TECHNICAL ACTIVITIES 1998 - NISTIR 6268

HIGHLIGHTS

• High Resolution FTS Upgrade Completed. Operating characteristics of our high resolution Fourier transform spectrometer (FTS) have been greatly improved by replacement of a number of electronic and mechanical components. Our new data acquisition system has been completely redesigned, tested, and installed on the spectrometer. This completes the modernization of all components of the electronic systems of the instrument. We have also replaced the linear motors and motion control system, giving us a simpler and more easily maintained system that eliminates one of the auxiliary interferometers with no reduction in performance. These instrument upgrades in conjunction with software improvements have produced a significant improvement in signal-to-noise ratios and have reduced ghosts in the system to a negligible level. (G. Nave, U. Griesmann, and R. Kling)

   

• Rare Earth FTS Spectroscopy for Lighting Applications. We have used our high resolution FTS to measure branching ratios for over 300 lines of Dy I and II in the range 400 nm to 2500 nm. This work has been combined with complementary work at the University of Wisconsin for a comprehensive set of branching ratios in these spectra. The results are important for the development of more efficient commercial lighting. Rare earth admixtures in high-pressure lamps are being utilized by the lighting industry both to increase luminosity as well as to achieve better color rendering, and atomic data for rare earth spectra are needed for the modeling of future lamp designs. We have also taken spectra of a Mn hollow cathode lamp in the VIS/IR region. This will be combined with UV data to obtain branching ratios for Mn II lines of interest to space astronomy groups. (G. Nave, R. Kling, and U. Griesmann)

   

• High Resolution Spectroscopy for Space Astronomy. We made new observations of the spectrum of singly-ionized mercury (Hg II) in the visible and near infrared with a pulsed radio-frequency discharge on our 10.7 m air Eagle spectrograph. These observations provide the first accurate wavelengths for Hg II in this region. We have now classified nearly 500 lines as transitions between 90 energy levels. Some of these are transitions in the visible that originate from levels of the 5d⁹6s5f configuration lying well above the ionization limit and that terminate on levels of 5d⁹6s6d lying just below the limit, a most unusual occurrence. From our new level values an accurate value for the ionization energy was determined. All of our results for Hg II are being assembled into a comprehensive report that will contain a complete quantum mechanical interpretation of the Hg II level structure. This report, which will also provide calculated transition probabilities, will constitute the first modern description of the spectrum and energy levels of this important atomic ion.

  A portion of our results for Hg II have been incorporated into a collaborative report with several astronomers that analyzes the abundance of mercury in the chemically peculiar stars chi Lupi and HR7775. These stars have an abundance of Hg about 10⁵ times the solar abundance. They also exhibit isotopic abundance anomalies. In chi Lupi, for example, the observed Hg is all in the form of isotope 204, the heaviest stable isotope, which comprises only 7% of terrestrial Hg. Many of our results for Hg II as well as for Bi I, II, and III, Hg III, Pb III, Zr II and III, Y III, and Sb II have been used in an atlas of observations of chi Lupi from the Goddard High Resolution Spectrograph prepared by Hubble Space Telescope scientists. (C. Sansonetti and J. Reader)

• Quantum Electrodynamic Effects in Low Energy Levels of Helium. Accurately determined ionization energies for the low 1sns and 1snp (nS and nP) levels of helium furnish excellent tests of calculations for this important three-body system, including two-electron quantum-electrodynamic (QED) effects. By combining a variety of high-accuracy measurements of transitions in helium, we derived ionization energies for several low nS and nP levels. The uncertainties for the n = 1 and n = 2 levels vary from 8 parts in 10⁹ for the ¹S ground level to 5 parts in 10¹¹ for the 2 ³S level. Corresponding theoretical energies including QED shifts of order α³ atomic units and higher were calculated by Gordon Drake of the University of Windsor, Canada, except for the three lowest levels. For these most accurate levels the main part of the QED shift of order α³ was based on a preliminary calculation of the Bethe logarithm which was completed in final form by an NRC Postdoctoral Associate at NIST. This particular calculation has been the object of theoretical studies for over 40 years. This latest calculation provides the definitive result, with an improvement in accuracy over previous calculations of about four orders of magnitude for the Bethe logarithm and two orders of magnitude for the energy levels.

