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High resolution krypton M/sub 4,5/ x-ray emission spectra

Description: High resolution M/sub 4,5/ (3d ..-->.. 4p) x-ray emission spectra from a krypton plasma were measured using a recently developed grazing-incidence reflection-grating monochromator/spectrometer with very high flux rates at extreme ultraviolet and soft x-ray wave lengths. The nominal resolving power of the instrument, E/..delta..E, is about 300 in this energy range (approx.80 eV). Three dipole-allowed 3d ..-->.. 4p emission lines were observed at 80.98 eV, 80.35 eV and 79.73 eV. A broad peak at about 82.3 eV is tentatively assigned to transitions resulting from Kr/sup 2 +/, and effects of excitation energy on M/sub 4,5/ x-ray emission were observed. 9 refs., 3 figs., 1 tab.
Date: October 1, 1987
Creator: Perera, R.C.C.; Hettrick, M.C. & Lindle, D.W.
Partner: UNT Libraries Government Documents Department

A method for sizing sub-micron particles using small angle diffraction of soft x-rays

Description: The purpose of this work is to develop a method of sizing sub-micron particles using small-angle soft x-ray diffraction. Solid poly-styrene spheres of known sizes were used as scattering samples, with C-K..cap alpha.. (44.8/angstrom/) and V-L..cap alpha.. (24.3/angstrom/) radiation from a conventional x-ray source. Two devices were used to diffract the x-rays. One collimates the radiation using a series of pinholes immediately preceding the sample, and directs the unfocussed radiation onto film placed far from the sample. The other utilizes radiation from a single pinhole above the source onto the sample and a spherical multilayer mirror in series, so that the radiation passes twice through the sample and is focused onto film immediately above the pinhole assembly to increase the effective sample area. Using the latter device, two types of diffraction patterns were obtained: a sharp, relatively small pattern from spheres which form a hexagonal lattice structure, and a diffuse, larger pattern from an unordered, or random, array of spheres. Both patterns are presented in this work, along with the calculations, based upon light scattering from an unordered and an ordered array of particles, of the scattering patterns. 9 refs., 32 figs., 2 tabs.
Date: October 1, 1988
Creator: Berkeland, D.J.; Underwood, J.H. & Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

Report on the workshop on new directions in soft x-ray near-threshold phenomena

Description: The ''Workshop on New Directions in Soft X-Ray Near-Threshold Phenomena'' was held at the Asilomar Conference Center in Pacific Grove, CA on March 1--4, 1987. It was attended by 59 scientists from 8 countries, representing 27 institutions. Major funding for the meeting was donated by L-Division of the Lawrence Livermore National Laboratory, who hosted and organized two previous workshops on photoabsorption and scattering in the soft x-ray energy range. Additional funding was provided by the User's Group of the Advanced Light Source. The Workshop, as its name suggests, emphasized physical phenomena in atoms, molecules, and solids near inner-shell thresholds. Of particular interest were threshold ionization, post-collisional interaction, resonant photoemission and fluorescence, and multi-electron effects such as shake-up and shake-off. In these areas and others, special consideration was given to presenting recent discoveries and potential ''new directions'' for future work.
Date: July 1, 1988
Creator: Lindle, D.W. & Perera, R.C.C. (eds.)
Partner: UNT Libraries Government Documents Department

Photon excitation for satellite free x-ray spectroscopy: Instrumentation challenges

Description: First systematic study of satellites in x-ray emission spectra was performed by Deslattes using quasi-monochromatic photon excitation from a group of L{alpha} x-ray sources lying close to the K edge of Cl. He observed significant alterations in the Cl K{beta} spectrum of KCl depending on the character of the excitation radiation and identified the initial state of these satellites as a double vacancy state. Recently, the valence electronic structure of the chlorofluoromethanes were analyzed by chlorine K x-ray emission under satellite-free conditions. These studies were based on the use of synchrotron radiation to eliminate the multivacancy effects that are inherent in conventional x-ray spectroscopy. In this report, satellite free x-ray emission spectra from chlorofluoromethanes will be presented to demonstrate that the simplified spectra can be obtained using selective photon excitation. Results from various research groups world wide, utilizing the tunable photon excitation form synchrotron sources to eliminate the obscuring features in x-ray emission spectra of rare-gas solids (RGS) and metals will be discussed. Also, the technical challenges in utilizing the small phase-space attributes of high brightness from third generation SR sources producing x-ray and vacuum ultra-violet wavelengths to study weak features like satellites in x-ray emission spectra will be presented.
Date: October 1, 1991
Creator: Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

