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Atomic, Molecular and Optical Physics

Series

2001

Articles 1 - 19 of 19

Full-Text Articles in Physical Sciences and Mathematics

Large Nondipole Effects In The Angular Distributions Of K-Shell Photoelectrons From Molecular Nitrogen, Oliver Hemmers, H. Wang, P. Focke, I. A. Sellin, Dennis W. Lindle, J. C. Arce, J. A. Sheehy, P. W. Langhoff Dec 2001

Large Nondipole Effects In The Angular Distributions Of K-Shell Photoelectrons From Molecular Nitrogen, Oliver Hemmers, H. Wang, P. Focke, I. A. Sellin, Dennis W. Lindle, J. C. Arce, J. A. Sheehy, P. W. Langhoff

Environmental Studies Faculty Publications

Measurements of angular distributions of K-shell electrons photoejected from molecular nitrogen are reported which reveal large deviations at relatively low photon energies ( ħω≤500eV) from emission patterns anticipated from the dipole approximation to interactions between radiation and matter. A concomitant theoretical analysis incorporating the effects of electromagnetic retardation attributes the observed large nondipole behaviors in N2 to bond-length-dependent terms in the E1⊗(E2,M1) photoelectron emission amplitudes which are indicative of a potentially universal nondipole behavior in molecular photoionization.

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Observation Of The Kapitza-Dirac Effect, Daniel L. Freimund, Kayvan Aflatooni, Herman Batelaan Sep 2001

Observation Of The Kapitza-Dirac Effect, Daniel L. Freimund, Kayvan Aflatooni, Herman Batelaan

Department of Physics and Astronomy: Faculty Publications

In their famous 1927 experiment, Davisson and Germer observed the diffraction of electrons by a periodic material structure, so showing that electrons can behave like waves. Shortly afterwards, Kapitza and Dirac predicted that electrons should also be diffracted by a standing light wave. This Kapitza-Dirac effect is analogous to the diffraction of light by a grating, but with the roles of the wave and matter reversed. The electron and the light grating interact extremely weakly, via the ‘ponderomotive potential,’ so attempts to measure the Kapitza-Dirac effect had to wait for the development of the laser. The idea that the underlying …

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Quantal Density Functional Theory Of Excited States, Viraht Sahni, Lou Massa, Ranbir Singh, Marlina Slamet Sep 2001

Quantal Density Functional Theory Of Excited States, Viraht Sahni, Lou Massa, Ranbir Singh, Marlina Slamet

Physics Faculty Publications

We explain by quantal density functional theory the physics of mapping from any bound nondegenerate excited state of Schrödinger theory to an S system of noninteracting fermions with equivalent density and energy. The S system may be in a ground or excited state. In either case, the highest occupied eigenvalue is the negative of the ionization potential. We demonstrate this physics with examples. The theory further provides a new framework for calculations of atomic excited states including multiplet structure.

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Dynamical Relativistic Effects In Photoionization: Spin-Orbit-Resolved Angular Distributions Of Xenon 4d Photoelectrons Near The Cooper Minimum, H. Wang, G. Snell, Oliver Hemmers, M. M. Sant'anna, I. A. Sellin, N. Berrah, Dennis W. Lindle, P. C. Deshmukh, N. Haque, S. T. Manson Sep 2001

Dynamical Relativistic Effects In Photoionization: Spin-Orbit-Resolved Angular Distributions Of Xenon 4d Photoelectrons Near The Cooper Minimum, H. Wang, G. Snell, Oliver Hemmers, M. M. Sant'anna, I. A. Sellin, N. Berrah, Dennis W. Lindle, P. C. Deshmukh, N. Haque, S. T. Manson

Environmental Studies Faculty Publications

Two decades ago, it was predicted [Y. S. Kim et al., Phys. Rev. Lett. 46, 1326 (1981)] that relativistic effects should alter the dynamics of the photoionization process in the vicinity of Cooper minima. The present experimental and theoretical study of the angular distributions of Xe 4d3/2 and 4d5/2 photoelectrons demonstrates this effect for the first time. The results clearly imply that relativistic effects are likely to be important for intermediate- Z atoms at most energies.

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Critique Of The Wigner Tunneling Speed And A Proposed Alternative, P Krekora, Q Su, Rainer Grobe Aug 2001

Critique Of The Wigner Tunneling Speed And A Proposed Alternative, P Krekora, Q Su, Rainer Grobe

Faculty publications – Physics

In the context of superluminal propagation of wave packets through potential barriers, the tunneling speed is usually characterized by the Wigner velocity. We propose an alternative speed that takes into account the interference between the incoming and the reflected waves and leads to a better estimation of arrival time for a wave packet entering the tunneling region. This arrival time is derived by an extrapolation from inside the barrier. The analytical theory is based on the stationary phase approximation whose validity is justified by a comparison with the numerical solution of the time-dependent Dirac equation.

