Technical Highlights
- High-Resolution Spectroscopy for the Lighting Industry and Space
Astronomy. Our high-resolution Fourier Transform Spectrometer has become
fully operable in its vacuum tank with its new data acquisition system. The
three years of development invested in the FTS have begun to pay dividends as
we apply it to obtain atomic data for development of more efficient lighting,
for interpretation of astrophysical spectra, and as collaborations with outside
user-groups are starting.
As a first component of our lighting CRADA with the Electric Power Research
Institute, we have made extensive observations of the spectra of neutral and
singly-ionized dysprosium. Dysprosium is one of the rare earth elements added
to high-intensity discharge lamps to improve their visible output and color
rendering. In order to model such lamps accurate wavelengths, energy levels,
and oscillator strengths are needed.
We observed the dysprosium spectrum with a high-current hollow cathode lamp
from 330 nm to 1.2 µm. The interpretation of the spectra of
natural dysprosium is hindered by unresolved structure due to four naturally
occurring isotopes. To assist in the analysis we also recorded spectra using an
electrodeless discharge lamp containing the single isotope 162Dy.
Over 4000 newly measured spectral lines have been used to revise the energies
for 376 levels of Dy I and 228 levels of Dy II.
Accurate radiometric calibration of the dysprosium spectra was obtained by
using a tungsten strip lamp as a standard source to calibrate the response of
the FTS. Initial branching ratios have been obtained for some low-lying levels
of both Dy I and Dy II and have been found to be in good agreement
with values from the University of Wisconsin that were calibrated using
branching ratios of argon lines excited in the hollow-cathode lamp.
Spectral observations have been carried out with our 10.7 m
normal-incidence spectrograph for elements of interest for the interpretation
of spectra of chemically peculiar stars obtained with the Hubble Space
Telescope (HST). The over-abundance (factors of 104 or
105) of certain heavy elements in the atmospheres of such stars is a
major puzzle of stellar astrophysics. We have continued to provide rapid
response to requests for data from HST investigators. Responding to an
urgent need of accurate wavelengths for a Pb III line at 1553 Å
for individual isotopes of Pb, we obtained small (~20 mg) samples of
separated isotopes of 204Pb and 207Pb. We excited their
spectra in a hollow cathode lamp and measured the wavelengths. The results will
be used to substantiate a possible isotope anomaly for Pb in the chemically
peculiar star chi Lupi. New measurements for Bi I, II, and III in the VUV
have also been provided to our astronomical collaborators for determination of
the abundance of Bi in the star HR7775.
In completing our energy level analysis of Hg II, a very important species
in the star chi Lupi, we have discovered a large group of autoionizing-type
lines in the 80 nm to 110 nm region. We interpret these lines as
transitions from the 5d86s26p configuration which lies
almost entirely above the ionization limit. (G. Nave, U. Griesmann,
J. Reader, and C. Sansonetti)
- High-Precision Laser Spectroscopy. We measured Doppler-free
near-infrared lines in a neon positive column discharge to obtain transfer
standards for an absolute calibration for the measurement of the
He 1 1S - 2 1S two-photon transition.
This transition gives a strong test of two-electron QED (quantum
electrodynamics) contributions to the ground state energy of He. Our values for
the neon lines were determined with sub-MHz absolute accuracies and make only a
minor contribution to the overall error budget of the He experiment.
We have also made Doppler-free observations of the green and yellow lines of
natural mercury and 198Hg. This work was prompted by our ongoing
investigation of discrepancies between the best wavelength measurements by
Fabry-Perot interferometry and measurements with Fourier Transform
Spectroscopy. The Doppler-free laser results in combination with our study of
the pressure dependence of the mercury wavelengths should provide a sensitive
test of the ultimate accuracy of our FTS measurements. (C. Sansonetti and
D. Veza)
- Observation of Spectra of Highly Ionized Atoms. We have continued
our studies of spectra of highly ionized atoms for tokamak diagnostics, now
focused on ions that will appear in the cooler edge and divertor regions. We
investigated the spectrum of Mo5+ in a sliding-spark discharge on
our 10.7 m normal-incidence vacuum spectrograph and observed strong
transitions involving levels of high angular momentum, for example 7i-8k. These
transitions occur at visible wavelengths. We have determined a new set of
energy levels for this ion and have calculated a number of highly accurate
Ritz-type wavelength standards in the 20 nm region. (J. Reader)
- Critical Compilations of Atomic Transition Probabilities, Energy Levels
and Wavelengths. Our large one-volume compilation of Spectral Data for
Highly Ionized Atoms is in press; it includes wavelengths, energy-level
classifications, and transition probabilities for Ti through Cu, and Kr and Mo.
