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Observation of the dynamics leading to a conical intersection in dissociative electron attachment to water

Description: Following prior work on the lower-energy resonances, we apply techniques of momentum imaging and ab initio scattering calculations to the process of dissociative electron attachment to water via the highest-energy {sup 2}B{sub 2} resonance. We focus on the H{sup -} anion fragment, which is produced via dynamics passing through and avoiding the conical intersection with the lower A{sub 1} state, leading to OH ({sup 2}{Pi}#5;) and OH ({sup 2}{Sigma}#6;), respectively. The momentum imaging technique, when combined with theoretical calculations on the attachment amplitude and dissociation dynamics, demonstrates that the angular distributions provide a signature of the location of the conical intersection in the space of nuclear configurations.
Date: August 11, 2011
Creator: Haxton, D.; Adaniya, H.; Slaughter, D. S.; Rudek, B.; Osipov, T.; Weber, T. et al.
Partner: UNT Libraries Government Documents Department

Three-body breakup in dissociative electron attachment to the water molecule

Description: We report the results of {\em ab initio} calculations on dissociative electron attachment (DEA) to water that demonstrate the importance of including three-body breakup in the dissociation dynamics. While three-body breakup is ubiquitous in the analogous process of dissociative recombination, its importance in low-energy dissociative electron attachment to a polyatomic target has not previously been quantified. Our calculations, along with our earlier studies of DEA into two-body channels, indicate that three-body breakup is a major component of the observed O- cross section. The local complex potential model provides a generally accurate picture of the experimentallyobserved features in this system, reproducing some quantitatively, others qualitatively, and one not at all.
Date: August 28, 2008
Creator: Haxton, Daniel J.; Rescigno, Thomas N. & McCurdy, C. William
Partner: UNT Libraries Government Documents Department

Enhanced Electron Attachment to Highly-Excited Molecules and Its Applications in Pulsed Plasmas

Description: Studies conducted over the past several years have shown that electron attachment to highly-excited states of molecules have extremely large cross sections. We will discuss the implications of this for pulsed discharges used for H<sup>-</sup> generation, material processing, and plasma remediation.
Date: June 27, 1999
Creator: Ding, W. X.; Ma, C. Y.; McCorkle, D. L. & Pinnaduwage, L. A.
Partner: UNT Libraries Government Documents Department

Angular dependence of dissociative electron attachment topolyatomic molecules: application to the 2B1 metastable state of the H2Oand H2S anions

Description: The angular dependence of dissociative electron attachment (DEA) to polyatomic targets is formulated in the local complex potential model, under the assumption that the axial recoil approximation describes the dissociation dynamics. An additional approximation, which is found to be valid in the case of H2O but not in the case of H2S, makes it possible to describe the angular dependence of DEA solely from an analysis of the fixed-nuclei entrance amplitude, without carrying out nuclear dynamics calculations. For H2S, the final-vibrational-state-specific angular dependence of DEA is obtained by incorporating the variation of the angular dependence of the entrance amplitude with nuclear geometry into the nuclear dynamics. Scattering calculations using the complex Kohn method and, for H2S, full quantum calculations of the nuclear dynamics using the Multi-Configuration Time-Dependent Hartree method, are performed.
Date: January 12, 2006
Creator: Haxton, Daniel J.; McCurdy, C. William & Rescigno, Thomas N.
Partner: UNT Libraries Government Documents Department

Dissociative electron attachment to the H2O molecule II: nucleardynamics on coupled electronic surfaces within the local complexpotential model

Description: We report the results of a first-principles study of dissociative electron attachment (DEA) to H{sub 2}O. The cross sections were obtained from nuclear dynamics calculations carried out in full dimensionality within the local complex potential model by using the multi-configuration time-dependent Hartree method. The calculations employ our previously obtained global, complex-valued, potential energy surfaces for the three ({sup 2}B{sub 1}, {sup 2}A{sub 1}, and {sup 2}B{sub 2}) electronic Feshbach resonances involved in this process. These three metastable states of H{sub 2}O{sup -} undergo several degeneracies, and we incorporate both the Renner-Teller coupling between the {sup 2}B{sub 1} and {sup 2}A{sub 1} states, as well as the conical intersection between the {sup 2}A{sub 1} and {sup 2}B{sub 2} states, into our treatment. The nuclear dynamics are inherently multi-dimensional and involve branching between different final product arrangements as well as extensive excitation of the diatomic fragment. Our results successfully mirror the qualitative features of the major fragment channels observed, but are less successful in reproducing the available results for some of the minor channels. We comment on the applicability of the local complex potential model to such a complicated resonant system.
Date: December 21, 2006
Creator: Haxton, Daniel J.; Rescigno, Thomas N. & McCurdy, C. William
Partner: UNT Libraries Government Documents Department

