Gordon Gallup Publications
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Recent documents in Gordon Gallup Publications
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Thu, 01 Feb 2018 10:56:34 PST
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Electron attachment to molecules in a cluster environment
https://digitalcommons.unl.edu/physicsgallup/55
https://digitalcommons.unl.edu/physicsgallup/55
Fri, 19 Oct 2012 10:11:42 PDT
Lowenergy dissociative electron attachment (DEA) to the CF_{2}Cl_{2} and CF_{3}Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF_{2}Cl_{2} molecule placed on the surface of H_{2}O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.
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Ilya I. Fabrikant et al.

Semiclassical complextime method for tunneling ionization: Molecular suppression and orientational dependence
https://digitalcommons.unl.edu/physicsgallup/54
https://digitalcommons.unl.edu/physicsgallup/54
Mon, 07 Jun 2010 13:24:29 PDT
We apply a previously developed semiclassical complex time method to the calculation of tunneling ionization of several diatomic molecules and CO_{2}.We investigate the presence or absence of the molecular suppression effect by calculating ionization rates of N_{2} versus Ar, O_{2} versus Xe, F_{2} versus Ar, and CO versus Kr. Comparisons with other theories, including the molecularorbitalAmmosovDeloneKrainov (MOADK) model and the strongfield approximation, are given. We also analyze the dependence of the ionization rate on the angle θ_{F} between the molecular axis and the field direction. The theoretical results agree quite well with experiment for N2 and O2 but give too low a value of the peak angle θ_{F} for CO_{2}. Our calculations give small values of the ionization rates for O_{2} and CO_{2} at θ_{F} = 0 and 90°, in agreement with experiment. Other calculations, including the MOADK model and methods involving a numerical integration of the timedependent Schrodinger equation, exhibit substantially weaker suppression at these angles.
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Gordon A. Gallup et al.

Scattering and boundstate solutions to the wave equation for one electron in the presence of a physical dipole
https://digitalcommons.unl.edu/physicsgallup/53
https://digitalcommons.unl.edu/physicsgallup/53
Thu, 03 Dec 2009 13:12:34 PST
This paper presents a quantummechanical analysis of the interaction of one electron with a physical dipole (two physically separated charges). Aspects of threshold laws in the continuous spectrum are treated. In addition there are determinations of energies of some dipole bound states. The boundstate calculations are used as a model for certain high dipole moment molecules, where, together with empirical data, an equivalent dipole length may be determined. These model calculations predict that the bound dipole orbital in high dipole molecule ions should have a node between its valence part and the more remote part where the majority of the charge resides. Some results from the two parts of the treatment are brought together to calculate the asymmetry factor of the photoelectron signal for uracil, measured by Schiedt et al.
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Gordon A. Gallup

Reply to “Comment on ‘Electroninduced bond breaking at low energies in HCOOH and glycine: The role of very shortlived σ anion states’ ”
https://digitalcommons.unl.edu/physicsgallup/52
https://digitalcommons.unl.edu/physicsgallup/52
Thu, 03 Dec 2009 13:10:41 PST
Rescigno et al. [Phys. Rev. A 80, 046701 (2009)] criticized our theoretical treatment of dissociative electron attachment in formic acid in which we show that this process can be explained by electron attachment into the temporary negative ion state formed by occupation of the σ^{*}(OH) orbital. We argue that their objections do not hold up to scrutiny.
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Gordon A. Gallup et al.

ElectronInduced Bond Breaking at Low Energies in HCOOH and Glycine: The Role of Very ShortLived σ* Anion States
https://digitalcommons.unl.edu/physicsgallup/51
https://digitalcommons.unl.edu/physicsgallup/51
Mon, 06 Apr 2009 13:56:08 PDT
Cross sections for dissociative electron attachment (DEA) to formic acid and glycine are calculated by the resonant Rmatrix theory. A model with one effective reaction coordinate close to the OH stretch motion is employed. The choice of the anion Rmatrix pole and the surface amplitude as functions of the reaction coordinate for formic acid are based on scattering phaseshift calculations using the finite element discrete model. For glycine the input parameters are adjusted to reproduce experimental data. The results show that the mechanism of DEA in these molecules is similar to that for the hydrogen halides and involves electron capture into a σ*(OH) orbital so that no σ* &#;π&#; coupling is required. Nonlocal effects play an important role for both molecules. In particular, pronounced steps and cusps are seen at the vibrational excitation thresholds. A sharp threshold peak in glycine is interpreted as a vibrational Feshbach resonance.
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Gordon A. Gallup et al.

