Lyman G. Parratt

Bio: Lyman G. Parratt is an academic researcher from Cornell University. The author has contributed to research in topics: Absorption spectroscopy & Absorption edge. The author has an hindex of 7, co-authored 10 publications receiving 4675 citations.

Surface Studies of Solids by Total Reflection of X-Rays

Abstract: Analysis of the shape of the curve of reflected x-ray intensity vs glancing angle in the region of total reflection provides a new method of studying certain structural properties of the mirror surface about 10 to several hundred angstroms deep. Dispersion theory, extended to treat any (small) number of stratified homogeneous media, is used as a basis of interpretation.Curves for evaporated copper on glass at room temperature are studied as an example. These curves may be explained by assuming that the copper (exposed to atmospheric air at room temperature) has completely oxidized about 150A deep. If oxidation is less deep, there probably exists some general reduction of density (e.g., porosity) and an electron density minimum just below an internal oxide seal. This seal, about 25A below the nominal surface plane, arrests further oxidation of more deeply-lying loose-packed copper crystallites.All measurements to date have been carried out under laboratory atmospheric conditions which do not allow satisfactory separation or control of the physical and chemical variables involved in the surface peculiarities. The method, under more controlled conditions of preparation and treatment of the surface, promises to be useful.

Anomalous Dispersion and Scattering of X-Rays

Abstract: The theoretical expressions for the anomalous dispersion of x-rays have been integrated for any positive value of ${p}_{q}$ in a ${C}_{q}{\ensuremath{\lambda}}^{{p}_{q}}$ term in the distribution of "dispersion" oscillators for the $q$ shell of electrons This distribution may be written generally as the sum of $n$ terms of the form ${C}_{\mathrm{qn}}{\ensuremath{\lambda}}^{{p}_{\mathrm{qn}}}$ or, as is commonly done, as a single term Damping has been retained, its effect evaluated and shown to be negligible except for $\ensuremath{\lambda}$ extremely close to the wavelength of an absorption discontinuity With damping neglected, universal dispersion curves are presented If ${p}_{q}$ and ${g}_{q}$ (the oscillator strength) are known, the anomalous part of the refractive index or of the atomic scattering factor can be readily deduced from the universal curvesComparison of the more exact theoretical values with experiment shows less satisfactory agreement than before

"Thickness Effect" in Absorption Spectra near Absorption Edges

Abstract: The details of an observed absorption spectrum are shown to depend upon the thickness of the absorber This effect is present, at least in principle, regardless of the type of radiation involved It is discussed here specifically for x-rays, and specifically for the neighborhood of an absorption edge Measurements of widths and of relative intensities of the component structure are the most sensitively involved, but wavelengths are also slightly affected The explanation lies in the r\^ole of the effective spectral window of the spectrometer The effect may be serious when the "tails" of the window are extensive, as is inevitably the case with two-crystal x-ray spectrometers Features of the extent and shape of the spectral window for the (1, +1) position of the instrument used in this work are roughly determined from the thickness effect

K α Satellite Lines

Abstract: With a two-crystal vacuum spectrometer, ionization curves of the $K{\ensuremath{\alpha}}_{3, 4}$ group of x-ray satellite lines have been recorded for elements S(16) to Ge(32). In the $K{\ensuremath{\alpha}}_{3, 4}$ satellite group are found five component lines for $16\ensuremath{\le}Z\ensuremath{\le}28$ and four components for $29\ensuremath{\le}Z\ensuremath{\le}32$. The wave-length position, relative intensity, and line width at half-maximum intensity of each satellite component has been measured. The widths and shapes of the $K{\ensuremath{\alpha}}_{1, 2}$ lines were determined and a brief discussion of these data is given.

X-Ray Resonance Absorption Lines in the Argon K Spectrum

Abstract: Recorded with a two-crystal vacuum spectrometer, the absorption curve for argon gas near the $K$ absorption edge, 3866 x.u., is analyzed in terms of the main edge and the $1S\ensuremath{\rightarrow}\mathrm{nP} (ng3)$ series of resonance absorption lines. The main edge is determined uniquely as follows: the wave-length position is taken as at the series limit; the width is obtained from the measured width of the most intense absorption line; and the shape is given by the theoretical arctangent curve. The wave-length positions of all the resonance absorption lines are taken from the optical terms of potassium, and, with the width and shape of each line known, the various members of the series $1S\ensuremath{\rightarrow}\mathrm{nP}$ are determined uniquely. The relative intensities of these lines are 100: 34: 18: 8.5, etc. This analysis illustrates the type and completeness of the analysis we should like to make for solid absorbers, but, unfortunately, the definition and meaning of the main edge of a solid absorber are rather obscure.

Relativistic Calculation of Anomalous Scattering Factors for X Rays

Abstract: Anomalous scattering factors Δf′ and Δf″ have been calculated relativistically for Cr, Fe, Cu, Mo, and Ag Kα radiations for the atoms Li through Cf. An interpolation scheme for other wavelengths is included in a separate report. Relativistic calculations of the photoelectric cross section have been made and the integral for the principal contribution to Δf′ has been evaluated numerically without approximation to the form of the cross section‐vs‐energy curve, as has been done in previous calculations. Many of the results are significantly different from previous calculations. Where experimental values exist, agreement for Δf″ is improved. For the rare gases, except for xenon, agreement between Δf′ and experiment is improved. Because of the more rigorous evaluation of Δf′ from cross‐section information, it is presumed that the present Δf′ values are more accurate than previous calculated values. Calculated mass absorption coefficients for the elements are included as incidental information.

Many-electron singularity in X-ray photoemission and X-ray line spectra from metals

Abstract: It is pointed out that the spectra of X-ray induced fast photoelectrons from metal should have a characteristic skew line shape resulting from Kondo-like many-electron interactions of the metallic conduction electrons with the accompanying deep hole in the final state. The same line shape should also occur for the discrete line spectra of X-rays emitted from metals. This mechanism could account for the well-known asymmetries observed for K alpha lines.

Gan : processing, defects, and devices

Abstract: The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

X-Ray Fluorescence Yields, Auger, and Coster-Kronig Transition Probabilities

Abstract: The present status of the field of fluorescence yields, radiationless (Auger and Coster-Kronig) and radiative transition probabilities is summarized. Tables of experimental and theoretical results are included, and tables of "best values" of important quantities are presented.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

Abstract: The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.