  Comparisons of the experimental energies with the less accurate calculated values for the seven 1S, 2S, and 2P levels give agreements well within the estimated theoretical uncertainties of 1 to 3 parts in 10⁸. The results verify the usefulness of the Kabir-Salpeter formalism for calculating QED shifts at least up to order α⁴. Much work is still needed, however, to obtain a two-electron theory of higher-order relativistic and QED contributions approaching the accuracy in hydrogen. (W. Martin and J. Baker)

• Atomic Interactions and Collisions of Cold, Trapped Atoms. The control of atomic interaction parameters and collision rates by magnetic or optical fields is an important goal of research on cold atoms. Applications include manipulation of the properties of Bose-Einstein condensates, cold molecule formation, and quantum computing. We have started new calculations for the properties of magnetically or optically induced scattering resonance states near zero collision energy. Our calculations quantitatively explain the strength and width of several such resonances recently measured in a sodium Bose-Einstein condensate. We also have set up the time-dependent Gross-Pitiaevskii equation and have predicted nonlinear four-wave mixing of matter wavepackets generated from Bose-Einstein condensates, an effect that has now been observed experimentally at NIST (see also the division's cover picture and the highlight on "nonlinear matter wave optics with Bose-Einstein Condensates" which contains another figure). Calculations of the matter wave coherence agree with experiment and other recent calculations. (C. Williams, P. Julienne, E. Tiesinga, P. Leo, F. Mies, M. Doery, M. Trippenbach, and Y. Band)

   

• Electron Impact Ionization Cross Sections. The semiconductor industry is shifting toward theoretical modeling of etching by plasma processing to save time and expenses in designing new chips. One critical need for such modeling is the ionization cross section of halogen molecules used in etching. We have developed a Binary-Encounter-Bethe model (BEB) for calculating such cross sections. It is the only ab initio theory in the world that can distinguish reliable experimental data from less reliable data for large neutral molecules of interest to the semiconductor industry. The BEB model has also been found to be effective for atomic and molecular ions of low charge states. A new collaboration with a quantum chemistry group at NIST has been initiated in addition to the existing collaboration with researchers at CalTech and NASA Ames Research Center to predict reliable ionization cross sections for large and complex molecules. New results are being posted on the Physics Laboratory Web site (physics.nist.gov/ionxsec) as they are published. Modelers of plasma processing in the semiconductor industry (e.g., Intel, Motorola, Phillips) have started to request theoretical cross sections for molecules of interest to them. (Y.-K. Kim, M. Ali)

• Complex Quantum Nanostructures. Quantum nanostructures are being studied by many labs to realize their promise of enhanced optoelectronic devices. We have implemented realistic, empirical, tight-binding models in our theory of quantum dot structures and used these models to study CdS/HgS and CdTe/ZnTe quantum-dot quantum-wells and Ge nanocrystallites. Our atomistic models allow us to study quantum nanostructures down to the smallest sizes, such as quantum-dot quantum-wells with layers as thin as one monolayer and tightly confined systems with indirect gaps (Ge) or strong valence-band/conduction-band mixing (InAs), where effective mass models are expected to break down. We have also extended our theory of T-shaped quantum wires to include the effects of magnetic fields. This has allowed us to explain recent magneto-photoluminescence experiments on these systems and provides a compelling description of confinement effects in these structures. (G. Bryant and P. Julienne)

• Theory of Near-Field Optical Microscopy. Near-field microscopy offers optical resolution much better than the diffraction limit. Detailed theory and modeling is needed to interpret and analyze near-field images. We developed a coupled dipole theory for imaging with transmission near-field optical microscopy (NSOM) and applied it to accurately model experimental NSOM images of Au nanoparticles. It is critical to model the entire imaging process because it allows us to clearly identify the contribution of the near-field optical excitation source, the coupling to the sample local fields, and the collection optics to NSOM image formation. We find that field enhancement under the metal cladding of the NSOM probe critically determines the structure in the NSOM images. We also modeled the near-field nonlinear optical response of nanoscale structures to understand how near-field optics can be used to extend the capabilities of nonlinear optical spectroscopy. (G. Bryant and P. Julienne)

