molecular beam experiment using infrared multiphoton absorption to energize the molecule. The translational energy distributions obtained in this work show that the average translational energy released to the fragments is around 8-12 kcal/mole, except for the three center elimination reaction from CHClCF{sub 2}, which gives a value of 1 kcal/mole. In four center eliminations, the translational energy released is less than 20% of the potential energy barrier of the back reaction. This is somewhat less than previous indications that approximately 30% of the potential energy barrier of the exit channel in four center reactions should be released into translation.
Date: April 1, 1978
Creator: Sudbo, Aa. S.; Schulz, P.A.; Shen, Y.R. & Lee, Y.T.
Partner: UNT Libraries Government Documents Department

Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

Description: In this work we report on thevacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuumultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH + (n=1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H + (n=2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH +, (CH 3OH)2 +, (CH3OH)nH + (n=1-9), and (CH 3OH)n(H2O)H + (n=2-9 ) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.
Date: May 12, 2008
Creator: Ahmed, Musahid; Ahmed, Musahid; Wilson, Kevin R.; Belau, Leonid & Kostko, Oleg
Partner: UNT Libraries Government Documents Department

Quantum Monte Carlo study of the reaction: C1 + CH3OH -->CH2OH+ HCl

Description: A theoretical study is reported of the Cl + CH{sub 3}OH {yields} CH{sub 2}OH + HCl reaction based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using a DMC trial function constructed as a product of Hartree-Fock and correlation functions, we have computed the barrier height, heat of reaction, atomization energies and heats of formation of reagents and products. The DMC heat of reaction, atomization energies, and heats of formation are found to agree with experiment to within the error bounds of computation and experiment. Moller-Plesset second order perturbation theory (MP2) and density functional theory, the latter in the B3LYP generalized gradient approximation, are found to overestimate the experimental heat of reaction. Intrinsic reaction coordinate calculations at the MP2 level of theory demonstrate that the reaction is predominantly direct, i.e., proceeds without formation of intermediates, which is consistent with a recent molecular beam experiment. The reaction barrier as determined from MP2 calculations is found to be 2.24 kcal/mol and by DMC it is computed to be 2.39(49) kcal/mol.
Date: December 1, 2003
Creator: Kollias, A.C.; Couronne, O. & Lester Jr., W.A.
Partner: UNT Libraries Government Documents Department

Using Allele-Specific PCR with Molecular Beams as a Means for Genotyping the Diallelic Indels

Description: The first Specific Aim for this grant was to identify and characterize an average of 500 human insertion/deletion polymorphisms per grant year (1500 total). This task was carried out entirely at MMRF. They substantially exceeded this goal by confirming about 2,300 diallelic indels. Complete characterization information for these polymorphisms is available from the Marshfield web site. A manuscript describing results for the first 2,000 diallelic indels was published earlier this year in the American Journal of Human Genetics. The Second Specific Aim of the grant was to investigate and develop improved methods for analysis of diallelic polymorphisms using miniaturized DNA arrays. The initial genotyping technology efforts focused on various hybridization and extension protocols with oligo arrays on flow-through channel glass. Channel glass is a porous material that permits reagents to be passed through the arrays. They devoted roughly 19 months at the beginning of the grant in pursuit of this methodology, but for various technological reasons, progress was limited.
Date: June 1, 2000
Creator: Doktycz, M.J. & Weber, J.L. (Marshfield Medical Research Foundation)
Partner: UNT Libraries Government Documents Department

Neutron sputtering of gold

Description: From surface effects in controlled thermonuclear fusion devices and reactors meeting; Argonne, Illinois, USA (10 Jan 1974). The sputtering yield of gold bombarded with fission neutrons was studied in the CP-5 reactor. Bulk single crystals were used at near liquid helium temperature. Overall sputtering yields ranged from 1 x 10/sup -3/ to 6 x 10/sup -3/ sputtered gold atoms per incident neutron for doses ranging from 2.1 x 10/sup 17/ to 5.5 x 10/sup 17/ (nvt > 0.1 MeV). Results are roughly an order of magnitude larger than many previoualy reported sputtering yields. Sputtering patterns have not been pursued yet, due to sufficiently high background radiation. (auth)
Date: February 1, 1974
Creator: Kirk, M.A.; Blewitt, T.H.; Klank, A.C.; Malewicki, R. & Scott, T.L.
Partner: UNT Libraries Government Documents Department

Intermolecular potential functions and high resolution molecular spectroscopy of weakly bound complexes. Final progress report

