Bulletin of the American Physical Society, May 18, 2005
We present the results of an {\ em ab initio} study of elastic scattering and vibrational excitat... more We present the results of an {\ em 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. We employ a nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model. We also present cross sections for dissociative electron attachment to NO leading to ground state products, ${\ rm O}^-(^ 2 {\ rm P})+{\ rm N}(^ 4 {\ rm S}) $. The ...
Glutarimidedioxime (H 2 A), a cyclic imide dioxime ligand that has implications in sequestering u... more Glutarimidedioxime (H 2 A), a cyclic imide dioxime ligand that has implications in sequestering uranium from seawater, forms strong tridentate complexes with UO 2 . The stability constants and the enthalpies of complexation for five U(VI) complexes were measured by potentiometry and microcalorimetry. The crystal structure of the 1:2 metal/ligand complex, UO 2 (HA) 2 H 2 O, was determined. The re-arrangement of the protons of the oxime groups (-CH=N-OH) and the deprotonation of the imide group (-CH-NH-CH-) results in a conjugated system with delocalized electron density on the ligand (-O-N-C-N-C-N-O-) that coordinates to UO 2 2+ via its equatorial plane.
Threshold structures, reminiscent of those seen in the polar hydrogen halides, have recently been... more Threshold structures, reminiscent of those seen in the polar hydrogen halides, have recently been observed in the cross sections for electron impact excitation of certain vibrational levels of the nonpolar CO 2 molecule. These structures occur at energies outside the range where shape resonances dominate the dynamics. We propose a virtual state model that describes the multidimensional nuclear dynamics during the collision and explains quantitatively the selectivity observed in the excitation of the Fermi dyad, as well as the pattern of threshold peaks and oscillations seen in the upper levels of the higher polyads.
A number of physical processes involving quantum dots depend critically upon the "evanescent" ele... more A number of physical processes involving quantum dots depend critically upon the "evanescent" electron eigenstate wave function that extends outside of the material surface into the surrounding region. These processes include electron tunneling through quantum dots, as well as interactions between multiple quantum dot structures. In order to unambiguously determine these evanescent fields, appropriate boundary conditions have been developed to connect the electronic solutions interior to the semiconductor quantum dot to exterior vacuum solutions. In standard envelope function theory, the interior wave function consists of products of band edge and envelope functions, and both must be considered when matching to the external solution. While the envelope functions satisfy tractable equations, the band edge functions are generally not known. In this work, symmetry arguments in the spherically symmetric approximation are used in conjunction with the known qualitative behavior of bonding and antibonding orbitals to catalog the behavior of the band edge functions at the unit cell boundary. This physical approximation allows consolidation of the influence of the band edge functions to two simple surface parameters that are incorporated into the boundary conditions and are straightforwardly computed by using numerical first-principles quantum techniques. These new boundary conditions are employed to analyze an isolated spherically symmetric semiconductor quantum dot in vacuum within the analytical model of Sercel and Vahala ͓Phys. Rev. Lett. 65, 239 ͑1990͒; Phys. Rev. B 42, 3690 ͑1990͔͒. Results are obtained for quantum dots made of GaAs and InP, which are compared with ab initio calculations that have appeared in the literature.
The potential-energy surface corresponding to the complex resonance energy of the 2 B 1 Feshbach ... more The potential-energy surface corresponding to the complex resonance energy of the 2 B 1 Feshbach resonance state of the water anion is constructed in its full dimensionality. Complex Kohn variational scattering calculations are used to compute the resonance width, while large-scale configuration interaction calculations are used to compute the resonance energy. Near the equilibrium geometry, an accompanying ground-state potential surface is constructed from configuration interaction calculations that treat correlation at a level similar to that used in the calculations on the anion.
Development of several new computational approaches within the framework of multi-reference ab in... more Development of several new computational approaches within the framework of multi-reference ab initio molecular electronic structure methodology and their implementation in the COLUMBUS program system are reported. These new features are : calculation of the analytical MR-CI gradient for excited states based on state-averaged MCSCF orbitals, the extension of the MR-ACPF/AQCC methods to excited states in the framework of linear-response theory, spinÈorbit CI for molecules containing heavy atoms and the development of a massively-parallel code for the computation of the one-and two-particle density matrix elements. Illustrative examples are given for each of these cases.
