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Full-Text Articles in Physics

Dissociative Electron Attachment In Nonplanar Chlorocarbons With Π**-Coupled Molecular Orbitals, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow Jan 2010

Dissociative Electron Attachment In Nonplanar Chlorocarbons With Π*/Σ*-Coupled Molecular Orbitals, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Total absolute cross sections for the dissociative electron attachment (DEA) process are reported for a series of nonplanar ethylenic and phenylic compounds monosubstituted with (CH2)nCl groups, where n=1–4. Coupling between the local π* molecular orbitals provided by the unsaturated moieties and the σ* (C-Cl) orbital is thus examined as a function of the separation of these groups. In particular, the coupling is viewed from the perspective of the interacting temporary negative ions formed by short lived occupation of these orbitals and their decay into the DEA channel. A theoretical treatment of “remote” bond breaking, presented ...


Negative-Ion Resonances In Zinc: A Study In Elastic Electron Scattering And Electron Transmission, S. A. Napier, D. Cvejanović, Paul D. Burrow, J. F. Williams, J. A. Michejda, L. Pravica Jan 2009

Negative-Ion Resonances In Zinc: A Study In Elastic Electron Scattering And Electron Transmission, S. A. Napier, D. Cvejanović, Paul D. Burrow, J. F. Williams, J. A. Michejda, L. Pravica

Paul Burrow Publications

The energies, widths, and angular behavior of negative-ion resonances in zinc atoms are determined using the complementary experimental techniques of electron transmission and angular differential elastic scattering.


Remote Bond Breaking By Interacting Temporary Anion States, Paul Burrow, Gordon A. Gallup Oct 2006

Remote Bond Breaking By Interacting Temporary Anion States, Paul Burrow, Gordon A. Gallup

Paul Burrow Publications

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 longer-lived 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 ...


Total Dissociative Electron Attachment Cross Sections For Molecular Constituents Of Dna, Kayvan Aflatooni, A.M. Scheer, Paul Burrow Aug 2006

Total Dissociative Electron Attachment Cross Sections For Molecular Constituents Of Dna, Kayvan Aflatooni, A.M. Scheer, Paul Burrow

Paul Burrow Publications

Total cross sections for the dissociative electron attachment process are presented for the DNA bases thymine, cytosine, and adenine and for three compounds used as surrogates for the ribose and phosphate groups, tetrahydrofuran, 3-hydroxytetrahydrofuran, and trimethylphosphate, respectively. Cross section magnitudes are obtained by observation of positive ion production and normalization to ionization cross sections calculated elsewhere using the binary-encounter-Bethe method. The average cross section of the three bases is 3–10 times smaller than the effective cross section per nucleotide reported for single strand breaks in surface-bound super-coiled DNA. Consequently, damage to the bases alone does not appear to account ...


Vibrational Feshbach Resonances In Uracil And Thymine, Paul Burrow, Gordon A. Gallup, Adam M. Scheer, G. Denifl, S. Ptasinska, T. Märk, P. Scheier Mar 2006

Vibrational Feshbach Resonances In Uracil And Thymine, Paul Burrow, Gordon A. Gallup, Adam M. Scheer, G. Denifl, S. Ptasinska, T. Märk, P. Scheier

Paul Burrow Publications

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest σ* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the σ * orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted ...


Dissociative Electron Attachment To Uracil Deuterated At The N1 And N3 Positions, A. M. Scheer, C. Silvernail, John A. Belot Jr., Gordon A. Gallup, Paul Burrow Aug 2005

Dissociative Electron Attachment To Uracil Deuterated At The N1 And N3 Positions, A. M. Scheer, C. Silvernail, John A. Belot Jr., Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Deuteration at the nitrogen sites of uracil produces a striking change in the dissociative attachment cross-section at electron energies below 3 eV. In particular, sharp structures observed in uracil are absent or too small to observe. This result, as well as theoretical modeling, supports the earlier assignment of the sharp structure to vibrational Feshbach resonances that decay by tunneling of the N1 hydrogen atom through the barrier created by the avoided crossing of the dipole bound anion potential surface by that of the lowest 2Σ valence anion state.


