TECHNICAL ACTIVITIES 1998 - NISTIR 6268

CURRENT DIRECTIONS

• Generation of Atomic Reference Data. We are producing atomic structure and collision data through innovative theoretical and experimental approaches, concentrating on neutral and low-ionization spectra as well as on highly ionized atoms of scientific and technological interest.

  On the theoretical side, we have developed sophisticated atomic structure codes and are calculating very accurate transition probabilities (≈1 %) for light atoms (atomic numbers Z < 20) so that these theoretical results may serve as benchmarks for experiments and other theories.

  A theory developed by us for the electron-impact ionization of atoms provides reliable cross sections for polyatomic molecules as well, including those used in microchip etching containing fluorine and chlorine atoms.

  On the experimental side, with our computer-automated electron beam ion trap (EBIT) we can now directly measure lifetimes of highly charged ions. A metal-vapor vacuum arc injection source for the EBIT has broadened the range of elements (presently up to Z = 83) and charge states (Q > 70+) that we are studying.

  Our ultra-high resolution Fourier Transform Spectrometer has a greatly improved data acquisition system, and we are recording and analyzing complex spectra of selected heavy elements. We are also measuring numerous transition probabilities for lines of neutral and singly ionized atoms.

• Data Compilations. Data centers on atomic spectroscopy located in the Division are the principal resources for spectroscopic reference data in the world community. We are continuing critical evaluations and compilations of wavelengths (including an all-Z tabulation of x-ray wavelengths), atomic energy levels, and transition probabilities. The data assessment greatly benefits from our own atomic data research discussed above. We have put a large part of our comprehensive data tabulations on the World Wide Web and will update and expand this coverage.

• Ion-Surface Interactions. With a special EBIT ion interaction beamline we are investigating the atomic scale interaction of very highly charged ions (Q >> 30+) with surfaces. The work is proceeding by a combination of (a) observations with x-ray and electron spectrometers during surface exposure to the ions, and (b) with a new ultra-high vacuum in-situ scanning tunneling/atomic force microscope and a sample preparation chamber on the EBIT beamline. We are also using a new detector to observe particle induced x-ray emission from silicon surfaces bombarded with highly charged ions from EBIT. Also, we have demonstrated an approach to projection lithography using ions with extremely high charge states and are now studying this technique in detail.

• Properties of Nanoscale Systems. Our expertise in atomic systems is being extended to include properties of nanoscale systems. One direction is to develop and apply quantum-mechanical methods for calculating the electronic states and optical properties of quantum dots, wires, and wells. Such systems have a wide variety of technological applications, including semiconductor lasers and advanced semiconductor devices. Another direction is the modeling of images produced by scanning near field optical microscopy, which offers the prospect for nanometer-scale optical metrology.

• Physics of Cold, Trapped Gases of Neutral Atoms. We are investigating, both experimentally and theoretically, the properties of cold dense gases in the quantum degenerate regime. We are using atom optics techniques to study the properties of Bose-Einstein condensates, and are developing such devices as an atom laser. The interactions between atoms strongly influence the properties of the condensate. The effects of such interactions are being theoretically modeled and tested against the experimental data. The nonlinear Schrödinger equation is used to predict properties of matter waves produced from a condensate.

• Ion-Surface Interactions. With a special EBIT ion interaction beamline we are investigating the atomic scale interaction of very highly charged ions (Q >> 30+) with surfaces. The work makes use of a new ultra-high vacuum, in-situ scanning tunneling/atomic force microscope and a sample preparation chamber on the EBIT beamline. We are also using a new detector to observe particle induced X-ray emission from silicon surfaces bombarded with highly charged ions from EBIT. Also, we have demonstrated an approach to projection lithography using ions with extremely high charge states and are now studying this technique in detail.

• Plasma Measurements. The Division uses nonintrusive optical emission measurement techniques to determine the properties of radio frequency (rf) inductively coupled and arc discharge plasmas. We are engaged in the detailed time- and space-resolved analysis of rf plasmas used in production-line plasma etching, and are also applying our experience in plasma work to develop and improve plasma radiometric source standards for the vacuum ultraviolet region.

• Optical Manipulation of Neutral Particles. We study the physics of laser cooling, electromagnetic trapping, and other radiative manipulation of neutral atoms and dielectric particles. We are using these fundamental studies to develop new kinds of physics measurements in areas such as high-resolution spectroscopy, atomic collisions, and atom optics aiming, for example, at improving atomic clocks.

• Optical Manipulation of Biological Objects. We are developing techniques for the remote manipulation of biological objects. We use laser trapping to manipulate cells, chromosomes and microspheres in order to study bio-molecular interactions. Applications of these investigations include fundamental studies of adhesion processes in biological systems, development of highly effective inhibitors of adhesion, and very sensitive biosensors.

• Precision Crystal, X-ray, and Gamma-ray Measurements. This work has three components: Optically-based measurements of a silicon lattice period (XROI), inter-crystal comparisons (delta-d), and Bragg-Laue diffraction studies using two-crystal instruments here and at the high flux reactor in Grenoble, France. XROI measurements are part of the new sub-atomic displacement measurement project. Delta-d measurements are used to determine the lattice spacings of crystal samples used for x- and γ-ray diffraction, and to compare crystal samples from the standards laboratories that are making absolute lattice spacing measurements [Germany, Italy, Japan]. They also are used to characterize the starting material for the Avogadro project and the next generation of Si powder diffraction Standard Reference Materials (SRM's). The joint NIST-ILL GAMS4 precision double crystal spectrometer at the Institut Laue-Langevin (ILL) measures γ-rays leading to new, accurate determinations of the neutron mass, the molar Planck constant, N_Ah, and atomic mass differences. At Gaithersburg, measurements to complement the x-ray wavelength tables project are made, using a second two-crystal transmission spectrometer (high-Z region) and a vacuum spectrometer (energy region below 15 keV).

• High-Resolution X-Ray Probes of Geometrical and Electronic Structures. We have developed a well equipped high-resolution x-ray toolset for the study of electronic and geometrical structure of materials ranging from the most perfect silicon monocrystals, through epitaxial layers, thin films and multilayer structures, to surfaces and sub-surface damage in technical artifacts. Electronic structure questions are addressed by diverse spectroscopic techniques while the geometrical aspects are probed by several diffraction methods. Both areas are supported by high-performance dual-ion beam deposition capability. In this work we are attempting to understand structure-function relationships in the spirit of modern chemistry. The structural issues considered differ from those found in crystallographic representations of the systems studied. In the limit of highly perfect monocrystals, our interest is in the metrology of the remaining imperfection. In synthetic microstructures, we explore their relationship to scaled crystalline forms both in terms of theoretical pictures and practical techniques.

• Sub-atomic Displacement Metrology. Linear displacement is a primitive component of many fundamental physical and technical measurements, yet its realization with atomic scale refinement and accuracy is difficult. Optical interferometry is the means by which a displacement is related to a primary wavelength standard, and at the moment the accuracy of an interferometric measurement is at least two orders of magnitude below that of the reference laser. We have a competence project (in collaboration with MEL) aimed at improving understanding and application of interferometry. The one-dimensional displacement of a specially fabricated translation stage will be simultaneously measured by means of heterodyne Michelson interferometry, scanning Fabry-Perot interferometry, and x-ray interferometry. The redundancy inherent in this approach will allow robust control of the systematic errors that currently limit the accuracy of displacement measurements. We are currently working towards measuring a displacement of up to 5 cm with 50 pm resolution in a high-vacuum, vibration controlled environment.

TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999