Our new compilations of both energy levels and wavelengths with classifications
for all spectra of Ar and Ga are completed and are being prepared for
publication. All new data will be included in the Atomic Spectra Database as
the work is published. Critically evaluated transition probabilities for all
atoms and ions from H to Li (Z=1 to 3) and Na to Si (Z=11 to 14)
have been compiled to replace NIST’s previous tables published in the 1960’s. For
the large majority of Na to Si transitions, the only available data come from
the sophisticated modern calculations of the Opacity Project, which, however,
contain important approximations such as ignoring relativistic effects. By
comparing these results with experimental data and other sophisticated theories
in the limited cases where they are available, a systematic method for
estimating the errors for different types of transitions has been developed.
The most serious discrepancies have been found for fluorine-like and neon-like
species. (J. Fuhr, D. Kelleher, W. Martin, A. Robey,
C. Sansonetti, J. Sugar, and W. Wiese)
- Enlarging the NIST Atomic Spectra Database on the World Wide Web. We
have greatly extended the atomic spectra database by editing and adding data
from earlier NIST compilations, non-NIST compilations, and selected recent
publications or unpublished material. A new code has been written for the
search engine and WWW interface of the "Atomic Spectra Database"
(ASD) in collaboration with staff from ECSED (840) and SRD. This
version 2.0 of the database will contain significantly more extensive
coverage of atomic and ionic transitions and energy levels. It should be
available on the WEB within a few months. ASD has data for about
950 spectra, with about 66,000 energy levels and 72,000 lines. E
nergy-level data are included for most spectra of H through Kr (Z=1-36),
Mo (Z=42), and for 65 spectra of the rare-earth elements La through Lu
(Z=57-71). Wavelengths of observed transitions are given for 99
elements. Energy-level classifications and transition probabilities are
tabulated for the lines of most spectra of H-Ni (Z=1-28). Comprehensive
observed wavelengths with classifications based on critically compiled level
data are available for some elements, including all spectra of Mg, Al, S, and
Sc. Wavelengths without level classifications are included for prominent lines
of up to the first five spectra of Cu-Es (Z=29-99), with selected
transition probabilities for the first two spectra. Several extensive data sets
from recent NIST compilations are being prepared for loading into the database.
Version 2.0 offers a comprehensive range of user-specified options and
selection criteria, each with its own default. (D. Kelleher, P. Mohr,
W. Martin, A. Musgrove, K. Olsen, and W. Wiese)
- Atomic Interactions and Collisions of Ultracold Trapped Atoms. Our
new comprehensive predictive models of atomic interaction parameters and
collision rates for alkali atomic species in optical and magnetic atom traps
and condensates explain the observed formation of dual condensates of both
ground hyperfine levels of 87Rb by the process of sympathetic
cooling. Sympathetic cooling offers the prospects of cooling other atomic
species by thermalizing collisions with a species such as 87Rb, thus
extending the range of atomic species which can be cooled to extremely low
temperatures. We have initiated other calculations to investigate the control
of collision rates and condensate properties using tunable external magnetic or
optical fields. We also predict the possibility of making a "molecule
laser," the analog of the "atom laser," a coherent matter wave
derived from a Bose-Einstein condensate source. Raman photoassociation pulses
should be capable of producing a coherent pulse of ultracold molecules from
binary pair collisions in a condensate. (F. Mies, C. Williams,
E. Tiesinga, and P. Julienne)
- Complex Quantum Nanostructures. We have developed multiband
effective mass models to calculate accurate spectral positions and oscillator
strengths of quantum dot quantum wells and InAs nanocrystallites with strong
valence-band/conduction-band mixing. These models, which are much easier to use
than ab initio models, can accurately describe complex nanostructures
and account for previously unexplained measurements of optical properties of
such nanostructures. An accurate theory for these optical properties must treat
electron-hole correlation and use realistic electronic state models that can be
applied from the macroscopic limit, through the mesoscopic regime, down almost
to the atomic limit. Our models work best for large structures, but also model
successfully the smallest nanostructures. (G. Bryant and
W. Jaskolski)
- Theory of Near-Field Optical Microscopy. We developed a theory for
imaging with transmission near-field optical microscopy (NSOM) and applied it
to accurately model experimental NSOM images of nanochannel glass arrays. In
order to develop NSOM as a practical metrological tool, it is critical to model
the entire imaging process and compare directly to experimental images. The
model identifies the contribution to the NSOM image from the near-field optical
excitation source, the coupling to the sample, and the collection optics. We
also modeled the near-field optical response of nanoscale structures and
anisotropic dielectrics to understand how information obtained from a
near-field probe differs from the information gained with a far-field probe.