Imaging the molecular dynamics of dissociative electron attachment to water

Description: Momentum imaging experiments on dissociative electron attachment to the water molecule are combined with ab initio theoretical calculations of the angular dependence of the quantum mechanical amplitude for electron attachment to provide a detailed picture of the molecular dynamics of dissociation attachment via the two lowest energy Feshbach resonances. The combination of momentum imaging experiments and theory can reveal dissociation dynamics for which the axial recoil approximation breaks down and thus provides a powerful reaction microscope for DEA to polyatomics.
Date: October 19, 2009
Creator: Adaniya, Hidihito; Rudek, B.; Osipov, Timur; Haxton, Dan; Weber, Thorsten; Rescigno, Thomas N. et al.
Partner: UNT Libraries Government Documents Department

Magnetization reversal behaviour of nanogranular CoCrPt alloy thinfilms studied with magnetic transmission X-ray microscopy

Description: The angular dependence of dissociative electron attachment (DEA) to polyatomic targets is formulated in the local complex potential model, under the assumption that the axial recoil approximation describes the dissociation dynamics. An additional approximation, which is found to be valid in the case of H{sub 2}O but not in the case of H{sub 2}S, makes it possible to describe the angular dependence of DEA solely from an analysis of the fixed-nuclei entrance amplitude, without carrying out nuclear dynamics calculations. For H{sub 2}S, the final-vibrational-state-specific angular dependence of DEA is obtained by incorporating the variation of the angular dependence of the entrance amplitude with nuclear geometry into the nuclear dynamics. Scattering calculations using the complex Kohn method and, for H{sub 2}S, full quantum calculations of the nuclear dynamics using the Multi-Configuration Time-Dependent Hartree method, are performed.
Date: November 4, 2004
Creator: Fischer, P.; Im, M.-Y.; Eimuller, T.; Schutz, G. & Shin, S.-C.
Partner: UNT Libraries Government Documents Department

Enhanced Electron Attachment to Highly-Excited States of Molecules: Implications for Plasma Processing Discharges

Description: Recent studies show that large negative ion densities exist in plasma processing discharges, including those of weakly electronegative gases such as SiH{sub 4} and CF{sub 4}. Also, there is strong evidence that the negative ions could be the precursors for particulate formation in processing discharges. Even though it is now well established that large concentrations of negative ions exist in processing discharges, and that they play a crucial role in such discharges, the source of such high negative ion densities has not been clarified. In particular, gases like SiH{sub 4} and CH{sub 4}, which are commonly used in processing discharges, attach electrons only weakly in their ground electronic states (see the references). Due to the lack of an alternative mechanism, the origin of large negative ion densities in such weakly electronegative gases has been frequently attributed to electron attachment to radicals (molecular fragments) or other byproducts produced in the discharge. This hypothesis had not been tested in direct electron attachment measurements.
Date: June 29, 1998
Creator: Datskos, P.G.; Ding, W.; McCorkle, D.L. & Pinnaduwage, L.A.
Partner: UNT Libraries Government Documents Department

Elastic electron scattering from formic acid

Description: Following our earlier study on the dynamics of low energy electron attachment to formic acid, we report the results of elastic low-energy electron collisions with formic acid. Momentum transfer and angular differential cross sections were obtained by performing fixed-nuclei calculations employing the complex Kohn variational method. We make a brief description of the technique used to account for the polar nature of this polyatomic target and compare our results with available experimental data.
Date: July 31, 2006
Creator: Trevisan, Cynthia S.; Orel, Ann E. & Rescigno, Thomas N.
Partner: UNT Libraries Government Documents Department