   

• Comprehensive Spectra Database on the World Wide Web. The Atomic Spectra Database (ASD) interactive Web server has become accessible at the NIST Physics Laboratory Web site: physics.nist.gov/asd. The new version 2.0 of ASD contains significantly more extensive coverage than previous versions. It contains data on about 950 spectra, with about 70,000 energy levels and 90,000 lines from 1 Å to 200 µm, 40,000 of which have transition probabilities with estimated accuracies. Wavelengths of observed transitions are included for the first 99 elements in the periodic table. ASD offers a comprehensive range of user-specified options and selection criteria and includes a "Help" file, which also serves as a users manual. (P. Mohr, D. Kelleher, W. Martin, J. Fuhr, A. Robey, and W. Wiese)

   

• Fundamental Constants - Toward a Better Rydberg constant. Recently, there has been a dramatic increase in the frequency metrology of hydrogen, and it is expected that the 1S-2S transition in hydrogen will eventually be measured to 1 Hz, a relative uncertainty below 5 × 10^-16, possibly using trapped hydrogen atoms. In order for the anticipated improvement in experimental precision to provide better values of the fundamental constants, there must be a corresponding improvement in the precision of the theory of the energy levels in hydrogen, particularly in the Lamb shift. As a first step toward this goal, we have carried out a numerical calculation of the one-photon self energy of the 1S state. Numerical convergence acceleration techniques were developed to decrease the substantial computation time by about three orders of magnitude. The result is the first complete calculation of the self energy in hydrogen and provides a value that contributes an uncertainty of about 0.8 Hz. The result is a step toward an improved value of the Rydberg constant and possibly toward the use of hydrogen as a frequency standard over a wide range of frequencies. The calculation was done in collaboration with the Technical University of Dresden, Germany. (P. Mohr and U. Jentschura)

• Critical Compilations Uncover Serious Problems for Calculated Transition Probabilities. The vast majority of transition probability data for atoms and ions are computed. In comparing the sophisticated atomic structure codes (there are about half-a-dozen in existence), we have found that the agreement is usually excellent for the strongest transitions, but that disagreements typically become greater than 50% for oscillator strengths smaller than 0.1 and increase to one or more orders of magnitude with further decreasing strengths. Figure 1 shows an example of the severity of the problem.
   

  Figure 1. Comparison of two different theoretical data sources for oscillator strengths, showing order of magnitude discrepancies.

  We have alerted the data generators to the seriousness and extent of this problem. Also, we organized a special session on this problem at the Sixth International Conference on Atomic Spectra and Oscillator Strengths, August 1998, in Victoria, B.C. This has sparked renewed and more critical work at several institutions focusing on spectra and transitions that we recommend. The first new high-accuracy computations in response to our requests are already producing promising data for noble-gas-like spectra. (D. Kelleher, J. Fuhr, and W. Wiese)

• Atomic Spectral Line Broadening Bibliographic Database Issued. The first bibliographic database on atomic spectral line broadening has been completed and made available on the NIST Physics Lab Website. This database contains approximately 850 recent references for the time period 1993 to 1998, all collected after the last published NIST bibliography: [NIST Special Publications 366, Supplement 4, 1993]. The papers listed in the database contain either numerical data or general information, comments, and review articles and are part of the collection of the Data Center on Atomic Line Shapes and Shifts at NIST. The following search categories are included: chemical element, stage of ionization, broadening mechanism, experiment, theory, word in title, author, and year of publications. This database is patterned after the existing NIST Web-based bibliographic database on atomic transition probabilities. Our plan is to add all 5000 earlier references from the Data Center collection to the database in order to provide a complete set. (J. Fuhr and H. Felrice)