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Relativistic Effects On Interchannel Coupling In Atomic Photoionization: The Photoelectron Angular Distribution Of Xe, Oliver Hemmers, S. T. Manson, M. M. Sant'anna, P. Focke, H. Wang, I. A. Sellin, Dennis W. Lindle Jul 2001

Relativistic Effects On Interchannel Coupling In Atomic Photoionization: The Photoelectron Angular Distribution Of Xe, Oliver Hemmers, S. T. Manson, M. M. Sant'anna, P. Focke, H. Wang, I. A. Sellin, Dennis W. Lindle

Environmental Studies Faculty Publications

Measurements of the photoelectron angular-distribution asymmetry parameter β for Xe 5s photoionization have been performed in the 80–200 eV photon-energy region. The results show a substantial deviation from the nonrelativistic value of β=2 and provide a clear signature of significant relativistic effects in interchannel coupling.

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Parity Violation In Neutron Resonances Of Antimony And Iodine, Y Matsuda, J D. Bowman, Bret E. Crawford, P P J. Delheij, T Haseyama, J N. Knudsen, L Y. Lowie, A Masaike, G E. Mitchell, S I. Penttila, H Postma, N R. Roberson, S J. Seestrom, E I. Sharapov, Sharon L. Stephenson, Y-F Yen, V W. Yuan Jun 2001

Parity Violation In Neutron Resonances Of Antimony And Iodine, Y Matsuda, J D. Bowman, Bret E. Crawford, P P J. Delheij, T Haseyama, J N. Knudsen, L Y. Lowie, A Masaike, G E. Mitchell, S I. Penttila, H Postma, N R. Roberson, S J. Seestrom, E I. Sharapov, Sharon L. Stephenson, Y-F Yen, V W. Yuan

Physics and Astronomy Faculty Publications

Parity violation in p-wave neutron resonances of 121Sb, 123Sb, and 127I has been measured by transmission of a longitudinally polarized neutron beam through natural antimony and iodine targets. The measurements were performed at the pulsed spallation neutron source of the Los Alamos Neutron Science Center. Five statistically significant parity violation effects were observed in 121Sb, one effect in 123Sb, and seven effects in 127I. The weak interaction rms matrix elements and the corresponding spreading widths were determined.

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Parity Violation In Neutron Resonances Of 117 Sn, D A. Smith, J D. Bowman, Bret E. Crawford, C A. Grossmann, T Haseyama, Mikkel B. Johnson, A Masaike, Y Matsuda, G E. Mitchell, V A. Nazarenko, S I. Penttila, N R. Roberson, S J. Seestrom, E I. Sharapov, L M. Smotritsky, Sharon L. Stephenson, S Tomsovic, V W. Yuan Jun 2001

Parity Violation In Neutron Resonances Of 117 Sn, D A. Smith, J D. Bowman, Bret E. Crawford, C A. Grossmann, T Haseyama, Mikkel B. Johnson, A Masaike, Y Matsuda, G E. Mitchell, V A. Nazarenko, S I. Penttila, N R. Roberson, S J. Seestrom, E I. Sharapov, L M. Smotritsky, Sharon L. Stephenson, S Tomsovic, V W. Yuan

Physics and Astronomy Faculty Publications

Parity nonconservation (PNC) has been studied in neutron p-wave resonances of 117Sn. The longitudinal asymmetries were measured for 29 p-wave resonances in the neutron energy range 0.8 eV to 1100 eV. Statistically significant PNC effects were observed for four resonances. A statistical analysis determined the rms weak mixing matrix element and the weak spreading width. A weak spreading width of Γw=(0.28-0.15+0.56)×10-7 eV was obtained for117Sn.

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Anionic Photofragmentation Of Co: A Selective Probe Of Core-Level Resonances, Wayne C. Stolte, D. L. Hansen, Maria Novella Piancastelli, I. Dominguez-Lopez, A. Rizvi, Oliver Hemmers, H. Wang, Alfred S. Schlachter, M. S. Lubell, Dennis W. Lindle May 2001

Anionic Photofragmentation Of Co: A Selective Probe Of Core-Level Resonances, Wayne C. Stolte, D. L. Hansen, Maria Novella Piancastelli, I. Dominguez-Lopez, A. Rizvi, Oliver Hemmers, H. Wang, Alfred S. Schlachter, M. S. Lubell, Dennis W. Lindle

Environmental Studies Faculty Publications

Anion-yield spectroscopy using x rays is shown to be a selective probe of molecular core-level processes, providing unique experimental verification of shape resonances. For CO, partial anion and cation yields are presented for photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yields which also exhibit pronounced structure due to the well-known σ* shape resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding information unobtainable with absorption or electron spectroscopy.