Photon excitation for satellite free x-ray spectroscopy: Instrumentation challenges

Description: First systematic study of satellites in x-ray emission spectra was performed by Deslattes using quasi-monochromatic photon excitation from a group of L{alpha} x-ray sources lying close to the K edge of Cl. He observed significant alterations in the Cl K{beta} spectrum of KCl depending on the character of the excitation radiation and identified the initial state of these satellites as a double vacancy state. Recently, the valence electronic structure of the chlorofluoromethanes were analyzed by chlorine K x-ray emission under satellite-free conditions. These studies were based on the use of synchrotron radiation to eliminate the multivacancy effects that are inherent in conventional x-ray spectroscopy. In this report, satellite free x-ray emission spectra from chlorofluoromethanes will be presented to demonstrate that the simplified spectra can be obtained using selective photon excitation. Results from various research groups world wide, utilizing the tunable photon excitation form synchrotron sources to eliminate the obscuring features in x-ray emission spectra of rare-gas solids (RGS) and metals will be discussed. Also, the technical challenges in utilizing the small phase-space attributes of high brightness from third generation SR sources producing x-ray and vacuum ultra-violet wavelengths to study weak features like satellites in x-ray emission spectra will be presented.
Date: October 1, 1991
Creator: Perera, R. C. C.
Partner: UNT Libraries Government Documents Department

First results from the high-brightness x-ray spectroscopy beamline 9. 3.1 at ALS

Description: Beamline 9.3.1 at the Advanced Light Source (ALS) is a windowless beamline, covering the 1-6 keV photon-energy range. This beamline is designed to achieve the goal of high brightness at the sample for use in the X-ray Atomic and Molecular Spectroscopy (XAMS) science, surface and interface science, biology, and x-ray optical development programs at ALS. X-ray absorption and time of flight photoemission measurements in 2 - 5 keV photon energy along with the flux, resolution, spot size and stability of the beamline will be discussed. Prospects for future XAMS measurements will also be presented.
Date: October 1, 1995
Creator: Ng, W.; Jones, G. & Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

A program for calculating and plotting soft x-ray optical interaction coefficients for molecules

Description: Comprehensive tables for atomic scattering factor components, f1 and f2, were compiled by Henke et al. for the extended photon region 50 - 10000 eV. Accurate calculations of optical interaction coefficients for absorption, reflection and scattering by material systems (e.g. filters, multi-layers, etc...), which have widespread application, can be based simply upon the atomic scattering factors for the elements comprising the material, except near the absorption threshold energies. These calculations based upon the weighted sum of f1 and f2 for each atomic species present can be very tedious if done by hand. This led us to develop a user friendly program to perform these calculations on an IBM PC or compatible computer. By entering the chemical formula, density and thickness of up to six molecules, values of the f1, f2, mass absorption transmission efficiencies, attenuation lengths, mirror reflectivities and complex indices of refraction can be calculated and plotted as a function of energy or wavelength. This program will be available distribution. 7 refs., 1 fig.
Date: August 1, 1989
Creator: Thomas, M.M.; Davis, J.C.; Jacobsen, C.J. & Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

Sub-threshold excited Cl K. beta. (K-V) x-ray fluorescence from CF/sub 3/Cl molecule