Comment On “Radiation Damage Of Biosystems Mediated By Secondary Electrons: Resonant Precursors For Uracil Molecules” † [J. Chem. Phys. 120, 7446 „ (2004)], Paul Burrow Feb 2005

Comment On “Radiation Damage Of Biosystems Mediated By Secondary Electrons: Resonant Precursors For Uracil Molecules” † [J. Chem. Phys. 120, 7446 „ (2004)], Paul Burrow

Paul Burrow Publications

Gianturco and Lucchese have recently presented calculations reporting resonances in the electron scattering cross section of the RNA base uracil and discussed the role of these temporary negative ion states in various fragmentation processes. Such efforts in complex systems are highly commendable. However, the authors have not made a proper comparison of their calculated anion states with those measured and assigned earlier, although this latter work is cited in their paper. In particular, they omit any reference to the lowest-lying resonance observed previously.

The amplitudes and nodal characteristics of the resonance orbitals sat the geometry of the neutral moleculed certainly ...


Bibliography Of Studies In Electron Transmission Spectroscopy, Paul Burrow Jan 2005

Bibliography Of Studies In Electron Transmission Spectroscopy, Paul Burrow

Paul Burrow Publications

A variety of techniques can be used to locate temporary negative ion states in atoms and molecules. One of the most efficient methods passes an electron beam through a gas cell and observes structure appearing in the attenuated “transmitted” electron current as a function of the electron energy. In 1971 L. Sanche and G.J. Schulz introduced a scheme whereby the derivative with respect to energy of the transmitted current was measured. Combining this with the trochoidal electron monochromator devised by A. Stamatovic and G.J. Schulz in 1968 produced a compact, sensitive and easily used apparatus for the detection ...


Dna Strand Breaks Induced By 0–4 Ev Electrons: The Role Of Shape Resonances, Frédéric Martin, Paul Burrow, Zhongli Cai, Pierre Cloutier, Darel Hunting, Léon Sanche Aug 2004

Dna Strand Breaks Induced By 0–4 Ev Electrons: The Role Of Shape Resonances, Frédéric Martin, Paul Burrow, Zhongli Cai, Pierre Cloutier, Darel Hunting, Léon Sanche

Paul Burrow Publications

Collisions of 0– 4 eV electrons with thin DNA films are shown to produce single strand breaks. The yield is sharply structured as a function of electron energy and indicates the involvement of π* shape resonances in the bond breaking process. The cross sections are comparable in magnitude to those observed in other compounds in the gas phase in which π* electrons are transferred through the molecule to break a remote bond. The results therefore support aspects of a theoretical study by Barrios et al. [J. Phys. B 106, 7991 (2002)] indicating that such a mechanism could produce strand breaks ...


Bond Breaking And Temporary Anion States In Uracil And Halouracils: Implications For The Dna Bases, Adam M. Scheer, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow Feb 2004

Bond Breaking And Temporary Anion States In Uracil And Halouracils: Implications For The Dna Bases, Adam M. Scheer, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Low energy electrons are capable of breaking bonds in gas phase DNA bases by means of the dissociative electron attachment process.With the aid of new total scattering data in the halouracils and input from quantum chemical calculations, we describe the dipole bound and valence anion states in these compounds and present assignments for the two types of structure appearing in the cross sections. A clear distinction between the two mechanisms for bond breaking is necessary for an understanding of electron induced damage to DNA.


Dissociative Electron Attachment Near Threshold, Thermal Attachment Rates, And Vertical Attachment Energies Of Chloroalkanes, Gordon A. Gallup, Kayvan Aflatooni, Paul Burrow Feb 2003

Dissociative Electron Attachment Near Threshold, Thermal Attachment Rates, And Vertical Attachment Energies Of Chloroalkanes, Gordon A. Gallup, Kayvan Aflatooni, Paul Burrow

Paul Burrow Publications

The peaks appearing near zero energy in the dissociative electron attachment cross section of 18 chloroalkanes are studied by electron beam methods. Fits to the experimental data are made using model cross sections having appropriate energy dependences and inclusion of the broadening due to the electron energy distribution. The magnitudes of the zero peaks are found to be well correlated with the vertical attachment energies (VAE) associated with occupation of the lowest empty orbitals of the compounds. The magnitudes rise exponentially by more than five orders of magnitude as VAE decreases from 2 eV to a slightly negative value. This ...


