Atomic Physics Division
- The strategy of the Atomic Physics Division is to develop and apply
atomic physics research methods, and particulary the interaction between atoms
and electromagnetic fields, to achieve fundamental advances in measurement
science--some at the quantum limit--relevant to industry and the technical
community, and to produce and critically compile physical reference
data.
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GOAL: To determine
atomic properties and
investigate fundamental
quantum interactions |
Strategic Focus Are: |
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First |
Light-Matter Interactions and
Atom Optics - to advance the physics of
electromagnetic-matter interactions and to explore new applications for laser
cooled and trapped atoms, to study exotic states of matter, and to study
and control many-body quantum systems. |
Second |
Nanoscale and Quantum Metrology - to advance measurement science at the
atomic and nanometer scale, focusing on ultraprecise length-displacement measurements, x-ray and gamma-ray precision metrology, and nanooptics and
nanosystems modeling. |
Third |
Critically Evaluated Atomic Data - to
produce reference data on atomic structure, to critically compile
reference data for scientific and technological applications, and to
develop techniques to apply the data to further the understanding of
important plasma devices. |
to produce reference data on atomic structure, to critically compile reference
data for scientific and technological applications, and to develop techniques to apply the data to further the
understanding of important plasma devices.
INTENDED OUTCOME AND
BACKGROUND
The objective of this strategic element is to critically compile fundamental constants and atomic spectroscopy data
from the far infrared to the x-ray spectral regions. We disseminate these reference data on the Physics Laboratory website, produce high-quality data for
urgent scientific or technological needs,and resolve discrepancies in the body of the data. When reliable data do not exist for high-priority needs,
specific measurements or calculations are undertaken to produce them.
The NIST databases for atomic spectra and fundamental constants are recognized throughout the world. We regularly add new material to our flagship
database, the Atomic Spectra Database, which now contains data for 141,000 spectral lines and 77,000 energy levels. It experiences over 55,000 requests
for data each month. To assist in the diagnostics of a variety of plasmas, we added two new databases that contain benchmark data on plasma population
kinetics, i.e., properties of ionized gases. These databases provide researchers with the best available data on numerous plasma parameters, such as
mean ion charge state for a plasma under specific conditions. An online computational system for collisional-radiative modeling of hot plasmas under
diverse conditions was also added, developed with Lawrence Livermore National Laboratory.
Accomplishments
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Precision Wavelengths for New Telescopes
The Very Large Telescope No.1 is one of the largest of a new class of modern telescopes for ground-based astronomy. It is located at the European
Southern Observatory in Chile. One of its important missions is to observe spectra of stars and interstellar media at infrared wavelengths,
950 nm to 5500 nm. To do this it uses a major new infrared spectrometer, the Cryogenic High-Resolution Echelle Spectrograph (CRIRES).
The wavelength scale of this spectrometer is established by spectral lines from a thorium/argon hollow cathode lamp, similar to the platinum/neon
lamp used to calibrate spectrometers on the Hubble Space Telescope (HST). Unfortunately, the spectrum of the Th/Ar lamp has not been well studied in
the infrared and not enough accurate calibration lines are available.
To remedy this problem, we made precision measurements of Th/Ar lamps with our 2 m Fourier transform spectrometer. The wavelengths are accurate to
about 0.00004 nm. With these new measurements, CRIRES will be able to achieve its astronomical goals.
In related work for the Hubble Space Telescope, we made observations of Pt/ Ne lamps similar to those to be used to calibrate a new spectrograph to be
installed on HST in 2008, the Cosmic Origins Spectrograph (COS). Since the lamps will be used much more intensively on COS than on earlier space
spectrographs, there was concern as to whether they would last for the whole mission.
We performed accelerated aging tests on lamps from the same production run as the COS lamps by running them on an interval timer to simulate their use
in space. After each three hundred hours of aging, we used our 10 m vacuum ultraviolet spectrometer to quantitatively measure the spectral output of
each lamp. Each lamp was run until it failed (about 1000 h). Although the aging is continuing for some lamps, results to date indicate that the lamps
will perform as needed for COS.