Description: With the availability of tunable synchrotron radiation sources, unoccupied molecular orbits (below vacuum level) can be selectively populated producing highly excited neutral molecules. X-ray fluorescence spectra from molecules were obtained with excitation below the ionization threshold and were observed to have significant intensity changes, absolute and relative energy position shifts and line width changes as compared to fluorescence spectra excited above the threshold. As an example, the Cl K..beta.. (K-V) emission spectra from CF/sub 3/Cl vapor are presented. The energy shifts and intensity changes are explained in terms of perturbation effects due to the presence of an electron in an unoccupied molecular orbital. The narrow line widths obtained in the spectra excited below threshold are explained in terms of the ''effective'' hole production region in a core state limited by the broadening of the unoccupied level. The change in line widths as a function of below-threshold excitation energy is proposed as a novel technique to study the localized properties and reorganization effects of a hole in a core level. 10 refs., 4 figs., 1 tab.
Date: October 1, 1987
Creator: Perera, R.C.C.; Cowan, P.L.; Lindle, D.W. & LaVilla, R.E.
Partner: UNT Libraries Government Documents Department

Applications of photon-in, photon-out spectroscopy with third-generation, synchrotron-radiation sources

Description: This report discusses the following topics: Mother nature's finest test probe; soft x-ray emission spectroscopy with high-brightness synchrotron radiation sources; anisotropy and polarization of x-ray emission from atoms and molecules; valence-hole fluorescence from molecular photoions as a probe of shape-resonance ionization: progress and prospects; structural biophysics on third-generation synchrotron sources; ultra-soft x-ray fluorescence-yield XAFS: an in situ photon-in, photon-out spectroscopy; and x-ray microprobe: an analytical tool for imaging elemental composition and microstructure.
Date: January 1, 1991
Creator: Lindle, D.W. & Perera, R.C.C. (eds.)
Partner: UNT Libraries Government Documents Department

The Advanced Light Source at Lawrence Berkeley Laboratory

Description: The Advanced Light Source (ALS) at the Lawrence Berkeley Laboratory (LBL), scheduled to be operational in the spring of 1993 as a US Department of Energy national user facility, will be a next- generation source of soft x-ray and ultraviolet (XUV) synchrotron radiation. Undulators will provide the world's brightest synchrotron radiation at photon energies from below 10 eV to above 2 keV; wiggler and bend-magnet radiation will extend the spectral coverage with high fluxes above 10 keV. These capabilities will support an extensive research program in a broad spectrum of scientific and technological areas in which XUV radiation is used to study and manipulate matter in all its varied gaseous, liquid, and solid forms. The ALS will also serve those interested in developing the fabrication technology for micro- and nanostructures, as well as characterizing them.
Date: October 1, 1991
Creator: Robinson, A.L.; Perera, R.C.C. & Schlachter, A.S.
Partner: UNT Libraries Government Documents Department

X-ray L. beta. /sub 2/ /sub 15/ emission spectrum of Ru in Ru(NH/sub 3/)/sub 6/Cl/sub 3/

Description: One of the broader applications of synchrotron radiation has been to EXAFS studies for material structure determination, i.e., for an analysis of x-ray absorption over an extended energy region beyond a core ionization limit. Studies of the near edge structure (XANES) give a different type of information, characteristic of the local symmetry and electronic configuration of the absorbing atom. This type of information is reflected also in the x-ray emission spectra, in particular for transitions involving the valence levels. Examination of the near edge absorption or the emission spectrum does not require an instrument capable of scanning a wide energy range with high counting statistics, as does EXAFS; the needs are rather for good resolution and a reliable calibration of the energy scale. Some of the problems of near edge spectra were particularly evident in our investigation of Ru-L..beta../sub 2/ /sub 15/ emission from Ru(NH/sub 3/)/sub 6/Cl/sub 3/. The Ru-L..beta../sub 2/ /sub 15/ emission was measured with a laboratory Rowland circle x-ray spectrometer with a curved quartz (1010) crystal (radius = 22 inches) in a fixed position appropriate to the energy range, and a position sensitive detector which can be positioned along the Rowland circle. The Ru spectrum was excited mainly by Sn-L/sub ..cap alpha../ primary radiation from a Sn anode in a demountable x-ray tube operating at 13 kV and 120 mA. The resolution of the instrument in this region is 1.5 eV. An accurate calibration of the energy scale was conveniently obtained by measuring a reference x-ray emission line in the same instrumental configuration. In the present case the Pd-L/sub ..cap alpha../ emission line at 2838 eV was used to establish the energy scale. The energy dispersion of the instrument was determined from the Cl-K/sub ..beta../ emission spectrum of CH/sub 3/Cl between 2810 eV and 2830 eV and ...
Date: July 1, 1984
Creator: Perera, R.C.C.; Barth, J.; LaVilla, R.E. & Nordling, C.
Partner: UNT Libraries Government Documents Department