Dissociative Electron Attachment To Molecules In The Gas Phase And In Rare Gas Solids, Paul Burrow, Kayvan Aflatooni Apr 2002

Dissociative Electron Attachment To Molecules In The Gas Phase And In Rare Gas Solids, Paul Burrow, Kayvan Aflatooni

Paul Burrow Publications

Measurements of dissociative electron attachment (DEA) cross sections in chloroalkanes and chlorofluoromethanes have shown strong correlations between the peak DEA cross sections and the vertical attachment energies (VAEs) of these compounds. We explore the extent to which these gas phase data can be used to predict such cross sections for molecules embedded within or on the surface of a Kr solid. Effective VAEs are computed that include polarization of the solid by the anion and effects due to electron motion in the lattice. Comparisons are made with recent surface and bulk measurements and show good agreement for CF3Cl ...


Temporary Anion States Of Selected Amino Acids, Kayvan Aflatooni, B. Hitt, Gordon A. Gallup, Paul Burrow Oct 2001

Temporary Anion States Of Selected Amino Acids, Kayvan Aflatooni, B. Hitt, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Vertical attachment energies for the formation of low-lying temporary anion states of glycine, alanine, phenylalanine, tryptophan, and proline in the gas phase are reported using electron transmission spectroscopy. Electron attachment into the empty π* orbital of the –COOH group was observed in all the compounds. Temporary anion states associated with the side groups in phenylalanine and tryptophan are found to be stabilized with respect to those in the reference compounds toluene and indole, respectively, by approximately 0.2 eV. We attribute this to electrostatic effects and explore, using simple theoretical models, the extent to which such anion states could be ...


Total Cross Sections For Dissociative Electron Attachment In Dichloroalkanes And Selected Polychloroalkanes: The Correlation With Vertical Attachment Energies, Kayvan Aflatooni, Paul Burrow Aug 2000

Total Cross Sections For Dissociative Electron Attachment In Dichloroalkanes And Selected Polychloroalkanes: The Correlation With Vertical Attachment Energies, Kayvan Aflatooni, Paul Burrow

Paul Burrow Publications

Electron attachment into the lowest unoccupied molecular orbital of a typical polychloroalkane in the gas phase forms a temporary negative ion in which the impinging electron resides on a combination of local C–Cl σ* orbitals. Because of the antibonding character of these orbitals, these anions may dissociate, producing Cl- with cross sections that vary enormously over the chloroalkane family. In this work, we present absolute total dissociative electron attachment (DEA) cross sections for 33 of these compounds, and we show that the peak values of the cross sections correlate strongly with the vertical attachment energies (VAEs) for formation of ...


Low Energy Electron Scattering From Ch3Cl, X. Shi, V.K. Chan, Gordon A. Gallup, Paul Burrow Feb 1996

Low Energy Electron Scattering From Ch3Cl, X. Shi, V.K. Chan, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Differential cross section measurements for the elastic scattering of electrons from CH3Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2A1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are ...


Electron-Impact Ionization Of Na, A.R. Johnston, Paul Burrow Mar 1995

Electron-Impact Ionization Of Na, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Recent convergent close-coupling calculations of the ionization cross section of Na by Bray [Phys. Rev. Lett. 73, 1088 (1994)] are in poor agreement with the experimental determinations, most of which were carried out in the 1960s. We report here a measurement of the ratio of the peak ionization cross section to the cross section for excitation of the 32P state near threshold. By normalizing the latter to the theoretical cross section computed by Moores and Norcross [J. Phys. B 5, 1482 (1972)] and more recently by Trail et al. [Phys. Rev. A 49, 3620 (1994)], we find an ...


Dissociative Attachment In Hot Ch3Cl: Experiment And Theory, D. M. Pearl, Paul Burrow, Ilya I. Fabrikant, Gordon A. Gallup Feb 1995

Dissociative Attachment In Hot Ch3Cl: Experiment And Theory, D. M. Pearl, Paul Burrow, Ilya I. Fabrikant, Gordon A. Gallup

Paul Burrow Publications

The dissociative attachment (DA) cross section of hot CH3Cl has been measured in a crossed electron–molecule beam apparatus at temperatures up to 750 K and electron energies from 0–0.5 eV. The results are compared to cross sections computed using a mixed ab initio-semiempirical approach, treating CH3Cl as a quasidiatomic molecule. The theoretical treatment requires an anion potential curve in the stable region as a portion of the input data. Computations with three different basis sets show the results to be sensitive to the size of basis set from which the potential is ...


