Physics Division

Accelerator Based Atomic Physics

Investigator(s)	Dunford, R.W.; Gemmell, D.S.; Kanter, E.P.; Young, L.			$585,000
Phone	630-252-4052
E-mail	DUNFORD@ANLPHY.PHY.ANL.GOV

This program is focused primarily on atomic structure measurements with an emphasis on precision measurements useful in testing many-body relativistic atomic structure calculations. The experimental work is principally conducted at three accelerator facilities, ATLAS and BLASE (both located in the ANL Physics Division), and UNILAC/SIS at GSI, Darmstadt. Together these facilities provide access to essentially all charge states of all elements, ideal for the study of the atomic physics of highly-ionized atoms. In FY1996, the ATLAS-based heavy ion program completed a measurement of the spectral distribution of the two-photon decay in helium-like Kr. This work provides a test of the calculations of the transition probability for this decay and had never previously been measured although non-relativistic theoretical predictions exist. These measurements test our understanding of the entire structure of an ion since a sum over the complete set of intermediate states is required and both energy levels and wavefunctions must be understood. Also at ATLAS, a two-foil technique to measure ultrashort lifetimes in the 100 fs to 10 ps regime for highly-charged ions was investigated. In this technique, the first foil is used to excite the ion and the second foil is used to probe the excited state population. Measurements of the final charge state distribution as a function of foil separation provides an independent confirmation of power-law dependence of Rydberg-fed atomic transitions observed in beam-foil studies. The VUV spectroscopy program at ATLAS shifted emphasis from 2-electron systems to multielectron systems and studies of 3-,4- and 5-electron krypton are underway. The program to investigate collisions of fast highly-charged ions with C₆₀ found that the weak photon emission branch of the collisionally-excited system was masked by prominent photon emission from atomic C ions. Future efforts will include continued analysis of the uranium Lamb-shift data from GSI (complementary to work done on Ni and Kr at ATLAS) and development of a polarized beam of metastable (2s) ions of H-like argon at ATLAS.

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Physics Division

Synchrotron Radiation-Based Atomic Physics

Investigator(s)	Young, L.; Gemmell, D.S.; Kanter, E.P.; Krässig, B.; LeBrun, T.; Southworth, S.H.			$790,000
Phone	630-252-8878
E-mail	YOUNG@ANLPHY.PHY.ANL.GOV

Our research program in atomic, molecular and optical (AMO) physics using X-rays from synchrotron radiation sources has concentrated during FY1996 on preparations for AMO physics at the Argonne Advanced Photon Source (APS). The primary goal of the program is to increase the understanding of photon-atom interactions at high photon energies. Initial experiments will concentrate on two unique aspects of high-energy photon-atom interactions: the increasing importance of x-ray scattering processes relative to photoabsorption, which dominates at lower photon energies; and the coherent nature of excitation and decay processes involving resonance and threshold states of deep inner-shells. Characterization of x-ray scattering from a free atom will be useful in testing advanced scattering theories (second-order S-matrix) as well as the commonly used form-factor approximation in crystallography. A careful decomposition of the elastic and inelastic x-ray scattering channels will permit study of the effects of electron binding on electron momentum distributions deduced from Compton scattering, a widely-used tool in solid-state physics. Study of resonance and threshold phenomena using atomic x-ray fluorescence will yield a deeper understanding of this region where excitation and decay processes are strongly coupled. Preparations include design of a bent-crystal x-ray spectrometer, scattering cell, recoil-ion spectrometer and laser-excited targets. As part of on-going research activities, the group has published the first detailed study of non-dipole effects on the angular distribution of Ar 1s, and Kr 2s, 2p photoelectrons, covering electron energies up to 3000 eV; this demonstrated the current central-field calculations to be adequate for the estimation of non-dipole effects in regions away from resonance. The investigation of double ionization in photoabsorption and Compton scattering, studies done in collaboration with the University of Frankfurt and Kansas State University at ALS in Berkeley and ESRF in Grenoble, are of prime importance in understanding the role of electron correlations. In these studies a novel technique has been used, recoil ion momentum spectroscopy, which due to its large solid angle and unique capabilities, is expected to play a prominent role in the AMO program at the APS, and thus participation in these experiments lays the foundation for future studies.