Description: This report describes accomplishments over the past year in research supported by this grant. Two papers published in this period are briefly discussed. The general goal of the work is to consolidate the understanding of experimental results through a theoretical model of intermolecular potential energy surfaces. Progress in the experimental and theoretical phases of the program are presented and immediate goals outlined. The ability to construct analytic intermolecular potential functions that accurately predict the energy of interaction between small molecules will have great impact in many areas of chemistry, biochemistry, and biology.
Date: April 1, 1997
Creator: Muenter, J.S.
Partner: UNT Libraries Government Documents Department

Understanding gas-surface interactions from direct force measurements using a specialized torsion balance

Description: The first comprehensive measurements of the magnitude and direction of the forces exerted on surfaces by molecular beams are discussed and used to obtain information about the microscopic properties of the gas-surface interactions. This unique approach is not based on microscopic measurements of the scattered molecules. The reduced force coefficients are introduced as a new set of parameters that completely describe the macroscopic average momentum transfer to a surface by an incident molecular beam. By using a specialized torsion balance and molecular beams of N{sub 2}, CO, CO{sub 2}, and H{sub 2}, the reduced force coefficients are determined from direct measurements of the force components exerted on surface of a solar panel array material, Kapton, SiO{sub 2}-coated Kapton, and Z-93 as a function of the angle of incidence ranging from 0{degrees} to 85{degrees}. The absolute flux densities of the molecular beams were measured using a different torsion balance with a beam-stop that nullified the force of the scattered molecules. Standard time-of-flight techniques were used to determine the flux-weighted average velocities of the various molecular beams ranging from 1600 m/s to 4600 m/s. The reduced force coefficients can be used to directly obtain macroscopic average properties of the scattered molecules, such as the flux-weighted average velocity and translational energy, that can then be used to determine microscopic details concerning gas-surface interactions without the complications associated with averaging microscopic measurements.
Date: December 31, 1996
Creator: Cook, S.R. & Hoffbauer, M.A.
Partner: UNT Libraries Government Documents Department

Rotating Aperture System

Description: A rotating aperture system includes a low-pressure vacuum pumping stage with apertures for passage of a deuterium beam. A stator assembly includes holes for passage of the beam. The rotor assembly includes a shaft connected to a deuterium gas cell or a crossflow venturi that has a single aperture on each side that together align with holes every rotation. The rotating apertures are synchronized with the firing of the deuterium beam such that the beam fires through a clear aperture and passes into the Xe gas beam stop. Portions of the rotor are lapped into the stator to improve the sealing surfaces, to prevent rapid escape of the deuterium gas from the gas cell.
Date: January 18, 2005
Creator: Rusnak, Brian; Hall, James M.; Shen, Stewart & Wood, Richard L.
Partner: UNT Libraries Government Documents Department

Formation of atomic clusters through the laser ablation of refractory materials in a supersonic molecular beam source

Description: Concepts which guide the design of atomic cluster supersonic beam sources have been developed. These ideas are founded on the knowledge of laser ablation dynamics and are structured in order to take advantage of certain features of the ablation event. Some of the drawbacks of previous cluster source designs become apparent when the sequence of events following laser ablation are clarified. Key features of the new cluster source design include control of the cluster size distribution, uniform performance with a variety of solid materials and elements, high beam intensity, and significant removal of internal energy during the supersonic expansion.
Date: December 31, 1993
Creator: Haufler, R.E.; Compton, R.N. & Puretzky, A.A.
Partner: UNT Libraries Government Documents Department

Oxide degradation effects in dry patterning of resist using neutral oxygen beams

Description: Novel processing methods are being studied to address the highly selective and directional etch requirements of the ULSI manufacturing era; neutral molecular and atomic beams are two promising candidates. In this study, the potential of 5 eV neutral atomic oxygen beams for dry development of photoresist is demonstrated for application in patterning of CMOS devices. The patterning of photoresist directly on polysilicon gate layers enables the use of a self-contained dry processing strategy, with oxygen beams for resist etching and chlorine beams for polysilicon etching. Exposure to such reactive low-energy species and to the UV radiation from the line-of-sight, high-density plasma source can, however, after MOSFET gate oxide quality, impacting device performance and reliability. We have studied this processing related device integrity issue by subjecting polysilicon gas MOS structures to exposure treatments similar to those used in resist patterning using low energy oxygen beams. Electrical C-V characterization shows a significant increase in the oxide trapped charge and interface state density upon low energy exposure. I-V and dielectric breakdown characterization show increased low-field leakage characteristics for the same exposure. High-field electron injection studies reveal that the 0.25-V to 0.5-V negative flatband shifts can be partially annealed by the carrier injection. This could be due to positive charge annihilation or electron trapping, or some combination of both. Physical and analysis of patterned resist layers and electrical characterization data of MOS structures exposed to different neutral beam processing environments and following thermal annealing treatments is presented.
Date: July 1, 1992
Creator: Mlynko, W. E.; Kasi, S. R. & Manos, D.
Partner: UNT Libraries Government Documents Department