The Advanced Light Source at Lawrence Berkeley Laboratory

Description: The Advanced Light Source (ALS) at the Lawrence Berkeley Laboratory (LBL), scheduled to be operational in the spring of 1993 as a US Department of Energy national user facility, will be a next- generation source of soft x-ray and ultraviolet (XUV) synchrotron radiation. Undulators will provide the world`s brightest synchrotron radiation at photon energies from below 10 eV to above 2 keV; wiggler and bend-magnet radiation will extend the spectral coverage with high fluxes above 10 keV. These capabilities will support an extensive research program in a broad spectrum of scientific and technological areas in which XUV radiation is used to study and manipulate matter in all its varied gaseous, liquid, and solid forms. The ALS will also serve those interested in developing the fabrication technology for micro- and nanostructures, as well as characterizing them.
Date: October 1, 1991
Creator: Robinson, A. L.; Perera, R. C. C. & Schlachter, A. S.
Partner: UNT Libraries Government Documents Department

Applications of photon-in, photon-out spectroscopy with third-generation, synchrotron-radiation sources

Description: This report discusses the following topics: Mother nature`s finest test probe; soft x-ray emission spectroscopy with high-brightness synchrotron radiation sources; anisotropy and polarization of x-ray emission from atoms and molecules; valence-hole fluorescence from molecular photoions as a probe of shape-resonance ionization: progress and prospects; structural biophysics on third-generation synchrotron sources; ultra-soft x-ray fluorescence-yield XAFS: an in situ photon-in, photon-out spectroscopy; and x-ray microprobe: an analytical tool for imaging elemental composition and microstructure.
Date: December 31, 1991
Creator: Lindle, D. W. & Perera, R. C. C.
Partner: UNT Libraries Government Documents Department

High-brightness beamline for X-ray spectroscopy at the Advanced Light Source

Description: Beamline 9.3.1 at the Advanced Light Source (ALS) is a windowless beamline, covering the 1-6 keV photon-energy range, designed to achieve the goals of high energy resolution, high flux, and high brightness at the sample. When completed later this year, it will be the first ALS monochromatic hard-x-ray beamline, and its brightness will be an order-of-magnitude higher than presently available in this energy range. In addition, it will provide flux and resolution comparable to any other beamline now in operation. To achieve these goals, two technical improvements, relative to existing x-ray beamlines, were incorporated. First, a somewhat novel optical design for x-rays, in which matched toroidal mirrors are positioned before and after the double-crystal monochromator, was adopted. This configuration allows for high resolution by passing a collimated beam through the monochromator, and for high brightness by focusing the ALS source on the sample with unit magnification. Second, a new ''Cowan type'' double-crystal monochromator based on the design used at NSLS beamline X-24A was developed. The measured mechanical precision of this new monochromator shows significant improvement over existing designs, without using positional feedback available with piezoelectric devices. Such precision is essential because of the high brightness of the radiation and the long distance (12m) from the source (sample) to the collimating (focusing) mirror. This combination of features will provide a bright, high resolution, and stable x-ray beam for use in the x-ray spectroscopy program at the ALS.
Date: August 1, 1994
Creator: Perera, R. C. C.; Jones, G. & Lindle, D. W.
Partner: UNT Libraries Government Documents Department

A Preliminary Report on X-Ray Photoabsorption Coefficients andAtomic Scattering Factors for 92 Elements in the 10-10,000 eVRegion

Description: Based on currently available photoabsorption measurements and recent theoretical calculations by Doolen and Liberman (Physica Scripta 36, 77 (1987)), a revised (from ADNDT 27, 1 (1982)) best-fit determination of the photoabsorption cross sections is presented here for the elements Z=1 to Z=92 in the 10-10,000 eV range. The photoabsorption data used include those described in the Lockheed and DOE listings of research abstracts for the past ten years and those which have been recently added to the comprehensive NBS Measured Data Base (NBSIR 86-3461, Hubbell et al.). The best-fit curves are compared with both the compilation of measurements and the calculations by Doolen and Liberman. Using the photoabsorption curves, the atomic scattering factors have been calculated for the energy range 50-10,000 eV and are also presented in this report.
Date: November 1, 1988
Creator: Henke, B.L.; Davis, J.C.; Gullikson, E.M. & Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