Semiclassical Propagation Method for Tunneling Ionization
https://digitalcommons.unl.edu/physicsgallup/50
https://digitalcommons.unl.edu/physicsgallup/50
Mon, 06 Apr 2009 13:56:07 PDT
We apply the semiclassical propagation technique to tunneling ionization in atomic and molecular systems. Semiclassical wave functions and the tunneling flux are calculated from the solution of the classical equations of motion in the complex time plane. We illustrate this method by rederiving the known result for the decay rate of a negative ion in a weak electric field. We then obtain numerical results for atomic hydrogen, H_{2}^{+}, H_{2}, and Ar, and compare them with the results of the asymptotic [AmmosovDeloneKrainov (ADK)] theory. The asymptotic theory gives surprisingly good results for the atomic and molecular ionization rates. In particular, our calculations for the simplest case of molecular suppression, ionization of H_{2} versus Ar, confirms the ADK analysis of Tong et al. [Phys. Rev. A 66, 013409 (2002)], explaining that the suppression is mainly due to the different symmetries of the ionized orbitals, s in H_{2} and 3p_{z} in Ar.
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Ilya I. Fabrikant et al.

Total dissociative electron attachment cross sections of selected amino acids
https://digitalcommons.unl.edu/physicsgallup/49
https://digitalcommons.unl.edu/physicsgallup/49
Tue, 28 Aug 2007 12:43:38 PDT
Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binaryencounterBethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the –COOH π * orbital. The authors discuss evidence that direct attachment to the lowest σ * orbital may instead be responsible. A close correlation between the energies of the coreexcited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H−.
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A.M. Scheer et al.

Comparative studies of dissociative electron attachment to methyl halides
https://digitalcommons.unl.edu/physicsgallup/48
https://digitalcommons.unl.edu/physicsgallup/48
Fri, 04 May 2007 12:48:43 PDT
The dissociative electron attachment cross sections for the methyl halides vary in an enormous range from the virtually unmeasurable 10^{−23} cm^{2} for CH_{3}Cl at room temperature to 10^{−14} cm^{2} for CH_{3}I. In this paper we supplement our previous studies by calculations of dissociative electron attachment to CH_{3}Br and compare results for all methyl halides studied so far. The rate as a function of temperature for CH_{3}Cl and CH_{3}Br exhibits an exponential dependence on 1/T (Arrhenius law) with the activation energy lower for CH_{3}Br. CH_{3}I does not obey the Arrhenius law since the crossing point of the neutral and anion potential curves occurs near the lowest vibrational levels. The cross section as a function of electron energy for all of the methyl halides studied here exhibits structure at the vibrational excitation thresholds that is associated with a vibrational Feshbach resonance like that observed recently in CH_{3}I by Schramm et al (Schramm A, Fabrikant I I, Weber J M, Leber E, Ruf MW and Hotop H 1999 J. Phys. B: At. Mol. Opt. Phys. 32 2153).
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R. S. Wilde et al.

Semiempirical <i>R</i>matrix theory of low energy electron–CF<sub>3</sub>Cl inelastic scattering
https://digitalcommons.unl.edu/physicsgallup/47
https://digitalcommons.unl.edu/physicsgallup/47
Fri, 04 May 2007 12:39:21 PDT
We apply a semiempirical Rmatrix theory to calculations of vibrational excitation and dissociative attachment in the CF_{3}Cl molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the CF_{3}Cl^{−} potential curve. We find that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory.
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R. S. Wilde et al.

Errors in the application of the JWKB method to calculating the survival factor in dissociative electron attachment using the local complex potential
https://digitalcommons.unl.edu/physicsgallup/46
https://digitalcommons.unl.edu/physicsgallup/46
Thu, 05 Apr 2007 13:46:37 PDT
The accuracy of the JWKB method for determining the survival factor defined for dissociative electron attachment (DEA) processes is examined for a range of electronic resonance lifetimes within the local complex potential approximation. The author concludes that the accuracy is inadequate for molecules with properties commonly found for shape resonance induced DEA. More accurate methods using the uniform Airy function approximation give much better results, but the direct numerical integration of Schrödinger’s equation appears simpler still. © 2007 American Institute of Physics.
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Gordon A. Gallup

Remote bond breaking by interacting temporary anion states
https://digitalcommons.unl.edu/physicsgallup/45
https://digitalcommons.unl.edu/physicsgallup/45
Thu, 05 Apr 2007 13:44:55 PDT
The cross section for bond breaking at the site of a dissociative temporary negative ion state through the dissociative electron attachment process can be considerably enhanced by the presence of a second longerlived temporary negative ion state elsewhere in the molecule, even one quite remote from the first. In a series of chloroalkenes possessing both C–Cl and C=C bonds separated by various distances, we show that the cross sections are determined by the lifetime of the lower anion state created by the mixing of the anion states of these two moieties, with the wave function’s coefficients giving the probability that the electron is located at the dissociative site. Furthermore, the lifetime of the composite anion state can be expressed in terms of these same coefficients and the lifetimes of the unmixed resonances. We also discuss how these results may give insight into the means by which strand breaks are induced in DNA by the attachment of slow electrons. © 2006 American Institute of Physics.
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Paul Burrow et al.