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Critical Data for Fusion Energy Science: Ionization Energies for Tungsten
Construction of the International Thermonuclear Experimental Reactor (ITER) will soon start in France. ITER is expected to generate fusion power for
periods up to 1000 s. It will be the most expensive science project ever undertaken. An important part of ITER is the divertor, a region of the vessel
that exhausts the flow of energy from charged particles and removes helium and other impurities. Tiles of the divertor will be made of tungsten, a
material with very high melting point.
Although the tungsten tiles are able to withstand the high temperatures in ITER, atoms of tungsten will be sputtered into the active gases.
To understand the complex processes taking place in these gases, it is important to determine the populations of the various ions of tungsten in the
gas. For this it is necessary to have reliable values for the ionization energies of tungsten ions. The ionization energy is the amount of energy
required to eject an electron from a given ion so that it is transformed to the next higher ion.
Previously, only values for the ionization energies of tungsten from rough theoretical calculations were available. We developed a method to determine
accurate values for all tungsten ions—from neutral tungsten through almost fully stripped tungsten, W73+. The method is based on scaling
results of theoretical calculations according to experimental data. Uncertainties vary from 1.7 % for W2+ to 0.0008 % for W73+,
with typical values about 0.1 %. The results are published in Atomic Data and Nuclear Data Tables.
Critical Data for Fusion Energy Science: Spectra of Highly Ionized Tungsten
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Figure 9. Observed spectrum and theoretical modeling of x-ray spectra of tungsten. The labels Cu, ni, and Co refer to the isoelectronic
sequences of the stages of ionization of the lines (Cu: W45+; ni: W46+; Co: W47+). The letters 6f, 5f, etc. refer to
upper electronic configurations of W46+. The line at 0.79 nm is blend of two transitions forbidden to electric-dipole radiation; one is an
electric-quadrupole transition, the other a magnetic-octopole transition.
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In order to help meet the need for spectral data of wall materials in the divertor region of ITER, we excited spectra of tungsten ions with our
Electron Beam Ion Trap (EBIT). Spectra in the x-ray region were measured with a microcalorimeter, a new type of spectrometer that detects single
photons and measures the rise in temperature to deduce the energy deposited—and hence the wavelength. At longer wavelengths, a grazing-incidence
spectrometer was used.
A number of new spectral lines were identified. The observed spectra were interpreted by means of collisional-radiative modeling of the ionized gas in
EBIT. Excellent agreement with the observed spectra was obtained. Fig. 9 shows a spectrum from the microcalorimeter together with a spectrum predicted
by modeling calculations.
According to the calculations, an important strong line at 0.79 nm is actually a blend of two forbidden-type transitions. We showed that electron
densities in plasma devices like ITER could be determined by measuring the ratio of the intensities of these two lines. The results are reported in
Physical Review A and the Journal of Physics B.
Redefinition of the International System of Units (SI)
In an effort to improve the International System of Units (SI) to overcome deficiencies, the Fundamental Constants Data Center has published a number
of articles describing potential new definitions of the kilogram, ampere, kelvin, and mole, pointing out the merits of these definitions based on
prescribed values of the Planck constant, the elementary charge, the Boltzmann constant, and the Avogadro constant.
The possible new definitions would have advantages including providing a stable, precise, and universal measurement system. With fundamental
constant-based definitions of the SI units in place, the values of many of the fundamental physical constants, which are presently determined by
experiment and theory, would have exact values, and the uncertainties of many other fundamental constants would be significantly reduced.
The Fundamental Constants Data Center has worked with relevant organizations to promote the changes in the SI. This includes the Consultative Committee
for Units (CCU), the Committee on Data for Science and Technology (CODATA), and the International Union of Pure and Applied Physics (IUPAP).
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
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