Installation of the MAXIMUM microscope at the ALS

Description: The MAXIMUM scanning x-ray microscope, developed at the Synchrotron Radiation Center (SRC) at the University of Wisconsin, Madison was implemented on the Advanced Light Source in August of 1995. The microscope`s initial operation at SRC successfully demonstrated the use of multilayer coated Schwarzschild objective for focusing 130 eV x-rays to a spot size of better than 0.1 micron with an electron energy resolution of 250meV. The performance of the microscope was severely limited, because of the relatively low brightness of SRC, which limits the available flux at the focus of the microscope. The high brightness of the ALS is expected to increase the usable flux at the sample by a factor of 1,000. The authors will report on the installation of the microscope on bending magnet beamline 6.3.2 at the ALS and the initial measurement of optical performance on the new source, and preliminary experiments with surface chemistry of HF etched Si will be described.
Date: October 1, 1995
Creator: Ng, W.; Perera, R.C.C.; Underwood, J.H.; Singh, S.; Solak, H. & Cerrina, F.
Partner: UNT Libraries Government Documents Department

Diagnostic beam line for a third generation storage ring

Description: A knowledge of the position, size, and stability of the source and the angle of emission of synchrotron radiation (SR) from the storage ring are essential for optimizing the operation of storage ring, insertion device and monochromators. Berkeley's Advanced Light Source (ALS) has a natural emittance of 3.4 {times} 10{sup {minus}9} m-rad, and has beam sizes {sigma}{sub h} and {sigma}{sub v} (assuming a 10% emittance ratio into the vertical direction) in bending magnet 1 (BM1) of 44 {mu}m and 83 {mu}m respectively. Simple diffractive optical calculations show that imaging this beam using visible light optics is not feasible and imaging must be performed using photon energies greater than 50 eV. This will be the same for all third generation low emittance storage rings. The synchrotron radiation diagnostics at ALS will consist of an imaging system for 200 eV photons and a white beam'' port with a streak camera to obtain the timing information. The imaging system will employ two crossed spherical mirrors in a Kirkpatrick-Baez configuration, to eliminate astigmatism. Use of 1:1 imaging will eliminate coma, resulting in an image of the source which is only limited by the residual aberrations of the optics. Real time imaging of the beam is deemed feasible by the use of a high resolution CCD, and the associated electronics necessary to read the CCD. The design of the imaging system of the diagnostic beam line for ALS and the detection system will be discussed with a view toward applications in other third generation SR sources. 6 refs.
Date: May 15, 1991
Creator: Perera, R.C.C.; Melczer, M.E.; Warwick, A.; Jackson, A. & Kincaid, B.M.
Partner: UNT Libraries Government Documents Department

Multilayer-coated mirrors as power filters in synchrotron radiation beamlines

Description: Multilayer-coated mirrors, rather than conventional total-reflection mirrors, have been proposed as a means to reduce power incident on the first optical element of high resolution monochromators. We have designed, fabricated, installed and characterized a multilayer pre-mirror specifically for the 800 to 4000 eV range for the X24-A bending magnet beamline at the National Synchrotron Light Source. Various aspects of this application are discussed, including power and thermal considerations, beamline layout considerations and constraints, choice of multilayer materials and substrate, and techniques to ensure lateral uniformity of the multilayer. Results of a preliminary characterization of a mirror coated with a SiC/V multilayer and installed in the beamline are also discussed. 9 refs., 5 figs.
Date: July 1, 1987
Creator: Kortright, J.B.; Plag, P.; Perera, R.C.C.; Cowan, P.L.; Lindle, D.W. & Karlin, B.
Partner: UNT Libraries Government Documents Department

