Showing papers on "Extended X-ray absorption fine structure published in 1975"

Theory of the extended x-ray absorption fine structure

Abstract: The extended x-ray absorption fine structure is a consequence of the modification of the photoelectron final state due to scattering by the surrounding atoms. We present a theory of the absorption fine structure starting from theoretically obtained electron-atom scattering phase shifts. The electron scattering is treated using a spherical wave expansion which takes into account the finite size of the atoms. Multiple-scattering effects are included by classifying multiple-scattering paths by their total path lengths. Their effects are quite large but appear to make quantitative but not qualitative changes on the single-scattering contribution. The exceptional case is the fourth shell in fcc or bcc structure, where it is shadowed by the first-shell atom and is profoundly affected by forward scattering due to the first shell. This may account for the anomaly observed experimentally at the fourth-shell radius in metals. A detailed numerical calculation is carried out for copper and is shown to agree quite well with experiment.

Extended x-ray-absorption fine-structure technique. III. Determination of physical parameters

Abstract: Fourier transforms of extended x-ray-absorption fine structure (EXAFS) give structural information in the vicinity of each kind of atom, separately, in a wide variety of gaseous, liquid, and solid systems. A detailed description of the analysis of EXAFS data is presented including details of the Fourier transform of the data and the extraction of structural and other physical parameters from these transforms. Included in this description are the measurement of interatomic distances, coordination numbers, disorder effects (thermal and structural), energy-dependent electron scattering amplitudes, inelastic mean free paths, and phase shifts. EXAFS spectra of Ge, Cu, and Ge${\mathrm{O}}_{2}$ are analyzed in detail. Multiple-scattering effects between atoms are generally found to be small. There are no multiple-scattering effects in the first shell of the Fourier transform. The phase shifts introduced by both the absorbing and surrounding atoms empirically appear to be characteristic of the particular atoms and independent of the surroundings for a given class of material. This is of great practical importance because it indicates that EXAFS can be calibrated by measuring known structures and then used to determine unknown ones.

Extended x-ray-absorption fine-structure technique. II. Experimental practice and selected results

Abstract: A technique is presented for obtaining extended x-ray-absorption fine structure (EXAFS) using a conventional, horizontal, x-ray diffractometer. Preparation of monochromator crystals, spectrometer alignment, counting techniques, evaluation of the energy scale and data normalization techniques are discussed. EXAFS spectra from a wide variety of materials are then presented to show the variability of the effect and interplay between various parameters of the theory. A final section illustrates a simple graphical scheme to obtain a first-neighbor distance from EXAFS data.

The temperature-dependent absorption spectrum of the v3 band of SF6 at 10·6 μm

Abstract: Mixtures of SF 6 diluted by argon were heated behind an incident shock front in the range 400 to 1500K. Absorption measurements at 11 wavelengths near 10·6 μm were made on the post-shock, equilibrated gas using a CO 2 laser. A band contour model, which makes use of known spectroscopic constantd and the experimental results, satisfactorily accounts for the observed temperature dependence of the absorption coefficient. We are able to estimate the identity of the transitions responsible for absorption of the laser radiation.

Direct Observation of Atomic Energy Level Shifts in Two-Photon Absorption

Abstract: We report direct observation of optically induced shifts of atomic energy levels in sodium vapor that occur in two-photon absorption. These shifts must be considered in high-resolution two-photon spectroscopy. Two cw dye laser beams of different frequencies and propagating in opposite directions are utilized to obtain high-resolution spectra. The intensity and frequency dependence of the level shifts are examined.

The low-energy absorption edge in 2H-MoS2 and 2H-MoSe2

Abstract: The optical transmission of thick samples of 2H-MoS2 and 2H-MoSe2 has been measured below the A exciton energy for each material. The absorption coefficients have been calculated, and the low absorption starting at 1·1–1·2 eV has been attributed to an indirect d-d transition for both materials. There appears to be considerable d-p valence band overlap giving rise to further higher absorption before the strong A, B excitons, which have been associated with direct transitions from a p-like valence band. An energy band scheme has been sketched on the basis of these interpretations.

Determination of the iron-sulfur distances in rubredoxin by x-ray absorption spectroscopy.