• X-ray Spectroscopy on EBIT. In collaboration with Russian researchers who have developed expertise in fabricating high quality spherically curved crystals of mica and quartz, we have deployed these crystals as the heart of a new type of x-ray spectrometer for use on an Electron Beam Ion Trap (EBIT). This spectrometer has an advantage over all other x-ray spectrometers previously used on an EBIT: the ability to acquire spectra with both high light collection efficiency and relative insensitivity to source position. This simultaneously addresses the two main factors that have limited the precision of previous measurements on EBIT's-photon statistics and calibration systematics. Demonstration spectra from neon-like barium (Ba⁴⁶⁺) and helium-like argon (Ar¹⁶⁺) were obtained, paving the way for future high accuracy measurements. In parallel with this work, progress has been made using traditional NIST x-ray spectrometers to determine wavelengths in hydrogen-like and helium-like vanadium ions with an absolute accuracy that rivals the best previous measurements in this region of the one- and two-electron isoelectronic sequences. With an accuracy of 20 to 30 parts per million, these results critically challenge calculations of the atomic structure of highly charged ions, particularly considering recent significant revisions involving higher order quantum electrodynamic corrections. Our work is proceeding in collaboration with researchers from Australia; a preliminary report was recently submitted for publication, and a final report is under preparation. (J. Gillaspy and L. Hudson)

• Highly Charged Ions Used to Pattern Surfaces. Masked ion beam lithography using highly charged ions (Xe⁴⁴⁺) was demonstrated by the EBIT team by exposing a silicon wafer coated with a commercial resist material (PMMA). Subsequent chemical development of the resist revealed the imposed pattern -- a regular array of hundreds of 1 micrometer wide squares with better than 100 nm edge resolution. Atomic force microscopy was also used to image single ion impact sites, which appear as 24 nm wide holes in the surface. Although PMMA is widely used in the community of ion-beam lithography researchers, this is the first time that highly charged ions have been used and that atomic force microscopy has been deployed to reveal the effect of a single ion on this material. Some related work has just been completed using Xe⁴⁴⁺ ions to pattern an advanced ultrathin resist consisting of self-assembled monolayers of alkanethiolates. (L. Ratliff, J. Gillaspy, and R. Minniti)

Figure 2. Portion of an array of squares produced using highly charged ions to expose a self-assembled monolayer resist.

• 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. As the wafer diameters used in etching increases, monitoring and control of plasma uniformity become increasingly important. A new diagnostic technique for plasma uniformity measurements based on 2D optical tomography has been developed for vacuum chambers with restricted optical access. Optical tomography determines the two-dimensional distribution of plasma species in a plasma from line-integrated measurements, such as optical emission measurements or laser absorption measurements, without assuming radial symmetry of the plasma. This technique has been applied to optical emission measurements from the GEC-ICP RF Plasma Source. Several conditions creating radially asymmetric plasmas have been identified, such as gas flow rate, proximity to the induction coil, and feed gas composition.

  Operation of the GEC-ICP RF Plasma Source with pulsed RF power has been investigated. By momentarily interrupting the power to the inductive coil, the properties of an inductively coupled plasma can be significantly altered. With electronegative gases commonly used in commercial etching reactors, interruption of the RF power results in a rapid loss of electrons creating a decaying plasma composed of only positive and negative ions. The resulting ion-ion plasmas have the potential to improve etching performance and reduce surface damage on wafers. The decay and growth of the plasma during pulsed power operation of the GEC-ICP RF Plasma Source has been measured using a new, intensified CCD camera. In argon/oxygen mixtures, when the RF power is turned back on, the plasma first ignites as a dim capacitive discharge before switching back into a bright inductive discharge. (E. Benck and J. Roberts)