Temporary Negative-Ion States Of Na, K, Rb, And Cs, A.R. Johnston, Paul Burrow Jan 1995

Temporary Negative-Ion States Of Na, K, Rb, And Cs, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Electron-transmission spectroscopy is used to observe structure in the electron-scattering cross section associated with the formation of temporary negative ions and channel-opening effects. A strong ‘‘family’’ resemblance is apparent among the resonances of the alkali metals. From published calculations and angular scattering studies, the lower-lying features are known with some confidence. Based on these, plausible assignments for some of the higher-lying resonances are put forth. We propose that the absence of resonances near the Cs 5 2D and possibly the Na 4 2P and 5 2P thresholds is associated with the negative polarizabilities of these excited states.


Dissociative Attachment In Selected Monochloroalkanes, D. M. Pearl, Paul Burrow Aug 1994

Dissociative Attachment In Selected Monochloroalkanes, D. M. Pearl, Paul Burrow

Paul Burrow Publications

The total cross sections for the dissociative attachment (DA) process are reported for eight normal chloroalkanes CnH2n+1Cl for n=2–9, two secondary compounds 2-chloropropane and 2-chlorobutane, one tertiary compound t-butyl chloride, and three cyclic chloroalkanes c-CnH2n-1Cl for n=4–6, for electron energies below 4 eV. Using vertical attachment energies (VAEs) determined by electron transmission spectroscopy elsewhere and in this work, we show that the resonance widths vary linearly with VAE in this series of compounds and that the peak DA cross sections decline exponentially with ...


Temporary Negative Ions And Vibrational Excitation In Ch3Cl And Cd3Cl, X. Shi, T.M. Stephen, Paul Burrow Mar 1992

Temporary Negative Ions And Vibrational Excitation In Ch3Cl And Cd3Cl, X. Shi, T.M. Stephen, Paul Burrow

Paul Burrow Publications

Electron energy loss spectroscopy is used to study the vibrational modes of CH3Cl and CD3Cl excited through the formation of a temporary negative ion state centered at 3.5 eV. Using symmetry and charge distribution arguments together with the experimentally determined excitation functions for several vibrational modes, we confirm that this resonance results from filling of the 8a1 (C–Cl) σ* molecular orbital. Evidence for a broad higher-lying resonance which we attribute to the unfilled 4e orbital is also presented.


Low-Lying Negative-Ion States Of Calcium, A.R. Johnston, Gordon A. Gallup, Paul Burrow Oct 1989

Low-Lying Negative-Ion States Of Calcium, A.R. Johnston, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Studies of electron transmission through Ca vapor reveal two prominent resonances in the range 0–4 eV. The energies and widths of these temporary anion states suggest that they may be assigned to the strongly mixed (4s23d) 2D and (4s4p2)2D states of Ca-. This interpretation is supported by ab initio configuration-interaction calculations and is consistent with previously unassigned structure observed in the photodetachment cross section.


Magnetically Induced Circular Polarization, K.L. Stricklett, D.J. Burns, Paul Burrow Dec 1987

Magnetically Induced Circular Polarization, K.L. Stricklett, D.J. Burns, Paul Burrow

Paul Burrow Publications

Helium atoms excited to an aligned (1s3d) 3D state are produced by electron impact parallel to a magnetic field. The initial alignment of the atomic state evolves into an orientation with finite angular momentum along the magnetic field direction and thus gives rise to a circularly polarized component in the (1s3d) 3D→(1s2p) 3P radiation propagating parallel to the magnetic field. Circular-polarization fractions of a few percent are observed, in general agreement with theory. The measured polarization fraction is found to be pressure dependent, indicating that secondary excitation and cascade processes must be included for a complete ...


Electron Transmission Study Of The Temporary Negative Ion States Of Selected Benzenoid And Conjugated Aromatic Hydrocarbons, Paul Burrow, J.A. Michejda, K.D. Jordan Jan 1987

Electron Transmission Study Of The Temporary Negative Ion States Of Selected Benzenoid And Conjugated Aromatic Hydrocarbons, Paul Burrow, J.A. Michejda, K.D. Jordan

Paul Burrow Publications

Electron transmission spectroscopy is utilized to determine the vertical electron affinities and to characterize the temporary anion states of a series of alternant hydrocarbons including benzene, naphthalene, anthracene, tetracene, styrene, and cis- and trans-stilbene. The vibrational structure present in the low lying resonances is interpreted in light of the charge distributions of the temporarily occupied orbitals. The energies of the anion states are compared with the predictions of PPP, PPP-CI, and HAM calculations, the pairing theorem and with the results from optical absorption measurements on the ground state anions in rigid glasses.