First tests of a Micromegas TPC in a magnetic field

Description: Since the summer of 2003, a large Micromegas TPC prototype (1000 channels, 50 cm drift, 50 cm diameter) has been operated in a 2T superconducting magnet at Saclay. A description of this apparatus and first results from cosmic ray tests are presented. Additional measurements using simpler detectors with a laser source, an X-ray gun and radio-active sources are discussed. Drift velocity and gain measurements, electron attachment and aging studies for a Micromegas TPC are presented. In particular, using simulations and measurements, it is shown that an $Argon-CF_4$ mixture is optimal for operation at a future Linear Collider.
Date: December 10, 2004
Creator: Colas, P.; Giomataris, I.; Lepeltier, V. & Ronan, M.
Partner: UNT Libraries Government Documents Department

Behavior of excess electrons in supercritical fluids -- Electron attachment

Description: The behavior of excess electrons in supercritical ethane was investigated by measuring mobility and reaction rates. Mobilities wee measured by means of a time-of-flight method at 306--320 K as a function of pressure. Mobility values decreased at all temperatures with increasing pressure, but showed a small minimum or a shoulder at the pressure where the compressibility {chi}{sub T} has a peak. Electron attachment to CO{sub 2}, NO, pyrimidine and C{sub 2}F{sub 4} over the same temperature range was studied as a function of pressure. Both attachment rate constants k{sub a} for NO and C{sub 2}F{sub 4}, and equilibrium constants K({double_bond}k{sub a}/k{sub d}) for CO{sub 2} and pyrimidine increased sharply at pressures of {chi}{sub T} peaks. Activation volumes V{sub a}* and reaction volumes {Delta}V{sub r} are very large and negative in the critical region. The volume change is mainly due to electrostriction around ions formed. The results are compared to volume changes predicted by a compressible continuum model.
Date: July 1, 1999
Creator: Nishikawa, Masaru; Holroyd, R.A. & Itoh, Kengo
Partner: UNT Libraries Government Documents Department

Implications of electron attachment to highly-excited states in pulsed-power discharges

Description: The author points out the possible implications of electron attachment to highly-excited states of molecules in two pulsed power technologies. One involves the pulsed H{sub 2} discharges used for the generation of H ion beams for magnetic fusion energy and particle accelerators. The other is the power modulated plasma discharges used for material processing.
Date: August 1, 1997
Creator: Pinnaduwage, L.A.
Partner: UNT Libraries Government Documents Department

Linking the gaseous and the condensed phases of matter: The slow electron and its interactions

Description: The interfacing of the gaseous and the condensed phases of matter as effected by interphase and cluster studies on the behavior of key reactions involving slow electrons either as reacting initial particles or as products of the reactions themselves is discussed. Emphasis is placed on the measurement of both the cross sections and the energetics involved, although most of the available information to date is on the latter. The discussion is selectively focussed on electron scattering (especially the role of negative ion states in gases, clusters, and dense matter), ionization, electron attachment and photodetachment. The dominant role of the electric polarization of the medium is emphasized.
Date: December 31, 1993
Creator: Christophorou, L. G.
Partner: UNT Libraries Government Documents Department

A Nonlocal Model of Dissociative Electron Attachment andVibrational Excitation of NO

Description: We present the results of a study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy (0-2eV) region where the cross sections are dominated by resonance contributions. The {sup 3}{Sigma}{sup -}, {sup 1}{Delta} and {sup 1}{Sigma}{sup +} NO{sup -} resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711(2004)], but the resonance energies used here are obtained from new configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground state products, O{sup -}({sup 2}P) + N({sup 4}S). The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small ({approx} 10{sup -20}cm{sup 2}), the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to {nu} = 10 and above.
Date: January 1, 2005
Creator: Trevisan, Cynthia S.; Houfek, Karel; Zhang, Z.; Orel, Ann E.; McCurdy, C. William & Rescigno, Thomas N.
Partner: UNT Libraries Government Documents Department

A Novel Energy Efficient Plasma Chemical Process for the Destruction of Volatile Toxic Compounds

Description: The objective is to develop a novel plasma chemical process for the destruction of low concentrations (below several percent) of toxic volatile compounds from contaminated air streams. Such contaminated air streams are encountered in air stripping of highly-contaminated water and soil, and also in the incineration of combustible hazardous wastes. Our technique is based on the efficient dissociation of molecules via enhanced electron attachment to highly-excited states of the molecules produced in a glow discharge.
Date: June 1, 2000
Creator: Pinnaduwage, Lal A.
Partner: UNT Libraries Government Documents Department