Resonances and threshold effects in lowenergy electron collisions with methyl halides
https://digitalcommons.unl.edu/physicsgallup/44
https://digitalcommons.unl.edu/physicsgallup/44
Thu, 05 Apr 2007 13:42:08 PDT
Cross sections for elastic and inelastic electron collisions with CH_{3}X(X =Cl,Br, I) molecules are calculated. For the lowest partial wave, the resonance Rmatrix theory, and for the higher partial waves, the theory of scattering by dipolar plus polarization potential, are used. It is shown that the rotationally elastic scattering amplitude for a polar molecule in the fixednuclei approximation is logarithmically divergent for the forward direction, and a closure formula is derived to speed up the convergence at small angles. In treating the nuclear motion, only CX stretch vibrations are taken into account. The dipole moment as a function of the CX distance is modeled by a function incorporating the experimental value of the molecular dipole moments at the equilibrium distance and the derivatives of the dipole moments extracted from the experimental data on infrared intensities. This is supplemented by ab initio calculations of the dipole moment function for CH3Br using the multiconfigurational valence bond method. The results for scattering cross sections show pronounced features caused by vibrational Feshbach resonances and threshold cusps. The features are most noticeable at the v=6, 7, and 8 thresholds in CH_{3}Cl, at the v=3 and 4 thresholds in CH3Br, and at the v=1 threshold in CH_{3}I.
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Gordon A. Gallup et al.

A Short History of Valence Bond Theory
https://digitalcommons.unl.edu/physicsgallup/43
https://digitalcommons.unl.edu/physicsgallup/43
Wed, 21 Mar 2007 13:11:33 PDT
Shortly after quantum mechanics evolved Heitler and London [1] applied the then new ideas to the problem of molecule formation and chemical valence. Their treatment of the H_{2} molecule was qualitatively very successful, and this led to numerous studies by various workers applying the same ideas to other substances. Many of these involved refinements of the original HeitlerLondon procedure, and within three or four years, a group of ideas and procedures had become reasonably well codified in what was called the valence bond (VB)* method for molecular structure.
A few calculations were carried out earlier, but by 1929 Dirac [2] wrote:
The general theory of quantum mechanics is now almost complete, the imperfections that still remain being in connection with the exact fitting in of the theory with relativity ideas. These give rise to difficulties only when highspeed particles are involved, and are therefore of no importance in the consideration of atomic and molecular structure and ordinary chemical reactions. The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble . . . .
Since these words were written there has been no reason to feel that they are incorrect in any way. Perhaps the only difference between attitudes then and now are that, today, with visions of DNA chains dangling before our eyes, we are likely to have an even greater appreciation of the phrase “much too complicated to be soluble” than did early workers.
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Gordon A. Gallup

An investigation of electron helicity density in bromocamphor and dibromocamphor as a source of electron circular dichroism
https://digitalcommons.unl.edu/physicsgallup/42
https://digitalcommons.unl.edu/physicsgallup/42
Wed, 21 Mar 2007 13:06:58 PDT
We investigate the causes of electroncircular dichroism (ECD) in bromocamphor and dibromocamphor, focusing specifically on the electron helicity density of the target. Using electron transmission spectroscopy (ETS) and quantum chemical calculations, we have observed and assigned temporary negative ion states of bromocamphor and dibromocamphor. Further calculations were conducted to determine the helicity densities of these compounds. Large helicity densities are found in the regions of large wave function amplitude of the normally unoccupied molecular orbitals responsible for resonances in the scattering cross sections. We relate our ETS assignments and helicity density results to the chiral asymmetry data observed in electroncircular dichroism experiments by the Münster group (Nolting et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30, 5491). Our results support helicity density as a possible source of chiral asymmetry at certain resonance positions in bromocamphor and dibromocamphor.
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A.M. Scheer et al.