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Interchannel Coupling In The Photoionization Of The M-Shell Of Kr Well Above Threshold: Experiment And Theory, H. S. Chakraborty, D. L. Hansen, Oliver Hemmers, P. C. Deshmukh, P. Focke, I. A. Sellin, Clemens Heske, Dennis W. Lindle, S. T. Manson Apr 2001

Interchannel Coupling In The Photoionization Of The M-Shell Of Kr Well Above Threshold: Experiment And Theory, H. S. Chakraborty, D. L. Hansen, Oliver Hemmers, P. C. Deshmukh, P. Focke, I. A. Sellin, Clemens Heske, Dennis W. Lindle, S. T. Manson

Environmental Studies Faculty Publications

Photoionization cross sections and asymmetry (β) parameters for Kr 3s, 3p, and 3d subshells have been measured and calculated in the 300–1300-eV photon energy range. Good agreement between experiment and theory is found for both cross-section branching ratios and β parameters. Interchannel coupling among the channels arising from 3s, 3p, and 3d subshells is found to be necessary for quantitative accuracy of the theory. This shows that the interchannel coupling phenomenology far above threshold, found previously for outer shells of Ne and Ar, is also operative for inner atomic shells.

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Effects Of Relativity On The Time-Resolved Tunneling Of Electron Wave Packets, P Krekora, Q Su, Rainer Grobe Mar 2001

Effects Of Relativity On The Time-Resolved Tunneling Of Electron Wave Packets, P Krekora, Q Su, Rainer Grobe

Faculty publications – Physics

We solve numerically the time-dependent Dirac equation for a quantum wave packet tunneling through a potential barrier. We analyze the spatial probability distribution of the transmitted wave packet in the context of the possibility of effectively superluminal peak and front velocities of the electron during tunneling. Both the Dirac and Schrodinger theories predict superluminal tunneling speeds. However, in contrast to the Dirac theory the Schrodinger equation allows a possible violation of causality. Based on an analysis of the tunneling process in full temporal and spatial resolution, we introduce an instantaneous tunneling speed that can be computed inside the potential barrier.

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Dirac Theory Of Ring-Shaped Electron Distributions In Atoms, P Krekora, R E. Wagner, Q Su, Rainer Grobe Feb 2001

Dirac Theory Of Ring-Shaped Electron Distributions In Atoms, P Krekora, R E. Wagner, Q Su, Rainer Grobe

Faculty publications – Physics

The time-dependent Dirac equation is solved numerically on a space-time grid for an atom in a strong static magnetic field and a laser field. The resonantly induced relativistic motion of the atomic electron leads to a ringlike spatial probability density similar to the features that have been recently predicted [Wagner, Su, and Grobe, Phys. Rev. Lett. 84, 3282 (2000)] based on a phase-space method. We further demonstrate that spin-orbit coupling for a fast-moving electron in such an atom becomes significant and the time dependence of the spin can dephase even if initially aligned parallel to the direction of the static …

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Intermediate Spin, Schrödinger Cat States, And Nanomagnets, S. E. Barnes Jan 2001

Intermediate Spin, Schrödinger Cat States, And Nanomagnets, S. E. Barnes

Physics Articles and Papers

Quantum tunneling in nanomagnets finds a natural description in terms of intermediate spin. Periodic magnetic effects correspond to a change of flux by the flux quantum Φ0. Schrödinger cat states with different superpositions of the applied magnetic field occur. The molecular magnet Fe8 is discussed and new effects are predicted for Mn12.

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Two-Center Effect On Low-Energy Electron Emission In Collisions Of 1-Mev/U Bare Ions With Atomic Hydrogen, Molecular Hydrogen, And Helium: Ii. H2 And He, Lokesh C. Tribedi, P. Richard, L. Gulya´S, M. Eugene Rudd Jan 2001

Two-Center Effect On Low-Energy Electron Emission In Collisions Of 1-Mev/U Bare Ions With Atomic Hydrogen, Molecular Hydrogen, And Helium: Ii. H2 And He, Lokesh C. Tribedi, P. Richard, L. Gulya´S, M. Eugene Rudd

M. Eugene Rudd Publications

We have studied the energy and angular distributions of low-energy electron emission in collisions of bare carbon ions of 1-MeV/u energy with He and H2 targets. The double-differential cross sections (DDCS’s) are measured for electrons with energies between 0.5 and 300 eV emitted within an angular range of 15° to 160°. The large forward-backward asymmetry observed in the angular distributions is explained in terms of the two-center effect. Single differential cross sections (SDCS’s) and total cross sections are also derived by integrating the DDCS’s over emission angles and energies. The data are compared with different theoretical calculations based on …

