Atomic Physics Division

Technical Highlights

• Observation of Spectra of Importance for Tokamak Diagnostics. We have continued our collaboration with the University of Texas to observe spectra of highly ionized atoms needed for the diagnostics of tokamak plasmas. By injecting molybdenum atoms into the TEXT tokamak by laser ablation, we observed "forbidden" (electric quadrupole) transitions of Mo¹⁴⁺ and Mo¹⁵⁺. The lines occur at about 5 nm. Their upper levels have relatively long lifetimes - about 10⁷ sec. However, the electron density in the tokamak is so low that the levels radiate before being depleted by collisions, and the resulting lines are in fact among the strongest in that part of the spectrum. The intensities of the lines, being sensitive to the plasma density, are useful for density diagnostics. (J. Reader and J. Sugar)

• High Precision Measurements of the Lithium Resonance Lines. We have made highly accurate observations of the resonance lines of both the ⁶Li and ⁷Li isotopes using Doppler-free frequency modulation laser spectroscopy. The lithium atoms were excited in a low density atomic beam under field- and collision-free conditions. Our results, which have an uncertainty of about 1.2 parts in 10⁹, are more accurate than previous measurements of these transitions by a factor of almost 150. The results also provide a precise new value for the isotope shift of the lithium resonance lines. These experimental data provide a precise benchmark by which the atomic theory of 3-electron systems can be tested. (C. Sansonetti and B. Richou)
• High Resolution Measurements of Heavy-Element Spectra for Space Astronomy. Spectral analyses have been completed for elements of interest for interpretation of spectra of chemically peculiar stars obtained with the Hubble Space Telescope. The over-abundance of certain heavy elements (factors of 10⁴ or 10⁵) in the atmospheres of such stars is a major puzzle of stellar physics. As it was suspected that some lines observed in the peculiar A star Chi Lupi might belong to the rare earth ion Dy III, we began an analysis of this spectrum. No energy levels or identified lines had been known. We have found energy levels belonging to the 4f¹⁰ ground configuration as well as three excited configurations. From these results, 56 lines in the stellar spectrum were identified as Dy III by astronomers. Continued progress was also made for the similarly important ions Zr III and Hg III. (J. Reader, C. Sansonetti, and J. Sugar)
• Development of the Mercury Pencil Lamp as a Convenient Source of Wavelength and Radiometric Calibrations. As part of a CRADA with Oriel Instruments we have used a Fourier Transform Spectrometer to determine standard wavelengths for Hg pencil lamps. These lamps are used in large numbers for routine calibrations throughout industry and science. However, accurate measurements of their wavelengths had never been made. Our FTS work provides values accurate to 0.0001 nm for 19 lines in the 253 nm to 579 nm region.
  
  We also evaluated the Hg pencil lamp as a source of intensity calibration of spectral systems by comparing the lamps with NIST-calibrated continuum standards. Six lamps were studied under a variety of conditions to assess lamp-to-lamp variations and stability of the irradiances of individual lines with age of lamp and ambient temperature. Irradiances of 37 lines were measured. For the group of eleven most prominent lines we found that the absolute irradiances of a typical lamp could be specified to an uncertainty of about ±15 %. The ratios of the line irradiances, being more stable, could be specified to an uncertainty of about ±10 %. Hg pencil lamps can thus be used as a convenient source for both wavelength and irradiance calibrations. (J. Reader, C. Sansonetti, and J.M. Bridges)
• Installation of a Powerful Fourier Transform Spectrometer. NIST has completed the acquisition of a very high resolution Fourier Transform Spectrometer (FTS) built by Los Alamos National Laboratory. The FTS, shown schematically in Figure 1, has been moved to NIST and installed in a newly renovated laboratory designed especially to house it. It has been reassembled and all of its basic optical, servo control, and data acquisition systems are operable. Test spectra of white light and mercury emission sources have been acquired and are being used to diagnose and eliminate problems in the instrument. When fully operational the FTS will cover the range from 200 nm to 18.5 µm with a resolution of 0.0025 cm^-1 and a signal to noise ratio of 10⁵, and thus constitute an instrument equal to or better than any existing FTS.

  

  Figure 1. Schematic diagram of the Fourier Transform Spectrometer.