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Chemical Sciences Division

Search for the Electric Dipole Moment of the Electron

Investigator(s)	Commins, E.D.			$120,000
Phone	510-642-2321
E-mail	commins@physics.berkeley.edu

An elementary particle can possess an electric dipole moment (EDM) only by virtue of an interaction that violates parity (P) and time reversal (T) invariance. The possible existence of an electron EDM (hereafter called d_e) is an issue of great current interest in connection with the unsolved problem of CP violation. Although the unmodified standard model of elementary particle interactions predicts a value of d_e far too small to be observed experimentally, a number of other plausible theoretical models of CP violation predict values of d_e within experimental range. In particular, there is great interest lately in grand unified supersymmetric models, which predict that d_e should be in the range 10^-27 to 10^-28 e cm. For the last few years we have been engaged in an experimental search for d_e. Our present published experimental upper limit on d_e is 4x10^-27 e cm[1]. This result was obtained using ²⁰⁵Tl atoms in an atomic beam magnetic resonance experiment employing separated oscillating fields, laser optical pumping for state selection and analysis, and a very intense electric field. Since arriving at this result we have been preparing a new and improved version of our experiment, which still makes use of the same general method, but has important new features as follows: 1) In addition to ²⁰⁵Tl atomic beams, we now have sodium atomic beams, generated in the same ovens and collimated by the same slits. The essential point here is that sodium atoms are susceptible to the same major systematic effects as are thallium atoms and are in fact more sensitive to them. However, sodium atoms would not exhibit a measurable EDM effect because the atomic number of sodium is Z=11 and the EDM effect increases in proportion to the cube of the atomic number Z. Thus sodium is of great value for comparison and calibration purposes. 2) A new electric field assembly has been constructed which permits simultaneous comparison of two beams of thallium (and at the same time two beams of sodium) in equal and opposite electric fields. This feature will sharply reduce a major contribution to noise that limited the precision of our latest published limit. In addition to these major features, a number of other significant improvements have been made to increase signal and reduce noise. We have now completed construction of the new apparatus, and have been testing it extensively during the last half-year or so. These tests yield very encouraging results: all our design goals have been met or surpassed. We hope to achieve an experimental sensitivity of 2x10^-28 e cm for d_e in the coming year.
[1] "Improved Experimental Limit on the Electric Dipole Moment of the Electron," Commins, E. D.; Ross, S. B.; DeMille, D.; Regan,B. C.; Phys. Rev. A 1994, 50, 2960.

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Chemical Sciences Division

High-Energy Atomic Physics

Investigator(s)	Gould, H.A.; Belkacem, A.			$470,000
Phone	510-486-7777
E-mail	GOULD@LBL.GOV

The goals of the High Energy Atomic Physics program are to: (1) Achieve an experimental and theoretical understanding of charge changing collisions at relativistic energies including electron capture from pair production and heavy particle capture from pair production; (2) Determine if there exists a small charge-parity violating permanent electric dipole moment (EDM) of the electron. Recent results include the: (1) Measurement of charge changing cross sections, including electron capture from pair production at 10.8 GeV/nucleon (2) Integrated calculation of capture from pair production, ionization, and excitation in relativistic ion-atom collisions using massively parallel computing; (3) Invention of the orthotropic source - a source that produces a highly collimated beam of neutral atoms with 100% efficiency. Present activities include: (1) Extending capture from pairs to particles heavier than electrons, by using bremsstrahlung photons produced at the LBNL Advanced Light Source; (2) Theoretical support for this activity; (3) Building a new cold atom fountain experiment with a sensitivity to an electron EDM of 10^-30 e-cm.