(G. Bryant and P. Julienne)
- Fundamental Constants. In collaboration with B. Taylor (Lab.
Office), the 1997-1998 Least Squares Adjustment of the fundamental constants to
produce CODATA recommended values is expected to be completed in early 1998.
This adjustment will affect many more constants than previous adjustments. The
process has been highly automated by extensive use of Unix shell scripts to
carry out the needed tasks, such as producing Fortran code for the covariance
matrices with the computer algebra program MACSYMA or producing computer
generated tables in the final LaTeX document. In conjunction with this project,
we have produced a thorough review of the theory of the electron anomalous
magnetic moment, the muon anomalous magnetic moment, and the hyperfine
splitting in muonium. This theory is critical for the determination of the
fundamental constants from the corresponding measurements. Also, the f
undamental constants bibliographic database has been updated and the numerical
fundamental constants database is having its user interface completely
renovated. (P. Mohr)
- Electron-Impact Ionization Cross Sections. A new theory has been
developed for calculating electron-impact total ionization cross sections for
atoms and molecules. These basic cross section data are needed for modeling a
variety of environmental or technologically significant phenomena. The
Binary-Encounter-Bethe (BEB) model is very successful in producing reliable
total ionization cross sections and agrees very well with experimental data on
closed-shell molecules such as CF4 and C3F8.
Published ionization cross sections were put on a Physics Reference Data World
Wide Web page during the summer, and they are now available to researchers in
diverse fields, such as for plasma processing of semiconductors, radiation
effect modeling, and fusion plasmas in tokamaks. At the request of the Atomic
and Molecular Data Unit of the International Atomic Energy Agency in Vienna,
Austria, the BEB model will be used to calculate selected molecular ionization
cross sections for applications in magnetic fusion research. (Y.-K. Kim
and M. Ali)
- Characterization of the GEC-ICP RF Plasma Source. This new class of
high-density, low-pressure plasma sources is becoming increasingly important to
meet the demands of reducing the critical dimensions of etched structures in
the semiconductor industry. Unlike capacitively coupled discharges such as the
Gaseous Electronics Conference (GEC) Reference Cell, rf biasing of electrodes
in an inductively coupled plasma (ICP) provides control, independent of the
plasma production, of the ion energies involved in the etching process.
Time-resolved optical emission spectroscopy (OES), Langmuir probes and
electrical measurements were used to study the behavior of the sheath in an
argon discharge in this plasma source.
A new technique for the measurement of electron densities, the plasma
oscillation probe (POP), was developed and compared with Langmuir probe
measurements. A weak beam of electrons is injected into the plasma, exciting
electron plasma waves whose frequency is proportional to the square root of the
electron density. Reasonable agreement between the two measurement techniques
was obtained for all plasmas investigated at low pressures (<3 Pa). In
molecular gases at pressures above 3 Pa, the POP technique resulted in
electron densities that are higher than the Langmuir probe measurements.