The ground and lowest three adiabatic excited states of methylene are computed using the variatio... more The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo (DMC) methods using progressively larger Jastrow–Slater multideterminant complete active space (CAS) wave functions. The highest of these states has the same symmetry, A11, as the first excited state. The DMC excitation energies obtained using any of the CAS wave functions are in excellent agreement with experiment, but single-determinant wave functions do not yield accurate DMC energies of the states of A11 symmetry, indicating that it is important to include in the wave function Slater determinants that describe static (strong) correlation. Excitation energies obtained using recently proposed pseudopotentials [Burkatzki et al., J. Chem. Phys. 126, 234105 (2007)] differ from the all-electron excitation energies by at most 0.04 eV.
We present an ab initio study of elastic scattering and vibrational excitation of NO by low-energ... more We present an ab initio study of elastic scattering and vibrational excitation of NO by low-energy ͑0-2.0 eV͒ electron impact. The low-energy scattering cross sections are dominated by shape resonance contributions associated with the 3 ⌺ − , 1 ⌬, and 1 ⌺ + states of NO −. Resonance parameters for the three anion states were extracted from an analysis of fixed-nuclei variational (complex Kohn) calculations that employed elaborate trial wave functions. Independent estimates of the resonance parameters were obtained by analytically continuing the results of large-scale coupled-cluster calculations into the plane of complex momentum. The local complex potential model was used to calculate vibrational excitation cross sections, as well as the resonant portion of the vibrationally elastic cross sections. These results were combined with background contributions from the fixed-nuclei calculations to compute elastic and grand total cross sections. Our results capture the essential features of recent measurements of the cross sections, but suggest the need at lower energies for a more sophisticated, nonlocal treatment of nuclear dynamics.
We present the results of a study of elastic scattering and vibrational excitation of NO by elect... more We present the results of a 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 ^3\Sigma^-, ^1\Delta and ^1\Sigma^+ 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 ground state products, \rm O^-(^2\rm P) + \rm N(^4\rm S). The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small (\sim 10^-20 \rm cm^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.
Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogena... more Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls and such C-H bonds can be completely broken by heating to 600 o C. We demonstrate approximately 65 ± 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes which is equivalent to 5.1 ± 1.2 weight % hydrogen capacity. We also show that the hydrogenation is a reversible process.
Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nit... more Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nitrogendoped ͑8, 0͒ carbon nanotubes are carried out using the first-principles density functional theory method. ͑1͒ For pure carbon nanotubes, we have identified the energetically most favorable dissociative pathway for hydrogen adsorption, with a barrier height of 1.3 eV. We also found that the adsorbed hydrogen atoms can act as an autocatalyst for further dissociative adsorption of hydrogen molecules. ͑2͒ It is found that on pure carbon nanotubes the diffusion of hydrogen atoms is constrained by interaction with neighboring adsorbed hydrogen atoms. The diffusion barrier is around 0.7 eV for an isolated hydrogen atom, but becomes substantially higher at around 1.4 eV in the presence of adsorbed hydrogen in neighboring positions. ͑3͒ Doping the nanotube with nitrogen considerably alters the catalytic effects of the carbon nanotube for hydrogen dissociative adsorption. The dissociative adsorption of hydrogen on the carbon nanotube is greatly enhanced, with the barrier substantially reduced to ca. 0.9 eV. The differences in the barrier heights are explained through analysis of the electronic structure changes of the nanotube.
