Atomic, Molecular, and Optical Physics (1986) / Chapter Skim
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4 Atomic Physics
4 Atomic Physics
Pages 53-87
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From page 53... ...
This research merges naturally into molecular physics; examples can be found in both this chapter and the next. ELEMENTARY ATOMIC PHYSICS Research in elementary atomic physics has flowered during the last decade.
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From page 54... ...
It is successful in describing nature over a range of lengths spanning 25 decades, from subnuclear dimensions, 10-~6 cm, to distances as large as 109 cm, where satellite measurements have verified the cubic power law falloff of the Earth's magnetic field M~nv theories are patterned after QED; its study has been one of the most rewarding pursuits of modern theoretical physics. Atomic physics provided the first experimental evidence for QED, and it continues to provide the most demanding tests of the theory.
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From page 55... ...
The method works equally well with positrons, and the equality of the electron and positron magnetic moments has been confirmed to 5 parts in 10~. The calculation of the electron magnetic moment anomaly to a precision comparable with the experimental precision is one of the most demanding tests of QED, and one of the most demanding calculations ever made in physics.
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From page 56... ...
One of the most compelling arguments for quantum electrodynamics (QED) was the discovery that the magnetic moment of the electron disagrees with the value predicted by Dirac's theory.
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From page 57... ...
One way out of the dilemma is to think of the Lamb shift as a probe of the proton, thereby testing hadronic physics; another way is to determine the proton structure by high-energy experiments and then combine the highenergy and atomic results to test QED. There is a third alternative: one can avoid all the complexities of hadronic interactions by studying pure leptonic atoms.
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From page 58... ...
The most precise value of the muon's basic properties its mass and magnetic moment- have come from measurements of the Zeeman erect of muonium (the electron-muon atom)
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From page 59... ...
Neutral-Current Parity Violations in Atomic Physics A key element of the theory that has now unified the electromagnetic with the weak interaction is the prediction that the weak neutral current interactions between electrons and nucleons should produce a parity violation in atoms. The result is that the photons emitted by atoms should "prefer" one circular polarization over the other by a small amount.
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From page 60... ...
Bell showed that correlations in measurements on particles whose initial state was highly correlated would have to lie below a given limit if quantum mechanics were incomplete but that the limit would be somewhat larger if the description by quantum mechanics were complete. The distinction between the two alternatives was presented as an inequality between observables: the completeness of quantum mechanics could be determined by an experimental study of Bell's inequalities.
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From page 61... ...
ATOMIC PHYSICS 61 The experiments involve studies of the correlation in the polarizations of photons successively emitted by a single atom in a cascade of fluorescent steps. The experiments are difficult, and the results were at first ambiguous.
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From page 62... ...
During the next decade physicists can look forward to detailed measurements of the Lamb shift and relativistic effects in positronium. These advances will put increasing pressure on QED theory.
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From page 63... ...
Loosely Bound Atomic States The advent of the laser and the development of better sources of negative ions have made it practical to study systems in which one electron is bound loosely. These systems are interesting because it is only near the atomic core that the motion is complex.
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From page 64... ...
This high precision permits the accurate study of spin-orbit, correlation, and relativistic terms that would be far too small to see in conventional scattering experiments. Negative ions represent a second type of loosely bound system.
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From page 65... ...
The experiments have stimulated interest in the structure of atoms in strong electric and magnetic fields, a subject that bears on problems in astrophysics, in general dynamics, and in the transition from ordered to chaotic motion. The techniques have also been applied to study collisions, photoionization, superradiance, and electrodynamics.
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From page 66... ...
In contrast to magnetic fields, which tend to compress electrons close to the nuclei, electric fields tend to tear the electrons out of atoms. One expects that a strong electric field will ionize an atom, but not that it will produce any sort of periodic structure.
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From page 67... ...
Thus, double-well potentials can serve as a "magnifying glass" for studying effects of angular momentum coupling, electron correlations, and relaxation effects. Collective Atomic States Our understanding of atomic structure has been dominated by single-particle pictures in which each electron moves independently in an effective potential that is due to the rest of the system.
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From page 68... ...
The opening of new spectral ranges by synchrotron light sources and multiphoton absorption of laser light, however, has revealed excited states in which two or more electrons share the excitation energy. These states display a surprisingly wide variation of spectral properties such as decay widths, absorption coefficients, and quantum defects.
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From page 69... ...
Experimentalists will soon be able to measure inner-shell energies of atoms and energies of highly stripped ions with such precision that quantities such as Lamb shifts in many-electron systems can be systematically determined. At present, calculation of these QED effects presents immense difficulties and can only be carried out for simple systems.