(E. Benck, J. Roberts, A. Schwabedissen, and
K. Musiol)
- Improved Metastable Atom Lithography We have extended our
previous work in this area to investigate the influence of varying the key
atomic parameter metastable energy. Our previous work demonstrated that energy
stored in a metastable excited state of argon can be used to locally modify a
self-assembled monolayer surface. We exploited this fact to demonstrate a novel
method of lithographic patterning of silicon. We have now extended this work
from argon to helium, increasing the metastable energy by 75 %. As a
result, the efficiency of the process (number of atoms required to effect a
given change) was observed to increase by over an order of magnitude. With
further development, the technique may find application as a highly sensitive
two-dimensional detector of metastable atoms and as a new method of
microlithography. The work was done in collaboration with the Electron and
Optical Physics Division and Harvard University. (J. Gillaspy,
S. Rolston, and W. Phillips, with J. McClelland of
Div. 841)
- Mesoscopic Surface Structures Created with Highly Charged Ions. We
have quantified how the volume of ion-induced surface structures varies with
the charge state of a low-velocity highly charged ion beam. This is the first
time that such measurements have been performed under well-controlled
conditions of constant ion velocity and species. Figure 1 shows an example
of the sort of hillocks which a single highly charged ion induces with
virtually 100 % efficiency in an insulating aluminosilicate compound.
Above a possible threshold near 15 keV, the volume of these structures is
observed to increase nearly linearly with the potential energy stored in the
incident ion. At the highest potential energy studied (~100 keV), the
volume is over 150 nm3. (D. Parks, L. Ratliff, and
J. Gillaspy)
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Figure 1: Atomic force micrograph of mesoscopic structures induced in
mica by two Xe50+ ions. |
- New EBIT Technique to Measure Excited State Lifetimes of Highly Charged
Ions. By rapidly switching off the EBIT electron beam in mid-operation and
observing the temporal decay of the trapped ion fluorescence, we have been able
to measure lifetimes of excited levels within the ground terms of highly
charged Xe32+, Kr22+, and Ar13+. The lifetimes
range from 2 ms to 9 ms. The ions remain trapped during the
measurement because, without the electron beam on, the EBIT functions as a
3 T Penning trap. (F. Serpa and J. Gillaspy)
- Deep-UV Refractive Index Measurements. We have teamed with the
Optical Technology Division to make high-accuracy, deep-UV refractive-index
measurements of materials considered for use as the optical components of
photolithography steppers for future-generation integrated circuit (IC)
fabrication. This is part of a collaborative project with MIT Lincoln
Laboratory and SEMATECH. We have upgraded a precision refractometer to enable
minimum-deviation-angle, refractive-index measurements near 193 nm,
accurate to 1 part in 105, with a temperature control of
0.1 °C. We have measured the indices for various grades of fused silica
and calcium fluoride from the major suppliers and have characterized
significant grade and supplier differences. We have also determined the
dispersion and temperature dependence of these materials near 193 nm.
These measurements are needed for the design of the transmission optics for the
steppers for 0.18 m minimum-feature-size IC fabrication (1 Gbit
DRAM), which is scheduled (by the SIA roadmap) for production by the U.S.
semiconductor industry beginning in 2001. (J. Burnett and
J. Roberts)
- Plasma Radiation. We have carried out our first experiment with the
new NIST IR-vis-UV Fourier Transform Spectrometer (FTS) and measured branching
fractions in neutral Kr using a high-current hollow-cathode lamp. Combining
these with recent, very accurate lifetime data for Kr levels, we were able to
determine transition probabilities in Kr with uncertainties of a few percent.
Such measurements are important for tests of sophisticated atomic structure
calculations and they are needed for applications in industry, and fusion
research, where often the only way to obtain information about a plasma is
through analysis of the emitted radiation. (K. Dzierzega,
U. Griesmann, J. Bridges, and W. Wiese)
- Quantum Revivals in Optical Lattices. We have used a technique that
measures the stimulated redistribution of photons between laser beams forming a
one dimensional optical lattice to study coherent wave-packet motion of atoms
trapped in the lattice. We suddenly shift the positions of the lattice sites by
introducing a phase shift in one of the laser beams with an electro-optic
modulator. This sets all the atoms trapped in the lattice sites oscillating in
phase with one another. The amount of redistributed photons is proportional to
the acceleration felt by the wave packets in the wells. Figure 2 shows a
typical signal obtained with this method. After the motion decays away due to
dephasing caused by the anharmonic character of the wells, it revives at a
later time, a purely quantum phenomena. Using Monte Carlo simulation techniques
we can understand this revival, which in fact occurs at an unexpected time due
to the complex interplay between coherent and dissipative processes.
