Showing papers on "Extended X-ray absorption fine structure published in 1972"

Stimulated Emission and Absorption near Resonance for Driven Systems

Abstract: The rate of absorption of energy from a weak signal field by an atom driven by a strong pump field is evaluated. The pump field and the signal field are assumed to induce transitions between the same pair of states, and their frequencies are both assumed to lie near the atomic resonance frequency for the transition in question. We find that the signal-field absorption line-shape function takes on negative values, representing stimulated emission rather than absorption, even though population inversion does not occur. This amplification of the signal field, which is most pronounced at high pump intensities, is shown to occur primarily at the expense of the pump field, which suffers an increased rate of attenuation. The results are discussed in the context of a theorem which expresses the absorption line-shape function for general atomic systems in terms of a suitable atomic correlation function.

Absorption and Scattering of X-Rays

Abstract: X-rays are absorbed by materials in differing degrees; absorption coefficients are defined to quantitatively describe the magnitudes of this process. Let us assume that when an x-ray beam passes through a thin layer of material a fraction dN/N of the pulse rate N is absorbed. This fraction is proportional to the thickness dx of the layer $$\frac{{d N}} {N} = - \mu dx$$ where μ is a proportionality factor. The negative sign indicates that x-rays are reduced in intensity when passing through matter. If the factor μ is independent of x, we then obtain by integration $$N = {N_0}\exp \left[ { - \mu x} \right]$$ where N 0 is the pulse rate of the primary incident radiation and N the pulse rate after passage through the layer of the thickness x. The term μ is known as the linear absorption coefficient and, in the following, is therefore always written as μ l . It is $$\frac{{mu _l}} = - {\{{1}} {N}{{dN}} {dx}\left[ {{\mu _l}} \right] = c{m^-1}$$ where μ l describes the magnitude of absorption after passage through one centimeter of matter.

Absorption Spectra of Pure Liquid Benzene in the Ultraviolet Region

Abstract: Absorption spectra of pure liquid benzene were measured at several temperatures from room temperature to just below the freezing point of benzene, in the wavelength region 3000–1670 A. Because of the very large absorption coefficient of benzene in this region, the transmission measurements were made with the samples prepared as thin films. Three absorption bands of quite different intensities were observed, corresponding to the π – π* singlet transitions well known in the vapor phase spectra. The spectra obtained are compared with those in the vapor and solid phases.

Structure determination of amorphous Ge, GeO2 and GeSe by fourier analysis of extended x-ray absorption fine structure (EXAFS)☆

Abstract: Fourier inversion of EXAFS data is a powerful new technique for structural investigation of non-crystalline materials with the unique capability to determine the near neighbor environment of each type of atom in a complex material. The radial structure function so obtained contains information on the number, distance, and distribution of atoms surrounding the absorbing atom. The data on Ge and GeO 2 generally agree with previous findings but with added detail. The environment of both Ge and Se in amorphous GeSe show a significant change from the crystalline material with evidence for local satisfaction of the directed bonding tendencies of each atomic species.

Two‐Photon Absorption and Photoconductivity in GaAs and InP

Abstract: Absorption and photoconductivity in GaAs and InP have been studied as functions of light intensity I up to 20 MW/cm2 at 1.06 μ. The two‐photon‐absorption coefficient is determined to be ≈ 0.3I cm−1 for n‐GaAs and ≈ 0.2I cm−1 for n‐InP. These values are much larger than that predicted by the existing theories. For the n‐InP samples, the absorption by the two‐photon‐excited free carriers is found to be important, giving an absorption coefficient of ≈ 0.15I2 cm−1. From the photoconductivity measurements, the concentration and the absorption cross section of holes are determined.

Theory of Two-Photon Absorption by Polaritons

Abstract: We treat theoretically the simultaneous two-photon absorption and sum-frequency generation associated with exciton polaritons in noncentrosymmetric crystals. Expressions are derived for the two-photon absorption coefficient and for the intensity of the polariton beam. We describe the symmetry properties of two-photon absorption by longitudinal excitons and by transverse polaritons.