The structures of <i>c</i>C<sub>4</sub>F<sub>8</sub> and <i>c</i>C<sub>4</sub>F<sub>8</sub><sup>¯</sup> and the adiabatic electron affinity of <i>c</i>C<sub>4</sub>F<sub>8</sub>
https://digitalcommons.unl.edu/physicsgallup/41
https://digitalcommons.unl.edu/physicsgallup/41
Fri, 09 Mar 2007 08:21:31 PST
Calculations at the 6311G(dps) level have been used to elucidate the structures of octafluorocyclobutane cC_{4}F_{8} and its negative ion cC_{4}F_{8}^{¯}. With no empirical adjustments, we obtain 0.640 eV for the adiabatic electron affi nity of cC_{4}F_{8}. This may be compared with an experimental value of 0.63 ± 0.05 eV from the FALP method. cC_{4}F_{8} has unusual features in that there is an increase in symmetry, D_{2d} to D_{4h}, in going from the neutral molecule to the negative ion, and the singly occupied molecular orbital in the ion has an unexpected nodal structure.
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Gordon A. Gallup

Representation of <i>ab initio</i> energy surfaces by analytic functions
https://digitalcommons.unl.edu/physicsgallup/40
https://digitalcommons.unl.edu/physicsgallup/40
Fri, 02 Mar 2007 07:44:32 PST
A method for the analytical representation of ab initio energy surface results is described. For a threeatom surface, the potential function is written as a sum of atomic, twobody, and threebody terms. This allows an easy fitting of the whole surface, including the dissociated and united atom limits. The theory is applied to an H_{3} surface of limited accuracy calculated for these purposes. Comparisons are made, where possible, with more accurate calculations. Extensions to surfaces involving four or more atoms are also discussed.
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R.L. Vance et al.

QuantumMechanical Analysis of a Longitudinal SternGerlach Effect
https://digitalcommons.unl.edu/physicsgallup/39
https://digitalcommons.unl.edu/physicsgallup/39
Mon, 26 Feb 2007 08:05:58 PST
We present the results of a rigorous quantummechanical calculation of the propagation of electrons through an inhomogeneous magnetic field with axial symmetry. A complete spin polarization of the beam is demonstrated assuming that a Landau eigenstate can be inserted into the field. This is in contrast with the semiclassical situation, where the spin splitting is blurred.
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Gordon A. Gallup et al.

Dissociative electron attachment to vibrationally and rotationally excited H<sub>2</sub> and HF molecules
https://digitalcommons.unl.edu/physicsgallup/38
https://digitalcommons.unl.edu/physicsgallup/38
Mon, 26 Feb 2007 08:02:22 PST
The vibrational and rotational state dependence of dissociative attachment (DA) in lowenergy eH_{2} and eHF collisions is studied within the framework of nonlocal resonance theory. The dynamics of nuclear motion in the nonlocal complex potential is treated by the quasiclassical approach. The resonance energy, width function, and levelshift function are taken from our previous calculations based on the projection operator formalism. Results for all vibrational states up to the dissociative attachment thresholds and selected rotational states are presented. Our cross sections for H_{2} exhibit a plateau structure in the energy range above the dissociation threshold, and this structure gets more pronounced with higher v or J. The dissociative attachment cross section for vibrationally and rotationally hot HF (T=1150 K) agrees well with available experimental data. For H_{2} in thermal equilibrium at 1400 K, the agreement between experiment and theory is less satisfactory.
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Yinfan Xu et al.

Nonlocal theory of dissociative electron attachment to H<sub>2</sub> and HF molecules
https://digitalcommons.unl.edu/physicsgallup/37
https://digitalcommons.unl.edu/physicsgallup/37
Mon, 26 Feb 2007 08:02:19 PST
We develop a method for calculation of dissociative electron attachment cross sections based on the Fano Feshbach projectionoperator approach. The coupling between the diabatic state and electron continuum is calculated with the inclusion of orthogonality scattering and longrange electronmolecule interaction. The dynamics of nuclear motion in the nonlocal complex potential is treated by semiclassical theory. We apply the theory to the calculation of dissociative attachment to the H_{2} and HF molecules. Our results for attachment to groundstate hydrogen molecules and the nearthreshold vibrational enhancement of H_{2} are essentially the same as previous nonlocal results. However, the shape of the energy dependence of the cross section for attachment to vibrationally excited states of H_{2} is slightly different. The calculated value for the total attachment cross section to the ground state of HF is consistent with the little experimental data available; more definite conclusions are difficult because of the approximate nature of the experimental results. The results for the vibrational enhancement are in very good agreement with experiment.
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Gordon A. Gallup et al.

Projected Hartree product wavefunctions. VI. Natural orbital CI expansions in nonsinglet cases
https://digitalcommons.unl.edu/physicsgallup/36
https://digitalcommons.unl.edu/physicsgallup/36
Fri, 23 Feb 2007 11:10:09 PST
The NSO's and NO's have been determined for some wavefunctions for Li, Be^{1+}, B^{2+}, C^{3+} ^{2}S, and Be ^{3}S wavefunctions containing radial correlation. It is shown how the NO's may be utilized to form rapidly converging CI expansions in general. The role of the NSO's in this problem is discussed. ©1973 The American Institute of Physics
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R.D. Koller et al.