(G. Raithel, S. Rolston, and W. Phillips)
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Figure 2. Redistribution signal of cesium atoms trapped in a
1D optical lattice, showing a quantum revival in the motion at long times.
The solid line is the experimental result, and the dashed line is from a
quantum Monte Carlo simulation. |
- Suppression of Collisions in an Optical Lattice. If atoms are cooled
and trapped into the individual potential wells of an optical lattice, it is
reasonable to assume that they will no longer be able to collide with one
another, greatly suppressing the collision rate. We have tested such an
assumption by observing Penning ionizing collisions between metastable xenon
atoms trapped in a three dimensional optical lattice. In fact both enhancement
and suppression of Penning ionizing collisions due to the presence of the
periodic potential of the lattice has been observed. This correlates with how
well localized the atoms are - when there are many atoms traveling above the
potential barriers, collisions are enhanced, due to the restricted phase space
available in the potential. When they have cooled down into the individual
wells, the rate is suppressed, by as much as a factor of two. We can interpret
this result as a measure of the rate of atoms hopping from well to well.
(J. Lawall, C. Orzel, and S. Rolston)
- Optical Tweezers. We have used focused beams of laser light as
"optical tweezers" to grab onto and remotely manipulate microscopic
biological particles. In one application involving two optical tweezers, we
independently grab onto and collide two biological objects to quantitatively
study their adhesion. This novel assay allows us control over most external
parameters such as collision velocity, orientation, pH, etc. In recent
measurements, we have been studying the adhesion of influenza viruses
(covalently attached to glass spheres) to red blood cells, in the presence of
highly potent inhibitors, as a function of collision velocity. These
experiments are being done in collaboration with the Whitesides group at
Harvard. In collaboration with the Biotechnology Division at NIST, we have
used optical tweezers as a transducer in a biosensor to detect small quantities
of a free antigen (Fig. 3). This is accomplished by measuring the binding
force between an antigen and an antibody and detecting the reduction in this
binding force as free antigens take up binding sites on the antibodies. In a
third application, also with the Biotechnology division, we are using optical
tweezers to isolate individual mitochondria for PCR analysis, in order to
quantify the variation of the genetic information between mitochondria from a
single cell. (B. Davies, R. Kishore, W. Phillips, and K. Helmerson)
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Figure 3. Illustration of the optical tweezers-based immunosensor.
An antigen-coated microsphere is trapped and pulled away from an
antibody-coated surface with optical tweezers. The minimum force required to
break the antigen-antibody bond is measured. Free antigens are detected as a
reduction in this force due to their blocking of the antibody sites. |
- Photoassociation Spectroscopy. When two atoms collide in the
presence of a light field they can collectively absorb a photon and form a
bound, excited diatomic molecule. When ultracold atoms are used in this process
the energy resolution for the formation of specific states is very high and
such spectroscopy is a valuable tool for precision atomic and molecular physics
experiments. In this way we have been able to make a precise atomic lifetime
determination, observe retardation effects in the spectra, and measure the
low-energy scattering length that determines the very-low-energy elastic
scattering properties. We have continued this work by performing two-color
photoassociation spectroscopy in sodium. With one laser atoms are
photoassociated to a bound molecular state and with a second the molecules are
promoted to doubly-excited states of the molecule. In collaboration with a
group in Orsay, France, we are investigating a number of doubly-excited
long-range states of Na2 in order to increase our understanding of
such states and in an attempt to find evidence for predicted curve-crossings of
long-range molecular states with ion-pair potentials. (P. Lett, K. Jones, and
U. Volz)
- Precision Measurement of the Binding Energy of Light Nuclei.