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From page 70... ...
The long-range nature of the Coulomb field results in a rich and orderly structure of the continuum, manifesting itself as resonances in scattering cross sections. The mechanisms for energy transfer and other state changes during a collision can often be deciphered by comparing the time for the collision (the resonance lifetime)
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From page 71... ...
Photoionization is of considerable theoretical interest in atomic and molecular physics because the calculations provide sensitive tests of our understanding of atomic and molecular structure. The upper drawing shows the Photoionization cross section of xenon as a function of wavelength.
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From page 72... ...
For example, the concept of core-excited resonances completely fails to account for structures that were recently observed near the excited states of helium and the alkali metals. The measurements were achieved with new high-resolution electron scattering and negative-ion photodetachment techniques that have revolutionized our ability to study narrow resonance structures.
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From page 73... ...
Dielectronic recombination can occur when an electron hits an ion with slightly less energy than needed to excite the ion. The electron is attracted to the ion until it gets enough kinetic energy to excite one of the valence electrons; at this point the electron becomes trapped in a large highly excited state (a Rydberg level)
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From page 74... ...
For four of the five, the experimental cross sections are substantially larger than predicted by theory. Because of the many systems that are affected by dielectronic recombination, including plasma fusion, there is high interest in understanding the source of the discrepancy.
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From page 75... ...
Collisions with Rydberg Atoms The experimental art of creating and detecting highly excited atoms (Rydberg atoms) has rapidly developed to the point where a wide range of precisely controlled conditions are realizable, including orbital shapes and matches between energy levels and level spacings.
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From page 76... ...
Without the approximate conservation of these new quantum numbers, radiation in the x-ray range could not be understood. The promotion predicts the appearance of energetic electrons and high-frequency x rays, highly anisotropic shapes, and anomalously large ionization cross sections.
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From page 77... ...
Approximate conservation laws are a recurring theme of atomic physics; whenever they are discovered they help to unify and systematize widely diverse data. Toward the Complete Scattering Experiment Recent advances in experimental technology, including positionsensitive detection, polarized beams, improvements in energy resolution, and fast electronics, have made possible a new range of measurements approaching the complete scattering experiments in which every possible quantum number is specified.
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From page 78... ...
Also included in this classification are studies of accelerator-produced beams of H-, high-charged ions and muonic atoms discussed elsewhere in this report. A fast beam of highly charged ions can collisionally generate systems whose electronic ionization energies and excitation levels are several kiloelectron volts.
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From page 79... ...
This is exactly opposite to the behavior under photoexcitation in which the relative orientation is fixed because of angular momentum conservation. Quantum Electrodynamics of Highly Charged Systems Precise tests of quantum electrodynamics have been made in the regime where particles interact weakly either with each other, as in
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From page 80... ...
Because of the strong Z dependence of the QED effects, the precise tests in loosely bound low-, atoms provide no quantitative information on the validity of QED in strongly bound high-, atoms. Thus independent tests of QED and the renormalization prescription for highly relativistic strongly bound electrons are necessary.
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From page 81... ...
It is now known that this mechanism is frequently eclipsed by inner-shell electron promotion mechanisms: the inner atomic orbitals evolve into molecular orbitals during the collision, from which they undergo transitions at near degeneracies to vacant orbitals whose ultimate evolution is to excited atomic levels. A relativistic treatment is essential, as well as a careful treatment of the many-body aspects of the problem.
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From page 82... ...
Metastable ions, such as the heliumlike neon ion, have been observed to capture an electron from a background gas and to radiate x rays and light. The captured electron goes into a highly excited orbit, thus forming a sort of population inversion in the final-state ions.
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From page 83... ...
The x-ray spectrum reflects the electronic structure of the crystalline medium. ATOMIC PHYSICS REQUIRING LARGER FACILITIES Two areas require access to larger facilities than are usually employed in AMO research: accelerator-based atomic physics and AMO physics with synchrotron light sources.
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From page 85... ...
Recent advances in the design of ion sources now make it possible to produce intense beams of highly charged ions that, in contrast to ions from a high-energy source, move very slowly. These sources provide important new opportunities for investigating atomiccollision processes and to carry out spectroscopy on multiply charged species.
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From page 86... ...
Molecular physics has advanced through studies of vibrational autoionization, shape resonances, and the breakdown of the singleparticle model due to strong vibronic coupling for inner-valence ionization. The picosecond time structure of synchrotron light is only
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From page 87... ...
Finally, the joint use of lasers and synchrotron light has just been initiated, giving access to photoelectron spectroscopy of excited states of atoms in a manifold of hitherto inaccessible levels.
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