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Chemical Sciences Division

Atomic Physics

Investigator(s)	Prior, M.H.			$260,000
Phone	510-486-7838
E-mail	MHPrior@lbl.gov

This program conducts detailed studies of the structure and interactions of atomic systems to provide accurate and detailed descriptions of their behavior and to stimulate and challenge theoretical understanding. The program exploits the ability of two state-of-the art electron cyclotron resonance (ECR) ion sources at LBNL to produce intense, highly charged, continuous ion beams for the conduct of low energy ion-atom and ion-molecule collision studies. Current emphasis is upon determination of the complex amplitudes, and their scattering angle dependences, for excited substates produced in multiple electron transfer collisions, the production and properties of large, highly charged molecules, and momentum spectroscopic studies of the products of ion collisions with He atoms. The program benefits from collaborative efforts with colleagues from outside LBNL.

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Physics Department

Spectroscopy and Collision Studies with Highly Charged Ions Produced by Electron Beam Ion Traps

Investigator(s)	Schneider, D.; Beiersdorfer, P.; Marrs, R.			$200,000
Phone	510-423-5940
E-mail	schneider2@llnl.gov

The project focuses on the spectroscopy and interactions of highly charged ions produced in the LLNL Electron Beam Ion Traps (EBITs), which produce stationary ions up to fully stripped uranium. High-resolution, highly accurate measurements of transition energies of few-electron high-Z ions are performed to provide benchmarks for evaluating QED, nuclear, and relativistic correlation effects in high-Z systems. These include studies of the fine structure of few-electron uranium ions and of the hyperfine structure of hydrogenic ions such as Ho66+, which test the Bohr-Weisskopf effect and have resulted in revised values of the nuclear magnetic moment. Studies of electron interactions are performed to provide benchmarks for distorted-wave and close-coupling calculations. Current efforts investigate the magnetic sublevel populations generated by electron-impact excitation and dielectronic recombination processes that results in anisotropic line emission. The EBIT facilities also allow measurements of radiative lifetimes of highly charged ions in regimes inaccessible to traditional sources. We have developed techniques to measure very long-lived lifetimes in the regime 1 microsecond - 1 second, such as the 3.92-ms lifetime of the triplet level in heliumlike N5+, as well as ultrashort lifetimes <10 fs, including a 1.65-fs lifetime of a level in neonlike Cs45+.

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Physics Division

Accelerator Atomic Physics

Investigator(s)	Datz, S.; Krause, H.F.; Vane, C.R.			$825,000
Phone	423-574-4984
E-mail	datzs@ornl.gov

The project objective is to achieve a detailed understanding of the interactions of high-energy, multiply-charged ions with gas and solid targets, and with electrons. The ORNL facilities used for this research are the EN Tandem Accelerator and the Holifield Radioactive Ion Beam Facility (HRIBF). A unique feature of the EN Tandem Facility is the Elbek high resolution magnetic spectrograph. This is coupled with a new recoil momentum spectrometer to allow complete energy disposition studies. Using heavy ions of different Z but the same charge state, the Z dependence of the energy shift of the Binary Encounter peak energy has been studied. To determine the relative importance of electron-electron vs. electron-nucleus interactions in excitation and ionization, neutral and charged ions in collision with He gas are measured, as are the collision partners in coincidence. Electrons contained in a crystal channel can be quantitatively treated as a dense electron gas target. A swift ion passing through the channel can be excited by collisional excitation, by dielectronic processes, or by resonant coherent excitation in which the periodicity of the crystal lattice provides an oscillator, which can separately excite specific m states of the moving ions. The strong phase coherent electric fields that the projectile experiences inside the crystal can also be used to selectively cause constructive and destructive interferences. At CERN in Geneva, lead beams at energies of 33 TeV are used to study electron capture from the negative continuum and lepton pair production cross sections as a function of angle, lepton energy, and target Z. In collaboration with Swedish scientists, experiments have been performed at the Stockholm Heavy Ion Storage Ring to measure dissociative recombination between electrons and molecular ions, e.g., HeH⁺, H₃⁺.