Diagnostic beam line for a third generation storage ring

Description: A knowledge of the position, size, and stability of the source and the angle of emission of synchrotron radiation (SR) from the storage ring are essential for optimizing the operation of storage ring, insertion device and monochromators. Berkeley`s Advanced Light Source (ALS) has a natural emittance of 3.4 {times} 10{sup {minus}9} m-rad, and has beam sizes {sigma}{sub h} and {sigma}{sub v} (assuming a 10% emittance ratio into the vertical direction) in bending magnet 1 (BM1) of 44 {mu}m and 83 {mu}m respectively. Simple diffractive optical calculations show that imaging this beam using visible light optics is not feasible and imaging must be performed using photon energies greater than 50 eV. This will be the same for all third generation low emittance storage rings. The synchrotron radiation diagnostics at ALS will consist of an imaging system for 200 eV photons and a ``white beam`` port with a streak camera to obtain the timing information. The imaging system will employ two crossed spherical mirrors in a Kirkpatrick-Baez configuration, to eliminate astigmatism. Use of 1:1 imaging will eliminate coma, resulting in an image of the source which is only limited by the residual aberrations of the optics. Real time imaging of the beam is deemed feasible by the use of a high resolution CCD, and the associated electronics necessary to read the CCD. The design of the imaging system of the diagnostic beam line for ALS and the detection system will be discussed with a view toward applications in other third generation SR sources. 6 refs.
Date: May 15, 1991
Creator: Perera, R. C. C.; Melczer, M. E.; Warwick, A.; Jackson, A. & Kincaid, B. M.
Partner: UNT Libraries Government Documents Department

Design and performance of the ALS double-crystal monochromator

Description: A new ``Cowan type`` double-crystal monochromator, based on the boomerang design used at NSLS beamline X-24A, has been developed for beamline 9.3.1 at the ALS, a windowless UHV beamline covering the 1-6 keV photon-energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x-rays at NSLS beamline X-A, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length of beamline 9.3.1 (26 m).
Date: August 1, 1994
Creator: Jones, G.; Ryce, S.; Perera, R.C.C.; Lindle, D.W.; Karlin, B.A. & Woicik, J.C.
Partner: UNT Libraries Government Documents Department

X-ray Raman scattering in H-BN observed by soft x-ray fluorescence spectroscopy

Description: Raman scattering of soft x-rays is observed in h-BN using monochromatic soft x-rays just below the B K absorption edge. The inelastic features are visible below threshold, track with the excitation energy, go through a resonance as the excitation is tuned to the B ls core exciton energy, and finally evolve into normal fluorescence as the excitation is raised above the energy needed to excite the B ls electron into the conduction band. The inelastic energy loss is identified as an excitation of valence {sigma} electrons into the {pi}* valence exciton state; at resonance and above, {pi} {minus} {pi}* transitions are also observed. At resonance, a sideband on the elastic peak Ls observed, which gives evidence of additional electronic and phonon loss processes. Very similar results have also been observed for B{sub 2}O{sub 3}.
Date: March 1, 1995
Creator: Jia, J.J.; Callcott, T.A.; Carlisle, J.A.; Terminello, L.J.; Asfaw, A.; Ederer, D.L. et al.
Partner: UNT Libraries Government Documents Department

Resonant soft x-ray fluorescence studies of novel materials

Description: The authors are using resonant soft x-ray fluorescence at the Advanced Light Source to probe the electronic and geometric structure of novel materials. In the resonant process, a core electron is excited by a photon whose energy is near the core binding energy. In this energy regime the absorption and emission processes are coupled, and this coupling manifests itself in several ways. In boron nitride (BN), the resonant emission spectra reflect the influence of a ``spectator`` electron in an unoccupied excitonic state. The resonant emission can be used to distinguish between the various bulk phases of BN, and can also be used to probe the electronic structure of a monolayer of BN buried in a bulk environment, where it is inaccessible to electron spectroscopies. For highly-oriented pyrolytic graphite (HOPG) a coherent absorption-emission process takes place in the resonant regime, whereby crystalline momentum is conserved between the core excited electron and the valence hole which remains after emission.
Date: February 8, 1995
Creator: Carlisle, J.A.; Terminello, L.J.; Hudson, E.A.; Shirley, E.L.; Jia, J.J.; Callcott, T.A. et al.
Partner: UNT Libraries Government Documents Department