• Plasma Radiation. We have recently completed the rebuilding of the NIST FT700 ultraviolet Fourier transform spectrometer. The FT700 spectrometer is a unique resource. It has a wavelength coverage from the visible down to approximately 200 nm and a resolving power of 10⁶. (An upgrade of the interferometer optics is currently underway to extend the range to 140 nm.) We have used the new spectrometer to make accurate measurements of spectral line intensities and branching ratios in the ultraviolet in Kr II, Mn II, and Xe II. The measurements provide data to test recent, sophisticated, atomic structure calculations and are needed in the diagnostics of laboratory and stellar plasmas. (U. Griesmann, K. Dzierzega, R. Kling, and W. Wiese)

• High-Accuracy DUV and VUV Index of Refraction Measurements. As part of a collaborative project with MIT Lincoln Laboratory, SEMATECH, and the NIST Optical Technology Division, we have made the highest accuracy (~7 × 10^-6) measurements of the index of refraction, its dispersion, and its temperature dependence, of fused silica and calcium fluoride near 193 nm. These numbers are being used by the semiconductor electronics industry for the design of the transmissive optics of photolithographic steppers using 193 nm ArF excimer laser excitation. These will be used for the fabrication of 0.18 µm minimum-feature-size integrated circuits, scheduled for large-scale production by the U.S. semiconductor industry beginning in 2001. For future-generation integrated circuits, stepper manufacturers are designing steppers based on 157 nm F₂ excimer laser excitation and calcium fluoride optics. Responding to requests from all major stepper manufacturers, we have made the only measurements of index of refraction (to 7 × 10^-6), its dispersion, and its temperature dependence, of calcium fluoride near 157 nm. [J. Burnett, U. Griesmann, and R. Gupta (Div 844)]

• Ultracold Collisions in Metastable Xenon. We have investigated the effects of spin polarization and quantum statistics on ultracold inelastic collisions in metastable xenon. We found that, contrary to expectations, the rate of inelastic ionizing collisions was not at all suppressed by spin-polarizing the sample of atoms. The spin selection rules that might be expected to apply are voided by a molecular effect where the spins "lock" to the molecular axis instead of the laboratory axis. The atoms strongly depolarize during the collision, so that the initial polarization has no effect on the outcome. This result calls into question the likelihood of using metastable rare gases other than helium for Bose-Einstein condensation. We also measured the spin-polarized collision rates for fermionic and bosonic isotopes of xenon, and found a significant decrease in the collision rate of the fermions at low temperatures. This is directly ascribable to quantum statistics and the Pauli exclusion principle, which prevents two identical fermions from occupying the same state. This measurement is the first clear observation of quantum statistical suppression in cold collisions.

  Because ultracold atoms move so slowly, it is possible to observe the temporal dynamics of collisions. By preparing excited state atoms with a short pulse of laser light and measuring the arrival time of the ions produced in collisions, we were able to study the collision process in detail. We have observed the acceleration of the atoms on the attractive intermolecular potential and have clearly observed collisions that include the decay of the excited atom to the ground state during the collision. We have also been able to time-resolve the optical shielding process, where light excites a pair of atoms onto a repulsive molecular potential, preventing a short range, ionizing collision from occurring. (S. Rolston, C. Orzel, and S. Kulin)

• Large Bose-Einstein Condensation of Sodium in a TOP Trap. We have created a large Bose-Einstein condensate (BEC) of sodium atoms in a time-averaged orbiting potential (TOP) trap. A TOP trap is a magnetic trap consisting of a quadrupole magnetic field and a constant magnitude, rotating, bias field. The arrangement of our fields produces a tri-axial potential that is well matched for loading from the nearly spherical clouds of laser cooled atoms. We have developed two new strategies for evaporatively cooling atoms to Bose-Einstein condensation. The first strategy involves evaporative cooling using rf, with the atoms initially trapped in a quadrupole field. This is then followed by rapidly transferring them into the TOP trap and further cooling of the sample to condensation, again using rf-induced evaporation. The second strategy involves starting with atoms in the TOP trap and evaporatively cooling the atoms with the "circle-of-death" (the zero field region rotating around the center of the trap) all the way to condensation. Both strategies produce approximately the same number of final condensate atoms, about 3 × 10⁶, at a BEC transition temperature of 1.2 µK. (L. Deng, E.W. Hagley, K. Helmerson, M. Kozuma, R. Lutwak, J.-H. Müller, W.D. Phillips, S.L. Rolston, and J. Wen)