Neutral-beam research and development at LBL, Berkeley

Description: The neutral-beam research and development effort at LLL has been concerned mainly with design, construction, and testing of ion sources needed in present and planned experiments, chiefly at LLL. Development of techniques for producing reactor-type neutral beam systems with higher voltage, current, and duty cycle proceeds as resources permit. An important part of this program is the attempt to increase the physics understanding and technology needed for the design of beams with precisely controlled trajectories, e.g., converging beams. (auth)
Date: November 1, 1973
Creator: Baker, W.R.; Berkner, K.H.; Cooper, W.S.; Ehlers, K.W.; Kunkel, W.B.; Pyle, R.V. et al.
Partner: UNT Libraries Government Documents Department

MULTIPHOTON DISSOCIATION PRODUCTS FROM HALOGENATED HYDROCARBONS

Description: The recent interest in multiphoton dissociation (MPD) of polyatomic molecules has revealed quite a bit of confusion as to what the major dissociation channels of some molecules are, and whether the dissociation process can be described by a statistical, e.g., RRKM, theory of unimolecular reactions. In a gas cell experiment an experimenter often finds great difficulties in answering questions such as: do vibrationally excited molecules react with one another, how do reactions involving free radicals produced in the dissociation proceed, can these radicals decompose further in the presence of the laser field, and does the detection method used actually probe the primary dissociation channel. Using the molecular beam setup previously described, we have overcome most of these difficulties. We have studied MPD of a number of compounds, mainly halogenated methanes, ethanes and ethylenes, and identified their major dissociation channels. To identify an MPD product, we realize that a unique set of ratios between mass spectrometer signals of various ion fragments must first be established for a given molecular species or radical, and preferably the velocity distributions of the dissociation products should be measured and checked for consistency. In our setup, we could measure both angular and time-of-flight distributions for the fragments, and pereorm this check. The low density of molecules in the beam and the extremely low duty cycle in our experiment prevented us from observing the competing minor channels which have very small branching ratios (< 0.1).
Date: November 1, 1977
Creator: Sudbo, Aa. S.; Schulz, P.A.; Grant, E.R & Lee, Y.T.
Partner: UNT Libraries Government Documents Department

Reconstruction of the Electron Density of Molecules with Single-Axis Alignment

Description: Diffraction from the individual molecules of a molecular beam, aligned parallel to a single axis by a strong electric field or other means, has been proposed as a means of structure determination of individual molecules. As in fiber diffraction, all the information extractable is contained in a diffraction pattern from incidence of the diffracting beam normal to the molecular alignment axis. We present two methods of structure solution for this case. One is based on the iterative projection algorithms for phase retrieval applied to the coefficients of the cylindrical harmonic expansion of the molecular electron density. Another is the holographic approach utilizing presence of the strongly scattering reference atom for a specific molecule.
Date: August 12, 2011
Creator: Starodub, Dmitri
Partner: UNT Libraries Government Documents Department

Measurement of neutral beam profiles at DIII-D. Revision 1

Description: The neutral beam systems of DIII-D, a National Fusion Facility at General Atomics, are used both for heating the plasma, and as tools for plasma diagnostics. The spatial distribution (profile) and energy of the beam is used in the absolute calibration of both the Charge Exchange Recombination (CER) and Motional Stark Effect (MSE) diagnostics. In the past, the beam spatial profile used in these calibrations was derived from beam divergence calculations and IR camera observations on the tokamak centerpost target tiles. Two experimental methods are now available to better determine the beam profile. In one method, the Doppler shifted D{sub {alpha}} light from the energetic neutrals are measured, and the full-width at half-maximum (FWHM) of the beam can be inferred from the measured divergence of the D{sub {alpha}} light intensity. The other method for determining the beam profile uses the temperature gradients measured by the thermocouples mounted on the calorimeter. A new iterative fitting routine for the measured thermocouple data has been developed to fit theoretical models on the dispersion of the beam. The results of both methods are compared, and used to provide a new experimental verification of the beam profile.
Date: August 1, 1998
Creator: Chiu, H.
Partner: UNT Libraries Government Documents Department