Effective-impurity model of optical absorption edges☆

Abstract: A simple model of an amorphous or disordered semiconductor is presented in which the disorder is approximated by a distribution of effective impurities. In this model the disorder potential in a given region of the semiconductor is characterized by an effective charge, and the optical absorption in that region is calculated. The total absorption is then a statistical average (over effective charge distributions) of the absorption in the presence of a single effective-impurity. Thus the model leads to an expression for e2(ω) as a functional of the density-of-states, which accounts for electron-hole correlations due to both the final-state (exciton) interaction and the spatial fluctuations of the disorder potential. These effects lead to an exponential suppression of the optical absorption in the edge region, beyond that calculated in an uncorrelated non-direct transition model. Finally, the model is complementary to Franz-Keldysh theories of absorption edge shapes and can be used to describe exponential edges in a variety of non-conducting materials.

Relations between X-Ray L III Absorption Limits and Lβ 2,15 Lines of Mo, Rh, Pd and Ag

Abstract: With a curved crystal spectrometer X-ray L III absorption limits of Mo, Rh, Pd and Ag were studied in relation to the Lβ 2,15 lines, from the two points of view assigning the absorption limit to correspond to transitions from the L III level to (i) the outer lowest unoccupied level and (ii) the Fermi level. For the first standpoint a method of passing the lines through foils of the respective elements was used in combination with the ordinary relative wavelength measurement of the line and the absorption limit. From the results it was concluded that the reverse process of emission of the Lβ 2,15 line is closely involved in the appearance of the L III absorption limit for Mo, Rh and Pd. Next, from the second standpoint the points corresponding to the binding energy of the absorption limit were located on the absorption spectra by using the date of the measurements of photo-electrons and ESCA.

Dominant Second-Order Dipole-Moment Contribution in the Infrared Absorption of III-V Compounds

Abstract: It is shown that a relationship exists between the coefficient describing the infrared lattice absorption and the ionic contribution to the second-order electric susceptibility in a polar noncentrosymmetric crystal. By analyzing the available data of the latter it is concluded that two-phonon sidebands in the infrared absorption are due to the second-order dipole moment in all the III-V compounds and to the third-order potential in ZnO and CdS. Further, the magnitude of the integrated infrared absorption coefficient in a cubic homopolar IV-IV semiconductor is discussed.

Infrared Absorption Coefficients for Certain Pollutant Gases

Abstract: Quantitative analysis of gas mixtures by infrared spectroscopy requires a knowledge of the absorption coefficients,: κν, as a function of optical path length and temperature. For the majority of pollutant gases, information is scarce or incomplete. The objective of this work was to determine absorption coefficients for CH4, C2H4, CO2, CO, SO2, NO2, NO, and H2S. Measurements were made at room temperature and elevated temperatures for certain wave lengths, and κν was determined over a very wide range of optical path length, x. The validity of the Bouguer-Beer law was confirmed for a limiting range of x, and for larger values of x, κν decreased and was correlated as a function of x and T.

Optical Absorption Spectroscopy of ThO2

Abstract: The optical absorption spectra from 200 to 2700 nm were determined for arc-fused ThO2 crystals of much higher purity than any available hitherto. The fundamental absorption edge for these crystals lies at a much higher energy (∼5.9 eV) than the apparent edge reported previously for less pure specimens. This apparent edge is shown to result from one or more prominent high-energy absorption bands superimposed on the true edge for such specimens. By comparing the spectra for different specimens subjected to thermal and γ-irradiation treatments, 31 distinct absorption bands were revealed in the ThO2 spectrum. A full analysis of certain crystals by spark-source mass spectrometry gave no strong correlation between specific band areas and individual impurity-element concentrations. Most of the bands, however, are believed to be associated with the impurity content.