• Bragg Diffraction of a Bose-Einstein Condensate. We have coherently split and deflected a Bose-Einstein condensate (BEC) of sodium atoms using Bragg diffraction by a moving, optical standing wave, comprised of two counterpropagating laser beams with a frequency difference. The condensate atoms, initially at rest, will simultaneously absorb photons from the higher frequency laser beam and be stimulated to emit photons into the lower frequency beam acquiring several units of photon momentum in the process. Hence the momentum transfer is uni-directional and coherent. The increase in kinetic energy of the Bragg diffracted atoms comes from the energy difference between the absorbed and emitted photons from the two different frequency laser beams.

  In our experiments we start with an adiabatically expanded BEC with no discernable thermal fraction present. The momentum spread of the condensate atoms released from the trap is much less than the momentum of a single photon. We then expose the atoms to a short pulse of the moving, optical standing wave while they are either still in the TOP trap or shortly after releasing them from the trap. We detect the momentum transferred to the atoms from the diffraction process by taking an absorption image after a sufficient time delay, such that the various atomic wave-packets with different momenta have spatially separated. Figure 3 shows first, second and third order Bragg diffraction of Bose condensed atoms, corresponding to momentum transfer of 2, 4 and 6 times the single photon momentum. We have observed up to 6th order Bragg diffraction. The direction of the momentum transfer can be reversed by changing the sign of the frequency difference. We have observed first order Bragg diffraction of 100% of the condensate atoms. (L. Deng, E.W. Hagley, K. Helmerson, M. Kozuma, R. Lutwak, W.D. Phillips, S.L. Rolston, and J. Wen)

  Figure 3. 1st, 2nd and 3rd order Bragg diffraction of a BEC by a moving, optical standing wave.

• Non-Linear Matter-Wave Optics with Bose-Einstein Condensates. Due to the relatively strong influence of the atom-atom interactions in a Bose-Einstein condensate, non-linear effects in matter-wave optics can occur. These non-linear effects are analogous to non-linear optical wave phenomena. Specifically, the theory predicts that an interacting condensate is analogous to optical waves interacting with a third order, non-linear medium. The resulting process from such an interaction is 4-wave mixing. In 4-wave mixing, three waves are sent into a non-linear medium and a fourth wave emerges. We have observed a similar phenomenon with matter-waves, where the non-linear medium is the interacting atoms themselves. (See also the Division's cover picture.)

  Figure 4. Image of the distribution of atoms resulting from 4-wave mixing of matter waves.

  In order to observe the generation of a fourth wave due to 4-wave mixing, the three incident waves must have the appropriate momenta to satisfy energy and momentum conservation. We use two Bragg diffraction pulses to produce condensates in the three appropriate momentum states to observe 4-wave mixing of matter-waves. The pulses are applied rapidly enough that the atoms in the three momentum states still overlap. The non-linear interaction between the atoms produces a fourth state with a different momentum. Figure 4 is an image of the distribution of atoms resulting from 4-wave mixing of matter-waves, taken after the different momentum states have spatially separated. This represents the first example of non-linear atom optics. The smallest peak is the fourth matter-wave, generated by the 4-wave mixing process. We have observed up to 12% of the initial condensate atoms appearing in the fourth wave. We have also confirmed that the process depends on the product of the densities of atoms in the three initial momentum states. (L. Deng, E.W. Hagley, K. Helmerson, W.D. Phillips, S.L. Rolston, and J.E. Simsarian)