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Physics Division

Collisions of Low-Energy Multiply Charged Ions

Investigator(s)	Meyer, F.W.; Havener, C.C.			$250,000
Phone	423-574-4705
E-mail	meyerfw@ornl.gov

In this activity, multicharged ion interactions with atoms, molecules, and surfaces are studied at the lowest attainable energies. At such energies, the stored electronic potential energy of the multicharged ions becomes an appreciable fraction of the total interaction energy, and inelastic collisions depend strongly on the detailed quasi-molecular potentials of the interacting systems. Emphasis is currently on merged-beam measurements of absolute electron-capture and ionization cross sections in the energy range from 0.01 to 1000 eV/amu, to provide benchmark data for the evaluation of theoretical approaches under development for this still poorly characterized energy regime, as well as to investigate low-collision-energy phenomena such as orbiting resonances and other cross section enhancements arising from trajectory effects. Experimental studies of the neutralization of multicharged ions during grazing interactions with metal, semiconductor, and insulator surfaces are also in progress. The current focus is on measurements of the angular and charge state distributions of scattered ions, as well as the characterization of the energy and angular distributions of ejected electrons, in order to better understand the detailed mechanisms by which the multicharged ions' potential energy is dissipated as the ions are neutralized at the surface.

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Physics Division

Theoretical Atomic Physics

Investigator(s)	Schultz, D.R.; Reinhold, C.O.; Strayer, M.R.			$325,000
Phone	423-576-9461
E-mail	schultzd@ornl.gov

The atomic theory program studies the dynamics of strongly perturbed atomic systems which requires the development of new physical models and new mathematical and computational techniques. The program is intimately related to state-of-the-art experimental programs at ORNL and other laboratories. Particular attention is focused on both fundamental and complex systems which play a role in many branches of energy research such as collisions of ions with atoms, solids, and surfaces, as well as atoms subject to high-intensity electromagnetic radiation. Because of the nonperturbative nature characterizing these interactions, and the nonseparability and high dimensionality of the equations describing them, this project is quite computationally intensive and requires the development of efficient numerical algorithms and their implementation on high-performance computers. These studies are interdisciplinary as the methods developed provide a link between atomic, solid state, plasma sciences, and chemical sciences. Specifically, the time-dependent dynamics of atomic systems is investigated using quantum mechanical, semiclassical, and classical approaches. Efforts to develop and apply fully quantum treatments involve direct numerical solution of the Schrodinger equation on a lattice, methods related to the time-dependent Hartree-Fock approach, and coupled channels techniques. Partial support is also provided through this program for a joint appointment of a Distinguished Scientist/Professor at ORNL and the University of Tennessee, Knoxville.

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Physics Division

EN Tandem Operations

Investigator(s)	Vane, C.R.			$250,000
Phone	423-574-4497
E-mail	vanecr@ornl.gov

The EN Tandem Van de Graaff is operated for atomic physics research. A wide variety of light ions and multiply-charged heavy ions are furnished by the EN Tandem at MeV energies for the accelerator atomic physics group and for outside users from other divisions of Oak Ridge National Laboratory (ORNL), universities, and industry. Neutral beams of 0.1 to 0.5 MeV/nucleon carbon and oxygen have recently been added. Terminal voltages up to 7 MV are routinely available and ion sources are sufficiently versatile to provide beams of all ions from protons through fluorine and silicon through chlorine, as well as beams of many heavier ions including nickel, iodine, gold, and uranium. A VAX-11/750 and Macintosh PC/CAMAC-based data acquisition systems, an Elbek magnetic spectrograph with position sensitive ion detectors, high-resolution electron spectrometers, Si(Li) X-ray detectors, and a curved crystal X-ray spectrometer are available to users. Recent major beam usage has included the channeling of carbon and nitrogen ions through thin crystals to examine resonant coherent excitation of one-electron ions, coincidence measurements of emitted electrons with recoiling target ions and various projectile charge states, and measurements of Auger electrons in coincidence with recoiling helium, neon, and argon atoms.

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