Novel Analytical techniques based on an enhanced electron attachment process

Description: The objective of this research program is to develop new analytical techniques based on the large cross sections associated with dissociative electron attachment to highly-excited molecular states. Such highly-excited states can be populated by using fixed frequency lasers or in glow discharges via excitation transfer from high-lying, metastable states of rare gases. It is anticipated that these studies will lead to negative-ion based analytical techniques for the detection of DNAPLs (dense non-aqueous phase liquids), VOCs (volatile organics) and other toxic compounds at DOE waste sites. We have made significant advances toward this goal as described below.
Date: June 1, 2000
Creator: Pinnaduwage, Lal A.; Buchanan, Michelle V. & B., Hurst Gregory
Partner: UNT Libraries Government Documents Department

A nonlocal, ab initio model of dissociative electron attachment and vibrational excitation of NO

Description: We present the results of an ab initio study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy (0-2 eV) region where the cross sections are dominated by resonance contributions. The 3Sigma-, 1Delta and 1Sigma+ NO- resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from new configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to groundstate products. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small, the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to v = 10 and above.
Date: February 1, 2005
Creator: Trevisan, Cynthia S.; Houfek, Karel; Zhang, Zhiyong; Orel, Ann E.; McCurdy, C. William & Rescigno, Thomas N.
Partner: UNT Libraries Government Documents Department

Topology of the Adiabatic Potential Energy Surfaces for theResonance States of the Water Anion

Description: The potential energy surfaces corresponding to the long-lived fixed-nuclei electron scattering resonances of H{sub 2}O relevant to the dissociative electron attachment process are examined using a combination of ab initio scattering and bound-state calculations. These surfaces have a rich topology, characterized by three main features: a conical intersection between the {sup 2}A{sub 1} and {sup 2}B{sub 2} Feshbach resonance states; charge-transfer behavior in the OH ({sup 2}{Pi}) + H{sup -} asymptote of the {sup 2}B{sub 1} and {sup 2}A{sub 1} resonances; and an inherent double-valuedness of the surface for the {sup 2}B{sub 2} state the C{sub 2v} geometry, arising from a branch-point degeneracy with a {sup 2}B{sub 2} shape resonance. In total, eight individual seams of degeneracy among these resonances are located.
Date: April 15, 2005
Creator: Haxton, Daniel J.; Rescigno, Thomas N. & McCurdy, C. William
Partner: UNT Libraries Government Documents Department

Interaction of Slow Electrons With High-Pressure Gases (`Quasi-Liquids`); Synthesis of Our Knowledge on Slow Electron-Molecule Interactions; Final Progress Report, March 15, 1986--August 14, 1987

Description: The rate constant for electron attachment to F{sub 2} was measured at 233 to 373 K in nitrogen at 0.04--0.75 eV and in argon at 298 K at 0.4--2.95 eV. The unfolded cross section was compared with calculations. The ionization threshold of N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine (TMPD) was measured in ethane over the density range 0.15--13.3 M/l at 295--413 K using multiphoton ionization conductivity.
Date: 1987-23~
Creator: McCorkle, D. L. & Christophorou, L. G.
Partner: UNT Libraries Government Documents Department

The role of high Rydberg states in the generation of negative ions in negative-ion discharges

Description: The generation of substantial yields of H{sup {minus}} ions in a laser excited H{sub 2} gas has been reported by Pinnaduwage and Christoforu. These H{sup {minus}} yields have been attributed to (2 + 1) REMP photoexcitation processes leading to dissociative attachment of doubly-excited or superexcited states (SES), or dissociative attachment of high Rydberg product states. The new feature of these experiments is the implied large dissociative attachment rates, of order 10{sup {minus}6} cm{sup 3} sec{sup {minus}1}, values that are orders-of-magnitude larger than the dissociative attachment of the vibrationally excited levels of the ground electronic state. While these laser excitations are not directly applicable to a hydrogen negative-ion discharge, the implication of large dissociative attachment rates to the high Rydberg states may affect both the total negative-ion density and the interpretation of discharge performance. Within the discharge energetic electrons will collisionally excite the higher Rydberg states, and the relative contribution of the dissociative attachment of these states when compared with the dissociative attachment to the ground state vibrational levels, is the topic of this paper.
Date: November 28, 1995
Creator: Hiskes, J.R.
Partner: UNT Libraries Government Documents Department