Low-Energy Optical Absorption in ReO3

Abstract: The absorptivity of polycrystalline samples of Re${\mathrm{O}}_{3}$ has been measured in the range 0.11 to 2.8 eV using a calorimetric technique. New structures have been found in the absorptivity, appearing at about 0.4 and 1.0 eV. The data have been Kramers-Kronig analyzed to obtain the optical constants. Band-calculation results of Mattheiss are presented and a tentative identification of the origin of the new structures is made. A comparison of the present data with those of Feinleib, Scouler, and Ferretti is made.

a New Technique to Determine Amorphous Structure Using Extended X-Ray Absorption Fine Structure.

Abstract: : A new theory for extended x-ray absorption fine structure (EXAFS) is presented which treats EXAFS as arising from the scattering of the ejected photoelectron by atoms surrounding the absorbing atom The surrounding atoms are treated as a system of weak point scatterers which allows the scattered waves to be calculated using standard scattering theory These scattered waves are included in the wave function of the ejected photoelectron and this is used to calculate the dipole transition matrix element from the x-ray absorption, which gives rise to oscillations in the absorption coefficient which depends on the positions of the surrounding atoms The final result includes scattering factor, range and temperature terms and is general enough to handle any symmetry of atoms about the absorbing atom and any polarization state of the x-rays

The Electronic Absorption Spectra of Some 2,3-Benzcarbazoloquinones

Abstract: The absorption spectra of quinones were the subject of many investigations; however, the studies did not include heterocyclic quinones. MORTON and ERLAM [1] showed that band positions and intensities are largely influenced by the nature of substituents. PETER and SUMMER [2] ascribed these changes to the electronic character of the substituent. In an investigation on substituted anthraquinone, YOSHIDA andTAKABAYASKi [3] assigned all absorption bands to n — n* transitions within the phenyl rings and the quinone nucleus. Similar assignments were given in a study on some hydroxy anthraquinones [4], In previous studies [5, 6], we discussed the absorption spectra of some carbazoloand dibenzfuranoquinones. In such studies, it was found that substituents on the quinone ring influence considerably the quinone bands. Similarly some changes in the visible region were observed on changing the substituents and type of heterocyclic moeities. However, in these studies no complete band assignment was given. The present work includes the results of an investigation on 2,3-benzcarbazoloquinones in organic solvents of different polarities. The effect of substitution at the 6-position of the carbazole moeity is also considered.

Infrared Absorption Bands of Nitrous Oxide

Abstract: : All of the infrared absorption bands of N2O that are sufficiently intense to absorb significantly in the earth's atmosphere are summarized. The information required to calculate the positions of most of the absorption lines is tabulated along with the intensities of the band systems. The required data on line positions that are not yet available are pointed out. Spectral plots of transmittance and of the absorption coefficient are included for several bands between 530 and 760/cm and between 1585 and 4000/cm.

X-ray mass absorption applied to mineral and rock analysis

Abstract: A method of determining mineral compositions by measurement of the X-ray mass absorption coefficient has been investigated. Absorption measurements have been made on several simple two and three component systems. For a two component system, the total mass absorption coefficient is linearly related to the composition, and rapid calibration may be achieved on the basis of data for the two pure components. The method may be extended in particular circumstances to allow determination of three component systems. Several examples are presented, and the applicability of the technique is discussed. Previous calculations have indicated that the absorption of a rock relative to some standard is almost constant over a large range in wavelength. Direct measurements suggest that this relationship holds only if the relative absorption has a value close to 1.0. The relationship has considerable potential for use in rapid matrix corrections in X-ray fluorescence spectrography, since a single measurement at a co...

Hydrogen plasma absorption coefficients at laser frequencies

Abstract: Detailed numerical calculations of hydrogen-plasma absorption coefficients are performed for a number of laser wavelengths between 0·3371 and 10·6 μ, in a wide range of plasma densities (10 16 -10 21 cm -3 ) and temperatures (0·5–10 5 eV). The numerical data are then utilized to determine plasma temperatures in pulsed linear discharge experiments which measure CO 2 -laser i.r. absorption. Some features of dense plasma laser-heating are also discussed.