  Initial applications of the FTS will include studies of rare earth spectra of interest for high efficiency lighting and for interpretation of the spectra of chemically peculiar stars. We are entering into a CRADA with Osram Sylvania to analyze spectra and determine branching ratios for atoms and ions important for lighting applications. These data are needed for accurate modelling of lamp discharges. Work on spectra of astrophysical interest will extend current work supported by NASA. (G. Nave and C. Sansonetti)
• World Wide Web (WWW) Server for the Physics Lab. We have developed and installed a WWW Mosaic server for the Physics Laboratory to provide information and data to the public over the internet. The server has been accessible to the public since June 20, 1994, and since then more than 2100 distinct computers have made over 15,000 accesses to documents. The rate of use is steadily increasing, and the current rate is over 40 new users per day and over 200 accesses to documents per day. The Physics Laboratory information accessible through WWW includes the following parts:

  Staff and Organization

  Technical Activities

  Physical Reference Data

  Major Research Facilities

  Research Opportunities

  Publications

  Calibration Services*

  Keyword Index*

  Seminar Schedule*

  Time and Frequency Standards*

  Standard Reference Materials*

  (Those marked * are planned entries.) According to access statistics, the Physical Reference Data part is by far the most often accessed section. (P. Mohr)
• Atomic Spectroscopic Databases. Late 1994 we released a large atomic database for dissemination via the Internet (beta version, accessible on the World Wide Web with a Mosaic browser). This database includes many of the existing critically evaluated data on wavelengths, energy levels, and transition probabilities that are reasonably up-to-date. A separate Database for Atomic spectroscopy was completed in 1994, which will run on PCs as well as UNIX based work stations. It contains energy levels, wavelengths and transition probabilities for most ionization stages of the elements hydrogen through nickel. The database includes 27,000 lines, from about 1 through 200,000 angstroms. We also completed work for a major publication "Atomic Transition Probabilities of Carbon, Nitrogen and Oxygen, A Critical Data Compilation." This book will contain critically evaluated numerical data for about 13000 transitions. A bibliographic database on atomic transition probabilities, which contains approximately 2500 references, is now also accessible through Mosaic using the WWW network mentioned above. Another bibliographic database on atomic spectral line broadening parameters has been completed in collaboration with A. Lesage of Meudon Observatory, France. A prototype database is also electronically accessible through Internet. (G. Dalton, J. Fuhr, D. Kelleher, A. Kramida, W. Martin, A. Musgrove, and W. Wiese)
• New Theory for Electron-Impact Ionization Cross Sections of Polyatomic Molecules. A new theory for electron-impact ionization cross sections of atoms (called binary-encounter-Bethe, or BEB model), which was developed in collaboration with M.E. Rudd (U. Nebraska), has been extended to complex molecules. The BEB model was found to provide reliable cross sections (better than 15 percent from threshold to several keV in incident energy) for a variety of polyatomic molecules, using minimal input data readily available from public domain quantum chemistry codes. This new theory will provide a simple, yet powerful, tool for estimating ionization cross sections critical to the modeling of diverse processes such as air pollution, microchip manufacturing, and divertor heat-load in tokamaks. The BEB cross section for SF₆ is compared with available experimental data in Figure 2. At present, this is the only nonempirical theory in the world that provides useful ionization cross sections for polyatomic molecules. (Y.-K. Kim and W. Hwang)



Figure 2. Comparison of the BEB model and experiment. The solid curve is the theoretical result, and the broken curves are contributions from different molecular orbitals.