To determine if carbon-based materials can be used for hydrogen storage, we have studied hydrogen... more To determine if carbon-based materials can be used for hydrogen storage, we have studied hydrogen chemisorption in single-walled carbon nanotubes. Using atomic hydrogen as the hydrogenation agent, we demonstrated that maximal degree of nanotube hydrogenation depends on the nanotube diameter, and for the diameter values around 2.0 nm nanotube-hydrogen complexes with close to 100% hydrogenation exist and are stable at room temperature. This means that specific carbon nanotubes can have a hydrogen storage capacity of more than 7 wt % through the formation of reversible C−H bonds
The reactivity of NH2BH2 in the presence of ammonia−borane (AB) is investigated using ab initio C... more The reactivity of NH2BH2 in the presence of ammonia−borane (AB) is investigated using ab initio CCSD(T) simulations to answer the following three questions: How do AB and NH2BH2 react? How do aminoborane species oligomerize apart from catalytic centers? Can the formation of experimentally observed products, especially cyclic N2B2H7−NH2BH3, be explained through the kinetics of NH2BH2 oligomerization in the presence of AB? AB is shown to react with NH2BH2 by the addition of NH3−BH3 across the NB double bond, generating linear NH3BH2NH2BH3. This species decomposes by surmounting a reasonable barrier to produce two NH2BH2 and H2. The generation of additional NH2BH2 from NH2BH2 and AB provides a pathway for autocatalytic NH2BH2 production. The important intermediates along the oligomerization pathway include cyclic (NH2BH2)2 and linear NH3BH2NH2BH3, both of which have been observed experimentally. Calculations show cyclic N2B2H7−NH2BH3, an aminoborane analogue of ethylcyclobutane, to be ...
The ground and lowest three adiabatic excited states of methylene are computed using the variatio... more The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo DMC methods using progressively larger Jastrow–Slater multideterminant complete active space CAS wave functions. The highest of these states has the same symmetry, 1A1, as the first excited state. The DMC excitation energies obtained using any of the CAS wave functions are in excellent agreement with experiment, but single-determinant wave functions do not yield accurate DMC energies of the states of 1A1 symmetry, indicating that it is important to include in the wave function Slater determinants that describe static strong correlation. Excitation energies obtained using recently proposed pseudopotentials (Burkatzki et al., J. Chem. Phys. 126, 234105 (2007)) differ from the all-electron excitation energies by at most 0.04 eV.
Bulletin of the American Physical Society, May 18, 2005
We present the results of an {\ em ab initio} study of elastic scattering and vibrational excitat... more We present the results of an {\ em 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. We employ a nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model. We also present cross sections for dissociative electron attachment to NO leading to ground state products, ${\ rm O}^-(^ 2 {\ rm P})+{\ rm N}(^ 4 {\ rm S}) $. The ...
Glutarimidedioxime (H 2 A), a cyclic imide dioxime ligand that has implications in sequestering u... more Glutarimidedioxime (H 2 A), a cyclic imide dioxime ligand that has implications in sequestering uranium from seawater, forms strong tridentate complexes with UO 2 . The stability constants and the enthalpies of complexation for five U(VI) complexes were measured by potentiometry and microcalorimetry. The crystal structure of the 1:2 metal/ligand complex, UO 2 (HA) 2 H 2 O, was determined. The re-arrangement of the protons of the oxime groups (-CH=N-OH) and the deprotonation of the imide group (-CH-NH-CH-) results in a conjugated system with delocalized electron density on the ligand (-O-N-C-N-C-N-O-) that coordinates to UO 2 2+ via its equatorial plane.
Threshold structures, reminiscent of those seen in the polar hydrogen halides, have recently been... more Threshold structures, reminiscent of those seen in the polar hydrogen halides, have recently been observed in the cross sections for electron impact excitation of certain vibrational levels of the nonpolar CO 2 molecule. These structures occur at energies outside the range where shape resonances dominate the dynamics. We propose a virtual state model that describes the multidimensional nuclear dynamics during the collision and explains quantitatively the selectivity observed in the excitation of the Fermi dyad, as well as the pattern of threshold peaks and oscillations seen in the upper levels of the higher polyads.