Temporary Negative Ions In The Chloromethanes Chcl2F And Ccl2F2: Characterization Of The Σ* Orbitals, Paul Burrow, A. Modelli, N.S. Chiu, K.D. Jordan Sep 1982

Temporary Negative Ions In The Chloromethanes Chcl2F And Ccl2F2: Characterization Of The Σ* Orbitals, Paul Burrow, A. Modelli, N.S. Chiu, K.D. Jordan

Paul Burrow Publications

The electron transmission spectra of chlorometers CHCl2F and CCl2F2 are presented. The electron affinities are evaluted compared with thoses calculated using self-consistent field methods.


Detection Of Vibrationally Excited Nitrogen By Trapped Electron And Electron Transmission Methods, J.A. Michejda, L.J. Dube, Paul Burrow May 1981

Detection Of Vibrationally Excited Nitrogen By Trapped Electron And Electron Transmission Methods, J.A. Michejda, L.J. Dube, Paul Burrow

Paul Burrow Publications

Two techniques for detection of vibrationally excited nitrogen using low-energy electron scattering are described. The first technique uses the trapped electron method to observe excitation from N2ν into the B3Пg state near threshold. From the known Franck-Condon factors, the contribution from excited vibrational levels may be unfolded. The second technique employs the electron transmission method to detect sharp structure in the total scattering cross section resulting from the formation of temporary negative ions. Because of uncertainties in the total scattering cross sections from each of the vibrational levels, the transmission method is less accurate than ...


Comment On The Negative Ion States Of Fluorobenzenes, K.D. Jordan, Paul Burrow Dec 1979

Comment On The Negative Ion States Of Fluorobenzenes, K.D. Jordan, Paul Burrow

Paul Burrow Publications

It is contended that several assignments given by the model employed by Frazier et al1 should be reconsidered especially of the anion states aniline phenol and anisole. The symmetry assignments of the anion states of fluorobenzene and p difluorobenzend as given by this model are incorrect.


Observation Of Vibrationally Excited Nitrogen With A Simplified Electron Transmission Apparatus, J.A. Michejda, Paul Burrow Jun 1976

Observation Of Vibrationally Excited Nitrogen With A Simplified Electron Transmission Apparatus, J.A. Michejda, Paul Burrow

Paul Burrow Publications

A simple electron transmission apparatus is used to observe vibrationally excited N2 in the first two vibrational levels. The method is based on the pronounced peaks appearing in the total electron scattering cross section in the range of energy 2–4 eV. Because of the wide spacing between these peaks, good energy resolution is not required and measurements can be performed without an electron monochromator.


Dissociative Attachment From The O2 (A1 Δ G) State, P.D. Burrow Nov 1973

Dissociative Attachment From The O2 (A1 Δ G) State, P.D. Burrow

Paul Burrow Publications

The dissociative attachment cross section for production of O- by electron impact on the metastable O2 (a1 Δ g) state is studied. The cross section is found to be 4.6 ± 1.3 × 10-18 cm2 at its maximum. From the measured energy dependence, we infer that the dissociative attachment process takes place through the O (2 Пu) state as in the case of O- production from the O2 ground state. The information thus obtained is used to estimate the portion of the cross section for excitation of the a1 Δg state ...


Electron Transmission Spectroscopy In Atomic Hydrogen, L. Sanche, Paul Burrow Dec 1972

Electron Transmission Spectroscopy In Atomic Hydrogen, L. Sanche, Paul Burrow

Paul Burrow Publications

An electron transmission experiment is used to study the resonances in the total scattering cross section of atomic hydrogen below the threshold of the first excited state. The three lowest resonances, designated 1S, 3P, and 1D, are observed and their energies and decay widths are found to be in good agreement with the values predicted theoretically using close coupling with correlation.


Elastic Scattering Of Low-Energy Electrons At 180° In Co2, Paul Burrow, L. Sanche Feb 1972

Elastic Scattering Of Low-Energy Electrons At 180° In Co2, Paul Burrow, L. Sanche

Paul Burrow Publications

A technique is described for the study of electrons which are elastically scattered from a gas target into a small solid angle centered at 180° with respect to the direction of the incident electron beam. The method is applied to electron scattering from CO2. Information is obtained concerning the nature of the compound state in the 4-eV region.