A NOVEL ENERGY EFFICIENT PLASMA CHEMICAL PROCESS FOR THE DESTRUCTION OF VOLATILE TOXIC COMPOUNDS

Description: Destruction of low-concentrations (below several percent) of toxic volatile compounds from contaminated air stream is encountered at the DOE waste sites in the following instances: (i) off-gases resulting from air-stripping of highly-contaminated water and soil (contaminated with DNAPLs, VOCs, etc), (ii) effluent from the incineration of highly-concentrated combustible hazardous wastes. If the contaminated air stream has concentrations of a few parts per million (ppm) then passive methods, such as activated carbon, can be used successfully. If the concentration level is more than several percent, then thermal incineration can be successfully used. However, in the intermediate range neither of the above processes are suitable [1]. During the past decade or so, non-thermal plasma techniques have been investigated (see the references in [1, 2]) for the remediation of waste streams with concentrations in this intermediate range (to our knowledge, our project was the only one that addressed this problem under the EMSP sponsorship). Our approach is a completely different from those previous non-thermal plasma techniques. The main objective was to investigate the feasibility of a novel plasma chemical process for the remediation of toxic gas mixtures based on an enhanced dissociative electron attachment process that had been discovered in a basic research program: Prior to this project, it had been found that molecules excited to highly-excited states via laser irradiation efficiently attached electrons leading to the dissociation of the molecule (see, [3-6] and references therein). The following tasks were needed to be completed during the three year duration of the project in order to illustrate the feasibility of the new technology: 1. Since the usage of lasers is not cost effective, two discharge based schemes were planned to be investigated for molecular excitation. 2. The basic mechanisms involving dissociative electron attachment to highly-excite d molecules needed to be studied, in order to ...
Date: December 31, 2000
Creator: Pinnaduwage, Lal A.
Partner: UNT Libraries Government Documents Department

Novel Analytical Techniques Based on an Enhanced Electron Attachment Process

Description: Sensitive and selective detection of dense non-aqueous liquids (DNAPL), volatile organic compounds (VOC), and other organics, such as polychlorinated biphenyls (PCB), is an area of importance to the DOE's environmental restoration program. Current negative-ion based analytical methods for the detection of these compounds rely on their propensity to form negative ions (in their ground electronic states). However, for compounds with smaller electron capture cross sections, including PCBs with few chlorine atoms and non-chlorinated VOCs, these methods are considerably less sensitive. The objective of our research program is to develop novel analytical techniques that can be expected to be applicable for a wide variety of molecules with high sensitivity. These are based on the recently discovered enhanced negative ion formation process involving electron attachment to highly-Rydberg states of molecules. We are using two approaches to produce the precursor highly-excited states; one is laser excitation, and the other is discharge based.
Date: June 1, 1999
Creator: Pinnaduwage, Lal A. & Buchanan, Michelle V.
Partner: UNT Libraries Government Documents Department

MEASUREMENT OF ELECTRON ATTACHMENT IN OXYGEN-METHANE AND OXYGEN-CARBON DIOXIDE MIXTURES

Description: The formation of heavy negative ions by the attachment of low-energy electrons to oxygen molecules was studied for small amounts of oxygen mixed with methane or carbon dioxide. The rate of attachment in both cases was found to depend on the electron energy, the pressure of the oxygen and the non-attaching gas, and on the kind of non-attaching gas. In general, the attachment increases as electron enprgy decreases or as either oxygen or total pressure increases. The value of the attachment coefficient in oxygencarbon dioxide mixtures is about 100 times its value in oxygen-methane mixtures. This large difference is probably due in part to differences in electron energy and partly to differences in the stabilizing qualities of the two molecules. Dissociative attachment, which should be pressure independent, does not occur at the low energies that were used in this work. Both methane and carbon dioxide are to differences in the stabilizing qualities of the two molecules. Dissociative attachment, which should be pressure independent, does not occur at the low energies that were used in this work. Both methane and carbon dioxide are sometimes used as filling gases for Geiger and proportional counters. The high sensitivity of carbon dioxide to oxygen contamination indicates that very pure gas should be used if the best operation is to be obtained. The low sensitivity of methane recommends it for counters where careful purification of the gas is difficult and particularly for flow counters where the possibility of contamination by atmospheric oxygen exists. (auth)
Date: January 25, 1960
Creator: O'Kelly, L. B.; Hurst, G. S. & Bortner, T. E.
Partner: UNT Libraries Government Documents Department