• Accurate Theoretical Transition Probabilities for Atoms. Extremely accurate transition probabilities (≤ ±2 %) for small atoms (Be and C) have been calculated to serve as standards for experiments and other theories. This effort is being expanded to the second-row atoms (Na through Ar) using both relativistic and nonrelativistic formalisms. Although the expected accuracy for the theoretical data on second-row atoms will not be as good as that for small atoms, our final data will still be among the most accurate ones available. Work is in progress at present on the Be, B, C, Na, and Al sequences. The development of a new, flexible and powerful relativistic wave function code, which takes advantage of the new generation of parallel computers, is underway as part of the HPCC program at NIST. Also, we have entered a CRADA with IBM to share software and algorithms for our new wavefunction codes. (Y.-K. Kim, A. Weiss, H. Saha, and J. Biero)
• UV Source Radiometry for Semiconductor Lithography. Maintaining the correct UV exposure, or dose, is critical for high throughput production in the semiconductor industry. In a collaborative project funded by SEMATECH, the Atomic Physics and Radiometric Physics divisions have designed, built and delivered two spectroradiometers for monitoring UV dose in a commercial lithographic manufacturing tool. These tools operate in the deep UV region, near 250 nm, where there were no suitable commercial instruments available for exposure monitoring. As with most instruments for measuring UV radiation, some change in response was experienced for our radiometers due to degradation of the light collector from strong irradiation. In order to get a more stable response, we tested several alternate materials for the light collector. Aluminum oxide was selected for its good optical properties and resistance to aging. The spectroradiometers were calibrated for absolute spectral response based on standard UV light sources at NIST. The calibration was effected by first setting up and calibrating a highflux source, which was then used to calibrate the spectroradiometer response. This calibration was checked by additional measurements using a standard detector, and good agreement was obtained. (J.M. Bridges and J. Roberts)
• Development of a New Infrared Source. The infrared (IR) is a spectral region of increasing technological activity, e.g., the monitoring of earth resources and environment, and defense applications such as surveillance. The accuracy of IR measurements is usually limited by inadequate signal-to-noise ratios. For many applications, such as testing and calibrating detectors or detection systems, a stronger source than those presently available is essential. The Atomic Physics and Radiometric Physics divisions are collaborating on a project to develop and characterize a brighter IR source. This is a wall-stabilized argon arc, similar to one previously developed at NIST for use in the ultraviolet. On a comparator for calibrating IR detectors, signals obtained from the arc source were up to 17 times greater than those from a typical source used in this spectral region. A calibration of the arc radiance was carried out from 2 µm to 10 µm by comparing its signals with those from a 1100 K blackbody. In this spectral region, the radiance of the arc was comparable to that of blackbodies ranging from 5000 K to 10,000 K. A high-resolution spectral scan from 1 µm to 3 µm shows many strong atomic lines, which also could be useful for wavelength calibrations. (J.M. Bridges).
• Branching Ratios and Transition Probabilities. Accurate measurements (≈ ±10 %) of transition probabilities in oxygen, nitrogen and carbon, which have recently been done by us and are now being extended to neon, are providing some key data for our data compilations. Relative transition probabilities are measured by observing lines emitted by a wall stabilized arc operating in helium with a small admixture of neon. These data have recently become of interest in fusion research for the blanketing of divertor regions with noble gases. Furthermore, advanced calculations of transition probabilities of neon have now become available. Our improved data will allow testing and guidance toward further improvement of these calculations. Thus, the determination of accurate experimental data for some typical atomic transitions can have great leverage in guiding the calculations for many more transitions. (U. Griesmann and W. Wiese)
• Characterization of the GEC rf Reference Cell. We have observed axial spatially-resolved optical emission measurements from a pure argon rf discharge. These time-averaged spectroscopic measurements showed that spectral lines emitted from higher upper energy states appear closer to the powered electrode. A relatively simple explanation of this apparent inverted distribution of the emission, which has puzzled observers of similar discharges since the 1930s, has been hypothesized. The explanation results from the fact that the electron energy distribution function changes with the distance from the powered electrode. As a result there are fewer and fewer fast electrons further away from the powered electrode due to their collisions with the gas particles, and therefore, the maximum of emission from higher excitation energy levels is closer to the powered electrode.

  We have also developed a new method to determine the electron density N_e and electron temperature T_e in a helium discharge by applying the non-intrusive laser-collision induced fluorescence method. Using spectral line intensities from collisionally redistributed populations, we obtained N_e and T_e from the solution of the set of rate equations.