A number of physical processes involving quantum dots depend critically upon the "evanescent" ele... more A number of physical processes involving quantum dots depend critically upon the "evanescent" electron eigenstate wave function that extends outside of the material surface into the surrounding region. These processes include electron tunneling through quantum dots, as well as interactions between multiple quantum dot structures. In order to unambiguously determine these evanescent fields, appropriate boundary conditions have been developed to connect the electronic solutions interior to the semiconductor quantum dot to exterior vacuum solutions. In standard envelope function theory, the interior wave function consists of products of band edge and envelope functions, and both must be considered when matching to the external solution. While the envelope functions satisfy tractable equations, the band edge functions are generally not known. In this work, symmetry arguments in the spherically symmetric approximation are used in conjunction with the known qualitative behavior of bonding and antibonding orbitals to catalog the behavior of the band edge functions at the unit cell boundary. This physical approximation allows consolidation of the influence of the band edge functions to two simple surface parameters that are incorporated into the boundary conditions and are straightforwardly computed by using numerical first-principles quantum techniques. These new boundary conditions are employed to analyze an isolated spherically symmetric semiconductor quantum dot in vacuum within the analytical model of Sercel and Vahala ͓Phys. Rev. Lett. 65, 239 ͑1990͒; Phys. Rev. B 42, 3690 ͑1990͔͒. Results are obtained for quantum dots made of GaAs and InP, which are compared with ab initio calculations that have appeared in the literature.
The potential-energy surface corresponding to the complex resonance energy of the 2 B 1 Feshbach ... more The potential-energy surface corresponding to the complex resonance energy of the 2 B 1 Feshbach resonance state of the water anion is constructed in its full dimensionality. Complex Kohn variational scattering calculations are used to compute the resonance width, while large-scale configuration interaction calculations are used to compute the resonance energy. Near the equilibrium geometry, an accompanying ground-state potential surface is constructed from configuration interaction calculations that treat correlation at a level similar to that used in the calculations on the anion.
Development of several new computational approaches within the framework of multi-reference ab in... more Development of several new computational approaches within the framework of multi-reference ab initio molecular electronic structure methodology and their implementation in the COLUMBUS program system are reported. These new features are : calculation of the analytical MR-CI gradient for excited states based on state-averaged MCSCF orbitals, the extension of the MR-ACPF/AQCC methods to excited states in the framework of linear-response theory, spinÈorbit CI for molecules containing heavy atoms and the development of a massively-parallel code for the computation of the one-and two-particle density matrix elements. Illustrative examples are given for each of these cases.
The ground and lowest three adiabatic excited states of methylene are computed using the variatio... more The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo (DMC) methods using progressively larger Jastrow–Slater multideterminant complete active space (CAS) wave functions. The highest of these states has the same symmetry, A11, as the first excited state. The DMC excitation energies obtained using any of the CAS wave functions are in excellent agreement with experiment, but single-determinant wave functions do not yield accurate DMC energies of the states of A11 symmetry, indicating that it is important to include in the wave function Slater determinants that describe static (strong) correlation. Excitation energies obtained using recently proposed pseudopotentials [Burkatzki et al., J. Chem. Phys. 126, 234105 (2007)] differ from the all-electron excitation energies by at most 0.04 eV.
We present an ab initio study of elastic scattering and vibrational excitation of NO by low-energ... more We present an ab initio study of elastic scattering and vibrational excitation of NO by low-energy ͑0-2.0 eV͒ electron impact. The low-energy scattering cross sections are dominated by shape resonance contributions associated with the 3 ⌺ − , 1 ⌬, and 1 ⌺ + states of NO −. Resonance parameters for the three anion states were extracted from an analysis of fixed-nuclei variational (complex Kohn) calculations that employed elaborate trial wave functions. Independent estimates of the resonance parameters were obtained by analytically continuing the results of large-scale coupled-cluster calculations into the plane of complex momentum. The local complex potential model was used to calculate vibrational excitation cross sections, as well as the resonant portion of the vibrationally elastic cross sections. These results were combined with background contributions from the fixed-nuclei calculations to compute elastic and grand total cross sections. Our results capture the essential features of recent measurements of the cross sections, but suggest the need at lower energies for a more sophisticated, nonlocal treatment of nuclear dynamics.