Abstract: The high intensity x-ray flux from the synchrotron radiation at the Stanford Synchroton Radiation Project has been used to study the extended x-ray absorption fine structure (EXAFS) of the iron-sulfur protein Peptococcus aerogenes rubredoxin. Absorption measurements were made from 7080 eV, which is below the K-edge of iron, to about 650 eV above the edge and structure was obtained over the entire region. By means of a model iron-sulfur compound for evaluating the phase shifts, the variation of the absorption above the edge of lyophilized, oxidized rubredoxin was converted to iron-sulfur distances. The data were fitted with a least squares program to a model in which three distances R3 were kept equal and the fourth R1 was allowed to differ. The mean square error was constant over a region of this parameter space, becoming twice as large at R3 = 2.217, R1 = 2.389 and R3 = 2.268, R1 = 2.108 A. These values, which are the extreme differences allowed by the present data, are definitely closer to being equal than those found by the determination of the x-ray diffraction crystal structure of the similar protein from Clostridium pasteurianum. However, the average distance from our experiment is in excellent agreement with the average distance from the crystal structure determination. Preliminary EXAFS measurements were also made on the oxidized rubredoxin in solution at pH 7.0. The spectra were unchanged, indicating that the average iron-sulfur distance change is less than 0.02 A. Upon reduction the average iron-sulfur bond length increased by about 0.05 A. Since the EXAFS measurements can give accurate determinations of distances in proteins both in crystals and solution, the technique should be widely applicable.

Measurement of very low absorption coefficients by laser calorimetry.

Abstract: Laser calorimetric or thermal rise techniques are useful for the determination of very low absorption coefficients in solids. A number of improvements in this technique are described of which the most important is a means of separating surface and bulk absorption. These techniques have been applied to study alkali halides in the ir but are applicable where laser sources of sufficient power are available.

X-ray absorption spectroscopy using synchrotron radiation for structural investigation of organometallic molecules of biological interest.

Abstract: The technique of x-ray absorption spectroscopy using tuneable, very intense x-rays from a high energy electron storage ring has been applied to study of the estended x-ray absorption fine structure for Cu and Ni tetraphenylporphyrin and methemoglobin. Preliminary analysis shows that the spectra may be interpreted as a super-position of modulations arising from the nearest neighbor nitrogen and pyrrole alpha-carbon coordination sheels of the metal atoms. We estimate that with the observed magnitude of noise to modulation amplitude, relative shifts of 0,5% in the metal-nitrogen to metal-carbon bond distances in the prophyrins should be observable using extended x-ray absorption fine structure and that this technique may provide a method of observing these types of structural changes in solution.

The absorption spectrum of Sr I between 40 and 95 Å

Abstract: Structures due to excitation from the 3d and 3p subshells have been observed in the absorption spectrum of Sr I, as well as some window resonances attributed to excitation of two electrons, one from the 3d and one from the 4p subshell. The latter resonances are believed to be the first example to date of simultaneous excitation from two inner shells. The structure is complicated by the collapse of the 4d wave function in the presence of an excited core. A pronounced delayed onset of continuous absorption beyond the 3d limits is compared with theoretical predictions for Z = 36.

Method and apparatus for background signal reduction in opto-acoustic absorption measurement

Abstract: The sensitivity of an opto-acoustic absorption detector is increased to make it possible to measure trace amounts of constituent gases (approaching 0.01 parts per billion) in a sample by creating a second beam radiation path through the sample cell identical to a first path except as to length, alternating the beam through the two paths and minimizing the detected pressure difference for the two paths while the beam wavelength is tuned away from the absorption lines of the sample. Then with the beam wavelength tuned to the absorption line of any constituent of interest, the pressure difference is a measure of trace amounts of the constituent. The same improved detector may also be used for measuring the absorption coefficient of known concentrations of absorbing gases.

Measurements of NH 3 absorption coefficients with a C 13 O 2 16 laser

Abstract: Measurements of NH3 absorption coefficients are presented for several transitions of a C-13(O-16)2 laser for small concentrations of NH3(p less than 1 torr) for absorption lines broadened to 1 atm with N2. NH3 absorption coefficients were determined for laser transitions R(8)(920.2194 wavelengths/cm) to R(28)(933.8808 wavelengths/cm) of the 00 1 - (10 0,02 0)I band. The strongest absorption coefficient K = 36.09 + or - 1.43 per (atm-cm) was measured for the R(18) transition for the NH3 line, aQ(6,6), and is larger than has been found in any previous measurements with a CO2 laser. The dependence of K on total pressure was also obtained for select transitions, and the frequency separation between the R(18) laser transition and the neighboring NH3 line aQ(6,6) was determined to be 550 + or - 50 MHz. These results are significant for long path absorption monitoring of NH3 with CO2 lasers since the path length can be reduced by approximately 40% and for heterodyne detection of NH3 since the relative position of the laser transition to the NH3 absorption line is well within the bandpass of Hg-Cd-Te photomixers.

Measurements of High‐Temperature Absorption Coefficients of Glasses

Abstract: An instrument for measuring spectral transmittance at wavelengths from 0.25 to 7 μm of samples at temperatures between ambient and 1300°C is described. Data showing the effect of temperature on the fundamental, Fe, and water absorption bands in glass are given and discussed.