  A new inductively-coupled rf-powered version of the GEC rf reference cell has been installed in our laboratory next to the original reference cell. (K. Dzierzega, E. Benck, and J. Roberts)
• Electron Beam Ion Trap (EBIT). Data on highly charged matter, previously only accessible at large accelerator, laser, and fusion confinement facilities, can now be obtained in-house with our EBIT source for extended periods of well-controlled conditions appropriate for obtaining accurate results. By using multiple view-ports into the trap core, several independent experiments are underway concurrently. During the first full year of operation, data have been produced to determine (a) precise x-ray wavelengths, (b) to determine the dependence of x-ray polarization on electron impact energy, and (c) to determine the spectral location and behavior of unusual dielectronic recombination processes. Also obtained was an experimental confirmation of a predicted set of unusual electric-dipole-forbidden M1 transitions that radiate in the visible region of the spectrum through an extended sequence of isoelectronic ions (Figure 3). This work opens up the prospect of further spectroscopic activity for applications to the diagnostics of Tokamak fusion plasmas as well as for fundamental studies. Design and construction work for an external beamline to extract and transport EBIT ions is underway so that a new range of experiments involving ion-surface interactions will be possible (see "Future Directions"). (J. Gillaspy, E. Bell, C. Morgan, A. Pikin, L. Ratliff, and G. Serpa)



Figure 3. The first successful observation of this and other forbidden (M1) lines with the NIST EBIT has moved highly charged trapped ion spectroscopy into the visible and near-uv range of the spectrum. The insert shows detail of Ba³⁴⁺ spectrum on a 10x expanded horizontal scale.

• Record Low Temperature. Last year we confined and cooled atoms in an optical lattice, a regular and stable 3-D array of potential wells for atoms formed by the interference of several laser beams (see figure on cover page of this division). Using the lattice we have cooled cesium atoms to 1.2 µK, about half the temperature in previous laser cooling experiments. By reducing the laser intensity we adiabatically reduce the potential well depth, allowing the trapped atoms to expand and cool. In this way we reached 700 nK, the lowest kinetic temperature ever measured for a 3D sample of atoms (Figure 4). The results are in good agreement with a band structure model treating the atoms in a manner analogous to electrons in a periodic crystal lattice. We hope to apply this cooling technique in the atomic fountain clock that is now under construction. (W. Phillips, P. Jessen, A. Kastberg, S. Rolston, and R. Spreeuw)



Figure 4. Adiabatic cooling Cs in an optical lattice: the left-hand figure shows the fluorescence as a function of time as an adiabatically cooled sample of Cs atoms falls through a probe laser beam. The width of the distribution corresponds to a temperature of 700 nK. The right-hand figure shows the 3-D temperature as a function of time during the adiabatic cooling. The dashed curve represents the intensity of the lattice laser beams (in arbitrary units).

• Photoassociative Spectroscopy of Na. Colliding laser cooled atoms can be selectively laser excited to specific bound molecular states. The resolution of the freebound photoassociative spectroscopy is very high because the kinetic energy of the initially free atoms is so low. In the past two years we have demonstrated this kind of spectroscopy and seen molecular states normally inaccessible by other means. During the past year we have extended these studies by using a second color to excite the molecules to more highly excited, possibly autoionizing states, or to the continuum, as well as back to the ground state. In this way we have observed the lowest lying vibrational levels of the O_g- purely long-range state (inner and outer turning points larger than 60 atomic units). Also, with the Molecular Physics Division, we have performed a detailed comparison of resolved hyperfine and rotational structure on forbidden transitions. This leads to detailed information about the ground state potential, information important for Bose-Einstein condensation. (P. Lett, S. Maleki, W. Phillips, S. Rolston, and M. Wagshul)
• Optical Shielding of Collisions. Although collisions between laser cooled atoms present an interesting and unique regime for fundamental studies, they frequently result in the ejection of atoms from a trap, and therefore limit the achievable atomic density. By illuminating atoms with a laser tuned blue of the free atom resonance transition we can couple the colliding atoms to a repulsive interatomic potential that prevents the collision. Using this technique with metastable xenon, we have reduced Penning ionization collisions by as much as a factor of 30. (S. Rolston, M. Hoogerland, C. Orzel, and M. Walhout)
• Optical Tweezers. A focused laser beam can trap a dielectric particle at its focus much in the way that atoms are trapped by a near-resonant, focused beam. We use such optical tweezers to manipulate micrometer-size dielectric spheres. With such spheres sparsely coated with biological molecules, such as an antigen, we have measured the force between a single one of these molecules and a substrate coated with the corresponding antibody. This work, done in collaboration with the Biotechnology Division, should allow improved design of biosensors and the development of techniques for ultrasensitive assay of tiny samples of biochemicals. (K. Helmerson and R. Kishore)

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