Gamma-rays produced in the reaction n + 35Cl =
36Cl + γ'’s were measured in collaboration with
M. Dewey of Division 846, using the NIST-ILL GAMS4 crystal diffraction
spectrometer at the Institut Laue Langevin (ILL) in Grenoble, France. The
binding energy is 8.579 MeV and is obtained by measuring and summing
gamma-rays at 517 keV, 1951 keV, and 6111 keV. The accuracy of
these measurements appears to be ~0.5 × 10-6. This is
the first time that gamma-rays in the 6 MeV region (first order Bragg
angle equals ~0.3°) have been diffracted with sufficient intensity to make
measurements with an accuracy <1 × 10-6. We now
feel confident that other important high-energy gamma-rays can be measured with
an uncertainty below 1 × 10-6. (E. Kessler and
R. Deslattes)
- Limited New X-Ray Wavelength Table Available. Our efforts to produce
a new all-Z tabulation of x-ray wavelengths have led to the publication of a
highly useful all-Z compilation of strong transitions and absorption edges. At
the request of the crystallography community, we produced an "X-ray
Wavelengths" section for the next edition of The International Tables
for Crystallography. This publication describes our procedures, gives
tables for intense K and L x-ray lines and absorption edges
(Kα1, Kα2, Kβ1,
Kβ3, Lα1, Lα2,
Lβ1 lines, and the K, LI, LII,
LIII absorption edges), and
graphically shows regions of the periodic table in which the values are less
reliable. The approach that we have followed to produce this table combines a
sparse network of robust experimental measurements which are linked to the SI
with advanced relativistic calculations to achieve the needed comprehensive
elemental coverage. Some of the more robust measurements were made at NIST/NBS,
and the theoretical work proceeds through a collaboration involving the
Université Pierre et Marie Curie, Paris (Paul Indelicato) and Stockholm
University (Eva Lindroth). (R. Deslattes and E. Kessler)
- Progress in Standardization and Characterization of Si
Powder-Diffraction Reference Material. X-ray powder
diffraction is not only a widely used analytical technique, but one which has
evolved in recent years to become a major approach to crystal structure
determination. NIST is the world’s principal source of powder-diffraction
reference materials (SRMs) which are used as internal or external standards in
both routine and advanced structural investigations. However, the present
reserve of certified silicon powder-diffraction SRMs is inadequate for even
current demand and will not satisfy forseeable future needs. In response to
this situation, we have undertaken a major effort (in collaboration with
Materials Science and Engineering Laboratory) to produce and certify a new
generation of powder-diffraction reference materials. We have now completed and
tested an apparatus to certify the new SRM and have completed uniformity tests
on 30 kg of single-crystal silicon material which will be crushed to form
the new SRM. Powder-diffraction scans show good peak to background ratios,
symmetric profiles, and counting rates sufficient to determine the lattice
spacing of powdered silicon with a relative uncertainty of
1 × 10-6. (J.L. Staudenmann, L. Hudson,
A. Henins, R. Deslattes, and E. Kessler)
Figure 4. Two dimensional experimental x-ray scattering map of C/Ni
multilayer. The multilayer is 50 pairs of C and Ni on a fused silica substrate.
The experimental plot yields layer thicknesses of dC=2.33 nm
and dNi=1.7 nm, and an interfacial roughness of 0.40 nm.
- Thin film and Multilayer Deposition and Characterization. To enhance
the surface and interface characterization of substrates and depositions, we
have expanded our capabilities to include the measurement and modeling of the
diffuse scatter of x-rays. The multiaxis diffractometer has been modified to
perform fully automated sequential x-ray scatter measurements, and software has
been developed to process and present the reciprocal space analysis and two
dimensional intensity plots of these x-ray scatter data and models. (Fig. 4)
These recent upgrades provide the tools to separate material diffusion and
interface roughness when characterizing buried interfaces. The enhanced x-ray
characterizations along with the dual ion beam deposition apparatus which
produces thin films and highly regular multilayer devices have permitted us to
make valuable contributions in materials development, biological studies, and
reactor facility experiments. (J. Pedulla and S. Owens)
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