• More Precise Value of the Neutron Mass. The absolute wavelength of the gamma-ray produced in the reaction n+p→d+γ (2.2 MeV) was measured with a relative uncertainty of 2 × 10^-7 using the NIST ILL GAMS4 crystal diffraction facility at the Institut Laue-Langevin in Grenoble, France. This wavelength measurement, expressed in energy units and corrected for recoil, is the binding energy of the neutron in deuterium. A previous crystal diffraction measurement of the deuteron binding energy has an uncertainty 5 times larger than this new result. The neutron mass follows directly from the reaction expressed in atomic mass units: m(n) = m(²H) - m(¹H) + S(d) where S(d) is the separation energy of the neutron in deuterium. The uncertainties of the atomic mass difference, m(²H) - m(¹H), and the new determination of S(d) are 0.71 × 10^-9 u and 0.42 × 10^-9 u, respectively, where u is unified atomic mass unit. The new, more precise value for the neutron mass, m(n) = 1.008 664 916 37(82) u, has an uncertainty which is ≈ 2.5 times smaller than the previous best value. [E. Kessler and M.S. Dewey (Div 846)]

• New High-flux X-ray Diffractometer/Reflectometer. Using a novel optical design, a new x-ray analysis facility has been built which provides a peak count rate of 10⁷ photons/s for the study of materials of interest to the semiconductor industry. Industry standard 200 mm wafers can now be examined using the new instrument. Films as thin and light as 1.5 nm of Si₃N₄ and as thick and dense as 100 nm of Pt have been successfully characterized by this facility. The high counting rate also allows large batches of films to be studied in short order. Recent materials of interest characterized for various industrial partners include SiO_xN_y, Ba_(1-x)Sr_xTi_yO₃ (Fig. 5), Ta₂O₅ and TaN_x. (S. Owens, J. Pedulla, and R. Deslattes)

Figure 5. Grazing incidence x-ray reflectivity data and modeled fits for Ba_(1-x)Sr_xTi_yO₃ thin films.

• Development of Thin Film Reference Materials. Accurately characterized, highly uniform thin films are in great demand for fluorescence measurement calibration. Using a Dual Ion Beam Assisted Deposition facility, a wide variety of materials are being grown with excellent lateral thickness uniformity, high density and low interfacial roughness. These films are characterized in-house by Grazing Incidence X-ray Reflectivity (GIXR) which provides film thickness and interface roughness to 0.1 nm resolution and density to a few percent resolution. Films on 7.5 cm × 7.5 cm float-glass and 7.5 cm diameter silicon substrates are currently being shipped to both internal and industrial partners. (J. Pedulla)

• Recertification of Si SRM-640b Powder-Diffraction Reference Material. X-ray powder diffraction is a widely used analytical method for which NIST is the world's principal supplier of powder-diffraction reference materials (SRMs). Current inventory and previous certification accuracy are inadequate to future need. In collaboration with the Materials Science and Engineering Laboratory, a major effort has been undertaken to produce and certify a new generation of these reference materials. Diffractometer calibrations and uniformity tests on 30 kg of single-crystal silicon material were completed. This material was crushed and sized to form the new silicon SRM-640c; its packaging and certification await selection of a surface stabilization process. Meanwhile, we have re-certified the previous material, silicon SRM 640b, with a relative uncertainty close to 1 × 10^-6 Å. (J.-L. Staudenmann, L. Hudson, and R. Deslattes)

• Picometer Heterodyne Interferometery Demonstrated. Heterodyne Michelson interferometry is the most widely used technique for accurate displacement measurements. Its accuracy has traditionally been limited to a few nm by well-known periodic systematic errors arising from optical crosstalk. Brute-force improvement of a traditional interferometer in our laboratory brought the amplitude of the periodic error down to 500 pm in 1992, but in order to go beyond this level, new techniques are required. We have recently developed and demonstrated two new schemes for doing heterodyne interferometry in which the amount of optical crosstalk can be greatly reduced. In one such scheme, the residual periodic error has an amplitude of 20 pm. More recent results with the second scheme suggest that the periodic error is even lower, and in fact beyond our ability to measure. In addition, we have demonstrated a new digital phase meter with a 10 kHz bandwidth capable of splitting optical fringes by a factor of 32,000. (C.-M. Wu, J. Lawall, and R. Deslattes)

TECHNICAL ACTIVITIES 1998 - Contents

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