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The Procedure For Determining And Quality Assurance Program For The Calculation Of Dose Coefficients Using Dcal Software, A. Arndt, John P. Shanahan, C. Gold, R. Brey, T. Gesell, Phillip W. Patton, Mark Rudin, K. Eckerman, V. Rusetski, S. Pagava Jan 2001

The Procedure For Determining And Quality Assurance Program For The Calculation Of Dose Coefficients Using Dcal Software, A. Arndt, John P. Shanahan, C. Gold, R. Brey, T. Gesell, Phillip W. Patton, Mark Rudin, K. Eckerman, V. Rusetski, S. Pagava

Transmutation Sciences Physics (TRP)

The development of a spallation neutron source with a mercury target may lead to the production of rare radionuclides. The dose coefficients for many of these radionuclides have not yet been published. A collaboration of universities and national labs has taken on the task of calculating dose coefficients for the rare radionuclides using the software package: DCAL. The working group developed a procedure for calculating dose coefficients and a quality assurance (QA) program to verify the calculations completed. The first portion of this QA program was to verify that each participating group could independently reproduce the dose coefficients for a …

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Photoelectron Spectroscopy And The Dipole Approximation, Oliver Hemmers, Dennis W. Lindle Jan 2001

Photoelectron Spectroscopy And The Dipole Approximation, Oliver Hemmers, Dennis W. Lindle

Environmental Studies Faculty Publications

Over the past three decades, the dipole approximation has facilitated a basic understanding of the photoionization process in atoms and molecules. Advances in gas-phase photoemission experiments using synchrotron radiation have recently highlighted nondipole effects at relatively low photon energies while probing the limits of the dipole approximation. Breakdowns in this approximation are manifested primarily as deviations from dipolar angular distributions of photoelectrons. Detailed new results demonstrate nondipolar angular-distribution effects are easily observable in atomic gases at energies well below 1 keV, and, in molecules, a previously unexpected phenomenon greatly enhances the breakdown of the dipole approximation just above the core-level …

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Epitaxial Growth Of Pb(Zr0.2ti0.8)O3 On Si And Its Nanoscale Piezoelectric Properties, A. Lin, X. Hong, V. Wood, A. A. Verevkin, C. H. Ahn, R. A. Mckee, F. J. Walker, E. D. Specht Jan 2001

Epitaxial Growth Of Pb(Zr0.2ti0.8)O3 On Si And Its Nanoscale Piezoelectric Properties, A. Lin, X. Hong, V. Wood, A. A. Verevkin, C. H. Ahn, R. A. Mckee, F. J. Walker, E. D. Specht

Xia Hong Publications

We have demonstrated a route to epitaxial Pb(Zr0.2Ti0.8)O3 on (001) Si that exhibits a uniform piezoelectric response down to nanoscale levels through the utilization of an insulating, single-crystalline SrTiO3 transition layer. These structures, which were grown by a combination of molecular-beam epitaxy and off-axis magnetron sputtering, have a surface roughness of <5 Å, with piezoelectric microscopy measurements revealing a piezoelectric coefficient of ~50 pm/V that is switchable down to sub-100-nm dimensions.

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Electron Heating In Atmospheric Pressure Glow Discharges, Robert H. Stark, Karl H. Schoenbach Jan 2001

Electron Heating In Atmospheric Pressure Glow Discharges, Robert H. Stark, Karl H. Schoenbach

Bioelectrics Publications

The application of nanosecond voltage pulses to weakly ionized atmospheric pressure plasmas allows heating the electrons without considerably increasing the gas temperature, provided that the duration of the pulses is less than the critical time for the development of glow-to-arc transitions. The shift in the electron energy distribution towards higher energies causes a temporary increase in the ionization rate, and consequently a strong rise in electron density. This increase in electron density is reflected in an increased decay time of the plasma after the pulse application. Experiments in atmospheric pressure air glow discharges with gas temperatures of approximately 2000 K …

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A New Compensating Element For A Femtosecond Photoelectron Gun, Bao-Liang Qian, Hani E. Elsayed-Ali Jan 2001

A New Compensating Element For A Femtosecond Photoelectron Gun, Bao-Liang Qian, Hani E. Elsayed-Ali

Electrical & Computer Engineering Faculty Publications

Design and analysis of a new compensating element for improving the electron pulse front and compressing the pulse duration in a femtosecond photoelectron gun are described. The compensating element is a small metallic cylindrical cavity in which an external voltage is applied in such a way that a special electric field forms and interacts with the electron pulse. This electric field reduces the distances between the faster and slower electrons inside the cavity and efficiently compensates for electron pulse broadening caused by the photoelectron energy spread and space charge effects. Poisson's equation and the equation of motion are solved to …

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