We present the results of a study of elastic scattering and vibrational excitation of NO by elect... more We present the results of a 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 ^3\Sigma^-, ^1\Delta and ^1\Sigma^+ 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 ground state products, \rm O^-(^2\rm P) + \rm N(^4\rm S). The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small (\sim 10^-20 \rm cm^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.
Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogena... more Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls and such C-H bonds can be completely broken by heating to 600 o C. We demonstrate approximately 65 ± 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes which is equivalent to 5.1 ± 1.2 weight % hydrogen capacity. We also show that the hydrogenation is a reversible process.
Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nit... more Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nitrogendoped ͑8, 0͒ carbon nanotubes are carried out using the first-principles density functional theory method. ͑1͒ For pure carbon nanotubes, we have identified the energetically most favorable dissociative pathway for hydrogen adsorption, with a barrier height of 1.3 eV. We also found that the adsorbed hydrogen atoms can act as an autocatalyst for further dissociative adsorption of hydrogen molecules. ͑2͒ It is found that on pure carbon nanotubes the diffusion of hydrogen atoms is constrained by interaction with neighboring adsorbed hydrogen atoms. The diffusion barrier is around 0.7 eV for an isolated hydrogen atom, but becomes substantially higher at around 1.4 eV in the presence of adsorbed hydrogen in neighboring positions. ͑3͒ Doping the nanotube with nitrogen considerably alters the catalytic effects of the carbon nanotube for hydrogen dissociative adsorption. The dissociative adsorption of hydrogen on the carbon nanotube is greatly enhanced, with the barrier substantially reduced to ca. 0.9 eV. The differences in the barrier heights are explained through analysis of the electronic structure changes of the nanotube.
To determine if carbon-based materials can be used for hydrogen storage, we have studied hydrogen... more To determine if carbon-based materials can be used for hydrogen storage, we have studied hydrogen chemisorption in single-walled carbon nanotubes. Using atomic hydrogen as the hydrogenation agent, we demonstrated that maximal degree of nanotube hydrogenation depends on the nanotube diameter, and for the diameter values around 2.0 nm nanotube-hydrogen complexes with close to 100% hydrogenation exist and are stable at room temperature. This means that specific carbon nanotubes can have a hydrogen storage capacity of more than 7 wt % through the formation of reversible C−H bonds
The reactivity of NH2BH2 in the presence of ammonia−borane (AB) is investigated using ab initio C... more The reactivity of NH2BH2 in the presence of ammonia−borane (AB) is investigated using ab initio CCSD(T) simulations to answer the following three questions: How do AB and NH2BH2 react? How do aminoborane species oligomerize apart from catalytic centers? Can the formation of experimentally observed products, especially cyclic N2B2H7−NH2BH3, be explained through the kinetics of NH2BH2 oligomerization in the presence of AB? AB is shown to react with NH2BH2 by the addition of NH3−BH3 across the NB double bond, generating linear NH3BH2NH2BH3. This species decomposes by surmounting a reasonable barrier to produce two NH2BH2 and H2. The generation of additional NH2BH2 from NH2BH2 and AB provides a pathway for autocatalytic NH2BH2 production. The important intermediates along the oligomerization pathway include cyclic (NH2BH2)2 and linear NH3BH2NH2BH3, both of which have been observed experimentally. Calculations show cyclic N2B2H7−NH2BH3, an aminoborane analogue of ethylcyclobutane, to be ...
The ground and lowest three adiabatic excited states of methylene are computed using the variatio... more The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo DMC methods using progressively larger Jastrow–Slater multideterminant complete active space CAS wave functions. The highest of these states has the same symmetry, 1A1, as the first excited state. The DMC excitation energies obtained using any of the CAS wave functions are in excellent agreement with experiment, but single-determinant wave functions do not yield accurate DMC energies of the states of 1A1 symmetry, indicating that it is important to include in the wave function Slater determinants that describe static strong correlation. Excitation energies obtained using recently proposed pseudopotentials (Burkatzki et al., J. Chem. Phys. 126, 234105 (2007)) differ from the all-electron excitation energies by at most 0.04 eV.
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