Far-Infrared Impurity Absorption in Highly Doped n-Type Silicon

Abstract: Far-infrared absorption spectra of uncompensated phosphorus-doped silicon with donor concentrations between 6.8×10 17 and 2.0×10 18 cm -3 have been measured at 6 K. These crystals show an absorption edge in the photon energy region lower than the lowest (1s→2p) excitation line of isolated phosphorus atoms. The position of this absorption energy shifts to lower energy with increasing donor concentration. The thermal donor ionization energy e 1 determined from measurements of the electrical property is situated near the low-energy threshold in the absorption cross-section spectrum, and therefore the absorption is ascribed to the ionization transition of donor electrons into the conduction band states perturbed by interactions with impurities. The absorption cross-section spectrum shows no anomaly at the photon energy equal to the thermal activation energy e 2 , which is characteristic of the intermediate concentration range of impurity conduction.

The free-free absorption coefficient of the negative helium ion

Abstract: The free-free absorption coefficient of the negative ion of atomic helium is evaluated using a multichannel theory based upon the R-matrix method for electron-atom collisions. The results are expected to be accurate to at least 10%.

Molecular — impurity absorption in KC1 for infrared laser windows

Abstract: Residual impurity absorption in the 10µm region severely limits the usefulness of KCl as a high-power CO2 laser window. In order to investigate possible origins for this absorption, KCl was doped with carbonate and hydroxyl impurities, and grown in various atmospheres. Characteristic absorption bands in the infrared and ultraviolet were used to determine relative dopant concentrations. Growth conditions introducing CO/ 3 = exclusively, led to relatively low and featureless absorption spectra in the 10.6µm region, while conditions introducing both CO/ 3 = and OH− led to enhanced absorption. (Additionally, a correlation is indicated between the strength of absorption at 10.6µm and that of the oxygen band at 0.26µm in samples grown in atmospheres in which oxygen was present). Our observations suggest the possibility of absorption due to CO/ 3 ≽ complexes, NO 3 − , HCO 3 − and/or ClO 3 − in the 10µm region. While the effects of ultralow concentration of these impurities remain uncertain, the present results suggest that their presence may well contribute to extrinsic absorption in the 10µm region.

Influence of molecular absorption on propagation of CO2 laser radiation in terrestrial atmosphere (review)

Abstract: The following topics are reviewed from the point of view of the molecular absorption in the atmosphere: 1) propagation of the P20 line of the 0001–1000 transition (λ≈10.6 μ) emitted by a C12O216 laser; 2) propagation of various lines of the 0001–1000 and 0001–0200 transitions emitted by C12O216 lasers; 3) propagation of radiation emitted by C13O216 and C12O218 lasers (0001–1000 and 0001–0200 transitions). It is stressed that three main factors should be considered in reviewing the atmospheric absorption: The continuous absorption of the water vapor, resonance absorption by CO2 molecules, and absorption due to the accidental coincidence of a laser emission line with a telluric absorption line of some gas component of the terrestrial atmosphere. The relative role of these factors depends on the actual laser and on the conditions in the atmosphere, such as temperature, humidity, and pressure. Preference should be given to those lasers for which the absorption due to the last two factors is small compared with the continuous absorption by the water vapor. The latter is constant to within (±20%) throughout the wavelength range 9–11 μ, where the laser radiation under consideration is located. It follows that the C13O216 and C12O218 lasers should be preferred because the concentration of the corresponding isotopes in the atmosphere is approximately 100 times lower than those of the isotopes in C12O216.

The production of photon antibunching by two-photon absorption

Abstract: The two-photon absorption process is, in principle, capable of producing photon beams with second-order coherence less than unity. The dependence of the observability of such values on the initial state of the beam, the absorption path length and the detector sensitivity is outlined. It is concluded that the main problem is not the low intensity involved but rather the low two-photon absorption cross section.

Determining absorption coefficients for trans-conformation bands of polyethylene

Abstract: IR absorption spectra of normal paraffins and PE were used in the frequency range of 680–750 and 1850–1950 cm−1 to determine coefficients of absorption (in absolute units) for amorphous and crystalline components of a band at 720 cm−1, for bands of PE at 730 and 1894 cm−1 and their conformation structure was more accurately defined.

The total absorption coefficient of the negative hydrogen ion

Abstract: The free-free absorption coefficient of the negative hydrogen ion is calculated in both the length and velocity formulations using the method of polarized orbitals to describe the free electron. The bound-free cross section of Bell and Kingston (1967) is converted to an absorption coefficient and combined with the present free-free results. Comparison with other calculations and the use of certain oscillator strength sum rules suggests that the total H- absorption coefficient values of the present paper are accurate to better than 10%.